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Lagrangian observations of the near-surface circulation in the North Pacific, 1990-1995 1998

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LAGRANGIAN OBSERVATIONS OF T H E NEAR-SURFACE CIRCULATION IN T H E NORTH PACIFIC, 1990-1995 b y S t e v e n J . B o g r a d B . S . , B . S . ( P h y s i c s , A t m o s p h e r i c S c i e n c e s ) U n i v e r s i t y o f A r i z o n a , 1 9 8 5 M . S . ( A t m o s p h e r i c S c i e n c e s ) U n i v e r s i t y o f W a s h i n g t o n , 1 9 8 9 A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T OF T H E R E Q U I R E M E N T S F O R T H E D E G R E E OF D O C T O R OF P H I L O S O P H Y ' i n T H E F A C U L T Y OF G R A D U A T E STUDIES E A R T H A N D O C E A N SCIENCES W e a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d T H E U N I V E R S I T Y OF BRITISH C O L U M B I A J u n e 1998 © S t e v e n J . B o g r a d , 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Earth and Ocean Sciences The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1Z1 Date: Abstract A n analysis and interpretation of the position time series, and accompanying derived velocities, obtained from a large set of satellite-tracked surface drifters deployed and tracked throughout the Nor th Pacific Ocean over the period 1990-1995 is presented. As part of Canada's contribution to the World Ocean Circulation Experiment 's Surface Velocity Program ( W O C E - S V P ) , 102 drifters were released in the Nor th Pacific in 12 deployments between August 1990 and November 1994. Each of the drifters was equipped with a drogue centered 15 m (62 drifters) or 120 m (40 drifters) below the surface, i.e., within the mixed layer or in the underlying pycnocline. The principal objective of this work was to describe the observed circulation and its variability at both drogue depths. This was accomplished through several independent analyses, each focusing on a particular suite of statistical methods or subset of drifters. As a first step, the effects of reduced sampling schedules (duty cycles) on derived ve- locity statistics were investigated by degrading continuous data segments from a subset of drifters to match three different duty cycles. It was found that strong high-frequency (primarily inertial) motions prevalent in the drifter records resulted in significantly bi- ased statistics derived from the degraded series, and that reproduction of the original prime and rotary spectral statistics required an interpolation which took into account the oscillatory component of the drifter motions. The trajectories of all drifters were used to characterize the upper-ocean mean cir- culation and eddy variability in the North Pacific Ocean over the period 1990-1995. A l l branches of the Alaskan Gyre were well-sampled at both drogue depths, revealing a weak Subarctic Current and strong, variable flow in the Alaska Current and Alaskan Stream. i i At 15 m depth, the bifurcation of the Subarctic Current was observed near 48°N, 130°W, while at 120 m, northward flow in the Alaska Current occurred much further offshore. A minimum in eddy kinetic energy was observed in the northern subtropical gyre (the "eddy desert"). Eddy kinetic energies were nearly twice as high in the mixed layer com- pared to below, and 2-3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. Taylor's theory of single particle dispersion was applied to the drifter ensembles to es- timate Lagrangian decorrelation scales and eddy diffusivities. Both the initial dispersion and random walk regimes predicted by Taylor's theory were identified in the dispersion time series computed for several regions of both ensembles. The consistency of the re- sults with previous studies suggests that the simplifying assumptions of Taylor (1921) are reasonably valid throughout the upper ocean, which bodes well for the effective pa- rameterization of near-surface diffusivities in general circulation models. Subsets of drifters were used to examine eddy activity in the vicinity of the Em- peror Seamount Chain (ESC) and the Kuril-Kamchatka Trench (KKT) in the western North Pacific. In both regions, drifters were trapped within topographically-controlled mesoscale eddies. The trajectories of two deep-drogued drifters revealed a pair of counter- rotating mesoscale eddies attached to the leeside of Ojin/Jingu Seamount. One of the drifters made five loops within the cyclonic eddy over a period of 62 days, providing one of the first observations to demonstrate an extended attachment of a topographically- generated eddy to a seamount. The observations of attached leeside eddies (or lack thereof) at the ESC match the predictions of numerical and analytical models very well. Further west, drifter trajectories revealed the presence of large anticyclonic eddies posi- tioned over the deepest part of the KKT. These observations support the implication from historical data that long-lived anticyclonic eddies are common in this region. However, the origin and longevity of these eddies remain uncertain. iii Table of Contents Abstract ii List of Tables vii List of Figures x Acknowledgments xxii 1 Introduction 1 1.1 S t u d y i n g O c e a n C i r c u l a t i o n U s i n g L a g r a n g i a n I n s t r u m e n t s 1 1.1.1 L a g r a n g i a n v s . E u l e r i a n M e a s u r e m e n t s 1 1.1 .2 B o t t l e s , C a r d s , D r i f t e r s a n d F l o a t s 2 1.2 T h e s i s O b j e c t i v e s 9 1.3 T h e s i s O v e r v i e w 11 2 Data and Methods 13 2.1 T h e W O C E / S V P D a t a s e t 13 2 .2 I n i t i a l D a t a P r o c e s s i n g 17 2 . 3 D e t e r m i n a t i o n o f D r o g u e L o s s 19 3 Sampling Strategies and Interpolation Schemes 25 3.1 I n t r o d u c t i o n 2 5 3 .2 D a t a a n d M e t h o d s 2 7 3.2 .1 D r i f t e r S e r i e s 2 7 iv 3 . 2 . 2 T i m e se r i es d e g r a d a t i o n 2 7 3 . 2 . 3 T i m e s e r i e s i n t e r p o l a t i o n 2 9 3 .3 R e s u l t s 35 3 .3 .1 P r i m e s t a t i s t i c s 3 5 3 . 3 . 2 R o t a r y s p e c t r a l a n a l y s i s 39 3 .4 D i s c u s s i o n 4 3 3 . 5 C o n c l u s i o n s 50 4 Mean Circulation and Energy Distribution in the North Pacific 53 4 . 1 I n t r o d u c t i o n 5 3 4 . 2 O c e a n o g r a p h i c S e t t i n g o f t h e N o r t h e a s t P a c i f i c 5 4 4 . 3 L a g r a n g i a n D e c o r r e l a t i o n S c a l e s 57 4 .4 S e l e c t i o n o f G r i d G e o m e t r i e s 59 4 . 5 M e a n C i r c u l a t i o n a n d E n e r g y D i s t r i b u t i o n 6 7 4 . 5 . 1 T h e o v e r v i e w 6 7 4 . 5 . 2 T h e A l a s k a n S t r e a m 75 4 . 5 . 3 V a r i a b i l i t y i n t h e A l a s k a n G y r e 77 4 . 6 H i g h F r e q u e n c y E n e r g y D i s t r i b u t i o n 8 4 4 . 7 A p p l i c a t i o n s 9 3 5 Eddy Statistics in the North Pacific 95 5.1 I n t r o d u c t i o n 9 5 5 .2 T h e o r y a n d M e t h o d s 9 5 5 .3 A p p l i c a b i l i t y t o t h e d r i f t e r e n s e m b l e s 9 7 6 Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 113 6.1 I n t r o d u c t i o n 113 v 6 .2 T h e o r e t i c a l B a c k g r o u n d 113 6 .3 D a t a a n d M e t h o d s 119 6 .3 .1 T h e E S C e n s e m b l e 1 1 9 6 . 3 . 2 T h e r o t a r y m u l t i p l e f i l t e r t e c h n i q u e 119 6 .4 O b s e r v a t i o n s 121 6 .4 .1 1991 d r i f t e r s 121 6 . 4 . 2 1 9 9 2 d r i f t e r s 1 2 2 6 . 4 . 3 1 9 9 3 d r i f t e r 139 6 . 4 . 4 S a t e l l i t e a l t i m e t r y 142 6.5 D i s c u s s i o n 143 6 . 5 . 1 C o m p a r i s o n w i t h m o d e l s 1 4 3 6 . 5 . 2 E d d y d i m e n s i o n s 149 6.6 C o n c l u s i o n s 151 7 Eddies II: Anticyclonic Eddies at the Kuri l -Kamchatka Trench 153 7.1 I n t r o d u c t i o n 153 7 .2 O c e a n o g r a p h i c S e t t i n g 1 5 3 7.3 T h e D a t a 158 7.4 T h e 1 9 9 0 E d d y 160 7.5 T h e 1 9 9 3 E d d y 171 7 .6 D i s c u s s i o n 1 7 2 7.7 C o n c l u s i o n 1 7 7 8 Summary and Synthesis I ? 9 8.1 S u m m a r y • • 179 8.2 A S y n t h e s i s 184 v i List o f Tables 2.1 Total and drogue life expectancies by drifter manufacturer. One of the Aanderaa shallow, two of the Seimac shallow, and two of the Technocean deep drifters failed on launch. Note that four of the Technocean shallow drifters were still active on M a y 1, 1996 17 3.1 Means (overbar) and standard deviations of the east-west (u) and north- south (v) currents, including the 95% confidence intervals, derived from the uninterpolated, spline-interpolated and MFF-interpolated series of drifter 1310 36 3.2 As in Table 3.1, but for drifter 15366 38 3.3 Clockwise (S~(UJ)), counterclockwise (5 +(a;)) and total (Stot{w)) rotary variance in four frequency bands derived from the MFF2-interpolated series of the Station P ensemble. Numbers in parentheses refer to the percentage of the total rotary variance. The frequency bands are low mesoscale (periods of 2-8 days), high mesoscale (periods of 17 hours-1.9 days), inertial (periods of 14.7-16.8 hours) and high (periods of 6-14.5 hours). 47 3.4 The rotary coefficient, r(ui), in four frequency bands of the MFF2-interpola ted series of the Station P drifter ensemble. The frequency bands are the same as in Table 3.3 48 vi i 3 .5 T h e c l o c k w i s e (S ) s p e c t r a l a m p l i t u d e r a t i o s i n f o u r f r e q u e n c y b a n d s , a n d f o r t o t a l c l o c k w i s e e n e r g y , o f t h e o r i g i n a l t o t h e d e g r a d e d s e r i e s f o r t h e s p l i n e - i n t e r p o l a t e d a n d M F F - i n t e r p o l a t e d se r i es o f t h e S t a t i o n P d r i f t e r e n s e m b l e . T h e f r e q u e n c y b a n d s a r e t h e s a m e as i n T a b l e 3 .3 4 9 4 .1 D e c o r r e l a t i o n t i m e a n d s p a c e s c a l e s d e r i v e d f r o m t h e g l o b a l a n d r e g i o n a l a u t o c o r r e l a t i o n f u n c t i o n s o f e a c h e n s e m b l e 63 4 . 2 Clockwise (S~ (w)), counterclockwise (5 + (w) ) and total (S t o t (w)) composite rotary vari- ances (cm 2 / s2) in five frequency bands derived from the (a) shallow (previous page) and (b) deep drifter deployments. Numbers in parentheses refer to the percentage of the total rotary variance. The frequency bands are low mesoscale (periods of 6.4-32 days), high mesoscale (periods of 1-5.3 days), near-inertial (periods of 13-24 hours), semidiur- nal (periods of 11.5-12.6 hours) and high (periods of 6-11 hours). Boxed numbers refer to the deployments (see Figure 4.19) 9 2 4 . 3 T h e t o t a l ( S t o t ( u ; ) ) c o m p o s i t e s p e c t r a l a m p l i t u d e r a t i o s ( s h a l l o w / d e e p ) i n five f r e q u e n c y b a n d s f o r e a c h o f t h e d e p l o y m e n t s c o n t a i n i n g s h a l l o w a n d d e e p d r i f t e r s . T h e f r e q u e n c y b a n d s a r e t h e s a m e as i n T a b l e 4 . 2 . B o x e d n u m b e r s r e f e r t o t h e d e p l o y m e n t s (see F i g u r e 4 . 1 9 ) 9 4 5.1 M e a n a n d r . m . s . v e l o c i t i e s i n t h e " T a y l o r " b o x e s . S t a n d a r d e r r o r s ( a t t h e 9 5 % c o n f i d e n c e l e v e l ) a r e b a s e d o n d i f f e r e n t i n t e g r a l t i m e s c a l e s i n t h e z o n a l a n d m e r i d i o n a l d i r e c t i o n s . M e a n p o s i t i o n s r e f e r t o t h e c e n t e r - o f - m a s s l o c a t i o n s 101 5.2 I n t e g r a l t i m e (Tk) a n d l e n g t h (Lk) s c a l e s a n d e d d y d i f f u s i v i t i e s (Kkk), w i t h s t a n d a r d e r r o r s , d e r i v e d f o r t h e " T a y l o r " b o x e s i n t h e s h a l l o w a n d d e e p d r i f t e r e n s e m b l e s . M e a n p o s i t i o n s r e f e r t o c e n t e r - o f - m a s s l o c a t i o n s . . . . 109 v i i i 6.1 Clockwise (S~(w)), counterclockwise (S+(u)) and total (Stot{u)) rotary- variance (cm2 / s2) in three frequency bands derived from the drifter tra- jectories in the vicinity of the Emperor Seamount Chain. Numbers in parentheses refer to percent of total variance. Estimates were derived from the periods M a y 23 to July 27, 1991 (64 days) for drifter 1314, March 26 to August 1, 1991 (128 days) for drifter 1316, and June 23 to December 23, 1992 (180 days) for drifters 1417, 4859, and 8098 128 6.2 Period, diameter, and mean rotational speed of the cyclonic eddy (eddy C) as delineated by the trajectories of drifters 1417 and 4859 132 6.3 Parameters of flow incident on the Emperor Seamount Chain as derived from the drifter trajectories and assuming a horizontal eddy diffusivity of 4 x 10 7 cm21s and a seamount fractional height of 0.83 147 7.1 Period, radius, mean rotational speed, mean 3-hourly speed, and maxi- mum 3-hourly speed of the K K T anticyclonic eddies as delineated by the trajectories of drifters 1315 (Eddy A l ) and 15371 (Eddy A2) 163 7.2 Clockwise (^-(u;)), counterclockwise ( ^ ( w ) ) and total (Stot{w)) rotary variance (cm2/s2) in four frequency bands derived from (a) drifter 1315 over the period November 8, 1990 to February 5, 1991 and (b) drifter 15371 over the period September 5 to December 3, 1993. Numbers in parentheses refer to the percentage of the total rotary variance. The frequency bands are low mesoscale (periods greater than 2 days), high mesoscale (periods of 19.2h - 2 days), near-inertial (periods of 16.1 - 18.7 hours) and high (periods of 6-15.6 hours) 166 ix L i s t o f F i g u r e s 1.1 S c h e m a t i c s h o w i n g t h e d e s i g n o f a m o d e r n s a t e l l i t e - t r a c k e d d r i f t e r . T h e d r o g u e is a h o l e y s o c k c e n t e r e d 15 m b e l o w t h e s u r f a c e . T h e s u r f a c e f l o a t h o u s e s t h e a n t e n n a , b a t t e r y p a c k , a n d s e n s o r s . F r o m S y b r a n d y a n d N i i l e r ( 1 9 9 1 ) 6 2 .1 P r o f i l e s o f t e m p e r a t u r e ( T ) , B r u n t - V a i s a l a f r e q u e n c y ( N ) , a n d d e n s i t y (crt) t a k e n n e a r S t a t i o n P (50° N, 145°W) i n A u g u s t 1 9 9 0 . L o c a t i o n s o f t h e t w o d r o g u e d e p t h s a r e a l s o s h o w n . F r o m T h o m s o n e t a l . ( 1 9 9 8 ) 14 2 .2 D r i f t e r d e p l o y m e n t p o s i t i o n s f o r t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n - s e m b l e s 15 2 . 3 C o m p l e t e t r a j e c t o r i e s o f t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . M a r k s r e f e r t o t h e d e p l o y m e n t p o s i t i o n s , a n d c o r r e s p o n d t o t h e l e g e n d i n F i g u r e 2 .2 2 0 2 .4 T i m e s e r i e s o f d a i l y - a v e r a g e d d r i f t e r s p e e d ( t o p ) a n d a c c e l e r a t i o n d i f f e r e n c e ( b o t t o m ) f o r d e e p d r i f t e r 1319 2 2 2 .5 T i m e l i n e s o f e a c h o f t h e d r i f t e r s i n t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . D a s h e d l i n e s c o r r e s p o n d t o u n d r o g u e d p o r t i o n s o f t h e d r i f t e r t r a j e c t o r i e s . T h e d o t t e d c u r v e is t h e t i m e se r i es o f t o t a l n u m b e r o f d a i l y d r i f t e r o b s e r v a t i o n s i n e a c h e n s e m b l e 2 4 x 3.1 M a p of the region showing the first 90 days of the trajectories of (a) six drifters deployed near Station P (50°N, 145°W) in September 1990 which had strong inertial motions, and (b) a drifter deployed at the head of the G u l f of Alaska in September 1992 which had weaker inertial motions. . . 28 3.2 The 90-day uninterpolated trajectories of drifter 1310 for the (a) original continuous series, and the (b) 48-24h, (c) 32-16h, and (d) 16-8h degraded series. Each mark represents an observation point. The number of ob- servations contained in each series is (a) 947, (b) 287, (c) 239, and (d) 265 30 3.3 The 90-day uninterpolated trajectories of drifter 15366 for the (a) original continuous series, and the (b) 48-24h, (c) 32-16h, and (d) 16-8h degraded series. Each mark represents an observation point. The number of ob- servations contained in each series is (a) 793, (b) 236, (c) 250, and (d) 235 31 3.4 The 90-day interpolated trajectories of drifter 1310. (a) The spline-interpolated continuous series, (b) the spline-interpolated 32-16h degraded series, (c) the MFF2-interpolated continuous series, and (d) the MFF2-interpola ted 32-16h degraded series 33 3.5 The 90-day interpolated trajectories of drifter 15366. (a) The spline- interpolated continuous series, (b) the spline-interpolated 32-16h degraded series, (c) the MFF2-interpolated continuous series, and (d) the M F F 2 - interpolated 32-16h degraded series 34 xi 3.6 The clockwise (S~; solid line) and counterclockwise (S+; dashed line) rotary energy density spectra (m2s~2cpd~1) derived from the first 80-day period of the spline-interpolated series of drifter 1310 for the (a) original continuous series, and the (b) 48-24h, (c) 32-16h, and (d) 16-8h degraded series. The 95% confidence limits and the inertial (/) and semidiurnal ( M 2 ) peaks are shown in (a) 41 3.7 As in Figure 3.6, but for the MFFl - in te rpo la ted original and degraded series of drifter 1310 42 3.8 As in Figure 3.6, but for the MFF2-interpolated original and degraded series of drifter 1310 44 3.9 As in Figure 3.6, but for the MFF2-interpolated original and degraded series of drifter 15366 45 4.1 M a p of the Nor th Pacific Ocean showing the major surface currents. From Tabata (1975) 55 4.2 The global average autocorrelation functions and eddy diffusivities as a function of time lag derived from the (a) shallow and (b) deep drifter ensembles 60 4.3 The regional average autocorrelation functions derived from the shallow drifter ensemble for (a) the Line P/bifurcation region (4795 drifter days used in the estimate), (b) the Subarctic Current (5835 days), (c) the north- ern Gul f of Alaska (1018 days), and (d) the Subtropical Gyre (3842 days). 61 4.4 The regional average autocorrelation functions derived from the deep drifter ensemble for (a) the Line P/bifurcation region (770 drifter days used in the estimate), (b) the Subarctic Current (2251 days), (c) the northern Gul f of Alaska (456 days), and (d) the Subtropical Gyre (2759 days) 62 xi i 4 . 5 T h e t r a j e c t o r i e s o f t h e d r o g u e d s e g m e n t s o f t h e (a ) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s , w i t h o v e r l y i n g g r i d g e o m e t r i e s . T h e b o x m a r k e d " E S C E d d i e s " r e f e r s t o t h e s u b j e c t o f C h a p t e r 6 a n d t h e b o x m a r k e d " K K T E d d i e s " r e f e r s t o t h e s u b j e c t o f C h a p t e r 7 65 4 . 6 M a p s s h o w i n g a n n u a l h i s t o g r a m s o f n u m b e r o f d r i f t e r d a y s i n g r i d b o x e s f o r t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . T h e s c a l e i s g i v e n i n u p p e r r i g h t 66 4 . 7 M a p s s h o w i n g (a ) t h e n u m b e r o f d e g r e e s o f f r e e d o m c o n t a i n e d i n t h e g r i d b o x e s o f t h e s h a l l o w d r i f t e r e n s e m b l e , a n d (b ) t h e d e r i v e d m e a n v e l o c - i t y a n d m e a n k i n e t i c e n e r g y ( M K E ; cm2/s2). B o x e s w i t h f e w e r t h a n 30 d e g r e e s o f f r e e d o m a r e d a s h e d , a n d m a r k s i n ( b ) r e f e r t o t h e b o x c e n t e r - o f - m a s s p o s i t i o n s . M K E c o n t o u r s e x t r a p o l a t e d b e y o n d t h e d a t a r a n g e i n t h e n o r t h w e s t p o r t i o n o f t h e m a p ( B e r i n g S e a ) a r e n o t v a l i d 69 4 .8 M a p s s h o w i n g (a) t h e n u m b e r o f d e g r e e s o f f r e e d o m c o n t a i n e d i n t h e g r i d b o x e s o f t h e d e e p d r i f t e r e n s e m b l e , a n d (b ) t h e d e r i v e d m e a n v e l o c i t y a n d m e a n k i n e t i c e n e r g y ( M K E ; cm2/s2). B o x e s w i t h f e w e r t h a n 30 d e g r e e s o f f r e e d o m a r e d a s h e d , a n d m a r k s i n (b ) r e f e r t o t h e b o x c e n t e r - o f - m a s s p o s i t i o n s . T h e c l o s e d c o n t o u r n e a r 48°N, 150°W is a m a x i m u m 70 4 .9 M a p s s h o w i n g v e l o c i t y v a r i a n c e e l l i p s e s ( g i v e n as ((u'k2J1/2)) i n g r i d b o x e s d e r i v e d f r o m t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . T h e s c a l e i s g i v e n i n u p p e r r i g h t 72 4 . 1 0 Z o n a l v s . m e r i d i o n a l r . m . s . s p e e d s d e r i v e d f r o m t h e s h a l l o w a n d d e e p d r i f t e r e n s e m b l e s . T h e l i n e s a r e l e a s t - s q u a r e s f i t s . T h e d e e p A l a s k a n S t r e a m b o x , w h i c h h a d f e w e r t h a n 10 d e g r e e s o f f r e e d o m , i s n o t i n c l u d e d i n t h i s p l o t 73 X l l l 4.11 Close-up of the mean circulation and eddy kinetic energy (cm2/s2) in the Alaskan Gyre derived from the (a) shallow and (b) deep drifter ensembles. 74 4.12 Maps showing the mean residence times (days) in the grid boxes of the (a) shallow and (b) deep drifter enesmbles 76 4.13 Locations of daily-mean drifter speeds in excess of 40 cm/s in the Alaskan Stream region for (a) winter, 0 m (undrogued), (b) summer, 0 m, (c) winter, 15 m, (d) summer, 15 m, (e) winter, 120 m, and (f) summer, 120 m. Winter is defined as October through March , summer as A p r i l through September. Plusses (winter) and triangles (summer) mark the positions of the high-speed observations, and small dots mark all other daily-mean positions. The 6000 m and 8000 m bathymetry contours (dashed fines) delineate the postion of the Aleutian Trench 78 4.14 Maps showing the mean velocities and eddy kinetic energies (cm2/s2) in the Alaskan Gyre derived from the shallow drifter ensemble for (a) winter and (b) summer. Seasons have the same definition as in Figure 4.13. . . . 80 4.15 Maps showing the 1990-1995 (a) winter and (b) summer mean E k m a n vertical velocity, WE ( X 1 0 - 5 cm/s) , derived from C O A D S winds. Seasons have the same definition as in Figure 4.13. WE is positive upwards. . . . 82 4.16 Time series of C O A D S monthly mean sea level pressure (heavy line) at 53°N, 155°W and its 3-month running mean (dashed line). Also plotted are the monthly values of the Pacific North American ( P N A ) index (octagons), which is defined as a linear combination of the normalized 500 mb height anomalies at four centers located near Hawaii, over the Nor th Pacific, over Alber ta and over the U . S . Gul f Coast. The P N A index is a measure of the strength of the Aleutian Low (high positive P N A corresponds to a stronger Aleutian Low) 83 x i v 4.17 Maps showing the winter mean Ekman vertical velocity, WE (x 10 5 cm/s) , derived from C O A D S winds, for (a) 1991-1992 and (b) 1992-1993. 85 4.18 Maps showing the winter mean Ekman vertical velocity, WE ( X 1 0 - 5 cm/s) , derived from C O A D S winds, for (a) 1993-1994 and (b) 1994-1995. 86 4.19 M a p showing the first 90-day trajectories of the (a) shallow and (b) deep drifter deployments (indicated by numbers) 87 4.20 Composite rotary energy density spectra (clockwise, S~, is solid; coun- terclockwise, S+, is dashed), and 95% confidence hmits, corresponding to each of the shallow drifter deployments. Deployment numbers and dates are given in lower left. A l l spectra are for the first 90-day period after ini t ial deployment 88 4.21 As in Figure 4.20, but for the deep drifter deployments 89 5.1 Spaghetti diagrams for the (a) shallow and (b) deep drifter ensembles. "Taylor" boxes are regions in which Taylor's theories of single particle dispersion are tested . 98 5.2 Displacement "plumes" in the (a) zonal and (b) meridional directions for 247 pseudotrajectories from the shallow box centered at 49.3°N, 145.3°W, within the Subarctic Current region. The mean flow has not been removed from this plot 100 5.3 First 10 days of dispersion (zonal-solid, meridional-dotted) for the shallow "Taylor" boxes. Straight lines (zonal-solid, meridional-dashed) are the theoretical values from equation 5.2. Coordinates refer to center-of-mass positions 102 5.4 As in Figure 5.3, except for the deep "Taylor" boxes 103 xv 5.5 T i m e se r i es o f e d d y d i f f u s i v i t i e s , u s i n g a 5 - d a y r u n n i n g m e a n f i l t e r , d e r i v e d d i r e c t l y f r o m t h e d e r i v a t i v e o f m e a n s q u a r e d i s p e r s i o n f o r o n e (a ) s h a l l o w a n d o n e ( b ) d e e p " T a y l o r " b o x f r o m t h e S u b a r c t i c C u r r e n t r e g i o n 104 5.6 M e a n s q u a r e d i s p e r s i o n ( z o n a l - d a s h e d , m e r i d i o n a l - d o t t e d ) o v e r t h e f i r s t 100 d a y s f o r t h e s h a l l o w " T a y l o r " b o x e s . S o l i d c u r v e s a r e t h e t h e o r e t i c a l v a l u e s f r o m e q u a t i o n 5 .3 . C o o r d i n a t e s r e f e r t o c e n t e r - o f - m a s s p o s i t i o n s . . 106 5 .7 A s i n F i g u r e 5 .6 , e x c e p t f o r t h e d e e p " T a y l o r " b o x e s 1 0 7 5.8 M a p s s h o w i n g t h e z o n a l / m e r i d i o n a l i n t e g r a l t i m e s c a l e s ( d a y s ) a n d e d d y d i f f u s i v i t i e s ( x 1 0 7 cm2/s) f o r t h e " T a y l o r " b o x e s i n t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s 110 5 .9 (a ) I n t e g r a l t i m e s c a l e a n d (b ) e d d y d i f f u s i v i t y v s . r . m . s . s p e e d d e r i v e d f o r t h e " T a y l o r " b o x e s . S t r a i g h t l i n e s a r e l e a s t - s q u a r e s f i t s . T h e f i t s a r e b a s e d o n l y o n t h e r a n g e o f r . m . s . s p e e d s s a m p l e d h e r e , a n d s h o u l d n o t b e e x t r a p o l a t e d t o t h e y - i n t e r c e p t I l l 6.1 M a p o f t h e N o r t h P a c i f i c O c e a n s h o w i n g l o c a t i o n o f t h e E m p e r o r S e a m o u n t C h a i n . L i g h t a n d d a r k s h a d i n g i n t h i s a n d s u b s e q u e n t f i g u r e s r e p r e s e n t s w a t e r d e p t h s o f 2 0 0 0 - 4 0 0 0 m a n d s h a l l o w e r t h a n 2 0 0 0 m , r e s p e c t i v e l y . T h e i n s e t s h o w s t h e b a t h y m e t r i c s e c t i o n a l o n g t h e b o x e d r e g i o n , w h i c h i s t h e c r e s t o f t h e s o u t h e r n p o r t i o n o f t h e E m p e r o r S e a m o u n t C h a i n ( a d a p t e d f r o m R o d e n e t a l . ( 1 9 8 2 ) ) 1 1 7 6.2 M a p o f d y n a m i c t o p o g r a p h y ( J / k g ) o f t h e 150 d b a r s u r f a c e r e l a t i v e t o 8 0 0 d b a r f r o m c r u i s e s o f t h e R V Thomas G. Thompson ( d o t s ) a n d R V Hokusei Maru ( t r i a n g l e s ) i n J u n e / J u l y 1 9 8 2 . F r o m R o d e n ( 1 9 8 7 ) 118 xvi 6.3 Trajectories of shallow-drogued drifters 1314 (dotted line), 1315 (dashed Une) and 1316 (dash-dot Une) which crossed the Emperor Seamount Chain in the summer of 1991. Selected dates are marked by crosses 123 6.4 Trajectory of deep-drogued drifter 1417 which crossed the Emperor Seamount Chain in the summer and fall of 1992. Selected dates are marked by crosses, and the anticyclonic (A) and cyclonic (C) eddies are labeled. Boxes outUne the immediate vicinity of the E S C , which is shown in Figure 6.8. 124 6.5 Trajectory of deep-drogued drifter 4859 which crossed the Emperor Seamount Chain in the summer and faU of 1992. Selected dates are marked by crosses. Boxes outUne the immediate vicinity of the E S C , which is shown in Figure 6.10 125 6.6 Trajectory of shaUow-drogued drifter 8098 which crossed the Emperor Seamount Chain in the summer and fall of 1992. Selected dates are marked by crosses. Boxes outUne the immediate vicinity of the E S C , which is shown in Figure 6.12 126 6.7 Total Stot (solid Une), clockwise S~ (long-dashed Une), and counterclock- wise S+ (short-dashed line) rotary energy density spectra (cm2/s2/cpd) derived from the trajectories of drifters (a) 1417, (b) 4859, and (c) 8098 for the period June 23 to December 23, 1992 129 6.8 Trajectory of drifter 1417 in the vicinity of Oj in /J ingu and K i n m e i Seamounts in the summer and fall of 1992. Each mark represents the daily drifter po- sition at 1200Z, and selected dates are labeled. The anticyclonic (A) and cyclonic (C) eddies are labeled 131 xvii 6.9 A m p l i t u d e e v o l u t i o n o f t h e c l o c k w i s e a n d c o u n t e r c o c k w i s e r o t a r y v e l o c i t y c o m p o n e n t s f o r d r i f t e r s ( a , b ) 1 4 1 7 , ( c , d ) 4 8 5 9 , a n d (e , f ) 8 0 9 8 f o r t h e p e r i o d J u n e 23 t o D e c e m b e r 2 3 , 1992 . A m p l i t u d e c o n t o u r s a r e g i v e n i n c m / s . R o t a r y c o m p o n e n t s f o r t h e l o w - f r e q u e n c y r a n g e a r e s h o w n ( l o g ( - 0 . 6 ) = p e r i o d o f 4 d a y s , l o g ( - l . O ) = p e r i o d o f 10 d a y s , l o g ( - 1 . 4 ) = p e r i o d o f 2 5 d a y s ) . T h e d a s h e d U n e n e a r t h e b o t t o m o f e a c h p l o t i n d i c a t e s t h e t i m e p e r i o d t h a t t h e d r i f t e r w a s i n t h e v i c i n i t y o f t h e E S C ( b e t w e e n 166°E a n d 174°E) 133 6 .10 T r a j e c t o r y o f d r i f t e r 4 8 5 9 i n t h e v i c i n i t y o f O j i n / J i n g u a n d K i n m e i S e a m o u n t s i n t h e s u m m e r a n d f a l l o f 1992 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o - s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d 134 6 .11 (a) T r a j e c t o r y o f d r i f t e r 4 8 5 9 i n t h e c y c l o n i c e d d y ( e d d y C ) i n t h e l e e o f O j i n / J i n g u S e a m o u n t b e t w e e n A u g u s t 4 a n d O c t o b e r 7, 1 9 9 2 . E a c h d i f f e r - e n t U n e t y p e r e p r e s e n t s a s e p a r a t e l o o p a r o u n d t h e e d d y . T h e a p p r o x i m a t e c e n t e r o f t h e e d d y , as d e t e r m i n e d b y t h e d r i f t e r t r a c k , i s m a r k e d w i t h a n o c t a g o n f o r e a c h l o o p , ( b ) T i m e se r i es o f r o t a t i o n a l s p e e d d e r i v e d f r o m d r i f t e r 4 8 5 9 i n e d d y C . E a c h o f t h e f i v e l o o p s o f e d d y C is m a r k e d a t b o t t o m . 136 6 . 1 2 T r a j e c t o r y o f d r i f t e r 8 0 9 8 i n t h e v i c i n i t y o f O j i n / J i n g u a n d K i n m e i S e a m o u n t s i n t h e s u m m e r a n d f a U o f 1992 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o - s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d 140 6 . 1 3 T r a j e c t o r y o f s h a U o w - d r o g u e d d r i f t e r 4 8 5 6 , w h i c h c r o s s e d t h e E m p e r o r S e a m o u n t C h a i n i n t h e w i n t e r o f 1 9 9 3 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d 141 xvin 6.14 (a) M a p of sea surface height anomalies (cm) from merged T O P E X / E R S - 1 altimetry for T O P E X cycle 2 (October 3-12, 1992) for the Nor th Pa- cific basin from 20°JV-55°/V and 150°£-130°W. The axis of the Emperor Seamount Chain is marked with the double-dashed line, (b) A close-up of the boxed region around the E S C from T O P E X cycle 2, with the 10-day trajectories (October 3-12) of drifters 1417 (squares), 4859 (stars), and 8098 (octagons) included and the anticyclonic (A) and cyclonic (C) eddies revealed by the tracks of drifters 1417 and 4859 labeled 144 7.1 (top) M a p of the Nor th Pacific Ocean and surrounding area, showing the location of the Kur i l -Kamchatka Trench ( K K T ) . (bottom) Chart of the K K T region. From Kono (1996) 154 7.2 (a) The track of Kuroshio warm-core ring 86B over a five-year period derived from satellite and ship data. The inset shows the translation velocity of the eddy. From Lobanov et al. (1991). (b) The tracks of several eddies translating northeastward in the K K T region. From Lobanov and Bulatov (1993) 157 7.3 M a p of the region showing the trajectories of drifter 1315 in eddy A l and drifter 15371 in eddy A 2 . Hexagons mark the deployment locations. The bathymetry contours in this and subsequent maps are in meters 159 7.4 Trajectory of drifter 1315 within Eddy A l over (a) loop 1 (Nov.8-12), (b) loop 2 (Nov.13-18), (c) loop 3 (Nov.19-27), (d) loop 4 (Nov.28-Dec.6), and (e) loop 5 (Dec.7-Dec.21). Small 'x's denote daily positions at 1200Z and stars mark the position of the eddy center (as determined by the mean drifter position during that loop.) 161 xix 7.5 T i m e s e r i e s o f (a) l o n g i t u d e a n d (b ) l a t i t u d e f r o m d r i f t e r 1315 i n E d d y A l d u r i n g N o v e m b e r - D e c e m b e r , 1990 . T h e p e r i o d s o f e a c h l o o p a r e m a r k e d a t b o t t o m 162 7.6 C l o s e - u p o f t h e t r a j e c t o r y o f d r i f t e r 1315 w i t h i n E d d y A l o v e r (a ) l o o p 1 a n d ( b ) l o o p 2. A h e x a g o n m a r k s t h e l o o p ' s s t a r t p o s i t i o n , ' x ' s d e n o t e 3 - h o u r l y p o s i t i o n s , a n d s t a r s d e n o t e p o s i t i o n s w h e r e 3 - h o u r l y d r i f t e r s p e e d s w e r e g r e a t e r t h a n 100 c m / s 164 7.7 T i m e s e r i e s o f z o n a l ( U ) a n d m e r i d i o n a l ( V ) 3 - h o u r l y s p e e d s f r o m d r i f t e r 1 3 1 5 i n E d d y A l o v e r l o o p s 1 a n d 2 , N o v e m b e r 8 - 1 9 , 1 9 9 0 165 7.8 C l o c k w i s e S~ ( s o l i d U n e ) a n d c o u n t e r c l o c k w i s e S+ ( d a s h e d U n e ) r o t a r y e n e r g y d e n s i t y s p e c t r a (m2/s2/cpd) d e r i v e d f r o m t h e t r a j e c t o r i e s o f d r i f t e r s (a ) 1 3 1 4 , (b ) 1 3 1 5 , a n d (c ) 1316 o v e r t h e p e r i o d N o v e m b e r 8 , 1 9 9 0 t o F e b r u a r y 6, 1 9 9 1 , a n d (d ) 15371 o v e r t h e p e r i o d S e p t e m b e r 4 t o D e c e m b e r 3 , 1 9 9 3 . T h e 9 5 % c o n f i d e n c e U m i t s a r e s h o w n i n (a ) , a n d t h e i n e r t i a l ( / ) a n d s e m i d i u r n a l ( M 2 ) p e a k s a r e s h o w n i n ( b ) a n d (d ) 167 7.9 A m p U t u d e e v o l u t i o n o f t h e c l o c k w i s e r o t a r y v e l o c i t y c o m p o n e n t f o r d r i f t e r 1 3 1 5 i n E d d y A l a t (a) h i g h f r e q u e n c i e s ( p e r i o d s o f 11 h o u r s t o 1.25 d a y s ) a n d ( b ) l o w f r e q u e n c i e s ( p e r i o d s o f 2 t o 20 d a y s ) . C o n t o u r s a r e i n c m / s . T h e s t a r t d a t e i s N o v e m b e r 8, 1990 , a n d t h e d o t t e d U n e i n (a ) r e f e r s t o t h e l o c a l v a l u e o f p l a n e t a r y v o r t i c i t y ( / ) a t t h e d r i f t e r p o s i t i o n 170 7 .10 T i m e se r i es o f (a) l o n g i t u d e a n d (b ) l a t i t u d e f r o m d r i f t e r 1 5 3 7 1 i n E d d y A 2 d u r i n g S e p t e m b e r - O c t o b e r , 1 9 9 3 173 x x 7.11 Ampli tude evolution of the clockwise rotary velocity component for drifter 15371 in Eddy A 2 at (a) high frequencies (periods of 10 hours to 2 days) and (b) low frequencies (periods of 2 to 20 days). Contours are in cm/s. The start date is September 5, 1993, and the dotted lines in (a) refer to the semidiurnal t idal frequency ( M 2 ) , the local value of planetary vorticity at the drifter position ( / ) , and the two dominant diurnal t idal constituents (Kx and Oi) 7.12 (top) Acceleration potential anomaly (10 m2js2) on the — 26.8 surface referred to 1500 db, derived from C T D casts taken from the R / V Hokko M a r u i n August-September, 1990. (bottom) Potential temperature (°C) on the ae = 26.8 surface, from the same C T D survey as (top). From Kono (1996) xxi A c k n o w l e d g m e n t s Firs t , I would like to thank Dr . Pau l LeBlond and Dr . Rick Thomson for supporting me financially and for providing me with the opportunity to work with an exciting data set. I am grateful for their patience and for the encouragement they have given me throughout my studies. I am also grateful for their willingness to send me to conferences in exotic places like Honolulu, San Diego, and Kelowna. I thank my other committee members, Dr . Steve Pond and Dr . Phyllis Stabeno, who took the time to attend committee meetings and carefully review my work. I feel very fortunate to have worked with this committee, and to have had the rewarding experience of drawing from their vast store of oceanographic knowledge. I greatly appreciate the help and guidance of Dr . Sasha Rabinovich, who provided spectral analysis code and who, in meticulously reviewing much of my work, was never afraid to tell me when I was mistaken. I am also very grateful to Jane Eert , who helped out with data processing with remarkable proficiency. I am also thankful for the com- puter support provided by Denis Laplante, who was always there to mediate when I had conflicts with the computers. I thank my colleagues and friends in the U B C Oceanography Department, David , Fred, A d a m , A n a and many others, who have helped to make this more than just an educational experience. Finally, I want to extend my deepest appreciation to L i Xuhua , whose patience, kindness, love and support have been essential to my well-being. She has given me the right perspective on life, even though I don't always demonstrate it. W o ai ni , fei chang, fei chang! xx i i Chapter 1 Introduction 1.1 Studying Ocean Circulation Using Lagrangian Instruments 1.1.1 Lagrangian vs. Eulerian Measurements In an oceanographic context, Lagrangian instruments are defined as free-floating devices which measure water velocity directly by passively drifting with the prevailing currents. Because of their potentially unlimited spatial range, they have a tremendous advantage over their Eulerian, or moored, counterparts. The interpretation of Eulerian velocity measurements, which generally consist of data from a set of current meters spaced ver- tically along a mooring Une, requires a great deal of extrapolation in order to place the point measurements into a broader spatial perspective. Lagrangian instruments are free to move and, given enough of them and an appropriate deployment distribution, are ca- pable of mapping current velocities over an entire ocean basin in a relatively short period of time. There is, of course, a flip side to the Lagrangian instrument's mobiUty. A n y particular location in the ocean is not Ukely to be sampled repeatedly (or even twice), hindering temporal resolution. Furthermore, Lagrangian devices are typicaUy restricted to current measurements at one vertical level, i.e., the level of a subsurface drogue which has a geom- etry designed to offset the wind effects on a surface buoy. Eulerian instruments describe vertical structure and resolve its temporal variability at a point. Lagrangian instruments, however, are the most efficient tools for studying the kinematics of mesoscale circulation 1 Chapter 1. Introduction 2 f e a t u r e s a n d f o r d e s c r i b i n g t h e l a r g e - s c a l e g e n e r a l c i r c u l a t i o n . W i t h t h e d e p l o y m e n t o f e n o u g h f r e e - f l o a t i n g i n s t r u m e n t s (a t m u l t i p l e d r o g u e d e p t h s i f v e r t i c a l r e s o l u t i o n i s d e - s i r e d ) , t h e r e w i l l b e s u f f i c i e n t d a t a t o d e r i v e r o b u s t v e l o c i t y s t a t i s t i c s w i t h a s p a t i a l a n d t e m p o r a l r e s o l u t i o n w h i c h c a p t u r e s t h e d o m i n a n t s c a l e s o f t h e h o r i z o n t a l c u r r e n t s . F o r - t u n a t e l y , L a g r a n g i a n i n s t r u m e n t s a r e r e l a t i v e l y s i m p l e a n d i n e x p e n s i v e d e v i c e s , m a k i n g s u c h l a r g e - s c a l e d e p l o y m e n t s f e a s i b l e . 1.1.2 Bottles, Cards, Drifters and Floats T h e e a r l i e s t c u r r e n t m e a s u r e m e n t s w e r e m a d e u s i n g t h e s i m p l e s t L a g r a n g i a n i n s t r u m e n t s : o b j e c t s p l a c e d o n t h e o c e a n s u r f a c e a n d v i s u a l l y t r a c k e d . T h e n a t u r e o f t h e o b j e c t s w a s u n i m p o r t a n t ( a n d i n c l u d e d e v e r y t h i n g f r o m w o o d e n p o l e s t o c o m p u t e r c a r d s t o p a r s n i p s ) , as l o n g as t h e y f l o a t e d a n d r e m a i n e d v i s i b l e ( D a v i s , 1 9 9 1 ) . S u c h m e a s u r e m e n t s w e r e m a d e i n t h e G u l f S t r e a m o v e r 2 0 0 y e a r s a g o , w h e r e b u o y s w i t h d r o g u e s t o m i n i m i z e w i n d a g e e f f e c t s w e r e t r a c k e d f r o m a n c h o r e d s h i p s ( F r a n k l i n , 1 7 8 5 ) . S t o m m e l ( 1 9 4 9 ) u s e d a e r i a l p h o t o g r a p h y o f f l o a t i n g p a p e r s h e e t s t o s t u d y s u r f a c e t u r b u l e n c e a n d d i f f u s i o n . T h e r e q u i r e m e n t o f v i s u a l t r a c k i n g , h o w e v e r , l i m i t e d t h e s c a l e s o v e r w h i c h o b s e r v a t i o n s c o u l d b e e x t e n d e d . E a r l y o b s e r v a t i o n s o n l a r g e r s c a l e s w e r e m a d e u s i n g d r i f t c a r d s a n d b o t t l e s , w h i c h c a r r i e d i n s t r u c t i o n s f o r n o t i f i c a t i o n u p o n d i s c o v e r y . W i t h o n l y s t a r t a n d e n d p o i n t s ( a n d o f t e n m i s c h i e v o u s d i s c o v e r e r s ) , i n t e r p r e t a t i o n o f t h e c a r d a n d b o t t l e r e s u l t s w a s n e a r l y i m p o s s i b l e ( D a v i s , 1 9 9 1 ) . S h i p s a r e a l s o d e f a u l t L a g r a n g i a n c u r r e n t m e a s u r e r s . I n f a c t , m u c h o f t h e e a r l y i n f o r m a t i o n r e g a r d i n g o c e a n s u r f a c e c u r r e n t s w a s d e r i v e d f r o m o b s e r v a t i o n s o f s h i p d r i f t ( R i c h a r d s o n , 1 9 9 7 ) . C o m p a r i s o n s b e t w e e n t h e a c t u a l a n d i n t e n d e d t r a c k s o f a s h i p g i v e a d i r e c t m e a s u r e o f t h e s u r f a c e c u r r e n t s , a n d s h i p ' s n a v i g a t i o n l o g s g o i n g b a c k t o t h e 1 9 t h c e n t u r y h a v e b e e n u s e d t o e x t r a c t s u c h i n f o r m a t i o n . M o r e r e c e n t l y , r a d a r - t r a c k i n g i n c r e a s e d t h e s p a t i a l a n d t e m p o r a l r a n g e s o f d r i f t i n g d e v i c e s , a l t h o u g h s i g n i f i c a n t w i n d d r a g o n t h e r a d a r r e f l e c t o r l i m i t e d t h e i r Chapter 1. Introduction 3 current-following ability (Davis, 1991). W i t h a limited tracking range, their most effective use was in mapping mesoscale features in near-coastal waters (e.g., Re id et al. , 1963). The observation of ocean circulation was revolutionized with the advent of satellite technology, which made global tracking of Lagrangian instruments possible. Presently, surface buoys with satellite transmitters are tracked with the A R G O S satellite system, which uses the Doppler shift of the buoy's 400 M H z signal, received from polar orbiting satellites, to provide position fixes several times per day with an accuracy of 200-300 m at the 95% confidence interval. These data are transmitted to a centralized ground station and to the principal investigators in near real-time. As satellite technology has matured and transmitters have become less expensive, large numbers of surface drifters have been deployed to derive the "mean" near-surface circulation in all of the world's ocean basins. Ear ly large-scale drifter deployments provided descriptions of the mean circulation and its variability in the Nor th Atlantic (Richardson, 1983; Krauss and Kase, 1984; Krauss, 1986), the subtropical gyre of the North Pacific (Kirwan et al. , 1978; McNal ly , 1981; M c N a l l y et al . , 1983), and the Antarctic Circumpolar Current of the Southern Ocean (Patterson, 1985; Hofmann, 1985; Daniault and Menard, 1985; Large and van Loon, 1989). Richardson (1983) used 110 drifters as "mobile current meters" (Davis, 1991) to calculate Eulerian mean velocity and its variance in 2° x 2° grid boxes through- out the Nor th Atlant ic , setting the standard for large-scale Lagrangian circulation mea- surements. Brugge (1995), also using a North Atlantic drifter data set, modified this approach by defining an objective criterion based on statistical significance to determine an optimal grid geometry, which may include boxes of unequal dimension. Richardson and Reverdin (1987), Mol inar i et al. (1990), and Reverdin et al. (1994) had sufficient drifter data to resolve the seasonal cycle of currents in the equatorial At lant ic Ocean, the tropical Indian Ocean, and the equatorial Pacific Ocean, respectively. Part icularly well- sampled regions are the California Current System off the west coast of Nor th America , Chapter 1. Introduction 4 w h e r e a s p e c t s o f i t s m e s o s c a l e s t r u c t u r e ( e d d i e s a n d f i l a m e n t s ) a n d t e m p o r a l v a r i a b i l i t y h a v e b e e n r e s o l v e d ( D a v i s , 1 9 8 5 a , b ; P o u l a i n a n d N i i l e r , 1989 ; B r i n k et a l . , 1 9 9 1 ; S w e n s o n e t a l . , 1 9 9 2 ; S w e n s o n a n d N i i l e r , 1 9 9 6 ) , a n d t h e t r o p i c a l P a c i f i c , w h e r e w e l l o v e r 1 0 0 0 d r i f t e r s h a v e b e e n d e p l o y e d s i n c e 1988 , r e s u l t i n g i n d e t a i l e d e x a m i n a t i o n s o f t h e n e a r - s u r f a c e o c e a n i c r e s p o n s e t o v a r i o u s p h a s e s o f t h e E l N i n o - S o u t h e r n O s c i l l a t i o n ( N i i l e r , 1 9 9 6 ; B i a n d N i i l e r , 1 9 9 8 ; R a l p h et a l , 1 9 9 7 ) . L a g r a n g i a n i n s t r u m e n t s a r e i d e a l l y s u i t e d f o r t h e s t u d y o f d i s p e r s i o n b e h a v i o r i n t h e o c e a n . D r i f t e r t r a j e c t o r i e s f r o m t h e N o r t h a n d S o u t h A t l a n t i c a n d t h e n o r t h e a s t P a c i f i c h a v e b e e n u s e d t o d e t e r m i n e t h e d i s p e r s i o n c h a r a c t e r i s t i c s o f t h e d r o g u e - l e v e l f l o w , a n d t o s u b s e q u e n t l y d e r i v e e d d y m i x i n g s c a l e s ( d e c o r r e l a t i o n l e n g t h a n d t i m e s c a l e s ) a n d h o r i z o n t a l e d d y d i f f u s i v i t i e s ( C o l i n d e V e r d i e r e , 1 9 8 3 ; D a v i s , 1 9 8 5 b ; K r a u s s a n d B o n i n g , 1 9 8 7 ; F i g u e r o a a n d O l s o n , 1989 ; T h o m s o n e t a l . , 1 9 9 0 ; S c h a f e r a n d K r a u s s , 1 9 9 5 ) . D i s p e r s i o n b y t h e e d d y f i e l d h a s a p r o f o u n d i m p a c t o n t h e d i s t r i b u t i o n o f p a s s i v e t r a c e r s i n t h e o c e a n . Q u a n t i f y i n g t h e o c e a n ' s e d d y - d i s p e r s i v e c h a r a c t e r i s t i c s c a n l e a d t o i m p r o v e d p a r a m e t e r i z a t i o n s i n e d d y - r e s o l v i n g g e n e r a l c i r c u l a t i o n m o d e l s , a n d a b e t t e r u n d e r s t a n d i n g o f t u r b u l e n t m i x i n g p r o c e s s e s . A n o t h e r i m p o r t a n t a p p l i c a t i o n o f s u r f a c e d r i f t e r s i s i n s t u d y i n g t h e d y n a m i c s a n d k i n e m a t i c s o f m e s o s c a l e c i r c u l a t i o n f e a t u r e s . E d d i e s a r e u b i q u i t o u s f e a t u r e s o f t h e w o r l d ' s o c e a n s , a n d i t i s n o t s u r p r i s i n g t h a t m a n y d r i f t e r s h a v e b e e n e n t r a i n e d i n t o t h e m , e i t h e r b y d e s i g n o r s e r e n d i p i t y . N u m e r o u s d e s c r i p t i o n s o f t h e d i m e n s i o n s ( s i z e a n d r o t a t i o n a l s p e e d ) , p r o p a g a t i o n , a n d k i n e m a t i c s ( e . g . , d i v e r g e n c e , v o r t i c i t y , s t r e t c h i n g d e f o r m a t i o n a n d s h e a r i n g d e f o r m a t i o n ) b a s e d o n d r i f t e r t r a j e c t o r i e s h a v e b e e n p r e s e n t e d ( K i r w a n et a l , 1 9 8 4 ; B o o t h , 1 9 8 8 ; T h o m s o n et a l , 1990 ; H a n s e n a n d M a u l , 1 9 9 1 ; P i n g r e e a n d L e C a n n , 1 9 9 1 ; P i n g r e e a n d L e C a n n , 1 9 9 2 a , b ; P i n g r e e , 1 9 9 4 ; P i n g r e e , 1 9 9 5 ; S a n d e r s o n , 1 9 9 5 ; P i n g r e e , 1 9 9 6 ; P i n g r e e et a l . , 1996 ; P i n g r e e , 1 9 9 7 ) . D r i f t e r s h a v e a l s o s a m p l e d f i l a m e n t s o f h i g h - s p e e d , o f f s h o r e - f l o w i n g w a t e r s a l o n g t h e w e s t c o a s t o f N o r t h A m e r i c a Chapter 1. Introduction 5 (Thomson and Papadakis, 1987; Swenson et al., 1992). Eddies are vitally important features of the ocean circulation, as they impact mixing processes and the distribution of tracers, and affect the large-scale mean circulation in ways that are still not entirely un- derstood. They are also regions of enhanced biological activity, so they play an important role in the exploitation of resources. The most critical issue related to the reliability of drifter measurements is the effec- tiveness of the drogue. The effects of wind, surface waves, and vertical shear can cause drifters to "slip" through the water, making them imperfect Lagrangian (i.e., water fol- lowing) instruments. Thus, drogue designs have sought to increase the water drag and maximize coupling with the currents at the drogue level. Ear ly drogue designs included parachutes and window shades, which were not particularly stable nor easy to deploy. More recent developments are the T R I S T A R and holey sock drogues, which have a large drogue-to-surface-float area ratio to reduce wind slippage, and a subsurface float between the drogue and surface and low tension on the connecting tether to reduce wave effects on the drogue (Niiler et a l , 1987; Kennan et a l , 1998). Niiler et al. (1995) found that the slippage induced by winds and vertical shear over the length of the drogue could be reduced to < 1 cm/s in 10 m/s winds i f the ratio of the drag area of the drogue to sum of the drag areas of the tether and surface float is at least 40. (Drag area is the product of the object's drag coefficient and surface area.) The standard drogue assembly currently being used for near-surface drifter deployments is the holey sock, which is lightweight, durable, inexpensive to manufacture and easy to deploy. Figure 1.1 shows a schematic of the modern satellite-tracked drifter, with a holey sock drogue centered 15 m below the surface (Sybrandy and Niiler, 1991). As of early 1997, more than 750 of these drifters were being tracked globally (Kennan et al., 1998). Drogued surface floats are impractical for measuring deep ocean currents. In the early 1950's, John Swallow invented the neutrally buoyant float ("Swallow float"), which Chapter 1. Introduction 6 F i g u r e 1 .1 : S c h e m a t i c s h o w i n g t h e d e s i g n o f a m o d e r n s a t e l l i t e - t r a c k e d d r i f t e r . T h e d r o g u e i s a h o l e y s o c k c e n t e r e d 15 m b e l o w t h e s u r f a c e . T h e s u r f a c e float h o u s e s t h e a n t e n n a , b a t t e r y p a c k , a n d s e n s o r s . F r o m S y b r a n d y a n d N i i l e r ( 1 9 9 1 ) . Chapter 1. Introduction 7 i s a d j u s t e d b e f o r e l a u n c h t o d e s c e n d t o a n d r e m a i n a t a s p e c i f i c d e n s i t y l e v e l . T h e f l o a t e m i t s a c o u s t i c s i g n a l s , w h i c h a r e p i c k e d u p b y h y d r o p h o n e s o n a s h i p ( o r s h i p s ) w h i c h " c h a s e " t h e f l o a t ( S w a l l o w , 1 9 5 5 ) . A t i m e se r i es o f f l o a t p o s i t i o n i s t h e n o b t a i n e d f r o m t h e s h i p ' s t r a c k . S w a l l o w f l o a t s h a d a n i m m e d i a t e a n d d r a m a t i c i m p a c t o n t h e c o n v e n t i o n a l c o n c e p t i o n s o f d e e p o c e a n c i r c u l a t i o n . I n s t e a d o f f i n d i n g v e r y l o w s p e e d s a t g r e a t d e p t h , as h a d b e e n g e n e r a l l y a s s u m e d i n c o m p u t i n g g e o s t r o p h i c v e l o c i t i e s f r o m h y d r o g r a p h i c d a t a , S w a l l o w f l o a t s u n d e r t h e G u l f S t r e a m r e v e a l e d a v i g o r o u s , s o u t h w a r d - f l o w i n g u n d e r c u r r e n t ( S w a l l o w a n d W o r t h i n g t o n , 1 9 6 1 ) . L a t e r f l o a t e x p e r i m e n t s r e v e a l e d s u r p r i s i n g l y e n e r g e t i c f l o w s o n s c a l e s o f 0 ( 5 0 k m ) , i . e . t h e m e s o s c a l e ( S w a l l o w , 1 9 7 1 ) . A l t h o u g h S w a l l o w f l o a t s g r e a t l y a d v a n c e d u n d e r s t a n d i n g o f d e e p o c e a n c i r c u l a t i o n , t h e i r n e e d f o r c o n t i n u o u s s h i p t r a c k i n g m a d e d e p l o y m e n t s i n l a r g e n u m b e r s i m p r a c t i c a l a n d p r o h i b i t i v e l y e x p e n s i v e . T h e n e x t d e v e l o p m e n t i n f l o a t t e c h n o l o g y r e l i e d o n t h e f o r t u i t o u s c i r c u m s t a n c e s t h a t t h e r e i s l i t t l e a b s o r p t i o n o f l o w - f r e q u e n c y s o u n d b y s e a w a t e r , a n d t h a t t h e r e i s a m i d - d e p t h m i n i m u m o f s o u n d s p e e d i n t h e o c e a n , w i t h i n w h i c h r e f r a c t e d s o u n d w a v e s c a n t r a v e l g r e a t d i s t a n c e s . T h i s i s t h e S O F A R ( S O u n d F i x i n g A n d R a n g i n g ) c h a n n e l , a n d f l o a t s o p e r a t i n g w i t h i n o r n e a r i t ( S O F A R f l o a t s ) h a v e t r a c k i n g r a n g e s o f 1 5 0 0 k m o r m o r e ( D a v i s , 1 9 9 1 ) . T r a c k i n g o f l a r g e n u m b e r s o f f l o a t s s i m u l t a n e o u s l y o v e r a b r o a d r e g i o n c o u l d n o w b e a c h i e v e d f r o m a n a r r a y o f o n s h o r e a n d m o o r e d o f f s h o r e l i s t e n i n g d e v i c e s . S O F A R f l o a t s w e r e u s e d e x t e n s i v e l y i n t h e 1 9 7 0 ' s a n d 1 9 8 0 ' s , w i t h a m a j o r i t y o f d e p l o y m e n t s b e i n g i n t h e N o r t h A t l a n t i c , p r i m a r i l y a t d e p t h s o f 700 m , 1 5 0 0 m ( n e a r t h e a x i s o f t h e S O F A R c h a n n e l ) a n d 2 0 0 0 m ( F r e e l a n d e t a l . , 1 9 7 5 ; R i s e r a n d R o s s b y , 1 9 8 3 ; O w e n s , 1 9 8 4 ; R i c h a r d s o n , 1 9 8 5 ; O w e n s , 1 9 9 1 ; R i c h a r d s o n , 1 9 9 3 ) . A s w i t h t h e s u r f a c e d r i f t e r s t u d i e s , m o s t o f t h e s e a n a l y s e s f o c u s e d o n t h e d e s c r i p t i o n o f t h e E u l e r i a n m e a n s t a t i s t i c s , t h e d i s p e r s i o n c h a r a c t e r i s t i c s , a n d t h e m e s o s c a l e s t r u c t u r e o f t h e d e e p f l o w . O n e o f t h e m o s t s u c c e s s f u l a p p l i c a t i o n s o f S O F A R f l o a t s w a s t h e i r d e p l o y m e n t Chapter 1. Introduction 8 w i t h i n s m a l l , l o n g - l i v e d m e s o s c a l e e d d i e s w h i c h c o n t a i n e d w a t e r o f M e d i t e r r a n e a n o r i g i n , M e d d i e s ( M c D o w e l l a n d R o s s b y , 1 9 7 8 ) . S o m e S O F A R f l o a t s r e m a i n e d i n M e d d i e s f o r y e a r s , r e v e a l i n g t h e i r s l o w w e s t w a r d p r o p a g a t i o n a c r o s s t h e A t l a n t i c a n d e v e n t u a l d e c a y ( A r m i e t a l , 1 9 8 9 ; R i c h a r d s o n et a l , 1 9 8 9 ) . I n o r d e r t o e m i t l o w f r e q u e n c y (250 H z ) s o u n d , S O F A R f l o a t s h a d t o b e f o r m i d a b l y l a r g e ( a b o u t 5 m i n l e n g t h a n d w e i g h i n g o v e r 4 0 0 k g ) , m a k i n g t h e m e x p e n s i v e a n d d i f f i c u l t t o d e p l o y . T h i s l e d t o t h e d e v e l o p m e n t o f t h e R A F O S float ( R A F O S is S O F A R s p e l l e d b a c k w a r d s ) , w h i c h r e v e r s e s t h e r o l e s b y listening t o a c o u s t i c e m i s s i o n s f r o m s t a t i o n a r y t r a n s m i t t e r s ( R o s s b y e t a l . , 1 9 8 6 ) . P o s i t i o n fixes o b t a i n e d f r o m t h e t r a n s m i t t e r s a r e s t o r e d i n t e r n a l l y , a n d a f t e r a p r e - d e t e r m i n e d l i f e s p a n ( t y p i c a l l y ~ 1-2 y e a r s ) , t h e float b e c o m e s b u o y a n t a n d floats t o t h e s u r f a c e , w h e r e i t s b o u n t y o f d a t a , i n c l u d i n g d a i l y r e c o r d s o f t e m p e r a t u r e a n d p r e s s u r e , i s t r a n s m i t t e d t o s a t e l l i t e s v i a S e r v i c e A R G O S . R A F O S floats a r e m u c h s m a l l e r ( ~ 10 k g ) a n d , t h e r e f o r e , c h e a p e r a n d m o r e p r a c t i c a l , a n d h a v e b e c o m e t h e w o r k h o r s e d e v i c e f o r L a g r a n g i a n m e a s u r e m e n t s o f d e e p c u r r e n t s . T h e s e floats a r e e q u i p p e d w i t h e n o u g h b a t t e r y p o w e r t o p e r f o r m a b o u t 60 c y c l e s , g i v i n g t h e m a l i f e t i m e o f ~ 4 . 5 y e a r s . A m o r e r e c e n t d e v e l o p m e n t is t h e A u t o n o m o u s L A g r a n g i a n C i r c u l a t i o n E x p l o r e r ( A L A C E ) , w h i c h uses a s m a l l h y d r a u l i c p u m p t o c h a n g e i t s v o l u m e , a l l o w i n g i t t o s u r f a c e a n d t r a n s m i t d a t a t o A R G O S s a t e l l i t e s a t p r e s c r i b e d t i m e i n t e r v a l s ( t y p i c a l l y e v e r y ~ 30 d a y s ) a n d t h e n c h a n g e i t s b u o y a n c y a g a i n f o r r e - d e p l o y m e n t a t t h e o r i g i n a l float d e p t h ( D a v i s et a l . , 1 9 9 1 ) . T h i s a l l o w s f o r s o m e d e g r e e o f c o n t i n u a l t r a c k i n g . T h e P r o f i l i n g A L A C E ( P A L A C E ) float, t h e l a t e s t i n c a r n a t i o n , t a k e s p r o f i l e m e a s u r e m e n t s o f t e m p e r a t u r e a n d s a l i n i t y o n i t s a s c e n t b e t w e e n t h e float l e v e l a n d t h e s u r f a c e . P o w e r - l i m i t e d h a l f - l i v e s f o r A L A C E a n d P A L A C E floats a r e o n t h e o r d e r o f 6 y e a r s , a l t h o u g h c o r r o s i o n a n d p u m p f a i l u r e s o n s o m e o f t h e floats d e p l o y e d i n t h e S o u t h P a c i f i c a n d L a b r a d o r S e a h a v e s h o r t e n e d t h e i r fives ( D a v i s , 1 9 9 8 ) . D r i f t e r a n d float t e c h n o l o g y i s e v o l v i n g r a p i d l y . Y e a r s o f e x p e r i e n c e h a v e l e d t o t h e Chapter 1. Introduction 9 present standard surface drifter, which is inexpensive (~ $2200US per unit), has a long half-life (~ 500 days), and has a stable tether and drogue assembly which makes the drifter an excellent current follower (Kennan et al., 1998). A wide variety of physical parameters are now being measured in the Lagrangian frame with sensors attached to the surface buoys. Almost all drifters currently in operation have a sea surface temperature (SST) sensor on the underside of the surface float, and the global drifter population has contributed greatly to global SST climatologies (Reynolds and Smith, 1994). Other sensors which have been used on drifter deployments include subsurface thermistors and conductivity cells for measuring mixed layer temperature and salinity, barometers for measuring surface atmospheric pressure, wind vanes for obtaining wind stress along the drifter path, and radiometers for radiance and irradiance measurements (Kennan et al. , 1998). Al though surface wave breaking has interfered with some of the pressure and wind measurements, an improvement in design could revolutionize weather observing and forcasting by providing a wealth of data in remote portions of the globe. New float designs include attached "wings" and the capability of two-way communication, which would allow floats to sample prescribed sections or travel to evolving features of interest (Davis, 1998). Present-day Lagrangian instruments are providing absolute current velocity measurements on a global scale. Future developments in drifter and float technology should make available a suite of high-quality physical measurements in real-time throughout the world's oceans. 1.2 Thesis Objectives The objective of this thesis is to present an analysis and interpretation of the position (latitude, longitude) time series, and accompanying derived velocities, obtained from a large set of satellite-tracked drifters deployed and tracked throughout the Nor th Pacific Chapter 1. Introduction 10 Ocean over the period 1990-1995. Most of these drifters were deployed as part of the Surface Velocity Program ( S V P ) , which is a component of the World Ocean Circulation Experiment ( W O C E ) . The principal objective of W O C E is "to develop models useful for predicting climate change and to collect the data necessary to test them" (Canadian National Committee for W O C E , 1992). To accomplish this goal, a 10-year field program involving 40 nations and utilizing hydrographic sampling (trans-oceanic sections), current meter moorings, drifters, floats, and satellite altimetry was initiated in 1990. A secondary objective of W O C E is to determine the representativeness of the data sets collected during the field programs. The specific objectives of the S V P are "to provide basin-scale observations of mixed layer velocity and sea surface temperature, which will subsequently be used to test global models of surface circulation and to study the advection of ocean surface properties" (Canadian National Committee for W O C E , 1992). This is to be accomplished by de- ployment over 5-year periods of arrays of satellite-tracked drifters within all major ocean basins, at a spatial resolution of approximately 500 km x 500 km. The data set presented in this thesis represents the Nor th Pacific component of the S V P as well as an additional set of deployments at a drogue level below the mixed layer. As with any large-scale deployment of Lagrangian instruments, this data set has its incumbent deficiencies and advantages. There are regions which are poorly sampled, and minimal temporal resolution even in regions of high data density. Some drifters failed long before expected, and drogues became detached from surface floats. However, the observations presented here represent the most spatially extensive current measurements ever obtained in the northeast Pacific Ocean. These data allow for descriptions of the mean and eddy velocity fields, as well as the dynamic characteristics of specific mesoscale circulation features. In a general sense, the goal of this work is to take advantage of the inherent assets of the data set, i.e., to take what the drifters have given. Considering Chapter 1. Introduction 11 this, and with the broader W O C E / S V P objectives in mind, the specific objectives of the work presented in the thesis are: (a) As a first step, to determine optimal sampling strategies and interpolation schemes for the raw drifter time series. A R G O S data is not uniform in time, and the type of interpolation used on the non-uniform position time series could affect derived velocity statistics. The effects of different data sampling intervals and interpolation schemes are explored using a subset of drifters which revealed strong high-frequency motions. (b) To derive the Eulerian (general) circulation and its eddy variability in the Nor th Pacific Ocean from the data ensembles at both drogue depths, per S V P objectives. The emphasis is on the subpolar gyre of the northeast Pacific, which has the highest data density. (c) To analyze the dispersion characteristics revealed by the drifter trajectories and use classical theories of single particle dispersion to derive eddy statistics. (d) To describe the characteristics and dynamics of prominent topographically-influenced mesoscale eddies revealed by subsets of the drifter trajectories near the Emperor Seamount Chain (~ 170°E) and the Kur i l -Kamchatka Trench in the western Nor th Pacific. (e) To provide an overview of the North Pacific S V P data set which wil l be sufficient for putting constraints on the planning of future drifter deployments in the region, and for providing improved paramaterizations in numerical circulation models, per W O C E objectives. 1.3 Thesis Overview Each chapter in the thesis stands as an independent analysis, focusing on a particular suite of statistical methods or subset of drifters. Taken together, however, the chapters provide an overview of the mean near-surface circulation and its eddy variability in the Chapter 1. Introduction 12 Nor th Pacific on a wide range of scales during the W O C E - S V P field phase, and should provide important constraints on numerical models of the region. The thesis is organized as follows: In Chapter 2, the data set is introduced and the ini t ial data processing is described. In Chapter 3, various sampling strategies and interpolation schemes are applied to a subset of the drifter position time series with the aim of quantifying their effects on the derived velocity statistics. A n overview of the low-frequency (Eulerian) mean circulation and energy distribution derived from the entire data set is provided in Chapter 4, while in Chapter 5 classical theories of single particle dispersion are tested and used to derive eddy statistics (eddy mixing scales and horizontal eddy diffusivities). In Chapter 6, the focus is shifted to the mesoscale, with an analysis of a subset of drifters, deployed near the Emperor Seamount Chain (~ 170°E), which revealed a pair of counter- rotating, seamount-trapped eddies. Finally, Chapter 7 provides an analysis of the subset of drifters which sampled anticyclonic eddies trapped over the Kur i l -Kamchatka Trench in the western Nor th Pacific. A summary and synthesis of the results are given in Chapter 8. Chapter 2 Data and Methods 2.1 The W O C E / S V P Dataset S a t e l l i t e - t r a c k e d d r i f t e r s h a v e b e c o m e i n v a l u a b l e t o o l s f o r s t u d y i n g o c e a n c i r c u l a t i o n . T h e y a r e r e l a t i v e l y i n e x p e n s i v e , e a s y t o d e p l o y , a n d h a v e l o n g l i v e s ( t y p i c a l l y > 1 y e a r ) . W h e n d e p l o y e d i n l a r g e n u m b e r s , t h e y e n a b l e a m a p p i n g o f t h e n e a r - s u r f a c e c i r c u l a t i o n o v e r a n e n t i r e o c e a n b a s i n . T h e S u r f a c e V e l o c i t y P r o g r a m ( S V P ) , a c o m p o n e n t o f t h e W o r l d O c e a n C i r c u l a t i o n E x p e r i m e n t ( W O C E ) , w a s d e s i g n e d t o p r o v i d e m i x e d - l a y e r v e l o c i t y a n d t e m p e r a t u r e o b s e r v a t i o n s o v e r 5 - y e a r p e r i o d s i n a l l m a j o r o c e a n b a s i n s . A s p a r t o f C a n a d a ' s c o n t r i b u t i o n t o t h e W O C E - S V P , 102 s a t e l l i t e - t r a c k e d d r i f t e r s w e r e r e l e a s e d i n t h e N o r t h P a c i f i c i n 12 d e p l o y m e n t s b e t w e e n A u g u s t 1 9 9 0 a n d N o v e m b e r 1 9 9 4 . E a c h o f t h e d r i f t e r s w a s e q u i p p e d w i t h a h o l e y - s o c k d r o g u e c e n t e r e d 15 m (62 d r i f t e r s ) o r 120 m (40 d r i f t e r s ) b e l o w t h e s u r f a c e , i . e . , w i t h i n t h e m i x e d - l a y e r o r n e a r t h e t o p o f t h e p e r m a n e n t h a l o c l i n e ( T a b a t a , 1 9 7 5 ; F i g u r e 2 .1 ) . A l l d e p l o y m e n t s w e r e m a d e f r o m s h i p s - o f - o p p o r t u n i t y , w i t h a m a j o r i t y o c c u r r i n g w i t h i n t h e G u l f o f A l a s k a ( F i g u r e 2 . 2 ) . T h e s t a n d a r d S V P s a t e l l i t e - t r a c k e d d r i f t e r w a s d e s i g n e d t o f o l l o w w a t e r p a r c e l s v e r t i - c a l l y a v e r a g e d o v e r a d r o g u e o f 6 - 7 m h e i g h t , c e n t e r e d 15 m b e l o w t h e s u r f a c e . T h e a i m w a s a d r i f t e r w i t h k n o w n w a t e r - f o l l o w i n g c h a r a c t e r i s t i c s , w i t h p r e d i c t a b l e s l i p t h r o u g h w a t e r , r u g g e d e n o u g h t o s u r v i v e m a n y m o n t h s i n t h e o p e n o c e a n , a n d p r e f e r a b l y o f l o w c o s t a n d e a s y t o d e p l o y ( S y b r a n d y a n d N i i l e r , 1 9 9 1 ) . T h e d r i f t e r a d o p t e d as t h e s t a n d a r d 13 Chapter 2. Data and Methods 14 Temperature (°C) Brunt - Vaisala Frequency (cph) 24!o ' 2SA ' 26.0 27.0 Density (a t) F i g u r e 2 . 1 : P r o f i l e s o f t e m p e r a t u r e ( T ) , B r u n t - V a i s a l a f r e q u e n c y ( N ) , a n d d e n s i t y (cr t ) t a k e n n e a r S t a t i o n P (50°iV, 145°W) i n A u g u s t 1990 . L o c a t i o n s o f t h e t w o d r o g u e d e p t h s a r e a l s o s h o w n . F r o m T h o m s o n e t a l . ( 1 9 9 8 ) . Chapter 2. Data and Methods 15 60°NH 5 0 ° N 4 0 ° N 3 0 ° N J I I J I I I I I I I L Shallow Deployment Positions 1990 - 1994 . x 1990 A 1991 + 1992 • 1993 O 1994 2 0 ° N - | 1 1 1 1 1 1 1 1 1 1 1 1—i 1 1 1 1 1 r 1 4 0 ° E 1 6 0 ° E 1 8 0 ° 160°W 140°W 120°W 5 0 ° N J I I J I I I I I I L 4 0 ° N | Deep Deployment Positions 1990 - 1994 x 1990 A 1991 + 1992 • 1993 O 1994 ~i 1 — i — i — i 1 — i — i — i — i — i — i — 1 1 1 1 1 1 r~ 1 4 0 ° E 1 6 0 ° E 1 8 0 ° 160°W 140°W 120°W F i g u r e 2 . 2 : D r i f t e r d e p l o y m e n t p o s i t i o n s f o r t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m - b l e s . Chapter 2. Data and Methods 16 h a s a t h r e e - d i m e n s i o n a l l y s y m m e t r i c s u r f a c e f l o a t ( a s p h e r e ) , w h i c h k e e p s s u r f a c e w a v e a l i a s i n g o f n e t h o r i z o n t a l f o r c e s o n t h e f l o a t t o a m i n i m u m . T h e f l o a t i s 3 5 c m i n d i a m e t e r a n d h o u s e s a n a n t e n n a , a t r a n s m i t t e r , b a t t e r i e s a n d a s u b m e r g e n c e s e n s o r (see F i g u r e 1 .1) . T h e w i r e t e t h e r s a r e t h i n a n d s t i f f t o r e d u c e s l i p c a u s e d b y u n d e r w a t e r d r a g , a n d s u b s u r f a c e f l o a t s a r e u s e d t o m i n i m i z e w a v e e f f e c t s o n t h e d r o g u e . T h e d r o g u e u s e d , a h o l e y s o c k , i s d i m e n s i o n a l l y s t a b l e a n d h a s a h i g h d r a g c o e f f i c i e n t . I n a n e f f o r t t o d e s c r i b e b o t h t h e m i x e d l a y e r f l o w and t h e f l o w i n t h e u n d e r l y i n g p y c n o c l i n e , a s i g n i f i c a n t p o r t i o n o f t h e d r i f t e r s i n t h e d a t a set d e s c r i b e d h e r e w e r e d r o g u e d a t 120 m . T h e g e o m e t r y a n d c o n s t r u c t i o n o f t h e s u r f a c e f l o a t a n d d r o g u e w e r e t h e s a m e f o r t h e d e e p d r i f t e r s as f o r t h e s t a n d a r d S V P d r i f t e r s . N i i l e r et a l . ( 1 9 9 5 ) m o d e l l e d t h e v e c t o r s l i p o f 1 5 - m - d r o g u e d d r i f t e r s (Ua) as a l i n e a r f u n c t i o n o f w i n d s p e e d a t 10 m ( W ) , v e r t i c a l c u r r e n t s h e a r a c r o s s t h e l e n g t h o f t h e d r o g u e ( A U ) , d r a g a r e a r a t i o ( R ; t h e r a t i o o f t h e d r a g a r e a o f t h e d r o g u e t o t h a t o f t h e t e t h e r p l u s s u r f a c e f l o a t ) , a n d t h e a n g l e s r e l a t i v e t o t h e w i n d a n d s h e a r d i r e c t i o n s ( a a n d / ? , r e s p e c t i v e l y ) : U. = {aeiaW + bei/3AU) ( 2 .1 ) M e a s u r e m e n t s o f s l i p a n d v e r t i c a l s h e a r w e r e m a d e b y v e c t o r - m e a s u r i n g c u r r e n t m e t e r s p l a c e d a t t h e t o p a n d b o t t o m o f t h e d r o g u e s . M o s t o f t h e v a r i a n c e ( 8 4 % ) i n t h e d r i f t e r s l i p w a s a c c o u n t e d f o r b y l i n e a r f i t s t o t h e f o u r c o e f f i c i e n t s ( a , b , a , /3), w i t h t h e r e s u l t t h a t a d r a g a r e a r a t i o o f 40 o r g r e a t e r w o u l d y i e l d a w i n d s l i p p a g e o f l e s s t h a n 1 c m / s i n 10 m / s w i n d s . S V P d r i f t e r s h a v e t h e r e f o r e b e e n d e s i g n e d t o h a v e a d r a g a r e a r a t i o o f ~ 4 0 , g i v i n g t h e m a s m a l l a n d p r e d i c t a b l e s l i p . A l t h o u g h t h e s t a n d a r d d r o g u e d e s i g n m i n i m i z e s w i n d - a n d w a v e - i n d u c e d d r i f t e r s l i p p a g e , i t s h o u l d b e r e m e m b e r e d t h a t d r i f t e r s a r e o n l y q u a s i - L a g r a n g i a n i n s t r u m e n t s . N o t o n l y d o e s s o m e s l i p p a g e r e m a i n , b u t t h e d r o g u e r e m a i n s Chapter 2. Data and Methods 17 D r i f t e r T y p e N u m b e r D e p l o y e d T o t a l L i f e ( d a y s ) D r o g u e L i f e ( d a y s ) Aanderaa Shallow 8 2 4 2 2 3 2 LCD Shallow 8 49 46 Seimac Shallow 19 291 2 1 0 Seimac Deep 2 196 1 7 7 Technocean Shallow 28 5 6 7 * 3 5 2 Technocean Deep 38 3 4 0 156 T a b l e 2 . 1 : T o t a l a n d d r o g u e l i f e e x p e c t a n c i e s b y d r i f t e r m a n u f a c t u r e r . O n e o f t h e A a n - d e r a a s h a l l o w , t w o o f t h e S e i m a c s h a l l o w , a n d t w o o f t h e T e c h n o c e a n d e e p d r i f t e r s f a i l e d o n l a u n c h . N o t e t h a t f o u r o f t h e T e c h n o c e a n s h a l l o w d r i f t e r s w e r e s t i l l a c t i v e o n M a y 1, 1 9 9 6 . c e n t e r e d a t a p a r t i c u l a r d e p t h , a n d d o e s n o t f o l l o w t h e t h r e e - d i m e n s i o n a l p a t h o f a w a t e r p a r c e l o n a n i s o p y c n a l s u r f a c e . N o t a l l d r i f t e r s a r e c r e a t e d e q u a l ( T a b l e 2 .1 ) . D r i f t e r l i f e t i m e s r a n g e d f r o m 11 t o > 1 4 0 0 d a y s , w i t h s i g n i f i c a n t v a r i a b i l i t y a m o n g m a n u f a c t u r e r s . T h e L C D ( L o w - C o s t D r i f t e r , D r a p e r L a b s ) d r i f t e r s w e r e l e a s t r e l i a b l e , w h i l e t h e T e c h n o c e a n d r i f t e r s , o n a v - e r a g e , o u t l i v e d t h e o t h e r s b y 5 0 % o r m o r e . O v e r a l l , t h e a v e r a g e d r i f t e r l i f e t i m e ( w i t h o r w i t h o u t d r o g u e ) w a s 3 7 7 (333 ) d a y s f o r t h e s h a l l o w ( d e e p ) d e p l o y m e n t s , w h i c h i s s h o r t e r t h a n t h e d e s i g n e d h a l f - l i f e o f 4 0 0 - 5 0 0 d a y s ( K e n n a n et a l . , 1 9 9 8 ) . 2 . 2 I n i t i a l D a t a P r o c e s s i n g T h e S e r v i c e A R G O S s a t e l l i t e t r a c k i n g s y s t e m w a s u s e d f o r d r i f t e r l o c a t i o n a n d r e t r i e v a l o f s e n s o r d a t a . D r i f t e r s t e l e m e t e r t h e i r d a t a ( i d e n t i f i c a t i o n n u m b e r a n d a n y m e a s u r e d p a r a m e t e r s , s u c h as S S T a n d d e g r e e o f s u b m e r g e n c e ) t o p o l a r o r b i t i n g s a t e l l i t e s . S e r v i c e A R G O S d e t e r m i n e s t h e d r i f t e r p o s i t i o n s a n d p r o d u c e s t i m e s e r i e s o f r a w d a t a , w h i c h a r e s e n t t o a c e n t r a l i z e d l o c a t i o n , t h e G l o b a l D r i f t e r D a t a C e n t e r ( G D D C ) a t N O A A ' s Chapter 2. Data and Methods 18 A t l a n t i c O c e a n o g r a p h i c a n d M e t e o r o l o g i c a l L a b o r a t o r y , as w e l l as t o t h e p r i n c i p a l i n - v e s t i g a t o r s . T h e G D D C t h e n p r o c e s s e s , a r c h i v e s a n d d i s t r i b u t e s t h e d a t a . F i n a l d a t a a r c h i v a l i s d o n e a t C a n a d a ' s M a r i n e E n v i r o n m e n t a l D a t a S e r v i c e ( M E D S ) . M o s t o f t h e d r i f t e r d a t a a n a l y z e d i n t h i s t h e s i s w a s p r o c e s s e d a t t h e I n s t i t u t e o f O c e a n S c i e n c e s ( I O S ) f r o m t h e r a w A R G O S d a t a . T h e d a t a o b t a i n e d f r o m S e r v i c e A R G O S c o n s i s t e d o f i r r e g u l a r l y - s p a c e d t i m e s e r i e s o f l o n g i t u d e a n d l a t i t u d e , w i t h t h e t i m e s b e t w e e n s u c c e s s i v e p o s i t i o n fixes r a n g i n g f r o m s e v e r a l m i n u t e s t o s e v e r a l h o u r s . A t t h e l a t i t u d e s o c c u p i e d b y m o s t o f t h e s e d r i f t e r s ( > 40°N) , 10 o r m o r e p o s i t i o n fixes p e r d a y w a s t y p i c a l . T o r e d u c e v e l o c i t y e r r o r s w h i c h c o u l d a r i s e f r o m s h o r t t i m e s t e p s , a l l s e q u e n t i a l fixes f o r At < 1 h o u r w e r e a v e r a g e d t o g e t h e r . O u t l y i n g p o i n t s , e x p r e s s e d b y u n r e a s o n a b l e first d i f f e r e n c e s p e e d s , w e r e r e - m o v e d . A c c u r a c i e s i n t h e A R G O S p o s i t i o n f i x e s a r e a p p r o x i m a t e l y 2 0 0 - 3 0 0 m , w h i c h w o u l d r e s u l t i n v e l o c i t y e r r o r s less t h a n 5 c m / s . A s t h e s e e r r o r s a r e r a n d o m , t h e i r n e t e f fec t o n E u l e r i a n a v e r a g e s c a n b e n e g l e c t e d ( T h o m s o n et a l . , 1 9 9 8 ) . T h e r a w t i m e s e r i e s w e r e i n t e r p o l a t e d t o o b t a i n e s t i m a t e s o f p o s i t i o n a n d v e l o c i t y a t s y n c h r o n i z e d 3 - h o u r l y i n t e r v a l s . T h e fitting r o u t i n e , d e s c r i b e d i n t h e n e x t c h a p t e r a n d i n B o g r a d et a l . ( 1 9 9 8 a ) , c o n s i s t e d o f a f o u r t h - o r d e r p o l y n o m i a l p l u s a n o s c i l l a t o r y c o m p o n e n t w h o s e f r e q u e n c y w a s d e t e r m i n e d b y t h e i n t e r p o l a t i o n . S i n c e i n e r t i a l m o t i o n s w e r e t y p i c a l l y t h e m o s t e n e r g e t i c c o m p o n e n t o f t h e d r i f t e r m o t i o n s ( T h o m s o n e t a l . , 1 9 9 8 ) , t h e o s c i l l a t o r y c o m p o n e n t o f t h e fit w a s u s u a l l y a t t h e l o c a l i n e r t i a l f r e q u e n c y . A s a c o s t - s a v i n g m e a s u r e , a l l o f t h e d r i f t e r s a l t e r n a t e d b e t w e e n c o n t i n u o u s d a t a r e - c e i v e / t r a n s m i t m o d e a n d a r e d u c e d s a m p l i n g s c h e d u l e ( d u t y c y c l e ) . T h e s t a n d a r d d u t y c y c l e u s e d i n t h e S V P c o n s i s t e d o f 48 h o u r s o f n o d a t a t r a n s m i s s i o n f o l l o w e d b y 24 h o u r s o f r e c e i v e d t r a n s m i s s i o n ( 4 8 - 2 4 h ) . S o m e o f t h e d r i f t e r s a l s o u s e d a 1 6 - 8 h d u t y c y c l e . T h e d e r i v a t i o n o f v e l o c i t y s t a t i s t i c s o v e r d u t y c y c l e s e g m e n t s w a s s e n s i t i v e t o t h e i n t e r p o l a t i o n s c h e m e u s e d a n d , i n d e e d , t h e fitting p r o c e d u r e d e s c r i b e d a b o v e w a s c u s t o m i z e d i n o r d e r Chapter 2. Data and Methods 19 t o o b t a i n r e l i a b l e s t a t i s t i c a l e s t i m a t e s f r o m b o t h c o n t i n u o u s a n d d u t y c y c l e s e g m e n t s o f t h e d r i f t e r t r a j e c t o r i e s . T h e e f f e c t s o f t h e d u t y c y c l e a n d v a r i o u s i n t e r p o l a t i o n s c h e m e s o n d e r i v e d v e l o c i t y s t a t i s t i c s a r e t h e s u b j e c t o f t h e f o l l o w i n g c h a p t e r . T h e f u l l i n t e r p o l a t e d t r a j e c t o r i e s o f a l l d r i f t e r s i n t h e s h a l l o w a n d d e e p e n s e m b l e s a r e s h o w n i n F i g u r e 2 . 3 . 2 . 3 D e t e r m i n a t i o n o f D r o g u e L o s s T h e r e l i a b i l i t y o f t h e s t a t i s t i c s p r e s e n t e d i n t h e f o l l o w i n g c h a p t e r s d e p e n d s c r i t i c a l l y o n t h e d e t e r m i n a t i o n o f t h e d a t e o f d r o g u e l o s s f o r e a c h d r i f t e r . F o r t h e s h a l l o w d r i f t e r s , w h i c h a r e a s s u m e d t o r e p r e s e n t t h e m i x e d - l a y e r flow, i t i s q u i t e d i f f i c u l t t o d e t e r m i n e p r e c i s e l y w h e n a d r o g u e m a y h a v e b e c o m e d e t a c h e d f r o m t h e s u r f a c e b u o y . A l t h o u g h a l l S V P d r i f t e r s w e r e e q u i p p e d w i t h s u b m e r g e n c e s e n s o r s , w h i c h a r e d e s i g n e d t o i n d i c a t e t h e f r a c t i o n o f t i m e a s u r f a c e b u o y i s s u b m e r g e d as s u r f a c e w a v e s b r e a k a n d s w e l l p a s s b y ( a n d t h u s w h e t h e r a d r o g u e i s l i k e l y p r e s e n t o r a b s e n t ) , t h i s m e t h o d i s n o t e n t i r e l y r e l i a b l e . F o r t h e s h a l l o w d r i f t e r s , d a t e o f d r o g u e l o s s w a s d e t e r m i n e d f r o m a r e v i e w o f t h e s u b m e r g e n c e s e n s o r d a t a a n d t i m e se r ies o f d r i f t e r s p e e d s a n d a c c e l e r a t i o n s , as w e l l as a c a r e f u l p e r u s a l o f t h e i n d i v i d u a l d r i f t e r t r a j e c t o r i e s . A l t h o u g h p o r t i o n s o f t h e s h a l l o w e n s e m b l e s m a y b e c o n t a m i n a t e d w i t h u n d r o g u e d d a t a , t h e a m o u n t o f s u c h d a t a i s s m a l l so t h a t t h e d e r i v e d m i x e d - l a y e r s t a t i s t i c s a r e n o t l i k e l y t o b e s e r i o u s l y a f f e c t e d . F o r t h e d e e p d r i f t e r e n s e m b l e , w h i c h r e p r e s e n t s t h e flow b e l o w t h e m i x e d l a y e r , i n t h e u n d e r l y i n g p y c n o c l i n e , i t i s e s s e n t i a l t h a t a l l s t a t i s t i c s b e d e r i v e d f o r d r o g u e d s e g m e n t s o n l y . B r u g g e ( 1 9 9 5 ) s h o w e d t h a t e s t i m a t e s o f e d d y k i n e t i c e n e r g y i n t h e N o r t h A t l a n t i c f r o m u n d r o g u e d d r i f t e r s a r e 2 - 3 t i m e s h i g h e r t h a n f r o m d r i f t e r s d r o g u e d a t 100 m . F o r t h e d e e p e n s e m b l e , a t e c h n i q u e , d e s c r i b e d b y P i n g r e e ( 1 9 9 3 ) , w a s e m p l o y e d w h i c h l o o k s f o r a n a b r u p t o r d e r - o f - m a g n i t u d e i n c r e a s e i n d r i f t e r a c c e l e r a t i o n s a n d s u s t a i n e d h i g h s p e e d s . A 1 2 h r u n n i n g m e a n w a s a p p l i e d t o t h e 3 - h o u r l y p o s i t i o n t i m e s e r i e s t o r e m o v e h i g h Chapter 2. Data and Methods 20 Figure 2.3: Complete trajectories of the (a) shallow and (b) deep drifter ensembles. Marks refer to the deployment positions, and correspond to the legend in Figure 2.2. Chapter 2. Data and Methods 2 1 f r e q u e n c y s i g n a l s , a n d v e l o c i t i e s w e r e d e r i v e d o v e r a 1 2 h i n t e r v a l a n d a c c e l e r a t i o n s o v e r a p e r i o d o f o n e d a y . S i n c e d r i f t e r t r a j e c t o r i e s w i t h i n e d d i e s c a n h a v e a c c e l e r a t i o n s r e s u l t i n g f r o m s t e a d y c i r c u l a r m o t i o n s ( P i n g r e e , 1 9 9 3 ) , s c a l a r a c c e l e r a t i o n d i f f e r e n c e s o v e r a 1 2 h p e r i o d w e r e t h e n c o m p u t e d a n d p l o t t e d a g a i n s t t i m e , as i s s h o w n f o r d r i f t e r 1 3 1 9 ( F i g u r e 2 . 4 ) . I n t h i s p l o t , a v e l o c i t y i n c r e a s e o f 10 c m / s o v e r a p e r i o d o f o n e d a y , f o r e x a m p l e , w o u l d r e s u l t i n a n a c c e l e r a t i o n d i f f e r e n c e p u l s e o f ± 10 cm/s/day2. T h e s a m e v e l o c i t y i n c r e a s e o v e r a s h o r t e r p e r i o d o f t i m e w o u l d y i e l d a l a r g e r p u l s e . L a r g e a n d s u s t a i n e d a c c e l e r a t i o n d i f f e r e n c e s ( a n d c o r r e s p o n d i n g i n c r e a s e s i n t h e m e a n a n d v a r i a n c e o f d r i f t e r s p e e d ) a r e o b s e r v e d a p p r o x i m a t e l y 50 d a y s a f t e r d e p l o y m e n t , w h i c h i s t a k e n t o b e t h e d a t e o f d r o g u e l o s s . N o t a l l d r i f t e r s s h o w e d s u c h a n o b v i o u s c h a n g e i n a c c e l e r a t i o n as d r i f t e r 1 3 1 9 , h o w e v e r , a n d t h e e x e c u t i o n o f t h e m e t h o d w a s a t t i m e s q u i t e s u b j e c t i v e . A p a r t i c u l a r c o m p l i c a t i o n w a s t h e d u t y c y c l e s e g m e n t s , t h r o u g h w h i c h t h e i n t e r p o l a t i o n s o f t e n c r e a t e d r e l a t i v e l y l a r g e a n d n o i s y a c c e l e r a t i o n s . I n a d d i t i o n t o t h e a c c e l e r a t i o n d i f f e r e n c e s a n d d a t a f r o m t h e s u b m e r g e n c e s e n s o r s , t i m e se r i es o f d r i f t e r s p e e d a n d t h e i d i o s y n c r a s i e s o f t h e i n d i v i d u a l t r a j e c t o r i e s w e r e a l s o u s e d t o d e t e r m i n e t h e d a t e o f d r o g u e l o s s f o r t h e d e e p d r i f t e r s . L a r g e a c c e l e r a t i o n d i f f e r e n c e s w e r e c h e c k e d a g a i n s t d r i f t e r e n t r y i n t o a s t r o n g c u r r e n t . I n g e n e r a l , s e l e c t i o n o f d a t e o f d r o g u e l o s s f o r t h e d e e p d r i f t e r s w a s d o n e c o n s e r v a t i v e l y , w i t h a p r e f e r e n c e t o e r r o n t h e s i d e o f r e m o v i n g g o o d d a t a r a t h e r t h a n k e e p i n g w i n d - c o n t a m i n a t e d d a t a . T h e r e m o v a l o f u n d r o g u e d p o r t i o n s o f t h e d r i f t e r t r a j e c t o r i e s r e s u l t e d i n a r e d u c t i o n f r o m 2 3 , 6 5 0 ( 1 3 , 0 6 4 ) d r i f t e r d a y s t o 1 6 , 0 2 2 ( 6 , 1 7 2 ) f o r t h e s h a l l o w ( d e e p ) e n s e m b l e . F i g u r e 2 .5 s h o w s t i m e l i n e s f o r a l l d r i f t e r s i n e a c h e n s e m b l e , a l o n g w i t h t h e t i m e s e r i e s o f n u m b e r o f d r o g u e d d r i f t e r d a y s o v e r t h e S V P p e r i o d . T h e a v e r a g e d r o g u e l i f e t i m e w a s 2 7 2 d a y s a t 15 m d e p t h a n d 167 d a y s a t 120 m d e p t h , c o n s i d e r a b l y s h o r t e r t h a n t h e a v e r a g e d r i f t e r l i f e t i m e s . T h u s , t h e m e c h a n i c s o f d r o g u e a t t a c h m e n t a p p e a r s t o b e t h e l i m i t i n g f a c t o r i n o b t a i n i n g l o n g r e c o r d s f r o m s u r f a c e d r i f t e r s . Chapter 2. Data and Methods 22 T i m e F r o m D e p l o y m e n t ( d a y s ) 0 20 40 60 80 100 D R I F T E R 1 3 1 9 Figure 2.4: Time series of daily-averaged drifter speed (top) and acceleration difference (bottom) for deep drifter 1319. Chapter 2. Data and Methods 2 3 A D e p a r t m e n t o f E a r t h a n d O c e a n S c i e n c e s D a t a R e p o r t ( B o g r a d a n d E e r t , 1 9 9 6 ) c o n t a i n s i n d i v i d u a l p l o t s o f a l l d r i f t e r t r a j e c t o r i e s , as w e l l as p r e l i m i n a r y E u l e r i a n s t a t i s - t i c s d e r i v e d f r o m t h e t r a j e c t o r i e s t h r o u g h D e c e m b e r 1994 . Chapter 2. Data and Methods 2 4 60 50 40 30 Q 2 0 10 111111111111 i 1111111111 1111 1111111111111111111 ( a ) S h a l l o w ( 1 5 m ) N buoys = 58 Xdays = 16022 11111111111111111111111111111111111111111111111111111111111111111111111111111111111 1990 1991 1992 1993 1994 1995 1996 600 500 £ cd > u CD 400 co h- 200 o h 100 40 30 20 -\ 10 1111 11111 1111111111111111111 I I 11111111111111111111 1111 ( b ) D e e p ( 1 2 0 m ) ^buoys ~ 34 Ndays = 6172; 111111111111111111111111111111111111111111111111 r 1111111111111111111111111111111111 T 400 cd 300 > 200 & 100 1990 1991 1992 1993 1994 1995 1996 F i g u r e 2 . 5 : T i m e l i n e s o f e a c h o f t h e d r i f t e r s i n t h e (a) s h a l l o w a n d ( b ) d e e p d r i f t e r e n s e m b l e s . D a s h e d l i n e s c o r r e s p o n d t o u n d r o g u e d p o r t i o n s o f t h e d r i f t e r t r a j e c t o r i e s . T h e d o t t e d c u r v e i s t h e t i m e se r i es o f t o t a l n u m b e r o f d a i l y d r i f t e r o b s e r v a t i o n s i n e a c h e n s e m b l e . Chapter 3 Sampling Strategies and Interpolation Schemes 3.1 Introduction A l t h o u g h d r i f t e r s a r e r e l a t i v e l y i n e x p e n s i v e c o m p a r e d t o c u r r e n t m e t e r s , l a r g e n u m b e r s a r e n e e d e d t o e f f e c t i v e l y s a m p l e b a s i n - s c a l e o c e a n i c r e g i o n s . M o r e i m p o r t a n t l y , t h e c o s t o f l o c a t i n g t h e d r i f t e r s a n d t r a n s m i t t i n g t h e d a t a ( p a i d t o S e r v i c e A R G O S ) c a n b e p r o - h i b i t i v e l y e x p e n s i v e . A s p o i n t e d o u t b y H a n s e n a n d H e r m a n ( 1 9 8 9 ) , o p e r a t i n g c o s t s ( i . e . , d a i l y d a t a c o l l e c t i o n ) f a r e x c e e d t h e p r o d u c t i o n c o s t s o f a d r i f t e r . A n i d e a l a r r a n g e m e n t w o u l d i n c l u d e a s u f f i c i e n t n u m b e r o f c o n c u r r e n t l y - t r a n s m i t t i n g d r i f t e r s m a i n t a i n e d a t a r e d u c e d s a m p l i n g s c h e d u l e , so as t o d e r i v e s t a t i s t i c a l l y - r e l i a b l e c i r c u l a t i o n a n d t e m p e r a - t u r e m e a s u r e m e n t s a t a m i n i m u m c o s t . U s i n g d a t a f r o m d r i f t e r s d r o g u e d a t 1 0 - 3 0 m d e p t h i n t h e t r o p i c a l P a c i f i c b e t w e e n 1 9 7 9 - 8 4 , H a n s e n a n d H e r m a n ( 1989 ) s t u d i e d t h e e f fec t o f r e d u c i n g t h e n u m b e r a n d f r e - q u e n c y o f A R G O S t r a n s m i s s i o n s o n t h e r e t r i e v e d c u r r e n t a n d t e m p e r a t u r e i n f o r m a t i o n . T h e y f o u n d t h a t t r a n s m i s s i o n r a t e s o f o n e d a y i n t h r e e y i e l d e d a c c e p t a b l e v e l o c i t y e r r o r s o f ± 5 c m / s f o r e q u a t o r i a l r e g i o n s j u s t s o u t h o f t h e e q u a t o r . N o r t h o f t h e e q u a t o r , d u e t o e n h a n c e d m e s o s c a l e v a r i a b i l i t y , t h i s s t a n d a r d c o u l d n o t b e m e t w i t h a r e d u c t i o n t o e v e n o n e d a y i n t w o . T h e s t a n d a r d f o r s e a s u r f a c e t e m p e r a t u r e c o u l d b e m e t t h r o u g h - o u t t h e s a m p l e d r e g i o n w i t h t r a n s m i s s i o n s o n o n e d a y i n f o u r . B a s e d o n t h e s e r e s u l t s , t h e t w o - d a y s - o f f / o n e - d a y - o n t r a n s m i s s i o n s c h e d u l e ( d u t y c y c l e ) w a s a d o p t e d as t h e S V P s t a n d a r d . 25 Chapter 3. Sampling Strategies and Interpolation Schemes 26 W h i l e t h i s s a m p l i n g s t r a t e g y h a s l e d t o a s i g n i f i c a n t r e d u c t i o n i n o p e r a t i n g c o s t s a n d h a s e n a b l e d t h e d e p l o y m e n t o f a l a r g e n u m b e r o f d r i f t e r s i n e a c h o c e a n b a s i n , p e r S V P o b j e c t i v e s , i t i s n o t n e c e s s a r i l y t h e b e s t d u t y c y c l e f o r a l l b a s i n s . R e s u l t s a r e c l e a r l y d e p e n d e n t o n t h e d y n a m i c s o f t h e p a r t i c u l a r r e g i o n s a m p l e d ( e . g . , H a n s e n a n d H e r m a n , 1 9 8 9 ) . L o w - f r e q u e n c y m o t i o n s d o m i n a t e t h e c i r c u l a t i o n i n t h e t r o p i c a l P a c i f i c . I n m i d d l e a n d h i g h l a t i t u d e s , h o w e v e r , i n e r t i a l a n d t i d a l m o t i o n s c a n d o m i n a t e t h e f l o w v a r i a b i l i t y ( M c N a l l y e t a l , 1 9 8 9 , T h o m s o n et a l . , 1 9 9 8 ) . T h u s , i t m i g h t b e e x p e c t e d t h a t t h e s t a n d a r d d u t y c y c l e , w i t h i t s r e g u l a r 4 8 - h o u r d a t a g a p s , w i l l n o t p r o v i d e s u f f i c i e n t l y a c c u r a t e c u r r e n t m e a s u r e m e n t s w h e n s t r o n g h i g h - f r e q u e n c y m o t i o n s p r e v a i l . I n t h i s c h a p t e r , t h e e f f e c t s o f t h e s t a n d a r d d u t y c y c l e , as w e l l as t w o o t h e r d u t y c y c l e s ( 3 2 - 1 6 h a n d 1 6 - 8 h ) , o n t h e v e l o c i t y s t a t i s t i c s d e r i v e d f r o m S V P d r i f t e r s i n t h e n o r t h e a s t P a c i f i c a r e e x a m i n e d , a n d t h e n e c e s s i t y o f a c c o u n t i n g f o r t h e h i g h - f r e q u e n c y m o t i o n s w h e n i n t e r p o l a t i n g o v e r d u t y c y c l e s e g m e n t s i s e m p h a s i z e d . T h i s i s d o n e b y d e g r a d i n g c o n t i n u o u s s e g m e n t s ( i n w h i c h a l l a v a i l a b l e s a t e l l i t e p o s i t i o n f i x e s a r e r e c o r d e d a n d p r o c e s s e d b y S e r v i c e A R G O S ) o f e a c h d r i f t e r se r i es t o m a t c h t h o s e o f t h e d u t y c y c l e s , a n d t h e n c o m p a r i n g t h e p r i m e ( m e a n a n d s t a n d a r d d e v i a t i o n ) a n d r o t a r y s p e c t r a l s t a t i s t i c s d e r i v e d f r o m t h e s e r i e s . I t i s s h o w n t h a t r e p r o d u c t i o n o f t h e o r i g i n a l p r i m e a n d s p e c t r a l s t a t i s t i c s r e q u i r e s a n i n t e r p o l a t i o n w h i c h t a k e s i n t o a c c o u n t t h e o s c i l l a t o r y c o m p o n e n t o f t h e d r i f t e r m o t i o n s . T h e f o l l o w i n g s e c t i o n d e s c r i b e s t h e d a t a se t u s e d i n t h i s s t u d y , a n d t h e d e g r a d a t i o n a n d i n t e r p o l a t i o n p r o c e d u r e s a p p l i e d t o t h e o r i g i n a l d r i f t e r s e r i e s . T h i s i s f o l l o w e d b y a c o m p a r i s o n o f t h e d e r i v e d p r i m e a n d s p e c t r a l v e l o c i t y s t a t i s t i c s , a n d a r e c o m m e n d a t i o n r e g a r d i n g s a m p l i n g a n d i n t e r p o l a t i o n s t r a t e g i e s t o b e u s e d i n f u t u r e d r i f t e r s t u d i e s . Chapter 3. Sampling Strategies and Interpolation Schemes 27 3.2 Data and Methods 3.2.1 Drifter Series The effects of the duty cycles on the derived velocity statistics are demonstrated by focusing on the trajectories of six drifters deployed in the vicinity of Station P (50°N, 145°W) in the northeast Pacific Ocean in September 1990 (Figure 3.1a). Each drifter was equipped with a holey-sock drogue centered 15 m below the surface. Strong high- frequency oscillations occur in the trajectories of each of these drifters. Inertial currents accounted for 58% of the total variance measured by these drifters, and semidiurnal currents accounted for another 11% (Thomson et al., 1998). For comparison, the tra- jectory of another shallow-drogued drifter deployed near the head of the Gu l f of Alaska in September 1992, which appeared to have weaker inertial motions, is also analyzed (Figure 3.1b). The drifters studied here alternated between continuous data receive/transmit mode and the 48-24h duty cycle at 90-day intervals. After the second 90-day period of con- tinuous transmission, they remained on the 48-24h duty cycle for the remainder of their lifetimes. Consequently, all reference statistics are derived using only the first 90-day period of each drifter's record. 3.2.2 Time series degradation Each of the original drifter trajectories consisted of irregularly-spaced time series of longi- tude and latitude, from which first-differenced zonal and meridional velocity components were computed. During the first 90-day period analyzed, the drifters in the Station P Group averaged 10.7 position fixes per day, for an average of 964 data points per drifter record. Each time series was degraded to match the standard duty cycle, 48-24h, as well as duty cycles of 32-16h and 16-8h. The 48-24h duty cycle reduced the average number Chapter 3. Sampling Strategies and Interpolation Schemes 28 50°N- (a) Station P 48°N- 146°W 1311 PACIFIC OCEAN 1198 S e p t e m b e r — N o v e m b e r 1 9 9 0 1 r 142°W 1 r 138°W 1 r 134°W 130°W F i g u r e 3 . 1 : M a p o f t h e r e g i o n s h o w i n g t h e f i r s t 90 d a y s o f t h e t r a j e c t o r i e s o f (a ) s i x d r i f t e r s d e p l o y e d n e a r S t a t i o n P (50°N, 145°W) i n S e p t e m b e r 1 9 9 0 w h i c h h a d s t r o n g i n e r t i a l m o t i o n s , a n d (b ) a d r i f t e r d e p l o y e d a t t h e h e a d o f t h e G u l f o f A l a s k a i n S e p t e m b e r 1 9 9 2 w h i c h h a d w e a k e r i n e r t i a l m o t i o n s . Chapter 3. Sampling Strategies and Interpolation Schemes 29 o f o b s e r v a t i o n s b y 7 0 % , w h i l e t h e 3 2 - 1 6 h a n d 1 6 - 8 h d u t y c y c l e s r e s u l t e d i n a n a v e r a g e r e d u c t i o n o f 7 2 % a n d 7 6 % , r e s p e c t i v e l y . A s i l l u s t r a t e d b y t r a j e c t o r y p l o t s o f t h e o r i g i n a l a n d d e g r a d e d s e r i e s o f d r i f t e r s 1310 a n d 1 5 3 6 6 ( F i g u r e s 3 .2 a n d 3 . 3 ) , s o m e o f t h e g a p s i n t h e d u t y c y c l e d a t a a r e q u i t e e x t e n s i v e . E s p e c i a l l y l a r g e g a p s a r e f o u n d i n t h e 4 8 - 2 4 h d e g r a d a t i o n o f d r i f t e r 1 5 3 6 6 w h e r e i t w a s w i t h i n t h e f a s t - f l o w i n g A l a s k a n S t r e a m ( F i g u r e 3 . 3 b ) . 3.2.3 T ime series interpolation T w o d i s t i n c t i n t e r p o l a t i o n r o u t i n e s w e r e u s e d o n t h e o r i g i n a l a n d d e g r a d e d s e r i e s o f e a c h d r i f t e r i n a n a t t e m p t t o r e p r o d u c e t h e p r i m e a n d s p e c t r a l s t a t i s t i c s o f t h e o r i g i n a l s e r i e s . T h e f i r s t r o u t i n e u s e d w a s a H e r m i t e s p l i n e , w h i l e t h e s e c o n d a s s u m e d a n o s c i l l a t i o n o f u n k n o w n f r e q u e n c y i n t h e d r i f t e r m o t i o n s a n d c o n s t r u c t e d a d a t a - d e n s i t y - d e p e n d e n t l o c a l f i t t o t h e o b s e r v a t i o n s . T h e l a t t e r i n t e r p o l a t i o n , r e f e r r e d t o h e r e as t h e m u l t i - f u n c t i o n a l f i t ( M F F ) , i s d e s c r i b e d b e l o w . F i r s t , t h e a v e r a g e d a t a d e n s i t y ( n u m b e r o f o b s e r v a t i o n s p e r d a y ) o v e r a 3 - d a y p e r i o d w a s c a l c u l a t e d s t a r t i n g a t e a c h o b s e r v a t i o n p o i n t i n t h e t i m e s e r i e s . T h e " l o c a l " i n t e r - p o l a t i o n o f a s e l e c t e d s e g m e n t o f t h e se r ies w a s t h e n d e p e n d e n t o n i t s d a t a d e n s i t y . F o r a d e n s i t y o f g r e a t e r t h a n 6 ( f e w e r t h a n 6) o b s e r v a t i o n s p e r d a y , t h e m a x i m u m n u m b e r o f o b s e r v a t i o n s a l l o w e d i n t h e l o c a l i n t e r p o l a t i o n w a s 30 (18 ) . T h e m a x i m u m t i m e a n d d i s t a n c e a l l o w e d i n t h e s e g m e n t w a s set a t 10 d a y s a n d 3 0 0 k m , r e s p e c t i v e l y . T h e s e c r i t e r i a , b a s e d o n t r i a l a n d e r r o r w i t h t h e d a t a s e t s , w e r e i n t e n d e d t o e n s u r e g o o d f i t s b o t h f o r c o n t i n u o u s a n d d u t y c y c l e s e g m e n t s . O n c e a n y o n e o f t h e l i m i t s ( d a t a d e n s i t y , d i s t a n c e o r t i m e ) w a s m e t , t h e s e l e c t e d s e g m e n t w a s t r a n s f e r r e d t o t h e f i t t i n g r o u t i n e . T h e f u n c t i o n , f>(t), c h o s e n t o f i t t h e s e l e c t e d s e g m e n t c o n s i s t e d o f a p o l y n o m i a l p l u s a n o s c i l l a t i o n w h o s e f r e q u e n c y , u>, w a s d e t e r m i n e d b y t h e i n t e r p o l a t i o n : Chapter 3. Sampling Strategies and Interpolation Schemes 30 5 0 ° N 4 9 ° N - 5 0 ° N 49°NH 5 0 ° N 4 9 ° N H 5 0 ° N 49°NH ft. ^*c*$T 1 3 1 0 V O r i g i n a l 1 1 1 1 1 f 1 3 1 0 4 8 - 2 4 h (e) * J> 1 3 1 0 3 2 - 1 6 h *' * •% 1 3 1 0 1 6 - 8 h 1 4 6 ° W 144°W 1 4 2 ° W 1 4 0 ° W F i g u r e 3 . 2 : T h e 9 0 - d a y u n i n t e r p o l a t e d t r a j e c t o r i e s o f d r i f t e r 1 3 1 0 f o r t h e (a ) o r i g i n a l c o n t i n u o u s s e r i e s , a n d t h e (b ) 4 8 - 2 4 h , (c ) 3 2 - 1 6 h , a n d (d ) 1 6 - 8 h d e g r a d e d s e r i e s . E a c h m a r k r e p r e s e n t s a n o b s e r v a t i o n p o i n t . T h e n u m b e r o f o b s e r v a t i o n s c o n t a i n e d i n e a c h s e r i e s i s (a ) 9 4 7 , (b ) 2 8 7 , (c ) 2 3 9 , a n d (d ) 2 6 5 . Chapter 3. Sampling Strategies and Interpolation Schemes 31 54°N- (a) 5 2 ° N - I i i 50°N- # t 15366 Original i i i i i 54°N- (b) S 5 2 ° N - 50°N- y { t 1 1 1 5 3 6 6 48 - 2 4 h I I I I I 5 4 ° N - 5 2 ° N - (c) J <m * J • 0 V 50°N- r <* I 1 1 1 15366 32 - 1 6 h I I I I I 54°N- (d) rf . -V"". s • » . r % • 5 2 ° N - 5 0 ° N - " I jr. rf 15366 16 - 8 h T 1 I I I I I I ' "I " I I I 1 7 4 ° W 1 7 0 ° W 1 6 6 ° W 1 6 2 ° W F i g u r e 3 . 3 : T h e 9 0 - d a y u n i n t e r p o l a t e d t r a j e c t o r i e s o f d r i f t e r 1 5 3 6 6 f o r t h e (a ) o r i g i n a l c o n t i n u o u s s e r i e s , a n d t h e (b ) 4 8 - 2 4 h , (c ) 3 2 - 1 6 h , a n d (d ) 1 6 - 8 h d e g r a d e d s e r i e s . E a c h m a r k r e p r e s e n t s a n o b s e r v a t i o n p o i n t . T h e n u m b e r o f o b s e r v a t i o n s c o n t a i n e d i n e a c h s e r i e s i s (a ) 7 9 3 , ( b ) 2 3 6 , (c ) 2 5 0 , a n d (d ) 2 3 5 . Chapter 3. Sampling Strategies and Interpolation Schemes 3 2 p(t-) = a + bt + ct2 + dt3 + et4 + hsin(u;t + cb), (3 .1 ) w h e r e t i s t i m e , a - e a r e c o n s t a n t s o f t h e p o l y n o m i a l c o m p o n e n t , a n d h i s a c o n s t a n t a n d 4> i s t h e p h a s e l a g f o r t h e o s c i l l a t o r y c o m p o n e n t . A g r a d i e n t - e x p a n s i o n a l g o r i t h m w a s a p p l i e d ( B e v i n g t o n , 1 9 6 9 ) . I n a h e u r i s t i c s e n s e , t h e p o l y n o m i a l f i t s t h e g e n e r a l t r e n d o f t h e s e g m e n t , w h i l e t h e o s c i l l a t i o n f i t s t h e d o m i n a n t h i g h - f r e q u e n c y f l u c t u a t i o n s . T h e o r d e r o f t h e p o l y n o m i a l d e p e n d s o n t h e d a t a d e n s i t y o f t h e s e g m e n t . I f t h e s e l e c t i o n p r o c e s s r e s u l t e d i n o n l y a f e w o b s e r v a t i o n s i n t h e s e g m e n t ( fo r e x a m p l e , i f t h e d i s t a n c e o r t i m e l i m i t w a s m e t d u r i n g a d u t y c y c l e p o r t i o n o f t h e s e r i e s ) , a s m a l l e r o r d e r p o l y n o m i a l w o u l d s u f f i c e . F o r t h e t r a j e c t o r i e s f i t t e d h e r e , a q u a r t i c p o l y n o m i a l w a s g e n e r a l l y r e q u i r e d . A n e w s e g m e n t w a s t h e n s e l e c t e d b y a d v a n c i n g t h e s t a r t t i m e b y 8 p o i n t s f o r a h i g h - d e n s i t y s e g m e n t ( > 6 o b s e r v a t i o n s p e r d a y ) o r b y 2 p o i n t s f o r a l o w - d e n s i t y s e g m e n t ( < 6 o b s e r v a t i o n s p e r d a y ) , a n d r e p e a t i n g t h e s e l e c t i o n a n d f i t t i n g p r o c e d u r e d e s c r i b e d a b o v e . T h e o v e r l a p p i n g s e g m e n t s y i e l d e d m u l t i p l e p r e d i c t e d f i t s a t e a c h 3 - h o u r l y t i m e . O n c e t h e e n t i r e se r i es h a d b e e n c o v e r e d , t w o d i f f e r e n t a p p r o a c h e s w e r e u s e d t o o b t a i n t h e f i n a l i n t e r p o l a t e d s e r i e s . I n t h e f i r s t m e t h o d ( M F F 1 ) , a l l o v e r l a p p i n g f i t t e d s e g m e n t s w e r e a v e r a g e d t o g e t 3 - h o u r l y e s t i m a t e s o f d r i f t e r p o s i t i o n ( l a t i t u d e , l o n g i t u d e ) , a n d s p e e d s w e r e a g a i n c a l c u l a t e d b y f i r s t - d i f f e r e n c i n g t h e p o s i t i o n d a t a . I n t h e s e c o n d m e t h o d ( M F F 2 ) , i n s t e a d o f a v e r a g i n g o v e r l a p p i n g s e g m e n t s , t h e p r e d i c t e d l o c a t i o n a t a g i v e n t i m e w h i c h h a d t h e l o w e s t r o o t - m e a n - s q u a r e d i s t a n c e t o t h e n e i g h b o r i n g t h r e e o b s e r v a t i o n p o i n t s o n e i t h e r s i d e o f t h a t t i m e ( i . e . , t h e " b e s t " f i t ) w a s c h o s e n . T h e r e s p e c t i v e s p l i n e a n d M F F 2 i n t e r p o l a t i o n s a p p l i e d t o t h e c o n t i n u o u s a n d 3 2 - 1 6 h d e g r a d e d s e r i e s o f d r i f t e r s 1 3 1 0 a n d 1 5 3 6 6 a r e s h o w n i n F i g u r e s 3 .4 a n d 3 .5 . O s c i l l a t i o n s o f i n e r t i a l p e r i o d w e r e r e p r o d u c e d i n t h e M F F 2 - i n t e r p o l a t e d d e g r a d e d se r ies o f d r i f t e r 1 3 1 0 i n r e g i o n s w h e r e s t r o n g i n e r t i a l m o t i o n s w e r e o r i g i n a l l y o b s e r v e d ( F i g u r e 3 .4 ) . Chapter 3. Sampling Strategies and Interpolation Schemes 33 F i g u r e 3 .4 : T h e 9 0 - d a y i n t e r p o l a t e d t r a j e c t o r i e s o f d r i f t e r 1 3 1 0 . (a ) T h e s p l i n e - i n t e r p o l a t e d c o n t i n u o u s s e r i e s , (b ) t h e s p l i n e - i n t e r p o l a t e d 3 2 - 1 6 h d e g r a d e d s e r i e s , (c ) t h e M F F 2 - i n t e r p o l a t e d c o n t i n u o u s s e r i e s , a n d (d ) t h e M F F 2 - i n t e r p o l a t e d 3 2 - 1 6 h d e - g r a d e d s e r i e s . Chapter 3. Sampling Strategies and Interpolation Schemes 3 4 • • I I I I I I ' ' I I 5 4 ° N - (a) 5 2 ° N - /• 1 5 3 6 6 /r^^irj C o n t i n u o u s 5 0 ° N - i i i i S p l i n e i i i i i i i i 5 4 ° N - (b) 5 2 ° N - _ 1 5 3 6 6 3 2 - 1 6 h 5 0 ° N - i i i i S p l i n e i i < i i i i i 5 4 ° N - (c) 5 2 ° N - r 1 5 3 6 6 C o n t i n u o u s 5 0 ° N - i i i i M F F 2 5 4 ° N - (d) 5 2 ° N - . 1 5 3 6 6 / ) 3 2 - 1 6 h 5 0 ° N - M F F 2 T 1 1 1 1 1 1 1 1 1 1 r 1 7 4 ° W 1 7 0 ° W 1 6 6 ° W 1 6 2 ° W F i g u r e 3 .5 : T h e 9 0 - d a y i n t e r p o l a t e d t r a j e c t o r i e s o f d r i f t e r 1 5 3 6 6 . (a ) T h e s p l i n e - i n t e r p o l a t e d c o n t i n u o u s s e r i e s , (b ) t h e s p l i n e - i n t e r p o l a t e d 3 2 - 1 6 h d e g r a d e d s e r i e s , (c ) t h e M F F 2 - i n t e r p o l a t e d c o n t i n u o u s s e r i e s , a n d (d ) t h e M F F 2 - i n t e r p o l a t e d 3 2 - 1 6 h d e - g r a d e d s e r i e s . Chapter 3. Sampling Strategies and Interpolation Schemes 3 5 P o s i t i o n s w e r e a l s o e s t i m a t e d a n d s p e e d s c a l c u l a t e d a t 6 - h o u r l y i n t e r v a l s ( n o t s h o w n ) , w h i c h i s c o n s i s t e n t w i t h t h e s t a n d a r d p r o d u c t a v a i l a b l e f r o m t h e S V P D a t a A s s e m b l y C e n t e r . H o w e v e r , b e c a u s e 10 o r m o r e o b s e r v a t i o n s p e r d a y a r e t y p i c a l l y a v a i l a b l e f r o m c o n t i n u o u s d r i f t e r s e g m e n t s a t e x t r a t r o p i c a l l a t i t u d e s , i t w a s m o r e a p p r o p r i a t e t o s u b - s a m p l e t h e i n t e r p o l a t i o n a t 3 - h o u r l y i n t e r v a l s . F u r t h e r m o r e , 6 - h o u r l y d a t a c o n t a i n 2 . 5 d a t a p o i n t s o r l e s s p e r i n e r t i a l p e r i o d a t m i d d l e t o h i g h l a t i t u d e s , a n d h a v e a N y q u i s t p e r i o d o f 12 h o u r s , i . e . s e m i d i u r n a l . G i v e n t h e s i g n i f i c a n t e n e r g y l e v e l s a t t h e i n e r t i a l a n d s e m i d i u r n a l p e r i o d s w h i c h a r e c o m m o n l y o b s e r v e d i n t h e u p p e r o c e a n , t h e h i g h e r s a m p l i n g r a t e s e e m s p a r t i c u l a r l y d e s i r a b l e . 3.3 Results 3.3.1 Prime statistics P r i m e v e l o c i t y s t a t i s t i c s p r o v i d e a d i r e c t c o m p a r i s o n b e t w e e n t h e o r i g i n a l a n d d e g r a d e d s e r i e s . A s T a b l e 3.1 i n d i c a t e s , t h e d e g r a d a t i o n s , w h e n l e f t u n i n t e r p o l a t e d , g i v e s i g n i f i - c a n t l y d i f f e r e n t m e a n s p e e d s f r o m t h e o r i g i n a l s e r i e s . F o r i n s t a n c e , t h e 1 6 - 8 h d e g r a d a t i o n o f d r i f t e r 1310 r e s u l t e d i n a n e s t i m a t e d m e a n z o n a l s p e e d 6 0 % g r e a t e r t h a n t h e o r i g i n a l u n i n t e r p o l a t e d s e r i e s . H o w e v e r , t h e o r i g i n a l m e a n s p e e d s w e r e q u i t e a d e q u a t e l y r e p r o - d u c e d f o r e a c h o f t h e d e g r a d a t i o n s a f t e r t h e y w e r e i n t e r p o l a t e d w i t h t h e s p l i n e , M F F 1 o r M F F 2 a l g o r i t h m s ( T a b l e 3 .1 ) . T h e 1 6 - 8 h d e g r a d a t i o n o f d r i f t e r 1 3 1 0 h a s m e a n z o n a l s p e e d s w i t h i n 1 % o f t h e o r i g i n a l f o r e a c h o f t h e i n t e r p o l a t i o n s . S i m i l a r c o m p a r i s o n s c a n b e m a d e f o r e a c h o f t h e S t a t i o n P d r i f t e r s ( n o t s h o w n ) , a l t h o u g h t h e r e w a s a s m a l l , s y s - t e m a t i c u n d e r e s t i m a t i o n o f t h e m e r i d i o n a l s p e e d s i n e a c h o f t h e i n t e r p o l a t i o n r o u t i n e s . F o r d r i f t e r 1 5 3 6 6 , t h e s p l i n e a n d M F F 1 i n t e r p o l a t i o n s s l i g h t l y o v e r e s t i m a t e d t h e z o n a l s p e e d s , w h i l e t h e M F F 2 i n t e r p o l a t i o n y i e l d e d s l i g h t u n d e r e s t i m a t i o n s ( T a b l e 3 . 2 ) . E a c h o f t h e i n t e r p o l a t i o n s n i c e l y r e p r o d u c e d t h e m e a n m e r i d i o n a l s p e e d s . Chapter 3. Sampling Strategies and Interpolation Schemes 36 DRIFTER 1310 Uninterp. Spline MFF1 MFF2 u (cm/s) O r i g i n a l 3 . 93 ± 1.47 3 .89 ± 1.37 3 .91 ± 1.44 3 .90 ± 1 .35 4 8 - 2 4 h 4 . 7 3 ± 2 . 3 4 3 .87 ± 0 .69 3 .85 ± 1 .02 3 . 8 9 ± 1.41 3 2 - 1 6 h 4 .31 ± 2 . 7 4 3 .91 ± 0 .68 3 .90 ± 0 .96 3 . 9 4 ± 1 .23 16 - 8 h 6 .30 ± 3 . 0 4 3 .90 ± 0 .75 3 . 8 9 ± 1.09 3 . 9 2 ± 1 .24 v (cm/s) O r i g i n a l - 2 . 1 4 ± 1.39 - 1 . 8 0 ± 1.40 - 1 . 8 4 ± 1 .47 - 1 . 9 0 ± 1 .38 48 - 2 4 h - 1 . 3 1 ± 2 .20 - 1 . 8 1 ± 0 .70 - 1 . 8 1 ± 0 . 9 4 - 1 . 8 9 ± 1 .37 3 2 - 1 6 h - 2 . 7 2 ± 2 . 6 7 - 1 . 8 2 ± 0 .73 - 1 . 8 1 ± 1.08 - 1 . 8 9 ± 1 .32 16 - 8 h - 4 . 3 2 ± 2 .68 - 1 . 8 2 ± 0 .69 - 1 . 8 2 ± 1.20 - 1 . 8 8 ± 1 .33 Vu'2 (cm/s) O r i g i n a l 2 3 . 0 7 ± 1.04 1 8 . 5 7 ± 0 .97 1 9 . 4 3 ± 1.02 1 8 . 1 9 ± 1 .35 4 8 - 2 4 h 2 0 . 2 5 ± 1.66 9 .38 ± 0 .49 1 3 . 7 5 ± 0 .72 1 8 . 9 1 ± 0 .99 32 - 1 6 h 2 1 . 6 2 ± 1.94 9 .22 ± 0 .48 13 .01 ± 0 .68 1 6 . 4 9 ± 0 . 8 7 16 - 8 h 2 5 . 2 4 ± 2 .15 10 .15 ± 0 .53 14 .80 ± 0 . 7 7 1 6 . 6 4 ± 0 .88 v V 2 (cm/s) O r i g i n a l 2 1 . 8 8 ± 0 .99 1 8 . 9 2 ± 0 .99 1 9 . 9 2 ± 1 .04 1 8 . 5 6 ± 1 .38 4 8 - 2 4 h 1 9 . 0 3 ± 1.56 9 .49 ± 0 .50 1 2 . 7 0 ± 0 . 6 7 1 8 . 4 5 ± 0 . 9 7 32 - 1 6 h 2 1 . 0 3 ± 1.89 9 .83 ± 0 .51 1 4 . 6 1 ± 0 .76 1 7 . 7 4 ± 0 . 9 3 16 - 8 h 2 2 . 2 4 ± 1.89 9 .36 ± 0 .49 1 6 . 1 7 ± 0 .85 1 7 . 9 1 ± 0 . 9 4 T a b l e 3 . 1 : M e a n s ( o v e r b a r ) a n d s t a n d a r d d e v i a t i o n s o f t h e e a s t - w e s t (u ) a n d n o r t h - s o u t h ( v ) c u r r e n t s , i n c l u d i n g t h e 9 5 % c o n f i d e n c e i n t e r v a l s , d e r i v e d f r o m t h e u n i n t e r p o l a t e d , s p l i n e - i n t e r p o l a t e d a n d M F F - i n t e r p o l a t e d se r i es o f d r i f t e r 1 3 1 0 . Chapter 3. Sampling Strategies and Interpolation Schemes 3 7 T h e s t a n d a r d d e v i a t i o n s ( v a r i a n c e s ) o f t h e c u r r e n t s p e e d s d e r i v e d f r o m t h e s p l i n e - i n t e r p o l a t e d d e g r a d e d se r i es o f d r i f t e r 1310 w e r e a p p r o x i m a t e l y 5 0 % ( 7 0 % ) s m a l l e r t h a n t h o s e d e r i v e d f r o m t h e s p l i n e - i n t e r p o l a t e d o r i g i n a l s e r i e s , w h i l e t h o s e c a l c u l a t e d f r o m t h e M F F 1 i n t e r p o l a t i o n w e r e 3 0 % ( 5 0 % ) s m a l l e r ( T a b l e 3 .1 ) . S i m i l a r r e s u l t s w e r e f o u n d f o r d r i f t e r 1 5 3 6 6 , a l t h o u g h t h e d i f f e r e n c e s b e t w e e n t h e o r i g i n a l a n d d e g r a d e d v a r i a n c e s w e r e s m a l l e r ( T a b l e 3 . 2 ) . T h e M F F 2 i n t e r p o l a t i o n , h o w e v e r , y i e l d e d s t a n d a r d d e v i a t i o n s ( v a r i a n c e s ) w i t h i n 5 % ( 1 0 % ) o f t h e o r i g i n a l f o r b o t h d r i f t e r s 1 3 1 0 a n d 1 5 3 6 6 . O n l y t h e m e r i d i o n a l v a r i a n c e o f t h e 1 6 - 8 h d e g r a d a t i o n o f d r i f t e r 1 5 3 6 6 w a s s t r o n g l y o v e r e s t i m a t e d . T h u s , a l t h o u g h e a c h o f t h e i n t e r p o l a t i o n s o f t h e d e g r a d e d s e r i e s r e p r o d u c e d t h e m e a n s p e e d s o f t h e o r i g i n a l s e r i e s , o n l y t h e M F F 2 i n t e r p o l a t i o n y i e l d e d v a r i a n c e s c o m p a r a b l e t o t h o s e o f t h e o r i g i n a l . T h i s w a s m o s t d r a m a t i c a l l y d e m o n s t r a t e d b y t h e d r i f t e r s c o n t a i n i n g s t r o n g i n e r t i a l o s c i l l a t i o n s . I n m a n y a p p l i c a t i o n s ( e . g . , i n t h e S V P ) i t is t h e l o n g - t e r m c i r c u l a t i o n s t a t i s t i c s d e r i v e d f r o m a n e n s e m b l e o f d r i f t e r s ( n o t n e c e s s a r i l y o p e r a t i n g s i m u l t a n e o u s l y ) w h i c h a r e d e s i r e d . A q u e s t i o n i s w h e t h e r t h e g a p s f r o m t h e d u t y c y c l e , a n d t h e i r s u b s e q u e n t i n t e r p o l a t i o n , c a n a f f e c t t h e s e e n s e m b l e s t a t i s t i c s . F o r e a c h o f t h e u n i n t e r p o l a t e d a n d i n t e r p o l a t e d s e r i e s i n t h e S t a t i o n P G r o u p , t h e e n s e m b l e v e l o c i t y s t a t i s t i c s w e r e d e r i v e d u s i n g t h e d a i l y - a v e r a g e d s e r i e s f r o m a l l s i x d r i f t e r s ( i . e . , a l l d a t a p o i n t s f r o m a g i v e n J u l i a n d a y , i f a n y , w e r e a v e r a g e d ) . T h e r e s u l t s ( n o t s h o w n ) a r e s i m i l a r t o t h o s e d e r i v e d f r o m t h e i n d i v i d u a l d r i f t e r s . T h e d e g r a d a t i o n s d i d i n t r o d u c e e r r o r s i n t h e e n s e m b l e m e a n s p e e d s a n d s t a n d a r d d e v i a t i o n s , b u t e a c h o f t h e i n t e r p o l a t i o n s c h e m e s w a s a b l e t o a d e q u a t e l y r e p r o d u c e t h e s t a t i s t i c s ( m e a n s and v a r i a n c e s ) d e r i v e d f r o m t h e o r i g i n a l e n s e m b l e . I t s h o u l d b e n o t e d t h a t i n c r e a s e d t i m e - a v e r a g i n g w i l l y i e l d l o w v a r i a n c e s , a n d t h a t a n y o f t h e i n t e r p o l a t i o n s c h e m e s a p p l i e d t o t h e d u t y c y c l e s e g m e n t s w i l l p r o d u c e s i m i l a r s t a t i s t i c s f o r a v e r a g e s o v e r o n e d a y o r l o n g e r . Chapter 3. Sampling Strategies and Interpolation Schemes D R I F T E R 15366 Uninterp. Spline MFF1 MFF2 u (cm/s) O r i g i n a l - 2 . 9 4 ± 2 . 1 5 - 3 . 2 7 ± 2 .26 - 3 . 3 3 ± 2 . 2 2 - 2 . 3 0 ± 2 . 2 7 48 - 2 4 h - 4 . 2 8 ± 4 . 1 9 - 3 . 0 5 ± 1.89 - 2 . 6 1 ± 2 . 0 3 - 2 . 5 0 ± 2 . 1 9 32 - 1 6 h - 3 . 5 9 ± 3 .86 - 3 . 1 0 ± 1.95 - 3 . 3 3 ± 1.94 - 2 . 4 4 ± 2 . 3 9 16 - 8 h - 4 . 6 1 ± 4 . 2 3 - 3 . 2 7 ± 1.95 - 3 . 2 7 ± 2 . 0 0 - 2 . 3 5 ± 2 . 4 3 v (cm/s) O r i g i n a l - 3 . 7 8 ± 1.97 - 3 . 6 5 ± 2 .12 - 3 . 5 1 ± 2 . 1 0 - 3 . 8 3 ± 2 . 1 3 4 8 - 2 4 h - 4 . 3 9 ± 3 .63 - 3 . 6 6 ± 1.63 - 3 . 4 6 ± 1 .87 - 3 . 5 7 ± 2 .31 3 2 - 1 6 h - 1 . 0 2 ± 3 .51 - 3 . 8 9 ± 1.65 - 3 . 7 2 ± 1.68 - 3 . 6 6 ± 2 . 3 0 16 - 8 h - 1 . 5 2 ± 3 .71 - 3 . 6 4 ± 1.69 - 3 . 5 6 ± 1.82 - 3 . 6 1 ± 2 . 8 9 y/u'2 (cm/s) O r i g i n a l 3 0 . 8 3 ± 1.52 2 8 . 2 2 ± 1.60 2 7 . 5 5 ± 1 .57 2 8 . 4 5 ± 1.60 4 8 - 2 4 h 3 2 . 8 6 ± 2 .96 2 3 . 6 1 ± 1.34 2 5 . 2 8 ± 1 .44 2 7 . 3 8 ± 1 .55 3 2 - 1 6 h 3 1 . 1 4 ± 2 .73 2 4 . 4 1 ± 1.38 2 4 . 2 6 ± 1 .37 2 9 . 9 7 ± 1.69 16 - 8 h 3 3 . 0 5 ± 2 .99 2 4 . 3 7 ± 1.38 2 4 . 9 6 ± 1.41 3 0 . 4 7 ± 1 .72 v V 2 (cm/s) O r i g i n a l 2 8 . 2 3 ± 1.39 2 6 . 5 0 ± 1.50 2 6 . 1 7 ± 1.49 2 6 . 7 3 ± 1.51 4 8 - 2 4 h 2 8 . 4 6 ± 2 .57 2 0 . 3 5 ± 1.15 2 3 . 3 1 ± 1 .33 2 8 . 8 4 ± 1 .63 3 2 - 1 6 h 2 8 . 3 5 ± 2 .49 2 0 . 6 6 ± 1.17 2 1 . 0 2 ± 1.19 2 8 . 8 1 ± 1 .63 16 - 8 h 2 9 . 0 0 ± 2 .62 2 1 . 1 6 ± 1.20 2 2 . 7 8 ± 1.29 3 6 . 3 0 ± 2 . 0 5 T a b l e 3 .2 : A s i n T a b l e 3 . 1 , b u t f o r d r i f t e r 1 5 3 6 6 . Chapter 3. Sampling Strategies and Interpolation Schemes 39 3.3.2 R o t a r y s p e c t r a l ana lys i s A decomposition of Cartesian velocity components (u(t),v(t)) into polarized rotary com- ponents (u~(t), u+(t)) is a convenient method for analysing motions in which one com- ponent is expected to be dominant (e.g. Gonella, 1972, Mooers, 1973). Because rotary vector quantities are invariant under coordinate transformation, components from dif- ferent regions can be compared without concern for the degree of flow reorientation by bottom topography or coastal boundaries (Emery and Thomson, 1997). Here, u~(t) and u+(t) are the clockwise and counterclockwise components, respectively. Spectral esti- mates, S(u>; u±), were obtained for each drifter, with each data segment weighted using a Kaiser-Bessel window (Harris, 1978) prior to the Fourier transform and half-window over- lapping performed to increase the number of degrees of freedom. The rotary variances of the components, Var(u-,u+) = [ [S(w;u-),S{w;u+)]du, (3.2) J Au Vartot = Var(u~) + Var{u+), (3.3) were then estimated, where the bandwidth AOJ encompasses a specified range of frequen- cies. Rotary energy density spectra of the spline-interpolated original and degraded series of drifter 1310 are provided in Figure 3.6. For the original series, the motions are nearly isotropic at the lowest frequencies (periods > 4 days), and are anticyclonically (clockwise) polarized at periods of 12 hours to 4 days (Figure 3.6a). There is a significant clockwise peak at the inertial frequency (period approximately 15.7 hours at this latitude), and a weaker semidiurnal (M2) peak (12.4 hours). The spectra are noisy at high frequencies. The spectra derived from the spline-interpolated degraded series reveal a very different picture (Figures 3.6b,c,d). The clockwise spectra have numerous spurious peaks which Chapter 3. Sampling Strategies and Interpolation Schemes 40 a r e n e a r l y as s t r o n g as t h e i n e r t i a l p e a k . T h e s e p e a k s c o r r e s p o n d t o i n e r t i a l e n e r g y w h i c h h a s b e e n a l i a s e d t o m u l t i p l e s o f t h e d u t y c y c l e f r e q u e n c y , / , ± n / d c , w h e r e / ; i s t h e i n e r t i a l f r e q u e n c y ( a p p r o x i m a t e l y 1 .53 c p d ) , fdc i s t h e d u t y c y c l e f r e q u e n c y ( e . g . , 3 2 / t ^ 1 6 / l = 0 .5 c p d f o r t h e 3 2 - 1 6 h d e g r a d a t i o n ) , a n d n = 1 , 2 , . . . . S o m e e n e r g y f r o m t h e w e a k e r M 2 p e a k h a s l i k e l y b e e n a l i a s e d t o l o w e r f r e q u e n c i e s as w e l l . T h e a l i a s i n g i s e s p e c i a l l y p r o b l e m a t i c i n t h e s p e c t r a o f t h e 1 6 - 8 h se r i es ( F i g u r e 3 . 6 d ) , w h e r e a s t r o n g c l o c k w i s e p e a k c e n t e r e d a t 2 d a y s c o n t a i n s a s i g n i f i c a n t p o r t i o n o f t h e t o t a l e n e r g y . T h u s , t h e n e a r - m e s o s c a l e e n e r g y d i s t r i b u t i o n i s s i g n i f i c a n t l y a f f e c t e d b y t h i s d u t y c y c l e . D ' A s a r o ( 1 9 9 2 ) f o u n d s p e c t r a l l e a k a g e d u e t o t h e i n h e r e n t l y c l u s t e r e d n a t u r e o f t h e A R G O S p o s i t i o n f i x e s f o r d r i f t e r s d e p l o y e d d u r i n g t h e O C E A N S T O R M S e x p e r i m e n t . T h i s i s r e l a t e d t o t h e o r b i t a l c h a r a c t e r i s t i c s o f t h e s a t e l l i t e . T h e a l i a s i n g o b s e r v e d h e r e i s d u e s o l e l y t o t h e i n t e r a c t i o n o f s t r o n g i n e r t i a l m o t i o n s w i t h t h e d u t y c y c l e , w h i c h i s a p o t e n t i a l l y t r a c t a b l e p r o b l e m . S p e c i f i c a l l y , t h e u s e r c a n n o t c o n t r o l when t h e s a t e l l i t e p o - s i t i o n f i x e s a r e a v a i l a b l e , b u t c a n , t o s o m e d e g r e e , c o n t r o l t h e d u t y c y c l e w h i c h o p t i m i z e s t h e d a t a r e c o r d s . T h e M F F i n t e r p o l a t i o n s s o l v e t h e p r o b l e m o f a b a s i n g b y t h e d u t y c y c l e . T h e M F F 1 i n t e r p o l a t i o n a d e q u a t e l y r e p r o d u c e s t h e i n e r t i a l p e a k i n t h e d e g r a d e d s e r i e s , b u t f a i l s i n t h a t t h e c l o c k w i s e e n e r g y a t p e r i o d s less t h a n i n e r t i a l a n d f r o m 1-5 d a y s i s t w o o r d e r s o f m a g n i t u d e l o w e r t h a n f o r t h e o r i g i n a l se r ies ( F i g u r e 3 .7 ) . T h i s i s t h e " m i s s i n g e n e r g y " s e e n i n t h e s t a n d a r d d e v i a t i o n s o f t h e d e g r a d e d se r i es ( T a b l e 3 . 1 ) . T h e r o t a r y s p e c t r a o f t h e M F F 2 - i n t e r p o l a t e d o r i g i n a l a n d d e g r a d e d s e r i e s o f d r i f t e r 1 3 1 0 ( F i g u r e 3 .8) d e m o n s t r a t e t h e e f f e c t i v e n e s s o f t h e M F F 2 i n t e r p o l a t i o n . T h e s p e c t r a o f e a c h o f t h e d e g r a d e d se r i es ( F i g u r e s 3 . 8 b , c , d ) m a t c h t h e s p e c t r a o f t h e o r i g i n a l s e r i e s ( F i g u r e 3 . 8 a ) q u i t e w e l l . T h e p r i m a r y d i f f e r e n c e s b e t w e e n t h e o r i g i n a l a n d d e g r a d e d M F F 2 s p e c t r a a r e t h e w e a k S+ i n e r t i a l p e a k s i n t h e l a t t e r ( t w o o r d e r s o f m a g n i t u d e s m a l l e r t h a n t h e S~ p e a k ) , w h i c h a r e d u e t o f i t t i n g t h e z o n a l a n d m e r i d i o n a l c o m p o n e n t s Chapter 3. Sampling Strategies and Interpolation Schemes 41 SPLINE INTERPOLATION F r e q u e n c y ( c p d ) F i g u r e 3 .6 : T h e c l o c k w i s e ( 5 ~ ; s o l i d U n e ) a n d c o u n t e r c l o c k w i s e (S+; d a s h e d U n e ) r o - t a r y e n e r g y d e n s i t y s p e c t r a (m2s~2cpd~1) d e r i v e d f r o m t h e f i r s t 8 0 - d a y p e r i o d o f t h e s p l i n e - i n t e r p o l a t e d se r i es o f d r i f t e r 1310 f o r t h e (a) o r i g i n a l c o n t i n u o u s s e r i e s , a n d t h e ( b ) 4 8 - 2 4 h , (c ) 3 2 - 1 6 h , a n d (d ) 1 6 - 8 h d e g r a d e d s e r i e s . T h e 9 5 % c o n f i d e n c e U m i t s a n d t h e i n e r t i a l ( / ) a n d s e m i d i u r n a l ( M 2 ) p e a k s a r e s h o w n i n (a ) . Chapter 3. Sampling Strategies and Interpolation Schemes 4 2 MFF1 INTERPOLATION Frequency (cpd) F i g u r e 3 .7 : A s i n F i g u r e 3 . 6 , b u t f o r t h e M F F l - i n t e r p o l a t e d o r i g i n a l a n d d e g r a d e d s e r i e s o f d r i f t e r 1 3 1 0 . Chapter 3. Sampling Strategies and Interpolation Schemes 4 3 i n d e p e n d e n t l y u s i n g (1 ) , t h e r e b y l o s i n g t h e p h a s e i n f o r m a t i o n o f t h e i n e r t i a l m o t i o n s . T h e s p e c t r a o f t h e 4 8 - 2 4 h a n d 1 6 - 8 h d e g r a d a t i o n s a r e n o i s y a t p e r i o d s l e s s t h a n 12 h o u r s , w h i l e t h o s e o f t h e 3 2 - 1 6 h d e g r a d a t i o n a p p e a r t o m o s t c l o s e l y r e s e m b l e t h e s p e c t r a o f t h e o r i g i n a l s e r i e s , w i t h a n e n e r g y r o l l - o f f a t h i g h f r e q u e n c i e s . T h e c l o c k w i s e s p e c t r u m o f t h e 1 6 - 8 h d e g r a d a t i o n d o e s a p p e a r t o c a p t u r e a w e a k s e m i d i u r n a l p e a k . T h e r o t a r y s p e c t r a o f t h e M F F 2 - i n t e r p o l a t e d o r i g i n a l s e r i e s o f d r i f t e r 1 5 3 6 6 ( F i g u r e 3 . 9 a ) h a v e a b r o a d c l o c k w i s e p e a k c e n t e r e d b e t w e e n t h e i n e r t i a l a n d M 2 f r e q u e n c i e s , a n d a r e w e a k l y c l o c k w i s e p o l a r i z e d a t l o w f r e q u e n c i e s , w i t h S+ t r o u g h s a t p e r i o d s o f 2 .5 a n d 5 .5 d a y s . T h e l o w f r e q u e n c i e s a r e s i g n i f i c a n t l y m o r e e n e r g e t i c h e r e t h a n i n t h e S t a t i o n P G r o u p . T h e s p e c t r a o f t h e 3 2 - 1 6 h d e g r a d a t i o n ( F i g u r e 3 . 9 c ) a p p e a r t o m o s t c l o s e l y r e s e m - b l e t h e s p e c t r a l c h a r a c t e r i s t i c s o f t h e o r i g i n a l s e r i e s , a l t h o u g h t h e S~ n e a r - i n e r t i a l p e a k is o v e r e s t i m a t e d a n d t o o n a r r o w . T h i s i s d u e t o f i t t i n g o n l y o n e o s c i l l a t i o n i n t h e i n t e r - p o l a t i o n . A d d i n g a p r e s c r i b e d M 2 o s c i l l a t i o n a n d f i t t i n g t h e ( v a r y i n g ) i n e r t i a l o s c i l l a t i o n w o u l d l i k e l y i m p r o v e t h e s p e c t r a l c h a r a c t e r i s t i c s . T h e s p e c t r a o f t h e 4 8 - 2 4 h d e g r a d a t i o n a r e v e r y n o i s y a t h i g h f r e q u e n c i e s ( F i g u r e 3 . 9 b ) , w h i l e only t h e l o w f r e q u e n c i e s ( p e r i o d s > 1 d a y ) o f t h e o r i g i n a l s p e c t r a a r e r e p r o d u c e d i n t h e 1 6 - 8 h d e g r a d a t i o n ( F i g u r e 3 . 9 d ) . T h e s p e c t r a o f t h e s p l i n e i n t e r p o l a t i o n s o f d r i f t e r 1 5 3 6 6 ( n o t s h o w n ) a r e a d e q u a t e f o r t h e o r i g i n a l s e r i e s , b u t a g a i n y i e l d s i g n i f i c a n t s p e c t r a l l e a k a g e i n t h e d e g r a d e d s e r i e s . 3.4 Discussion A q u e s t i o n r a i s e d b y t h i s a n a l y s i s i s w h e t h e r o n e o f t h e t h r e e i n v e s t i g a t e d d u t y c y c l e s i s b e t t e r s u i t e d f o r m i d - l a t i t u d e d r i f t e r s i n r e g i o n s o f s t r o n g i n e r t i a l o s c i l l a t i o n s . A c o m p a r i s o n o f t h e r o t a r y v a r i a n c e s c o m p u t e d i n f o u r f r e q u e n c y b a n d s f o r t h e M F F 2 - i n t e r p o l a t e d o r i g i n a l a n d d e g r a d e d se r ies o f t h e S t a t i o n P e n s e m b l e a l l o w s f o r a m o r e q u a n t i t a t i v e c o m p a r i s o n o f t h e s p e c t r a l s t a t i s t i c s ( T a b l e 3 . 3 ) . T h e e n e r g y c o n t a i n e d i n t h e Chapter 3. Sampling Strategies and Interpolation Schemes F i g u r e 3 .8 : A s i n F i g u r e 3 .6 , b u t f o r t h e M F F 2 - i n t e r p o l a t e d o r i g i n a l a n d d e g r a d e d o f d r i f t e r 1 3 1 0 . Chapter 3. Sampling Strategies and Interpolation Schemes F i g u r e 3 .9 : A s i n F i g u r e 3 .6 , b u t f o r t h e M F F 2 - i n t e r p o l a t e d o r i g i n a l a n d d e g r a d e d o f d r i f t e r 1 5 3 6 6 . Chapter 3. Sampling Strategies and Interpolation Schemes 4 6 l o w f r e q u e n c y p o r t i o n o f t h e m e s o s c a l e b a n d (2 -8 d a y p e r i o d ) , a n d i t s p a r t i t i o n b e t w e e n t h e c l o c k w i s e a n d c o u n t e r c l o c k w i s e c o m p o n e n t s , i s s i m i l a r f o r t h e o r i g i n a l a n d e a c h o f t h e d e g r a d e d s e r i e s . T h e 4 8 - 2 4 h d e g r a d a t i o n h a s t o o m u c h c o u n t e r c l o c k w i s e e n e r g y a t t h e h i g h e s t f r e q u e n c i e s , w h i l e t h e 1 6 - 8 h d e g r a d a t i o n h a s t o o m u c h h i g h - f r e q u e n c y e n e r g y i n b o t h r o t a r y c o m p o n e n t s . T h e o v e r - e s t i m a t e d h i g h f r e q u e n c y e n e r g i e s a r e a l s o s e e n i n t h e M F F 2 - i n t e r p o l a t e d s p e c t r a o f d r i f t e r 1310 ( F i g u r e s 3 . 8 b , d ) . T h e 1 6 - 8 h d e g r a d a t i o n a l s o h a s t o o m u c h c o u n t e r c l o c k w i s e e n e r g y i n t h e h i g h f r e q u e n c y p o r t i o n o f t h e m e s o s c a l e b a n d ( 1 7 h - 1 . 9 d p e r i o d ) . I n g e n e r a l , i t a p p e a r s t h a t t h e 3 2 - 1 6 h d e g r a d a t i o n r e p r o d u c e s t h e t o t a l e n e r g y o f t h e o r i g i n a l s e r i e s , a n d i t s f r e q u e n c y p a r t i t i o n , v e r y w e l l . F u r t h e r c o m p a r i s o n o f t h e d u t y c y c l e s c a n b e m a d e b y c o m p u t i n g t h e r o t a r y c o e f f i - c i e n t , g i v e n b y b(<jj]u ) + b{u);u+) f o r t h e S t a t i o n P e n s e m b l e ( T a b l e 3 .4 ) . F o r c l o c k w i s e ( c o u n t e r c l o c k w i s e ) r o t a r y m o t i o n s , r i s p o s i t i v e ( n e g a t i v e ) , a n d (—1 < r < 1) ( T h o m s o n e t a l . , 1 9 9 8 ) . F o r e a c h o f t h e d u t y c y c l e s , t h e r o t a r y c o e f f i c i e n t is t o o l o w i n a l l f r e q u e n c y b a n d s , i n d i c a t i n g a n e x c e s s ( d e f i c i e n c y ) o f c o u n t e r c l o c k w i s e ( c l o c k w i s e ) e n e r g y i n t h e i n t e r p o l a t i o n s o f t h e d e g r a d e d s e r i e s . T h u s , e f f e c t s o f t h e d u t y c y c l e a r e p r e v a l e n t e v e n a t r e l a t i v e l y l o w f r e q u e n c i e s , w h e r e c l o c k w i s e e n e r g i e s a r e u n d e r e s t i m a t e d i n t h e l o w m e s o s c a l e b a n d a n d c o u n t e r c l o c k - w i s e e n e r g i e s a r e o v e r e s t i m a t e d i n t h e h i g h m e s o s c a l e b a n d (see T a b l e 3 . 3 ) . T h e r o t a r y c o e f f i c i e n t s d e r i v e d f r o m t h e 3 2 - 1 6 h d e g r a d a t i o n s a r e i n c l o s e s t a g r e e m e n t w i t h t h o s e o f t h e o r i g i n a l s e r i e s , p a r t i c u l a r l y i n t h e i n e r t i a l f r e q u e n c y b a n d , w h i c h c o n t a i n e d n e a r l y 8 0 % o f t h e t o t a l e n e r g y o f t h e m o t i o n s w i t h p e r i o d s less t h a n 8 d a y s . S i n c e c l o c k w i s e o s c i l l a t o r y m o t i o n s d o m i n a t e t h e S t a t i o n P e n s e m b l e , t h e c l o c k w i s e s p e c t r a l a m p l i t u d e r a t i o s b e t w e e n t h e o r i g i n a l a n d d e g r a d e d s e r i e s c a n b e c o m p a r e d i n Chapter 3. Sampling Strategies and Interpolation Schemes 4 7 E N S E M B L E Original 48 - 24h 32 - 16h 16- 8h S~{u>) {cm2/s2) (%) l o w m e s o s c a l e 23 .1 (2 .4) 13 .4 (1 .5 ) 14 .4 (1 .6 ) 13 .1 (1 .2 ) h i g h m e s o s c a l e 6 3 . 9 (6 .6) 3 4 . 6 (4 .0) 4 3 . 3 (4 .7 ) 5 6 . 4 (5 .0 ) i n e r t i a l 7 6 5 . 5 (79 .0 ) 6 2 5 . 0 (71 .8 ) 7 0 5 . 1 ( 75 .9 ) 6 7 3 . 4 ( 59 .5 ) h i g h 6 4 . 7 (6 .7) 6 0 . 8 (7 .0 ) 4 7 . 7 (5 .1 ) 1 2 5 . 0 ( 11 .0 ) S+{w) (cm2/s2) (%) l o w m e s o s c a l e 11 .1 (1 .1) 13 .1 (1 .5) 10 .4 (1 .1 ) 10 .0 (0 .9 ) h i g h m e s o s c a l e 7.8 (0 .8 ) 2 7 . 7 (3 .2 ) 2 7 . 5 (3 .0 ) 4 8 . 3 (4 .3 ) i n e r t i a l 2 .0 (0 .2) 3 0 . 8 (3 .5 ) 2 9 . 8 (3 .2 ) 7 7 . 3 (6 .8 ) h i g h 15 .8 (1 .6) 4 8 . 8 (5 .6 ) 3 4 . 9 (3 .8 ) 1 1 2 . 5 (9 .9 ) Stot(oj) [cm2Is2) (%) l o w m e s o s c a l e 3 4 . 2 (3 .5) 2 6 . 5 (3 .0 ) 2 4 . 8 (2 .7 ) 2 3 . 1 (2 .1 ) h i g h m e s o s c a l e 7 1 . 7 (7 .4 ) 6 2 . 3 (7 .2 ) 70 .8 (7 .7 ) 1 0 4 . 7 (9 .3 ) i n e r t i a l 7 6 7 . 5 (79 .2 ) 6 5 5 . 8 (75 .3 ) 7 3 4 . 9 ( 79 .1 ) 7 5 0 . 7 ( 66 .3 ) h i g h 8 0 . 5 (8 .3) 109 .6 (12 .6 ) 8 2 . 6 (8 .9 ) 2 3 7 . 5 ( 20 .9 ) T a b l e 3 . 3 : C l o c k w i s e (S~(u>)), c o u n t e r c l o c k w i s e (S+(u;)) a n d t o t a l (Stot{<*>)) r o t a r y v a r i - a n c e i n f o u r f r e q u e n c y b a n d s d e r i v e d f r o m t h e M F F 2 - i n t e r p o l a t e d s e r i e s o f t h e S t a t i o n P e n s e m b l e . N u m b e r s i n p a r e n t h e s e s r e f e r t o t h e p e r c e n t a g e o f t h e t o t a l r o t a r y v a r i a n c e . T h e f r e q u e n c y b a n d s a r e l o w m e s o s c a l e ( p e r i o d s o f 2 -8 d a y s ) , h i g h m e s o s c a l e ( p e r i o d s o f 17 h o u r s - 1 . 9 d a y s ) , i n e r t i a l ( p e r i o d s o f 1 4 . 7 - 1 6 . 8 h o u r s ) a n d h i g h ( p e r i o d s o f 6 - 1 4 . 5 h o u r s ) . Chapter 3. Sampling Strategies and Interpolation Schemes 4 8 E N S E M B L E Original 48 - 24h 32 - 16h 16 - 8h r(uj) l o w m e s o s c a l e 0 .351 0 .011 0 .161 0 . 1 3 4 h i g h m e s o s c a l e 0 .782 0 .111 0 . 2 2 3 0 . 0 7 7 i n e r t i a l 0 . 995 0 .906 0 . 9 1 9 0 . 7 9 4 h i g h 0 . 6 0 7 0 . 1 0 9 0 . 1 5 5 0 . 0 5 3 T a b l e 3 .4 : T h e r o t a r y c o e f f i c i e n t , r(oj), i n f o u r f r e q u e n c y b a n d s o f t h e M F F 2 - i n t e r p o l a t e d s e r i e s o f t h e S t a t i o n P d r i f t e r e n s e m b l e . T h e f r e q u e n c y b a n d s a r e t h e s a m e as i n T a b l e 3 . 3 . e a c h o f t h e f o u r f r e q u e n c y b a n d s , a n d f o r a l l t h r e e i n t e r p o l a t i o n a l g o r i t h m s ( T a b l e 3 . 5 ) . T h e s p l i n e i n t e r p o l a t i o n c l e a r l y d o e s n o t r e p r o d u c e t h e i n e r t i a l m o t i o n s . T h e M F F 1 i n t e r p o l a t i o n y i e l d s t h e i n e r t i a l p e a k , b u t d o e s n o t h a v e n e a r l y e n o u g h c l o c k w i s e e n e r g y i n t h e h i g h m e s o s c a l e a n d h i g h f r e q u e n c y b a n d s . T h e M F F 2 i n t e r p o l a t i o n a d e q u a t e l y r e p r o d u c e s t h e c l o c k w i s e r o t a r y e n e r g y d i s t r i b u t i o n i n a l l f r e q u e n c y b a n d s , w i t h t h e 3 2 - 1 6 h d e g r a d a t i o n y i e l d i n g t h e b e s t c o m p a r i s o n . T h e M F F 2 i n t e r p o l a t i o n d o e s n o t r e s o l v e t h e i n e r t i a l a n d s e m i d i u r n a l p e a k s i n t h e d e g r a d e d s e r i e s ( e .g . F i g u r e 3 .8 ) . S i n c e i n e r t i a l m o t i o n s w e r e n e a r l y t w o o r d e r s o f m a g n i - t u d e s t r o n g e r t h a n s e m i d i u r n a l m o t i o n s i n t h e S t a t i o n P d r i f t e r r e c o r d s , i t w a s n o t d e e m e d n e c e s s a r y t o a t t e m p t t h i s r e s o l u t i o n . T h e m a i n o b j e c t i v e w a s s i m p l y t o r e p r o d u c e t h e dominant m o t i o n s i n t h e d e g r a d e d s e g m e n t s . T h e M F F i n t e r p o l a t i o n c a n r e a d i l y b e m a d e m o r e s o p h i s t i c a t e d , d e p e n d i n g o n t h e n a t u r e o f t h e d a t a set a n d t h e u s e r ' s i n t e r e s t s . F o r e x a m p l e , o n e c o u l d a d d o s c i l l a t i o n s o f k n o w n f r e q u e n c y i f p a r t i c u l a r t i d a l c o m p o n e n t s a r e Chapter 3. Sampling Strategies and Interpolation Schemes 49 E N S E M B L E O r i g i n a l 48 - 24fc O r i g i n a l 3 2 - l%h O r i g i n a l 16 - 8h Spline l o w m e s o s c a l e 1:7 1.2 0 .7 h i g h m e s o s c a l e 0.8 0 .9 1.0 i n e r t i a l 12 .8 16 .4 2 9 . 8 h i g h 1.7 2 .0 4 . 2 t o t a l 3 .8 4 .4 5 .8 MFF1 l o w m e s o s c a l e 4 .6 4 .4 5 .5 h i g h m e s o s c a l e 18 .2 14 .8 11 .1 i n e r t i a l 1.2 1.4 1.3 h i g h 2 4 . 7 3 2 . 2 5 .6 t o t a l 1.6 1.8 1.7 MFF2 l o w m e s o s c a l e 1.7 1.6 1.8 h i g h m e s o s c a l e 1.8 1.5 1.1 i n e r t i a l 1.2 1.1 1.1 h i g h 1.1 1.4 0 .5 t o t a l 1.2 1.1 1.1 T a b l e 3 . 5 : T h e c l o c k w i s e (S~) s p e c t r a l a m p l i t u d e r a t i o s i n f o u r f r e q u e n c y b a n d s , a n d f o r t o t a l c l o c k w i s e e n e r g y , o f t h e o r i g i n a l t o t h e d e g r a d e d se r i es f o r t h e s p l i n e - i n t e r p o l a t e d a n d M F F - i n t e r p o l a t e d se r i es o f t h e S t a t i o n P d r i f t e r e n s e m b l e . T h e f r e q u e n c y b a n d s a r e t h e s a m e as i n T a b l e 3 .3 . Chapter 3. Sampling Strategies and Interpolation Schemes 50 k n o w n t o d o m i n a t e t h e m o t i o n s , as w a s d o n e b y P e a s e et a l . ( 1 9 9 5 ) . H o w e v e r , t h e r e s u l t s s h o w n h e r e d e m o n s t r a t e t h a t a r e l a t i v e l y s i m p l e p r o c e d u r e c a n ( a n d s h o u l d ) b e a p p l i e d t o g e n e r a l d r i f t e r d a t a s e t s , w h e r e h i g h - f r e q u e n c y m o t i o n s m a y o r m a y n o t b e s t r o n g ( o r m a y o r m a y n o t b e known t o b e s t r o n g ) , a n d t h e p r i m e a n d s p e c t r a l s t a t i s t i c s c a n b e c o n f i d e n t l y e s t i m a t e d f o r b o t h c o n t i n u o u s a n d d u t y - c y c l e s e g m e n t s . I t i s r e c o m m e n d e d t h a t u s e r s " l e a r n " f r o m t h e c o n t i n u o u s s e g m e n t s o f t h e i r d r i f t e r t r a j e c t o r i e s ( i . e . , t h o s e n o t s p o i l e d b y t h e d u t y c y c l e ) , a n d s u b s e q u e n t l y c u s t o m i z e t h e i r i n t e r p o l a t i o n r o u t i n e i n o r d e r t o a d e q u a t e l y a c c o u n t f o r t h e d o m i n a n t m o d e s o f v a r i a b i l i t y o v e r t h e d u t y c y c l e s e g m e n t s . S i n c e a l l S V P d r i f t e r s w e r e d r o g u e d a t a d e p t h o f 15 m , i n e r t i a l o s c i l l a t i o n s a r e e x p e c t e d t o b e t h e d o m i n a n t m o d e o f v a r i a b i l i t y a t m i d - t o h i g h l a t i t u d e s . 3.5 Conclusions C h e r e s k i n et a l . ( 1989 ) e x p l o r e d t h e e f fec ts o f u p p e r o c e a n v e r t i c a l s h e a r o n d r i f t e r s l i p p a g e , w h i l e D ' A s a r o ( 1992 ) a n a l y z e d t h e e f fec ts o f i n h e r e n t S e r v i c e A R G O S s a m - p l i n g e r r o r s o n d r i f t e r v e l o c i t y e s t i m a t e s . I n t h i s s t u d y , t h e e f f e c t s o f r e d u c e d s a m p l i n g s c h e d u l e s ( d u t y c y c l e s ) o n v e l o c i t y s t a t i s t i c s d e r i v e d f r o m s a t e l l i t e - t r a c k e d d r i f t e r s i n t h e n o r t h e a s t P a c i f i c O c e a n h a v e b e e n e x a m i n e d . T h e f i n d i n g s s h o w t h a t b o t h s p l i n e i n t e r - p o l a t i o n a n d m o r e s o p h i s t i c a t e d m u l t i - f u n c t i o n a l ( M F F ) i n t e r p o l a t i o n o f d e g r a d e d ( d u t y c y c l e ) s e g m e n t s o f c o n t i n u o u s d r i f t e r t r a j e c t o r y r e c o r d s a d e q u a t e l y r e p r o d u c e t h e m e a n v e l o c i t i e s o f t h e o r i g i n a l d a t a s e r i e s . T h i s a p p l i e s t o t h e s t a n d a r d d u t y c y c l e u s e d i n m o s t l a r g e - s c a l e d r i f t e r d e p l o y m e n t s c o n s i s t i n g o f 48 h o u r s o f n o A R G O S d a t a t r a n s m i s s i o n f o l l o w e d b y 24 h o u r s o f A R G O S t r a n s m i s s i o n r e c e i v e d ( 4 8 - 2 4 h ) , as w e l l as t o d u t y c y c l e s h a v i n g s h o r t e r b u t m o r e f r e q u e n t g a p s ( 3 2 - 1 6 h a n d 1 6 - 8 h ) . H o w e v e r , t h e a b i l i t y t o r e p r o - d u c e t h e r o t a r y s p e c t r a l c h a r a c t e r i s t i c s o f t h e o r i g i n a l t i m e s e r i e s , as w e l l as t h e v e l o c i t y v a r i a n c e s , i s s t r o n g l y d e p e n d e n t o n t h e i n t e r p o l a t i o n s c h e m e a p p l i e d t o t h e d u t y c y c l e Chapter 3. Sampling Strategies and Interpolation Schemes 51 r e c o r d . T h e s p l i n e i n t e r p o l a t i o n p r o d u c e d n u m e r o u s s p u r i o u s s p e c t r a l p e a k s , w i t h i n e r - t i a l e n e r g y a l i a s e d t o m u l t i p l e s o f t h e d u t y c y c l e f r e q u e n c y . O n t h e o t h e r h a n d , t h e M F F r o u t i n e d e s c r i b e d h e r e , w h i c h a l l o w s f o r a n o s c i l l a t o r y c o m p o n e n t t o t h e d r i f t e r m o t i o n s , w a s a b l e t o a d e q u a t e l y r e p r o d u c e t h e r o t a r y s p e c t r a l f e a t u r e s o f t h e o r i g i n a l d a t a s e r i e s f o r e a c h o f t h e t h r e e t y p e s o f d e g r a d e d se r i es e x a m i n e d . B e s t r e p r o d u c t i o n o f t h e r o t a r y s p e c t r a l a m p l i t u d e s a n d a s s o c i a t e d f r e q u e n c y p a r t i t i o n i n g r e q u i r e d t h a t t h e b e s t f i t t e d s e g m e n t s i n t h e i n t e r p o l a t i o n r o u t i n e b e s e l e c t e d , r a t h e r t h a n t h e a v e r a g e o f o v e r l a p p i n g s e g m e n t s . N e v e r t h e l e s s , e v e n a t s u b i n e r t i a l f r e q u e n c i e s , t h e d u t y c y c l e r e s u l t s i n b i a s e d e s t i m a t e s o f t h e r o t a r y v e l o c i t y v a r i a n c e s . I t is e x p e c t e d t h a t a m o r e s o p h i s t i c a t e d f i t t i n g r o u t i n e , w h i c h w o u l d a l l o w f o r o s c i l l a t i o n s o f k n o w n f r e q u e n c y i n r e g i o n s w h e r e s p e c i f i c f r e q u e n c y c o m p o n e n t s ( s u c h as s e m i d i u r n a l t i d a l m o t i o n s ) a r e k n o w n t o d o m i n a t e (cf . P e a s e et a l . , 1 9 9 5 ) , w o u l d f u r t h e r i m p r o v e e s t i m a t e s o f t h e s p e c t r a l c h a r a c t e r i s t i c s . R e s u l t s o f t h e a n a l y s i s a l s o i n d i c a t e t h a t , f o r t h e t h r e e d u t y c y c l e s i n v e s t i g a t e d a n d f o r m i d - t o - h i g h l a t i t u d e d r i f t e r m o t i o n s h a v i n g s t r o n g i n e r t i a l a n d / o r s e m i d i u r n a l m o - t i o n s , t h e s p e c t r a l c h a r a c t e r i s t i c s o f t h e o r i g i n a l c o n t i n u o u s r e c o r d s a r e m o s t c o n s i s t e n t l y r e p r o d u c e d b y t h e 3 2 - 1 6 h d u t y c y c l e . T h i s p r e s u m a b l y i s b e c a u s e t h e 1 6 h t r a n s m i s s i o n - r e c e i v e d s e g m e n t s o f d r i f t e r t r a c k s a r e s u f f i c i e n t l y l o n g t o d e f i n e s u p e r i n e r t i a l m o t i o n s w h i l e t h e 3 2 h t r a n s m i s s i o n - b l a c k o u t s e g m e n t s a r e s h o r t e n o u g h t h a t t h e m a i n f e a t u r e s o f t h e m o t i o n s r e m a i n r e l a t i v e l y u n c h a n g e d . U n f o r t u n a t e l y , t h i s d u t y c y c l e i s n o t a p e r m i s s i b l e o p t i o n w i t h S e r v i c e A R G O S . T h e d u t y c y c l e w i t h t h e s h o r t e s t a n d m o s t f r e q u e n t g a p s ( 1 6 - 8 h ) y i e l d e d t h e w o r s t r e p r o d u c t i o n , w i t h s i g n i f i c a n t l y o v e r e s t i m a t e d s u p e r i n e r t i a l e n e r g i e s . D r i f t e r e n g i n e e r i n g h a s p r o g r e s s e d r a p i d l y i n r e c e n t y e a r s . M o d e r n d r i f t e r s a r e c a - p a b l e o f m e a s u r i n g v e l o c i t i e s a n d s e a s u r f a c e t e m p e r a t u r e t o a c c u r a c i e s o f 5 cm/s a n d 0.1° ( H a n s e n a n d P o u l a i n , 1 9 9 6 ) , r e s p e c t i v e l y , o v e r a p e r i o d g r e a t e r t h a n a y e a r , a n d , i n s o m e c i r c u m s t a n c e s , o f o b t a i n i n g u s e f u l i n f o r m a t i o n a c r o s s e n t i r e b a s i n s w e l l o v e r a y e a r Chapter 3. Sampling Strategies and Interpolation Schemes 52 a f t e r d e p l o y m e n t ( T h o m s o n et al., 1 9 9 7 , B o g r a d et a l . , 1 9 9 8 b ) . C o n s e q u e n t l y , d r i f t e r d e p l o y m e n t s h a v e b e c o m e a n i n t e g r a l c o m p o n e n t o f m a n y l a r g e - s c a l e a n d m e s o s c a l e o b - s e r v a t i o n a l s t u d i e s . T h e u s e r h a s t h e m a i n o b l i g a t i o n t o g e n e r a t e r e l i a b l e s t a t i s t i c a l i n f o r m a t i o n f r o m t h e d r i f t e r d a t a . I n t h i s r e g a r d , u s e r s a r e s t r o n g l y e n c o u r a g e d t o t a k e i n t o a c c o u n t h i g h - f r e q u e n c y m o t i o n s d u r i n g t i m e se r i es i n t e r p o l a t i o n . I n l i g h t o f t h e r e s u l t s p r e s e n t e d h e r e , i t i s f u r t h e r r e c o m m e n d e d t h a t S e r v i c e A R G O S : (1) l o w e r t h e c o s t s o f d r i f t e r t r a c k i n g a n d d a t a t r a n s m i s s i o n ; a n d (2) p r o v i d e a s e l e c t i o n o f d u t y c y c l e o p t i o n s w h i c h d e p e n d o n t h e l a t i t u d e o f t h e d r i f t e r d e p l o y m e n t s ( s p e c i f i c a l l y , e q u a t o r i a l v e r s u s n o n - e q u a t o r i a l d e p l o y m e n t s ) . Chapter 4 Mean Circulation and Energy Distribution in the North Pacific 4.1 Introduction T h e n e a r - s u r f a c e c i r c u l a t i o n o f t h e A l a s k a n G y r e i m p a c t s t h e l o c a l d i s t r i b u t i o n o f n o n - p a s s i v e ( e . g . , t e m p e r a t u r e , s a l t ) a n d p a s s i v e t r a c e r s ( e . g . , n u t r i e n t s ) . T h i s h a s p r o f o u n d e f f e c t s o n t h e d i s t r i b u t i o n a n d m a g n i t u d e o f p r i m a r y ( P o l o v i n a e t a l . , 1995 ) a n d s e c o n d a r y p r o d u c t i o n ( B r o d e u r a n d W a r e , 1 9 9 2 ) , w h i c h i n t u r n a f f e c t s t h e r a t e s a n d m e c h a n i s m s o f c a r b o n e x p o r t t o t h e d e e p o c e a n ( K n a u e r et a l . , 1 9 9 0 ) . Q u a n t i f y i n g t h e s e r a t e s i s t h e p r i m a r y g o a l o f t h e C a n a d i a n J o i n t G l o b a l O c e a n F l u x S t u d y ( C J G O F S ) , w h o s e f i e l d m e a s u r e m e n t s i n t h e n o r t h e a s t P a c i f i c o v e r l a p t h e W O C E d r i f t e r m e a s u r e m e n t s s t u d - i e d h e r e . R e g i o n a l c i r c u l a t i o n v a r i a b i l i t y i s a l s o a n i m p o r t a n t f a c t o r i n t h e s u r v i v a b i l i t y o f m a n y n o r t h e r n f i s h p o p u l a t i o n s a n d t h e i r e v e n t u a l r e c r u i t m e n t t o e c o n o m i c a l l y v i t a l f i s h e r i e s ( B e a m i s h a n d B o u i l l o n , 1 9 9 3 ; M a n t u a et a l . , 1997 ; W a l t e r e t a l , 1 9 9 7 ) . C l e a r l y , s u f f i c i e n t k n o w l e d g e o f t h e n o r t h e a s t P a c i f i c c i r c u l a t i o n v a r i a b i l i t y i s a p r e r e q u i s i t e t o a m o r e c o m p l e t e u n d e r s t a n d i n g o f t h e e c o s y s t e m d y n a m i c s , a n d i t s c l i m a t o l o g i c a l s i g n i f i - c a n c e , i n t h i s b i o l o g i c a l l y p r o d u c t i v e r e g i o n ( G a r g e t t , 1 9 9 1 ) . I n t h i s c h a p t e r , t h e d r i f t e r e n s e m b l e s a r e a n a l y z e d i n o r d e r t o p r o v i d e a s t a t i s t i c a l r e a l i z a t i o n o f t h e s p a t i a l l y v a r y i n g m e a n f l o w a n d e n e r g y d i s t r i b u t i o n o v e r a b r o a d r e g i o n o f t h e n o r t h e a s t P a c i f i c O c e a n f o r t h e p e r i o d 1 9 9 0 - 1 9 9 5 , a n d t o a l l o w f o r a c o m p a r i s o n o f t h e c i r c u l a t i o n a t t w o d e p t h s r e p r e s e n t i n g t h e m i x e d - l a y e r a n d t h e u n d e r l y i n g p y c n o c l i n e . T h e s e r e s u l t s c a n s u b s e q u e n t l y b e u s e d i n c o n s t r a i n i n g p a r a m e t e r i z a t i o n s a n d v e r i f y i n g 53 Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 54 r e s u l t s i n b o t h g e n e r a l c i r c u l a t i o n a n d c o u p l e d b i o p h y s i c a l m o d e l s o f t h e r e g i o n . 4.2 O c e a n o g r a p h i c S e t t i n g o f t h e N o r t h e a s t P a c i f i c T h e A l a s k a n G y r e i s a w i n d - a n d b u o y a n c y - f o r c e d , p a r t i a l l y c l o s e d c i r c u l a t i o n r e g i m e l o c a t e d i n t h e G u l f o f A l a s k a a n d e x t e n d i n g w e s t w a r d t o n e a r t h e d a t e l i n e ( V a n S c o y e t a l . , 1 9 9 1 ) . I t i s b o u n d e d t o t h e s o u t h b y t h e s l o w ( ~ 10 c m / s ) , e a s t w a r d - f l o w i n g S u b a r c t i c C u r r e n t a t 45° -50°N, t o t h e e a s t b y t h e h i g h l y v a r i a b l e A l a s k a C u r r e n t , a n d t o t h e n o r t h a n d w e s t b y t h e s t r o n g ( > 30 c m / s ) s o u t h w e s t w a r d - f l o w i n g A l a s k a n S t r e a m a l o n g t h e c o n t i n e n t a l s l o p e . S o m e o f t h e w a t e r f r o m t h e A l a s k a n S t r e a m r e c i r c u l a t e s i n t o t h e S u b a r c t i c C u r r e n t o r e n t e r s t h e B e r i n g S e a t h r o u g h v a r i o u s p a s s e s , a l t h o u g h b o t h s c e n a r i o s a r e e x t r e m e l y v a r i a b l e ( T h o m s o n , 1 9 7 2 ; R e e d , 1984 ; R e e d a n d S t a b e n o , 1 9 9 4 ) . B o t h t h e A l a s k a n S t r e a m a n d t h e S u b a r c t i c C u r r e n t h a v e b e e n s h o w n t o b e c o n t i n u o u s t o t h e o c e a n b o t t o m ( W a r r e n a n d O w e n s , 1 9 8 8 ) . T h e S u b a r c t i c C u r r e n t b i f u r c a t e s i n t h e e a s t e r n N o r t h P a c i f i c , w i t h t h e b u l k o f i t s t r a n s p o r t t u r n i n g e i t h e r n o r t h w a r d i n t h e A l a s k a C u r r e n t o r s o u t h w a r d i n t h e C a l i f o r n i a C u r r e n t o f t h e s u b t r o p i c a l g y r e ( C h e l t o n a n d D a v i s , 1 9 8 2 ) . A s c h e m a t i c o f t h e m a j o r s u r f a c e c u r r e n t s i n t h e N o r t h P a c i f i c O c e a n is s h o w n i n F i g u r e 4 . 1 . T h e r e g i o n i s d o m i n a t e d i n w i n t e r b y t h e p r e s e n c e o f a n i n t e n s e l o w - p r e s s u r e s y s t e m , t h e A l e u t i a n L o w . R e l a t i v e l y h i g h p r e c i p i t a t i o n a n d l o w e v a p o r a t i o n , a l o n g w i t h n u - m e r o u s f r e s h w a t e r s o u r c e s a l o n g i t s p e r i m e t e r , y i e l d a n u p p e r l a y e r m a r k e d b y a s t r o n g , s h a l l o w h a l o c l i n e , w i t h w i n t e r m i x e d - l a y e r d e p t h s i n t h e c e n t e r o f t h e g y r e r e a c h i n g o n l y 7 5 - 1 0 0 m ( L e v i t u s , 1 9 8 2 ; T a b a t a , 1 9 7 5 ) . T h e A l e u t i a n L o w r e p r e s e n t s t h e i n t e g r a t e d e f f e c t o f n u m e r o u s w i n t e r c y c l o n e s , w h i c h r e s u l t i n u p w e l l i n g a n d a n e x c h a n g e o f s e n s i b l e h e a t f r o m t h e w a r m e r o c e a n t o t h e a t m o s p h e r e ( V a n S c o y e t a l . , 1 9 9 1 ) . I t i s t h i s E k m a n p u m p i n g i n t h e G u l f o f A l a s k a w h i c h m a i n t a i n s h i g h n u t r i e n t c o n c e n t r a t i o n s , a n d w h i c h Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 5 5 F i g u r e 4 . 1 : M a p o f t h e N o r t h P a c i f i c O c e a n s h o w i n g t h e m a j o r s u r f a c e c u r r e n t s . F r o m T a b a t a ( 1 9 7 5 ) . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 56 p e r m i t s r e l a t i v e l y h i g h l e v e l s o f b i o - p r o d u c t i v i t y i n t h e n e a r - s u r f a c e w a t e r s ( G a r g e t t , 1 9 9 1 ) . T h e a t m o s p h e r i c c i r c u l a t i o n o v e r t h e N o r t h P a c i f i c h a s b e e n o b s e r v e d t o v a r y o n a d e c a d a l t i m e s c a l e ( e . g . , T r e n b e r t h a n d H u r r e l l , 1994 ; M a n t u a et a l . , 1 9 9 7 ) . I n p a r t i c u l a r , a s o u t h w a r d d i s p l a c e m e n t a n d i n t e n s i f i c a t i o n o f t h e w i n t e r t i m e A l e u t i a n L o w b e g a n i n t h e m i d - 1 9 7 0 ' s a n d c o n t i n u e d t h r o u g h t h e 1 9 8 0 ' s , i n w h a t h a s b e e n c a l l e d a " r e g i m e s h i f t " ( T r e n b e r t h a n d H u r r e l l , 1 9 9 4 ) . S i m i l a r v a r i a b i l i t y h a s b e e n o b s e r v e d o n s h o r t e r t i m e s c a l e s , m o s t n o t a b l e b e i n g a n i n t e n s i f i c a t i o n a n d s o u t h e a s t w a r d d i s p l a c e m e n t o f t h e A l e u t i a n L o w f r o m i t s m e a n p o s i t i o n d u r i n g E l N i n o y e a r s . T h i s c o u l d h a v e s i g n i f i c a n t c o n s e q u e n c e s f o r t h e u p p e r - l e v e l c i r c u l a t i o n a n d h e a t f l u x i n t h e r e g i o n . N o r t h w a r d t r a n s - p o r t i n t h e e a s t e r n N o r t h P a c i f i c ( A l a s k a C u r r e n t ) a p p e a r s t o d o m i n a t e o v e r s o u t h w a r d t r a n s p o r t ( C a l i f o r n i a C u r r e n t ) d u r i n g E l N i n o y e a r s ( C h e l t o n a n d D a v i s , 1 9 8 2 ) . K e l l y et a l . ( 1 9 9 3 ) u s e d s a t e l l i t e a l t i m e t r y d a t a t o o b s e r v e a n i n t e n s i f i c a t i o n o f t h e A l a s k a n G y r e d u r i n g t h e m o d e r a t e E l N i f i o o f 1 9 8 7 , a n d a w e a k e n i n g o f t h e A l a s k a n G y r e d u r i n g t h e f o l l o w i n g n o n - E l N i f i o y e a r . R o y e r a n d E m e r y ( 1987 ) o b s e r v e d a s i g n i f i c a n t w e s t w a r d d i s p l a c e m e n t o f t h e A l a s k a n G y r e i n t h e s u m m e r o f 1 9 8 1 , a y e a r p r e c e d i n g a s t r o n g E l N i f i o e v e n t , w i t h v i r t u a l l y n o n o r t h w a r d f l o w e a s t o f 145°W. T h e y a t t r i b u t e d t h i s t o a n a n o m a l o u s w i n d s t r e s s c u r l c o m b i n e d w i t h t h e s h o a l i n g b o t t o m t o p o g r a p h y i n t h e n o r t h e a s t P a c i f i c . K n o w l e d g e o f t h e r e g i o n a l c i r c u l a t i o n i s b a s e d l a r g e l y o n a r o b u s t h i s t o r y o f h y d r o - g r a p h i c m e a s u r e m e n t s , p a r t i c u l a r l y i n t h e n e a r - c o a s t a l r e g i o n s ( D o d i m e a d et a l . , 1 9 6 3 ; F a v o r i t e e t a l . , 1976 ) a n d a l o n g t h e f r e q u e n t l y o c c u p i e d L i n e P b e t w e e n 50°N, 145°W a n d t h e s o u t h e r n t i p o f V a n c o u v e r I s l a n d ( T a b a t a , 1 9 9 1 ) . L a g r a n g i a n o b s e r v a t i o n s i n t h e N o r t h P a c i f i c h a v e l a r g e l y b e e n c o n f i n e d t o t h e e a s t e r n b o u n d a r y c u r r e n t r e g i m e s , p a r t i c - u l a r l y t h e C a l i f o r n i a C u r r e n t s y s t e m ( D a v i s , 1 9 8 5 a , b ; P o u l a i n a n d N i i l e r , 1 9 8 9 ; B r i n k e t a l . , 1 9 9 1 ; S w e n s o n a n d N i i l e r , 1996 ) a n d t h e b i f u r c a t i o n z o n e o f f t h e w e s t c o a s t o f B r i t i s h Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 5 7 C o l u m b i a ( T h o m s o n e t a l . , 1990 ; P a d u a n a n d N i i l e r , 1 9 9 3 ; N i i l e r a n d P a d u a n , 1 9 9 5 ; v a n M e u r s a n d N i i l e r , 1 9 9 7 ) . T h e o n l y d r i f t e r d e p l o y m e n t s d e s i g n e d t o m a p a b r o a d r e g i o n o f t h e N o r t h P a c i f i c s u r f a c e c i r c u l a t i o n w e r e u n d e r t a k e n as p a r t o f t h e N o r t h P a c i f i c E x p e r i m e n t ( N O R P A X ) A n o m a l y D y n a m i c s S t u d y ( A D S ) i n 1 9 7 6 - 8 0 , d u r i n g w h i c h 75 d r i f t e r s , o s t e n s i b l y d r o g u e d a t 30 m , w e r e d e p l o y e d t h r o u g h o u t t h e m i d l a t i t u d e N o r t h P a c i f i c , t h o u g h p r i m a r i l y w i t h i n t h e S u b t r o p i c a l G y r e ( K i r w a n e t a l . , 1 9 7 8 ; M c N a l l y , 1 9 8 1 ; M c N a l l y et a l . , 1 9 8 3 ) . M o s t o f t h e s e d r i f t e r s w e r e o b s e r v e d t o m o v e n e a r l y p a r a l l e l t o t h e s e a l e v e l p r e s s u r e i s o b a r s , s u g g e s t i n g a f l o w t o t h e r i g h t o f t h e s u r f a c e w i n d s . T h e e f f e c t i v e n e s s o f t h e d r o g u e s u s e d i n t h e s e e a r l y e x p e r i m e n t s i s q u e s t i o n a b l e , h o w e v e r ( v a n M e u r s a n d N i i l e r , 1 9 9 7 ) . H i g h - q u a l i t y d r i f t e r m e a s u r e m e n t s a r e f a r m o r e a b u n d a n t i n t h e N o r t h A t l a n t i c a n d e q u a t o r i a l P a c i f i c t h a n i n t h e n o r t h e a s t P a c i f i c . I t i s t h i s v o i d w h i c h h a s b e e n filled b y t h e S V P d r i f t e r d a t a a n a l y z e d h e r e . 4.3 L a g r a n g i a n D e c o r r e l a t i o n S c a l e s A s a p r e l i m i n a r y s t e p t o o b t a i n i n g a n E u l e r i a n d e s c r i p t i o n o f t h e c i r c u l a t i o n , t h e d e c o r r e - l a t i o n s c a l e s o f t h e m o t i o n s a t b o t h d r o g u e d e p t h s n e e d t o b e d e t e r m i n e d . T h e L a g r a n g i a n i n t e g r a l t i m e s c a l e , w h i c h i s a m e a s u r e o f t h e l e n g t h o f t i m e b e y o n d w h i c h t h e v e l o c i t y f l u c t u a t i o n s c a n b e c o n s i d e r e d t o b e s t a t i s t i c a l l y i n d e p e n d e n t , i s d e f i n e d as /•oo Tk = / Rk{r)dT, (4 .1 ) Jo • Mr) = , , a s r / uk(t)uk(t + r)dr, (4 .2 ) ) i m a x JO w h e r e Rk i s t h e n o r m a l i z e d a u t o c o r r e l a t i o n f u n c t i o n f o r m o t i o n s i n t h e fc-direction, uk a r e t h e v e l o c i t y f l u c t u a t i o n s , r i s t h e t i m e l a g , Tmax i s t h e l e n g t h o f t h e t i m e s e r i e s , a n d a n g l e b r a c k e t s r e f e r t o e n s e m b l e a v e r a g e s . I d e a l l y , t h e a u t o c o r r e l a t i o n f u n c t i o n f o r a g i v e n t i m e s e r i e s w i l l a p p r o a c h z e r o a t t h e i n t e g r a l t i m e s c a l e . H o w e v e r , as h a s b e e n Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 58 p o i n t e d o u t i n e a r l i e r d r i f t e r s t u d i e s (e .g . , K r a u s s a n d B o n i n g , 1 9 8 7 ) , t h e a u t o c o r r e l a t i o n f u n c t i o n o f t e n d o e s n o t a p p r o a c h z e r o w h e n t h e m e a n c u r r e n t i s n o t w e l l d e f i n e d a n d / o r t h e t i m e s e r i e s i s t o o s h o r t . K r a u s s a n d B o n i n g ( 1987 ) d e r i v e d a u t o c o r r e l a t i o n f u n c t i o n s i n d i r e c t l y u s i n g t h e s t r u c t u r e f u n c t i o n , g i v e n b y Dk{r) = {[uh{t + T)-uk{t)]2). (4 .3 ) T h e s t r u c t u r e f u n c t i o n i s r e l a t e d t o t h e a u t o c o r r e l a t i o n f u n c t i o n t h r o u g h t h e n o n - n o r m a l i z e d c o r r e l a t i o n f u n c t i o n B(T), Dh(r) = 2[Bh(0) - Bk(r)] = 2[<u?> - 2 f c ( r ) ] , ( 4 .4 ) Bk(r) = (u'?)Rk{T). (4 .5 ) T h e a u t o c o r r e l a t i o n f u n c t i o n s d e r i v e d f r o m a l l d r i f t e r s i n b o t h e n s e m b l e s a r e s h o w n i n F i g u r e 4 . 2 . T h e i n t e g r a l t i m e s c a l e s w e r e d e r i v e d b y i n t e g r a t i n g t h e a u t o c o r r e l a t i o n f u n c t i o n f r o m z e r o t o t h e t i m e o f t h e f i r s t z e r o c r o s s i n g , y i e l d i n g g l o b a l i n t e g r a l t i m e s c a l e s o f 2 .5 (2 .8 ) a n d 1.5 (2 .4 ) d a y s i n t h e z o n a l a n d m e r i d i o n a l d i r e c t i o n s , r e s p e c t i v e l y , f o r t h e s h a l l o w ( d e e p ) e n s e m b l e ( T a b l e 4 . 1 ) . D e r i v a t i o n s b a s e d o n i n t e g r a t i o n s o f t h e s t r u c t u r e a n d a u t o c o r r e l a t i o n f u n c t i o n s g a v e n e a r l y i d e n t i c a l r e s u l t s . T h e i n t e g r a l l e n g t h s c a l e , i . e . t h e d i s t a n c e t r a v e r s e d o v e r o n e i n t e g r a l t i m e s c a l e p e r i o d , w a s t h e n f o u n d f r o m , L„ = <ti?>*Tfcl ( 4 .6 ) y i e l d i n g g l o b a l e d d y l e n g t h s c a l e s o f 29 (27) a n d 17 (23) k m i n t h e z o n a l a n d m e r i d i o n a l d i r e c t i o n s , r e s p e c t i v e l y , f o r t h e s h a l l o w ( d e e p ) e n s e m b l e ( T a b l e 4 . 1 ) . A s n o t e d b y T h o m - s o n e t a l . ( 1 9 9 0 ) , t h e d e c o r r e l a t i o n l e n g t h s c a l e s a r e s h o r t e r i n t h e N o r t h P a c i f i c , w i t h i t s w e a k e r e d d y f i e l d , t h a n i n t h e N o r t h A t l a n t i c ( K r a u s s a n d B o n i n g , 1 9 8 7 ) . T h e p r e - d o m i n a n t l y z o n a l ( e a s t w a r d ) f l o w r e s u l t s i n a n i s o t r o p i c d e c o r r e l a t i o n s c a l e s , p a r t i c u l a r l y Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 59 a t 15 m d e p t h . I n t e g r a l t i m e s c a l e s a r e s l i g h t l y l o n g e r b e l o w t h e m i x e d l a y e r . I t i s i n t e r e s t i n g t o n o t e t h a t t h e r e i s s p a t i a l a n d t e m p o r a l ( n o t s h o w n ) v a r i a b i l i t y i n t h e d e c o r r e l a t i o n s c a l e s ( F i g u r e s 4 . 3 , 4 . 4 ; T a b l e 4 .1 ) . I n m o s t r e g i o n s , a n d a t b o t h d e p t h s , t h e s c a l e s a r e s l i g h t l y l o n g e r i n t h e z o n a l d i r e c t i o n . N e a r t h e b i f u r c a t i o n z o n e , s t r o n g e r m e r i d i o n a l f l o w a t 15 m d e p t h y i e l d s n e a r l y i s o t r o p i c d e c o r r e l a t i o n s c a l e s . T h e m o s t s i g n i f i c a n t a n i s o t r o p y o c c u r s i n t h e n o r t h e r n G u l f o f A l a s k a . A t 15 m , t h i s s u b - e n s e m b l e c o n s i s t s p r i m a r i l y o f d r i f t e r s d e p l o y e d i n t h e A l a s k a n S t r e a m , a n d t h e r e s u l t i s l o n g t i m e a n d l e n g t h s c a l e s . A t 120 m , t h e s e d r i f t e r s w e r e d e p l o y e d n e a r t h e h e a d o f t h e G u l f , a n d s u b s e q u e n t l y r e v e a l e d a s l o w r e c i r c u l a t i o n . A l s o s h o w n i n F i g u r e 4 .2 a r e t h e g l o b a l e d d y d i f f u s i v i t y c o m p o n e n t s as a f u n c t i o n o f t i m e l a g , d e r i v e d f r o m Kkk = (uk2) / Rk(r)dr. (4 .7 ) Jo P e a k v a l u e s o c c u r a t a l a g o f a p p r o x i m a t e l y 10 d a y s a n d a r e o n t h e o r d e r o f 2 X 1 0 r cm2Is, w h i c h i s c o m p a r a b l e t o t h e e s t i m a t e s o b t a i n e d f r o m 1 0 0 - m d r o g u e d d r i f t e r s i n t h e b i f u r c a t i o n r e g i o n ( T h o m s o n et a l . , 1 9 9 0 ) . T h e d i f f u s i v i t i e s a r e l a r g e r i n t h e z o n a l d i r e c t i o n , p a r t i c u l a r l y f o r t h e m i x e d - l a y e r d r i f t e r s . T h e s e v a l u e s a r e b a s e d o n t h e g l o b a l m e a n v e l o c i t y s t a t i s t i c s , w h i c h i n f a c t h a v e s i g n i f i c a n t s p a t i a l s t r u c t u r e (see b e l o w ) . T h e s p a t i a l v a r i a b i l i t y o f t h e e d d y d i f f u s i v i t i e s i s a d d r e s s e d i n C h a p t e r 5 . 4.4 Selection of G r i d Geometries D r i f t e r - d e r i v e d m e a n v e l o c i t y m a p s r e q u i r e a n e n s e m b l e g r i d g e o m e t r y w h i c h p r o v i d e s h i g h s p a t i a l r e s o l u t i o n , w h i l e e v e r y w h e r e m a i n t a i n i n g a d a t a p o p u l a t i o n s u f f i c i e n t f o r r o b u s t s t a t i s t i c a l e s t i m a t e s . T h e s e c r i t e r i a r u n c o u n t e r t o e a c h o t h e r , s i n c e a m i n i m u m n u m b e r o f d a t a p o i n t s m a y r e q u i r e a b o x s i z e c o n s i d e r a b l y l a r g e r t h a n t h e d o m i n a n t Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 60 F i g u r e 4 . 2 : T h e g l o b a l a v e r a g e a u t o c o r r e l a t i o n f u n c t i o n s a n d e d d y d i f f u s i v i t i e s as a f u n c - t i o n o f t i m e l a g d e r i v e d f r o m t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 61 SHALLOW DEPLOYMENTS 0 10 20 30 40 0 10 20 30 40 Lag (days) F i g u r e 4 . 3 : T h e r e g i o n a l a v e r a g e a u t o c o r r e l a t i o n f u n c t i o n s d e r i v e d f r o m t h e s h a l l o w d r i f t e r e n s e m b l e f o r (a) t h e L i n e P / b i f u r c a t i o n r e g i o n ( 4 7 9 5 d r i f t e r d a y s u s e d i n t h e e s t i m a t e ) , ( b ) t h e S u b a r c t i c C u r r e n t ( 5 8 3 5 d a y s ) , (c ) t h e n o r t h e r n G u l f o f A l a s k a ( 1 0 1 8 d a y s ) , a n d (d ) t h e S u b t r o p i c a l G y r e ( 3 8 4 2 d a y s ) . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 6 2 DEEP DEPLOYMENTS 1—i—i—i—i— i —i—i—r i—r Lag (days) F i g u r e 4 . 4 : T h e r e g i o n a l a v e r a g e a u t o c o r r e l a t i o n f u n c t i o n s d e r i v e d f r o m t h e d e e p d r i f t e r e n s e m b l e f o r (a ) t h e L i n e P / b i f u r c a t i o n r e g i o n ( 770 d r i f t e r d a y s u s e d i n t h e e s t i m a t e ) , ( b ) t h e S u b a r c t i c C u r r e n t ( 2251 d a y s ) , (c ) t h e n o r t h e r n G u l f o f A l a s k a (456 d a y s ) , a n d (d ) t h e S u b t r o p i c a l G y r e ( 2 7 5 9 d a y s ) . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 63 R E G I O N Tx ( d a y s ) Ty ( d a y s ) Lx ( k m ) Ly ( k m ) SHALLOW L i n e P 3.0 2 .7 2 4 . 5 2 3 . 5 S u b a r c t i c C u r r e n t 2.1 1.2 2 0 . 1 10 .8 G u l f o f A l a s k a 5.1 1.2 8 9 . 3 17 .6 S u b t r o p i c a l G y r e 2 .8 2 .2 3 8 . 9 3 1 . 2 G l o b a l 2 . 5 1 . 5 2 8 . 8 1 6 . 5 DEEP L i n e P 2.8 2 .4 19 .3 1 4 . 4 S u b a r c t i c C u r r e n t 3.8 2 .6 2 8 . 0 1 8 . 4 G u l f o f A l a s k a 1.4 2 .6 16 .5 2 9 . 5 S u b t r o p i c a l G y r e 3.0 2 .2 3 1 . 6 2 4 . 0 G l o b a l 2 . 8 2 . 4 2 7 . 4 2 2 . 7 T a b l e 4 . 1 : D e c o r r e l a t i o n t i m e a n d s p a c e s c a l e s d e r i v e d f r o m t h e g l o b a l a n d r e g i o n a l a u t o c o r r e l a t i o n f u n c t i o n s o f e a c h e n s e m b l e . h o r i z o n t a l s c a l e s o f t h e c u r r e n t v a r i a b i l i t y . W i t h s u f f i c i e n t d a t a , o n e c a n s a t i s f y a p r e - d e t e r m i n e d o b j e c t i v e c r i t e r i o n f o r d e r i v i n g m i n i m u m b o x s i z e s . T h i s a p p r o a c h w a s r e - c e n t l y t a k e n f o r d r i f t e r s t u d i e s i n t h e N o r t h A t l a n t i c ( B r u g g e , 1995 ) a n d S o u t h A t l a n t i c ( S c h a f e r a n d K r a u s s , 1 9 9 5 ) , i n w h i c h t h e n u m b e r o f d a t a p o i n t s r e q u i r e d f o r t h e s t a t i s t i c s i n a g i v e n b o x t o c o n v e r g e o n t h e b o x m e a n w a s c o m p u t e d . T h e s e s t u d i e s p o i n t e d o u t t h a t t h e s t a n d a r d e r r o r s ( a t t h e 9 5 % c o n f i d e n c e l e v e l ) i n t h e m e a n v e l o c i t y , (uk), a n d t h e r o o t - m e a n - s q u a r e v e l o c i t y , ( % 2 ) 2 , g i v e n b y (uk)± (4 .8 ) (4 .9 ) a r e d e p e n d e n t n o t o n l y o n t h e n u m b e r o f d e g r e e s o f f r e e d o m , A ^ , ( i . e . , t h e n u m b e r o f i n d e p e n d e n t e s t i m a t e s , a s s u m i n g a d e c o r r e l a t i o n t i m e s c a l e o f t h e c i r c u l a t i o n ) , b u t a l s o Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 64 o n t h e e d d y k i n e t i c e n e r g y , (EKE) = \((u?+u?)). ( 4 .10 ) T h u s , r e g i o n s h a v i n g d i f f e r e n t e n e r g y l e v e l s w o u l d h a v e d i f f e r e n t r e q u i r e m e n t s f o r a c h i e v - i n g s t a t i s t i c a l s t a b i l i t y , a n d t h e r e is n o b e n e f i t i n c h o o s i n g g r i d b o x e s o f e q u a l d i m e n s i o n o r s i z e . B r u g g e ( 1 9 9 5 ) f o u n d t h a t t h i s s t a b i l i t y c r i t e r i o n c o u l d b e m e t i n a m a j o r i t y o f 3° x 2° (A x 4>) b o x e s i n t h e N o r t h A t l a n t i c . H o w e v e r , a p p l y i n g B r u g g e ' s ( 1995 ) c o n v e r g e n c e c r i t e r i o n t o t h e e n s e m b l e s p r e s e n t e d h e r e o f t e n r e q u i r e d b o x e s w h i c h w e r e c o n s i d e r a b l y l a r g e r t h a n t h e h o r i z o n t a l s c a l e s o f t h e c i r c u l a t i o n . A s a n e x a m p l e , c o n s i d e r t h e r e g i o n n e a r t h e t e r m i n u s o f t h e A l a s k a n S t r e a m i n t h e s h a l l o w e n s e m b l e ( F i g u r e 2 .3 ) . R e q u i r i n g a b o x s i z e l a r g e e n o u g h f o r s t a b i l i t y w o u l d r e s u l t i n a m e a n v e l o c i t y w h i c h i s r e p r e s e n t a t i v e o f n e i t h e r t h e A l a s k a n S t r e a m n o r i t s o b s e r v e d r e c i r c u l a t i o n n e a r 170°W. T w o b o x e s a r e r e q u i r e d h e r e , e v e n t h o u g h t h e r e s u l t a n t 9 5 % c o n f i d e n c e l i m i t s o n t h e m e a n s p e e d s a r e l a r g e . T h e a p p r o a c h t a k e n , t h e r e f o r e , w a s t o c o n s t r u c t a s a t i s f a c t o r y c o m p r o m i s e b e t w e e n r e s o l u t i o n a n d s t a b i l i t y b y c h o o s i n g b o x e s w h i c h r e s o l v e d b u t w e r e n o t s i g n i f i c a n t l y l a r g e r t h a n t h e h o r i z o n t a l s c a l e s o f t h e d o m i n a n t c u r r e n t s , w h i l e t r y i n g t o m a i n t a i n a m i n i m u m o f 30 d e g r e e s o f f r e e d o m i n e a c h b o x . T h i s i s a n a r b i t r a r y m i n i m u m , b u t s t a t i s t i c s i n b o x e s w i t h c o n s i d e r a b l y f e w e r t h a n 30 d e g r e e s o f f r e e d o m t e n d e d t o h a v e n o n - c o n v e r g e n t m e a n s a n d s h o u l d b e t r e a t e d w i t h c a u t i o n . T h e g r i d g e o m e t r i e s f o r b o t h e n s e m b l e s , a l o n g w i t h a n n u a l d a t a d e n s i t y h i s t o g r a m s , a r e s h o w n i n F i g u r e s 4 .5 a n d 4 . 6 , r e s p e c t i v e l y . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 6 5 6 0 ° N - 50°hH 4 0 ° N 3 0 ° N 2 0 ° N J I I J I I I I I I I L Shallow Drifters (15m) Drogued Segments T i i i i i i i i i i i T i i i i i r~ 1 4 0 ° E 1 6 0 ° E 1 8 0 ° 160°W 140°W 120°W 6 0 ° N 5 0 ° N 4 0 ° N 3 0 ° N 2 0 ° N 1 J I I J, ,,i I j I I I J I I I I I I I L ESC Eddies Deep Drifters (120m) Drogued Segments 40 1 I I I I I I I I I I I I I I I I I r~ °E 1 6 0 ° E 1 8 0 ° 160°W 140°W 120°W F i g u r e 4 . 5 : T h e t r a j e c t o r i e s o f t h e d r o g u e d s e g m e n t s o f t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s , w i t h o v e r l y i n g g r i d g e o m e t r i e s . T h e b o x m a r k e d " E S C E d d i e s " r e f e r s t o t h e s u b j e c t o f C h a p t e r 6 a n d t h e b o x m a r k e d " K K T E d d i e s " r e f e r s t o t h e s u b j e c t o f C h a p t e r 7. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 6 6 60°N J 50°N J | 40°N J 30°N 20 °N J 1 1—J I I I I I I I L Shallow (15m) Annual Data Density Histograms (days) n — r 140 160°E ~i—i—r 180°E ~i—i—i—|—i—i—r 160°W 140°W " i — r 120°W F i g u r e 4 . 6 : M a p s s h o w i n g a n n u a l h i s t o g r a m s o f n u m b e r o f d r i f t e r d a y s i n g r i d b o x e s f o r t h e (a ) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . T h e s c a l e i s g i v e n i n u p p e r r i g h t . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 6 7 4.5 Mean Circulation and Energy Distribution 4.5.1 The overview I n c o m p u t i n g E u l e r i a n s t a t i s t i c s , t h e i n t e g r a l t i m e s c a l e w a s a s s u m e d t o b e 5 d a y s , w h i c h i s l a r g e r ( b y a f a c t o r o f ~ 2) t h a n t h e g l o b a l e s t i m a t e s f o r b o t h t h e s h a l l o w a n d d e e p e n s e m b l e s ( T a b l e 4 . 1 ) . F o l l o w i n g t h e m e t h o d o f S t a b e n o a n d R e e d ( 1 9 9 4 ) , t h e n u m b e r o f i n d e p e n d e n t s a m p l e s ( d e g r e e s o f f r e e d o m ) w a s d e f i n e d as t h e n u m b e r o f i n t e g r a l - t i m e - s c a l e p e r i o d s i n w h i c h d a t a , f r o m a n y n u m b e r o f d r i f t e r s , w e r e a v a i l a b l e i n a g i v e n b o x . T h i s i s a r a t h e r c o n s e r v a t i v e d e f i n i t i o n , b o t h b e c a u s e t h e a s s u m e d i n t e g r a l t i m e s c a l e is l i k e l y a n o v e r e s t i m a t e a n d b e c a u s e all d r i f t e r s a c t i v e i n a b o x o v e r a 5 - d a y p e r i o d c o n t r i b u t e a s i n g l e i n d e p e n d e n t o b s e r v a t i o n . A t 15 m d e p t h , t h e n u m b e r o f d e g r e e s o f f r e e d o m r a n g e d f r o m f e w e r t h a n 20 i n t h e n o r t h c e n t r a l G u l f o f A l a s k a , w h e r e d a t a w a s s p a r s e a n d t h e s c a l e s o f m o t i o n l a r g e , t o g r e a t e r t h a n 60 i n m o s t o f t h e b o x e s i n t h e d a t a - r i c h e a s t e r n S u b a r c t i c C u r r e n t ( F i g u r e 4 . 7 a ) . T h e m e a n c i r c u l a t i o n d e r i v e d f r o m t h e d a t a c o n t a i n e d i n t h e s e b o x e s r e v e a l s t h e e n t i r e A l a s k a n G y r e , as w e l l as p o r t i o n s o f t h e n o r t h e r n a n d e a s t e r n b r a n c h e s o f t h e S u b t r o p i c a l G y r e ( F i g u r e 4 . 7 b ) . M e a n s p e e d s i n t h e S u b a r c t i c C u r r e n t a r e g e n e r a l l y < 10 c m / s , a n d p e a k n e a r 150°W. A t 15 m d e p t h , t h e b i f u r c a t i o n o f t h e S u b a r c t i c C u r r e n t o c c u r s n e a r 48°N, 130°-135°W, w i t h a c c e l e r a t e d n o r t h w a r d f l o w i n t h e A l a s k a C u r r e n t ( m o s t y e a r s ) , o r s o u t h w a r d f l o w i n t h e C a l i f o r n i a C u r r e n t ( 1 9 9 0 - 9 1 ) . T h e A l a s k a n S t r e a m i s c l e a r l y e v i d e n t b e t w e e n 150°-170°W, w i t h m e a n s p e e d s e x c e e d i n g 25 c m / s n e a r 155°W, a n d a r e c i r c u l a t i o n i n t o t h e S u b a r c t i c C u r r e n t n e a r 170°W. S o u t h o f 45°N, t h e K u r o s h i o E x t e n s i o n / N o r t h P a c i f i c C u r r e n t i s w e l l - s a m p l e d , w i t h m e a n s p e e d s o f 1 0 - 1 5 c m / s . T h e r e is e v i d e n c e o f a w e a k s o u t h w a r d r e c i r c u l a t i o n o f t h e w e s t e r n S u b t r o p i c a l G y r e a t 160°- 170°W, a l t h o u g h n o r t h o f 40°N t h e flow is s t i l l e a s t w a r d . M e a n k i n e t i c e n e r g i e s a r e o n t h e o r d e r o f 20 cm2/s2 t h r o u g h o u t t h e e a s t e r n N o r t h P a c i f i c , b u t r e a c h 60 cm2/s2 w i t h i n Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 68 t h e K u r o s h i o E x t e n s i o n a n d 120 cm2/s2 i n t h e A l a s k a n S t r e a m . T h e d a t a c o v e r a g e a t 120 m d e p t h i s m o r e c o n f i n e d t h a n a t 15 m , b u t n o n e t h e l e s s s a m p l e s t h e A l a s k a n G y r e q u i t e w e l l ( F i g u r e 4 . 8 a ) . A g a i n , t h e n u m b e r o f d e g r e e s o f f r e e d o m g e n e r a l l y e x c e e d s 3 0 , w i t h t h e e a s t e r n S u b a r c t i c C u r r e n t b e i n g m o s t e f f e c t i v e l y s a m p l e d . A s t r i k i n g f e a t u r e o f t h e m e a n v e l o c i t y m a p i s t h e t i g h t c i r c u l a t i o n o f t h e A l a s k a n G y r e a t t h i s d e p t h ( F i g u r e 4 . 8 b ) . A l t h o u g h d e e p a n d s h a l l o w d r i f t e r s w e r e g e n e r a l l y d e p l o y e d a t t h e s a m e p o s i t i o n s a n d t i m e s , o n l y o n e d e e p d r i f t e r w e n t e a s t o f ~ 140°W i n t h e S u b a r c t i c C u r r e n t . I n s t e a d , t h e r e i s a n o r t h w a r d v e l o c i t y c o m p o n e n t n e a r 52°N e v e r y w h e r e e a s t o f 155°W, a n d a b r o a d , s t r o n g n o r t h w a r d flow i n t h e A l a s k a C u r r e n t . O n l y t h r e e d e e p d r i f t e r s s a m p l e d t h e c o r e o f t h e A l a s k a n S t r e a m , n e a r 165°W. F l o w s o u t h o f 45°N, i n t h e N o r t h P a c i f i c C u r r e n t , i s w e a k a n d e a s t w a r d , as i t i s w i t h i n t h e m i x e d l a y e r . M e a n k i n e t i c e n e r g i e s a r e s i m i l a r as w e l l , w i t h l o w v a l u e s ( ~ 20 cm2/s2) i n t h e e n t i r e A l a s k a n G y r e e x c e p t w i t h i n t h e A l a s k a n S t r e a m ( ~ 120 cm2/s2). V e l o c i t y v a r i a n c e e l l i p s e s r e v e a l n e a r l y i s o t r o p i c v e l o c i t y f l u c t u a t i o n s t h r o u g h o u t m o s t o f t h e o p e n o c e a n , b u t s i g n i f i c a n t a n i s o t r o p y a l o n g t h e p e r i m e t e r o f t h e A l a s k a n G y r e a n d i n t h e K u r o s h i o E x t e n s i o n r e g i o n ( F i g u r e 4 . 9 ) . V a r i a n c e s a t b o t h d e p t h s a r e p a r t i c u l a r l y s m a l l a n d n e a r l y i s o t r o p i c i n t h e w e s t e r n S u b a r c t i c C u r r e n t b e t w e e n 40° -50°N, w h e r e r a t i o s o f e d d y t o m e a n k i n e t i c e n e r g y a r e n e a r , o r e v e n l e s s t h a n , u n i t y . T h i s m a t c h e s t h e m i d - l a t i t u d e h y d r o g r a p h i c a n d X B T t r a n s e c t o b s e r v a t i o n s o f B e r n s t e i n a n d W h i t e ( 1 9 7 7 ) , w h o r e p o r t e d a n a b r u p t d r o p i n n e a r - s u r f a c e e d d y e n e r g y e a s t o f 170°W, as w e l l as c u r r e n t m e t e r o b s e r v a t i o n s a t m i d - l a t i t u d e s t a t i o n s a l o n g 152°W ( N i i l e r a n d H a l l , 1 9 8 8 ; H a l l e t a l . , 1 9 9 7 ) . T h i s r e g i o n h a s b e e n t e r m e d a n " e d d y d e s e r t " ( e . g . , H a l l et a l . , 1 9 9 7 ) , a m o n i k e r s u p p o r t e d b y t h e d r i f t e r m e a s u r e m e n t s . O v e r a l l , t h e v e l o c i t y fluctuations a r e s l i g h t l y m o r e a n i s o t r o p i c i n t h e m i x e d l a y e r ( F i g u r e 4 . 1 0 ) . I t s h o u l d b e k e p t i n m i n d t h a t t h e d e r i v a t i o n o f t h e e d d y k i n e t i c e n e r g i e s r e q u i r e s t h e m e a n flow t o b e k n o w n , w h i c h i s n o t n e c e s s a r i l y t h e c a s e . I n s o m e g r i d b o x e s , t h e m e a n v e l o c i t i e s h a v e l a r g e a c c o m p a n y i n g Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 69 F i g u r e 4 . 7 : M a p s s h o w i n g (a) t h e n u m b e r o f d e g r e e s o f f r e e d o m c o n t a i n e d i n t h e g r i d b o x e s o f t h e s h a l l o w d r i f t e r e n s e m b l e , a n d (b ) t h e d e r i v e d m e a n v e l o c i t y a n d m e a n k i n e t i c . e n e r g y ( M K E ; cm2/s2). B o x e s w i t h f e w e r t h a n 30 d e g r e e s o f f r e e d o m a r e d a s h e d , a n d m a r k s i n ( b ) r e f e r t o t h e b o x c e n t e r - o f - m a s s p o s i t i o n s . M K E c o n t o u r s e x t r a p o l a t e d b e y o n d t h e d a t a r a n g e i n t h e n o r t h w e s t p o r t i o n o f t h e m a p ( B e r i n g S e a ) a r e n o t v a l i d . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 70 F i g u r e 4 . 8 : M a p s s h o w i n g (a) t h e n u m b e r o f d e g r e e s o f f r e e d o m c o n t a i n e d i n t h e g r i d b o x e s o f t h e d e e p d r i f t e r e n s e m b l e , a n d (b ) t h e d e r i v e d m e a n v e l o c i t y a n d m e a n k i n e t i c e n e r g y ( M K E ; cm2/s2). B o x e s w i t h f e w e r t h a n 30 d e g r e e s o f f r e e d o m a r e d a s h e d , a n d m a r k s i n (b ) r e f e r t o t h e b o x c e n t e r - o f - m a s s p o s i t i o n s . T h e c l o s e d c o n t o u r n e a r 48°N, 150°W i s a m a x i m u m . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 71 s t a n d a r d e r r o r s . T h e m i x e d - l a y e r v a r i a n c e s i n t h e S u b a r c t i c C u r r e n t i n c r e a s e a n d b e c o m e m o r e a n i s o t r o p i c w i t h p r o x i m i t y t o t h e c o a s t . H i g h e s t v a r i a n c e s a r e s e e n i n t h e K u r o s h i o E x t e n s i o n r e g i o n a t 15 m d e p t h , a n d i n t h e n o r t h e r n G u l f o f A l a s k a a t b o t h d e p t h s . T h e m a j o r a x e s a r e a l i g n e d w i t h t h e A l a s k a n S t r e a m , b u t a r e p e r p e n d i c u l a r t o t h e m e a n f l o w o f t h e K u r o s h i o E x t e n s i o n . T h e h i g h e n e r g i e s i n t h e s h a l l o w b o x c e n t e r e d a t 47°N, 152°E a n d t h e d e e p b o x c e n t e r e d a t 39°N, 172°E r e f l e c t t h e l o n g - l i v e d , t o p o g r a p h i c a l l y - g e n e r a t e d e d d i e s o b - s e r v e d o v e r t h e K u r i l - K a m c h a t k a T r e n c h ( C h a p t e r 7) a n d t h e E m p e r o r S e a m o u n t C h a i n ( C h a p t e r 6; B o g r a d e t a l . , 1 9 9 7 ) , r e s p e c t i v e l y . A c l o s e - u p o f t h e m e a n c i r c u l a t i o n a t b o t h d e p t h s i n t h e A l a s k a n G y r e i s g i v e n i n F i g u r e 4 . 1 1 . E d d y k i n e t i c e n e r g i e s a r e r e l a t i v e l y w e a k i n t h i s r e g i o n , b e i n g o n t h e o r d e r o f 4 0 - 8 0 cm21s2 a t b o t h d e p t h s i n t h e S u b a r c t i c C u r r e n t . A m i n i m u m i n e d d y k i n e t i c e n e r g y i s s e e n i n t h e n o r t h e r n S u b t r o p i c a l G y r e n e a r 45°N, 160°W, t h e " e d d y d e s e r t " , w h i l e h i g h e r v a l u e s a r e s e e n i n t h e b i f u r c a t i o n r e g i o n a t 15 m d e p t h a n d w i t h i n t h e A l a s k a n S t r e a m a t b o t h d e p t h s . T h i s p a t t e r n i s a l s o e v i d e n t i n t h e v a r i a n c e e l l i p s e m a p s . T h e h i g h e r e d d y k i n e t i c e n e r g i e s n e a r t h e c o a s t l i n e m a y b e a r e f l e c t i o n o f i n s t a b i l i t i e s i n t h e c o a s t a l c u r r e n t s , a n d t h e s u b s e q u e n t i n t r u s i o n o f f s h o r e o f d e c a y i n g e d d i e s . T h o m s o n a n d G o w e r ( 1 9 9 8 ) h a v e o b s e r v e d t h e f o r m a t i o n a n d d e c a y o f m e s o s c a l e e d d i e s i n t h i s r e g i o n i n s a t e l l i t e i m a g e r y , a t t r i b u t i n g i t t o w i n d - i n d u c e d i n s t a b i l i t i e s i n t h e N o r t h e a s t P a c i f i c C o a s t a l C u r r e n t . N u m e r o u s e d d i e s o f s i m i l a r d i m e n s i o n t o t h o s e o b s e r v e d i n t h e s a t e l l i t e i m a g e r y w e r e a l s o o b s e r v e d i n t h e d r i f t e r t r a c k s , p a r t i c u l a r l y i n t h e s h a l l o w e n s e m b l e . O n e m u s t i n t e r p r e t t h e E u l e r i a n m e a n v e l o c i t y s t a t i s t i c s w i t h c a u t i o n , as t h e y a r e n o t a s y n o p t i c c h a r a c t e r i z a t i o n o f t h e flow field. T h e r e p r e s e n t e d t i m e i n t e r v a l s a n d t h e s a m p l i n g d e n s i t y v a r y c o n s i d e r a b l y f r o m b o x t o b o x ( F i g u r e 4 . 6 ) . D a v i s ( 1 9 8 5 b ) p o i n t e d o u t t h a t n o n - u n i f o r m d e p l o y m e n t s ( i n t i m e a n d s p a c e ) c a n l e a d t o a s a m p l i n g Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 7 2 60°N J 50°N J 40 °N J 30°N J 20°N J 1—J I I I I I i i i Shallow (15m) Velocity Variance Ellipses 0.1 m/s 140 ^ i r E 180°E 60°N 50°N J 40°N J 30°N J ° ' ' 'l60°w' 1 1 1 1 — r 140°W 120°W 20°N J 1 1 J I I ' ' I i i i 0.1 m/s © © <8 e © ® ® © Deep ( 1 2 0 m ) Velocity Variance Ellipses 140 " i — r i I — i — i — i — r — i — i — i — I — r 160°E 180°E 160°W 'l40oW i — i — r 126"W F i g u r e 4 .9 : M a p s s h o w i n g v e l o c i t y v a r i a n c e e l l i p s e s ( g i v e n as (u'k2)1/2) i n g r i d b o x e s d e r i v e d f r o m t h e (a) s h a l l o w a n d ( b ) d e e p d r i f t e r e n s e m b l e s . T h e s c a l e i s g i v e n i n u p p e r r i g h t . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 73 0 5 10 15 20 25 U r m s (cm/s) F i g u r e 4 . 1 0 : Z o n a l v s . m e r i d i o n a l r . m . s . s p e e d s d e r i v e d f r o m t h e s h a l l o w a n d d e e p d r i f t e r e n s e m b l e s . T h e l i n e s a r e l e a s t - s q u a r e s fits. T h e d e e p A l a s k a n S t r e a m b o x , w h i c h h a d f e w e r t h a n 10 d e g r e e s o f f r e e d o m , i s n o t i n c l u d e d i n t h i s p l o t . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 74 60°M 50°t\H 40°N 175°E 165°W 145°W 125°W 60oNH 50°NH 40°N 175°E 165°W 145°W 125°W F i g u r e 4 . 1 1 : C l o s e - u p o f t h e m e a n c i r c u l a t i o n a n d e d d y k i n e t i c e n e r g y (cm2/s2) i n t h e A l a s k a n G y r e d e r i v e d f r o m t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 75 b i a s , e v i d e n t as e r r o r s o n t h e m e a n v e l o c i t i e s o f u p t o a f e w c m / s . T h i s i s a r e f l e c t i o n o f t h e f a c t t h a t t h e f l o w f i e l d i n a g i v e n l o c a t i o n ( g r i d b o x ) i s t e m p o r a l l y v a r i a b l e , a n d t h e s a m p l i n g o f t h a t l o c a t i o n b y a d r i f t e r i s b i a s e d b y t h e t i m e d u r i n g w h i c h s a m p l i n g o c c u r s a n d t h e d i r e c t i o n f r o m w h i c h t h e d r i f t e r a r r i v e d . H o w e v e r , t h e i n t e g r a l t i m e s c a l e s c o m p u t e d f r o m t h e d r i f t e r v e l o c i t i e s a r e q u i t e s h o r t , so a d r i f t e r ' s h i s t o r y i s n o t w e l l - c o r r e l a t e d w i t h i t s p r e s e n t v e l o c i t y a n d l o c a t i o n . T h e m e a n r e s i d e n c e t i m e s i n t h e g r i d b o x e s ( F i g u r e 4 .12 ) a r e g e n e r a l l y m u c h l o n g e r t h a n t h e i n t e g r a l t i m e s c a l e o f t h e m o t i o n s , t h u s i t c a n b e a s s u m e d t h a t t h e s a m p l i n g b i a s e s a r e s m a l l o n t h e c h o s e n g r i d s . I t s h o u l d a l s o b e k e p t i n m i n d t h a t t h e v e l o c i t y s t a t i s t i c s p r e s e n t e d h e r e a r e a s i n g l e s t a t i s t i c a l r e a l i z a t i o n . D i f f e r e n t g r i d g e o m e t r i e s c o u l d h a v e b e e n c h o s e n , w h i c h w o u l d h a v e r e s u l t e d i n d i f f e r e n t m e a n a n d e d d y v e l o c i t y f i e l d s . T h e s t a t i s t i c s p r e s e n t e d i n t h e D a t a R e p o r t ( B o g r a d a n d E e r t , 1996 ) w e r e d e r i v e d u s i n g u n i f o r m 5° x 5° g r i d b o x e s , a n d t h e m e a n s a l o n g t h e p e r i m e t e r o f t h e A l a s k a n G y r e d i f f e r c o n s i d e r a b l y f r o m t h o s e o n t h e n o n - u n i f o r m g r i d . T h i s r e s u l t s p r i m a r i l y f r o m a l a c k o f s u f f i c i e n t s p a t i a l r e s o l u t i o n u s i n g 5° b o x e s i n t h e h i g h e d d y e n e r g y r e g i o n s . U s e o f a d i f f e r e n t i n t e g r a l t i m e s c a l e c o u l d a l s o a f f e c t t h e d e r i v e d s t a t i s t i c s . T e s t s w e r e d o n e ( n o t s h o w n ) o n t h e s a m e g r i d g e o m e t r i e s u s i n g i n t e g r a l t i m e s c a l e s o f 2 d a y s , 5 d a y s a n d 10 d a y s , r e v e a l i n g s l i g h t v a r i a t i o n s i n t h e m e a n v e l o c i t y f i e l d s i n e a c h c a s e . 4 . 5 . 2 T h e A l a s k a n S t r e a m T h e a x i s o f t h e A l a s k a n S t r e a m is c l e a r l y v i s i b l e i n t h e " s p a g h e t t i " d i a g r a m , w h i c h i n c l u d e s d a t a f r o m u n d r o g u e d ( i . e . , s u r f a c e ) t r a j e c t o r i e s ( F i g u r e 2 . 3 ) . A n a r r o w a x i s i s s e e n o f f s h o r e o f K o d i a k I s l a n d a n d e x t e n d i n g t o n e a r 170°W, w h e r e d r i f t e r s w e r e o b s e r v e d t o e i t h e r r e c i r c u l a t e s o u t h w a r d i n t o t h e S u b a r c t i c C u r r e n t o r e n t e r t h e B e r i n g S e a t h r o u g h o n e o f s e v e r a l P a s s e s a l o n g t h e A l e u t i a n I s l a n d a r c . T h e a x i s o f t h e A l a s k a n S t r e a m c a n a l s o b e s e e n f r o m t h e l o c a t i o n s o f a l l h i g h ( e . g . , > 40 c m / s ; S t a b e n o a n d R e e d , 1991 ) Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 76 F i g u r e 4 . 1 2 : M a p s s h o w i n g t h e m e a n r e s i d e n c e t i m e s ( d a y s ) i n t h e g r i d b o x e s o f t h e (a ) s h a l l o w a n d (b ) d e e p d r i f t e r e n e s m b l e s . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 77 d a i l y - a v e r a g e d d r i f t e r s p e e d s ( F i g u r e 4 . 1 3 ) . T h e s e l o c a t i o n s g e n e r a l l y Ue o n t h e s h e l f b r e a k , w h i c h i s f u r t h e r o f f s h o r e e a s t o f 160°W. A l t h o u g h t h e t w o s e a s o n s h a d n e a r l y t h e s a m e n u m b e r o f t o t a l o b s e r v a t i o n s i n t h e r e g i o n , t h e w i n t e r m o n t h s h a d t w i c e as m a n y h i g h - s p e e d o b s e r v a t i o n s as t h e s u m m e r m o n t h s , w i t h m u c h o f t h e d i f f e r e n c e c o m i n g f r o m t h e u n d r o g u e d d r i f t e r s . T h e 15 m a n d u n d r o g u e d d r i f t e r s h a d a l a r g e r p e r c e n t a g e o f h i g h s p e e d d a y s ( 1 1 % ) t h a n t h o s e d r o g u e d a t 120 m ( 7 % ) . R e e d a n d S t a b e n o ( 1 9 9 4 , 1997 ) r e p o r t e d a n i n t e n s i f i e d A l a s k a n S t r e a m i n S e p t e m b e r 1 9 9 3 a n d t h e s u m m e r o f 1 9 9 5 , r e s p e c t i v e l y . T h e d r i f t e r m e a s u r e m e n t s c o n f i r m t h i s , a n d s u g g e s t t h a t t h e i n t e n s i f i c a t i o n e x t e n d e d i n t o t h e f a U p e r i o d s o f 1 9 9 3 a n d 1 9 9 5 , w h e n m a n y o f t h e u n d r o g u e d h i g h - s p e e d o b s e r v a t i o n s w e r e m a d e . A t 120 m d e p t h , n e a r l y a l l o f t h e h i g h - s p e e d o b s e r v a t i o n s w e r e f r o m O c t o b e r 1 9 9 1 , a y e a r i n w h i c h t h e r e w a s a n a n o m a l o u s l a c k o f i n f l o w i n t o t h e B e r i n g S e a ( S t a b e n o a n d R e e d , 1 9 9 2 ) . I n d e e d , t h e o n l y o b s e r v a t i o n s f r o m t h i s p e r i o d s h o w a r e c i r c u l a t i o n a t 167°W ( F i g u r e 4 . 1 3 e ) . T h e r e w a s a s t r o n g s u r f a c e i n f l o w i n t o t h e B e r i n g S e a t h r o u g h A m u t k a P a s s ( n e a r 172°W) i n D e c e m b e r 1992 t o J a n u a r y 1 9 9 3 ( F i g u r e 4 . 1 3 a ) , a n d a s u b s e q u e n t e n t r y i n t o t h e B e r i n g S l o p e C u r r e n t ( S t a b e n o a n d R e e d , 1 9 9 4 ) . U n d r o g u e d d r i f t e r s a l s o e n t e r e d t h e B e r i n g S e a f u r t h e r w e s t , t h r o u g h A m c h i t k a a n d N e a r P a s s e s ( F i g u r e 2 . 3 ) . R e c i r c u l a t i o n e v e n t s a t 15 m d e p t h w e r e o b s e r v e d n e a r 170°W i n O c t o b e r 1992 a n d n e a r 160°W i n D e c e m b e r 1 9 9 2 ( F i g u r e 4 . 1 3 c ) . N o n e o f t h e d r o g u e d . d r i f t e r s r e c i r c u l a t e d o u t o f t h e S t r e a m i n t h e s u m m e r m o n t h s . O n e s h a l l o w d r i f t e r e n t e r e d t h e A l a s k a n S t r e a m f r o m t h e s o u t h w e s t i n t h e s u m m e r o f 1 9 9 2 , r e v e a U n g a n a n o m a l o u s c i r c u l a t i o n s i m i l a r t o t h a t d e s c r i b e d f o r s u m m e r 1 9 8 1 ( R o y e r a n d E m e r y , 1 9 8 7 ) . 4.5.3 Variability in the Alaskan Gyre T h e r e w e r e s u f f i c i e n t d a t a i n t h e s h a U o w e n s e m b l e t o a t t e m p t t o r e s o l v e s e a s o n a l v a r i - a b i U t y o f t h e m i x e d - l a y e r f l o w ( F i g u r e 4 . 1 4 ) . T h e m o s t p r o n o u n c e d s e a s o n a l d i f f e r e n c e Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 78 WINTER SUMMER N l 1 1 1 1 1 r 1 1 1 1 1 1 r 180° 170°W 160°W 150°W180° 170°W 160°W 150°W F i g u r e 4 . 1 3 : L o c a t i o n s o f d a i l y - m e a n d r i f t e r s p e e d s i n e x c e s s o f 4 0 c m / s i n t h e A l a s k a n S t r e a m r e g i o n f o r (a ) w i n t e r , 0 m ( u n d r o g u e d ) , (b ) s u m m e r , 0 m , (c ) w i n t e r , 15 m , ( d ) s u m m e r , 15 m , (e) w i n t e r , 120 m , a n d ( f ) s u m m e r , 120 m . W i n t e r i s d e f i n e d as O c t o b e r t h r o u g h M a r c h , s u m m e r as A p r i l t h r o u g h S e p t e m b e r . P l u s s e s ( w i n t e r ) a n d t r i a n g l e s ( s u m m e r ) m a r k t h e p o s i t i o n s o f t h e h i g h - s p e e d o b s e r v a t i o n s , a n d s m a l l d o t s m a r k a l l o t h e r d a i l y - m e a n p o s i t i o n s . T h e 6 0 0 0 m a n d 8 0 0 0 m b a t h y m e t r y c o n t o u r s ( d a s h e d l i n e s ) d e l i n e a t e t h e p o s t i o n o f t h e A l e u t i a n T r e n c h . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 79 i s a l o n g 50°N e a s t o f 150°W, w h e r e t h e flow i s w e a k a n d v a r i a b l e i n s u m m e r . M e a n k i n e t i c e n e r g i e s i n t h e S u b a r c t i c C u r r e n t ( n o t s h o w n ) a r e < 20 cm2/s2 i n s u m m e r , b u t a r e 2 0 - 3 0 cm2/s2 i n w i n t e r , p e a k i n g a t 150°W ( ~ 40 cm2/s2). L a r g e r s e a s o n a l d i f f e r - e n c e s a r e s e e n i n t h e e d d y k i n e t i c e n e r g y field, w h e r e v a l u e s n e a r 90 cm2/s2 a r e s e e n n e a r 150°W i n w i n t e r . E d d y k i n e t i c e n e r g i e s a r e n e a r l y t w i c e as h i g h i n w i n t e r t h a n s u m m e r i n t h e b i f u r c a t i o n r e g i o n , b u t a r e c o m p a r a b l e i n t h e t w o s e a s o n s i n t h e A l a s k a n S t r e a m r e c i r c u l a t i o n r e g i o n . A n i n t e r e s t i n g f e a t u r e s e e n i n t h e s p a g h e t t i d i a g r a m s is a z o n e l o c a t e d n e a r 52° -53°N, 150°-155°W w h e r e n o d r i f t e r s e n t e r e d a t e i t h e r d r o g u e d e p t h o r a t t h e s u r f a c e ( F i g u r e 2 . 3 ) . T h i s i s a p p a r e n t l y a r e g i o n o f d i v e r g e n t n e a r - s u r f a c e flow, a n d c a n b e t a k e n as t h e c e n t e r o f t h e A l a s k a n G y r e f o r t h e t i m e p e r i o d s r e p r e s e n t e d b y t h e s e d r i f t e r t r a j e c t o r i e s . T h i s a r e a i s l o c a t e d s l i g h t l y f u r t h e r t o t h e w e s t a t 120 m d e p t h t h a n a t 15 m , w h i c h i s c o n s i s t e n t w i t h a w e s t w a r d d i s p l a c e m e n t o f t h e g y r e w i t h d e p t h p r e v i o u s l y o b s e r v e d i n h y d r o g r a p h i c d a t a ( R o y e r a n d E m e r y , 1 9 8 7 ) . U s i n g m o n t h l y m e a n w i n d d a t a f r o m t h e C o m p r e h e n s i v e O c e a n - A t m o s p h e r e D a t a S e t ( C O A D S ) ( W o o d r u f f e t a l . , 1 9 8 7 ) , a n d t h e w i n d - s p e e d - d e p e n d e n t d r a g c o e f f i c i e n t s o f L a r g e a n d P o n d ( 1 9 8 1 ) , t h e E k m a n v e r t i c a l v e l o c i t y a t t h e b a s e o f t h e m i x e d l a y e r w a s d e r i v e d f r o m , WB = 1^, ( 4 .11 ) PJ w h e r e T*. i s t h e w i n d s t r e s s , p i s d e n s i t y , / i s t h e l o c a l C o r i o l i s p a r a m e t e r , a n d WE i s p o s i t i v e u p w a r d s . A t t h e g y r e " c e n t e r " (53°N, 155°W) , t h e m e a n w i n t e r E k m a n v e r t i c a l v e l o c i t y o v e r t h e p e r i o d 1 9 9 0 - 9 5 i s c o n s i s t e n t l y p o s i t i v e , w i t h a n a v e r a g e r a t e o f u p w e l l i n g o f 7 x 1 0 - 5 c m / s ( ~ 20 m / y r ) . S e a s o n a l m a p s o f m e a n E k m a n v e r t i c a l v e l o c i t y f o r t h e e n t i r e N o r t h P a c i f i c o v e r t h e 1 9 9 0 - 9 5 p e r i o d r e v e a l a d o u b l e u p w e l l i n g m a x i m u m i n w i n t e r , w i t h p e a k s i n t h e w e s t e r n S u b a r c t i c G y r e a n d t h e e a s t e r n G u l f o f A l a s k a ( F i g u r e 4 . 1 5 a ) . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 80 1 7 5 ° E 1 6 5 ° W 1 4 5 ° W 1 2 5 ° W j i i J i i i i i i i i 1 7 5 ° E 1 6 5 ° W 1 4 5 ° W 1 2 5 ° W F i g u r e 4 . 1 4 : M a p s s h o w i n g t h e m e a n v e l o c i t i e s a n d e d d y k i n e t i c e n e r g i e s (cm2/s2) i n t h e A l a s k a n G y r e d e r i v e d f r o m t h e s h a l l o w d r i f t e r e n s e m b l e f o r (a ) w i n t e r a n d (b ) s u m m e r . S e a s o n s h a v e t h e s a m e d e f i n i t i o n as i n F i g u r e 4 . 1 3 . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 81 U p w e l l i n g o c c u r s n o r t h o f 50°N i n s u m m e r , a l t h o u g h i t i s c o n s i d e r a b l y w e a k e r i n t h e e a s t e r n G u l f o f A l a s k a ( F i g u r e 4 . 1 5 b ) . T h e p a t t e r n s i n F i g u r e 4 . 1 5 a r e c o n s i s t e n t w i t h c l i m a t o l o g i c a l e s t i m a t e s o f u p w e l l i n g b a s e d o n t h e c u r l o f t h e w i n d s t r e s s (e .g . , T a l l e y , 1 9 8 5 ) , b u t t h e y m a s k s i g n i f i c a n t i n t e r - a n n u a l v a r i a b i l i t y o v e r t h e s t u d y p e r i o d . A c o m p a r i s o n o f t h e s e a l e v e l p r e s s u r e p a t t e r n s d u r i n g t h e 1 9 9 0 - 9 5 p e r i o d w i t h t h e m a p s p r e p a r e d b y E m e r y a n d H a m i l t o n ( 1 9 8 5 ) f o r t h e p e r i o d 1 9 4 7 - 8 3 s u g g e s t s t h a t t h e A l e u t i a n L o w w a s p a r t i c u l a r l y s t r o n g d u r i n g w i n t e r 1 9 9 1 - 9 2 , n e a r n o r m a l d u r i n g t h e w i n t e r s o f 1 9 9 0 - 9 1 a n d 1 9 9 3 - 9 4 , a n d w e a k e r t h a n u s u a l d u r i n g w i n t e r 1 9 9 2 - 9 3 . T h e 1 9 9 4 - 9 5 w i n t e r S L P f i e l d w a s n e a r n o r m a l , b u t t h e A l e u t i a n L o w w a s s h i f t e d a n o m a l o u s l y e a s t w a r d . T h i s v a r i a b i l i t y , w h i c h m a y h a v e b e e n a s s o c i a t e d w i t h E l N i f i o e v e n t s i n t h e t r o p i c a l P a c i f i c ( T r e n b e r t h a n d H o a r , 1 9 9 6 ) , c a n a l s o b e s e e n i n t h e t i m e s e r i e s o f C O A D S m o n t h l y m e a n s e a l e v e l p r e s s u r e a t t h e g y r e " c e n t e r " , 53°N, 155°W ( F i g u r e 4 . 1 6 ) . T h e P N A i n d e x , w h i c h i s a m e a s u r e o f t h e s t r e n g t h o f t h e A l e u t i a n L o w ( W a l l a c e a n d G u t z l e r , 1 9 8 1 ) , a l s o r e v e a l s t h i s i n t e r a n n u a l v a r i a b i l i t y . T h e h i g h e s t P N A i n d e x o v e r t h e 5 - y e a r p e r i o d o c c u r r e d i n t h e w i n t e r o f 1 9 9 1 - 9 2 . T h e s e p a t t e r n s r e s u l t e d i n d i v e r g e n c e a n d s t r o n g E k m a n p u m p i n g ( u p t o 20 x 1 0 - 5 c m / s ) i n t h e e a s t e r n G u l f o f A l a s k a d u r i n g t h e w i n t e r s o f 1 9 9 1 - 9 2 a n d 1 9 9 2 - 9 3 , a n d i n t h e c e n t r a l G u l f o f A l a s k a d u r i n g w i n t e r 1 9 9 3 - 9 4 ( F i g u r e s 4 . 1 7 , 4 . 1 8 ) . U p w e l l i n g w a s w e a k t h r o u g h o u t t h e G u l f d u r i n g w i n t e r 1 9 9 4 - 9 5 , w h e n t h e A l e u t i a n L o w a p p e a r e d t o b e s h i f t e d e a s t w a r d o f i t s c l i m a t o l o g i c a l p o s i t i o n . T h e d o u b l e u p w e l l i n g m a x i m u m s e e n i n t h e 5 - y e a r w i n t e r m e a n u p w e l l i n g p a t t e r n ( F i g u r e 4 . 1 5 a ) i s a r e f l e c t i o n o f t h i s i n t e r a n n u a l v a r i a b i l i t y . T h e d r i f t e r s a l s o r e v e a l e d a n e a r - s u r f a c e r e s p o n s e t o t h e v a r i a b l e a t m o s p h e r i c f o r c i n g , e v i d e n t f r o m a p e r u s a l o f t h e i n d i v i d u a l t r a j e c t o r i e s ( e . g . , B o g r a d a n d E e r t , 1 9 9 6 ) . T h e r e w a s a s t r o n g n o r t h w a r d f l o w t h r o u g h o u t t h e e a s t e r n G u l f o f A l a s k a a t b o t h d e p t h s d u r i n g t h e w i n t e r s o f 1 9 9 1 - 9 2 a n d 1 9 9 2 - 9 3 , w h e r e a s t h e S u b a r c t i c C u r r e n t h a d a s i g n i f i c a n t s o u t h e r l y c o m p o n e n t d u r i n g w i n t e r 1 9 9 4 - 9 5 . T h e s e o b s e r v a t i o n s a r e c o n s i s t e n t w i t h Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 8 2 F i g u r e 4 . 1 5 : M a p s s h o w i n g t h e 1 9 9 0 - 1 9 9 5 (a) w i n t e r a n d (b ) s u m m e r m e a n E k m a n v e r t i c a l v e l o c i t y , WE ( X I O - 5 c m / s ) , d e r i v e d f r o m C O A D S w i n d s . S e a s o n s h a v e t h e s a m e d e f i n i t i o n as i n F i g u r e 4 . 1 3 . WE i s p o s i t i v e u p w a r d s . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 8 3 1991 1992 1993 1994 1995 F i g u r e 4 . 1 6 : T i m e s e r i e s o f C O A D S m o n t h l y m e a n s e a l e v e l p r e s s u r e ( h e a v y U n e ) a t 53°N, 155°W a n d i t s 3 - m o n t h r u n n i n g m e a n ( d a s h e d U n e ) . A l s o p l o t t e d a r e t h e m o n t h l y v a l u e s o f t h e P a c i f i c N o r t h A m e r i c a n ( P N A ) i n d e x ( o c t a g o n s ) , w h i c h i s d e f i n e d as a U n e a r c o m b i n a t i o n o f t h e n o r m a h z e d 5 0 0 m b h e i g h t a n o m a U e s a t f o u r c e n t e r s l o c a t e d n e a r H a w a i i , o v e r t h e N o r t h P a c i f i c , o v e r A l b e r t a a n d o v e r t h e U . S . G u l f C o a s t . T h e P N A i n d e x i s a m e a s u r e o f t h e s t r e n g t h o f t h e A l e u t i a n L o w ( h i g h p o s i t i v e P N A c o r r e s p o n d s t o a s t r o n g e r A l e u t i a n L o w ) . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 8 4 e a r l i e r s t u d i e s w h i c h s h o w e d i n c r e a s e d n o r t h w a r d t r a n s p o r t i n t h e b i f u r c a t i o n r e g i o n d u r i n g E l N i n o y e a r s ( C h e l t o n a n d D a v i s , 1982 ; K e l l y e t a l , 1 9 9 3 ) . A l t h o u g h t h e r e a r e n o t s u f f i c i e n t d a t a t o d e t e r m i n e t h e r e p r e s e n t a t i v e n e s s o f t h e d r i f t e r m e a s u r e m e n t s a n a l y z e d h e r e , t h e r e was s i g n i f i c a n t y e a r - t o - y e a r v a r i a b i l i t y i n t h e l a r g e - s c a l e a t m o s p h e r i c f o r c i n g o v e r t h e G u l f o f A l a s k a d u r i n g t h e e a r l y 1 9 9 0 ' s . T h e m a g n i t u d e o f t h i s v a r i a b i l i t y , a n d o f t h e c o r r e s p o n d i n g u p p e r - o c e a n r e s p o n s e , i s o f t h e s a m e o r d e r o f m a g n i t u d e as t h e r e g i m e s h i f t s w h i c h h a v e b e e n r e l a t e d t o l a r g e - s c a l e f l u c t u a t i o n s i n N o r t h P a c i f i c fish s t o c k s ( F r a n c i s a n d H a r e , 1994 ; M a n t u a et a l . , 1 9 9 7 ) . 4.6 High Frequency Energy Distribution T h e g e o g r a p h i c a l d i s t r i b u t i o n o f m e a n a n d e d d y k i n e t i c e n e r g y g i v e n i n t h e e a r l i e r s e c - t i o n s r e p r e s e n t s a l o n g - t e r m m e a n , o r " p s e u d o - E u l e r i a n " , p a t t e r n . E x c e p t f o r a s e a s o n a l s e p a r a t i o n o f t h e s h a l l o w e n s e m b l e i n t h e G u l f o f A l a s k a , d a t a w e r e n o t s u f f i c i e n t t o a t - t e m p t a n y k i n d o f t e m p o r a l r e s o l u t i o n . H o w e v e r , v a r i a b i l i t y a t t h e h i g h f r e q u e n c y e n d o f t h e e n e r g y s p e c t r u m c a n b e d e t e r m i n e d b y c o m p u t i n g t h e r o t a r y s p e c t r a o v e r t h e f i r s t 90 d a y s ( i . e . , p r i o r t o t h e d u t y c y c l e ) o f t h e d r i f t e r t r a j e c t o r i e s ( F i g u r e 4 . 1 9 ) . F i g u r e s 4 . 2 0 a n d 4 . 2 1 s h o w c o m p o s i t e r o t a r y s p e c t r a o v e r t h e first 90 d a y s o f i n d i v i d u a l d e p l o y m e n t s f o r t h e s h a l l o w a n d d e e p e n s e m b l e s , r e s p e c t i v e l y . O n l y t h o s e t r a j e c t o r i e s w h i c h w e r e r e l a - t i v e l y c l u s t e r e d , i n t i m e a n d s p a c e , w e r e u s e d i n t h i s a n a l y s i s . T h e f r e q u e n c y - p a r t i t i o n e d c o m p o s i t e r o t a r y v a r i a n c e s f o r e a c h o f t h e d e p l o y m e n t s a r e g i v e n i n T a b l e 4 . 2 . A t t h e l o w e r f r e q u e n c i e s ( p e r i o d s o f ~ 5 - 3 0 d a y s ) , t h e h i g h e s t m i x e d - l a y e r e n e r g i e s a r e a l o n g t h e p e r i m e t e r o f t h e A l a s k a n G y r e ( d e p l o y m e n t s 7 a n d 9 , w i t h i n t h e A l a s k a n S t r e a m ) a n d i n t h e S u b t r o p i c a l G y r e ( d e p l o y m e n t 6 ) . E n e r g i e s i n t h e l o w m e s o s c a l e f r e q u e n c y b a n d a r e n e a r l y a n o r d e r o f m a g n i t u d e l o w e r w i t h i n t h e S u b a r c t i c C u r r e n t ( d e p l o y m e n t s 1, 5 , 1 1 , 12 ) . L o w m e s o s c a l e e n e r g y w a s n e a r l y i s o t r o p i c e v e r y w h e r e , Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 8 5 1 7 5 ° E 1 6 5 ° W 1 4 5 ° W 1 2 5 ° W 1 7 5 ° E 1 6 5 ° W 1 4 5 ° W 1 2 5 ° W F i g u r e 4 . 1 7 : M a p s s h o w i n g t h e w i n t e r m e a n E k m a n v e r t i c a l v e l o c i t y , WE (x 10 5 c m / s ) , d e r i v e d f r o m C O A D S w i n d s , f o r (a) 1 9 9 1 - 1 9 9 2 a n d (b ) 1 9 9 2 - 1 9 9 3 . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 86 1 7 5 ° E 1 6 5 ° W 1 4 5 ° W 1 2 5 ° W 1 7 5 ° E 1 6 5 ° W 1 4 5 ° W 1 2 5 ° W F i g u r e 4 . 1 8 : M a p s s h o w i n g t h e w i n t e r m e a n E k m a n v e r t i c a l v e l o c i t y , WE (x 10 5 c m / s ) , d e r i v e d f r o m C O A D S w i n d s , f o r (a) 1 9 9 3 - 1 9 9 4 a n d (b ) 1 9 9 4 - 1 9 9 5 . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 8 7 60°N 50°N-fl 40°Ni 30°N Shallow Drifters (15m) First 90 Days x 1990 A 1991 + 1992 • 1993 O 1994 20°N-| 1 1 1 1 1 1 1 1 1 1 1 1—T 1 1 1 1 1 r 140°E 160°E 180° 160°W 140°W 120°W 60°N- 50°N 40°N 30°N J I L J I L Deep Drifters (120m) First 90 Days x 1990 A 1991 + 1992 • 1993 O 1994 20°N-| i i i i i i i i i i i i i i i i i i r 140°E 160°E 180° 160°W 140°W 120°W F i g u r e 4 . 1 9 : M a p s h o w i n g t h e f i r s t 9 0 - d a y t r a j e c t o r i e s o f t h e (a) s h a l l o w a n d ( b ) d e e p d r i f t e r d e p l o y m e n t s ( i n d i c a t e d b y n u m b e r s ) . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 88 S H A L L O W D R I F T E R S x) sx o CQ I w f~, h-> o CD OH m u cO -t-> o P H 10" 10° 10-' 10- 2 10" 3 10" 4 10" 5 10° 10-' 10" 2 io~ 3 -J 10" 10' 10" 5 10° 10-' 10" 2 10" 3-^ 10" 4 10" 5 10° io- ' 4 10" 2 10" 3 . I . I i mill "1 • !(a) j -i fl R r \ \ r • September 1990 1—1 1 M i l l ) 1—1 1 1 1 1111in i ' '1 i r i i m i l 1 = (c) 1 M W J 9 5 3 f r r 1 6 r - June 1992 1—r "I II I ill 1—i i i mil 1 11 mn - W) J95?£ 9 June 1993 10 - | — I I I I III[ 1—I I I IMI| 1— I I II111] 111 I • i i m i l ' I _5 ~\ September 1994 10 I-2 • • • • i • |— i i • • • • • • | 10" 10u 10' 10" 2 10"1 10° 101 ( • I . I I mill • ' • "'"I I (t>) November 1991 —I "TTITTTTJ- I 11 III) • 1 ' 'I » ' • | (d) 9551 September 1992 —I" T 1 TIIII) ri llllll| l"l T I i mill—1,-I.XUJHI i—IJLJi.niJ 9551 11 May 1994 i 1 '"""I 10° 10-' 10" 2 10~3 10" 4 10" 5 10° 10"1 10" 2 10" 3 10" 10" 10° I O " 1 10" 2 10~3 I O " 4 10" Frequency (cpd) F i g u r e 4 . 2 0 : C o m p o s i t e r o t a r y e n e r g y d e n s i t y s p e c t r a ( c l o c k w i s e , S~, i s s o l i d ; c o u n t e r - c l o c k w i s e , S+, i s d a s h e d ) , a n d 9 5 % c o n f i d e n c e l i m i t s , c o r r e s p o n d i n g t o e a c h o f t h e s h a l l o w d r i f t e r d e p l o y m e n t s . D e p l o y m e n t n u m b e r s a n d d a t e s a r e g i v e n i n l o w e r l e f t . A l l s p e c t r a a r e f o r t h e first 9 0 - d a y p e r i o d a f t e r i n i t i a l d e p l o y m e n t . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 89 D E E P D R I F T E R S 10 10"*-J / 10 September 1990 — I — I I I M i l ] r m 10-1 4 10" 10" 10-* 10 -5 • 10° 10"1 10"2 10"3 IO"4' 10 -5 IO- 1 ^ \ - ^\ - *\ t I 9 5 *r r - June 1992 ft f w r 1 i i 1 ! (e) I 9 5 ^ r II r V \ ^ \ 1 1 \ V i jjf 1 / / • May 1994 1 1 1 T rill] 1 1 1 M I N I i «L ^ r l I I IIIII 10" 10° 10 1 10" 10" 10° 101 (b) —,\ \ \ \ * I\A J 9 5 3 3 ' TF December 1990 10° 1 1 1 1 IIIIJ I I I I M i l ) -1—l l l l l l l (d) \ 9 June 1993 1 — 1 I 1 IIII! 1 — 1 1 ITI III— — \ — 1 1 1 1 I I I %- 10 10- I I I l l l l j I I I I I • 111 1 I I l l l l l | 10" 10' 10' k IO"* 10" . L i , I,UJHII ' 1 (f) -I • i i i"t J 9 5 3 V 12 September 1994 — i i I I I I I I I — i T r n u i ] — i — i 11 n u 10" 10"3 10"4 10"5 10- 10" 10u 10' Frequency (cpd) F i g u r e 4 . 2 1 : A s i n F i g u r e 4 . 2 0 , b u t f o r t h e d e e p d r i f t e r d e p l o y m e n t s . Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 90 e x c e p t f o r t h e c l o c k w i s e - p o l a r i z e d e n e r g y p a r t i t i o n o f d e p l o y m e n t 11 . A t p e r i o d s o f ~ 1-5 d a y s ( h i g h m e s o s c a l e ) , e n e r g y l e v e l s a r e c o m p a r a b l e i n a l l r e g i o n s a n d r e p r e s e n t e d t i m e p e r i o d s , b u t a r e h i g h e s t w i t h i n t h e A l a s k a n S t r e a m ( d e p l o y m e n t 7 ) . A l l c o m p o s i t e s p e c t r a r e v e a l a s i g n i f i c a n t n e a r - i n e r t i a l c l o c k w i s e p e a k , t h e s t r o n g e s t o c c u r r i n g f o r t h e F a l l 1 9 9 0 L i n e P d r i f t e r s ( d e p l o y m e n t 1) . S t r o n g n e a r - i n e r t i a l p e a k s a l s o o c c u r f o r t h e S u b t r o p i c a l G y r e d r i f t e r s a n d t h e F a l l 1991 ( d e p l o y m e n t 5) a n d F a l l 1 9 9 4 ( d e p l o y m e n t 12) S u b a r c t i c C u r r e n t d e p l o y m e n t s . T h e M a y 1994 L i n e P d e p l o y m e n t h a s a n o r d e r - o f - m a g n i t u d e l e s s n e a r - i n e r t i a l e n e r g y t h a n t h e d e p l o y m e n t i n t h e s a m e r e g i o n i n F a l l 1 9 9 0 , r e v e a l i n g a n e x p e c t e d s t r o n g s e a s o n a l d i f f e r e n c e i n a t m o s p h e r i c f o r c i n g i n t h i s r e g i o n . T h e A l a s k a n S t r e a m d r i f t e r s h a v e r e l a t i v e l y w e a k n e a r - i n e r t i a l p e a k s , b u t c o n t a i n t h e h i g h e s t e n e r g i e s a t t h e s e m i d i u r n a l f r e q u e n c y . H i g h f r e q u e n c y e n e r g y l e v e l s ( p e r i o d s < 12 h o u r s ) a r e o f c o m p a r a b l e m a g n i t u d e f o r a l l d e p l o y m e n t s . F o r d e p l o y m e n t 1, w h i c h h a d t h e h i g h e s t t o t a l e n e r g y l e v e l a t 15 m , 87 % o f t h e t o t a l e n e r g y a t p e r i o d s less t h a n ~ 30 d a y s i s c o n t a i n e d i n t h e n e a r - i n e r t i a l p e a k ( p e r i o d s o f 1 3 - 2 4 h o u r s ) . T h i s i m p l i e s t h a t t h e w i n d i n f l u e n c e i s c o n c e n t r a t e d a t h i g h f r e q u e n c i e s . ( N o t e t h a t t h e e d d y k i n e t i c e n e r g y f i e l d s p r e s e n t e d i n S e c t i o n 4 .5 .1 w e r e o b t a i n e d f r o m f i l t e r e d - d a i l y - a v e r a g e d - t i m e s e r i e s , a n d t h u s r e p r e s e n t t h e g e o s t r o p h i c e d d y f i e l d . ) T h e n e x t h i g h e s t t o t a l e n e r g y l e v e l ( d e p l o y m e n t 6) r e v e a l s t h e s t r o n g e d d y i n g m o t i o n s o f t h e N o r t h P a c i f i c C u r r e n t i n t h a t p a r t o f t h e S u b t r o p i c a l G y r e , w h i l e t h e l o w e s t t o t a l e n e r g y l e v e l s w e r e c o n t a i n e d i n t h e l a t e S p r i n g / e a r l y S u m m e r L i n e P d r i f t e r s ( d e p l o y m e n t 11 ) . A l t h o u g h t h e e n e r g y l e v e l s a r e c o n s i d e r a b l y l o w e r , a s i m i l a r f r e q u e n c y - p a r t i t i o n i n g o f t h e h i g h f r e q u e n c y e n e r g y i s s e e n a t 120 m as a t 15 m ( F i g u r e 4 .21 a n d T a b l e 4 . 2 ) . T h e r e i s a w i d e r r a n g e o f e n e r g y l e v e l s a t t h e l o w e r f r e q u e n c i e s ( p e r i o d s o f ~ 5 - 3 0 d a y s ) , w i t h t h e l o w m e s o s c a l e r o t a r y v a r i a n c e s r a n g i n g f r o m 160 cm2/s2 f o r d e p l o y m e n t 6 t o < 5 cm2 js2 f o r d e p l o y m e n t 1. E x c e p t f o r t h e n o r t h e r n G u l f o f A l a s k a d r i f t e r s o f d e p l o y m e n t Chapter 4. Mean Circulation and Energy Distribution in the North Pacific ( a ) S h a l l o w D e p l o y m e n t F r e q u e n c y s-{«) Stot{u) l o w m e s o s c a l e 9 .3 (0 .9) 10 .1 (1 .0 ) 1 9 . 4 (2 .0 ) m h i g h m e s o s c a l e 4 3 . 3 (4 .4) 11 .9 (1 .2 ) 5 5 . 2 (5 .6 ) n e a r - i n e r t i a l 8 5 5 . 0 (87 .1 ) 8 .0 (0 .8 ) 8 6 3 . 0 (87 .9 ) s e m i d i u r n a l 19 .4 (2 .0) 3 .2 (0 .3 ) 2 2 . 6 (2 .3 ) h i g h 11 .4 (1 .2) 9.9 (1 .0 ) 2 1 . 3 (2 .2 ) l o w m e s o s c a l e 2 8 . 2 (6 .7) 26 .1 (6 .2 ) 5 4 . 4 ( 1 2 . 9 ) h i g h m e s o s c a l e 39 .1 (9 .3 ) 2 8 . 3 (6 .7 ) 6 7 . 4 ( 16 .0 ) 5 n e a r - i n e r t i a l 2 4 9 . 6 (59 .2 ) 15 .7 (3 .7 ) 2 6 5 . 3 ( 6 3 . 0 ) s e m i d i u r n a l 8 .0 (1 .9 ) 1.7 (0 .4 ) 9 .7 (2 .3 ) h i g h 13 .2 (3 .1 ) 11 .5 (2 .7 ) 2 4 . 6 (5 .8 ) l o w m e s o s c a l e 122 .9 (17 .6 ) 106 .1 ( 15 .2 ) 2 2 9 . 1 ( 3 2 . 8 ) h i g h m e s o s c a l e 2 9 . 5 (4 .2 ) 15 .2 (2 .2 ) 4 4 . 8 (6 .4 ) 6 n e a r - i n e r t i a l 2 7 8 . 9 (39 .9 ) 6 6 . 5 (9 .5 ) 3 4 5 . 4 (49 .5 ) s e m i d i u r n a l 6 .0 (0 .9 ) 4 .2 (0 .6 ) 10 .2 (1 .5 ) h i g h 3 5 . 7 (5 .1 ) 3 3 . 4 (4 .8 ) 6 9 . 1 (9 .9 ) l o w m e s o s c a l e 8 5 . 8 (14 .3 ) 1 0 0 . 5 ( 16 .7 ) 1 8 6 . 3 ( 31 .0 ) in h i g h m e s o s c a l e 32 .6 (5 .4 ) 4 0 . 1 (6 .7 ) 7 2 . 7 ( 12 .1 ) n e a r - i n e r t i a l 2 2 0 . 1 (36 .6 ) 7.4 (1 .2 ) 2 2 7 . 5 ( 37 .9 ) s e m i d i u r n a l 79 .5 (13 .2 ) 4 . 3 (0 .7 ) 8 3 . 8 ( 14 .0 ) h i g h 18 .5 (3 .1 ) 12 .1 (2 .0 ) 3 0 . 7 (5 .1 ) l o w m e s o s c a l e 3 8 . 8 (10 .7 ) 8 3 . 8 ( 23 .2 ) 1 2 2 . 6 ( 34 .0 ) h i g h m e s o s c a l e 13 .5 (3 .7 ) 8 .3 (2 .3 ) 2 1 . 8 (6 .0 ) 9 n e a r - i n e r t i a l 133 .0 (36 .8 ) 8 .3 (2 .3 ) 1 4 1 . 3 ( 39 .1 ) s e m i d i u r n a l 4 3 . 9 (12 .2 ) 3 .8 (1 .1 ) 4 7 . 8 ( 13 .2 ) h i g h 18 .3 (5 .1 ) 9 .4 (2 .6 ) 2 7 . 7 (7 .7 ) l o w m e s o s c a l e 2 2 . 2 (7 .8 ) 5 .0 (1 .7 ) 2 7 . 1 (9 .5 ) h i g h m e s o s c a l e 7.6 (2 .7 ) 6.0 (2 .1 ) 13 .6 ( 4 .8 ) ES n e a r - i n e r t i a l 108 .0 (37 .8 ) 3 4 . 0 ( 11 .9 ) 1 4 2 . 1 ( 49 .7 ) s e m i d i u r n a l 2 3 . 7 (8 .3 ) 8 .4 (2 .9 ) 3 2 . 1 ( 1 1 . 2 ) h i g h 3 7 . 3 (13 .1 ) 3 3 . 7 ( 11 .8 ) 7 1 . 0 ( 2 4 . 8 ) l o w m e s o s c a l e 15 .3 (3 .3 ) 14 .7 (3 .1 ) 2 9 . 9 ( 6 .4 ) h i g h m e s o s c a l e 2 0 . 2 (4 .3 ) 9.1 (1 .9 ) 2 9 . 3 ( 6 .2 ) 12 n e a r - i n e r t i a l 3 0 2 . 6 (64 .4 ) 3 9 . 0 (8 .3 ) 3 4 1 . 5 ( 7 2 . 7 ) s e m i d i u r n a l 13 .9 (3 .0 ) 4 .6 (1 .0 ) 1 8 . 5 ( 3 .9 ) h i g h 2 5 . 7 (5 .5 ) 2 4 . 7 (5 .3 ) 5 0 . 4 ( 1 0 . 7 ) Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 9 2 ( b ) D e e p D e p l o y m e n t F r e q u e n c y Stot{v) l o w m e s o s c a l e 1.9 (1 .7 ) 1.7 (1.4) 3.6 (3 .1 ) 0 h i g h m e s o s c a l e 2 .5 (2 .2 ) 2 .3 (2 .0 ) 4 .8 (4 .2 ) n e a r - i n e r t i a l 4 5 . 2 (39 .2 ) 5 .7 (4 .9 ) 5 0 . 9 ( 4 4 . 1 ) s e m i d i u r n a l 3 7 . 0 (32 .1 ) 3 .2 (2 .8 ) 4 0 . 2 ( 3 4 . 9 ) h i g h 8 .7 (7 .5 ) 7 .2 (6 .2 ) 15 .8 ( 13 .7 ) l o w m e s o s c a l e 2 9 . 2 (9 .1 ) 6 9 . 7 ( 21 .8 ) 9 8 . 8 ( 30 .9 ) h i g h m e s o s c a l e 18 .0 (5 .6 ) 2 8 . 0 (8 .7 ) 4 6 . 0 ( 1 4 . 4 ) 3 n e a r - i n e r t i a l 5 8 . 6 (18 .3 ) 2 1 . 4 (6 .7 ) 8 0 . 0 ( 2 5 . 0 ) s e m i d i u r n a l 3 6 . 8 (11 .5 ) 5 .9 (1 .8 ) 4 2 . 7 ( 13 .3 ) h i g h 2 2 . 4 (7 .0) 3 0 . 4 (9 .5 ) 5 2 . 9 ( 16 .5 ) l o w m e s o s c a l e 9 3 . 5 (46 .7 ) 6 6 . 4 ( 33 .2 ) 1 6 0 . 0 ( 80 .0 ) h i g h m e s o s c a l e 4 .0 (2 .0) 2 .1 (1 .1 ) 6 .2 (3 .1 ) 6 n e a r - i n e r t i a l 18 .0 (9 .0) 3 .8 (1 .9 ) 2 1 . 8 ( 1 0 . 9 ) s e m i d i u r n a l 2 .3 (1 .1) 0 .7 (0 .4 ) 3 .0 (1 .5 ) h i g h 4 .9 (2 .5 ) 4 .4 (2 .2 ) 9 .3 (4 .7 ) l o w m e s o s c a l e 6.6 (8 .4 ) 6.6 (8 .4 ) 1 3 . 3 ( 16 .8 ) h i g h m e s o s c a l e 1.1 (1.4) 1.0 (1 .3 ) 2 .1 (2 .7 ) 9 n e a r - i n e r t i a l 3 8 . 2 (48 .4 ) 3 .9 (4 .9 ) 4 2 . 0 ( 53 .3 ) s e m i d i u r n a l 9 .9 (12 .6 ) 1.6 (2 .0 ) 1 1 . 5 ( 14 .7 ) h i g h 5 .0 (6 .4 ) 4 . 9 (6 .2 ) 9 .9 ( 12 .5 ) l o w m e s o s c a l e 5 .0 (4 .0) 3 .3 (2 .6 ) 8 .3 (6 .6 ) h i g h m e s o s c a l e 3 .5 (2 .8) 2 .6 (2 .1 ) 6 .2 (4 .9 ) 11 n e a r - i n e r t i a l 37 .1 (29 .5 ) 7.6 (6 .1 ) 4 4 . 8 ( 35 .6 ) s e m i d i u r n a l 4 3 . 7 (34 .7 ) 5 .6 (4 .5 ) 4 9 . 3 ( 39 .2 ) h i g h 8 .4 (6 .7 ) 8 .8 (7 .0 ) 17 .2 ( 13 .7 ) l o w m e s o s c a l e 2 8 . 3 (5 .5 ) 23 .1 (4 .5 ) 5 1 . 3 ( 10 .0 ) h i g h m e s o s c a l e 20 .6 (4 .0 ) 18 .1 (3 .5 ) 3 8 . 6 (7 .5 ) 12 n e a r - i n e r t i a l 3 1 5 . 5 (61 .6 ) 4 5 . 4 (8 .9 ) 3 6 0 . 9 ( 7 0 . 4 ) s e m i d i u r n a l 8 .7 (1 .7 ) 5 .2 (1 .0 ) 13 .9 (2 .7 ) h i g h 26 .0 (5 .1 ) 2 1 . 8 (4 .3 ) 4 7 . 8 (9 .3 ) T a b l e 4 . 2 : Clockwise (S~ (w)), counterclockwise (S +(o;)) and total (Sto t(w)) composite rotary variances ( cm 2 / s 2 ) in five frequency bands derived from the (a) shallow (previous page) and (b) deep drifter deployments. Numbers in parentheses refer to the percentage of the total rotary variance. The frequency bands are low mesoscale (periods of 6.4-32 days), high mesoscale (periods of 1-5.3 days), near-inertial (periods of 13-24 hours), semidiurnal (periods of 11.5-12.6 hours) and high (periods of 6-11 hours). Boxed numbers refer to the deployments (see Figure 4.19). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 93 3 , t h e r e i s a h i g h m e s o s c a l e e n e r g y t r o u g h s e e n i n a l l o f t h e r o t a r y s p e c t r a . T h e n e a r - i n e r t i a l c l o c k w i s e p e a k i s s t r o n g e s t f o r t h e F a l l 1994 S u b a r c t i c C u r r e n t d e p l o y m e n t , a n d w e a k e s t i n t h e S u b t r o p i c a l G y r e . R e l a t i v e l y s t r o n g s e m i d i u r n a l p e a k s o c c u r f o r e a c h o f t h e e a s t e r n N o r t h e a s t P a c i f i c d e p l o y m e n t s ( 1 , 3 , 9 , 11 ) . F o r f i v e o f t h e d e p l o y m e n t s , t h e f r e q u e n c y - p a r t i t i o n e d s p e c t r a l a m p l i t u d e s c a n b e c o m p a r e d f o r d r i f t e r m o t i o n s w i t h i n a n d b e l o w t h e m i x e d l a y e r ( T a b l e 4 . 3 ) . S h a l l o w - t o - d e e p t o t a l s p e c t r a l a m p l i t u d e r a t i o s a r e h i g h e s t a t t h e n e a r - i n e r t i a l f r e q u e n c i e s f o r d e p l o y m e n t s 1 a n d 6, w h i c h h a d v e r y s t r o n g m i x e d - l a y e r i n e r t i a l m o t i o n s . T h e s h a l l o w d r i f t e r s w i t h i n d e p l o y m e n t 9 h a d s i g n i f i c a n t l y h i g h e r m e s o s c a l e e n e r g y l e v e l s , p r e s u m a b l y d u e t o t h e e n t r a i n m e n t o f o n e o f t h e s e d r i f t e r s i n t o t h e A l a s k a n S t r e a m . M i x e d - l a y e r e n - e r g y l e v e l s w e r e o n l y s l i g h t l y h i g h e r f o r t h e M a y 1994 L i n e P d e p l o y m e n t , a n d w e r e c o m p a r a b l e t o t h e p y c n o c l i n e e n e r g y l e v e l s f o r t h e F a l l 1994 S u b a r c t i c C u r r e n t d e p l o y - m e n t . E n e r g y a t t h e s e m i d i u r n a l f r e q u e n c y w a s s l i g h t l y h i g h e r a t 120 m f o r b o t h L i n e P d e p l o y m e n t s . 4.7 A p p l i c a t i o n s F i n a l l y , t h e d r i f t e r v e l o c i t y s t a t i s t i c s c a n b e u s e d t o a d d r e s s s o m e o f t h e r e l e v a n t i s s u e s r a i s e d d u r i n g t h e C J G O F S f i e l d s t u d i e s . F o r e x a m p l e , i r o n - r i c h n e a r - s u r f a c e w a t e r w a s o b s e r v e d a t s o m e o f f s h o r e s t a t i o n s a l o n g L i n e P , a n d i t s o r i g i n i s u n c l e a r . A t S t a t i o n P (50°N, 145°W) , t h e n e a r - s u r f a c e m e a n f l o w is c o n s i s t e n t l y f r o m t h e w e s t ( e . g . , F i g u r e 4 . 1 1 ) . A n y w a t e r o f r e c e n t c o a s t a l o r i g i n o b s e r v e d h e r e , o r a l o n g t h e o f f s h o r e p o r t i o n o f L i n e P , i s m o r e l i k e l y t o h a v e a r r i v e d v i a a s t r o n g r e c i r c u l a t i o n f r o m t h e h e a d o f t h e G u l f , a t l e a s t i n t h e w i n t e r m o n t h s , r a t h e r t h a n f r o m t h e c o a s t a l w a t e r s o f f V a n c o u v e r I s l a n d . H o w e v e r , i t i s p o s s i b l e t h a t w a t e r o f i m m e d i a t e c o a s t a l o r i g i n m a y b e d e p o s i t e d o f f s h o r e n e a r L i n e P b y w e s t w a r d - p r o p a g a t i n g e d d i e s . T h i s s c e n a r i o w o u l d b e m o r e l i k e l y Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 9 4 D e p l o y m e n t l o w m e s o s c a l e h i g h m e s o s c a l e n e a r - i n e r t i a l s e m i d i u r n a l h i g h B 5.4 11 .5 17 .0 0.6 1.3 6 1.4 7.2 15 .8 3 .4 7 .4 9 9 .2 10 .4 3 .4 4 . 2 2 .8 11 3 .3 2 .2 3 .2 0 .7 4 .1 12 0.6 0 .8 1.0 1.3 1.1 T a b l e 4 . 3 : T h e t o t a l ( S t o t ( u ; ) ) c o m p o s i t e s p e c t r a l a m p l i t u d e r a t i o s ( s h a l l o w / d e e p ) i n f i v e f r e q u e n c y b a n d s f o r e a c h o f t h e d e p l o y m e n t s c o n t a i n i n g s h a l l o w a n d d e e p d r i f t e r s . T h e f r e q u e n c y b a n d s a r e t h e s a m e as i n T a b l e 4 . 2 . B o x e d n u m b e r s r e f e r t o t h e d e p l o y m e n t s (see F i g u r e 4 . 1 9 ) . i n s p r i n g a n d s u m m e r , w h e n t h e A l a s k a n G y r e c i r c u l a t i o n i s w e a k e r . T h e r e g i o n t o t h e s o u t h w e s t o f t h e Q u e e n C h a r l o t t e I s l a n d s ( ~ 51°N, 134°W) i s a s i t e w h e r e e d d i e s , l i k e l y f o r m e d b y t i d a l r e c t i f i c a t i o n ( T h o m s o n a n d W i l s o n , 1 9 8 7 ) , h a v e f r e q u e n t l y b e e n o b s e r v e d i n d r i f t e r t r a j e c t o r i e s ( i n c l u d i n g f r o m t h e s e d a t a ) , s a t e l l i t e i m a g e r y , a n d h y d r o g r a p h y . E d d i e s f o r m e d b y c o a s t a l c u r r e n t i n s t a b i l i t i e s h a v e b e e n o b s e r v e d a l l a l o n g t h e e a s t e r n p e r i m e t e r o f t h e G u l f o f A l a s k a ( T h o m s o n a n d G o w e r , 1 9 9 8 ) . T h u s , w h i l e t h e m e a n f l o w c e r t a i n l y a d v e c t s o p e n - o c e a n w a t e r t o t h e o f f s h o r e s t a t i o n s a l o n g L i n e P , i t i s p o s s i b l e t h a t m e s o s c a l e e d d i e s m a y b e e v e n t - s c a l e t r a n s p o r t e r s o f c o a s t a l w a t e r s t o t h e r e g i o n . S a t e l l i t e a l t i m e t r y , a l o n g w i t h t h e r e g u l a r s e e d i n g o f s u r f a c e d r i f t e r s i n t h e s l o p e w a t e r s o f f t h e B r i t i s h C o l u m b i a n c o a s t a n d n e a r t h e h e a d o f t h e G u l f , w o u l d b e i d e a l t o o l s f o r r e s o l v i n g t h i s i s s u e . Chapter 5 Eddy Statistics in the North Pacific 5.1 Introduction L a g r a n g i a n d r i f t e r s a r e i d e a l l y s u i t e d f o r t h e a n a l y s i s o f d i s p e r s i o n b e h a v i o r . F r o m a d i s p e r s i o n a n a l y s i s , a n d s o m e s i m p l i f y i n g a s s u m p t i o n s , o n e c a n d e r i v e d e c o r r e l a t i o n t i m e a n d l e n g t h s c a l e s ( i . e . , " e d d y m i x i n g " s c a l e s ) a n d h o r i z o n t a l e d d y d i f f u s i v i t i e s , w h i c h a r e v i t a l p a r a m e t e r s f o r n u m e r i c a l o c e a n c i r c u l a t i o n m o d e l s . O b t a i n i n g r e l i a b l e v a l u e s f o r t h e s e e d d y s t a t i s t i c s c a n l e a d t o i m p r o v e d p a r a m e t e r i z a t i o n s i n e d d y - r e s o l v i n g c i r c u l a - t i o n m o d e l s a n d u l t i m a t e l y t o a b e t t e r u n d e r s t a n d i n g o f l a t e r a l m i x i n g p r o c e s s e s i n t h e o c e a n . I n t h i s c h a p t e r , T a y l o r ' s ( 1921 ) t h e o r i e s o f s i n g l e p a r t i c l e d i s p e r s i o n a r e a p p l i e d t o t h e d r i f t e r e n s e m b l e s i n o r d e r t o d e r i v e t h e m a g n i t u d e o f t h e e d d y m i x i n g s c a l e s a n d d i f f u s i v i t i e s o v e r a b r o a d r e g i o n o f t h e N o r t h P a c i f i c O c e a n a n d a t b o t h d r o g u e d e p t h s . 5.2 Theory and Methods I n t h e s e m i n a l w o r k o f T a y l o r ( 1 9 2 1 ) , t h e d i s p e r s i o n o f a p a r t i c l e f r o m i t s o r i g i n i n h o m o g e n e o u s , i s o t r o p i c t u r b u l e n c e w a s r e l a t e d t o t h e v e l o c i t y f l u c t u a t i o n s . T h e m e a n s q u a r e d i s p e r s i o n c a n b e w r i t t e n a s , (x'k2(t)) = 2(u*) (t-r)Rk(r)dr, (5 .1 ) Jo w h e r e x'k(t) i s t h e d i s p l a c e m e n t o f t h e p a r t i c l e d u e t o u'k. T h e a u t o c o r r e l a t i o n f u n c t i o n h a s t w o l i m i t s , i . e . Rk = 1 a t r = 0 a n d Rk —> 0 f o r r l a r g e . C o r r e s p o n d i n g l y , t h e r e a r e 95 Chapter 5. Eddy Statistics in the North Pacific 96 t w o l i m i t s o n t h e d i s p e r s i o n : (x'k2) = (u'2)t2 for * « T f c , (5 .2 ) (x'2) = 2(u2)Tkt for i » T f c , (5 .3 ) w h e r e Tk i s t h e L a g r a n g i a n i n t e g r a l t i m e s c a l e . I n t h e f i r s t l i m i t ( i n i t i a l d i s p e r s i o n ) , d i s p e r s i o n i s a l i n e a r f u n c t i o n o f t i m e . F o r t i m e s g r e a t e r t h a n t h e i n t e g r a l t i m e s c a l e ( r a n d o m - w a l k r e g i m e ) , d i s p e r s i o n v a r i e s as t1!2. E q u a t i o n (5 .3 ) c a n b e u s e d t o e s t i m a t e t h e i n t e g r a l t i m e s c a l e i n d e p e n d e n t l y f r o m e q u a t i o n (4 .1 ) . T h e L a g r a n g i a n e d d y d i f f u s i v i t y , 2 dt c a n t h e n b e w r i t t e n a s , kk i f ° ° (u2) / Rk(r)dr, ( 5 .5 ) Jo w h i c h y i e l d s Kkk = (u2)t for t<Th, (5 .6 ) Kkk = (u'2)Tk for t ^ T k . (5 .7 ) T h e e d d y d i f f u s i v i t y c o m p o n e n t s c a n b e f o u n d d i r e c t l y t h r o u g h e q u a t i o n 5 .4 o r i n d i r e c t l y t h r o u g h e q u a t i o n 5 .7 . T h e a s s u m p t i o n s o f h o m o g e n e i t y a n d s t a t i o n a r i t y a r e r a r e l y i f e v e r m e t i n t h e o c e a n s . H o w e v e r , T a y l o r ' s t u r b u l e n t m i x i n g t h e o r i e s h a v e p r o v e n t o b e s u c c e s s f u l i n p r e d i c t i n g p a r t i c l e d i s p e r s i o n f r o m d r i f t e r e n s e m b l e s ( K r a u s s a n d B o n i n g , 1 9 8 7 ; T h o m s o n e t a l . , 1 9 9 0 ; S c h a f e r a n d K r a u s s , 1 9 9 5 ) . T h e a s s u m p t i o n s a r e c e r t a i n t o b r e a k d o w n i n r e g i o n s Chapter 5. Eddy Statistics in the North Pacific 9 7 o f s t r o n g e d d y a c t i v i t y , r e g i o n s o f t o p o g r a p h i c s t e e r i n g , a n d b o u n d a r y c u r r e n t r e g i m e s . R e g i o n s ( b o x e s ) w h i c h a r e r e p r e s e n t a t i v e o f t h e m o r e h o m o g e n e o u s ( a n d q u i e s c e n t ) o c e a n i n t e r i o r w e r e t h e r e f o r e s e l e c t e d t o t e s t T a y l o r ' s t h e o r i e s o n t h e m i x e d l a y e r a n d p y c n o c l i n e d r i f t e r e n s e m b l e s ( F i g u r e 5 .1 ) . A l t h o u g h t h e q u e s t i o n o f s t a t i o n a r i t y c a n n o t b e a d d r e s s e d w i t h t h i s d a t a s e t , t h e a s s u m p t i o n o f i s o t r o p y i s a sa fe o n e ( F i g u r e s 4 . 9 , 4 . 1 0 ) . S i n c e m o s t o f t h e e a r l i e r a p p l i c a t i o n s o f T a y l o r ' s t h e o r y i n v o l v e d t h e u s e o f d e e p - d r o g u e d ( ~ 100 m ) s u r f a c e d r i f t e r s o r s u b - t h e r m o c l i n e f l o a t s , t h e i r a p p l i c a t i o n t o d r i f t e r t r a j e c t o r i e s f r o m t h e s a m e r e g i o n s a n d a t t w o d e p t h s , w i t h i n t h e s t r o n g l y w i n d - i n f l u e n c e d m i x e d l a y e r a n d b e l o w i t , w i l l b e i n s t r u c t i v e . 5.3 Applicability to the drifter ensembles D e c o r r e l a t i o n s c a l e s h a v e b e e n d e r i v e d f o r t h e g l o b a l e n s e m b l e a n d f o r r e g i o n a l s u b - e n s e m b l e s ( C h a p t e r 4 ) . I n t h i s c h a p t e r , t h e d e c o r r e l a t i o n s c a l e s a r e r e - d e r i v e d b a s e d o n T a y l o r ' s t h e o r y o f s i n g l e p a r t i c l e d i s p e r s i o n . T h e a p p r o a c h t a k e n h a s b e c o m e s t a n d a r d i n p a r t i c l e d i s p e r s i o n s t u d i e s u s i n g L a - g r a n g i a n t r a j e c t o r i e s , a n d m a k e s u s e o f t h e a s s u m p t i o n s o f h o m o g e n e i t y a n d s t a t i o n a r i t y ( C o l i n d e V e r d i e r e , 1 9 8 3 ) . D a t a w e r e i n c r e a s e d i n e a c h b o x b y s p l i t t i n g e a c h d r i f t e r ' s t i m e s e r i e s i n t o s e p a r a t e s e r i e s , e a c h r e - s e t t o a n o r i g i n p o i n t a f t e r a m u l t i p l e o f 10 d a y s ( i . e . , l o n g e r t h a n t h e i n t e g r a l t i m e s c a l e s e s t i m a t e d i n C h a p t e r 4 ) . T i m e s e r i e s o f less t h a n 10 d a y s w e r e n o t i n c l u d e d i n t h e a n a l y s i s . D r i f t e r s w h i c h l e f t a b o x f o r l o n g e r t h a n o n e d a y a n d r e - e n t e r e d w e r e t r e a t e d as s e p a r a t e t r a j e c t o r i e s . T h e r e s u l t i n g n u m b e r o f s e p a r a t e t r a j e c t o r i e s c o n t a i n e d i n t h e " T a y l o r " b o x e s r a n g e d f r o m 4 7 t o 2 4 7 . F i g u r e 5 .2 s h o w s a n e x a m p l e o f t h e d i s p l a c e m e n t " p l u m e s " d e r i v e d f r o m t h i s e x e r c i s e . T h e n e t m e a n p a r t i c l e d i s p l a c e m e n t i n t h e s h a l l o w b o x c e n t e r e d a t 49.3°N, 145.3°W w a s e a s t w a r d ( p o s i t i v e z o n a l d i s p l a c e m e n t ) , w h i c h is c o n s i s t e n t w i t h t h e d e r i v e d m e a n v e l o c i t y ( T a b l e Chapter 5. Eddy Statistics in the North Pacific 98 F i g u r e 5 . 1 : S p a g h e t t i d i a g r a m s f o r t h e (a) s h a l l o w a n d (b ) d e e p d r i f t e r e n s e m b l e s . " T a y - l o r " b o x e s a r e r e g i o n s i n w h i c h T a y l o r ' s t h e o r i e s o f s i n g l e p a r t i c l e d i s p e r s i o n a r e t e s t e d . Chapter 5. Eddy Statistics in the North Pacific 99 5 . 1 ) . F r o m t h e s e d i s p l a c e m e n t p l u m e s , t h e b o x - m e a n d i s p e r s i o n s {(x'k2(t))) c a n b e c a l c u - l a t e d . T i m e s e r i e s o f m e a n d i s p e r s i o n o v e r t h e f i r s t 10 d a y s f o r a l l o f t h e s h a l l o w a n d d e e p T a y l o r b o x e s a r e s h o w n i n F i g u r e s 5 .3 a n d 5 .4 , r e s p e c t i v e l y . T h e t h e o r e t i c a l i n i t i a l d i s p e r s i o n c u r v e s ( e q u a t i o n 5 .2) a r e a l s o g i v e n . I n a l l o f t h e b o x e s t h e r e w a s a r a p i d i n i t i a l d i s p e r s i o n o v e r t h e f i r s t 2 - 4 d a y s , f o l l o w e d b y a s l o w e r d i s p e r s i o n . A f t e r t h e f i r s t 10 d a y s , t h e m e a n d i s p e r s i o n s r a n g e d f r o m 3 0 - 9 0 k m , w i t h z o n a l d i s p e r s i o n e x c e e d i n g m e r i d i o n a l d i s p e r s i o n i n a l l b o x e s e x c e p t f o r t h o s e i n t h e n o r t h e a s t e r n G u l f o f A l a s k a a n d t h e n o r t h e r n S u b t r o p i c a l G y r e . H i g h e r v e l o c i t y f l u c t u a t i o n s y i e l d e d l a r g e r d i s p e r - s i o n i n t h e b o x e s o f t h e S u b t r o p i c a l G y r e ( e . g . , F i g u r e s 5 . 3 g , h a n d F i g u r e 5 . 4 e ) . T a y l o r ' s p r e d i c t i o n o f a n i n i t i a l d i s p e r s i o n r e g i m e a p p e a r s t o h o l d q u i t e w e l l i n t h e a l l o f t h e " T a y - l o r " b o x e s , i n c l u d i n g t h e o n e w i t h t h e f e w e s t t r a j e c t o r y s e g m e n t s ( c e n t e r e d a t 55.2°N, 141.4°W, F i g u r e 5 . 3 a ) . A l t h o u g h t h e h o r i z o n t a l e d d y d i f f u s i v i t y c o m p o n e n t s c a n b e f o u n d d i r e c t l y f r o m t h e r a t e o f c h a n g e o f m e a n s q u a r e d i s p e r s i o n ( e q u a t i o n 5 . 4 ) , t h i s c a l c u l a t i o n i s v e r y s e n s i t i v e t o s m a l l v a r i a t i o n s o f d i s p e r s i o n ( S c h a f e r a n d K r a u s s , 1 9 9 5 ) . F i g u r e 5 .5 p r e s e n t s t h i s c a l c u l a t i o n f o r o n e b o x w i t h i n t h e S u b a r c t i c C u r r e n t r e g i o n f r o m e a c h e n s e m b l e . W i t h i n c r e a s i n g t i m e , t h e r e a r e f e w e r d a t a p o i n t s i n t h e c a l c u l a t i o n a n d r a n d o m fluctuations m a y b e c o m e d o m i n a n t ( K r a u s s a n d B o n i n g , 1 9 8 7 ) . T h u s , e v e n s m o o t h i n g w i t h a 5 - d a y r u n n i n g m e a n f i l t e r l e a v e s a f a i r l y n o i s y t i m e s e r i e s . T h e m a x i m u m v a l u e s a t t a i n e d b y t h e c a l c u l a t i o n c a n b e a s s u m e d t o b e t h e u p p e r l i m i t o f t h e d i f f u s i v i t i e s f o r t h e s e b o x e s ( T h o m s o n e t a l . , 1 9 9 0 ) . A t b o t h d e p t h s , t h e z o n a l d i f f u s i v i t i e s a r e s l i g h t l y l a r g e r t h a n t h e m e r i d i o n a l d i f f u s i v i t i e s . P e a k v a l u e s a r e o n t h e o r d e r o f 3 - 5 (1 -2 ) x 1 0 7 cm2 / s i n t h e z o n a l ( m e r i d i o n a l ) d i r e c t i o n . T h e e d d y d i f f u s i v i t y c o m p o n e n t s c a n b e m o r e r e a d i l y o b t a i n e d t h r o u g h a p p l i c a t i o n o f T a y l o r ' s t h e o r y . A c c o r d i n g t o e q u a t i o n ( 5 . 3 ) , t h e r e s h o u l d e x i s t a t i m e r a n g e o v e r w h i c h Chapter 5. Eddy Statistics in the North Pacific 100 F i g u r e 5 . 2 : D i s p l a c e m e n t " p l u m e s " i n t h e (a) z o n a l a n d (b ) m e r i d i o n a l d i r e c t i o n s f o r 2 4 7 p s e u d o t r a j e c t o r i e s f r o m t h e s h a l l o w b o x c e n t e r e d a t 49.3°N, 145.3°W, w i t h i n t h e S u b a r c t i c C u r r e n t r e g i o n . T h e m e a n flow h a s n o t b e e n r e m o v e d f r o m t h i s p l o t . Chapter 5. Eddy Statistics in the North Pacific 101 Mean Position U V U r.m.s. V r.m.s. (°N° W) (cm/s) (cm/s ) (cm/s ) (cm/s ) SHALLOW 5 5 . 2 , 1 4 1 . 4 - 5 . 1 ± 2 .2 5 .7 ± 2 .5 13 .8 ± 1.5 15 .8 ± 1.8 4 8 . 0 , 1 7 3 . 3 6 .2 ± 2 .2 0 .3 ± 1.0 11.8 ± 1.5 9.9 ± 0 .7 4 9 . 4 , 1 5 9 . 6 6 .3 ± 1.2 - 0 . 1 ± 0.6 8 .7 ± 0.8 8.1 ± 0 .4 4 9 . 3 , 1 4 5 . 3 4 .6 ± 0 .9 - 0 . 1 ± 0 .5 8.8 ± 0 .7 8 .8 ± 0 .3 4 8 . 6 , 1 3 4 . 3 5 .3 ± 0 .9 - 0 . 3 ± 0 .5 8.0 ± 0.6 8 .2 ± 0 .4 4 2 . 4 , 1 5 4 . 4 5 .3 ± 1.0 - 0 . 4 ± 0.6 7.8 ± 0 .7 7.7 ± 0 .4 4 0 . 8 , 1 9 3 . 1 10 .7 ± 2 .7 - 0 . 8 ± 1.4 15 .6 ± 1.9 1 6 . 5 ± 1.0 3 7 . 8 , 175 .1 7.6 ± 1.8 - 2 . 4 ± 1.6 14 .5 ± 1.3 17 .6 ± 1.1 DEEP 5 4 . 9 , 145 .0 3.1 ± 1.3 2 .3 ± 1.6 11 .6 ± 0 .9 11 .8 ± 1.1 5 0 . 1 , 155 .9 5 .9 ± 1.2 0.1 ± 0 .7 9.1 ± 0 .8 8.8 ± 0 .5 5 0 . 0 , 1 4 4 . 4 3 .9 ± 1.0 1.0 ± 0.6 7.8 ± 0 .7 7.0 ± 0 .4 4 0 . 6 , 148 .9 5 .0 ± 1.2 0.0 ± 0.7 8.8 ± 0.8 8 .7 ± 0 .5 3 7 . 8 , 1 7 0 . 0 3 .8 ± 2 .3 - 1 . 4 ± 1.8 12 .6 ± 1.6 1 1 . 7 ± 1.3 T a b l e 5 . 1 : M e a n a n d r . m . s . v e l o c i t i e s i n t h e " T a y l o r " b o x e s . S t a n d a r d e r r o r s ( a t t h e 9 5 % c o n f i d e n c e l e v e l ) a r e b a s e d o n d i f f e r e n t i n t e g r a l t i m e s c a l e s i n t h e z o n a l a n d m e r i d i o n a l d i r e c t i o n s . M e a n p o s i t i o n s r e f e r t o t h e c e n t e r - o f - m a s s l o c a t i o n s . Chapter 5. Eddy Statistics in the North Pacific 102 S H A L L O W D R I F T E R S o •i—i S H CD a w Q CTJ CD 0 2 4 6 8 10 90 • 60 30 • 0 120 48.6°N, 134.3°W 1 1 1 1 T i m e ( d a y s ) F i g u r e 5 . 3 : F i r s t 10 d a y s o f d i s p e r s i o n ( z o n a l - s o l i d , m e r i d i o n a l - d o t t e d ) f o r t h e s h a l l o w " T a y l o r " b o x e s . S t r a i g h t l i n e s ( z o n a l - s o l i d , m e r i d i o n a l - d a s h e d ) a r e t h e t h e o r e t i c a l v a l u e s f r o m e q u a t i o n 5 .2 . C o o r d i n a t e s r e f e r t o c e n t e r - o f - m a s s p o s i t i o n s . Chapter 5. Eddy Statistics in the North Pacific 103 D E E P D R I F T E R S 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 T i m e ( d a y s ) F i g u r e 5 .4 : A s i n F i g u r e 5 . 3 , e x c e p t f o r t h e d e e p " T a y l o r " b o x e s . Chapter 5. Eddy Statistics in the North Pacific 104 F i g u r e 5.5: T i m e se r i es o f e d d y d i f f u s i v i t i e s , u s i n g a 5 -day r u n n i n g m e a n f i l t e r , d e r i v e d d i r e c t l y f r o m t h e d e r i v a t i v e o f m e a n s q u a r e d i s p e r s i o n f o r o n e (a ) s h a l l o w a n d o n e ( b ) d e e p " T a y l o r " b o x f r o m t h e S u b a r c t i c C u r r e n t r e g i o n . Chapter 5. Eddy Statistics in the North Pacific 105 t h e m e a n s q u a r e d i s p e r s i o n c a n b e r e p r e s e n t e d as a s t r a i g h t l i n e , i . e . d i s p e r s i o n v a r i e s as t1/2. T h i s r a n d o m w a l k r e g i m e a p p e a r s t o o c c u r i n a l l o f t h e s h a l l o w a n d d e e p b o x e s a f t e r a b o u t 10 d a y s ( F i g u r e s 5 .6 , 5 .7 ) . T i m e r a n g e s o v e r w h i c h t h i s a p p r o x i m a t i o n h o l d s w e r e s u b j e c t i v e l y d e t e r m i n e d ( s o l i d l i n e s ) a n d , f o l l o w i n g S c h a f e r a n d K r a u s s ( 1 9 9 5 ) , t h e i n t e g r a l t i m e s c a l e s w e r e t h e n e s t i m a t e d f r o m t h e m e a n s q u a r e d i s p e r s i o n o v e r t h i s t i m e r a n g e , %=wrr^- (5-8) E q u a t i o n s (4 .6 ) a n d (5 .7 ) w e r e t h e n u s e d t o d e t e r m i n e t h e i n t e g r a l l e n g t h s c a l e s a n d e d d y d i f f u s i v i t i e s , r e s p e c t i v e l y , f r o m t h e d e r i v e d t i m e s c a l e s ( T a b l e 5 . 2 ) . T h e g l o b a l m e a n t i m e s c a l e s d e r i v e d f r o m d i s p e r s i o n a r e 2 .5 (2 .8 ) d a y s i n t h e z o n a l d i r e c t i o n a n d 1.7 (2 .3 ) d a y s i n t h e m e r i d i o n a l d i r e c t i o n , r e s p e c t i v e l y , f o r t h e s h a l l o w ( d e e p ) e n s e m b l e s . T h e l o n g e s t z o n a l t i m e s c a l e s a t 15 m d e p t h a r e i n t h e c e n t r a l S u b a r c t i c C u r r e n t r e g i o n , w h i l e t h e m e r i d i o n a l t i m e s c a l e s a r e 1 .5 -2 d a y s t h r o u g h o u t t h e n e a r - s u r f a c e N o r t h P a c i f i c ( T a b l e 5 . 2 , F i g u r e 5 .8 ) . T h e s e e s t i m a t e s c o m p a r e v e r y w e l l w i t h t h e e s t i m a t e s d e r i v e d e a r l i e r f r o m t h e g l o b a l - a n d r e g i o n a l - m e a n a u t o c o r r e l a t i o n f u n c t i o n s ( T a b l e 4 . 1 ) . D i f f u s i v i t i e s a r e h i g h e s t a t b o t h d e p t h s i n t h e S u b t r o p i c a l G y r e , p a r t i c u l a r l y i n t h e r e g i o n j u s t e a s t o f t h e E m p e r o r S e a m o u n t C h a i n . T h e e d d y s t a t i s t i c s a r e s l i g h t l y a n i s o t r o p i c , w i t h t h e z o n a l s c a l e s a n d d i f f u s i v i t i e s e v e r y w h e r e e x c e e d i n g t h e m e r i d i o n a l s t a t i s t i c s , e x c e p t i n t h e n o r t h e a s t e r n G u l f o f A l a s k a . S c h a f e r a n d K r a u s s ( 1 9 9 5 ) f o u n d t h e s a m e a n i s o t r o p y i n t h e S o u t h A t l a n t i c , a n d a t t r i b u t e d t h e e n h a n c e d p a r t i c l e d i s p e r s i o n i n t h e z o n a l d i r e c t i o n t o t h e / 3 - e f f e c t . A s w a s f o u n d e a r l i e r , t h e d e c o r r e l a t i o n s c a l e s a r e s l i g h t l y l o n g e r , a n d m o r e n e a r l y i s o t r o p i c , b e l o w t h e m i x e d l a y e r . T h e d i f f u s i v i t i e s d e r i v e d h e r e a r e c o m p a r a b l e t o t h o s e d e r i v e d f r o m 1 0 0 - m d r o g u e d d r i f t e r s i n t h e e a s t e r n N o r t h P a c i f i c ( T h o m s o n et a l , 1990 ; r o u g h l y t h e r e g i o n o f t h e s h a l l o w A l a s k a C u r r e n t Chapter 5. Eddy Statistics in the North Pacific 106 C\2 a o • 1—I tn S H CD OH OT • i—i Q CD S H cd GO cd S H A L L O W D R I F T E R S 10" 1 « . 101 10z _J 1 (a ) io5 -J 104 103 -s 102-T 101 55.2°N, 141.41 —\—i—i 111111 -i—i—iin 106 105 10* -J 103 102 101 _ l I I I ( c ) 49.4°N, 159.6°W 10B 105 104 io3 -A 10: 10 10( 10s 104 103 102 10 -1—I—I I I I I I I 1—I—I I I I I I -1 1 I I • ' • • 1(e) 48.6°N, 134.3°Wt n6 . -i—i—i i 11 ii| 1—i i i 11 I I —i—i—i i 11111 i i • (g) -|1 40.8°N, 166.9°E I I—I I I I 111 1 1 1 I I I I I 10° 101 102 T i m e 10° 101 i i ' i 11111 102 (b) 48.0°N, 173.3°Wf n—i—i I I I I I ) r—i—n~r !<*> i r r r 49.3°N, r 145.3°W : :( f) |—i  nii inj' 1 III II II — I'T II II II \ i . X mui i ii iiii j :'" 42.4°N, 154.4°W : ' • 1 ' I I I I I I I 1(h) 37.8°N, 175.1°W - I 1—I I I I 111 1 1 I I II 11 106 105 104 103 102 101 106 105 104 103 102 101 106 10s 104 103 102 101 106 105 104 103 102 101 10° 101 102 (days) F i g u r e 5 .6 : M e a n s q u a r e d i s p e r s i o n ( z o n a l - d a s h e d , m e r i d i o n a l - d o t t e d ) o v e r t h e f i r s t 100 d a y s f o r t h e s h a l l o w " T a y l o r " b o x e s . S o l i d c u r v e s a r e t h e t h e o r e t i c a l v a l u e s f r o m e q u a t i o n 5 . 3 . C o o r d i n a t e s r e f e r t o c e n t e r - o f - m a s s p o s i t i o n s . Chapter 5. Eddy Statistics in the North Pacific 1 0 7 D E E P D R I F T E R S 10° (a) 10° • 105 • 104- 103 • 102 -1''' 101 i±±J 102 11 10 106 105 54.9°N, 145.0°W 1(c) io* 4 103 102 101 106 50.0°N, 144.4°W —\ 1—I' I I III] 1 1—l l l 111 (e) 10 5^ 10* 103 102 10' 37.8°N, 170.0°W 10' 1—i—i i 11111 1—i—r r T i II ,o 1 0 i 10z 10" 10' IO'' 1(b) 50.1°N, 155.9° -1 1—I I M i l l 1 1—I I I I I I 10° 105 104 103 l r10 2 10' (d) fe- 106 105 10* 103 102 40.6°N, 148.9°W —i—i—1111111—i—i—i 11111 10° 101 101 102 T i m e (days) F i g u r e 5 .7 : A s i n F i g u r e 5 .6 , e x c e p t f o r t h e d e e p " T a y l o r " b o x e s . Chapter 5. Eddy Statistics in the North Pacific 108 b o x , c e n t e r e d a t 48.6°N, 134.3°W), b u t , d u e t o l o w e r e d d y k i n e t i c e n e r g i e s i n t h e N o r t h P a c i f i c , a r e a b o u t h a l f t h e m a g n i t u d e o f t h o s e d e r i v e d f r o m 1 0 0 - m d r o g u e d d r i f t e r s i n t h e N o r t h A t l a n t i c ( K r a u s s a n d B o n i n g , 1987 ) a n d S o u t h A t l a n t i c ( S c h a f e r a n d K r a u s s , 1 9 9 5 ) . I t s h o u l d b e k e p t i n m i n d t h a t t h e r e a r e i n h e r e n t l i m i t a t i o n s o f t h e a n a l y s i s p r e s e n t e d h e r e . F i r s t , t h e d i f f u s i v i t y e s t i m a t e s a r e d e p e n d e n t o n t h e c o m p l e t e r e m o v a l o f t h e m e a n f l o w ( (« ) ) a n d i t s e f f e c t s , w h i c h i s s u b j e c t t o s o m e u n c e r t a i n t y . O f c o u r s e , t h e e s t i m a t e s a r e a l s o d e p e n d e n t o n t h e v a l i d i t y o f t h e a s s u m p t i o n s o f h o m o g e n e i t y a n d s t a t i o n a r i t y . A s p o i n t e d o u t b y D a v i s ( 1 9 9 1 ) , E u l e r i a n s t a t i s t i c a l i n h o m o g e n e i t y ( w h i c h i s a p p a r e n t i n t h e s e d a t a ) c a n l e a d t o L a g r a n g i a n n o n s t a t i o n a r i t y . N o n e t h e l e s s , t h e m e t h o d y i e l d e d r e s u l t s w h i c h m a t c h e d T a y l o r ' s p r e d i c t i o n s q u i t e w e l l . C o n s i s t e n t w i t h p r e v i o u s s t u d i e s b a s e d o n s u r f a c e d r i f t e r s , a s c a l i n g o f i n t e g r a l t i m e s c a l e a n d e d d y d i f f u s i v i t y w i t h r . m . s . v e l o c i t y i s f o u n d ( F i g u r e 5 . 9 ) . T h e s e r e l a t i o n s h i p s h o l d b o t h w i t h i n a n d b e l o w t h e m i x e d l a y e r , a n d f i t w e l l w i t h t h e r e l a t i o n s h i p s d e r i v e d f r o m d r i f t e r d a t a i n t h e A t l a n t i c ( e . g . , F i g u r e s 8 a n d 12 o f S c h a f e r a n d K r a u s s , 1 9 9 5 ) . A n e x c e p t i o n i s t h e m e r i d i o n a l t i m e s c a l e , w h i c h a p p e a r s t o h a v e a w e a k e r d e p e n d e n c e o n vT.m.s.- A s f o r t h e d i f f u s i v i t i e s , t h e i n t e g r a l l e n g t h s c a l e s a r e s l i g h t l y a n i s o t r o p i c , a n d a r e l a r g e s t i n t h e s u b t r o p i c a l b o x e s . T h e g l o b a l m e a n l e n g t h s c a l e s a r e a b o u t 3 0 % s m a l l e r t h a n t h o s e d e r i v e d f r o m d r i f t e r s i n t h e A t l a n t i c ( K r a u s s a n d B o n i n g , 1 9 8 7 ; S c h a f e r a n d K r a u s s , 1 9 9 5 ) . T h u s , t h e r e a p p e a r s t o b e l i t t l e g e o g r a p h i c a l v a r i a t i o n i n u p p e r - o c e a n m i x i n g l e n g t h s . T h e s e a r e t h e f i r s t L a g r a n g i a n e s t i m a t e s o f p a r t i c l e d i s p e r s i o n o v e r a b r o a d r e g i o n o f t h e n e a r - s u r f a c e N o r t h P a c i f i c , a n d t h e c o n s i s t e n c y o f t h e r e s u l t s w i t h p r e v i o u s s t u d - i e s f r o m t h e A t l a n t i c s u g g e s t s t h a t t h e s i m p l i f y i n g a s s u m p t i o n s o f T a y l o r ( 1 9 2 1 ) a r e r e a s o n a b l y v a l i d t h r o u g h o u t t h e u p p e r o c e a n . S i n c e v e l o c i t y v a r i a n c e i s r e a d i l y a n d s y n - o p t i c a l l y m e a s u r e d b y s a t e l l i t e a l t i m e t r y ( fo r l o w - f r e q u e n c y m o t i o n s ) , u s e o f t h e s i m p l e Chapter 5. Eddy Statistics in the North Pacific 109 Mean Position (°N° W) T (days) T (days) Lx (km) Ly (km) Kxx (107 cm2Is) ^yy (107 cm2Is) S H A L L O W 5 5 . 2 , 1 4 1 . 4 1.5 ± 0.1 1.6 ± 0.2 17 .8 ± 2 .3 2 1 . 9 ± 3 .7 2 .4 ± 0 .5 3 .5 ± 0 .9 4 8 . 0 , 1 7 3 . 3 2 .5 ± 0.1 1.4 ± 0.1 2 5 . 4 ± 3 .4 12 .1 ± 1.2 3 .0 ± 0 .7 1.2 ± 0 .2 4 9 . 4 , 1 5 9 . 6 2 .4 ± 0.1 1.9 ± 0.2 17 .9 ± 1.8 13 .2 ± 1.5 1.5 ± 0 .3 1.1 dz 0 .2 4 9 . 3 , 1 4 5 . 3 3 .3 ± 0.1 1.9 ± 0.1 2 4 . 8 ± 2.1 14.1 ± 0 .9 2 .2 ± 0 .4 1.2 ± 0.1 4 8 . 6 , 1 3 4 . 3 2.1 ± 0 .2 1.8 ± 0.1 14 .2 ± 1.7 12 .6 ± 0 .9 1.1 ± 0 .2 1.0 ± 0.1 4 2 . 4 , 1 5 4 . 4 3.1 ± 0 .2 1.8 ± 0.1 2 1 . 2 ± 2 .3 11 .8 ± 0 .9 1.7 ± 0 .3 0 .9 ± 0.1 4 0 . 8 , 1 9 3 . 1 2 .4 ± 0 .2 1.1 ± 0.1 3 2 . 4 ± 4 .8 16 .0 ± 1.7 5.1 ± 1.3 2 .6 ± 0 .4 3 7 . 8 , 1 7 5 . 1 2 .7 ± 0.1 2.0 ± 0.1 3 3 . 3 ± 3.2 3 0 . 9 ± 2 .5 4 . 8 ± 0 .9 5 .5 ± 0 . 7 G l o b a l M e a n 2 .5 ± 0.1 1.7 ± 0.1 2 3 . 4 ± 2 .7 16 .6 ± 1.7 2 .7 ± 0.6 2 .1 ± 0 .3 DEEP 5 4 . 9 , 1 4 5 . 0 1.5 ± 0.1 2 .5 db 0 .3 14 .8 ± 1.5 2 5 . 6 ± 3 .9 1.7 ± 0 .3 3 .0 ± 0 . 7 5 0 . 1 , 1 5 5 . 9 2 .2 ± 0 .2 1.8 ± 0.1 17 .3 ± 2 .2 13 .8 ± 1.1 1.6 ± 0 .3 1.2 zb 0 .2 5 0 . 0 , 1 4 4 . 4 3 .5 ± 0 .2 2 .3 ± 0.1 2 3 . 4 ± 2 .5 14 .0 ± 1.0 1.8 ± 0 .3 1.0 ± 0.1 4 0 . 6 , 1 4 8 . 9 3 .5 ± 0 .2 2.1 ± 0.1 2 6 . 9 ± 2.9 15 .4 ± 1.1 2 .4 ± 0 .5 1.3 ± 0 .2 3 7 . 8 , 1 7 0 . 0 3 .3 ± 0.2 2 .8 ± 0.4 3 6 . 4 ± 5.1 2 8 . 8 ± 5 .2 4 . 6 ± 1.2 3 . 4 ± 0 .9 G l o b a l M e a n 2 .8 ± 0 .2 2 .3 ± 0.2 23 .8 ± 2.8 19 .5 ± 2 .5 2 .4 ± 0 .5 2 . 0 ± 0 .4 T a b l e 5 . 2 : I n t e g r a l t i m e (Tk) a n d l e n g t h (Lk) s c a l e s a n d e d d y d i f f u s i v i t i e s (Kkk), w i t h s t a n d a r d e r r o r s , d e r i v e d f o r t h e " T a y l o r " b o x e s i n t h e s h a l l o w a n d d e e p d r i f t e r e n s e m b l e s . M e a n p o s i t i o n s r e f e r t o c e n t e r - o f - m a s s l o c a t i o n s . Chapter 5. Eddy Statistics in the North Pacific 110 F i g u r e 5 .8 : M a p s s h o w i n g t h e z o n a l / m e r i d i o n a l i n t e g r a l t i m e s c a l e s ( d a y s ) a n d e d d y d i f f u s i v i t i e s ( x 1 0 7 cm2/s) f o r t h e " T a y l o r " b o x e s i n t h e (a ) s h a l l o w a n d ( b ) d e e p d r i f t e r e n s e m b l e s . Chapter 5. Eddy Statistics in the North Pacific 111 ^ 6 CO cd CD « 4 o in CD H a •i—( H 2 i— i cd (ao 1 H CD (a) ...••X-'•4 : o X _ T X vs. (15m) O—T y vs. V^, (15m) A — T x vs. (120m) + Ty V S . VJTBS (120m) O O r 8 12 16 20 r.m.s. speed (cm/s) F i g u r e 5 . 9 : (a ) I n t e g r a l t i m e s c a l e a n d (b ) e d d y d i f f u s i v i t y v s . r . m . s . s p e e d d e r i v e d f o r t h e " T a y l o r " b o x e s . S t r a i g h t l i n e s a r e l e a s t - s q u a r e s f i t s . T h e f i t s a r e b a s e d o n l y o n t h e r a n g e o f r . m . s . s p e e d s s a m p l e d h e r e , a n d s h o u l d n o t b e e x t r a p o l a t e d t o t h e y - i n t e r c e p t . Chapter 5. Eddy Statistics in the North Pacific 112 r e l a t i o n s h i p , Kkk = (u'2)l*Lh, (5 .9 ) m a y y i e l d g o o d e s t i m a t e s o f n e a r - s u r f a c e e d d y d i f f u s i v i t y f o r m o d e l i n g a p p l i c a t i o n s ( S c h a f e r a n d K r a u s s , 1 9 9 5 ) . Chapter 6 Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 6.1 Introduction T h e p r e v i o u s c h a p t e r s p r o v i d e d a s t a t i s t i c a l o v e r v i e w o f t h e n e a r - s u r f a c e c i r c u l a t i o n a n d i t s v a r i a b i l i t y i n t h e N o r t h P a c i f i c O c e a n b a s e d o n t h e t r a j e c t o r i e s o f n e a r l y 100 s a t e l l i t e - t r a c k e d d r i f t e r s d r o g u e d e i t h e r w i t h i n t h e m i x e d l a y e r o r i n t h e u n d e r l y i n g p y c n o c l i n e . A p e r u s a l o f t h e s e t r a j e c t o r i e s ( e . g . , F i g u r e 4 .5 ) r e v e a l s s o m e i n t e r e s t i n g m e s o s c a l e f e a t u r e s w h i c h w a r r a n t f u r t h e r a n a l y s i s . I n t h i s c h a p t e r , t h e f o c u s i s o n a s u b s e t o f d r i f t e r s w h i c h s a m p l e d t h e r e g i o n n e a r t h e E m p e r o r S e a m o u n t C h a i n ( n e a r 172°E) a n d w e r e e n t r a i n e d i n t o l o n g - l i v e d , t o p o g r a p h i c a l l y - g e n e r a t e d e d d i e s . 6.2 Theoretical Background S e a m o u n t s a r e u b i q u i t o u s f e a t u r e s o f t h e w o r l d ' s o c e a n s w h i c h c a n p r o f o u n d l y a f f e c t p h y s i c a l a n d b i o l o g i c a l p r o c e s s e s , g e n e r a t i n g m e s o s c a l e c i r c u l a t i o n f e a t u r e s a n d i n c r e a s - i n g p r i m a r y p r o d u c t i o n ( R o y e r , 1978 ; H o g g , 1980 ; B o e h l e r t a n d G e n i n , 1 9 8 7 ; L o b e l a n d R o b i n s o n , 1 9 8 6 ; R o d e n , 1987 ; R o d e n , 1 9 9 1 ; F r e e l a n d , 1994 ; C o m e a u e t a l . , 1 9 9 5 . ) M o d e l i n g t h e e f f e c t s o f s u b m e r g e d t o p o g r a p h y o n o c e a n c i r c u l a t i o n a n d t h e r m o h a l i n e s t r u c t u r e h a s t h e r e f o r e g a r n e r e d c o n s i d e r a b l e a t t e n t i o n i n t h e o c e a n o g r a p h i c c o m m u n i t y . T h e d o m i n a n t p r i n c i p l e i s t h e c o n s e r v a t i o n o f p o t e n t i a l v o r t i c i t y a l o n g a p a r t i c l e t r a j e c - t o r y : as a c o l u m n o f w a t e r m o v e s o v e r a n o b s t a c l e , v o r t e x c o m p r e s s i o n r e q u i r e s a d e c r e a s e i n l o c a l v o r t i c i t y , r e s u l t i n g i n a n t i c y c l o n i c c u r r e n t s ( a T a y l o r c o l u m n ) o v e r t h e f e a t u r e 113 Chapter 6. Eddies h Seamount-attached Eddies at the Emperor Seamount Chain 114 ( T a y l o r ( 1 9 2 3 ) ) . V o r t e x s t r e t c h i n g u p o n m o v i n g a w a y f r o m t h e f e a t u r e p r o d u c e s c y c l o n i c r o t a t i o n . E a r l y a n a l y t i c a l a n d n u m e r i c a l m o d e l s b y H o g g ( 1973 ) a n d H u p p e r t a n d B r y a n ( 1 9 7 6 ) , r e s p e c t i v e l y , d e m o n s t r a t e d t h e p o t e n t i a l i n i t i a l i z a t i o n o f a T a y l o r c o l u m n o v e r s u b m e r g e d i s o l a t e d t o p o g r a p h y . N u m e r i c a l m o d e l s d e s c r i b i n g q u a s i - g e o s t r o p h i c s t e a d y - s t a t e ( K o z l o v , 1 9 8 1 ; J o h n s o n , 1982 ) a n d t i m e - v a r i a b l e ( V e r r o n a n d L e P r o v o s t , 1 9 8 5 ; V e r r o n , 1986 ) f l o w o v e r a n i s o l a t e d s u b m e r g e d o b s t a c l e h a v e s h o w n t h a t a n t i c y c l o n i c a n d c y c l o n i c e d d i e s c a n b e f o r m e d , w i t h d y n a m i c a l c h a r a c t e r i s t i c s d e p e n d e n t o n t h e b a c k - g r o u n d f l o w a n d t h e o b s t a c l e g e o m e t r y . M o r e r e c e n t l y , C h a p m a n a n d H a i d v o g e l ( 1 9 9 2 ) a n d S m i t h ( 1 9 9 2 ) , u s i n g p r i m i t i v e e q u a t i o n n u m e r i c a l m o d e l s , h a v e b o t h s h o w n t h a t t h e f o r m a t i o n o f T a y l o r c a p s o r e d d i e s n e a r t h e t o p o g r a p h y d e p e n d s c r i t i c a l l y o n s e a m o u n t h e i g h t . L a b o r a t o r y m o d e l s h a v e a l s o b e e n d e v e l o p e d t o s t u d y t h e m o t i o n o f r o t a t i n g , s t r a t i f i e d , s t e a d y a n d o s c i l l a t o r y c u r r e n t s p a s t i s o l a t e d t o p o g r a p h y ( B o y e r a n d Z h a n g , 1 9 9 0 a , b ; Z h a n g a n d B o y e r , 1993 ) a n d i n t h e v i c i n i t y o f m u l t i p l e s e a m o u n t s ( Z h a n g a n d B o y e r , 1 9 9 1 ) , e a c h s h o w i n g t h e d e v e l o p m e n t o f m e s o s c a l e e d d i e s o n t o p o f o r i n t h e l e e o f t h e o b s t a c l e ( s ) . B r i n k ( 1990 ) u s e d a n a n a l y t i c a l m o d e l t o d e m o n s t r a t e t h e g e n e r a t i o n o f s e a m o u n t - t r a p p e d w a v e s b y t h e r e s o n a n t e x c i t a t i o n a t c e r t a i n f r e q u e n c i e s o f a h o r i - z o n t a l l y u n i f o r m a m b i e n t f l o w . R o d e n (1991 ) p r o v i d e s a c o m p r e h e n s i v e r e v i e w o f r e c e n t m o d e l i n g a n d o b s e r v a t i o n a l s t u d i e s o f f l o w - t o p o g r a p h y i n t e r a c t i o n , w i t h a n e m p h a s i s o n t h e N o r t h P a c i f i c . A l t h o u g h t h e m o d e l s o f flow o v e r s u b m e r g e d t o p o g r a p h y h a v e p r o d u c e d a n i n t r i g u i n g l i s t o f p r e d i c t e d m e s o s c a l e c i r c u l a t i o n f e a t u r e s , o b s e r v a t i o n s o f t h e s e f e a t u r e s h a v e b e e n less d e f i n i t i v e . O w e n s a n d H o g g (1980 ) u s e d h y d r o g r a p h i c d a t a f r o m t h e M i d - O c e a n D y n a m i c s E x p e r i m e n t ( M O D E ) i n t h e N o r t h A t l a n t i c t o o b s e r v e T a y l o r c o l u m n s o v e r s e a m o u n t s , w h i l e R i c h a r d s o n (1980 ) o b s e r v e d a n t i c y c l o n i c e d d i e s i n t h e t r a j e c t o r i e s o f s a t e l l i t e - t r a c k e d d r i f t e r s i n t h e l e e o f t h e C o r n e r R i s e S e a m o u n t s , d e s c r i b i n g t h e m as Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 115 T a y l o r c o l u m n s s h e d f r o m t h e t o p o g r a p h y . R e c e n t l y , i n t e n s i v e f i e l d i n v e s t i g a t i o n s w e r e u n d e r t a k e n a t t h e F i e b e r l i n g s e a m o u n t g r o u p i n t h e s u b t r o p i c a l N o r t h P a c i f i c . D i r e c t c u r r e n t m e a s u r e m e n t s r e v e a l e d d i u r n a l t i d a l a m p l i f i c a t i o n o v e r t h e s u m m i t a n d a r e s i d u a l a n t i c y c l o n i c c i r c u l a t i o n a r o u n d t h e s e a m o u n t ( E r i k s e n , 1 9 9 1 ; N o b l e e t a l . , 1 9 9 4 ; B r i n k , 1 9 9 5 ) . F l o w p a t t e r n s i n t h e v i c i n i t y o f t h e s e a m o u n t s w e r e d o m i n a t e d b y p a i r s o f t r a n s i e n t m e s o s c a l e ( 1 0 - 3 0 k m d i a m e t e r ) c y c l o n i c a n d a n t i c y c l o n i c e d d i e s a n d a c c o m p a n y i n g j e t s w i t h c o r e s p e e d s o f 2 0 - 5 0 c m / s ( R o d e n , 1 9 9 4 ) . I n t h e N o r t h P a c i f i c , o n e o f t h e m o s t p r o m i n e n t t o p o g r a p h i c f e a t u r e s i s t h e E m p e r o r S e a m o u n t C h a i n ( E S C ) , a m e r i d i o n a l l y - o r i e n t e d u n d e r w a t e r m o u n t a i n r a n g e , e x t e n d i n g a p p r o x i m a t e l y 2 4 0 0 k m n e a r 170°E f r o m t h e A l e u t i a n I s l a n d s a r c t o t h e H a w a i i a n I s l a n d s a r c a n d r i s i n g 3 t o 5 k m a b o v e t h e o c e a n f l o o r ( F i g u r e 6 .1 ) , w i t h s o m e i n d i v i d u a l p e a k s n e a r i t s s o u t h e r n e n d a p p r o a c h i n g 100 m o f t h e s e a s u r f a c e ( R o d e n e t a l . , 1 9 8 2 ) . T h e E S C e f f e c t i v e l y s p l i t s t h e N o r t h P a c i f i c i n t o s e p a r a t e w e s t e r n a n d e a s t e r n b a s i n s . U s i n g s p a r s e h i s t o r i c a l h y d r o g r a p h i c d a t a , R o d e n et a l . ( 1982 ) d e m o n s t r a t e d t h a t t h e K u r o s h i o E x t e n s i o n w e s t o f t h e E S C is a w e l l - d e f i n e d m e a n d e r i n g j e t , e x t e n d i n g n e a r l y t o t h e b o t t o m , w h i c h t e n d s t o t u r n n o r t h w a r d u p o n i t s a p p r o a c h t o t h e s o u t h e r n e n d o f t h e c h a i n a n d t h e n t u r n s s o u t h w a r d i n a n a n t i c y c l o n i c l o o p as i t c r o s s e s t h e a x i s o f t h e s e a m o u n t s , g e n e r a l l y i n t h e 6 0 0 0 m d e e p M a i n G a p a t 39°N, 171°E. E a s t o f t h e E S C , t h e K u r o s h i o E x t e n s i o n i s w e a k a n d p o o r l y d e f i n e d . A n e x t e n s i v e f i e l d s u r v e y o f t h e r e g i o n c a r r i e d o u t i n J u n e a n d J u l y , 1 9 8 2 ( R o d e n a n d T a f t , 1985) r e v e a l e d t h i s d e f l e c t i o n a n d w e a k e n i n g o f t h e K u r o s h i o E x t e n s i o n o v e r K i n m e i S e a m o u n t ( n e a r 35°N) , as c a n b e s e e n i n t h e d y n a m i c t o p o g r a p h y o f t h e 150 d b s u r f a c e r e l a t i v e t o 8 0 0 d b , n e a r t h e d e p t h o f t h e s e a m o u n t p e a k s ( F i g u r e 6 .2 ) . M e s o s c a l e p e r t u r b a t i o n s o f l a r g e v e r t i c a l a m p l i t u d e , s u g g e s t i v e o f e d d i e s o r T a y l o r c o l u m n s , w e r e a l s o o b s e r v e d n e a r s e v e r a l o f t h e s e a m o u n t s . A c u r r e n t m e t e r m o o r i n g i n t h e M a i n G a p b e t w e e n J u n e 1982 a n d N o v e m b e r 1 9 8 3 d e m o n s t r a t e d f l o w r e c t i f i c a t i o n w i t h d e p t h a n d a m e a n b o t t o m - r e f e r e n c e d n o r t h e a s t w a r d t r a n s p o r t o f 14 Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 116 S v ( H a m a n n a n d T a f t , 1 9 8 7 ) . R o d e n a n d T a f t ( 1985 ) s u g g e s t e d t h a t t h i s flow t h r o u g h t h e M a i n G a p m a y p r o v i d e o n e o f t h e f e w a v e n u e s t h r o u g h w h i c h n o r t h e a s t P a c i f i c a b y s s a l w a t e r s c o m m u n i c a t e w i t h t h e r e s t o f t h e P a c i f i c . V a s t a n o e t a l . ( 1 9 8 5 ) p r e s e n t e d o b s e r v a t i o n s o f t w o s a t e l l i t e - t r a c k e d d r i f t e r s , d r o g u e d a t 100 m , w h i c h c r o s s e d t h e E S C i n 1 9 7 7 . B o t h d r i f t e r s r e v e a l e d u p s t r e a m m e a n d e r s , c y c l o n i c e d d y a c t i v i t y j u s t w e s t o f t h e c h a i n , a n d p a s s a g e t h r o u g h t h e M a i n G a p . D u e i n p a r t t o i t s r e m o t e n e s s , n o s y s t e m a t i c fieldwork h a s b e e n f o r t h c o m i n g a t t h e E S C s i n c e t h e o b s e r v a t i o n s i n t h e e a r l y 1 9 8 0 ' s , n o r h a v e t h e r e b e e n a n y e x t e n s i v e L a g r a n g i a n o b s e r v a t i o n s . A l t h o u g h t h e s u m m e r 1 9 8 2 field s u r v e y p r o v i d e s a c h a r a c t e r i z a t i o n o f t h e u p p e r l e v e l flow p a t t e r n a t t h e E S C ( F i g u r e 6 . 2 ) , i t r e p r e s e n t s o n l y o n e r e a l i z a t i o n a n d t h e p e r s i s t e n c e o f t h e o b s e r v e d m e s o s c a l e f e a t u r e s i s u n k o w n . I n t h i s c h a p t e r , d a t a a r e p r e s e n t e d a n d d e s c r i b e d f r o m s e v e n s a t e l l i t e - t r a c k e d d r i f t e r s , five d r o g u e d w i t h i n a n d t w o b e l o w t h e w i n d - m i x e d l a y e r , w h i c h c r o s s e d t h e s o u t h e r n e n d o f t h e E S C i n t h e s u m m e r s o f 1991 a n d 1992 a n d t h e w i n t e r o f 1 9 9 3 . I n p a r t i c u l a r , o n e o f t h e first o b s e r v a t i o n s o f a n e x t e n d e d a t t a c h m e n t ( > 6 0 d a y s ) o f a t o p o g r a p h i c a l l y - g e n e r a t e d e d d y t o a s e a m o u n t is p r e s e n t e d . T h u s , a l t h o u g h l i m i t e d , t h i s d a t a se t f u r t h e r i l l u s t r a t e s t h e p r o n o u n c e d e d d y a c t i v i t y t h o u g h t t o b e p r e s e n t a t t h e E S C , a n d p r o v i d e s a m e a s u r e o f t h e c h a r a c t e r i s t i c s a n d p e r s i s t e n c e o f t h e m e s o s c a l e f e a t u r e s . C o m p a r i s o n s a r e m a d e b e t w e e n t h e o b s e r v a t i o n s a n d t h e flows p r e d i c t e d b y l a b o r a t o r y a n d n u m e r i c a l m o d e l s o f c u r r e n t - t o p o g r a p h y i n t e r a c t i o n s . T h e o b s e r v a t i o n s p r e s e n t e d h e r e r e v e a l t h e E S C as a g e n e r a t o r o f q u a s i - s t a t i o n a r y , l o n g - l i v e d u p p e r - l e v e l m e s o s c a l e e d d i e s , a n d p r o - v i d e a n e s p e c i a l l y u s e f u l t e s t o f m o d e l s o f flow o v e r t a l l s e a m o u n t s , d e m o n s t r a t i n g b o t h t h e i r g e n e r a l v a l i d i t y a n d s p e c i f i c l i m i t a t i o n s . Chapter 6. Eddies h Seamount-attached Eddies at the Emperor Seamount Chain 117 F i g u r e 6 . 1 : M a p o f t h e N o r t h P a c i f i c O c e a n s h o w i n g l o c a t i o n o f t h e E m p e r o r S e a m o u n t C h a i n . L i g h t a n d d a r k s h a d i n g i n t h i s a n d s u b s e q u e n t f i g u r e s r e p r e s e n t s w a t e r d e p t h s o f 2 0 0 0 - 4 0 0 0 m a n d s h a l l o w e r t h a n 2 0 0 0 m , r e s p e c t i v e l y . T h e i n s e t s h o w s t h e b a t h y m e t r i c s e c t i o n a l o n g t h e b o x e d r e g i o n , w h i c h is t h e c r e s t o f t h e s o u t h e r n p o r t i o n o f t h e E m p e r o r S e a m o u n t C h a i n ( a d a p t e d f r o m R o d e n e t a l . ( 1982 ) ) . Chapter 6. Eddies h Seamount-attached Eddies at the Emperor Seamount Chain 118 166' tea* 170°E 172* 174* 176* 166' 168° 170°E 172* 174« 176* F i g u r e 6 . 2 : M a p o f d y n a m i c t o p o g r a p h y ( J / k g ) o f t h e 150 d b a r s u r f a c e r e l a t i v e t o 8 0 0 d b a r f r o m c r u i s e s o f t h e R V Thomas G. Thompson ( d o t s ) a n d R V Hokusei Maru ( t r i a n - g l e s ) i n J u n e / J u l y 1982 . F r o m R o d e n (1987 ) . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 119 6.3 Data and Methods 6.3.1 The E S C ensemble O f t h e t o t a l o f 102 d r i f t e r s d e p l o y e d , s e v e n ( l i v e s h a l l o w - d r o g u e d a n d t w o d e e p - d r o g u e d ) w e r e o p e r a t i o n a l i n t h e v i c i n i t y o f t h e E S C . F o u r s h a l l o w - d r o g u e d d r i f t e r s d e p l o y e d n e a r t h e K u r i l - K a m c h a t k a T r e n c h o r w i t h i n t h e K u r o s h i o E x t e n s i o n s u r v i v e d l o n g e n o u g h t o c r o s s t h e E S C o n t h e i r e a s t w a r d j o u r n e y a c r o s s t h e m i d - l a t i t u d e N o r t h P a c i f i c . T h r e e o t h e r d r i f t e r s w e r e d e p l o y e d j u s t w e s t o f t h e E S C i n J u n e 1 9 9 2 w i t h t h e i n t e n t i o n o f o b s e r v i n g t h e t o p o g r a p h i c e f fec ts o n t h e n e a r - s u r f a c e c i r c u l a t i o n . T h e a n a l y s i s p r e s e n t e d b e l o w u s e s o n l y t h i s e n s e m b l e o f s e v e n d r i f t e r s . 6.3.2 The rotary multiple filter technique A s s t a t e d i n c h a p t e r 3 , t h e r o t a r y a n a l y s i s o f v e l o c i t y r e c o r d s i s p a r t i c u l a r l y u s e f u l w h e n o n e o r t h e o t h e r r o t a r y c o m p o n e n t d o m i n a t e s t h e e n e r g y s p e c t r u m . T h i s i s c e r t a i n l y t h e c a s e f o r L a g r a n g i a n i n s t r u m e n t s t r a p p e d w i t h i n a m e s o s c a l e e d d y . I n t h e r o t a r y s p e c t r a l a n a l y s i s o f c u r r e n t s ( G o n e l l a , 1972 ; M o o e r s , 1 9 7 3 ) , v e l o c i t y r e c o r d s a r e a n a l y z e d i n t e r m s o f t h e i r f r e q u e n c y - d e p e n d e n t r o t a r y c o m p o n e n t s i n s t e a d o f t h e s t a n d a r d t i m e - d e p e n d e n t C a r t e s i a n c o m p o n e n t s (u(t),v(t)). A F o u r i e r a n a l y s i s w a s p e r f o r m e d o n t h e C a r t e s i a n - f o r m a t t e d d r i f t e r r e c o r d s t o t r a n s f o r m t h e m i n t o c l o c k w i s e a n d c o u n t e r c l o c k - w i s e (u~,u+) r o t a r y c o m p o n e n t s . S p e c t r a l e s t i m a t e s , 5 ( w , u ± ) , w e r e o b t a i n e d f o r e a c h r o t a r y c o m p o n e n t . T o i m p r o v e t h e s p e c t r a l e s t i m a t e s , e a c h d a t a s e g m e n t w a s w e i g h t e d u s i n g a K a i s e r - B e s s e l w i n d o w ( H a r r i s , 1978 ) p r i o r t o t h e F o u r i e r t r a n s f o r m . H a l f - w i n d o w o v e r l a p p i n g w a s p e r f o r m e d t o i n c r e a s e t h e n u m b e r o f d e g r e e s o f f r e e d o m . T h e r o t a r y v a r i a n c e o f t h e c l o c k w i s e a n d c o u n t e r c l o c k w i s e c o m p o n e n t s w a s e s t i m a t e d as Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 120 Var{u-,u+)= f [ % , t t + ) , % , t t - ) ] d w , (6 .1 ) J Au Vartot = Var{u+) + Var{u~), (6 .2 ) w h e r e t h e b a n d w i d t h A u ; e n c o m p a s s e s a s p e c i f i e d r a n g e o f f r e q u e n c i e s . R o t a r y s p e c t r a l c o m p o n e n t s a n d r o t a r y v a r i a n c e s a r e i n v a r i a n t u n d e r c o o r d i n a t e t r a n s f o r m a t i o n , a l l o w i n g f o r a c o m p a r i s o n o f d r i f t e r m o t i o n s f r o m d i f f e r e n t r e g i o n s a n d t i m e p e r i o d s . H o w e v e r , a l t h o u g h t h e y p r o v i d e a f r e q u e n c y - d e p e n d e n t ( 5 , ( a > , t i ± ) ) o r g e n e r a l ( V ^ a r ( , u : t ) ) d e s c r i p t i o n o f c y c l o n i c a n d a n t i c y c l o n i c m o t i o n s , t h e y d o n o t a l l o w f o r a n e x a m i n a t i o n o f e d d y e v o l u t i o n i n t i m e o r s p a c e . A r e c e n t l y e l a b o r a t e d m e t h o d , t h e r o t a r y m u l t i p l e f i l t e r t e c h n i q u e ( R M F T ) ( K u l i k o v a n d R a b i n o v i c h , 1 9 9 8 ) , w a s u s e d t o i n v e s t i g a t e t e m p o r a l v a r i a t i o n s o f t h e c l o c k w i s e a n d c o u n t e r c l o c k w i s e r o t a r y c o m p o n e n t s o f t h e d r i f t e r m o t i o n s a n d e d d y a c t i v i t y i n t h e v i c i n i t y o f t h e E S C . T h e o r i g i n a l m u l t i p l e filter t e c h n i q u e , i n t r o d u c e d b y D z i e w o n s k i e t a l . ( 1 9 6 9 ) t o e x a m i n e s c a l a r s e i s m i c s i g n a l s , i s a p r o c e s s i n g t o o l w h i c h u s e s a set o f n a r r o w - b a n d d i g i t a l f i l t e r s t o e x a m i n e v a r i a t i o n s i n a n i n p u t s i g n a l ' s a m p l i t u d e a n d p h a s e as f u n c t i o n s o f f r e q u e n c y a n d t i m e ( T h o m s o n et a l . , 1997 ; K u l i k o v a n d R a b i n o v i c h , 1 9 9 8 ) . T h e m o d i f i c a t i o n o f t h e m e t h o d f o r v e c t o r p r o c e s s e s e n a b l e s s u c h a n e x a m i n a t i o n f o r t h e r o t a r y c o m p o n e n t s . T h e o r i g i n a l m u l t i p l e f i l t e r t e c h n i q u e h a s b e e n w i d e l y u s e d t o a n a l y z e t s u n a m i w a v e s (e .g . K u l i k o v et a l . , 1 9 9 6 ) , w h i l e t h e R M F T h a s r e c e n t l y b e e n u s e d e f f e c t i v e l y t o q u a n t i f y a w i d e r a n g e o f o c e a n o g r a p h i c p h e n o m e n a o b s e r v e d i n a s i n g l e d r i f t e r t r a j e c t o r y ( T h o m s o n et a l . , 1 9 9 7 ) . N a r r o w - b a n d filters w i t h a G a u s s i a n w i n d o w w e r e a p p l i e d t o i s o l a t e a s p e c i f i c c e n t e r f r e q u e n c y uin = 2irfn: Hn(u) = e - « [ ( « — « ) / ' - n ] a ) (6.3) Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 121 w h e r e t h e p a r a m e t e r a i s u s e d t o c o n t r o l t h e w i d t h o f t h e f i l t e r a n d d e p e n d s o n t h e d i s p e r s i o n c h a r a c t e r i s t i c s o f t h e o r i g i n a l s i g n a l a n d t h e d e g r e e o f f r e q u e n c y o r t i m e r e s o - l u t i o n d e s i r e d ( T h o m s o n et a l . , 1 9 9 7 ) . A h i g h e r v a l u e o f a g i v e s b e t t e r r e s o l u t i o n i n t h e f r e q u e n c y d o m a i n , b u t p o o r e r r e s o l u t i o n i n t h e t i m e d o m a i n . F o r t h e p r e s e n t a n a l y s i s , a v a l u e o f a = 2 5 h a s b e e n u s e d . D e m o d u l a t i o n o f t h e v e c t o r t i m e s e r i e s , V(ujn, t) = u{u>n, t) + iv(ujn, t), ( 6 .4 ) y i e l d s a m a t r i x o f a m p l i t u d e s , w i t h c o l u m n s r e p r e s e n t i n g t i m e a n d r o w s r e p r e s e n t i n g f r e q u e n c y , w h i c h c a n b e c o n t o u r e d t o g i v e a t h r e e - d i m e n s i o n a l p l o t o f t h e c l o c k w i s e a n d c o u n t e r c l o c k w i s e c o m p o n e n t s o f v e l o c i t y as a f u n c t i o n o f f r e q u e n c y a n d t i m e ( t h e f — t d i a g r a m ; T h o m s o n e t a l . , 1 9 9 7 ) . F u r t h e r d e t a i l s o f t h e R M F T c a n b e f o u n d i n E m e r y a n d T h o m s o n ( 1 9 9 7 ) . 6.4 Observations 6.4.1 1991 drifters T w o s e t s o f s a t e l l i t e - t r a c k e d d r i f t e r s d e p l o y e d as p a r t o f t h e S V P i n t h e w e s t e r n N o r t h P a c i f i c c r o s s e d t h e E S C . T h r e e S V P s h a l l o w - d r o g u e d (15 m ) d r i f t e r s , d e p l o y e d n e a r t h e K u r i l I s l a n d s (45°N, 150°E) i n t h e f a l l o f 1990 , a p p r o a c h e d t h e E S C i n t h e s u m m e r o f 1 9 9 1 ( F i g u r e 6 .3 ) . O n a p p r o a c h t o t h e E S C f r o m t h e w e s t , e a c h d r i f t e r w a s d e f l e c t e d n o r t h w a r d n e a r 166°E, a p p r o x i m a t e l y 3 5 0 k m w e s t o f t h e s e a m o u n t s . T h i s m a t c h e s t h e n e a r - s u r f a c e f l o w p a t t e r n d e r i v e d f r o m t h e h y d r o g r a p h i c o b s e r v a t i o n s o f R o d e n a n d T a f t ( 1 9 8 5 ) . E a c h d r i f t e r t h e n a r r i v e d a t t h e E S C n e a r 41°N-42°N a n d w a s d e f l e c t e d a n t i c y c l o n i c a l l y o v e r t h e p e a k o f N i n t o k u S e a m o u n t . D r i f t e r 1314 c r o s s e d n e a r t h e p e a k (170°E t o 171°E) i n 5 d a y s ( f r o m J u n e 30 t o J u l y 4 , 1 9 9 1 ) , w h i l e d r i f t e r s 1 3 1 5 a n d 1 3 1 6 t o o k 10 d a y s ( A u g u s t 4 - 1 4 , 1991 ) a n d 37 d a y s ( J u n e 2 2 t o J u l y 2 9 , 1 9 9 1 ) , r e s p e c t i v e l y , Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 122 t o c r o s s t h e s e a m o u n t s u m m i t . U n f o r t u n a t e l y , d r i f t e r 1316 c e a s e d t r a n s m i s s i o n s o o n a f t e r c r o s s i n g t h e E S C . D r i f t e r s 1 3 1 4 a n d 1 3 1 5 c o n t i n u e d a n e a s t w a r d t r e k , c r o s s i n g t h e d a t e l i n e i n A u g u s t 1 9 9 1 a n d O c t o b e r 1 9 9 1 , r e s p e c t i v e l y ( F i g u r e 6 .3 ) . N o n e o f t h e t h r e e d r i f t e r s w a s r e t a i n e d i n t h e v i c i n i t y o f t h e E S C f o r a n e x t e n d e d p e r i o d o f t i m e , n o r d i d t h e y r e v e a l a n y e d d y a c t i v i t y i n t h e l e e o f t h e s e a m o u n t s . 6 . 4 . 2 1 9 9 2 d r i f t e r s I n c o n t r a s t t o t h e 1 9 9 1 d r i f t e r s , t h r e e d r i f t e r s w h i c h w e r e d e p l o y e d t o g e t h e r j u s t w e s t o f t h e E S C (40°N, 166°E) o n J u n e 2 3 , 1992 w e r e e a c h r e t a i n e d i n t h e v i c i n i t y o f t h e E S C f o r s e v e r a l m o n t h s . T h e s e d r i f t e r s , o n e s t a n d a r d S V P s h a l l o w - d r o g u e d ( 8 0 9 8 ) a n d t w o d e e p - d r o g u e d ( 1 2 0 m ; 1 4 1 7 a n d 4 8 5 9 ) , b e g a n a n a n t i c y c l o n i c m e a n d e r t o w a r d s O j i n / J i n g u S e a m o u n t ( F i g u r e 6 . 4 , 6 . 5 , 6 . 6 ) . D r i f t e r 8 0 9 8 s l o w l y d r i f t e d s o u t h w e s t w a r d a n d w a s r e - t a i n e d i n a c y c l o n i c e d d y n e a r 33°N, 164°E b e f o r e s w e e p i n g a c r o s s K i n m e i S e a m o u n t f r o m t h e s o u t h w e s t i n O c t o b e r 1992 ( F i g u r e 6 .6 ) . I t t h e n m a d e o n e l o o p i n a l a r g e a n - t i c y c l o n i c e d d y a n d t w o l o o p s i n a s m a l l e r a n t i c y c l o n i c e d d y i n t h e g a p b e t w e e n K i n m e i a n d O j i n / J i n g u S e a m o u n t s b e f o r e c o n t i n u i n g e a s t w a r d i n D e c e m b e r . D r i f t e r s 1 4 1 7 a n d 4 8 5 9 , d r o g u e d b e l o w t h e m i x e d l a y e r , a p p r o a c h e d O j i n / J i n g u S e a m o u n t i n e a r l y J u l y 1 9 9 2 , s w e p t t h r o u g h t h e M a i n G a p i n m i d - J u l y a t a n a v e r a g e s p e e d o f 39 c m / s , a n d w e r e s u b s e q u e n t l y r e t a i n e d f o r f o u r a n d t w o m o n t h s , r e s p e c t i v e l y , w i t h i n e d d i e s i n t h e l e e o f O j i n / J i n g u S e a m o u n t ( F i g s . 6 . 4 , 6 . 5 ) . A n e x a m i n a t i o n o f t h e r o t a r y v a r i a n c e ( fo r m o t i o n s w i t h p e r i o d s o f 2 - 6 4 d a y s ) d e r i v e d f r o m t h e 1 9 9 2 d r i f t e r t r a j e c t o r i e s s h o w s t h a t t h e r e w a s c o n s i d e r a b l y m o r e e n e r g y n e a r t h e E S C i n t h e s u m m e r a n d f a l l o f 1992 t h a n i n t h e s u m m e r o f 1 9 9 1 ( T a b l e 6 . 1 ) . T h e s h a l l o w d r i f t e r ( 8 0 9 8 ) , l i k e t h e 1991 d r i f t e r s , h a d n e a r l y e q u a l c o n t r i b u t i o n s f r o m b o t h r o t a r y c o m p o n e n t s , b u t h a d m o r e t h a n t w i c e as m u c h e n e r g y as t h e m o s t e n e r g e t i c 1 9 9 1 Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 123 42°NH 38°NH v 3/26/91 7/27/91 A 34 °N- 3 0 ° N * 5/23/91 1071/91;'' ^1314 1 3 1 5 t l EMPEROR SEAMOUNTS 1991 DRIFTERS (15m) 1 6 0 ° E 1 7 0 ° E 1 8 0 ° 1 1 7 0 ° W F i g u r e 6 .3 : T r a j e c t o r i e s o f s h a l l o w - d r o g u e d d r i f t e r s 1314 ( d o t t e d U n e ) , 1 3 1 5 ( d a s h e d l i n e ) a n d 1 3 1 6 ( d a s h - d o t U n e ) w h i c h c r o s s e d t h e E m p e r o r S e a m o u n t C h a i n i n t h e s u m m e r o f 1 9 9 1 . S e l e c t e d d a t e s a r e m a r k e d b y c r o s s e s . Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 124 160°E 170°E 180° 170°W Figure 6.4: Trajectory of deep-drogued drifter 1417 which crossed the Emperor Seamount Chain in the summer and fall of 1992. Selected dates are marked by crosses, and the anticyclonic (A) and cyclonic (C) eddies are labeled. Boxes outline the immediate vicinity of the E S C , which is shown in Figure 6.8. Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 125 ' i i 1 1 1 1—- 160°E 170°E 180° 170°W F i g u r e 6 . 5 : T r a j e c t o r y o f d e e p - d r o g u e d d r i f t e r 4 8 5 9 w h i c h c r o s s e d t h e E m p e r o r S e a m o u n t C h a i n i n t h e s u m m e r a n d f a l l o f 1 9 9 2 . S e l e c t e d d a t e s a r e m a r k e d b y c r o s s e s . B o x e s o u t l i n e t h e i m m e d i a t e v i c i n i t y o f t h e E S C , w h i c h is s h o w n i n F i g u r e 6 .10 . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 126 F i g u r e 6 .6 : T r a j e c t o r y o f s h a l l o w - d r o g u e d d r i f t e r 8 0 9 8 w h i c h c r o s s e d t h e E m p e r o r S e a m o u n t C h a i n i n t h e s u m m e r a n d f a l l o f 1 9 9 2 . S e l e c t e d d a t e s a r e m a r k e d b y c r o s s e s . B o x e s o u t l i n e t h e i m m e d i a t e v i c i n i t y o f t h e E S C , w h i c h i s s h o w n i n F i g u r e 6 . 1 2 . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 127 d r i f t e r ( 1 3 1 4 ) . S i g n i f i c a n t e n e r g y i n b o t h r o t a r y c o m p o n e n t s w a s p r e s e n t a t f r e q u e n c i e s l e s s t h a n 0 . 2 5 c p d . T h e d e e p d r i f t e r s ( 1 4 1 7 a n d 4 8 5 9 ) w e r e d o m i n a t e d b y c l o c k w i s e a n d c o u n t e r c l o c k w i s e e n e r g y , r e s p e c t i v e l y , r e f l e c t i n g t h e d o m i n a n c e o f t h e e d d i e s s a m p l e d (see d i s c u s s i o n b e l o w ) . M o s t o f t h i s e n e r g y w a s c o n t r i b u t e d a t t h e l o w e s t f r e q u e n c i e s ( b y t h e m e s o s c a l e e d d i e s a t t h e E S C ) , w i t h v e r y l i t t l e e n e r g y b e t w e e n 0 . 2 5 - 0 . 5 c p d . T h e d i s t r i b u t i o n o f e n e r g y d e r i v e d f r o m t h e 1992 d r i f t e r m o t i o n s c a n a l s o b e s e e n i n t h e i r r o t a r y e n e r g y d e n s i t y s p e c t r a ( F i g u r e 6 .7 ) . T h e s p e c t r a o f d r i f t e r s 1 4 1 7 a n d 4 8 5 9 ( F i g s . 6 . 7 a , b ) h a v e a s i m i l a r p a t t e r n , w i t h c l o c k w i s e e n e r g y ( r e p r e s e n t i n g t h e a n t i c y c l o n i c e d d y i n t h e l e e o f t h e O j i n / J i n g u S e a m o u n t a n d t h e l a r g e - s c a l e m e a n d e r s i m m e d i a t e l y w e s t o f t h e E S C ) d o m i n a t i n g t h e l o w e s t f r e q u e n c i e s a n d a s h a r p e n e r g y d r o p - o f f w i t h n e a r l y e q u a l c l o c k w i s e a n d c o u n t e r c l o c k w i s e c o n t r i b u t i o n s a t f r e q u e n c i e s g r e a t e r t h a n 0.1 c p d . T h e d o m i n a n t f e a t u r e i n t h e s p e c t r a o f d r i f t e r 4 8 5 9 i s t h e l a r g e c o u n t e r c l o c k w i s e e n e r g y p e a k , c o n t r i b u t e d b y t h e c y c l o n i c e d d y i n t h e l e e o f O j i n / J i n g u S e a m o u n t , n e a r 0 .08 c p d ( 1 2 . 5 d a y p e r i o d ) . T h e c o u n t e r c l o c k w i s e c o n t r i b u t i o n a t t h i s f r e q u e n c y i s a n o r d e r o f m a g n i t u d e g r e a t e r t h a n t h e c l o c k w i s e c o n t r i b u t i o n . T h e s p e c t r a o f d r i f t e r 8 0 9 8 ( F i g u r e 6 . 7 c ) s h o w t h a t t h e r e w a s s i g n i f i c a n t l y m o r e e n e r g y i n b o t h r o t a r y c o m p o n e n t s a t 15 m t h a n a t 120 m , w i t h n e a r l y a n o r d e r o f m a g n i t u d e m o r e e n e r g y a t f r e q u e n c i e s b e t w e e n 0 . 1 - 0 . 5 c p d . W h i l e t h e l o w e s t f r e q u e n c i e s w e r e n e a r l y r e c t i l i n e a r , t h e 0 . 2 - 0 . 3 c p d f r e q u e n c y r a n g e ( p e r i o d o f 3 - 5 d a y s ) w a s d o m i n a t e d b y c o u n t e r c l o c k w i s e e n e r g y , w h i c h r e p r e s e n t s t h e c y c l o n i c e d d y a c t i v i t y e n c o u n t e r e d b y d r i f t e r 8 0 9 8 w e s t o f t h e E S C . A s i g n i f i c a n t p o r t i o n o f t h e t o t a l e n e r g y c o n t a i n e d i n t h e 1992 d r i f t e r t r a j e c t o r i e s w a s c o n t r i b u t e d b y m e s o s c a l e e d d i e s e n c o u n t e r e d o v e r o r o n t h e l e e s i d e o f t h e E S C . A c l o s e r e x a m i n a t i o n o f t h e 1992 d r i f t e r s i n t h e i m m e d i a t e v i c i n i t y o f t h e E S C i s t h e r e f o r e i n o r d e r . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 128 F r e q u e n c y ( c p d ) 1314 1316 1417 4859 8098 5"H (%) 0 . 0 1 5 - 0 .08 6 9 . 3 (12 .5 ) 6 4 . 3 (39 .5 ) 4 1 0 . 2 ( 36 .1 ) 2 5 7 . 8 ( 29 .3 ) 4 6 2 . 5 ( 36 .1 ) 0 . 0 8 - 0 . 2 5 2 2 8 . 4 (41 .1 ) 23 .1 (14 .2 ) 3 2 . 5 (4 .7 ) 2 8 . 1 (3 .2 ) 1 5 3 . 2 ( 1 2 . 0 ) 0 . 2 5 - 0 .5 12 .5 (2 .3 ) 3 .4 (2 .1) 0 .6 (0 .1 ) 0 .5 (0 .1 ) 6 .2 (0 .5 ) t o t a l 3 1 0 . 2 (55 .8 ) 9 0 . 8 (55 .8 ) 4 4 3 . 3 (64 .2 ) 2 8 6 . 4 ( 32 .5 ) 6 2 1 . 9 ( 48 .6 ) s+H (%) 0 . 0 1 5 - 0 .08 1 5 9 . 4 (28 .7 ) 6 2 . 6 (38 .4 ) 2 1 5 . 0 ( 31 .1 ) 5 2 2 . 8 ( 59 .3 ) 4 5 1 . 7 ( 35 .3 ) 0 .08 - 0 . 2 5 7 5 . 5 (13 .6 ) 8 .0 (4 .9 ) 3 1 . 5 (4 .6 ) 7 1 . 4 (8 .1 ) 1 9 0 . 3 ( 14 .9 ) 0 . 2 5 - 0 .5 11 .1 (2 .0 ) 1.5 (0 .9) 0 .4 (0 .1 ) 0 .5 (0 .1 ) 1 6 . 5 (1 .3 ) t o t a l 2 4 6 . 0 (44 .2 ) 72 .1 (44 .2 ) 2 4 6 . 9 (35 .8 ) 5 9 4 . 7 ( 67 .5 ) 6 5 8 . 5 ( 5 1 . 4 ) StoM (%) 0 . 0 1 5 - 0 .08 2 2 8 . 7 (41 .1 ) 126 .9 (77 .9 ) 6 2 5 . 2 (90 .6 ) 7 8 0 . 6 ( 8 8 . 6 ) 9 1 4 . 2 ( 71 .4 ) 0 .08 - 0 . 2 5 3 0 3 . 9 (54 .6 ) 31 .1 (19 .1 ) 6 4 . 0 (9 .3 ) 9 9 . 5 ( 1 1 . 3 ) 3 4 3 . 5 ( 26 .8 ) 0 . 2 5 - 0 .5 2 3 . 6 (4 .3 ) 4 .9 (3 .0) 1.0 (0 .2 ) 1.0 (0 .1 ) 2 2 . 7 (1 .8 ) t o t a l 5 5 6 . 2 (100 ) 162 .9 (100) 6 9 0 . 2 (100 ) 8 8 1 . 1 ( 100 ) 1 2 8 0 . 4 ( 100 ) T a b l e 6 . 1 : C l o c k w i s e (S~(u)), c o u n t e r c l o c k w i s e ( S + ( u ; ) ) a n d t o t a l (Stot(w)) r o t a r y v a r i - a n c e (cm21s2) i n t h r e e f r e q u e n c y b a n d s d e r i v e d f r o m t h e d r i f t e r t r a j e c t o r i e s i n t h e v i c i n i t y o f t h e E m p e r o r S e a m o u n t C h a i n . N u m b e r s i n p a r e n t h e s e s r e f e r t o p e r c e n t o f t o t a l v a r i - a n c e . E s t i m a t e s w e r e d e r i v e d f r o m t h e p e r i o d s M a y 2 3 t o J u l y 2 7 , 1 9 9 1 ( 64 d a y s ) f o r d r i f t e r 1 3 1 4 , M a r c h 26 t o A u g u s t 1, 1991 (128 d a y s ) f o r d r i f t e r 1 3 1 6 , a n d J u n e 2 3 t o D e c e m b e r 2 3 , 1992 (180 d a y s ) f o r d r i f t e r s 1 4 1 7 , 4 8 5 9 , a n d 8 0 9 8 . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 129 Frequency (cpd) F i g u r e 6 . 7 : T o t a l Stot ( s o l i d l i n e ) , c l o c k w i s e S ( l o n g - d a s h e d l i n e ) , a n d c o u n t e r c l o c k w i s e S+ ( s h o r t - d a s h e d l i n e ) r o t a r y e n e r g y d e n s i t y s p e c t r a (cm2/s2/cpd) d e r i v e d f r o m t h e t r a - j e c t o r i e s o f d r i f t e r s (a) 1 4 1 7 , (b ) 4 8 5 9 , a n d (c ) 8 0 9 8 f o r t h e p e r i o d J u n e 2 3 t o D e c e m b e r 2 3 , 1 9 9 2 . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 130 Drifter 1417 A f t e r d r i f t e r 1 4 1 7 m e a n d e r e d a n t i c y c l o n i c a l l y t h r o u g h t h e M a i n G a p , i t w a s r e t a i n e d i n a c y c l o n i c e d d y ( e d d y C ) i n t h e l e e o f O j i n / J i n g u S e a m o u n t ( t o i t s r i g h t , f a c i n g d o w n s t r e a m ) f o r 31 d a y s ( F i g u r e 6 .8 ) . I t m a d e 2 c o m p l e t e l o o p s i n e d d y C ( T a b l e 6 . 2 ) , t h e f i r s t i n 11 d a y s a t a r a d i u s o f n e a r l y 50 k m ( m e a n r o t a t i o n a l s p e e d , o f 2 8 . 5 c m / s ) a n d t h e s e c o n d i n 20 d a y s a t a r a d i u s o f a p p r o x i m a t e l y 40 k m ( m e a n r o t a t i o n a l s p e e d o f 15 c m / s ) . D r i f t e r 1 4 1 7 w a s v e r y c l o s e t o t h e u n n a m e d s e a m o u n t a t 36.3°N, 171.8°E as i t s l o w e d d o w n d u r i n g t h e s e c o n d l o o p , a n d m a y h a v e b e e n t r a c i n g o u t t h e o u t e r e d g e o f t h e e d d y . D r i f t e r 1 4 1 7 t h e n e x i t e d e d d y C a n d c r o s s e d o v e r t h e t o p o f O j i n / J i n g u S e a m o u n t i n S e p t e m b e r b e f o r e b e i n g r e t a i n e d i n a l a r g e r a n t i c y c l o n i c e d d y ( e d d y A ) , a g a i n i n t h e l e e o f O j i n / J i n g u S e a m o u n t ( t o i t s l e f t , f a c i n g d o w n s t r e a m ) . A f t e r m a k i n g o n e c o m p l e t e c i r c u i t o f e d d y A i n 26 d a y s , a t a r a d i u s o f a p p r o x i m a t e l y 100 k m ( m e a n r o t a t i o n a l s p e e d o f 29 c m / s ) , i t w a s r e t a i n e d i n a n o t h e r , m u c h s m a l l e r a n t i c y c l o n i c e d d y a t t h e s o u t h e r n t i p o f N i n t o k u S e a m o u n t (40°N, 170°E) b e f o r e l e a v i n g t h e r e g i o n i n m i d - D e c e m b e r . D r i f t e r 1 4 1 7 m a d e 2 l o o p s o v e r a 2 4 - d a y p e r i o d i n t h e l a t t e r e d d y , w h i c h h a d a d i a m e t e r o f a p p r o x i m a t e l y 35 k m a n d m e a n r o t a t i o n a l s p e e d o f 11 c m / s . T h e m e s o s c a l e e d d y a c t i v i t y d e l i n e a t e d b y d r i f t e r 1 4 1 7 i s r e a d i l y s e e n i n i t s / — t d i a g r a m s , w h i c h a l l o w f o r a m o r e d e t a i l e d e x a m i n a t i o n o f c y c l o n i c a n d a n t i c y c l o n i c f e a t u r e s i n t i m e a n d f r e q u e n c y s p a c e ( F i g s . 6 . 9 a , b ) . T h e f i r s t 40 d a y s ( J u n e 2 3 t o A u g u s t 2 , 1992 ) a r e d o m i n a t e d b y c l o c k w i s e e n e r g y c e n t e r e d a t a p e r i o d o f a b o u t 2 5 d a y s , w h i c h r e p r e s e n t s t h e l a r g e a n t i c y c l o n i c m e a n d e r j u s t w e s t o f t h e E S C a n d t h r o u g h t h e M a i n G a p . D r i f t e r 1 4 1 7 t h e n e n t e r e d e d d y C , a n d h a d s t r o n g c o u n t e r c l o c k w i s e e n e r g y c e n t e r e d f i r s t n e a r a p e r i o d o f 1 1 - 1 2 d a y s ( n e a r d a y 4 5 ; l o o p 1) a n d t h e n c e n t e r e d n e a r a p e r i o d o f 1 8 - 2 2 d a y s ( b e t w e e n d a y s 6 0 - 7 0 ; l o o p 2 ) . A b r o a d b a n d o f l o w - f r e q u e n c y c l o c k w i s e e n e r g y , c e n t e r e d a t a p e r i o d o f 1 6 - 2 6 d a y s , c a n b e s e e n a f t e r d r i f t e r 1 4 1 7 l e f t Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 131 1 6 6 ° E 1 6 8 ° E 1 7 0 ° E 1 7 2 ° E 1 7 4 ° E F i g u r e 6 .8 : T r a j e c t o r y o f d r i f t e r 1417 i n t h e v i c i n i t y o f O j i n / J i n g u a n d K i n m e i S e a m o u n t s i n t h e s u m m e r a n d f a l l o f 1992 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d . T h e a n t i c y c l o n i c ( A ) a n d c y c l o n i c ( C ) e d d i e s a r e l a b e l e d . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 132 L o o p P e r i o d ( d a y s ) D i a m e t e r ( k m ) S p e e d ( c m / s ) Drifter 1411 1 11 9 4 . 2 2 8 . 5 2 20 8 3 . 4 15 .2 Drifter 4859 1 12 7 1 . 7 2 1 . 7 2 12 7 7 . 3 2 3 . 4 3 15 6 0 . 7 1 4 . 7 4 13 8 8 . 3 2 4 . 7 5 10 6 6 . 2 2 4 . 1 T a b l e 6 . 2 : P e r i o d , d i a m e t e r , a n d m e a n r o t a t i o n a l s p e e d o f t h e c y c l o n i c e d d y ( e d d y C ) as d e l i n e a t e d b y t h e t r a j e c t o r i e s o f d r i f t e r s 1 4 1 7 a n d 4 8 5 9 . e d d y C a n d m a d e a l a r g e l o o p i n e d d y A ( d a y s 8 0 - 1 0 0 ) . T h e s m a l l a n t i c y c l o n i c e d d y a t t h e s o u t h e r n t i p o f N i n t o k u S e a m o u n t s h o w s u p as a w e a k c l o c k w i s e e n e r g y p e a k c e n t e r e d a t a p e r i o d o f 1 0 - 1 2 d a y s ( d a y s 1 4 5 - 1 7 0 ) . D r i f t e r 1 4 1 7 t h e n l o o p e d s o u t h w a r d o u t o f t h e r e g i o n , t o w a r d s H e s s R i s e , w i t h m o d e r a t e e n e r g y i n b o t h r o t a r y c o m p o n e n t s . D r i f t e r 4 8 5 9 T h e t r a j e c t o r y o f d r i f t e r 4 8 5 9 w a s a l m o s t c o i n c i d e n t w i t h t h a t o f d r i f t e r 1 4 1 7 as i t s p e d t h r o u g h t h e M a i n G a p , m a d e a l a r g e a n t i c y c l o n i c l o o p i n t h e r e g i o n w h e r e 1 4 1 7 w a s l a t e r r e t a i n e d i n e d d y A , a n d f i n a l l y w a s t r a p p e d w i t h i n e d d y C ( F i g u r e 6 . 1 0 ) . D r i f t e r 4 8 5 9 m a d e a n i n i t i a l l o o p i n 12 d a y s a t a r a d i u s o f a p p r o x i m a t e l y 36 k m ( m e a n r o t a t i o n a l s p e e d o f 2 2 c m / s ) , s o m e 1 0 - 1 5 k m c l o s e r t o t h e e d d y c e n t e r t h a n d r i f t e r 1 4 1 7 w a s . I t w a s s u b s e q u e n t l y r e t a i n e d i n e d d y C f o r a n o t h e r 50 d a y s , m a k i n g 4 m o r e c o m p l e t e c i r c u i t s w i t h p e r i o d s r a n g i n g f r o m 10 t o 15 d a y s a t r a d i i o f 30 t o 4 5 k m ( m e a n r o t a t i o n a l s p e e d s o f 1 5 - 2 5 c m / s ; T a b l e 6 .2 ) . F i g u r e 6 . 1 1 a a g a i n s h o w s t h e t r a j e c t o r y o f d r i f t e r 4 8 5 9 w i t h i n e d d y C , w i t h e a c h Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 133 t i m e ( d a y s ) 0 40 80 120 160 ' ' ' ' i i ' i t i m e ( d a y s ) 0 40 80 120 160 ' ' 40 80 120 160 t i m e ( d a y s ) 40 80 120 160 t i m e ( d a y s ) F i g u r e 6 .9 : A m p l i t u d e e v o l u t i o n o f t h e c l o c k w i s e a n d c o u n t e r c o c k w i s e r o t a r y v e l o c i t y c o m p o n e n t s f o r d r i f t e r s ( a , b ) 1 4 1 7 , ( c , d ) 4 8 5 9 , a n d (e , f ) 8 0 9 8 f o r t h e p e r i o d J u n e 2 3 t o D e c e m b e r 2 3 , 1 9 9 2 . A m p l i t u d e c o n t o u r s a r e g i v e n i n c m / s . R o t a r y c o m p o n e n t s f o r t h e l o w - f r e q u e n c y r a n g e a r e s h o w n ( l o g ( - 0 . 6 ) = p e r i o d o f 4 d a y s , l o g ( - l . O ) = p e r i o d o f 10 d a y s , l o g ( - 1 . 4 ) = p e r i o d o f 25 d a y s ) . T h e d a s h e d l i n e n e a r t h e b o t t o m o f e a c h p l o t i n d i c a t e s t h e t i m e p e r i o d t h a t t h e d r i f t e r w a s i n t h e v i c i n i t y o f t h e E S C ( b e t w e e n 166°E a n d 174°E). Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 134 F i g u r e 6 . 1 0 : T r a j e c t o r y o f d r i f t e r 4 8 5 9 i n t h e v i c i n i t y o f O j i n / J i n g u a n d K i n m e i S e a m o u n t s i n t h e s u m m e r a n d f a l l o f 1 9 9 2 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 135 l o o p p l o t t e d u n i q u e l y a n d w i t h t h e a p p r o x i m a t e e d d y c e n t e r (as d e t e r m i n e d f r o m t h e d r i f t e r t r a c k ) m a r k e d . A t i m e se r i es o f t h e r o t a t i o n a l s p e e d o f e d d y C d e r i v e d f r o m t h e t r a j e c t o r y o f d r i f t e r 4 8 5 9 is g i v e n i n F i g u r e 6 . 1 1 b . S p e e d s r a n g e d f r o m 2 5 - 6 0 c m / s d u r i n g t h e first t w o l o o p s , w i t h f a s t e r s p e e d s a t t h e n o r t h e r n e n d o f t h e e d d y a n d s l o w e r s p e e d s a t t h e s o u t h e r n e n d . T h e s a m e p a t t e r n i s s e e n i n l o o p s 3 a n d 4 , b u t p e a k s p e e d s a t t h e n o r t h e r n e n d w e r e o n l y 40 c m / s . D r i f t e r 4 8 5 9 t h e n a c c e l e r a t e d n o r t h w a r d ( s p e e d > 50 c m / s ) i n e a r l y O c t o b e r , o u t o f e d d y C , a f t e r h a v i n g s l o w e d t o l e s s t h a n 10 c m / s a t t h e s o u t h e r n e n d o f l o o p 5. W h e n d r i f t e r 1 4 1 7 w a s a t t h e s o u t h e r n t i p o f i t s s e c o n d l o o p i n e d d y C ( A u g u s t 2 9 ; F i g u r e 6 . 8 ) , d r i f t e r 4 8 5 9 w a s a t t h e n o r t h e r n t i p o f i t s s e c o n d l o o p ( F i g u r e 6 . 1 0 ) . I f t h e s e t w o p o s i t i o n s d e f i n e t h e n o r t h e r n a n d s o u t h e r n b o u n d a r i e s o f e d d y C , i t s n o r t h - s o u t h e x t e n t w a s a p p r o x i m a t e l y 2 0 0 k m , h i g h l y e l o n g a t e d p a r a l l e l t o t h e a x i s o f t h e s e a m o u n t s . A f t e r A u g u s t 2 9 , d r i f t e r 1 4 1 7 a c c e l e r a t e d n o r t h w a r d a n d l e f t e d d y C as d r i f t e r 4 8 5 9 m o v e d s o u t h w a r d i n i t s t h i r d l o o p , o n t h e w e s t e r n s i d e o f e d d y C a n d p r e s u m a b l y c l o s e r t o t h e e d d y c e n t e r . D r i f t e r 4 8 5 9 w a s t h e n r e t a i n e d w i t h i n e d d y C f o r a n o t h e r 2 3 d a y s , t h r o u g h l o o p s 4 a n d 5 , b e f o r e l e a v i n g t h e e d d y i n e a r l y O c t o b e r a n d l o o p i n g o v e r O j i n / J i n g u S e a m o u n t ( F i g u r e 6 . 1 0 ) . I t t h e n m a d e a s m a l l , s l o w a n t i c y c l o n i c l o o p i n t h e g a p b e t w e e n t h e O j i n / J i n g u a n d K i n m e i S e a m o u n t s b e f o r e a g a i n c r o s s i n g o v e r t h e t o p o f O j i n / J i n g u S e a m o u n t , l o o p i n g a n t i c y c l o n i c a l l y i n t h e p r e v i o u s l o c a t i o n o f e d d y A , a n d finally l e a v i n g t h e r e g i o n i n l a t e N o v e m b e r . T h e / — t d i a g r a m s f o r d r i f t e r 4 8 5 9 ( F i g s . 6 . 9 c , d ) a r e c o m p a r a b l e t o t h o s e o f d r i f t e r 1 4 1 7 f o r t h e f i r s t 40 d a y s , w h e n b o t h d r i f t e r s w e r e n e a r l y c o i n c i d e n t . T h e r e s t o f t h e s i x - m o n t h p e r i o d , h o w e v e r , i s d o m i n a t e d b y v e r y s t r o n g c o u n t e r c l o c k w i s e e n e r g y c e n t e r e d n e a r a p e r i o d o f 12 d a y s b e t w e e n d a y s 3 5 - 1 0 0 , w h i c h i s e d d y C ( F i g u r e 6 . 9 d ) . A s s e e n i n t h e r o t a r y s p e c t r a , e d d y C c o n t r i b u t e d a s i g n i f i c a n t c o u n t e r c l o c k w i s e e n e r g y p e a k a t t h i s p e r i o d ( F i g u r e 6 . 7 b ) . T h e r e w a s a l s o e n e r g y i n t h e c l o c k w i s e c o m p o n e n t a t a p e r i o d Chapter 6. Eddies h Seamount-attached Eddies at the Emperor Seamount Chain 136 NI 1 1 1 i 1 1 1 r 170°E 171°E 172°E 173°E 174°E F i g u r e 6 . 1 1 : (a ) T r a j e c t o r y o f d r i f t e r 4 8 5 9 i n t h e c y c l o n i c e d d y ( e d d y C ) i n t h e l e e o f O j i n / J i n g u S e a m o u n t b e t w e e n A u g u s t 4 a n d O c t o b e r 7, 1 9 9 2 . E a c h d i f f e r e n t l i n e t y p e r e p r e s e n t s a s e p a r a t e l o o p a r o u n d t h e e d d y . T h e a p p r o x i m a t e c e n t e r o f t h e e d d y , as d e t e r m i n e d b y t h e d r i f t e r t r a c k , i s m a r k e d w i t h a n o c t a g o n f o r e a c h l o o p , ( b ) T i m e s e r i e s o f r o t a t i o n a l s p e e d d e r i v e d f r o m d r i f t e r 4 8 5 9 i n e d d y C . E a c h o f t h e f i v e l o o p s o f e d d y C i s m a r k e d a t b o t t o m . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 1 3 7 o f a b o u t 12 d a y s d u r i n g t h i s t i m e ( F i g u r e 6 . 9 c ) , d e m o n s t r a t i n g t h e e l l i p t i c a l s t r u c t u r e o f e d d y C . C l o c k w i s e e n e r g y , c e n t e r e d a t a p e r i o d o f a b o u t 10 d a y s , t h e n i n c r e a s e d as d r i f t e r 4 8 5 9 l o o p e d o v e r O j i n / J i n g u S e a m o u n t a n d o u t o f t h e r e g i o n ( a f t e r d a y 1 5 0 ) . D u r i n g t h e p e r i o d t h a t d r i f t e r 4 8 5 9 w a s r e t a i n e d i n e d d y C , t h e e d d y m o v e d a p p r o x - i m a t e l y 150 k m i n a b o u t 60 d a y s t o t h e w e s t - s o u t h w e s t ( 2 4 0 T ) , a t a s p e e d o f 2 .9 c m / s ( F i g u r e 6 . 1 1 a ) . T h i s i s c o m p a r a b l e t o t h e m o t i o n o f t h e a n t i c y c l o n i c e d d i e s ( R i c h a r d - s o n , 1 9 8 0 ) o b s e r v e d i n t h e l e e o f t h e C o r n e r R i s e S e a m o u n t s , w h i c h t r a n s l a t e d t o t h e s o u t h w e s t , i n t h e d i r e c t i o n o f t h e p r e v a i l i n g b a c k g r o u n d flow, a t 4 - 5 c m / s . T h e o b s e r v e d m o t i o n i s a l s o c o n s i s t e n t w i t h s e l f - p r o p u l s i o n d u e t o t h e (3 - e f f e c t , w h i c h p r e d i c t s a w e s t - w a r d p r o p a g a t i o n o f u — —(3Qr2, w h e r e f30 i s t h e r o t a t i o n p a r a m e t e r (2 x 1 0 - 1 1 m - 1 s - 1 a t m i d - l a t i t u d e s ) a n d r i s t h e e d d y r a d i u s ( c f , M c W i l l i a m s a n d F l i e r l , 1 9 7 9 ) . F o r a n e d d y r a d i u s o f 4 1 . 5 k m ( t a k i n g t h e s e c o n d l o o p o f d r i f t e r 1 4 1 7 t o b e t h e m a x i m u m e a s t - w e s t e x t e n t o f e d d y C ) , t h e p r e d i c t e d s p e e d i s 3 .4 c m / s . A l t e r n a t i v e l y , e d d y C m a y h a v e s i m p l y b e e n a d v e c t e d w e s t w a r d b y a c o n v e r g e n t flow b e t w e e n e d d i e s A a n d C . T h e s h a p e o f e d d y C , as d e l i n e a t e d b y t h e t r a j e c t o r y o f d r i f t e r 4 8 5 9 , c h a n g e d c o n - s i d e r a b l y as t h e e d d y a p p r o a c h e d a n d c r o s s e d t h e s o u t h e r n e n d o f O j i n / J i n g u S e a m o u n t a n d t h e u n n a m e d s e a m o u n t a t 36.8°N, 171.3°E ( F i g s . 6 . 10 a n d 6 . 1 1 a ) . T h e t r a j e c t o r y t r a c e d o u t a m o r e c i r c u l a r p a t h t h r o u g h l o o p 4 , s u g g e s t i n g t h a t t h e b u l k o f e d d y C w a s s q u e e z i n g t h r o u g h t h e g a p b e t w e e n t h e s e a m o u n t s . T h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h p r e v i o u s o b s e r v a t i o n s o f m e d d y i n t e r a c t i o n s w i t h s e a m o u n t s i n t h e N o r t h A t l a n t i c . I n o n e c a s e , a m e d d y ( t r a c k e d w i t h S O F A R floats) w a s a p p a r e n t l y d e s t r o y e d u p o n c o l l i d i n g h e a d - o n w i t h H y e r e s S e a m o u n t ( R i c h a r d s o n et a l . , 1 9 8 9 ) , w h i l e i n a n o t h e r c a s e , a m e d d y s q u e e z e d t h r o u g h a g a p b e t w e e n H y e r e s a n d I r v i n g S e a m o u n t s , a l t e r i n g i t s s h a p e b u t m a i n t a i n i n g d y n a m i c a l s t a b i l i t y as a c o h e r e n t s t r u c t u r e ( S h a p i r o e t a l . , 1 9 9 2 ) . S c h u l t z T o k o s et a l . ( 1 9 9 4 ) , t r a c k i n g a m e d d y w i t h R A F O S floats, s p e c u l a t e d t h a t t h e J o s e p h i n e S e a m o u n t a c t e d as a w e d g e t o " b r e a k o f f t h e o u t e r p i e c e s " o f t h e m e d d y as i t s i d e - s w i p e d Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 138 t h e s e a m o u n t . E d d y C h a d a p p a r e n t l y d i s s i p a t e d b y t h e e n d o f O c t o b e r , w h e n d r i f t e r 4 8 5 9 m a d e t h e s m a l l a n t i c y c l o n i c l o o p i n t h e l o c a t i o n (37°N, 170°E) w h e r e e d d y C w o u l d h a v e b e e n e x p e c t e d f r o m i t s o b s e r v e d t r a n s l a t i o n . I t i s r e a s o n a b l e t o a s s u m e t h a t e d d y C d e c a y e d f r o m t h e b o t t o m u p b y f r i c t i o n as i t c r o s s e d o v e r t h e a x i s o f t h e E S C . T h e f a s t s p e e d s o f d r i f t e r 4 8 5 9 ( 57 c m / s ) o n i t s n o r t h w a r d t r a n s e c t o f l o o p 5 m a y b e a n i n d i c a t i o n o f t h i s d e c a y , as a n o r t h w a r d j e t - l i k e s q u i r t o f f l u i d l e a v i n g e d d y C w o u l d b e e x p e c t e d as i t r u n s i n t o t h e s e a m o u n t ( N o f , 1 9 8 8 ) . D r i f t e r 8 0 9 8 A l t h o u g h r e l e a s e d s i m u l t a n e o u s l y w i t h d r i f t e r s 1 4 1 7 a n d 4 8 5 9 , t h e s h a l l o w - d r o g u e d d r i f t e r 8 0 9 8 d i v e r g e d f r o m t h e i r m e a n d e r i n g p a t h t o w a r d s t h e E S C w i t h i n s e v e r a l d a y s ( F i g u r e 6 . 1 2 ) . A f t e r a s l o w d r i f t t o t h e s o u t h w e s t a n d r e t e n t i o n i n t w o c y c l o n i c e d d i e s n e a r 33°N, 164°E, d r i f t e r 8 0 9 8 a p p r o a c h e d K i n m e i S e a m o u n t f r o m t h e s o u t h w e s t i n e a r l y O c t o b e r , j u s t as d r i f t e r 4 8 5 9 w a s l e a v i n g e d d y C ( F i g u r e 6 . 1 0 ) . I t s l o w e d as i t c r o s s e d o v e r t h e t o p o f K i n m e i S e a m o u n t , s t a y i n g o v e r t h e s u m m i t f o r n e a r l y a w e e k . I t t h e n m a d e o n e l o o p i n a l a r g e a n t i c y c l o n i c e d d y s i t u a t e d o n t o p o f t h e u n n a m e d s e a m o u n t s a t 36.3°N, 171.8°E a n d 36.8°N, 171.3°E ( O c t o b e r 18 t o N o v e m b e r 10 ) , t h e n t w o l o o p s i n a s m a l l e r a n t i c y c l o n i c e d d y j u s t s o u t h o f O j i n / J i n g u S e a m o u n t ( N o v e m b e r 1 1 - 3 0 ) b e f o r e c o n t i n u i n g e a s t w a r d i n e a r l y D e c e m b e r . T h e e d d i e s w e r e a p p r o x i m a t e l y 175 k m a n d 90 k m i n d i a m e t e r , a n d b o t h h a d r o t a t i o n a l s p e e d s o f 2 7 - 2 8 c m / s . I t i s i n t e r e s t i n g t o n o t e t h a t t h e s m a l l e r e d d y w a s t h e s a m e s i z e a n d i n t h e s a m e l o c a t i o n as e d d y C a p p r o x i m a t e l y s i x w e e k s a f t e r e d d y C h a d a p p a r e n t l y d i s s i p a t e d . T h u s , i t a p p e a r s t h a t a c y c l o n i c e d d y ( e d d y C ) e x i s t e d i n t h e v i c i n i t y o f O j i n / J i n g u S e a m o u n t f o r a t l e a s t t w o m o n t h s a n d , u p o n d i s s i p a t i o n , w a s r e p l a c e d b y a n e d d y o f s i m i l a r d i m e n s i o n s b u t o p p o s i t e r o t a t i o n . I t i s u n c l e a r , h o w e v e r , w h e t h e r t h e a n t i c y c l o n i c e d d y r e v e a l e d b y d r i f t e r 8 0 9 8 h a d a s i g n a t u r e t h a t e x t e n d e d b e l o w t h e m i x e d l a y e r , w h e r e e d d y C w a s o b s e r v e d , o r w h e n o r w h e r e i t Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 139 m a y h a v e o r i g i n a t e d . T h e e x t e n s i v e e d d y a c t i v i t y r e v e a l e d b y t h e t r a j e c t o r y o f d r i f t e r 8 0 9 8 c a n a l s o b e s e e n i n i t s / — t d i a g r a m s ( F i g s . 6 . 9 e , f ) . S t r o n g c o u n t e r c l o c k w i s e ( a n d w e a k e r c l o c k w i s e ) e n e r g y c e n t e r e d f i r s t n e a r a p e r i o d o f 8 - 1 0 d a y s ( n e a r d a y 65 ) a n d t h e n a t l e s s t h a n 4 d a y s ( n e a r d a y 75) r e f l e c t t h e c y c l o n i c e d d i e s e n c o u n t e r e d n e a r 33°N, 164°E. T h i s e d d y a c t i v i t y s h o w s u p as s m a l l p e a k s i n t h e c o u n t e r c l o c k w i s e e n e r g y d e n s i t y s p e c t r u m ( F i g u r e 6 . 7 c ) . T h e t w o a n t i c y c l o n i c e d d i e s l o c a t e d i n t h e g a p b e t w e e n O j i n / J i n g u a n d K i n m e i S e a m o u n t s c a n b e s e e n as c l o c k w i s e e n e r g y p e a k s w i t h p e r i o d s o f 2 5 - 3 0 d a y s ( b e t w e e n d a y s 9 0 - 1 2 0 ) a n d 1 0 - 1 3 d a y s ( b e t w e e n d a y s 1 2 0 - 1 5 0 ) . D r i f t e r 8 0 9 8 t h e n m a d e a c y c l o n i c m e a n d e r o u t o f t h e r e g i o n a n d t o w a r d s H e s s R i s e , p r o d u c i n g a n o t h e r c o u n t e r c l o c k w i s e e n e r g y p e a k c e n t e r e d n e a r a p e r i o d o f 12 d a y s . I n g e n e r a l , d r i f t e r 8 0 9 8 , d r o g u e d a t 15 m , r e v e a l e d m o r e l o w - f r e q u e n c y e n e r g y i n b o t h r o t a r y c o m p o n e n t s t h a n d i d t h e d e e p - d r o g u e d d r i f t e r s . 6 . 4 . 3 1 9 9 3 d r i f t e r A n o t h e r s h a l l o w - d r o g u e d d r i f t e r ( 4856 ) w a s d e p l o y e d i n A u g u s t 1992 w i t h i n t h e K u r o s h i o E x t e n s i o n , a t 40°N, 153°E. I t s u b s e q u e n t l y d r i f t e d e a s t w a r d a t s p e e d s o f 2 5 - 3 0 c m / s , a p - p r o a c h i n g t h e r e g i o n o f t h e E S C a l o n g 39°N i n l a t e F e b r u a r y 1 9 9 3 ( F i g u r e 6 . 1 3 ) . A t a b o u t 100 k m f r o m t h e M a i n G a p , d r i f t e r 4 8 5 6 s l o w e d d o w n a n d w a s d e f l e c t e d f i r s t s o u t h e a s t - w a r d t o w a r d s O j i n / J i n g u S e a m o u n t t h e n n o r t h w a r d t o w a r d s N i n t o k u S e a m o u n t . I t m a d e a s l o w a n t i c y c l o n i c d e f l e c t i o n w i t h i n t h e M a i n G a p a n d o v e r t h e s o u t h e r n t i p o f N i n t o k u S e a m o u n t , s t a y i n g o v e r t h e E S C (170°E t o 171°E) f o r 9 d a y s . I t t h e n a c c e l e r a t e d e a s t - w a r d , o u t o f t h e r e g i o n , i n l a t e M a r c h 1993 . A s w i t h t h e s h a l l o w - d r o g u e d d r i f t e r s i n t h e s u m m e r o f 1 9 9 1 , t h i s d r i f t e r w a s d e c e l e r a t e d a n d d e f l e c t e d a n t i c y c l o n i c a l l y o v e r t h e s o u t h e r n t i p o f N i n t o k u S e a m o u n t , b u t s h o w e d n o e v i d e n c e o f e d d y a c t i v i t y i n t h e w a k e o f t h e E S C . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 140 166°E 168°E 170°E 172°E 174°E F i g u r e 6 . 1 2 : T r a j e c t o r y o f d r i f t e r 8 0 9 8 i n t h e v i c i n i t y o f O j i n / J i n g u a n d K i n m e i S e a m o u n t s i n t h e s u m m e r a n d f a l l o f 1992 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d . Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 141 F i g u r e 6 . 1 3 : T r a j e c t o r y o f s h a l l o w - d r o g u e d d r i f t e r 4 8 5 6 , w h i c h c r o s s e d t h e E m p e r o r S e a m o u n t C h a i n i n t h e w i n t e r o f 1993 . E a c h m a r k r e p r e s e n t s t h e d a i l y d r i f t e r p o s i t i o n a t 1 2 0 0 Z , a n d s e l e c t e d d a t e s a r e l a b e l e d . Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 142 6 . 4 . 4 S a t e l l i t e a l t i m e t r y U n f o r t u n a t e l y , n o h y d r o g r a p h i c s t a t i o n s w e r e o c c u p i e d o r X B T s c a s t i n t h e v i c i n i t y o f t h e E S C i n t h e s u m m e r o f 1 9 9 2 . H o w e v e r , t h e T O P E X s a t e l l i t e a l t i m e t e r w a s l a u n c h e d i n A u g u s t 1 9 9 2 , a n d w a s p r o v i d i n g r e l i a b l e s e a s u r f a c e h e i g h t d a t a b y O c t o b e r . T h e s e d a t a w e r e t h e n b l e n d e d w i t h d a t a f r o m t h e E R S - 1 a l t i m e t e r , w h i c h w e r e c o r r e c t e d u s i n g c r o s s o v e r d i f f e r e n c e d e t r e n d i n g ( D . C h a r t , p e r s . c o m m . ) T h e a b s o l u t e s e a s u r f a c e t o p o g - r a p h y w a s e s t i m a t e d r e l a t i v e t o t h e J o i n t G r a v i t y M o d e l - 3 ( J G M - 3 ) g e o i d ( e .g . D e s a i a n d W a h r , 1 9 9 5 ) . S t a n d a r d g e o p h y s i c a l d a t a r e c o r d c o r r e c t i o n s w e r e a p p l i e d ( w e t a n d d r y t r o p o s p h e r i c r a n g e d e l a y s , i o n o s p h e r i c d e l a y , s e a s t a t e b i a s ) , a n d t i d a l c o r r e c t i o n s w e r e c o m p u t e d u s i n g t h e t i d e m o d e l o f D e s a i a n d W a h r ( 1 9 9 5 ) . A c c u r a c y i s b e t t e r t h a n 5 c m p o i n t w i s e ( J . H e n d r i c k s , p e r s . c o m m . ) . F i g u r e 6 . 1 4 a s h o w s t h e T O P E X / E R S - 1 b l e n d e d s e a s u r f a c e h e i g h t a n o m a l i e s f o r t h e N o r t h P a c i f i c b a s i n (20°N-55°N, 150°E -130°W) f o r T O P E X c y c l e 2 ( O c t o b e r 3 - 1 2 , 1 9 9 2 ) . T h e s e a r e a n o m a l i e s f r o m t h e t w o - y e a r 1 9 9 3 - 9 4 m e a n s e a s u r f a c e h e i g h t f i e l d , d e r i v e d f r o m T O P E X , a n d i n t e r p o l a t e d t o a 1/4° r e s o l u t i o n g l o b a l g r i d . T h e r e w a s v e r y l i t t l e e d d y e n e r g y a n y w h e r e i n t h e b a s i n e x c e p t i n t h e v i c i n - i t y o f t h e K u r o s h i o E x t e n s i o n (150°E-170°E n e a r 35°N) a n d i n t h e r e g i o n j u s t e a s t o f t h e E S C . N o e v i d e n c e o f t h e K u r o s h i o E x t e n s i o n o r s e a m o u n t w a k e e f f e c t s c a n b e d i s c e r n e d e a s t o f t h e d a t e l i n e . A m o r e e x t e n s i v e a n a l y s i s o f T O P E X d a t a f o r t h e N o r t h P a c i f i c h a s c o n c l u d e d t h a t t h e E S C is a r e g i o n o f h i g h e d d y e n e r g y ( T h u r s t o n , 1 9 9 5 ) . T h i s y e a r - l o n g a n a l y s i s ( F e b r u a r y 1 9 9 3 - M a r c h 1994 ) f o u n d t h a t t h e m a j o r i t y o f m e s o s c a l e v a r i a b i l i t y w a s c o n t a i n e d i n t h e r e g i o n b e t w e e n t h e s o u t h e r n p o r t i o n o f t h e E S C a n d H e s s R i s e , w h e r e f o u r a n t i c y c l o n i c e d d i e s w e r e i d e n t i f i e d a n d t r a c k e d f o r 8 0 - 1 2 0 d a y s . T h e 1 0 - d a y t r a c k s ( O c t o b e r 3 - 1 2 ) o f t h e 1992 d r i f t e r s a r e a l s o p l o t t e d o n t h e s e a s u r f a c e h e i g h t a n o m a l y field ( F i g u r e 6 . 1 4 b ) . T h e r e w a s a p r o n o u n c e d " d e p r e s s i o n " o f a b o u t 20 c m i n t h e s a m e l o c a t i o n w h e r e d r i f t e r 4 8 5 9 h a d b e e n i n e d d y C ( l o o p 5 ; see Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 143 F i g u r e 6 . 1 0 ) , a n d a " h i l l " o f a b o u t 50 c m i n t h e l o c a t i o n w h e r e d r i f t e r 1 4 1 7 h a d b e e n i n e d d y A ( s o u t h w e s t p o r t i o n o f l o o p ; see F i g u r e 6 .8 ) . I t i s c l e a r t h a t t h e e d d i e s d e l i n e a t e d b y t h e d r i f t e r s a t 120 m h a d a s u r f a c e m a n i f e s t a t i o n as w e l l , w h i c h m a y h a v e b e e n m a i n t a i n e d a f t e r t h e d e c a y o f t h e e d d i e s a t d e p t h . B o t h f e a t u r e s w e r e s t i l l d i s c e r n a b l e i n t h e s e a s u r f a c e h e i g h t a n o m a l y f i e l d i n e a r l y N o v e m b e r , a l t h o u g h t h e a n o m a l i e s w e r e c o n s i d e r a b l y w e a k e r . D r i f t e r 8 0 9 8 , i n f a c t , r e v e a l e d a n a n t i c y c l o n i c e d d y a t 15 m n e a r t h e p r i o r l o c a t i o n o f e d d y C b y m i d - N o v e m b e r . T O P E X c y c l e s 17 ( F e b r u a r y 28 - M a r c h 10 , 1993 ) a n d 18 ( M a r c h 1 0 - 2 0 , 1 9 9 3 ) r e v e a l e d o n l y w e a k s e a s u r f a c e h e i g h t a n o m a l i e s n e a r t h e E S C b e t w e e n 39°N-41°N ( n o t s h o w n ) , c o r r o b o r a t i n g t h e l a c k o f e d d y a c t i v i t y e n c o u n t e r e d b y d r i f t e r 4 8 5 6 . 6 . 5 D i s c u s s i o n 6 . 5 . 1 C o m p a r i s o n w i t h m o d e l s E a c h o f t h e d r i f t e r s d e p l o y e d i n 1 9 9 2 w e r e r e t a i n e d i n t h e v i c i n i t y o f t h e E S C f o r n e a r l y 5 m o n t h s , w i t h i n e d d i e s t h a t w e r e a p p a r e n t l y a t t a c h e d t o t h e l e e s i d e o f O j i n / J i n g u S e a m o u n t . W h y w e r e l e e s i d e e d d i e s o b s e r v e d i n 1992 b u t n o t i n 1 9 9 1 o r 1 9 9 3 , a n d d o t h e d r i f t e r o b s e r v a t i o n s m a t c h t h e p r e d i c t i o n s o f l a b o r a t o r y a n d n u m e r i c a l m o d e l s o f f l o w p a s t i s o l a t e d t o p o g r a p h y o r s e a m o u n t c h a i n s ? Z h a n g a n d B o y e r ( 1 9 9 1 ) d e m o n s t r a t e d t h a t f o r t w o s e a m o u n t s s e p a r a t e d b y a t l e a s t o n e o b s t a c l e d i a m e t e r , t h e f l o w i m p i n g i n g o n e a c h c a n b e c o n s i d e r e d t o b e i n t e r a c t i n g w i t h i s o l a t e d t o p o g r a p h y . T h i s i s a p p r o x i m a t e l y t h e c a s e f o r t h e u p p e r - l e v e l flow a p p r o a c h i n g N i n t o k u a n d O j i n / J i n g u S e a m o u n t s (see F i g u r e 6.1 i n s e t ) . A b y s s a l c u r r e n t s i n c i d e n t u p o n t h e E S C , h o w e v e r , w i l l b e i n t e r a c t i n g w i t h a s e a m o u n t c h a i n . A l t h o u g h n o n l i n e a r i n t e r a c t i o n s r e s u l t i n g f r o m flow a r o u n d n e i g h b o r i n g s e a m o u n t s i n t h e u p p e r l e v e l s a r e l i k e l y i m p o r t a n t , i t w i l l b e a s s u m e d t h a t N i n t o k u a n d O j i n / J i n g u a r e i s o l a t e d s e a m o u n t s Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 144 a 150'E 160'E 17CTE 180-E 170'W 160'W ISO'W 140'W 130"W 150'E 160'E 170'E 180'E 170'W 160'W 150"W 140'W 130'W 166'E 167"E 168'E 169"E 170'E 171-E 172-E 173'E 174"E 166"E 167'E 168*E 169'E 170'E 17VE 172"E 173*E 174"E F i g u r e 6.14: (a ) M a p o f s e a s u r f a c e h e i g h t a n o m a l i e s ( c m ) f r o m m e r g e d T O P E X / E R S - 1 a l t i m e t r y f o r T O P E X c y c l e 2 ( O c t o b e r 3 - 1 2 , 1992 ) f o r t h e N o r t h P a c i f i c b a s i n f r o m 20°JV-55°iV a n d 150°E-130°W. T h e a x i s o f t h e E m p e r o r S e a m o u n t C h a i n i s m a r k e d w i t h t h e d o u b l e - d a s h e d l i n e , ( b ) A c l o s e - u p o f t h e b o x e d r e g i o n a r o u n d t h e E S C f r o m T O P E X c y c l e 2 , w i t h t h e 1 0 - d a y t r a j e c t o r i e s ( O c t o b e r 3 -12 ) o f d r i f t e r s 1 4 1 7 ( s q u a r e s ) , 4 8 5 9 ( s t a r s ) , a n d 8 0 9 8 ( o c t a g o n s ) i n c l u d e d a n d t h e a n t i c y c l o n i c ( A ) a n d c y c l o n i c ( C ) e d d i e s r e v e a l e d b y t h e t r a c k s o f d r i f t e r s 1 4 1 7 a n d 4 8 5 9 l a b e l e d . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 145 f o r t h e s a k e o f e a s y c o m p a r i s o n w i t h p r i o r t h e o r e t i c a l a n d m o d e l i n g w o r k . L a b o r a t o r y a n d n u m e r i c a l m o d e l s h a v e d e t e r m i n e d t h a t t h e p r i m a r y m e c h a n i s m s w h i c h d e t e r m i n e t h e f l o w p a t t e r n p a s t i s o l a t e d t o p o g r a p h y a r e t h e s t r e n g t h a n d t i m e d e p e n d e n c e o f t h e b a c k g r o u n d f l o w , t h e s t r a t i f i c a t i o n o f t h e w a t e r c o l u m n , t h e o b s t a c l e g e o m e t r y a n d b o t t o m f r i c t i o n . F o r e x a m p l e , t h e l a b o r a t o r y e x p e r i m e n t s o f B o y e r a n d Z h a n g ( 1 9 9 0 b ) f o u n d t h r e e d i s t i n c t flow r e g i m e s , ( i ) f u l l y a t t a c h e d flow, ( i i ) a t t a c h e d l e e s i d e e d d i e s , a n d ( i i i ) e d d y s h e d d i n g , w h i c h d e p e n d o n t h e s e m e c h a n i s m s . T h e r e l a t i v e i m p o r t a n c e o f e a c h m e c h a n i s m c a n b e q u a n t i f i e d w i t h t h e n o n - d i m e n s i o n a l p a r a m e t e r s t h e R o s s b y n u m b e r (R0), w h i c h i s t h e r a t i o o f t h e a d v e c t i v e t o t h e C o r i o l i s t e r m s i n t h e e q u a t i o n o f m o t i o n , t h e h o r i z o n t a l E k m a n n u m b e r (Ek), w h i c h i s t h e r a t i o o f t h e f r i c t i o n a l t o t h e C o r i o l i s t e r m s , a n d t h e B u r g e r n u m b e r (S), w h i c h i s t h e r a t i o o f a m e a s u r e m e n t o f s t r a t i f i c a t i o n t o t h e C o r i o l i s t e r m : R °~ fD' E« = T£F. (6-5) NH fD' w h e r e U0 i s t h e m e a n b a c k g r o u n d flow, / = 2Q,sin<b i s t h e C o r i o l i s p a r a m e t e r , w h e r e Q, i s t h e E a r t h ' s r o t a t i o n r a t e a n d (b i s l a t i t u d e , D i s t h e s e a m o u n t l e n g t h s c a l e ( d i a m e t e r ) , v i s t h e v i s c o s i t y , TV is t h e B r u n t - V a i s a l a f r e q u e n c y , a n d H i s t h e m e a n w a t e r d e p t h . I n p a r t i c u l a r , i t w a s f o u n d t h a t t h e r e s u l t i n g flow r e g i m e w a s m o s t s e n s i t i v e t o v a r i a t i o n o f t h e R o s s b y n u m b e r ( B o y e r a n d Z h a n g , 1 9 9 0 a , b ) . F o r s u b i n e r t i a l flow w i t h R o s s b y n u m b e r s o n t h e o r d e r 0 . 0 1 - 0 . 1 , t h e l a b o r a t o r y m o d e l s p r e d i c t a t t a c h e d l e e s i d e e d d i e s , w i t h a n a n t i c y c l o n i c e d d y o n t h e l e f t a n d a c y c l o n i c e d d y o n t h e r i g h t , f a c i n g d o w n s t r e a m ( e . g . , B o y e r a n d Z h a n g , 1 9 9 0 b , t h e i r F i g u r e 1 0 C ) . T h e n u m e r i c a l m o d e l s o f K o z l o v ( 1 9 8 1 ) , J o h n s o n ( 1982 ) a n d V e r r o n a n d L e P r o v o s t Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 146 ( 1 9 8 5 ) d e m o n s t r a t e d t h a t a n o t h e r r e l e v a n t q u a n t i t y f o r p r e d i c t i n g t h e s t e a d y - s t a t e q u a s i - g e o s t r o p h i c flow p a t t e r n o v e r s e a m o u n t s i s t h e b l o c k i n g p a r a m e t e r , HU0 R0 w h e r e h i s t h e o b s t a c l e h e i g h t a n d a = h/H i s t h e s e a m o u n t f r a c t i o n a l h e i g h t . B o t h N i n t o k u a n d O j i n / J i n g u S e a m o u n t s r i s e t o a b o u t 1000 m f r o m a w a t e r d e p t h o f a p p r o x - i m a t e l y 6 0 0 0 m , t h u s a ~ 0 . 8 3 . K o z l o v ( 1981 ) o b t a i n e d a t r a p p e d c y c l o n i c e d d y t o t h e r i g h t o f t h e o b s t a c l e ( l o o k i n g d o w n s t r e a m ) f o r B > 0(1), w h i l e n o e d d y w a s f o r m e d f o r B < 1. V e r r o n a n d L e P r o v o s t ( 1985 ) o b t a i n e d a t t a c h e d l e e s i d e e d d i e s a t a " m o d e r a t e " b l o c k i n g p a r a m e t e r o f 15 .6 ( t h e i r F i g u r e 5 ) . T o e s t i m a t e t h e r e l e v a n t n o n d i m e n s i o n a l p a r a m e t e r s f o r t h e o b s e r v e d flow, a s e g - m e n t o f e a c h d r i f t e r ' s t r a j e c t o r y w h i c h r e p r e s e n t e d e a s t w a r d flow i n c i d e n t u p o n t h e E S C w a s s e l e c t e d . T h e d a t e s c h o s e n t o r e p r e s e n t e a c h d r i f t e r ' s m e a n b a c k g r o u n d flow a r e g i v e n i n T a b l e 6 . 3 , a l o n g w i t h t h e c o m p u t e d m e a n s p e e d s a n d v a l u e s o f t h e C o r i o l i s p a r a m e t e r , t h e R o s s b y a n d h o r i z o n t a l E k m a n n u m b e r s , a n d t h e b l o c k i n g p a r a m e t e r . T h e s e a m o u n t l e n g t h s c a l e s w e r e t a k e n t o b e 50 k m a n d 100 k m f o r t h e N i n t o k u a n d O j i n / J i n g u S e a m o u n t s , r e s p e c t i v e l y . T h e v i s c o s i t y w a s t a k e n t o b e t h e h o r i z o n t a l e d d y d i f f u s i v i t y , KH- A l t h o u g h KH c a n v a r y b y s e v e r a l o r d e r s o f m a g n i t u d e i n t h e o c e a n ( P o n d a n d P i c k a r d , 1 9 8 3 ) , a v a l u e o f 4 x 1 0 r cm2/s h a s b e e n u s e d , w h i c h i s c o n s i s t e n t w i t h t h e e s t i m a t e s p r e s e n t e d i n C h a p t e r 5 . T h e 15 m flow i m p i n g i n g o n N i n t o k u S e a m o u n t h a d R o s s b y n u m b e r s r a n g i n g f r o m 0 . 0 3 t o 0 .11 i n t h e s u m m e r o f 1991 a n d 0 .06 i n t h e w i n t e r o f 1 9 9 3 ( T a b l e 6 .3 ) . T h e R o s s b y n u m b e r f o r t h e 120 m flow a t O j i n / J i n g u S e a m o u n t i n t h e s u m m e r o f 1 9 9 2 w a s l o w e r , 0 . 0 2 . I n a l l c a s e s , t h e flow i n c i d e n t u p o n t h e E S C w a s w e a k l y n o n l i n e a r . T h u s , t h e o b s e r v a t i o n s o f a t t a c h e d e d d i e s i n t h e l e e o f O j i n / J i n g u S e a m o u n t i n 1 9 9 2 m a t c h t h e p r e d i c t i o n s o f Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 1 4 7 D r i f t e r D a t e s U0 ( c m / s ) f ( 1 0 - 4 s-1) D ( k m ) R0 Ek B ta ( d a y s ) 1991 1 3 1 4 6 / 1 9 - 6 / 3 0 3 2 . 8 0 .954 50 0 .07 0 . 0 1 7 1 2 . 1 2 1.76 1 3 1 5 7 / 2 7 - 8 / 3 5 0 . 9 0 .964 50 0 .11 0 . 0 1 7 7 .89 1.14. 1 3 1 6 5 / 3 1 - 6 / 2 1 14 .8 0 . 9 7 3 50 0 . 0 3 0 . 0 1 6 2 7 . 3 9 3 .91 1992 1 4 1 7 7 / 9 - 7 / 1 3 2 2 . 0 0 .895 100 0 . 0 2 0 . 0 0 4 3 3 . 9 0 5 .26 4 8 5 9 7 / 9 - 7 / 1 3 2 2 . 0 0 .895 100 0 . 0 2 0 . 0 0 4 3 3 . 9 0 5 .26 1993 4 8 5 6 2 / 2 2 - 3 / 5 2 8 . 6 0 .919 50 0 .06 0 . 0 1 7 1 3 . 3 3 2 . 0 2 T a b l e 6 . 3 : P a r a m e t e r s o f f l o w i n c i d e n t o n t h e E m p e r o r S e a m o u n t C h a i n as d e r i v e d f r o m t h e d r i f t e r t r a j e c t o r i e s a n d a s s u m i n g a h o r i z o n t a l e d d y d i f f u s i v i t y o f 4 x 1 0 r cm2/s a n d a s e a m o u n t f r a c t i o n a l h e i g h t o f 0 . 8 3 . l a b o r a t o r y a n d n u m e r i c a l m o d e l s , w h i c h h a v e a t t a c h e d l e e s i d e e d d i e s f o r R o s s b y n u m b e r s b e t w e e n 0 . 0 1 - 0 . 1 . H i g h e r R o s s b y n u m b e r s g e n e r a l l y c o r r e s p o n d e d t o t h e e d d y - s h e d d i n g r e g i m e o f t h e l a b o r a t o r y m o d e l s . N o e d d i e s c a n b e s e e n i n t h e t r a j e c t o r i e s o f d r i f t e r s 1 3 1 4 , 1 3 1 5 o r 4 8 5 6 d o w n s t r e a m o r i n t h e v i c i n i t y o f t h e E S C , h o w e v e r , s u g g e s t i n g t h a t e d d i e s h a v i n g a n e a r - s u r f a c e m a n i f e s t a t i o n w e r e n o t b e i n g g e n e r a t e d a t t h e t i m e s o f d r i f t e r p a s s a g e i n t h e s u m m e r o f 1991 o r l a t e w i n t e r o f 1 9 9 3 . T h i s c o u l d b e d u e t o t h e d i f f e r e n t s e a m o u n t g e o m e t r y o f t h e N i n t o k u a n d O j i n / J i n g u S e a m o u n t s , o r s i m p l y t h a t t o p o g r a p h i c a l l y - g e n e r a t e d e d d y a c t i v i t y i n t h i s r e g i o n m a y h a v e b e e n p r e s e n t o n l y a t g r e a t e r d e p t h s , a n d n o t e x t e n d i n g i n t o t h e m i x e d l a y e r . T h e m i x e d - l a y e r d e p t h i n t h e N o r t h P a c i f i c r a r e l y e x c e e d s 100 m , a n d i n s u m m e r is m u c h s h a l l o w e r ( P i c k a r d a n d E m e r y , 1 9 9 0 ) . I t is e x p e c t e d t h a t i n a s t r a t i f i e d o c e a n , s e a m o u n t e f f e c t s w i l l b e b o t t o m - i n t e n s i f i e d ( R o d e n , 1 9 9 1 ) . A d d r e s s i n g t h i s q u e s t i o n w o u l d r e q u i r e k n o w l e d g e o f t h e s t r a t i f i c a t i o n o f t h e w a t e r c o l u m n a b o v e t h e s e a m o u n t s u m m i t s . T h e l a c k o f h y d r o g r a p h i c o b s e r v a t i o n s c o n c u r r e n t Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 148 w i t h t h e d r i f t e r o b s e r v a t i o n s , h o w e v e r , p r e c l u d e s a n y c a l c u l a t i o n s o f t h e B u r g e r n u m b e r f o r t h e o b s e r v e d f l o w s , o r o f t h e v e r t i c a l t r a p p i n g s c a l e , hf = ^f, ( 6 -7 ) w h i c h is a m e a s u r e o f t h e v e r t i c a l e x t e n t a b o v e t h e b o t t o m t o w h i c h s e a m o u n t e f f e c t s c a n b e f e l t ( F r e e l a n d , 1 9 9 4 ) . H o w e v e r , a n e s t i m a t e o f t h e s e p a r a m e t e r s c a n b e m a d e u s i n g t h e v a l u e s o f B r u n t - V a i s a l a f r e q u e n c y g i v e n i n E m e r y e t a l . ( 1 9 8 4 ) , w h i c h w e r e d e r i v e d f r o m h i s t o r i c a l h y d r o g r a p h i c d a t a i n 5° s q u a r e s f o r t h e N o r t h A t l a n t i c a n d N o r t h P a c i f i c . F r o m t h e s e d a t a , t h e m e a n d e p t h - a v e r a g e d ( u p p e r 1000 m ) v a l u e o f N i n t h e v i c i n i t y o f t h e E S C i s a p p r o x i m a t e l y 0 . 0 0 5 s - 1 i n s u m m e r a n d 0 . 0 0 3 s _ 1 i n w i n t e r , y i e l d i n g a B u r g e r n u m b e r o f 0 ( 1 ) f o r t h e flow i n c i d e n t o n t h e E S C a n d s u g g e s t i n g t h a t s t r a t i f i c a t i o n i s a n i m p o r t a n t e f f e c t h e r e . C o m p a r i n g t h e c r o s s i n g l a t i t u d e s o f t h e 1991 a n d 1 9 9 2 d r i f t e r s , s e a m o u n t e f f e c t s w o u l d b e e x p e c t e d t o r e a c h t h e 15 m l e v e l o v e r N i n t o k u S e a m o u n t ( s u m m i t d e p t h a p p r o x i m a t e l y 1000 m , d i a m e t e r 50 k m ) i f N < 0 . 0 0 5 s - 1 , w h i l e a b o v e O j i n / J i n g u S e a m o u n t ( s u m m i t d e p t h a p p r o x i m a t e l y 8 0 0 m , d i a m e t e r 100 k m ) s e a m o u n t e f f e c t s w o u l d b e f e l t a t 15 m i f N < 0 .011 s~\ a n d a t 120 m i f N < 0 . 0 1 3 a ' 1 . T h u s , a l t h o u g h d r i f t e r 8 0 9 8 d e m o n s t r a t e d t h a t e d d y a c t i v i t y a t t h e E S C c a n b e p r e s e n t n e a r t h e s u r f a c e , i t w o u l d n o t b e e n t i r e l y s u r p r i s i n g i f t h e 15 m flow w a s o f t e n d e c o u p l e d f r o m t h e t o p o g r a p h i c e f f e c t s , a t l e a s t o v e r N i n t o k u S e a m o u n t . D r i f t e r 8 0 9 8 c r o s s e d o v e r K i n m e i S e a m o u n t , w h o s e s u m m i t a p p r o a c h e s 100 m o f t h e o c e a n s u r f a c e . T h e i m p l i c a t i o n i s t h a t e d d y f o r m a t i o n w i t h i n t h e m i x e d l a y e r n e a r t h e E S C m a y b e c o n f i n e d t o t h e r e g i o n a r o u n d t h e O j i n / J i n g u a n d K i n m e i S e a m o u n t s . D r i f t e r s 1314 a n d 1315 ( i n 1991) a n d d r i f t e r 4 8 5 6 ( i n 1 9 9 3 ) , a l l o f w h i c h c r o s s e d t h e s u m m i t o f N i n t o k u S e a m o u n t r e l a t i v e l y q u i c k l y , h a d b l o c k i n g p a r a m e t e r s o n t h e o r d e r o f 8 - 1 3 ( T a b l e 6 .3 ) . D r i f t e r 1 3 1 6 , w h i c h t o o k o v e r a m o n t h t o c r o s s N i n t o k u S e a m o u n t , h a d Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 149 a b l o c k i n g p a r a m e t e r o f 2 7 , w h i l e t h e t w o d r i f t e r s r e t a i n e d i n t h e l e e s i d e e d d i e s ( 1 4 1 7 a n d 4 8 5 9 ) h a d a b l o c k i n g p a r a m e t e r o f 34 . T h e d r i f t e r o b s e r v a t i o n s a r e a g a i n c o n s i s t e n t w i t h t h e m o d e l s , a l t h o u g h p a r t i c l e r e t e n t i o n i n t h e w a k e o f t h e E S C a p p e a r s t o b e l i m i t e d t o h i g h e r b l o c k i n g p a r a m e t e r s t h a n s u g g e s t e d b y V e r r o n a n d L e P r o v o s t ( 1 9 8 5 ) . T h i s d i s c r e p a n c y i s p r o b a b l y d u e t o s t r a t i f i c a t i o n e f f e c t s , w h i c h w e r e i g n o r e d i n t h e i r m o d e l . T h e l o w v a l u e s o f t h e h o r i z o n t a l E k m a n n u m b e r ( 0 . 0 1 7 f o r t h e 1 9 9 1 a n d 1 9 9 3 b a c k - g r o u n d f l o w s a n d 0 . 0 0 4 i n 1992 ) s u g g e s t t h a t f r i c t i o n a l e f f e c t s i n t h e u p p e r l a y e r s a r e n o t s i g n i f i c a n t . T h i s i s e x p e c t e d t o b e t h e c a s e , c o n s i d e r i n g t h e l o n g l i v e s o f t h e o b s e r v e d e d d i e s . T h e m a g n i t u d e o f t h e h o r i z o n t a l e d d y d i f f u s i v i t y , h o w e v e r , w a s p r e s c r i b e d f r o m t h e r e s u l t s o f a n e a r l i e r s t u d y i n t h e n o r t h e a s t P a c i f i c O c e a n . I t s t r u e v a l u e i n t h e v i c i n i t y o f t h e E S C i s u n k n o w n . 6 . 5 . 2 E d d y d i m e n s i o n s C o m p a r i s o n b e t w e e n o b s e r v a t i o n s a n d t h e o r y c a n a l s o b e m a d e w i t h r e g a r d t o t h e t e m - p o r a l a n d s p a t i a l s c a l e s o f t h e a t t a c h e d e d d i e s . T h e d o m i n a n t t i m e s c a l e f o r m e s o s c a l e f l o w o v e r t o p o g r a p h y is t h e a d v e c t i v e t i m e s c a l e , « . = • £ (6 .8 ) w h i c h i s a l s o g i v e n i n T a b l e 6 .3 f o r t h e o b s e r v e d flows. A g a i n , t h e l o n g e r a d v e c t i v e t i m e s c a l e o c c u r s f o r t h e d r i f t e r s w h i c h w e r e r e t a i n e d f o r l o n g e r t h a n a m o n t h i n t h e v i c i n i t y o f t h e E S C . A r e c e n t n u m e r i c a l m o d e l o f t h e o c e a n r e s p o n s e o v e r a n i s o l a t e d s e a m o u n t i n d u c e d b y t h e c o m b i n a t i o n o f s l o w s t e a d y i n f l o w a n d w e a k d i u r n a l t i d e s ( G o l d n e r a n d C h a p m a n , 1997 ) h a s s h o w n t h a t p a r t i c l e s l o c a t e d a t m i d - d e p t h a n d s h a l l o w e r a r e r e t a i n e d f o r u p t o 5 a d v e c t i v e t i m e s c a l e s . D r i f t e r s 1 4 1 7 a n d 4 8 5 9 d e m o n s t r a t e t h a t p a r t i c l e s b e l o w t h e m i x e d l a y e r n e a r t h e E S C c a n b e r e t a i n e d f o r a t l e a s t u p t o 12 a d v e c t i v e t i m e s c a l e s Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 150 (62 d a y s f o r d r i f t e r 4 8 5 9 ) . T h e e v o l u t i o n i n t i m e o f t h e w a k e f l o w b e h i n d t h e s e a m o u n t s , h o w e v e r , i s r e l a t e d t o t h e v a r y i n g i n c i d e n t f l o w as w e l l as n o n l i n e a r v o r t i c i t y i n t e r a c t i o n ( V e r r o n a n d L e P r o v o s t , 1 9 8 5 ) , n e i t h e r o f w h i c h c a n b e e x a m i n e d w i t h t h e p r e s e n t d a t a s e t . T h e d i m e n s i o n s o f t h e a t t a c h e d l e e s i d e e d d i e s o b s e r v e d i n 1 9 9 2 a r e o n t h e s a m e o r d e r as t h e s e a m o u n t d i a m e t e r ( a b o u t 100 k m ) . T h i s i s c o m p a r a b l e t o t h e t o p o g r a p h i c a l l y - g e n e r a t e d e d d i e s o b s e r v e d b y R i c h a r d s o n ( 1 9 8 0 ) , P a t t i a r a t c h i e t a l . ( 1 9 8 7 ) , a n d H e y w o o d e t a l . ( 1 9 9 0 , 1 9 9 6 ) . T h e e d d i e s r e v e a l e d b y t h e d r i f t e r s d o w n s t r e a m o f t h e C o r n e r R i s e S e a m o u n t s ( w h i c h h a v e a d i a m e t e r o f r o u g h l y 150 k m a n d a f r a c t i o n a l h e i g h t o f 0 . 8 , s i m i l a r t o O j i n / J i n g u S e a m o u n t ) h a d d i a m e t e r s o f a p p r o x i m a t e l y 2 0 0 k m , s l i g h t l y l a r g e r t h a n t h e o b s t a c l e d i a m e t e r ( R i c h a r d s o n , 1 9 8 0 ) . T h e o b s e r v e d a n d m o d e l e d e d d i e s s h e d f r o m t h e t r o p i c a l i s l a n d o f A l d a b r a a r e a l s o r o u g h l y t h e s a m e s i z e as t h e i s l a n d ( H e y w o o d e t a l . , 1 9 9 0 , 1 9 9 6 ) . T h e s e o b s e r v a t i o n s d i f f e r f r o m t h e p r e d i c t i o n s o f t h e l a b o r a t o r y m o d e l o f B o y e r a n d Z h a n g ( 1 9 9 0 b ) , w h i c h h a v e t h e " l e e s i d e b u b b l e r e g i o n " ( t h e l e n g t h s c a l e o f t h e a t t a c h e d e d d i e s ) o n t h e o r d e r o f o n e - t h i r d t h e s e a m o u n t d i a m e t e r f o r a R o s s b y n u m b e r o n t h e o r d e r o f 0 . 0 2 . I t s h o u l d b e k e p t i n m i n d t h a t t h e p a r a m e t e r s u s e d f o r t h e c o m p a r i s o n b e t w e e n t h e o b s e r v a t i o n s a n d t h e m o d e l p r e d i c t i o n s ( T a b l e 6 .3) w e r e e s t i m a t e d f r o m t h e d r i f t e r t r a j e c t o r i e s a t t h e i r a p p r o a c h t o t h e E S C . T h e 1992 d r i f t e r s r e m a i n e d i n t h e r e g i o n f o r n e a r l y s i x m o n t h s , h o w e v e r , d u r i n g w h i c h t i m e t h e b a c k g r o u n d f l o w i n c i d e n t u p o n t h e E S C w a s u n m e a s u r e d . T h u s , c a u t i o n s h o u l d b e a p p l i e d i n m a k i n g c o n c l u s i o n s b a s e d o n t h e s e c o m p a r i s o n s . Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 151 6.6 Conclusions S a t e l l i t e - t r a c k e d d r i f t e r s w e r e u s e d t o i n v e s t i g a t e e d d y a c t i v i t y i n t h e v i c i n i t y o f t h e E m p e r o r S e a m o u n t C h a i n d u r i n g 1 9 9 1 - 1 9 9 3 . A p a i r o f c o u n t e r - r o t a t i n g m e s o s c a l e e d - d i e s w e r e o b s e r v e d a t t a c h e d t o t h e l e e s i d e o f O j i n / J i n g u S e a m o u n t i n t h e s u m m e r o f 1 9 9 2 . T h i s i s o n e o f t h e f i r s t o b s e r v a t i o n s d e m o n s t r a t i n g a n e x t e n d e d a t t a c h m e n t ( > 6 0 d a y s ) o f a t o p o g r a p h i c a l l y - g e n e r a t e d e d d y t o a s e a m o u n t . T h e flow i n c i d e n t o n t h e s e a m o u n t h a d a l o w R o s s b y n u m b e r (0 .02 ) a n d a h i g h b l o c k i n g p a r a m e t e r ( 3 4 ) . R e s u l t s o f l a b o r a t o r y a n d n u m e r i c a l m o d e l i n g p r e d i c t c o u n t e r - r o t a t i n g , a t t a c h e d l e e s i d e e d d i e s f o r flow o v e r i s o l a t e d t o p o g r a p h y w i t h p a r a m e t e r s e n v e l o p i n g t h o s e c a l c u l a t e d f o r t h e 1 9 9 2 flow. T h u s , a l t h o u g h t h e d r i f t e r o b s e r v a t i o n s a r e f e w a n d t e m p o r a l l y l i m i t e d , t h e y m a t c h t h e p r e d i c t i o n s a n d s u g g e s t t h a t t h e e x i s t i n g m o d e l s a r e a d d r e s s i n g t h e d o m i n a n t d y n a m i c a l c o n s t r a i n t s q u i t e w e l l . T h e l a c k o f e d d i e s o b s e r v e d i n 1 9 9 1 a n d 1 9 9 3 , e v e n t h o u g h t h e i n c i d e n t R o s s b y n u m b e r s w e r e w i t h i n t h e r a n g e o f t h e a t t a c h e d l e e s i d e e d d y o r e d d y - s h e d d i n g r e g i m e s o f t h e m o d e l s , w a s m o s t l i k e l y d u e t o a p a r t i a l d e c o u p l i n g , d u e t o s t r a t i f i c a t i o n , o f t h e t o p o g r a p h i c i n f l u e n c e o f N i n t o k u S e a m o u n t ( s u m m i t d e p t h n e a r 1 0 0 0 m ) o n t h e 15 m flow. M i x e d - l a y e r e d d y a c t i v i t y a t t h e E S C w a s p r e s e n t o v e r t h e t a l l e r O j i n / J i n g u a n d K i n m e i S e a m o u n t s . T h e flow d e p i c t e d b y t h e s a t e l l i t e - t r a c k e d d r i f t e r s i n t h e s u m m e r o f 1 9 9 2 - a m e a n d e r - i n g c u r r e n t a p p r o a c h i n g t h e E S C a t s p e e d s o f 2 0 - 2 5 c m / s - is c o n s i s t e n t w i t h h i s t o r i c a l o b s e r v a t i o n s f r o m t h e r e g i o n . T h u s , i t c a n b e e x p e c t e d t h a t e d d i e s w i t h s i z e s o n t h e o r d e r 100 k m , e x t e n d i n g f r o m t h e s u r f a c e t o w e l l b e l o w t h e m i x e d l a y e r , h a v i n g l i f e t i m e s o f w e e k s t o m o n t h s , a n d a t t a c h e d t o t h e lee s i d e o f t h e s e a m o u n t s m a y b e a c o m m o n p h e n o m e n o n . T h e E S C , a t l e a s t a t i t s s o u t h e r n e n d , m a y i n d e e d b e a n " e d d y g e n e r a t o r " ( R o d e n , 1 9 9 1 ) , w i t h i m p o r t a n t r e s u l t i n g e f fec ts o n b i o l o g i c a l p r o d u c t i o n a n d p l a n k t o n r e - t e n t i o n . F u r t h e r , a s i g n i f i c a n t f r a c t i o n o f t h e d e e p w a t e r e x c h a n g e d b e t w e e n t h e w e s t e r n Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 152 a n d e a s t e r n b a s i n s o f t h e N o r t h P a c i f i c i s t h r o u g h t h e M a i n G a p b e t w e e n t h e O j i n / J i n g u a n d N i n t o k u S e a m o u n t s . S i n c e t h i s e x c h a n g e m a y b e a f f e c t e d b y t h e p r e s e n c e o f a t t a c h e d l e e s i d e e d d i e s , w i t h p r o f o u n d i m p l i c a t i o n s f o r p r o p e r t y r e d i s t r i b u t i o n i n t o t h e s u b a r c t i c N o r t h P a c i f i c , i t is h i g h l y r e c o m m e n d e d t h a t f u r t h e r h y d r o g r a p h i c , c u r r e n t m e t e r , a l t i m e - t r y a n d d r i f t e r s t u d i e s b e p e r f o r m e d i n t h e v i c i n i t y o f t h e E S C t o q u a n t i f y t h e p r o d u c t i o n f r e q u e n c y a n d d y n a m i c c h a r a c t e r i s t i c s o f t h e s e e d d i e s . Chapter 7 Eddies II: Anticyclonic Eddies at the Kuri l -Kamchatka Trench 7.1 Introduction T h e a n a l y s i s o f m e s o s c a l e c i r c u l a t i o n f e a t u r e s i s c o n t i n u e d i n t h i s c h a p t e r , w i t h t h e f o c u s n o w s h i f t e d t o t w o g r o u p s o f d r i f t e r s d e p l o y e d i n t h e f a r w e s t e r n N o r t h P a c i f i c , n e a r t h e K u r i l - K a m c h a t k a T r e n c h ( ~ 45°N, 150°E), i n f a l l 1 9 9 0 a n d l a t e s u m m e r 1 9 9 3 . I n b o t h y e a r s , t h e d r i f t e r t r a j e c t o r i e s r e v e a l e d l a r g e a n t i c y c l o n i c e d d i e s p o s i t i o n e d o v e r t h e d e e p e s t p a r t o f t h e t r e n c h ( e . g . , F i g u r e 4 . 5 ) . S o m e o f t h e g e n e r a l c h a r a c t e r i s t i c s o f t h e s e e d d i e s a r e p r e s e n t e d b e l o w , a l o n g w i t h a d i s c u s s i o n o f t h e i r p o t e n t i a l i n f l u e n c e o n t h e s p r e a d i n g o f N o r t h P a c i f i c I n t e r m e d i a t e W a t e r . 7.2 Oceanographic Setting T h e K u r i l - K a m c h a t k a T r e n c h ( K K T ) , w h i c h l i e s j u s t e a s t o f t h e K a m c h a t k a P e n i n s u l a a n d t h e K u r i l I s l a n d c h a i n , i s o v e r 1 0 , 0 0 0 m a t i t s d e e p e s t p o i n t a n d d e e p e r t h a n 6 0 0 0 m o v e r a 2 2 0 0 k m l e n g t h ( F i g u r e 7 .1 ) . A s w i t h t h e o t h e r d e e p o c e a n t r e n c h e s ( e . g . , t h e A l e u t i a n T r e n c h , j u s t s e a w a r d o f t h e a x i s o f t h e A l a s k a n S t r e a m , o r t h e M a r i a n a T r e n c h ) , t h e K K T w a s f o r m e d a t t h e z o n e o f s u b d u c t i o n o f a n o c e a n i c p l a t e u n d e r a c o n t i n e n t a l l i t h o s p h e r i c p l a t e ( T h u r m a n , 1 9 9 4 ) . T o t h e n o r t h w e s t o f t h e K u r i l I s l a n d s i s t h e O k h o t s k S e a , a s e m i - e n c l o s e d b a s i n w h i c h f o r m s t h e w e s t e r n m o s t p a r t o f t h e P a c i f i c s u b p o l a r g y r e . ( T h e A l a s k a n G y r e , B e r i n g S e a G y r e , a n d W e s t e r n S u b a r c t i c G y r e a r e t h e o t h e r c o m p o n e n t s o f t h e P a c i f i c s u b p o l a r g y r e . ) 153 F i g u r e 7.1: ( t o p ) M a p o f t h e N o r t h P a c i f i c O c e a n a n d s u r r o u n d i n g a r e a , s h o w i n g t h e l o c a t i o n o f t h e K u r i l - K a m c h a t k a T r e n c h ( K K T ) . ( b o t t o m ) C h a r t o f t h e K K T r e g i o n . F r o m K o n o ( 1 9 9 6 ) . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 155 T h e O k h o t s k S e a i s g e n e r a l l y i c e - c o v e r e d t h r o u g h o u t w i n t e r , b u t i c e - f r e e i n t h e s u m m e r m o n t h s . T h e r e i s a d o m i n a n t c y c l o n i c c i r c u l a t i o n w i t h i n t h e b a s i n , a l t h o u g h s i g n i f i c a n t e d d y a c t i v i t y i s a l s o p r e v a l e n t ( T a l l e y , 1 9 9 6 ) . S e v e r a l p a s s e s c o n n e c t t h e O k h o t s k S e a t o t h e N o r t h P a c i f i c , t h e d e e p e s t b e i n g K r u z e n s h t e r n S t r a i t i n t h e n o r t h ( s i l l d e p t h o f 1 4 0 0 m ) a n d B u s s o l ' S t r a i t i n t h e c e n t e r ( s i l l d e p t h o f 2 3 0 0 m ) . T h e n e t e x c h a n g e b e t w e e n t h e O k h o t s k S e a a n d t h e N o r t h P a c i f i c h a s b e e n e s t i m a t e d a t 3 - 5 S v , a l t h o u g h a d e q u a t e d a t a f o r s u c h e s t i m a t i o n s a r e s c a r c e ( T a l l e y , 1 9 9 6 ) . T i d a l c u r r e n t s a r e v e r y s t r o n g ( u p t o 2 5 0 c m / s ) t h r o u g h t h e p a s s e s ( T a l l e y a n d N a g a t a , 1 9 9 5 ) . T h e E a s t K a m c h a t k a C u r r e n t ( E K C ) , w h i c h o r i g i n a t e s i n t h e B e r i n g S e a , f l o w s s o u t h - w e s t w a r d a l o n g t h e a x i s o f t h e K K T as t h e w e s t e r n b o u n d a r y c u r r e n t o f t h e W e s t e r n S u b a r c t i c G y r e . M e a n v o l u m e t r a n s p o r t s i n t h e E K C a r e o n t h e o r d e r o f 1 0 - 2 0 S v , a n d m a x i m u m s p e e d s r e a c h 8 0 - 1 0 0 c m / s ( T a l l e y a n d N a g a t a , 1 9 9 5 ; S t a b e n o et a l . , 1 9 9 4 ) . A p o r t i o n o f t h e E K C is t r a n s p o r t e d i n t o t h e O k h o t s k S e a , p r i m a r i l y t h r o u g h K r u z e n - s h t e r n S t r a i t . A n o t h e r p o r t i o n o f t h e E K C r e m a i n s e a s t o f t h e K u r i l I s l a n d s a n d m i x e s w i t h O k h o t s k S e a o u t f l o w , p r i m a r i l y f r o m t h e B u s s o l ' S t r a i t , t o e v e n t u a l l y f o r m t h e O y - a s h i o C u r r e n t . T h e O y a s h i o c o n t i n u e s s o u t h w a r d t o w a r d s H o k k a i d o , t h e n s e p a r a t e s f r o m t h e w e s t e r n b o u n d a r y a n d f l o w s e a s t w a r d i n t o t h e o p e n N o r t h P a c i f i c , j u s t n o r t h o f t h e K u r o s h i o E x t e n s i o n . S a t e l l i t e , h y d r o g r a p h i c a n d d r i f t e r d a t a h a v e c o n f i r m e d t h e p r e v a l e n c e o f l a r g e e d d i e s i n t h e a r e a e a s t o f t h e K u r i l I s l a n d s , w h i c h m o d i f y t h e t r a n s p o r t a n d p r o p e r t i e s o f t h e E K C a n d O y a s h i o ( e . g . , B u l a t o v a n d L o b a n o v , 1 9 8 3 ; R o g a c h e v et a l . , 1 9 9 3 ; S t a b e n o e t a l . , 1 9 9 4 ) . A t t h e s o u t h e r n e n d o f t h e i s l a n d s , t h e e d d i e s h a v e w a r m , s a l i n e c o r e s , a n d m o s t l i k e l y o r i g i n a t e as K u r o s h i o w a r m - c o r e r i n g s ( L o b a n o v et a l . , 1 9 9 1 ; L o b a n o v a n d B u l a t o v , 1 9 9 3 ) . T h e e d d i e s f u r t h e r n o r t h h a v e c o l d , f r e s h c o r e s , a n d m a y b e w a r m - c o r e r i n g s w h i c h h a v e m i g r a t e d n o r t h a n d e n t r a i n e d s u f f i c i e n t a m o u n t s o f E K C a n d O k h o t s k S e a w a t e r t o t r a n s f o r m t h e i r c o r e p r o p e r t i e s ( T a l l e y a n d N a g a t a , 1 9 9 5 ) . L o b a n o v e t Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 156 a l . ( 1 9 9 1 ) a n d L o b a n o v a n d B u l a t o v ( 1993 ) h a v e u s e d h y d r o g r a p h i c d a t a a n d s a t e l l i t e A V H R R i m a g e s t o t r a c k e d d i e s f r o m t h e i r o r i g i n n e a r t h e K u r o s h i o t o t h e K K T r e g i o n , d e m o n s t r a t i n g e d d y l i f e t i m e s o f u p t o s e v e r a l y e a r s ( F i g u r e 7 .2 ) . T h e i r n o r t h e a s t w a r d t r a n s l a t i o n v e l o c i t y i s o n t h e o r d e r o f 2 -6 c m / s . T h e r e g i o n h a s a t t r a c t e d r e n e w e d a t t e n t i o n i n t h e o c e a n o g r a p h i c c o m m u n i t y i n r e c e n t y e a r s ( e . g . , T a l l e y a n d N a g a t a , 1 9 9 5 ) . I t h a s b e e n p r o p o s e d t h a t t h e O k h o t s k S e a i s t h e p r i m a r y s o u r c e r e g i o n f o r N o r t h P a c i f i c I n t e r m e d i a t e W a t e r ( N P I W ) , a l o w - s a l i n i t y , h i g h - o x y g e n w a t e r m a s s f o u n d a t m i d - d e p t h s ( c o r r e s p o n d i n g t o a$ = 2 6 . 7 - 2 6 . 9 ) t h r o u g h o u t t h e s u b t r o p i c a l N o r t h P a c i f i c ( T a l l e y , 1 9 9 1 ; T a l l e y , 1 9 9 3 ; T a l l e y et a l , 1 9 9 5 ) . T h e l o w s a l i n i t y O k h o t s k S e a w a t e r ( i d e n t i f i e d as h a v i n g a l o w p o t e n t i a l v o r t i c i t y , Q = ( £ ) ( § f ) ) w h i c h o u t f l o w s t h r o u g h B u s s o l ' S t r a i t m i x e s w i t h w a r m e r , m o r e s a l i n e E K C w a t e r t o f o r m t h e O y a s h i o , a p o r t i o n o f w h i c h s u b s e q u e n t l y m i x e s w i t h w a t e r i n t h e K u r o s h i o E x t e n s i o n t o f o r m n e w N P I W ( T a l l e y , 1 9 9 3 ; T a U e y et a l . , 1 9 9 5 ; Y a s u d a , 1 9 9 7 ) . T h e u l t i m a t e s o u r c e f o r N P I W is a p p a r e n t l y n e w l y v e n t i l a t e d O k h o t s k S e a w a t e r . A l t h o u g h t h e s t u d i e s r e v i e w e d h e r e a c k n o w l e d g e d t h e u b i q u i t y o f l a r g e e d d i e s n e a r t h e o u t f l o w r e g i o n , t h e r o l e t h e s e e d d i e s p l a y i n t h e f o r m a t i o n , m o d i f i c a t i o n o r s p r e a d i n g o f O y a s h i o a n d N P I W w a t e r m a s s e s w a s n o t a d d r e s s e d . A l t h o u g h t h e r e g i o n c o n t a i n s r i c h fisheries r e s o u r c e s w h i c h h a v e b e e n e x p l o i t e d b y J a p a n a n d R u s s i a , a n d a n a c c o m p a n y i n g h i s t o r y o f h y d r o g r a p h i c s a m p l i n g , m u c h o f t h e d a t a a r e o l d a n d / o r i n a c c e s s i b l e . T h e r e h a v e b e e n r e l a t i v e l y f e w d e s c r i p t i o n s o f t h e p h y s i c a l o c e a n o g r a p h y o f t h e r e g i o n r e p o r t e d i n t h e w e s t e r n l i t e r a t u r e . T h e N o r t h P a c i f i c M a r i n e S c i e n c e O r g a n i z a t i o n ( P I C E S ) h a s e s t a b l i s h e d a w o r k i n g g r o u p t o p r o m o t e d a t a e x c h a n g e b e t w e e n g r o u p s a n d n a t i o n s a n d t o e s t a b l i s h p r i o r i t i e s f o r f u t u r e o c e a n o g r a p h i c r e s e a r c h i n t h e a r e a ( T a l l e y a n d N a g a t a , 1 9 9 5 ) . T h e i r r e c o m m e n d a t i o n s f o c u s e d o n (a ) i m p r o v e d u n d e r s t a n d i n g o f w a t e r m a s s m o d i f i c a t i o n s a n d t h e f o r m a t i o n o f i n t e r m e d i a t e w a t e r w i t h i n t h e O k h o t s k S e a , (b ) q u a n t i f i c a t i o n o f r a t e s a n d p r o c e s s e s o f e x c h a n g e Chapter 7. Eddies IE Anticyclonic Eddies at the Kuril-Kamchatka Trench 1 5 7 F i g u r e 7 .2 : (a) T h e t r a c k o f K u r o s h i o w a r m - c o r e r i n g 8 6 B o v e r a f i v e - y e a r p e r i o d d e r i v e d f r o m s a t e l l i t e a n d s h i p d a t a . T h e i n s e t s h o w s t h e t r a n s l a t i o n v e l o c i t y o f t h e e d d y . F r o m L o b a n o v e t a l . ( 1 9 9 1 ) . ( b ) T h e t r a c k s o f s e v e r a l e d d i e s t r a n s l a t i n g n o r t h e a s t w a r d i n t h e K K T r e g i o n . F r o m L o b a n o v a n d B u l a t o v ( 1 9 9 3 ) . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 158 b e t w e e n t h e O k h o s t s k S e a a n d t h e N o r t h P a c i f i c , i n c l u d i n g t h e r o l e o f e d d i e s i n t h e f o r m a t i o n a n d s p r e a d i n g o f N P I W , a n d (c ) i m p r o v e d f o r e c a s t i n g o f s e a i c e c o v e r i n t h e O k h o t s k S e a . T h e r e s u l t s p r e s e n t e d b e l o w a r e a c o n t r i b u t i o n t o t h e e f f o r t s t o b e t t e r u n d e r s t a n d t h e c h a r a c t e r i s t i c s a n d d y n a m i c s o f p e r s i s t e n t a n t i c y c l o n i c e d d i e s a t t h e K K T . 7.3 The Data I n N o v e m b e r 1 9 9 0 , t h r e e s h a l l o w - d r o g u e d d r i f t e r s w e r e d e p l o y e d a t t h e c e n t e r a n d e d g e s o f a n a n t i c y c l o n i c w a r m - c o r e r i n g ( h e r e d e s i g n a t e d E d d y A l ) , w h i c h c a n b e i n d e n t i f i e d i n e a r l i e r h y d r o g r a p h i c , X B T a n d r e m o t e l y - s e n s e d d a t a s e t s ( R o g a c h e v et a l . , 1 9 9 3 ; K o n o , 1 9 9 6 ) . T h e d r i f t e r d e p l o y e d a t t h e e d d y c e n t e r ( 1315 ) e x e c u t e d f i v e c o m p l e t e l o o p s o v e r a p e r i o d o f ~ 4 5 d a y s , e a c h l o o p a t a s u c c e s s i v e l y g r e a t e r d i s t a n c e f r o m t h e e d d y c e n t e r , b e f o r e e x i t i n g t h e e d d y a n d h e a d i n g f i r s t t o w a r d s t h e K u r i l I s l a n d s t h e n s e a w a r d i n t o t h e o p e n N o r t h P a c i f i c ( F i g u r e 7 .3 ) . S t r o n g i n e r t i a l o s c i l l a t i o n s w e r e s u p e r i m p o s e d o n t h e c l o c k w i s e r o t a t i o n o v e r t h e f i r s t t w o l o o p s ( ~ 11 d a y s ) . D r i f t e r s 1 3 1 4 a n d 1 3 1 6 ( n o t s h o w n ) w e r e d e p l o y e d a t t h e w e s t e r n a n d e a s t e r n e d g e s o f E d d y A l , r e s p e c t i v e l y , a n d p r o m p t l y e x i t e d t h e e d d y a n d m e a n d e r e d s o u t h w a r d a n d e a s t w a r d i n t h e O y a s h i o . B o t h o f t h e s e d r i f t e r s a l s o e x h i b i t e d s t r o n g i n e r t i a l o s c i l l a t i o n s , a l o n g t h e p e r i m e t e r o f E d d y A l , a t t h e s a m e t i m e d r i f t e r 1315 m a d e i t s i n i t i a l t w o l o o p s n e a r t h e c e n t e r . I n S e p t e m b e r 1 9 9 3 , t h r e e m o r e s h a l l o w - d r o g u e d d r i f t e r s w e r e d e p l o y e d i n t h e v i c i n i t y o f t h e K K T . D r i f t e r s 1 5 3 7 2 a n d 1 5 3 7 4 ( n o t s h o w n ) f a i l e d p r e m a t u r e l y , p r o v i d i n g o n l y 17 a n d 70 d a y s o f d a t a , r e s p e c t i v e l y . T h e t h i r d d r i f t e r , 1 5 3 7 1 , w a s r e t a i n e d i n a n a n t i c y l o n i c e d d y ( A 2 ) , m a k i n g t w o c o m p l e t e c i r c u i t s a n d p a r t o f a t h i r d o v e r a p p r o x i m a t e l y 4 0 d a y s ( F i g u r e 7 .3 ) . U p o n l e a v i n g E d d y A 2 , d r i f t e r 15371 w e n t o n t o t h e K u r i l I s l a n d s h e l f , t h r o u g h F r i z a S t r a i t a n d i n t o t h e O k h o t s k S e a , a n d u l t i m a t e l y b a c k t h r o u g h B u s s o l ' Chapter 7. Eddies lh Anticyclonic Eddies at the Kuril-Kamchatka Trench 159 150°E 152°E 154°E 156°E F i g u r e 7 .3 : M a p o f t h e r e g i o n s h o w i n g t h e t r a j e c t o r i e s o f d r i f t e r 1315 i n e d d y A l a n d d r i f t e r 1 5 3 7 1 i n e d d y A 2 . H e x a g o n s m a r k t h e d e p l o y m e n t l o c a t i o n s . T h e b a t h y m e t r y c o n t o u r s i n t h i s a n d s u b s e q u e n t m a p s a r e i n m e t e r s . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 160 S t r a i t a n d i n t o t h e o p e n N o r t h P a c i f i c , w h e r e i t s p e n t a n o t h e r 2 .5 y e a r s m a k i n g a t r a n s - P a c i f i c c r o s s i n g ( T h o m s o n et a l . , 1 9 9 7 ) . T h e t r a j e c t o r y o f t h i s d r i f t e r i n t h e v i c i n i t y o f t h e K u r i l I s l a n d s h a s b e e n e x t e n s i v e l y a n a l y z e d i n T h o m s o n et a l . ( 1 9 9 7 ) a n d R a b i n o v i c h a n d T h o m s o n ( 1 9 9 8 ) , w h o f o u n d e v i d e n c e o f d i u r n a l s h e l f w a v e s a n d v e r y s t r o n g t i d a l c u r r e n t s i n t h e p a s s e s . T h e a n a l y s i s h e r e is r e s t r i c t e d t o t h e 4 0 - d a y p e r i o d w i t h i n E d d y A 2 . 7.4 The 1990 Eddy A s d r i f t e r 1 3 1 5 s p i r a l e d o u t o f E d d y A l o v e r a 4 3 d a y p e r i o d , t h e e d d y ' s c e n t e r (as e s t i m a t e d f r o m t h e m e a n d r i f t e r p o s i t i o n w i t h i n e a c h l o o p ) t r a n s l a t e d n o r t h w a r d a p - p r o x i m a t e l y 50 k m , f o r a n e t t r a n s l a t i o n v e l o c i t y o f ~ 1.5 c m / s ( F i g u r e 7 .4 ) . T h i s i s c o m p a r a b l e t o t h e t r a n s l a t i o n v e l o c i t i e s e s t i m a t e d f r o m s a t e l l i t e i m a g e r y b y L o b a n o v e t a l . ( 1 9 9 1 ) , a l t h o u g h u n c e r t a i n t i e s i n d e t e r m i n i n g t h e e d d y c e n t e r w o u l d p u t l a r g e e r r o r b a r s o n t h i s e s t i m a t e . E a c h o f t h e f i v e s u c c e s s i v e l o o p s w a s l a r g e r t h a n t h e p r e v i o u s , t h e first b e i n g 2 7 k m a n d t h e fifth 115 k m f r o m t h e e d d y c e n t e r ( T a b l e 7 .1 ) . T i m e s e r i e s o f d r i f t e r l o n g i t u d e a n d l a t i t u d e c l e a r l y d e m o n s t r a t e t h e i n c r e a s i n g r a d i a l d i s t a n c e , p a r t i c - u l a r l y i n t h e m e r i d i o n a l d i r e c t i o n ( F i g u r e 7 .5 ) . A s s u m i n g t h e fifth l o o p t r a c e d i t s o u t e r e d g e s , t h i s w o u l d y i e l d a h o r i z o n t a l d i m e n s i o n o f a p p r o x i m a t e l y 2 3 0 k m f o r E d d y A l a t 15 m d e p t h , c o n s i d e r a b l y l a r g e r t h a n t h e e d d i e s o b s e r v e d a t t h e E m p e r o r S e a m o u n t C h a i n ( p r e v i o u s c h a p t e r ) . M e a n 3 - h o u r l y s p e e d s d e c r e a s e d f r o m 88 c m / s i n l o o p 1 t o < 50 c m / s i n l o o p s 3 - 5 , w h i l e t h e m e a n r o t a t i o n a l s p e e d s ( ^ p , w h e r e r i s e d d y r a d i u s a n d T i s r o t a t i o n a l p e r i o d ) i n c r e a s e d o u t w a r d f r o m t h e e d d y c e n t e r f r o m 30 c m / s a t ~ 30 k m t o 4 7 c m / s a t ~ 115 k m . H i g h - a m p l i t u d e i n e r t i a l o s c i l l a t i o n s a r e c l e a r l y e v i d e n t i n d r i f t e r 1 3 1 5 ' s t r a j e c t o r y o v e r i t s first t w o l o o p s , n e a r t h e e d d y c e n t e r ( F i g u r e 7 .6 ) . M a x i m u m 3 - h o u r l y s p e e d s Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 161 F i g u r e 7 .4 : T r a j e c t o r y o f d r i f t e r 1315 w i t h i n E d d y A l o v e r (a ) l o o p 1 ( N o v . 8 - 1 2 ) , ( b ) l o o p 2 ( N o v . 1 3 - 1 8 ) , (c ) l o o p 3 ( N o v . 1 9 - 2 7 ) , (d ) l o o p 4 (Nov.28-Dec.6), a n d (e) l o o p 5 ( D e c . 7 - D e c . 2 1 ) . S m a l l ' x ' s d e n o t e d a i l y p o s i t i o n s a t 1 2 0 0 Z a n d s t a r s m a r k t h e p o s i t i o n o f t h e e d d y c e n t e r (as d e t e r m i n e d b y t h e m e a n d r i f t e r p o s i t i o n d u r i n g t h a t l o o p . ) Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 162 F i g u r e 7 .5 : T i m e s e r i e s o f (a) l o n g i t u d e a n d (b ) l a t i t u d e f r o m d r i f t e r 1 3 1 5 i n E d d y A l d u r i n g N o v e m b e r - D e c e m b e r , 1990 . T h e p e r i o d s o f e a c h l o o p a r e m a r k e d a t b o t t o m . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 163 L o o p P e r i o d ( d a y s ) R a d i u s ( k m ) S r o t ( c m / s ) Smean ( c m / s ) Smax ( c m / s ) Drifter 1315 1 5 .0 2 7 . 3 2 9 . 6 8 8 . 3 1 2 8 . 6 2 5 .5 3 9 . 2 3 8 . 7 6 2 . 4 1 2 3 . 3 3 9 .0 5 9 . 9 36 .1 4 4 . 8 7 7 . 3 4 9 .0 7 8 . 4 4 7 . 3 4 5 . 7 7 9 . 5 5 13 .3 1 1 5 . 5 4 7 . 1 4 7 . 2 8 7 . 5 Drifter 15371 1 10 .3 81 .1 4 3 . 0 3 8 . 4 5 3 . 1 2 12 .8 1 0 0 . 2 4 2 . 7 3 7 . 1 6 1 . 5 T a b l e 7 . 1 : P e r i o d , r a d i u s , m e a n r o t a t i o n a l s p e e d , m e a n 3 - h o u r l y s p e e d , a n d m a x i m u m 3 - h o u r l y s p e e d o f t h e K K T a n t i c y c l o n i c e d d i e s as d e l i n e a t e d b y t h e t r a j e c t o r i e s o f d r i f t e r s 1 3 1 5 ( E d d y A l ) a n d 1 5 3 7 1 ( E d d y A 2 ) . d u r i n g t h i s p e r i o d w e r e g r e a t e r t h a n 120 c m / s , w i t h m o s t o f t h e h i g h a m p l i t u d e i n e r t i a l m o t i o n s o c c u r i n g w i t h i n t h e f i r s t l o o p , j u s t a f t e r d e p l o y m e n t ( F i g u r e s 7 .6 a n d 7 .7 , T a b l e 7 .1 ) . W i t h o u t t h e s e i n e r t i a l m o t i o n s , t h e m e a n 3 - h o u r l y s p e e d s o f d r i f t e r 1 3 1 5 w o u l d h a v e r e m a i n e d n e a r l y c o n s t a n t r e g a r d l e s s o f d i s t a n c e f r o m t h e e d d y c e n t e r , e v e n t h o u g h t h e m e a n r o t a t i o n a l s p e e d s i n c r e a s e d o u t w a r d . K a w a i ( 1 9 9 2 ) p o i n t e d o u t t h a t a l m o s t a l l p r e v i o u s o b s e r v a t i o n s h a v e s h o w n w a r m - c o r e a n t i c y c l o n i c r i n g s a n d e d d i e s t o b e i n s o l i d - b o d y r o t a t i o n , w i t h o n l y o n e o b s e r v a t i o n s h o w i n g a w a r m - c o r e r i n g w i t h a s t a t i c c o r e . T h e r o t a r y s p e c t r a f o r d r i f t e r 1315 a n d t h e t w o d r i f t e r s d e p l o y e d a t t h e e d g e s o f E d d y A l ( c a l c u l a t e d f o r a 9 0 - d a y p e r i o d a n d t h u s i n c l u d i n g t r a j e c t o r y s e g m e n t s o u t s i d e o f t h e e d d y ) a l l s h o w a s t r o n g n e a r - i n e r t i a l p e a k , s i g n i f i c a n t l o w - f r e q u e n c y c l o c k w i s e p e a k s ( i n d i c a t i v e o f E d d y A l ' s r o t a t i o n ) , a n d g e n e r a l l y n o i s y s p e c t r a a t p e r i o d s l e s s t h a n o n e d a y ( F i g u r e 7 .8 ) . T h e l a r g e s t s p e c t r a l p e a k o c c u r s f o r c l o c k w i s e m o t i o n s o f d r i f t e r 1 3 1 5 a t a p e r i o d o f ~ 10 d a y s . A f r e q u e n c y p a r t i t i o n i n g o f t h e r o t a r y v a r i a n c e s r e v e a l s t h a t Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 164 F i g u r e 7 .6 : C l o s e - u p o f t h e t r a j e c t o r y o f d r i f t e r 1315 w i t h i n E d d y A l o v e r (a ) l o o p 1 a n d (b ) l o o p 2. A h e x a g o n m a r k s t h e l o o p ' s s t a r t p o s i t i o n , ' x ' s d e n o t e 3 - h o u r l y p o s i t i o n s , a n d s t a r s d e n o t e p o s i t i o n s w h e r e 3 - h o u r l y d r i f t e r s p e e d s w e r e g r e a t e r t h a n 100 c m / s . Chapter 7. Eddies IT. Anticyclonic Eddies at the Kuril-Kamchatka Trench 1 6 5 F i g u r e 7 .7 : T i m e s e r i e s o f z o n a l ( U ) a n d m e r i d i o n a l ( V ) 3 - h o u r l y s p e e d s f r o m d r i f t e r 1 3 1 5 i n E d d y A l o v e r l o o p s 1 a n d 2 , N o v e m b e r 8 - 1 9 , 1990 . Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 166 Drifter s~H Stot(u) 1315 l o w m e s o s c a l e h i g h m e s o s c a l e n e a r - i n e r t i a l h i g h 1 0 5 0 . 8 (71 .4 ) 2 2 . 3 (1 .5 ) 1 8 2 . 7 (12 .4 ) 38 .1 (2 .6 ) 157 .9 (10 .7 ) 6.1 (0 .4 ) 2 .9 (0 .2 ) 11 .8 (0 .8 ) 1 2 0 8 . 7 ( 8 2 . 1 ) 2 8 . 4 (1 .9 ) 1 8 5 . 6 ( 1 2 . 6 ) 4 9 . 9 (3 .4 ) 15371 l o w m e s o s c a l e h i g h m e s o s c a l e n e a r - i n e r t i a l h i g h 4 5 5 . 9 (65 .1 ) 104 .1 (14 .9 ) 3 3 . 3 (4 .8 ) 2 5 . 4 (3 .6) 3 8 . 3 (5 .5 ) 2 0 . 5 (2 .9 ) 3 .0 (0 .4) 19 .6 (2 .8 ) 4 9 4 . 2 ( 7 0 . 6 ) 1 2 4 . 6 ( 17 .8 ) 3 6 . 4 (5 .2 ) 4 5 . 1 (6 .4 ) T a b l e 7 .2 : C l o c k w i s e (S~(u)), c o u n t e r c l o c k w i s e (5 +(u;)) a n d t o t a l (5t<>t(<*>)) r o t a r y v a r i - a n c e (cm2/s2) i n f o u r f r e q u e n c y b a n d s d e r i v e d f r o m (a ) d r i f t e r 1 3 1 5 o v e r t h e p e r i o d N o v e m b e r 8 , 1 9 9 0 t o F e b r u a r y 5 , 1991 a n d (b ) d r i f t e r 1 5 3 7 1 o v e r t h e p e r i o d S e p t e m b e r 5 t o D e c e m b e r 3 , 1 9 9 3 . N u m b e r s i n p a r e n t h e s e s r e f e r t o t h e p e r c e n t a g e o f t h e t o t a l r o t a r y v a r i a n c e . T h e f r e q u e n c y b a n d s a r e l o w m e s o s c a l e ( p e r i o d s g r e a t e r t h a n 2 d a y s ) , h i g h m e s o s c a l e ( p e r i o d s o f 1 9 . 2 h - 2 d a y s ) , n e a r - i n e r t i a l ( p e r i o d s o f 16 .1 - 1 8 . 7 h o u r s ) a n d h i g h ( p e r i o d s o f 6 - 1 5 . 6 h o u r s ) . m o s t o f t h e e n e r g y ( 7 1 % ) i n d r i f t e r 1315 ' s t r a j e c t o r y w a s c o n t a i n e d i n l o w f r e q u e n c y c l o c k w i s e m o t i o n s ( p e r i o d s g r e a t e r t h a n 2 d a y s , i . e . , t h e e d d y r o t a t i o n ) , a n d t h a t o v e r t h e t h r e e m o n t h p e r i o d , c l o c k w i s e m o t i o n s a t a l l f r e q u e n c i e s a c c o u n t e d f o r n e a r l y 9 0 % o f t h e t o t a l e n e r g y ( T a b l e 7 .2 ) . I n e r t i a l m o t i o n s i n d r i f t e r 1 3 1 5 ' s t r a j e c t o r y c o m p r i s e d 1 2 % o f t h i s t o t a l . O n e o f t h e m o r e i n t e r e s t i n g a s p e c t s o f d r i f t e r 1 3 1 5 ' s t r a j e c t o r y i s t h e r e l a t i o n s h i p b e t w e e n t h e p r e s u m a b l y w i n d - f o r c e d i n e r t i a l o s c i l l a t i o n s a n d t h e r o t a t i o n o f E d d y A l . K u n z e ( 1 9 8 5 ) d e r i v e d a d i s p e r s i o n r e l a t i o n f o r n e a r - i n e r t i a l i n t e r n a l w a v e s p r o p a g a t i n g i n g e o s t r o p h i c s h e a r w h i c h t o o k i n t o a c c o u n t i n t e r a c t i o n t e r m s i n v o l v i n g b o t h a d v e c t i o n b y t h e m e a n f l o w a n d s t r a i n i n g b y m e a n f l o w s h e a r , t e r m s w h i c h a r e o r d i n a r i l y n e g l e c t e d . I t w a s f o u n d t h a t a d v e c t i o n l e a d s t o a D o p p l e r s h i f t , a n d t h a t s t r a i n i n g b y t h e m e a n Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 1 6 7 Frequency (cpd) F i g u r e 7 .8 : C l o c k w i s e S~ ( s o l i d l i n e ) a n d c o u n t e r c l o c k w i s e S+ ( d a s h e d U n e ) r o t a r y e n e r g y d e n s i t y s p e c t r a (m2/s2/cpd) d e r i v e d f r o m t h e t r a j e c t o r i e s o f d r i f t e r s (a ) 1 3 1 4 , ( b ) 1 3 1 5 , a n d (c ) 1 3 1 6 o v e r t h e p e r i o d N o v e m b e r 8, 1990 t o F e b r u a r y 6, 1 9 9 1 , a n d ( d ) 1 5 3 7 1 o v e r t h e p e r i o d S e p t e m b e r 4 t o D e c e m b e r 3 , 1993 . T h e 9 5 % c o n f i d e n c e U m i t s a r e s h o w n i n ( a ) , a n d t h e i n e r t i a l ( / ) a n d s e m i d i u r n a l ( M 2 ) p e a k s a r e s h o w n i n ( b ) a n d ( d ) . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 168 flow v o r t i c i t y , £, s h i f t s t h e l o w e r b o u n d o f t h e i n e r t i a l w a v e b a n d f r o m t h e l o c a l p l a n e t a r y v o r t i c i t y ( C o r i o l i s f r e q u e n c y , / ) t o a n effective C o r i o l i s f r e q u e n c y , I n o t h e r w o r d s , a n i n t e r n a l w a v e e x p e r i e n c e s b o t h t h e e a r t h ' s a n d t h e fluid's r o t a t i o n . W i t h t h e b r o a d e n i n g o f t h e i n e r t i a l w a v e b a n d , m o r e i n e r t i a l e n e r g y i s a v a i l a b l e f o r t h e g e n e r a t i o n o f t u r b u l e n c e a n d m i x i n g i n t h e u p p e r w a t e r c o l u m n ( K u n z e , 1 9 8 5 ; R o g a c h e v a n d C a r m a c k , 1 9 9 8 ) . I n f a c t , t r a p p i n g a n d a m p l i f i c a t i o n o f i n e r t i a l e n e r g y c a n o c c u r i n r e g i o n s o f n e g a t i v e v o r t i c i t y ( e . g . , a n t i c y c l o n i c e d d i e s ) , w h e r e t h e i n t r i n s i c i n e r t i a l f r e - q u e n c y is l e s s t h a n t h e e f f e c t i v e C o r i o l i s f r e q u e n c y ( K u n z e , 1 9 8 5 ) . A s n e a r - i n e r t i a l w a v e s a r e i n t e r m i t t e n t , so t o o i s t h e e n h a n c e d w a v e a c t i v i t y . I n t e n s e d o w n w a r d - p r o p a g a t i n g p a c k e t s o f n e a r - i n e r t i a l e n e r g y h a v e b e e n o b s e r v e d a t t h e n e g a t i v e v o r t i c i t y s i d e s o f f r o n t s ( K u n z e a n d S a n f o r d , 1 9 8 4 a ) , a n t i c y c l o n i c e d d i e s ( K u n z e a n d S a n f o r d , 1 9 8 6 ; K u n z e a n d L u e c k , 1 9 8 6 ) , a n d w a r m - c o r e r i n g s ( K u n z e a n d S a n f o r d , 1 9 8 4 b ) . C l e a r e v i d e n c e o f t h e b r o a d e n i n g o f t h e i n e r t i a l w a v e b a n d i s f o u n d i n t h e f — t d i a g r a m f o r t h e c l o c k w i s e r o t a r y v e l o c i t y c o m p o n e n t o f d r i f t e r 1315 i n E d d y A l ( F i g u r e 7 .9 ) . O v e r t h e f i r s t f e w d a y s a f t e r d e p l o y m e n t , t h e r e i s a s i g n i f i c a n t c l o c k w i s e s p e c t r a l a m p l i t u d e p e a k c e n t e r e d a t ~ 1.28 c p d , a p p r o x i m a t e l y 0 . 1 3 c p d l o w e r t h a n t h e l o c a l p l a n e t a r y v o r t i c i t y ( F i g u r e 7 . 9 a ) . T h e s e h i g h - a m p l i t u d e i n e r t i a l m o t i o n s h a d d i s s i p a t e d b y N o v e m b e r 2 0 , t h e s t a r t o f l o o p 3 . I n e r t i a l m o t i o n s w i t h p e a k a m p l i t u d e s c e n t e r e d o n / w e r e o b s e r v e d a g a i n i n l a t e N o v e m b e r ( t h e e n d o f l o o p 3 a n d b e g i n n i n g o f l o o p 4 ; see F i g u r e 7 .4) a n d m i d - D e c e m b e r ( l o o p 5 ) . E x c e p t f o r a w e a k d i u r n a l p e a k i n l o o p 1, n o o t h e r h i g h f r e q u e n c y m o t i o n s a r e e v i d e n t f r o m d r i f t e r 1 3 1 5 ' s t r a j e c t o r y i n E d d y A l . A t l o w f r e q u e n c i e s ( F i g u r e 7 . 9 b ) , t h e c l o c k w i s e s p e c t r a l a m p l i t u d e p e a k s w e r e c e n t e r e d a t t h e r o t a t i o n r a t e s o f E d d y A l , f i r s t a t 0 .2 - 0 .24 c p d ( 4 - 5 d a y s ; l o o p s 1 a n d 2) a n d t h e n Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 169 n e a r 0.1 c p d (10 d a y s ; l o o p s 3 , 4 a n d 5 ) . K n o w i n g t h e l o c a l p l a n e t a r y v o r t i c i t y , a n d t a k i n g t h e c l o c k w i s e v e l o c i t y a m p l i t u d e p e a k d u r i n g l o o p 1 ( F i g u r e 7 .9a ) as t h e e f f e c t i v e C o r i o l i s f r e q u e n c y , t h e r e l a t i v e v o r t i c i t y f o r E d d y A l a t a r a d i u s o f ~ 30 k m is f o u n d t o b e £ = - 1 . 9 x 1 0 ~ 5 s"1. T h i s i s a p p r o x i m a t e l y — 0 . 2 / , a s m a l l e r m a g n i t u d e t h a n t h e — | / < ( < — | / p r e d i c t e d f r o m a l | l a y e r m o d e l o f a n a n t i c y c l o n i c r i n g i n s o l i d - b o d y r o t a t i o n p r e s e n t e d i n K a w a i ( 1 9 9 2 ) . T h i s d i s c r e p a n c y w i t h t h e m o d e l p r e d i c t i o n i s c o n s i s t e n t w i t h c o m p a r i s o n s m a d e f r o m t h e e s t i m a t e d v o r t i c i t i e s o f o t h e r w a r m - c o r e r i n g s , w h i c h w e r e g e n e r a l l y l e s s t h a n 5 0 % t h e m a g n i t u d e o f / ( J o y c e a n d K e n n e l l y , 1985 ; O l s o n et a l . , 1 9 8 5 ) . I t i s a l s o n o t s u r p r i s i n g b e c a u s e E d d y A l d o e s n o t a p p e a r t o b e i n s o l i d - b o d y r o t a t i o n . I t i s i n t e r e s t i n g t o n o t e t h a t R o g a c h e v a n d C a r m a c k ( 1 9 9 8 ) h a v e s u g g e s t e d t h a t t h e b r o a d e n i n g o f t h e i n e r t i a l w a v e b a n d b y t h e e d d y ' s r o t a t i o n r e s u l t e d i n r e s o n a n c e w i t h t h e d i u r n a l t i d e , a n d s u b s e q u e n t l y a n i n c r e a s e d e n e r g y i n p u t t o E d d y A l t h r o u g h w a v e t r a p p i n g a n d a m p l i f i c a t i o n a t t h e b a s e o f t h e e d d y ' s c o r e . T h i s s u p p l y o f e n e r g y , t h e y a r g u e , w o u l d c o u n t e r f r i c t i o n a l d e c a y a n d i n c r e a s e t h e l i f e o f t h e e d d y . T h u s , d i u r n a l t i d a l f o r c i n g o f n e a r - i n e r t i a l w a v e s w i t h i n E d d y A l (o r p r e s u m a b l y a n y e d d y i n t h e c e n t r a l K K T r e g i o n ) w o u l d p r o v i d e a r e g u l a r s u p p l y o f e n e r g y ( p r i n c i p a l l y a t s p r i n g t i d e s ) t o m a i n t a i n d y n a m i c a l c o h e r e n c e . T h e t h e o r y o f R o g a c h e v a n d C a r m a c k ( 1998 ) is e v i d e n t l y c o n t r a d i c t e d b y t h e s p e c t r a l a n a l y s i s p r e s e n t e d h e r e . T h e e f f e c t i v e i n e r t i a l f r e q u e n c y i n E d d y A l i s not d i u r n a l , b u t i s o n l y ~ 9 % l o w e r t h a n t h e l o c a l p l a n e t a r y v o r t i c i t y ( F i g u r e 7 . 9 a ) . F u r t h e r m o r e , t h e r e w e r e n o p e r c e p t i b l e m o t i o n s o f d i u r n a l p e r i o d o b s e r v e d a t a n y t i m e d u r i n g d r i f t e r 1 3 1 5 ' s e n t r a p m e n t i n E d d y A l . A l t h o u g h d i u r n a l t i d e s a n d d i u r n a l p e r i o d s h e l f w a v e s i n t h e K u r i l I s l a n d s r e g i o n a r e o f g e n e r a l l y h i g h a m p l i t u d e (see R a b i n o v i c h a n d T h o m s o n ( 1 9 9 8 ) , a n d s e c t i o n 7 .5 b e l o w ) , t h e r e is n o e v i d e n c e f r o m t h e d r i f t e r d a t a t o s u g g e s t t h a t t h e s e m o t i o n s h a d a n y i m p a c t o n t h e n e a r - s u r f a c e e n v i r o n m e n t o f E d d y A l , s i t u a t e d o v e r t h e Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 170 Nov Dec J a n 10 20 30 10 20 30 10 ' i • " i m i n i Drifter 1315 Clockwise (S~) I I I I I I I I I I I I M I I I n i i m I ' l M '11 ' 1111111 n 1111 11 Drifter /. Clockwise 0 . 0 5 ' f i n i'l i I I 111 i ^ i 1111111111111111111II11111 M 111II111111111111 i i 111 I i t 10 20 30 10 20 30 10 Nov Dec J a n F i g u r e 7 .9 : A m p l i t u d e e v o l u t i o n o f t h e c l o c k w i s e r o t a r y v e l o c i t y c o m p o n e n t f o r d r i f t e r 1 3 1 5 i n E d d y A l a t (a) h i g h f r e q u e n c i e s ( p e r i o d s o f 11 h o u r s t o 1.25 d a y s ) a n d (b ) l o w f r e q u e n c i e s ( p e r i o d s o f 2 t o 20 d a y s ) . C o n t o u r s a r e i n c m / s . T h e s t a r t d a t e i s N o v e m b e r 8 , 1 9 9 0 , a n d t h e d o t t e d l i n e i n (a) r e f e r s t o t h e l o c a l v a l u e o f p l a n e t a r y v o r t i c i t y ( / ) a t t h e d r i f t e r p o s i t i o n . Chapter 7. Eddies lh Anticyclonic Eddies at the Kuril-Kamchatka Trench 171 d e e p e s t p a r t o f t h e t r e n c h . I n s t e a d , t h e i n e r t i a l o s c i l l a t i o n s o b s e r v e d h e r e w e r e m o s t l i k e l y w i n d - i n d u c e d , as s u g g e s t e d b y t h e i n t e r m i t t e n t i n e r t i a l p e a k s i n F i g u r e 7 . 9 a . A l t h o u g h w i n d d a t a f r o m t h e i m m e d i a t e v i c i n i t y o f t h e K K T a r e n o t p r e s e n t l y a v a i l a b l e , i t i s k n o w n t h a t t h r e e s t o r m s p a s s e d t h r o u g h t h e O k h o t s k S e a b e t w e e n N o v e m b e r 6 - 1 2 , 1 9 9 0 ( M a r i n e r s ' W e a t h e r L o g , 1 9 9 1 ) . I n p a r t i c u l a r , t h e s t o r m o f N o v e m b e r 6 - 8 , 1 9 9 0 , j u s t p r i o r t o t h e d r i f t e r d e p l o y m e n t s , w a s c e n t e r e d r i g h t a l o n g t h e a x i s o f t h e K u r i l I s l a n d c h a i n . S t o r m s a l s o a f f e c t e d t h e r e g i o n a t t h e t i m e s o f t h e t w o l a t e r i n e r t i a l p e a k s . 7.5 The 1993 Eddy D r i f t e r s w e r e a g a i n d e p l o y e d a t t h e K K T i n S e p t e m b e r 1 9 9 3 , a n d a g a i n r e v e a l e d a l a r g e a n t i c y c l o n i c e d d y p o s i t i o n e d o v e r t h e d e e p e s t p a r t o f t h e t r e n c h ( F i g u r e 7 .3 ) . T h e e d d y r e v e a l e d b y d r i f t e r 1 5 3 7 1 i n S e p t e m b e r 1 9 9 3 ( A 2 ) h a d c h a r a c t e r i s t i c s s i m i l a r t o t h o s e o f E d d y A l ( T a b l e 7 .1 ) . A s w i t h A l , t h e p e r i o d o f r o t a t i o n a t 8 0 - 1 0 0 k m f r o m t h e e d d y c e n t e r w a s 1 0 - 1 3 d a y s a n d m e a n 3 - h o u r l y s p e e d s w e r e o n t h e o r d e r o f 4 0 c m / s . T h e e v i d e n c e s u g g e s t s a n e d d y i n s o l i d - b o d y r o t a t i o n , a l t h o u g h n o o b s e r v a t i o n s a r e a v a i l a b l e f r o m n e a r e r t h e e d d y c e n t e r . T h e t w o - a n d - a - h a l f l o o p s m a d e b y d r i f t e r 15371 w e r e r e l a t i v e l y s m o o t h , w i t h n o e v i d e n c e o f s i g n i f i c a n t h i g h - f r e q u e n c y e n e r g y u n t i l e a r l y O c t o b e r , w h e n t h e d r i f t e r w a s a b o u t t o l e a v e t h e e d d y a n d a p p r o a c h t h e K u r i l I s l a n d s h e l f ( F i g u r e 7 . 1 0 ) . T h e f — t d i a g r a m r e v e a l s t h e r e l a t i v e l a c k o f h i g h - f r e q u e n c y e n e r g y i n d r i f t e r 1 5 3 7 1 ' s t r a j e c t o r y w i t h i n E d d y A 2 , w i t h t h e o n l y c l o c k w i s e s p e c t r a l a m p l i t u d e p e a k s o c c u r r i n g n e a r 0 .1 c p d ( t h e e d d y ' s r o t a t i o n r a t e ) a n d a t t h e l o c a l i n e r t i a l f r e q u e n c y ( b u t not a l o w e r e f f e c t i v e C o r i o l i s f r e q u e n c y ; F i g u r e 7 .11 ) . T h e i n e r t i a l p e a k , w h i c h o c c u r r e d o n t h e d r i f t e r ' s f i n a l h a l f - l o o p o f E d d y A 2 , w a s f o l l o w e d b y s t r o n g p e a k s a t t h e d i u r n a l t i d a l f r e q u e n c i e s f r o m l a t e O c t o b e r t o m i d - N o v e m b e r ( F i g u r e 7 . 1 1 a ) . T h e s e d i u r n a l - p e r i o d m o t i o n s y i e l d e d t h e Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 172 r e l a t i v e l y s t r o n g c l o c k w i s e v a r i a n c e s o b s e r v e d f o r d r i f t e r 1 5 3 7 1 i n t h e h i g h m e s o s c a l e b a n d ( T a b l e 7 .2 ) . S h o r t p e r i o d s o f s u p e r i n e r t i a l m o t i o n s , i n c l u d i n g a t t h e s e m i d i u r n a l t i d a l f r e q u e n c y , a l s o o c c u r r e d a t t h e s e t i m e s . F u r t h e r d i s c u s s i o n o f t h e s e h i g h - f r e q u e n c y s h e l f m o t i o n s c a n b e f o u n d i n T h o m s o n et a l . ( 1997 ) a n d R a b i n o v i c h a n d T h o m s o n ( 1 9 9 8 ) . 7.6 Discussion H o w o f t e n c a n e d d i e s b e f o u n d a t t h e K K T , a n d w h a t i s t h e i r o r i g i n a n d l i f e e x p e c t a n c y ? T h e d r i f t e r d a t a d i s c u s s e d h e r e a r e i n s u f f i c i e n t f o r a n s w e r i n g t h e s e q u e s t i o n s . H o w e v e r , f r o m a p e r u s a l o f t h e J a p a n e s e a n d R u s s i a n l i t e r a t u r e , i t i s c l e a r t h a t l a r g e , l o n g - l i v e d , a n t i c y c l o n i c e d d i e s a r e c o m m o n l y o b s e r v e d i n t h e K K T r e g i o n . K o n o ( 1 9 9 6 ) a n a l y z e d h y d r o g r a p h i c d a t a c o l l e c t e d i n t h e K K T a r e a i n s u c c e s s i v e s u m m e r s b e t w e e n 1 9 8 9 a n d 1 9 9 2 , a n d f o u n d a n t i c y c l o n i c e d d i e s p r e s e n t i n e a c h y e a r . I n t h e A u g u s t - S e p t e m b e r 1 9 9 0 s u r v e y , E d d y A l c l e a r l y s h o w s u p i n m a p s o f a c c e l e r a t i o n p o t e n t i a l a n o m a l y ( r e f e r r e d t o 1 5 0 0 d b ) a n d p o t e n t i a l t e m p e r a t u r e ( F i g u r e 7 . 1 2 ) , p o s i t i o n e d i n p r e c i s e l y t h e s a m e l o c a t i o n w h e r e i t w a s s a m p l e d b y t h e d r i f t e r s t h r e e m o n t h s l a t e r . K o n o ( 1 9 9 6 ) s u g g e s t s t h a t t h e e d d y o b s e r v e d j u s t s o u t h e a s t o f B u s s o l ' S t r a i t i n 1 9 9 1 w a s t h e s a m e e d d y as t h e o n e o b s e r v e d t h e r e i n 1990 ( i . e . , E d d y A l ) , i m p l y i n g t h a t E d d y A l h a d b e e n n e a r l y s t a t i o n a r y d u r i n g t h a t y e a r , p e r h a p s l o c k e d t o t h e t o p o g r a p h y . W a t e r i n E d d y A l a t t h e erg — 2 6 . 8 i s o p y c n a l ( ~ 1 5 0 - 1 8 0 m ) w a s a b o u t 1° w a r m e r t h a n s u r r o u n d i n g w a t e r s , a l t h o u g h t h i s s i g n a l w a s n o t a p p a r e n t b e l o w a b o u t 3 5 0 m . A n X B T t r a n s e c t t h r o u g h E d d y A l t a k e n a t t h e t i m e o f t h e d r i f t e r d e p l o y m e n t s ( e a r l y N o v e m b e r 1990 ) r e v e a l e d a w a r m - c o r e e x t e n d i n g t o ~ 3 0 0 m ( R o g a c h e v a n d C a r m a c k , 1 9 9 8 ) . I n e a c h y e a r o f K o n o ' s ( 1 9 9 6 ) o b s e r v a t i o n s , t h e O y a s h i o w a s o b s e r v e d t o m e a n d e r a r o u n d t h e e d d i e s t o w a r d s t h e s o u t h w e s t . N u m e r o u s e d d i e s w e r e a l s o o b s e r v e d i n d r i f t e r t r a j e c t o r i e s i n t h e r e g i o n b e t w e e n 1 9 8 8 a n d 1 9 9 3 ( S t a b e n o et a l . , 1 9 9 4 ) , a n d h y d r o g r a p h i c o b s e r v a t i o n s c o n d u c t e d Chapter 7. Eddies IT. Anticyclonic Eddies at the Kuril-Kamchatka Trench 173 F i g u r e 7 .10 : T i m e se r i es o f (a) l o n g i t u d e a n d (b ) l a t i t u d e f r o m d r i f t e r 1 5 3 7 1 i n E d d y A 2 d u r i n g S e p t e m b e r - O c t o b e r , 1 9 9 3 . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 174 10 20 30 10 20 30 10 20 30 S e p Oct Nov Dec F i g u r e 7 . 1 1 : A m p l i t u d e e v o l u t i o n o f t h e c l o c k w i s e r o t a r y v e l o c i t y c o m p o n e n t f o r d r i f t e r 1 5 3 7 1 i n E d d y A 2 a t (a) h i g h f r e q u e n c i e s ( p e r i o d s o f 10 h o u r s t o 2 d a y s ) a n d ( b ) l o w f r e q u e n c i e s ( p e r i o d s o f 2 t o 20 d a y s ) . C o n t o u r s a r e i n c m / s . T h e s t a r t d a t e is S e p t e m b e r 5 , 1 9 9 3 , a n d t h e d o t t e d l i n e s i n (a) r e f e r t o t h e s e m i d i u r n a l t i d a l f r e q u e n c y (M2), t h e l o c a l v a l u e o f p l a n e t a r y v o r t i c i t y a t t h e d r i f t e r p o s i t i o n ( / ) , a n d t h e t w o d o m i n a n t d i u r n a l t i d a l c o n s t i t u e n t s (Ki a n d Oi). Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 175 i n s u m m e r 1994 r e v e a l e d t h r e e a n t i c y c l o n i c e d d i e s a l o n g t h e K K T , i n c l u d i n g o n e j u s t s o u t h e a s t o f B u s s o l ' S t r a i t ( R o g a c h e v a n d V e r k h u n o v , 1 9 9 6 ) . E d d y A l w a s a p p a r e n t l y s t i l l p r e s e n t s o u t h e a s t o f B u s s o l ' S t r a i t n e a r l y a y e a r a f t e r t h e d r i f t e r o b s e r v a t i o n s w e r e m a d e . I n f a c t , t h e r e i s e v i d e n c e t o s u g g e s t t h a t E d d y A l o r i g i n a t e d as K u r o s h i o w a r m - c o r e r i n g 8 6 B i n l a t e s u m m e r 1 9 8 6 , a n d w a s s a m p l e d b y d r i f t e r 1 3 1 5 m o r e t h a n f o u r y e a r s i n t o i t s l i f e ( F i g u r e 7 .2 ; L o b a n o v e t a l . , 1 9 9 1 ; Y a s u d a e t a l . , 1 9 9 2 ; R o g a c h e v a n d C a r m a c k , 1 9 9 8 ) . I f t h i s w e r e t h e c a s e , E d d y A l e x i s t e d f o r a t l e a s t f i v e y e a r s . W h a t a r e t h e m e c h a n i s m s r e s p o n s i b l e f o r s u c h a l o n g l i f e ? R o g a c h e v a n d C a r m a c k ( 1 9 9 8 ) h a v e s u g g e s t e d t h a t d i r e c t t i d a l f o r c i n g (a t t h e d i u r n a l f r e q u e n c y , a n d i n r e s o n a n c e w i t h t h e e f f e c t i v e C o r i o l i s f r e q u e n c y ) o f n e a r - i n e r t i a l w a v e s p r o v i d e s a c o n s t a n t s u p p l y o f e n e r g y t o t h e e d d y , a n d , b e c a u s e t i d a l n e a r - i n e r t i a l o s c i l l a t i o n s d o n o t e x t r a c t e n e r g y f r o m t h e m e a n f l o w , t h i s w o u l d s e r v e t o p r o l o n g t h e e d d y ' s l i f e . H o w e v e r , as p o i n t e d o u t i n S e c t i o n 7 .4 , t h e r e d o e s not a p p e a r t o b e r e s o n a n c e o f i n e r t i a l w a v e s w i t h t h e d i u r n a l t i d e , so t h i s m e c h a n i s m c a n b e r u l e d o u t . T h e y a l s o s u g g e s t t h a t i n e r t i a l w a v e t r a p p i n g a n d a m p l i f i c a t i o n a l o n g t h e e d d y ' s b o u n d a r y w o u l d l e a d t o m i x i n g b e t w e e n w a t e r s w i t h l a r g e t e m p e r a t u r e d i f f e r e n c e s , w h i c h i n t u r n w o u l d l e a d t o t h e c o n v e r s i o n o f p o t e n t i a l e n e r g y t o k i n e t i c e n e r g y d u e t o v o l u m e c o n t r a c t i o n u p o n m i x i n g . E v e n i f t h i s m e c h a n i s m d i d o c c u r , i t i s n o t c l e a r i f t h e t e m p e r a t u r e g r a d i e n t s o b s e r v e d h e r e w o u l d b e s u f f i c i e n t t o m a i n t a i n t h i s e n e r g y c o n v e r s i o n f o r f i v e y e a r s o r m o r e , a n d R o g a c h e v a n d C a r m a c k ( 1 9 9 8 ) d o n o t o f fe r a n y t i m e - s c a l e e s t i m a t e s . A s t h e r e h a v e b e e n n o c o n t i n u o u s l o n g - t e r m o b s e r v a t i o n s w i t h i n a n y K K T e d d y , o r m e a s u r e m e n t s o f f i n e - s c a l e m i x i n g a n d t u r b u l e n t d i s s i p a t i o n , t h e o r i e s o n t h e m e c h a n i s m s r e s p o n s i b l e f o r t h e i r l o n g e v i t y r e m a i n l a r g e l y u n t e s t e d . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 176 F i g u r e 7 .12 : ( t o p ) A c c e l e r a t i o n p o t e n t i a l a n o m a l y (10 m2/s2) o n t h e erg = 2 6 . 8 s u r f a c e r e f e r r e d t o 1500 d b , d e r i v e d f r o m C T D c a s t s t a k e n from t h e R / V H o k k o M a r u i n A u - g u s t - S e p t e m b e r , 1990 . ( b o t t o m ) P o t e n t i a l t e m p e r a t u r e ( ° C ) o n t h e erg = 2 6 . 8 s u r f a c e , f r o m t h e s a m e C T D s u r v e y as ( t o p ) . F r o m K o n o ( 1 9 9 6 ) . Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 1 7 7 7.7 Conclusion T h e c h a r a c t e r i s t i c s o f t w o l a r g e a n t i c y c l o n i c e d d i e s l o c a t e d o v e r t h e K u r i l - K a m c h a t k a T r e n c h h a v e b e e n d e s c r i b e d u s i n g p o s i t i o n d a t a o b t a i n e d f r o m s a t e l l i t e - t r a c k e d d r i f t e r s i n f a l l 1 9 9 0 a n d l a t e s u m m e r 1 9 9 3 . I n 1 9 9 0 , a d r i f t e r d e p l o y e d n e a r t h e e d d y ' s c e n t e r w a s r e t a i n e d f o r o v e r 40 d a y s , d u r i n g w h i c h i t m a d e f i v e l o o p s a t s u c c e s s i v e l y g r e a t e r d i s t a n c e s f r o m t h e e d d y c e n t e r . H i g h - a m p l i t u d e i n e r t i a l o s c i l l a t i o n s , w h i c h c o i n c i d e d w i t h a s t o r m p a s s a g e , w e r e o b s e r v e d d u r i n g t h e f i r s t t w o l o o p s . T h e r e l a t i v e v o r t i c i t y o f t h e e d d y r e s u l t e d i n a l o w e r i n g o f t h e i n e r t i a l w a v e b a n d , as p r e d i c t e d b y K u n z e ( 1 9 8 5 ) a n d o t h e r s . I n 1 9 9 3 , a d r i f t e r w a s a g a i n r e t a i n e d i n a n e d d y f o r ~ 4 0 d a y s . T h i s e d d y h a d c h a r a c t e r i s t i c s s i m i l a r t o t h e 1990 e d d y , b u t w a s r e l a t i v e l y d e v o i d o f s i g n i f i c a n t h i g h - f r e q u e n c y m o t i o n s u n t i l t h e d r i f t e r ' s f i n a l h a l f - l o o p . T h e s i z e a n d p r e v a l e n c e o f K K T e d d i e s i m p l i e s t h a t t h e y c o u l d h a v e a l a r g e i m p a c t o n t h e t r a n s p o r t d y n a m i c s o f t h e E a s t K a m c h a t k a a n d O y a s h i o C u r r e n t s . A l t h o u g h t h e O y a s h i o h a s b e e n o b s e r v e d t o m e a n d e r a r o u n d t h e r e l a t i v e l y s t a t i o n a r y e d d i e s ( K o n o , 1 9 9 6 ) , i t i s l i k e l y t h a t m i x i n g e v e n t s a r e o c c u r r i n g a l o n g t h e p e r i m e t e r s o f t h e e d d i e s . T h u s , i t s e e m s r e a s o n a b l e t o s u p p o s e t h a t K K T e d d i e s c o u l d a f f e c t t h e c h a r a c t e r i s t i c s a n d s p r e a d i n g o f O k h o t s k S e a w a t e r , a n d t h u s m o d i f y t h e d e n s i t y a n d v o l u m e o f n e w l y - p r o d u c e d N P I W . A l t h o u g h t h e d r i f t e r d a t a a l l o w e d f o r a d e s c r i p t i o n o f s o m e o f t h e g e n e r a l c h a r a c t e r - i s t i c s o f K K T e d d i e s , t h e r e a r e m a n y q u e s t i o n s w h i c h r e m a i n u n a n s w e r e d , i n c l u d i n g : • W h a t i s t h e o r i g i n a n d l i f e t i m e o f K K T e d d i e s ? W h a t m e c h a n i s m s c o u l d b e r e s p o n - s i b l e f o r t h e l o n g e v i t y i m p l i e d f r o m t h e a v a i l a b l e o b s e r v a t i o n s ? T h e r e a r e t w o m a i n i d e a s o n t h e n a t u r e o f t h e K K T e d d i e s : (1) T h e y a r e K u r o s h i o w a r m - c o r e r i n g s w h i c h h a v e m i g r a t e d n o r t h w a r d t o t h e r e g i o n , a n d (2) t h e y a r e f o r m e d a n d r e m a i n i n t h e v i c i n i t y Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 178 o f t h e c e n t r a l K u r i l I s l a n d s . A l t h o u g h t h e r e i s e v i d e n c e t o s u p p o r t t h e f o r m e r h y p o t h - es is ( c f . , L o b a n o v e t a l . , 1 9 9 1 ) , i t r e m a i n s u n c l e a r w h y t h e e d d i e s a r e m o s t c o m m o n l y o b s e r v e d a l o n g t h e t r e n c h a x i s a n d , i n p a r t i c u l a r , n e a r t h e d e e p e s t s t r a i t o f t h e K u r i l I s l a n d s ( B u s s o l ' S t r a i t ) . R a b i n o v i c h ( p e r s . c o m m . ) h a s s u g g e s t e d t h a t t r e n c h w a v e s m a y p l a y a r o l e i n t h e o r i g i n a n d b e h a v i o r o f K K T e d d i e s , a n d t h a t t h e K K T a n d B u s s o l ' S t r a i t m a y a c t t o t r a p t h e e d d i e s a l o n g t h e t r e n c h a x i s ( " a p i n g p o n g b a l l o n a f o u n t a i n j e t " ) . L o n g - t e r m o b s e r v a t i o n s , as w e l l as m i c r o s c a l e m e a s u r e m e n t s w h i c h c o u l d q u a n t i f y t u r b u l e n t d i s s i p a t i o n r a t e s w i t h i n t h e e d d i e s , a r e n e e d e d t o a d d r e s s t h e s e q u e s t i o n s . • W h a t r o l e d o e s e x t e r n a l l y - f o r c e d n e a r - i n e r t i a l e n e r g y p l a y i n t h e i n t e r n a l d y n a m i c s o f K K T e d d i e s ? A r e n e a r - i n e r t i a l w a v e s e v e r t i d a l l y - i n d u c e d ? W i n d d a t a f r o m t h e p e r i o d o f d r i f t e r 1 3 1 5 ' s s a m p l i n g c o u l d h e l p t o a d d r e s s t h i s i s s u e , as w e l l as t h e q u e s t i o n o f w h y t h e d r i f t e r s p i r a l e d o u t o f t h e e d d y . • W h a t i s t h e p r e c i s e r o l e o f K K T e d d i e s i n t h e m i x i n g a n d s p r e a d i n g o f O k h o t s k S e a w a t e r ? D o t h e y m o d i f y t h e c h a r a c t e r i s t i c s a n d t r a n s p o r t o f e v e n t u a l N P I W ? A l o n g e r r e c o r d o f h i g h - q u a l i t y h y d r o g r a p h i c d a t a c o u l d p r o v i d e t h e s e a n s w e r s . S u b s u r f a c e f l o a t s , w h i c h a r e n o t a f f e c t e d b y w i n d s l i p p a g e , h a v e b e e n u s e d w i t h g r e a t s u c c e s s t o e x p l o r e t h e o r i g i n , m o v e m e n t , d y n a m i c s , a n d l i f e - s t a g e c h a r a c t e r i s t i c s o f M e d - d i e s ( B o w e r e t a l . , 1 9 9 5 ) . I f s e e d e d a t a n a p p r o p r i a t e d e p t h n e a r t h e c e n t e r o f a K u r o s h i o w a r m - c o r e r i n g , a f l o a t c o u l d r e m a i n t h e r e f o r t h e e x t e n t o f t h e e d d y ' s l i f e . I t i s r e c - o m m e n d e d t h a t f l o a t s b e u t i l i z e d i n c o n j u n c t i o n w i t h m i c r o s c a l e m e a s u r e m e n t s , s a t e l l i t e A V H R R a n d a l t i m e t r y d a t a , s u r f a c e d r i f t e r d e p l o y m e n t s , a n d c o n t i n u e d h y d r o g r a p h i c s a m p l i n g i n o r d e r t o b e t t e r u n d e r s t a n d t h e d y n a m i c s a n d l i f e h i s t o r y o f K K T e d d i e s , a n d t h e i r r e l a t i v e i m p o r t a n c e i n l o c a l b i o l o g i c a l p r o d u c t i o n a n d t h e f o r m a t i o n a n d s p r e a d i n g o f N P I W . Chapter 8 Summary and Synthesis 8.1 Summary T h i s t h e s i s h a s p r e s e n t e d a n a n a l y s i s a n d i n t e r p r e t a t i o n o f t h e p o s i t i o n t i m e s e r i e s , a n d a c c o m p a n y i n g d e r i v e d v e l o c i t i e s , o b t a i n e d f r o m a l a r g e set o f s a t e l l i t e - t r a c k e d d r i f t e r s d e p l o y e d a n d t r a c k e d t h r o u g h o u t t h e N o r t h P a c i f i c O c e a n o v e r t h e p e r i o d 1 9 9 0 - 1 9 9 5 . A m a j o r i t y o f t h e d r i f t e r s w e r e d r o g u e d w i t h i n t h e m i x e d l a y e r ( 15 m d e p t h ) , a n d c o m p r i s e d t h e N o r t h P a c i f i c c o m p o n e n t o f t h e W O C E - S V P , w h i l e t h e r e s t w e r e d r o g u e d w i t h i n t h e u n d e r l y i n g p y c n o c l i n e ( 120 m d e p t h ) . T h e o v e r a l l o b j e c t i v e o f t h i s w o r k w a s t o d e s c r i b e t h e o b s e r v e d c i r c u l a t i o n a n d i t s v a r i a b i l i t y a t b o t h d r o g u e d e p t h s . T h i s w a s a c c o m p l i s h e d t h r o u g h s e v e r a l i n d e p e n d e n t a n a l y s e s , e a c h f o c u s i n g o n a p a r t i c u l a r s u i t e o f s t a t i s t i c a l m e t h o d s o r s u b s e t o f d r i f t e r s . A s u m m a r y o f t h e p r i n c i p a l r e s u l t s o f e a c h a n a l y s i s i s g i v e n b e l o w : (a ) SAMPLING STRATEGIES • A s a f i r s t s t e p , t h e e f f e c t s o f r e d u c e d s a m p l i n g s c h e d u l e s ( d u t y c y c l e s ) o n d r i f t e r - d e r i v e d v e l o c i t y s t a t i s t i c s w e r e i n v e s t i g a t e d b y d e g r a d i n g c o n t i n u o u s s e g m e n t s o f d r i f t e r r e c o r d s t o m a t c h t h e s t a n d a r d d u t y c y c l e u s e d i n t h e W O C E - S V P (48 h o u r s o f n o A R G O S d a t a t r a n s m i s s i o n f o l l o w e d b y 24 h o u r s o f r e c e i v e d t r a n s m i s s i o n ) a n d t w o o t h e r d u t y c y c l e s ( 3 2 - 1 6 h a n d 1 6 - 8 h ) . 179 Chapter 8. Summary and Synthesis 180 • I t w a s f o u n d t h a t s t r o n g h i g h - f r e q u e n c y ( p r i m a r i l y i n e r t i a l a n d s e m i d i u r n a l ) m o - t i o n s p r e v a l e n t i n t h e d r i f t e r r e c o r d s c o u l d r e s u l t i n s i g n i f i c a n t l y b i a s e d s t a t i s t i c s d e r i v e d f r o m t h e d e g r a d e d s e r i e s . I n p a r t i c u l a r , r e p r o d u c t i o n o f t h e v e l o c i t y v a r i a n c e s a n d r o - t a r y s p e c t r a l c h a r a c t e r i s t i c s o f t h e o r i g i n a l t i m e se r i es w a s s t r o n g l y d e p e n d e n t o n t h e i n t e r p o l a t i o n s c h e m e a p p l i e d t o t h e d u t y c y c l e r e c o r d . • T h e c o m m o n l y u s e d s p l i n e i n t e r p o l a t i o n p r o d u c e d n u m e r o u s s p u r i o u s s p e c t r a l p e a k s , w i t h h i g h - f r e q u e n c y e n e r g y a l i a s e d t o m u l t i p l e s o f t h e d u t y c y c l e f r e q u e n c y . R e p r o d u c t i o n o f t h e o r i g i n a l ( u n d e g r a d e d ) p r i m e a n d s p e c t r a l v e l o c i t y s t a t i s t i c s r e q u i r e d a n i n t e r p o - l a t i o n w h i c h t o o k i n t o a c c o u n t t h e o s c i l l a t o r y c o m p o n e n t o f t h e d r i f t e r m o t i o n s . T h e m u l t i - f u n c t i o n a l fit ( M F F ) , w h i c h f i t s a f o u r t h - o r d e r p o l y n o m i a l t o t h e o b s e r v a t i o n s p l u s a n o s c i l l a t i o n w h o s e f r e q u e n c y i s d e t e r m i n e d b y t h e i n t e r p o l a t i o n , w a s a b l e t o r e p r o - d u c e t h e r o t a r y s p e c t r a l f e a t u r e s o f t h e o r i g i n a l d a t a se r i es f o r e a c h o f t h e t h r e e t y p e s o f d e g r a d e d s e r i e s e x a m i n e d . • T h e M F F i n t e r p o l a t i o n m o s t c o n s i s t e n t l y r e p r o d u c e d t h e s p e c t r a l c h a r a c t e r i s t i c s o f t h e o r i g i n a l c o n t i n u o u s r e c o r d s f o r t h e 3 2 - 1 6 h d u t y c y c l e . T h i s i s p r e s u m a b l y b e c a u s e t h e 1 6 h t r a n s m i s s i o n - r e c e i v e d s e g m e n t s a r e l o n g e n o u g h t o c a p t u r e t h e l o c a l i n e r t i a l p e r i o d , w h i l e t h e 3 2 h t r a n s m i s s i o n - b l a c k o u t s e g m e n t s a r e s h o r t e n o u g h t o l e a v e t h e m a i n f e a t u r e s o f t h e m o t i o n s r e l a t i v e l y u n c h a n g e d . T h e d u t y c y c l e w i t h t h e s h o r t e s t a n d m o s t f r e q u e n t g a p s ( 1 6 - 8 h ) y i e l d e d t h e w o r s t r e p r o d u c t i o n , w i t h s i g n i f i c a n t l y o v e r e s t i m a t e d s u p e r i n e r t i a l e n e r g i e s . • R e c o m m e n d a t i o n s w e r e m a d e f o r u s e r s t o t a k e i n t o a c c o u n t t h e d o m i n a n t h i g h - f r e q u e n c y s i g n a l s i n t h e i r d r i f t e r r e c o r d s , a n d t o c u s t o m i z e t h e i r i n t e r p o l a t i o n s c h e m e s t o a c c o u n t f o r t h e s e m o t i o n s . I t w a s a l s o r e c o m m e n d e d t h a t S e r v i c e A R G O S p r o v i d e a s e l e c t i o n o f d u t y c y c l e o p t i o n s w h i c h d e p e n d o n t h e l a t i t u d e o f t h e d r i f t e r d e p l o y m e n t s ( s p e c i f i c a l l y , e q u a t o r i a l v e r s u s n o n - e q u a t o r i a l ) . Chapter 8. Summary and Synthesis 181 ( b ) EULERIAN STATISTICS • T h e t r a j e c t o r i e s o f a l l d r i f t e r s w e r e u s e d t o c h a r a c t e r i z e t h e u p p e r - o c e a n ( 15 m a n d 1 2 0 m ) m e a n c i r c u l a t i o n a n d e d d y v a r i a b i l i t y i n t h e N o r t h P a c i f i c O c e a n o v e r t h e p e r i o d 1 9 9 0 - 1 9 9 5 . E s t i m a t e s o f i n t e g r a l t i m e a n d l e n g t h s c a l e s w e r e d e r i v e d f r o m t h e e n s e m b l e - m e a n a u t o c o r r e l a t i o n f u n c t i o n s . E n s e m b l e - m e a n v e l o c i t i e s a n d ( m e a n a n d e d d y ) k i n e t i c e n e r g i e s w e r e d e r i v e d i n g r i d b o x e s w h i c h w e r e c h o s e n t o s a t i s f y a c o m p r o m i s e b e t w e e n h o r i z o n t a l r e s o l u t i o n a n d s t a t i s t i c a l r e l i a b i l i t y . • A l l b r a n c h e s o f t h e A l a s k a n G y r e w e r e w e l l - s a m p l e d a t b o t h d r o g u e d e p t h s . A t 15 m d e p t h , a w e a k S u b a r c t i c C u r r e n t w a s o b s e r v e d , a l o n g w i t h s t r o n g , v a r i a b l e f l o w i n t h e A l a s k a C u r r e n t a n d A l a s k a n S t r e a m . T h e b i f u r c a t i o n o f t h e S u b a r c t i c C u r r e n t w a s o b s e r v e d n e a r 48°N, 130°W. A t 120 m d e p t h , n o r t h w a r d f l o w i n t h e A l a s k a C u r r e n t o c c u r r e d m u c h f u r t h e r o f f s h o r e t h a n w i t h i n t h e m i x e d l a y e r , r e v e a l i n g a r e l a t i v e l y s m a l l g y r e . A r e g i o n d e v o i d o f d r i f t e r d a t a ( n e a r 52°N, 155°W) w a s i d e n t i f i e d as t h e " c e n t e r " o f t h e A l a s k a n G y r e d u r i n g t h e o b s e r v a t i o n p e r i o d . A n e s t i m a t e o f t h e w i n t e r m e a n E k m a n p u m p i n g v e l o c i t y a t t h i s l o c a t i o n w a s ~ 7 x 1 0 ~ 5 c m / s . • M e a n t r a n s i t t i m e s a r o u n d t h e A l a s k a n G y r e w e r e o n t h e o r d e r o f 1-2 y e a r s , a l t h o u g h a f e w d r i f t e r s r e v e a l e d m u c h l o n g e r r e t e n t i o n t i m e s ( 3 - 4 y e a r s ) i n t h e n e a r - s u r f a c e w a t e r s o f t h e G u l f o f A l a s k a . • A m i n i m u m i n e d d y k i n e t i c e n e r g y w a s o b s e r v e d i n t h e n o r t h e r n s u b t r o p i c a l g y r e ( t h e " e d d y d e s e r t " ) . E d d y k i n e t i c e n e r g i e s w e r e n e a r l y t w i c e as h i g h i n t h e m i x e d l a y e r c o m p a r e d t o b e l o w , a n d 2 - 3 t i m e s l a r g e r i n w i n t e r t h a n i n s u m m e r t h r o u g h o u t m o s t o f t h e n e a r - s u r f a c e A l a s k a n G y r e . T h e h i g h e d d y k i n e t i c e n e r g i e s o b s e r v e d a l o n g t h e p e r i m e t e r o f t h e A l a s k a n G y r e m a y b e d u e t o t h e o f f s h o r e i n t r u s i o n o f e d d i e s f o r m e d b y c o a s t a l c u r r e n t i n s t a b i l i t i e s . • T h e r e w a s e v i d e n c e o f i n t e r a n n u a l v a r i a b i l i t y i n t h e d r i f t e r t r a j e c t o r i e s , w i t h a n a p p a r e n t l y i n t e n s i f i e d A l a s k a n G y r e d u r i n g w i n t e r 1 9 9 3 - 9 4 a n d m o r e s o u t h e r l y t r a n s p o r t Chapter 8. Summary and Synthesis 182 t h e f o l l o w i n g y e a r . (c ) EDDY STATISTICS • T a y l o r ' s t h e o r y o f s i n g l e p a r t i c l e d i s p e r s i o n w a s a p p l i e d t o t h e d r i f t e r e n s e m b l e s i n o r d e r t o d e r i v e t h e m a g n i t u d e o f t h e e d d y m i x i n g s c a l e s a n d d i f f u s i v i t i e s o v e r a b r o a d r e g i o n o f t h e N o r t h P a c i f i c O c e a n a n d a t b o t h d r o g u e d e p t h s . • B o t h t h e i n i t i a l d i s p e r s i o n a n d r a n d o m w a l k r e g i m e s p r e d i c t e d b y T a y l o r ' s t h e - o r y w e r e i d e n t i f i e d i n t h e d i s p e r s i o n t i m e se r i es c o m p u t e d f o r s e v e r a l r e g i o n s o f b o t h e n s e m b l e s . • T h e i n t e g r a l t i m e s c a l e s a n d e d d y d i f f u s i v i t i e s c o m p u t e d f r o m t h e d i s p e r s i o n s c a l e l i n e a r l y w i t h r . m . s . v e l o c i t y . A n e x c e p t i o n i s t h e m e r i d i o n a l t i m e s c a l e , w h i c h a p p e a r s t o h a v e a w e a k e r d e p e n d e n c e o n f r . m . « . - • T h e m a g n i t u d e s o f t h e d e r i v e d e d d y s t a t i s t i c s a r e c o m p a r a b l e t o t h o s e d e r i v e d f r o m s u r f a c e d r i f t e r s i n o t h e r p a r t s o f t h e w o r l d o c e a n . T h e c o n s i s t e n c y o f t h e r e s u l t s w i t h p r e v i o u s s t u d i e s l e n d s c r e d e n c e t o t h e i d e a t h a t t h e s i m p l i f y i n g a s s u m p t i o n s o f T a y l o r ( 1 9 2 1 ) a r e r e a s o n a b l y v a l i d t h r o u g h o u t t h e u p p e r o c e a n , w h i c h b o d e s w e l l f o r t h e e f f e c t i v e p a r a m e t e r i z a t i o n o f n e a r - s u r f a c e d i f f u s i v i t i e s i n g e n e r a l c i r c u l a t i o n m o d e l s . ( d ) SEAMOUNT-ATTACHED EDDIES • A s u b s e t o f d r i f t e r s w e r e u s e d t o e x a m i n e e d d y a c t i v i t y i n t h e v i c i n i t y o f t h e E m p e r o r S e a m o u n t C h a i n ( ~ 170°E) d u r i n g s u m m e r a n d f a l l 1 9 9 2 . • T h e t r a j e c t o r i e s o f t w o d e e p - d r o g u e d d r i f t e r s r e v e a l e d a p a i r o f c o u n t e r r o t a t i n g m e s o s c a l e e d d i e s a t t a c h e d t o t h e l e e s i d e o f O j i n / J i n g u S e a m o u n t . T h e e d d i e s h a d d i a m - e t e r s o f 7 5 - 1 0 0 k m a n d m e a n r o t a t i o n a l s p e e d s o f 2 0 - 4 0 c m / s . O n e o f t h e d r i f t e r s m a d e Chapter 8. Summary and Synthesis 183 f i v e l o o p s w i t h i n t h e c y c l o n i c e d d y o v e r a p e r i o d o f 62 d a y s , d u r i n g w h i c h t i m e t h e e d d y t r a n s l a t e d w e s t w a r d a t ~ 2 .9 c m / s u n t i l c o l l i d i n g w i t h t h e s e a m o u n t s . T h i s i s o n e o f t h e f i r s t o b s e r v a t i o n s d e m o n s t r a t i n g a n e x t e n d e d a t t a c h m e n t o f a t o p o g r a p h i c a l l y - g e n e r a t e d e d d y t o a s e a m o u n t . • S h a l l o w - d r o g u e d d r i f t e r s w h i c h c r o s s e d t h e E S C i n t h e s u m m e r o f 1 9 9 1 a n d t h e w i n t e r o f 1 9 9 3 r e v e a l e d n o e d d y a c t i v i t y , m o s t l i k e l y b e c a u s e o f a d e c o u p l i n g o f t h e t o p o g r a p h i c i n f l u e n c e t o t h e 15 m f l o w a t t h e i r c r o s s i n g l a t i t u d e o v e r t h e s h o r t e r N i n t o k u S e a m o u n t ( s u m m i t d e p t h a t 1000 m ) . T h e i m p l i c a t i o n i s t h a t e d d y f o r m a t i o n w i t h i n t h e m i x e d l a y e r n e a r t h e E S C is c o n f i n e d t o t h e r e g i o n a r o u n d t h e t a l l e r O j i n / J i n g u a n d K i n m e i S e a m o u n t s . • T h e o b s e r v a t i o n s o f a t t a c h e d l e e s i d e e d d i e s a t t h e E S C m a t c h t h e p r e d i c t i o n s o f n u m e r i c a l a n d a n a l y t i c a l m o d e l s v e r y w e l l . (e) TRENCH-TRAPPED EDDIES • S u b s e t s o f d r i f t e r s n e a r t h e K u r i l - K a m c h a t k a T r e n c h i n t h e w e s t e r n N o r t h P a c i f i c r e v e a l e d t h e p r e s e n c e o f l a r g e a n t i c y c l o n i c e d d i e s p o s i t i o n e d o v e r t h e d e e p e s t p a r t o f t h e t r e n c h , b o t h i n f a l l 1990 a n d l a t e s u m m e r 1 9 9 3 . • I n f a l l 1 9 9 0 , a d r i f t e r r e l e a s e d n e a r t h e c e n t e r o f a p r e v i o u s l y i d e n t i f i e d e d d y r e m a i n e d i n t h e e d d y f o r 4 3 d a y s , d u r i n g w h i c h i t m a d e f i v e l o o p s a t s u c c e s s i v e l y g r e a t e r d i s t a n c e s f r o m t h e e d d y c e n t e r . H i g h - a m p l i t u d e i n e r t i a l o s c i l l a t i o n s , w h i c h c o i n c i d e d w i t h a s t o r m p a s s a g e , w e r e o b s e r v e d d u r i n g t h e f i r s t t w o l o o p s . T h e r e l a t i v e v o r t i c i t y o f t h e e d d y r e s u l t e d i n a l o w e r i n g o f t h e i n e r t i a l w a v e b a n d , as p r e d i c t e d b y K u n z e ( 1 9 8 5 ) a n d o t h e r s . • I t h a s b e e n p r o p o s e d t h a t t h e b r o a d e n i n g o f t h e i n e r t i a l w a v e b a n d b y t h e e d d y ' s r o t a t i o n r e s u l t e d i n r e s o n a n c e w i t h t h e d i u r n a l t i d e , a n d a s u b s e q u e n t e n e r g y i n p u t w h i c h i n c r e a s e d t h e e d d y ' s l i f e . A s p e c t r a l a n a l y s i s o f t h e d r i f t e r ' s r e c o r d w i t h i n t h e 1 9 9 0 e d d y Chapter 8. Summary and Synthesis 184 s u g g e s t e d t h a t t h e p r o p o s e d m e c h a n i s m is i n v a l i d . • T h e 1 9 9 0 a n d 1 9 9 3 e d d i e s h a d s i m i l a r c h a r a c t e r i s t i c s , w i t h d i a m e t e r s o f ~ 2 0 0 k m a n d m e a n r o t a t i o n a l s p e e d s o f ~ 4 5 c m / s a t 8 0 - 1 0 0 k m f r o m t h e e d d y c e n t e r . • T h e d r i f t e r o b s e r v a t i o n s s u p p o r t t h e i m p l i c a t i o n f r o m h i s t o r i c a l d a t a t h a t l a r g e , l o n g - l i v e d a n t i c y c l o n i c e d d i e s a r e c o m m o n i n t h e K K T r e g i o n . 8.2 A S y n t h e s i s I n o r d e r t o d e s c r i b e i n d i v i d u a l c i r c u l a t i o n f e a t u r e s o r t o d e f i n e a m e a n c i r c u l a t i o n o v e r a b r o a d r e g i o n , c a r e f u l p l a n n i n g m a y g o i n t o t h e d e p l o y m e n t o f a set o f s a t e l l i t e - t r a c k e d d r i f t e r s . T h e a c q u i r e d o b s e r v a t i o n s , h o w e v e r , w i l l g e n e r a l l y e v o l v e i n w a y s t h a t a r e n e i t h e r p r e d i c t a b l e n o r n e c e s s a r i l y r e p r e s e n t a t i v e o f t h e c i r c u l a t i o n f e a t u r e s i n t e n d e d t o b e s a m p l e d . W h a t L a g r a n g i a n i n s t r u m e n t s d o p r o v i d e is a snapshot o f t h e c i r c u l a t i o n , a g l i m p s e o f t h e s t a t e o f t h e o c e a n o v e r a p e r i o d o f t i m e w h i c h is s h o r t c o m p a r e d t o t h e t i m e s c a l e s o v e r w h i c h l a r g e - s c a l e c h a n g e s i n t h e o c e a n t y p i c a l l y t a k e p l a c e . W i t h t h e d r i f t e r d e p l o y m e n t s p r e s e n t e d h e r e , t h e p r i m a r y o b j e c t i v e w a s t o d e s c r i b e a n d c o m p a r e t h e m e a n c i r c u l a t i o n a n d i t s v a r i a b i l i t y i n t h e N o r t h P a c i f i c O c e a n a t d e p t h s r e p r e s e n t i n g t h e m i x e d l a y e r a n d t h e u n d e r l y i n g p y c n o c l i n e . T h e d r i f t e r s o b l i g e d , a n d , i n e t c h i n g o u t a s n a p s h o t o f t h e s t a t e o f t h e o c e a n a t t h e d r o g u e l e v e l s , p r o v i d e d n u m e r o u s s u r p r i s e s as w e l l . T h r o u g h o u t t h e t h e s i s , t h i s s n a p s h o t h a s b e e n v i e w e d f r o m v a r i o u s a n g l e s . I n C h a p t e r 3 , r e f i n e m e n t s t o t h e t o o l s u s e d t o d e f i n e t h e i m a g e ( t h e p o s i t i o n t i m e s e r i e s ) w e r e e x p l o r e d so t h a t t h e s n a p s h o t p r o d u c e d w o u l d b e a s t a t i s t i c a l l y r e l i a b l e o n e . I n C h a p t e r s 4 a n d 5 , t h e " w i d e - a n g l e " v i e w w a s p r e s e n t e d , i n w h i c h all t r a j e c t o r i e s w e r e u s e d t o d e r i v e e n s e m b l e - m e a n v e l o c i t y s t a t i s t i c s a n d a c h a r a c t e r i z a t i o n o f t h e n e a r - s u r f a c e c i r c u l a t i o n o v e r t h e o b s e r v a t i o n p e r i o d . I n C h a p t e r s 6 a n d 7, t h e s n a p s h o t w a s v i e w e d w i t h a Chapter 8. Summary and Synthesis 185 n a r r o w e r f o c u s , w i t h s o m e o f i t s m o r e i n t e r e s t i n g f e a t u r e s , i . e . , t o p o g r a p h i c a l l y - c o n t r o l l e d m e s o s c a l e e d d i e s , b e i n g e x a m i n e d i n g r e a t e r d e t a i l . T h e t h e s i s h a s i n t e g r a t e d t h e s e d i s p a r a t e v i e w s i n t o o n e d o c u m e n t , y i e l d i n g a c o m p r e h e n s i v e d e s c r i p t i o n o f t h e n e a r - s u r f a c e c i r c u l a t i o n i n t h e N o r t h P a c i f i c O c e a n o v e r t h e p e r i o d 1 9 9 0 - 1 9 9 5 . A l t h o u g h i t w i l l n o t b e k n o w n f o r a l o n g t i m e j u s t h o w " r e p r e s e n t a t i v e " t h i s s n a p s h o t i s , t h e s e o b s e r v a t i o n s c a n b e u s e d t o p u t c o n s t r a i n t s o n t h e p l a n n i n g o f f u t u r e d r i f t e r d e p l o y m e n t s i n t h e r e g i o n , a n d t o p r o v i d e i m p r o v e d p a r a m e t e r i z a t i o n s f o r , a n d t r u t h i n g of , g e n e r a l c i r c u l a t i o n m o d e l s . T h e t h e s i s h a s t h e r e f o r e m a d e a c o n t r i b u t i o n t o t h e p r i n c i p a l o b j e c t i v e s o f t h e W o r l d O c e a n C i r c u l a t i o n E x p e r i m e n t a n d t h e S u r f a c e V e l o c i t y P r o g r a m , w h i l e a m p l y s a t i s f y i n g t h e o b j e c t i v e s o u t l i n e d f o r t h i s s t u d y . Bibliography A r m i , L . J . , D . H e b e r t , N . O a k e y , J . F . P r i c e , P . L . R i c h a r d s o n , e t a l . ( 1 9 8 9 ) . 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