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Lagrangian observations of the near-surface circulation in the North Pacific, 1990-1995 Bograd, Steven J. 1998

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LAGRANGIAN OBSERVATIONS OF THE NEAR-SURFACE CIRCULATION IN THE NORTH PACIFIC, 1990-1995 by Steven J. Bograd B. S., B. S. (Physics, Atmospheric Sciences) University of Arizona, 1985 M. S. (Atmospheric Sciences) University of Washington, 1989 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY ' in THE FACULTY OF GRADUATE STUDIES EARTH AND OCEAN SCIENCES We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June 1998 © Steven J. Bograd, 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 An 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 North 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 (WOCE-SVP), 102 drifters were released in the North 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. All 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. ii 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 viList of Figures x Acknowledgments xxii 1 Introduction 1 1.1 Studying Ocean Circulation Using Lagrangian Instruments 1 1.1.1 Lagrangian vs. Eulerian Measurements 1 1.1.2 Bottles, Cards, Drifters and Floats 2 1.2 Thesis Objectives 9 1.3 Thesis Overview 11 2 Data and Methods 3 2.1 The WOCE/SVP Dataset 12.2 Initial Data Processing 7 2.3 Determination of Drogue Loss 19 3 Sampling Strategies and Interpolation Schemes 25 3.1 Introduction 23.2 Data and Methods 7 3.2.1 Drifter Seriesiv 3.2.2 Time series degradation 27 3.2.3 Time series interpolation 9 3.3 Results 35 3.3.1 Prime statistics 33.3.2 Rotary spectral analysis 39 3.4 Discussion 43 3.5 Conclusions 50 4 Mean Circulation and Energy Distribution in the North Pacific 53 4.1 Introduction 54.2 Oceanographic Setting of the Northeast Pacific 54 4.3 Lagrangian Decorrelation Scales 57 4.4 Selection of Grid Geometries 9 4.5 Mean Circulation and Energy Distribution 67 4.5.1 The overview 64.5.2 The Alaskan Stream 75 4.5.3 Variability in the Alaskan Gyre 77 4.6 High Frequency Energy Distribution 84 4.7 Applications 93 5 Eddy Statistics in the North Pacific 95 5.1 Introduction 95.2 Theory and Methods5.3 Applicability to the drifter ensembles 97 6 Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 113 6.1 Introduction 11v 6.2 Theoretical Background 113 6.3 Data and Methods 9 6.3.1 The ESC ensemble 116.3.2 The rotary multiple filter technique 119 6.4 Observations 121 6.4.1 1991 drifters 126.4.2 1992 drifters 2 6.4.3 1993 drifter 139 6.4.4 Satellite altimetry 142 6.5 Discussion 143 6.5.1 Comparison with models 146.5.2 Eddy dimensions 149 6.6 Conclusions 151 7 Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 153 7.1 Introduction 157.2 Oceanographic Setting 153 7.3 The Data 158 7.4 The 1990 Eddy 160 7.5 The 1993 Eddy 171 7.6 Discussion 2 7.7 Conclusion 177 8 Summary and Synthesis I?9 8.1 Summary • • 179 8.2 A Synthesis 184 vi List of 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 May 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-interpolated series of the Station P drifter ensemble. The frequency bands are the same as in Table 3.3 48 vii 3.5 The clockwise (S ) spectral amplitude ratios in four frequency bands, and for total clockwise energy, of the original to the degraded series for the spline-interpolated and MFF-interpolated series of the Station P drifter ensemble. The frequency bands are the same as in Table 3.3 49 4.1 Decorrelation time and space scales derived from the global and regional autocorrelation functions of each ensemble 63 4.2 Clockwise (S~ (w)), counterclockwise (5+(w)) and total (Stot(w)) composite rotary vari ances (cm2/ 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) 92 4.3 The total (Stot(u;)) composite spectral amplitude ratios (shallow/deep) in five frequency bands for each of the deployments containing shallow and deep drifters. The frequency bands are the same as in Table 4.2. Boxed numbers refer to the deployments (see Figure 4.19) 94 5.1 Mean and r.m.s. velocities in the "Taylor" boxes. Standard errors (at the 95% confidence level) are based on different integral time scales in the zonal and meridional directions. Mean positions refer to the center-of-mass locations 101 5.2 Integral time (Tk) and length (Lk) scales and eddy diffusivities (Kkk), with standard errors, derived for the "Taylor" boxes in the shallow and deep drifter ensembles. Mean positions refer to center-of-mass locations. . . . 109 viii 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 May 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 107 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 KKT anticyclonic eddies as delineated by the trajectories of drifters 1315 (Eddy Al) 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 List of Figures 1.1 Schematic showing the design of a modern satellite-tracked drifter. The drogue is a holey sock centered 15 m below the surface. The surface float houses the antenna, battery pack, and sensors. From Sybrandy and Niiler (1991) 6 2.1 Profiles of temperature (T), Brunt-Vaisala frequency (N), and density (crt) taken near Station P (50° N, 145°W) in August 1990. Locations of the two drogue depths are also shown. From Thomson et al. (1998) 14 2.2 Drifter deployment positions for the (a) shallow and (b) deep drifter en sembles 15 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 20 2.4 Time series of daily-averaged drifter speed (top) and acceleration difference (bottom) for deep drifter 1319 22 2.5 Time lines of each of the drifters in the (a) shallow and (b) deep drifter ensembles. Dashed lines correspond to undrogued portions of the drifter trajectories. The dotted curve is the time series of total number of daily drifter observations in each ensemble 24 x 3.1 Map 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 Gulf 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-interpolated 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 MFF2-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 (M2) peaks are shown in (a) 41 3.7 As in Figure 3.6, but for the MFFl-interpolated original and degraded series of drifter 1310 2 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 5 4.1 Map of the North 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 Gulf 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 Gulf of Alaska (456 days), and (d) the Subtropical Gyre (2759 days) 62 xii 4.5 The trajectories of the drogued segments of the (a) shallow and (b) deep drifter ensembles, with overlying grid geometries. The box marked "ESC Eddies" refers to the subject of Chapter 6 and the box marked "KKT Eddies" refers to the subject of Chapter 7 5 4.6 Maps showing annual histograms of number of drifter days in grid boxes for the (a) shallow and (b) deep drifter ensembles. The scale is given in upper right 66 4.7 Maps showing (a) the number of degrees of freedom contained in the grid boxes of the shallow drifter ensemble, and (b) the derived mean veloc ity and mean kinetic energy (MKE; cm2/s2). Boxes with fewer than 30 degrees of freedom are dashed, and marks in (b) refer to the box center-of-mass positions. MKE contours extrapolated beyond the data range in the northwest portion of the map (Bering Sea) are not valid 69 4.8 Maps showing (a) the number of degrees of freedom contained in the grid boxes of the deep drifter ensemble, and (b) the derived mean velocity and mean kinetic energy (MKE; cm2/s2). Boxes with fewer than 30 degrees of freedom are dashed, and marks in (b) refer to the box center-of-mass positions. The closed contour near 48°N, 150°W is a maximum 70 4.9 Maps showing velocity variance ellipses (given as ((u'k2J1/2)) in grid boxes derived from the (a) shallow and (b) deep drifter ensembles. The scale is given in upper right 72 4.10 Zonal vs. meridional r.m.s. speeds derived from the shallow and deep drifter ensembles. The lines are least-squares fits. The deep Alaskan Stream box, which had fewer than 10 degrees of freedom, is not included in this plot 73 Xlll 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 April 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 Ekman vertical velocity, WE (X 10-5 cm/s), derived from COADS winds. Seasons have the same definition as in Figure 4.13. WE is positive upwards. ... 82 4.16 Time series of COADS 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 (PNA) index (octagons), which is defined as a linear combination of the normalized 500 mb height anomalies at four centers located near Hawaii, over the North Pacific, over Alberta and over the U.S. Gulf Coast. The PNA index is a measure of the strength of the Aleutian Low (high positive PNA corresponds to a stronger Aleutian Low) 83 xiv 4.17 Maps showing the winter mean Ekman vertical velocity, WE (x 10 5 cm/s), derived from COADS winds, for (a) 1991-1992 and (b) 1992-1993. 85 4.18 Maps showing the winter mean Ekman vertical velocity, WE (X 10-5 cm/s), derived from COADS winds, for (a) 1993-1994 and (b) 1994-1995. 86 4.19 Map 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. All spectra are for the first 90-day period after initial 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 Time series of eddy diffusivities, using a 5-day running mean filter, derived directly from the derivative of mean square dispersion for one (a) shallow and one (b) deep "Taylor" box from the Subarctic Current region 104 5.6 Mean square dispersion (zonal-dashed, meridional-dotted) over the first 100 days for the shallow "Taylor" boxes. Solid curves are the theoretical values from equation 5.3. Coordinates refer to center-of-mass positions. . 106 5.7 As in Figure 5.6, except for the deep "Taylor" boxes 107 5.8 Maps showing the zonal/meridional integral time scales (days) and eddy diffusivities (x 107 cm2/s) for the "Taylor" boxes in the (a) shallow and (b) deep drifter ensembles 110 5.9 (a) Integral time scale and (b) eddy diffusivity vs. r.m.s. speed derived for the "Taylor" boxes. Straight lines are least-squares fits. The fits are based only on the range of r.m.s. speeds sampled here, and should not be extrapolated to the y-intercept Ill 6.1 Map of the North Pacific Ocean showing location of the Emperor Seamount Chain. Light and dark shading in this and subsequent figures represents water depths of 2000-4000 m and shallower than 2000 m, respectively. The inset shows the bathymetric section along the boxed region, which is the crest of the southern portion of the Emperor Seamount Chain (adapted from Roden et al. (1982)) 117 6.2 Map of dynamic topography (J/kg) of the 150 dbar surface relative to 800 dbar from cruises of the RV Thomas G. Thompson (dots) and RV Hokusei Maru (triangles) in June/July 1982. From Roden (1987) 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 ESC, 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 ESC, 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 ESC, 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 Ojin/Jingu and Kinmei 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 Amplitude evolution of the clockwise and countercockwise rotary velocity components for drifters (a,b) 1417, (c,d) 4859, and (e,f) 8098 for the period June 23 to December 23, 1992. Amplitude contours are given in cm/s. Rotary components for the low-frequency range are shown (log(-0.6) = period of 4 days, log(-l.O) = period of 10 days, log(-1.4) = period of 25 days). The dashed Une near the bottom of each plot indicates the time period that the drifter was in the vicinity of the ESC (between 166°E and 174°E) 133 6.10 Trajectory of drifter 4859 in the vicinity of Ojin/Jingu and Kinmei Seamounts in the summer and fall of 1992. Each mark represents the daily drifter po sition at 1200Z, and selected dates are labeled 134 6.11 (a) Trajectory of drifter 4859 in the cyclonic eddy (eddy C) in the lee of Ojin/Jingu Seamount between August 4 and October 7, 1992. Each differ ent Une type represents a separate loop around the eddy. The approximate center of the eddy, as determined by the drifter track, is marked with an octagon for each loop, (b) Time series of rotational speed derived from drifter 4859 in eddy C. Each of the five loops of eddy C is marked at bottom. 136 6.12 Trajectory of drifter 8098 in the vicinity of Ojin/Jingu and Kinmei Seamounts in the summer and faU of 1992. Each mark represents the daily drifter po sition at 1200Z, and selected dates are labeled 140 6.13 Trajectory of shaUow-drogued drifter 4856, which crossed the Emperor Seamount Chain in the winter of 1993. Each mark represents the daily drifter position at 1200Z, and selected dates are labeled 141 xvin 6.14 (a) Map of sea surface height anomalies (cm) from merged TOPEX/ERS-1 altimetry for TOPEX cycle 2 (October 3-12, 1992) for the North 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 ESC from TOPEX 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) Map of the North Pacific Ocean and surrounding area, showing the location of the Kuril-Kamchatka Trench (KKT). (bottom) Chart of the KKT 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 KKT region. From Lobanov and Bulatov (1993) 157 7.3 Map of the region showing the trajectories of drifter 1315 in eddy Al and drifter 15371 in eddy A2. 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 Al 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 Time series of (a) longitude and (b) latitude from drifter 1315 in Eddy Al during November-December, 1990. The periods of each loop are marked at bottom 162 7.6 Close-up of the trajectory of drifter 1315 within Eddy Al over (a) loop 1 and (b) loop 2. A hexagon marks the loop's start position, 'x's denote 3-hourly positions, and stars denote positions where 3-hourly drifter speeds were greater than 100 cm/s 164 7.7 Time series of zonal (U) and meridional (V) 3-hourly speeds from drifter 1315 in Eddy Al over loops 1 and 2, November 8-19, 1990 165 7.8 Clockwise S~ (solid Une) and counterclockwise S+ (dashed Une) rotary energy density spectra (m2/s2/cpd) derived from the trajectories of drifters (a) 1314, (b) 1315, and (c) 1316 over the period November 8, 1990 to February 6, 1991, and (d) 15371 over the period September 4 to December 3, 1993. The 95% confidence Umits are shown in (a), and the inertial (/) and semidiurnal (M2) peaks are shown in (b) and (d) 167 7.9 AmpUtude evolution of the clockwise rotary velocity component for drifter 1315 in Eddy Al at (a) high frequencies (periods of 11 hours to 1.25 days) and (b) low frequencies (periods of 2 to 20 days). Contours are in cm/s. The start date is November 8, 1990, and the dotted Une in (a) refers to the local value of planetary vorticity (/) at the drifter position 170 7.10 Time series of (a) longitude and (b) latitude from drifter 15371 in Eddy A2 during September-October, 1993 173 xx 7.11 Amplitude evolution of the clockwise rotary velocity component for drifter 15371 in Eddy A2 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 tidal frequency (M2), the local value of planetary vorticity at the drifter position (/), and the two dominant diurnal tidal constituents (Kx and Oi) 7.12 (top) Acceleration potential anomaly (10 m2js2) on the — 26.8 surface referred to 1500 db, derived from CTD casts taken from the R/V Hokko Maruin August-September, 1990. (bottom) Potential temperature (°C) on the ae = 26.8 surface, from the same CTD survey as (top). From Kono (1996) xxi Acknowledgments First, I would like to thank Dr. Paul 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 UBC Oceanography Department, David, Fred, Adam, Ana and many others, who have helped to make this more than just an educational experience. Finally, I want to extend my deepest appreciation to Li 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. Wo ai ni, fei chang, fei chang! xxii 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. Any 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 features and for describing the large-scale general circulation. With the deployment of enough free-floating instruments (at multiple drogue depths if vertical resolution is de sired), there will be sufficient data to derive robust velocity statistics with a spatial and temporal resolution which captures the dominant scales of the horizontal currents. For tunately, Lagrangian instruments are relatively simple and inexpensive devices, making such large-scale deployments feasible. 1.1.2 Bottles, Cards, Drifters and Floats The earliest current measurements were made using the simplest Lagrangian instruments: objects placed on the ocean surface and visually tracked. The nature of the objects was unimportant (and included everything from wooden poles to computer cards to parsnips), as long as they floated and remained visible (Davis, 1991). Such measurements were made in the Gulf Stream over 200 years ago, where buoys with drogues to minimize windage effects were tracked from anchored ships (Franklin, 1785). Stommel (1949) used aerial photography of floating paper sheets to study surface turbulence and diffusion. The requirement of visual tracking, however, limited the scales over which observations could be extended. Early observations on larger scales were made using drift cards and bottles, which carried instructions for notification upon discovery. With only start and end points (and often mischievous discoverers), interpretation of the card and bottle results was nearly impossible (Davis, 1991). Ships are also default Lagrangian current measurers. In fact, much of the early information regarding ocean surface currents was derived from observations of ship drift (Richardson, 1997). Comparisons between the actual and intended tracks of a ship give a direct measure of the surface currents, and ship's navigation logs going back to the 19th century have been used to extract such information. More recently, radar-tracking increased the spatial and temporal ranges of drifting devices, although significant wind drag on the radar reflector limited their Chapter 1. Introduction 3 current-following ability (Davis, 1991). With a limited tracking range, their most effective use was in mapping mesoscale features in near-coastal waters (e.g., Reid 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 ARGOS satellite system, which uses the Doppler shift of the buoy's 400 MHz 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. Early large-scale drifter deployments provided descriptions of the mean circulation and its variability in the North Atlantic (Richardson, 1983; Krauss and Kase, 1984; Krauss, 1986), the subtropical gyre of the North Pacific (Kirwan et al., 1978; McNally, 1981; McNally 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 North Atlantic, 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), Molinari et al. (1990), and Reverdin et al. (1994) had sufficient drifter data to resolve the seasonal cycle of currents in the equatorial Atlantic Ocean, the tropical Indian Ocean, and the equatorial Pacific Ocean, respectively. Particularly well-sampled regions are the California Current System off the west coast of North America, Chapter 1. Introduction 4 where aspects of its mesoscale structure (eddies and filaments) and temporal variability have been resolved (Davis, 1985a,b; Poulain and Niiler, 1989; Brink et al., 1991; Swenson et al., 1992; Swenson and Niiler, 1996), and the tropical Pacific, where well over 1000 drifters have been deployed since 1988, resulting in detailed examinations of the near-surface oceanic response to various phases of the El Nino - Southern Oscillation (Niiler, 1996; Bi and Niiler, 1998; Ralph et al, 1997). Lagrangian instruments are ideally suited for the study of dispersion behavior in the ocean. Drifter trajectories from the North and South Atlantic and the northeast Pacific have been used to determine the dispersion characteristics of the drogue-level flow, and to subsequently derive eddy mixing scales (decorrelation length and time scales) and horizontal eddy diffusivities (Colin de Verdiere, 1983; Davis, 1985b; Krauss and Boning, 1987; Figueroa and Olson, 1989; Thomson et al., 1990; Schafer and Krauss, 1995). Dispersion by the eddy field has a profound impact on the distribution of passive tracers in the ocean. Quantifying the ocean's eddy-dispersive characteristics can lead to improved parameterizations in eddy-resolving general circulation models, and a better understanding of turbulent mixing processes. Another important application of surface drifters is in studying the dynamics and kinematics of mesoscale circulation features. Eddies are ubiquitous features of the world's oceans, and it is not surprising that many drifters have been entrained into them, either by design or serendipity. Numerous descriptions of the dimensions (size and rotational speed), propagation, and kinematics (e.g., divergence, vorticity, stretching deformation and shearing deformation) based on drifter trajectories have been presented (Kirwan et al, 1984; Booth, 1988; Thomson et al, 1990; Hansen and Maul, 1991; Pingree and Le Cann, 1991; Pingree and Le Cann, 1992a,b; Pingree, 1994; Pingree, 1995; Sanderson, 1995; Pingree, 1996; Pingree et al., 1996; Pingree, 1997). Drifters have also sampled filaments of high-speed, offshore-flowing waters along the west coast of North America 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. Early drogue designs included parachutes and window shades, which were not particularly stable nor easy to deploy. More recent developments are the TRISTAR 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 al, 1987; Kennan et al, 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 if 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 Figure 1.1: Schematic showing the design of a modern satellite-tracked drifter. The drogue is a holey sock centered 15 m below the surface. The surface float houses the antenna, battery pack, and sensors. From Sybrandy and Niiler (1991). Chapter 1. Introduction 7 is adjusted before launch to descend to and remain at a specific density level. The float emits acoustic signals, which are picked up by hydrophones on a ship (or ships) which "chase" the float (Swallow, 1955). A time series of float position is then obtained from the ship's track. Swallow floats had an immediate and dramatic impact on the conventional conceptions of deep ocean circulation. Instead of finding very low speeds at great depth, as had been generally assumed in computing geostrophic velocities from hydrographic data, Swallow floats under the Gulf Stream revealed a vigorous, southward-flowing undercurrent (Swallow and Worthington, 1961). Later float experiments revealed surprisingly energetic flows on scales of 0(50 km), i.e. the mesoscale (Swallow, 1971). Although Swallow floats greatly advanced understanding of deep ocean circulation, their need for continuous ship tracking made deployments in large numbers impractical and prohibitively expensive. The next development in float technology relied on the fortuitous circumstances that there is little absorption of low-frequency sound by seawater, and that there is a mid-depth minimum of sound speed in the ocean, within which refracted sound waves can travel great distances. This is the SOFAR (SOund Fixing And Ranging) channel, and floats operating within or near it (SOFAR floats) have tracking ranges of 1500 km or more (Davis, 1991). Tracking of large numbers of floats simultaneously over a broad region could now be achieved from an array of onshore and moored offshore listening devices. SOFAR floats were used extensively in the 1970's and 1980's, with a majority of deployments being in the North Atlantic, primarily at depths of 700 m, 1500 m (near the axis of the SOFAR channel) and 2000 m (Freeland et al., 1975; Riser and Rossby, 1983; Owens, 1984; Richardson, 1985; Owens, 1991; Richardson, 1993). As with the surface drifter studies, most of these analyses focused on the description of the Eulerian mean statistics, the dispersion characteristics, and the mesoscale structure of the deep flow. One of the most successful applications of SOFAR floats was their deployment Chapter 1. Introduction 8 within small, long-lived mesoscale eddies which contained water of Mediterranean origin, Meddies (McDowell and Rossby, 1978). Some SOFAR floats remained in Meddies for years, revealing their slow westward propagation across the Atlantic and eventual decay (Armi et al, 1989; Richardson et al, 1989). In order to emit low frequency (250 Hz) sound, SOFAR floats had to be formidably large (about 5 m in length and weighing over 400 kg), making them expensive and difficult to deploy. This led to the development of the RAFOS float (RAFOS is SOFAR spelled backwards), which reverses the roles by listening to acoustic emissions from stationary transmitters (Rossby et al., 1986). Position fixes obtained from the transmitters are stored internally, and after a pre-determined life span (typically ~ 1-2 years), the float becomes buoyant and floats to the surface, where its bounty of data, including daily records of temperature and pressure, is transmitted to satellites via Service ARGOS. RAFOS floats are much smaller (~ 10 kg) and, therefore, cheaper and more practical, and have become the workhorse device for Lagrangian measurements of deep currents. These floats are equipped with enough battery power to perform about 60 cycles, giving them a lifetime of ~ 4.5 years. A more recent development is the Autonomous LAgrangian Circulation Explorer (ALACE), which uses a small hydraulic pump to change its volume, allowing it to surface and transmit data to ARGOS satellites at prescribed time intervals (typically every ~ 30 days) and then change its buoyancy again for re-deployment at the original float depth (Davis et al., 1991). This allows for some degree of continual tracking. The Profiling ALACE (PALACE) float, the latest incarnation, takes profile measurements of temperature and salinity on its ascent between the float level and the surface. Power-limited half-lives for ALACE and PALACE floats are on the order of 6 years, although corrosion and pump failures on some of the floats deployed in the South Pacific and Labrador Sea have shortened their fives (Davis, 1998). Drifter and float technology is evolving rapidly. Years of experience have led to the 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). Although 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 North Pacific Chapter 1. Introduction 10 Ocean over the period 1990-1995. Most of these drifters were deployed as part of the Surface Velocity Program (SVP), which is a component of the World Ocean Circulation Experiment (WOCE). The principal objective of WOCE is "to develop models useful for predicting climate change and to collect the data necessary to test them" (Canadian National Committee for WOCE, 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 WOCE is to determine the representativeness of the data sets collected during the field programs. The specific objectives of the SVP 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 WOCE, 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 North Pacific component of the SVP 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 WOCE/SVP 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. ARGOS 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 North Pacific Ocean from the data ensembles at both drogue depths, per SVP 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 Kuril-Kamchatka Trench in the western North Pacific. (e) To provide an overview of the North Pacific SVP data set which will 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 WOCE 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 North Pacific on a wide range of scales during the WOCE-SVP 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 initial 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. An 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 Kuril-Kamchatka Trench in the western North Pacific. A summary and synthesis of the results are given in Chapter 8. Chapter 2 Data and Methods 2.1 The WOCE/SVP Dataset Satellite-tracked drifters have become invaluable tools for studying ocean circulation. They are relatively inexpensive, easy to deploy, and have long lives (typically > 1 year). When deployed in large numbers, they enable a mapping of the near-surface circulation over an entire ocean basin. The Surface Velocity Program (SVP), a component of the World Ocean Circulation Experiment (WOCE), was designed to provide mixed-layer velocity and temperature observations over 5-year periods in all major ocean basins. As part of Canada's contribution to the WOCE-SVP, 102 satellite-tracked drifters were released in the North Pacific in 12 deployments between August 1990 and November 1994. Each of the drifters was equipped with a holey-sock drogue centered 15 m (62 drifters) or 120 m (40 drifters) below the surface, i.e., within the mixed-layer or near the top of the permanent halocline (Tabata, 1975; Figure 2.1). All deployments were made from ships-of-opportunity, with a majority occurring within the Gulf of Alaska (Figure 2.2). The standard SVP satellite-tracked drifter was designed to follow water parcels verti cally averaged over a drogue of 6-7 m height, centered 15 m below the surface. The aim was a drifter with known water-following characteristics, with predictable slip through water, rugged enough to survive many months in the open ocean, and preferably of low cost and easy to deploy (Sybrandy and Niiler, 1991). The drifter adopted as the standard 13 Chapter 2. Data and Methods 14 Temperature (°C) Brunt - Vaisala Frequency (cph) 24!o ' 2SA ' 26.0 27.0 Density (at) Figure 2.1: Profiles of temperature (T), Brunt-Vaisala frequency (N), and density (crt) taken near Station P (50°iV, 145°W) in August 1990. Locations of the two drogue depths are also shown. From Thomson et al. (1998). Chapter 2. Data and Methods 15 60°NH 50°N 40°N 30°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 20°N-| 1 1 1 1 1 1 1 1 1 1 1 1—i 1 1 1 1 1 r 140°E 160°E 180° 160°W 140°W 120°W 50°N J I I J I I I I I I L 40°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~ 140°E 160°E 180° 160°W 140°W 120°W Figure 2.2: Drifter deployment positions for the (a) shallow and (b) deep drifter ensem bles. Chapter 2. Data and Methods 16 has a three-dimensionally symmetric surface float (a sphere), which keeps surface wave aliasing of net horizontal forces on the float to a minimum. The float is 35 cm in diameter and houses an antenna, a transmitter, batteries and a submergence sensor (see Figure 1.1). The wire tethers are thin and stiff to reduce slip caused by underwater drag, and subsurface floats are used to minimize wave effects on the drogue. The drogue used, a holey sock, is dimensionally stable and has a high drag coefficient. In an effort to describe both the mixed layer flow and the flow in the underlying pycnocline, a significant portion of the drifters in the data set described here were drogued at 120 m. The geometry and construction of the surface float and drogue were the same for the deep drifters as for the standard SVP drifters. Niiler et al. (1995) modelled the vector slip of 15-m-drogued drifters (Ua) as a linear function of wind speed at 10 m (W), vertical current shear across the length of the drogue (AU), drag area ratio (R; the ratio of the drag area of the drogue to that of the tether plus surface float), and the angles relative to the wind and shear directions (a and /?, respectively): U. = {aeiaW + bei/3AU) (2.1) Measurements of slip and vertical shear were made by vector-measuring current meters placed at the top and bottom of the drogues. Most of the variance (84%) in the drifter slip was accounted for by linear fits to the four coefficients (a, b, a, /3), with the result that a drag area ratio of 40 or greater would yield a wind slippage of less than 1 cm/s in 10 m/s winds. SVP drifters have therefore been designed to have a drag area ratio of ~ 40, giving them a small and predictable slip. Although the standard drogue design minimizes wind-and wave-induced drifter slippage, it should be remembered that drifters are only quasi-Lagrangian instruments. Not only does some slippage remain, but the drogue remains Chapter 2. Data and Methods 17 Drifter Type Number Deployed Total Life (days) Drogue Life (days) Aanderaa Shallow 8 242 232 LCD Shallow 8 49 46 Seimac Shallow 19 291 210 Seimac Deep 2 196 177 Technocean Shallow 28 567* 352 Technocean Deep 38 340 156 Table 2.1: Total and drogue life expectancies by drifter manufacturer. One of the Aan deraa 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 May 1, 1996. centered at a particular depth, and does not follow the three-dimensional path of a water parcel on an isopycnal surface. Not all drifters are created equal (Table 2.1). Drifter lifetimes ranged from 11 to > 1400 days, with significant variability among manufacturers. The LCD (Low-Cost Drifter, Draper Labs) drifters were least reliable, while the Technocean drifters, on av erage, outlived the others by 50% or more. Overall, the average drifter lifetime (with or without drogue) was 377 (333) days for the shallow (deep) deployments, which is shorter than the designed half-life of 400-500 days (Kennan et al., 1998). 2.2 Initial Data Processing The Service ARGOS satellite tracking system was used for drifter location and retrieval of sensor data. Drifters telemeter their data (identification number and any measured parameters, such as SST and degree of submergence) to polar orbiting satellites. Service ARGOS determines the drifter positions and produces time series of raw data, which are sent to a centralized location, the Global Drifter Data Center (GDDC) at NOAA's Chapter 2. Data and Methods 18 Atlantic Oceanographic and Meteorological Laboratory, as well as to the principal in vestigators. The GDDC then processes, archives and distributes the data. Final data archival is done at Canada's Marine Environmental Data Service (MEDS). Most of the drifter data analyzed in this thesis was processed at the Institute of Ocean Sciences (IOS) from the raw ARGOS data. The data obtained from Service ARGOS consisted of irregularly-spaced time series of longitude and latitude, with the times between successive position fixes ranging from several minutes to several hours. At the latitudes occupied by most of these drifters (> 40°N), 10 or more position fixes per day was typical. To reduce velocity errors which could arise from short time steps, all sequential fixes for At < 1 hour were averaged together. Outlying points, expressed by unreasonable first difference speeds, were re moved. Accuracies in the ARGOS position fixes are approximately 200-300 m, which would result in velocity errors less than 5 cm/s. As these errors are random, their net effect on Eulerian averages can be neglected (Thomson et al., 1998). The raw time series were interpolated to obtain estimates of position and velocity at synchronized 3-hourly intervals. The fitting routine, described in the next chapter and in Bograd et al. (1998a), consisted of a fourth-order polynomial plus an oscillatory component whose frequency was determined by the interpolation. Since inertial motions were typically the most energetic component of the drifter motions (Thomson et al., 1998), the oscillatory component of the fit was usually at the local inertial frequency. As a cost-saving measure, all of the drifters alternated between continuous data re ceive/transmit mode and a reduced sampling schedule (duty cycle). The standard duty cycle used in the SVP consisted of 48 hours of no data transmission followed by 24 hours of received transmission (48-24h). Some of the drifters also used a 16-8h duty cycle. The derivation of velocity statistics over duty cycle segments was sensitive to the interpolation scheme used and, indeed, the fitting procedure described above was customized in order Chapter 2. Data and Methods 19 to obtain reliable statistical estimates from both continuous and duty cycle segments of the drifter trajectories. The effects of the duty cycle and various interpolation schemes on derived velocity statistics are the subject of the following chapter. The full interpolated trajectories of all drifters in the shallow and deep ensembles are shown in Figure 2.3. 2.3 Determination of Drogue Loss The reliability of the statistics presented in the following chapters depends critically on the determination of the date of drogue loss for each drifter. For the shallow drifters, which are assumed to represent the mixed-layer flow, it is quite difficult to determine precisely when a drogue may have become detached from the surface buoy. Although all SVP drifters were equipped with submergence sensors, which are designed to indicate the fraction of time a surface buoy is submerged as surface waves break and swell pass by (and thus whether a drogue is likely present or absent), this method is not entirely reliable. For the shallow drifters, date of drogue loss was determined from a review of the submergence sensor data and time series of drifter speeds and accelerations, as well as a careful perusal of the individual drifter trajectories. Although portions of the shallow ensembles may be contaminated with undrogued data, the amount of such data is small so that the derived mixed-layer statistics are not likely to be seriously affected. For the deep drifter ensemble, which represents the flow below the mixed layer, in the underlying pycnocline, it is essential that all statistics be derived for drogued segments only. Brugge (1995) showed that estimates of eddy kinetic energy in the North Atlantic from undrogued drifters are 2-3 times higher than from drifters drogued at 100 m. For the deep ensemble, a technique, described by Pingree (1993), was employed which looks for an abrupt order-of-magnitude increase in drifter accelerations and sustained high speeds. A 12h running mean was applied to the 3-hourly position time series to remove high 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 21 frequency signals, and velocities were derived over a 12h interval and accelerations over a period of one day. Since drifter trajectories within eddies can have accelerations resulting from steady circular motions (Pingree, 1993), scalar acceleration differences over a 12h period were then computed and plotted against time, as is shown for drifter 1319 (Figure 2.4). In this plot, a velocity increase of 10 cm/s over a period of one day, for example, would result in an acceleration difference pulse of ± 10 cm/s/day2. The same velocity increase over a shorter period of time would yield a larger pulse. Large and sustained acceleration differences (and corresponding increases in the mean and variance of drifter speed) are observed approximately 50 days after deployment, which is taken to be the date of drogue loss. Not all drifters showed such an obvious change in acceleration as drifter 1319, however, and the execution of the method was at times quite subjective. A particular complication was the duty cycle segments, through which the interpolations often created relatively large and noisy accelerations. In addition to the acceleration differences and data from the submergence sensors, time series of drifter speed and the idiosyncrasies of the individual trajectories were also used to determine the date of drogue loss for the deep drifters. Large acceleration differences were checked against drifter entry into a strong current. In general, selection of date of drogue loss for the deep drifters was done conservatively, with a preference to err on the side of removing good data rather than keeping wind-contaminated data. The removal of undrogued portions of the drifter trajectories resulted in a reduction from 23,650 (13,064) drifter days to 16,022 (6,172) for the shallow (deep) ensemble. Figure 2.5 shows timelines for all drifters in each ensemble, along with the time series of number of drogued drifter days over the SVP period. The average drogue lifetime was 272 days at 15 m depth and 167 days at 120 m depth, considerably shorter than the average drifter lifetimes. Thus, the mechanics of drogue attachment appears to be the limiting factor in obtaining long records from surface drifters. Chapter 2. Data and Methods 22 Time From Deployment (days) 0 20 40 60 80 100 DRIFTER 1319 Figure 2.4: Time series of daily-averaged drifter speed (top) and acceleration difference (bottom) for deep drifter 1319. Chapter 2. Data and Methods 23 A Department of Earth and Ocean Sciences Data Report (Bograd and Eert, 1996) contains individual plots of all drifter trajectories, as well as preliminary Eulerian statis tics derived from the trajectories through December 1994. Chapter 2. Data and Methods 24 60 50 40 30 Q 20 10 111111111111 i 1111111111 1111 1111111111111111111 (a) Shallow (15m) 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) Deep (120m) ^buoys ~ 34 Ndays = 6172; 111111111111111111111111111111111111111111111111 r 1111111111111111111111111111111111 T 400 cd 300 > 200 & 100 1990 1991 1992 1993 1994 1995 1996 Figure 2.5: Time lines of each of the drifters in the (a) shallow and (b) deep drifter ensembles. Dashed lines correspond to undrogued portions of the drifter trajectories. The dotted curve is the time series of total number of daily drifter observations in each ensemble. Chapter 3 Sampling Strategies and Interpolation Schemes 3.1 Introduction Although drifters are relatively inexpensive compared to current meters, large numbers are needed to effectively sample basin-scale oceanic regions. More importantly, the cost of locating the drifters and transmitting the data (paid to Service ARGOS) can be pro hibitively expensive. As pointed out by Hansen and Herman (1989), operating costs (i.e., daily data collection) far exceed the production costs of a drifter. An ideal arrangement would include a sufficient number of concurrently-transmitting drifters maintained at a reduced sampling schedule, so as to derive statistically-reliable circulation and tempera ture measurements at a minimum cost. Using data from drifters drogued at 10-30 m depth in the tropical Pacific between 1979-84, Hansen and Herman (1989) studied the effect of reducing the number and fre quency of ARGOS transmissions on the retrieved current and temperature information. They found that transmission rates of one day in three yielded acceptable velocity errors of ± 5 cm/s for equatorial regions just south of the equator. North of the equator, due to enhanced mesoscale variability, this standard could not be met with a reduction to even one day in two. The standard for sea surface temperature could be met through out the sampled region with transmissions on one day in four. Based on these results, the two-days-off/one-day-on transmission schedule (duty cycle) was adopted as the SVP standard. 25 Chapter 3. Sampling Strategies and Interpolation Schemes 26 While this sampling strategy has led to a significant reduction in operating costs and has enabled the deployment of a large number of drifters in each ocean basin, per SVP objectives, it is not necessarily the best duty cycle for all basins. Results are clearly dependent on the dynamics of the particular region sampled (e.g., Hansen and Herman, 1989). Low-frequency motions dominate the circulation in the tropical Pacific. In middle and high latitudes, however, inertial and tidal motions can dominate the flow variability (McNally et al, 1989, Thomson et al., 1998). Thus, it might be expected that the standard duty cycle, with its regular 48-hour data gaps, will not provide sufficiently accurate current measurements when strong high-frequency motions prevail. In this chapter, the effects of the standard duty cycle, as well as two other duty cycles (32-16h and 16-8h), on the velocity statistics derived from SVP drifters in the northeast Pacific are examined, and the necessity of accounting for the high-frequency motions when interpolating over duty cycle segments is emphasized. This is done by degrading continuous segments (in which all available satellite position fixes are recorded and processed by Service ARGOS) of each drifter series to match those of the duty cycles, and then comparing the prime (mean and standard deviation) and rotary spectral statistics derived from the series. It is shown that reproduction of the original prime and spectral statistics requires an interpolation which takes into account the oscillatory component of the drifter motions. The following section describes the data set used in this study, and the degradation and interpolation procedures applied to the original drifter series. This is followed by a comparison of the derived prime and spectral velocity statistics, and a recommendation regarding sampling and interpolation strategies to be used in future drifter studies. 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 Gulf 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 September — November 1990 1 r 142°W 1 r 138°W 1 r 134°W 130°W Figure 3.1: Map 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 Gulf of Alaska in September 1992 which had weaker inertial motions. Chapter 3. Sampling Strategies and Interpolation Schemes 29 of observations by 70%, while the 32-16h and 16-8h duty cycles resulted in an average reduction of 72% and 76%, respectively. As illustrated by trajectory plots of the original and degraded series of drifters 1310 and 15366 (Figures 3.2 and 3.3), some of the gaps in the duty cycle data are quite extensive. Especially large gaps are found in the 48-24h degradation of drifter 15366 where it was within the fast-flowing Alaskan Stream (Figure 3.3b). 3.2.3 Time series interpolation Two distinct interpolation routines were used on the original and degraded series of each drifter in an attempt to reproduce the prime and spectral statistics of the original series. The first routine used was a Hermite spline, while the second assumed an oscillation of unknown frequency in the drifter motions and constructed a data-density-dependent local fit to the observations. The latter interpolation, referred to here as the multi-functional fit (MFF), is described below. First, the average data density (number of observations per day) over a 3-day period was calculated starting at each observation point in the time series. The "local" inter polation of a selected segment of the series was then dependent on its data density. For a density of greater than 6 (fewer than 6) observations per day, the maximum number of observations allowed in the local interpolation was 30 (18). The maximum time and distance allowed in the segment was set at 10 days and 300 km, respectively. These criteria, based on trial and error with the data sets, were intended to ensure good fits both for continuous and duty cycle segments. Once any one of the limits (data density, distance or time) was met, the selected segment was transferred to the fitting routine. The function, f>(t), chosen to fit the selected segment consisted of a polynomial plus an oscillation whose frequency, u>, was determined by the interpolation: Chapter 3. Sampling Strategies and Interpolation Schemes 30 50°N 49°N-50°N 49°NH 50°N 49°NH 50°N 49°NH ft. ^*c*$T 1310 V Original 1 1 1 1 1 f 1310 48 - 24h (e) * J> 1310 32 -16h *' * •% 1310 16 - 8h 146°W 144°W 142°W 140°W Figure 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 observations contained in each series is (a) 947, (b) 287, (c) 239, and (d) 265. Chapter 3. Sampling Strategies and Interpolation Schemes 31 54°N- (a) 52°N-I i i 50°N-# t 15366 Original i i i i i 54°N-(b) S 52°N-50°N-y { t 1 1 15366 48 - 24h IIIII 54°N-52°N-(c) J <m * J •0 V 50°N-r <* I 1 1 1 15366 32 - 16h IIIII 54°N-(d) rf . -V"". s • » . r % • 52°N-50°N-"I jr. rf 15366 16 - 8h T 1 I I I I I I ' "I "III 174°W 170°W 166°W 162°W Figure 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 observations contained in each series is (a) 793, (b) 236, (c) 250, and (d) 235. Chapter 3. Sampling Strategies and Interpolation Schemes 32 p(t-) = a + bt + ct2 + dt3 + et4 + hsin(u;t + cb), (3.1) where t is time, a-e are constants of the polynomial component, and h is a constant and 4> is the phase lag for the oscillatory component. A gradient-expansion algorithm was applied (Bevington, 1969). In a heuristic sense, the polynomial fits the general trend of the segment, while the oscillation fits the dominant high-frequency fluctuations. The order of the polynomial depends on the data density of the segment. If the selection process resulted in only a few observations in the segment (for example, if the distance or time limit was met during a duty cycle portion of the series), a smaller order polynomial would suffice. For the trajectories fitted here, a quartic polynomial was generally required. A new segment was then selected by advancing the start time by 8 points for a high-density segment (> 6 observations per day) or by 2 points for a low-density segment (< 6 observations per day), and repeating the selection and fitting procedure described above. The overlapping segments yielded multiple predicted fits at each 3-hourly time. Once the entire series had been covered, two different approaches were used to obtain the final interpolated series. In the first method (MFF1), all overlapping fitted segments were averaged to get 3-hourly estimates of drifter position (latitude, longitude), and speeds were again calculated by first-differencing the position data. In the second method (MFF2), instead of averaging overlapping segments, the predicted location at a given time which had the lowest root-mean-square distance to the neighboring three observation points on either side of that time (i.e., the "best" fit) was chosen. The respective spline and MFF2 interpolations applied to the continuous and 32-16h degraded series of drifters 1310 and 15366 are shown in Figures 3.4 and 3.5. Oscillations of inertial period were reproduced in the MFF2-interpolated degraded series of drifter 1310 in regions where strong inertial motions were originally observed (Figure 3.4). Chapter 3. Sampling Strategies and Interpolation Schemes 33 Figure 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-interpolated 32-16h de graded series. Chapter 3. Sampling Strategies and Interpolation Schemes 34 • • I I I I I I ''II 54°N-(a) 52°N-/• 15366 /r^^irj Continuous 50°N-i i i i Spline i i i i i i i i 54°N-(b) 52°N-_ 15366 32 - 16h 50°N-i i i i Spline i i < i i i i i 54°N-(c) 52°N-r 15366 Continuous 50°N-i i i i MFF2 54°N-(d) 52°N-. 15366 /) 32 - 16h 50°N- MFF2 T 1 1 1 1 1 1 1 1 1 1 r 174°W 170°W 166°W 162°W Figure 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 MFF2-interpolated 32-16h de graded series. Chapter 3. Sampling Strategies and Interpolation Schemes 35 Positions were also estimated and speeds calculated at 6-hourly intervals (not shown), which is consistent with the standard product available from the SVP Data Assembly Center. However, because 10 or more observations per day are typically available from continuous drifter segments at extratropical latitudes, it was more appropriate to sub-sample the interpolation at 3-hourly intervals. Furthermore, 6-hourly data contain 2.5 data points or less per inertial period at middle to high latitudes, and have a Nyquist period of 12 hours, i.e. semidiurnal. Given the significant energy levels at the inertial and semidiurnal periods which are commonly observed in the upper ocean, the higher sampling rate seems particularly desirable. 3.3 Results 3.3.1 Prime statistics Prime velocity statistics provide a direct comparison between the original and degraded series. As Table 3.1 indicates, the degradations, when left uninterpolated, give signifi cantly different mean speeds from the original series. For instance, the 16-8h degradation of drifter 1310 resulted in an estimated mean zonal speed 60% greater than the original uninterpolated series. However, the original mean speeds were quite adequately repro duced for each of the degradations after they were interpolated with the spline, MFF1 or MFF2 algorithms (Table 3.1). The 16-8h degradation of drifter 1310 has mean zonal speeds within 1% of the original for each of the interpolations. Similar comparisons can be made for each of the Station P drifters (not shown), although there was a small, sys tematic underestimation of the meridional speeds in each of the interpolation routines. For drifter 15366, the spline and MFF1 interpolations slightly overestimated the zonal speeds, while the MFF2 interpolation yielded slight underestimations (Table 3.2). Each of the interpolations nicely reproduced the mean meridional speeds. Chapter 3. Sampling Strategies and Interpolation Schemes 36 DRIFTER 1310 Uninterp. Spline MFF1 MFF2 u (cm/s) Original 3.93 ± 1.47 3.89 ± 1.37 3.91 ± 1.44 3.90 ± 1.35 48 - 24h 4.73 ± 2.34 3.87 ± 0.69 3.85 ± 1.02 3.89 ± 1.41 32 - 16h 4.31 ± 2.74 3.91 ± 0.68 3.90 ± 0.96 3.94 ± 1.23 16 - 8h 6.30 ±3.04 3.90 ± 0.75 3.89 ± 1.09 3.92 ± 1.24 v (cm/s) Original -2.14 ± 1.39 -1.80 ± 1.40 -1.84 ± 1.47 -1.90 ± 1.38 48 - 24h -1.31 ± 2.20 -1.81 ± 0.70 -1.81 ± 0.94 -1.89 ± 1.37 32 - 16h -2.72 ± 2.67 -1.82 ± 0.73 -1.81 ± 1.08 -1.89 ± 1.32 16 - 8h -4.32 ± 2.68 -1.82 ± 0.69 -1.82 ± 1.20 -1.88 ± 1.33 Vu'2 (cm/s) Original 23.07 ± 1.04 18.57 ± 0.97 19.43 ± 1.02 18.19 ± 1.35 48 - 24h 20.25 ± 1.66 9.38 ± 0.49 13.75 ± 0.72 18.91 ± 0.99 32 - 16h 21.62 ± 1.94 9.22 ± 0.48 13.01 ± 0.68 16.49 ± 0.87 16 - 8h 25.24 ± 2.15 10.15 ± 0.53 14.80 ± 0.77 16.64 ± 0.88 vV2 (cm/s) Original 21.88 ± 0.99 18.92 ± 0.99 19.92 ± 1.04 18.56 ± 1.38 48 - 24h 19.03 ± 1.56 9.49 ± 0.50 12.70 ± 0.67 18.45 ± 0.97 32 - 16h 21.03 ± 1.89 9.83 ± 0.51 14.61 ± 0.76 17.74 ± 0.93 16 - 8h 22.24 ± 1.89 9.36 ± 0.49 16.17 ± 0.85 17.91 ± 0.94 Table 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. Chapter 3. Sampling Strategies and Interpolation Schemes 37 The standard deviations (variances) of the current speeds derived from the spline-interpolated degraded series of drifter 1310 were approximately 50% (70%) smaller than those derived from the spline-interpolated original series, while those calculated from the MFF1 interpolation were 30% (50%) smaller (Table 3.1). Similar results were found for drifter 15366, although the differences between the original and degraded variances were smaller (Table 3.2). The MFF2 interpolation, however, yielded standard deviations (variances) within 5% (10%) of the original for both drifters 1310 and 15366. Only the meridional variance of the 16-8h degradation of drifter 15366 was strongly overestimated. Thus, although each of the interpolations of the degraded series reproduced the mean speeds of the original series, only the MFF2 interpolation yielded variances comparable to those of the original. This was most dramatically demonstrated by the drifters containing strong inertial oscillations. In many applications (e.g., in the SVP) it is the long-term circulation statistics derived from an ensemble of drifters (not necessarily operating simultaneously) which are desired. A question is whether the gaps from the duty cycle, and their subsequent interpolation, can affect these ensemble statistics. For each of the uninterpolated and interpolated series in the Station P Group, the ensemble velocity statistics were derived using the daily-averaged series from all six drifters (i.e., all data points from a given Julian day, if any, were averaged). The results (not shown) are similar to those derived from the individual drifters. The degradations did introduce errors in the ensemble mean speeds and standard deviations, but each of the interpolation schemes was able to adequately reproduce the statistics (means and variances) derived from the original ensemble. It should be noted that increased time-averaging will yield low variances, and that any of the interpolation schemes applied to the duty cycle segments will produce similar statistics for averages over one day or longer. Chapter 3. Sampling Strategies and Interpolation Schemes DRIFTER 15366 Uninterp. Spline MFF1 MFF2 u (cm/s) Original -2.94 ± 2.15 -3.27 ± 2.26 -3.33 ± 2.22 -2.30 ± 2.27 48 - 24h -4.28 ± 4.19 -3.05 ± 1.89 -2.61 ± 2.03 -2.50 ± 2.19 32 - 16h -3.59 ± 3.86 -3.10 ± 1.95 -3.33 ± 1.94 -2.44 ± 2.39 16 - 8h -4.61 ± 4.23 -3.27 ± 1.95 -3.27 ± 2.00 -2.35 ± 2.43 v (cm/s) Original -3.78 ± 1.97 -3.65 ± 2.12 -3.51 ± 2.10 -3.83 ± 2.13 48 - 24h -4.39 ± 3.63 -3.66 ± 1.63 -3.46 ± 1.87 -3.57 ± 2.31 32 - 16h -1.02 ± 3.51 -3.89 ± 1.65 -3.72 ± 1.68 -3.66 ± 2.30 16 - 8h -1.52 ± 3.71 -3.64 ± 1.69 -3.56 ± 1.82 -3.61 ± 2.89 y/u'2 (cm/s) Original 30.83 ± 1.52 28.22 ± 1.60 27.55 ± 1.57 28.45 ± 1.60 48 - 24h 32.86 ± 2.96 23.61 ± 1.34 25.28 ± 1.44 27.38 ± 1.55 32 - 16h 31.14 ± 2.73 24.41 ± 1.38 24.26 ± 1.37 29.97 ± 1.69 16 - 8h 33.05 ± 2.99 24.37 ± 1.38 24.96 ± 1.41 30.47 ± 1.72 vV2 (cm/s) Original 28.23 ± 1.39 26.50 ± 1.50 26.17 ± 1.49 26.73 ± 1.51 48 - 24h 28.46 ± 2.57 20.35 ± 1.15 23.31 ± 1.33 28.84 ± 1.63 32 - 16h 28.35 ± 2.49 20.66 ± 1.17 21.02 ± 1.19 28.81 ± 1.63 16 - 8h 29.00 ± 2.62 21.16 ± 1.20 22.78 ± 1.29 36.30 ± 2.05 Table 3.2: As in Table 3.1, but for drifter 15366. Chapter 3. Sampling Strategies and Interpolation Schemes 39 3.3.2 Rotary spectral analysis 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 are nearly as strong as the inertial peak. These peaks correspond to inertial energy which has been aliased to multiples of the duty cycle frequency, /,±n/dc, where /; is the inertial frequency (approximately 1.53 cpd), fdc is the duty cycle frequency (e.g., 32/t^16/l = 0.5 cpd for the 32-16h degradation), and n = 1,2,.... Some energy from the weaker M2 peak has likely been aliased to lower frequencies as well. The aliasing is especially problematic in the spectra of the 16-8h series (Figure 3.6d), where a strong clockwise peak centered at 2 days contains a significant portion of the total energy. Thus, the near-mesoscale energy distribution is significantly affected by this duty cycle. D'Asaro (1992) found spectral leakage due to the inherently clustered nature of the ARGOS position fixes for drifters deployed during the OCEAN STORMS experiment. This is related to the orbital characteristics of the satellite. The aliasing observed here is due solely to the interaction of strong inertial motions with the duty cycle, which is a potentially tractable problem. Specifically, the user cannot control when the satellite po sition fixes are available, but can, to some degree, control the duty cycle which optimizes the data records. The MFF interpolations solve the problem of abasing by the duty cycle. The MFF1 interpolation adequately reproduces the inertial peak in the degraded series, but fails in that the clockwise energy at periods less than inertial and from 1-5 days is two orders of magnitude lower than for the original series (Figure 3.7). This is the "missing energy" seen in the standard deviations of the degraded series (Table 3.1). The rotary spectra of the MFF2-interpolated original and degraded series of drifter 1310 (Figure 3.8) demonstrate the effectiveness of the MFF2 interpolation. The spectra of each of the degraded series (Figures 3.8b,c,d) match the spectra of the original series (Figure 3.8a) quite well. The primary differences between the original and degraded MFF2 spectra are the weak S+ inertial peaks in the latter (two orders of magnitude smaller than the S~ peak), which are due to fitting the zonal and meridional components Chapter 3. Sampling Strategies and Interpolation Schemes 41 SPLINE INTERPOLATION Frequency (cpd) Figure 3.6: The clockwise (5~; solid Une) and counterclockwise (S+; dashed Une) ro tary 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 Umits and the inertial (/) and semidiurnal (M2) peaks are shown in (a). Chapter 3. Sampling Strategies and Interpolation Schemes 42 MFF1 INTERPOLATION Frequency (cpd) Figure 3.7: As in Figure 3.6, but for the MFFl-interpolated original and degraded series of drifter 1310. Chapter 3. Sampling Strategies and Interpolation Schemes 43 independently using (1), thereby losing the phase information of the inertial motions. The spectra of the 48-24h and 16-8h degradations are noisy at periods less than 12 hours, while those of the 32-16h degradation appear to most closely resemble the spectra of the original series, with an energy roll-off at high frequencies. The clockwise spectrum of the 16-8h degradation does appear to capture a weak semidiurnal peak. The rotary spectra of the MFF2-interpolated original series of drifter 15366 (Figure 3.9a) have a broad clockwise peak centered between the inertial and M2 frequencies, and are weakly clockwise polarized at low frequencies, with S+ troughs at periods of 2.5 and 5.5 days. The low frequencies are significantly more energetic here than in the Station P Group. The spectra of the 32-16h degradation (Figure 3.9c) appear to most closely resem ble the spectral characteristics of the original series, although the S~ near-inertial peak is overestimated and too narrow. This is due to fitting only one oscillation in the inter polation. Adding a prescribed M2 oscillation and fitting the (varying) inertial oscillation would likely improve the spectral characteristics. The spectra of the 48-24h degradation are very noisy at high frequencies (Figure 3.9b), while only the low frequencies (periods > 1 day) of the original spectra are reproduced in the 16-8h degradation (Figure 3.9d). The spectra of the spline interpolations of drifter 15366 (not shown) are adequate for the original series, but again yield significant spectral leakage in the degraded series. 3.4 Discussion A question raised by this analysis is whether one of the three investigated duty cycles is better suited for mid-latitude drifters in regions of strong inertial oscillations. A comparison of the rotary variances computed in four frequency bands for the MFF2-interpolated original and degraded series of the Station P ensemble allows for a more quantitative comparison of the spectral statistics (Table 3.3). The energy contained in the Chapter 3. Sampling Strategies and Interpolation Schemes Figure 3.8: As in Figure 3.6, but for the MFF2-interpolated original and degraded of drifter 1310. Chapter 3. Sampling Strategies and Interpolation Schemes Figure 3.9: As in Figure 3.6, but for the MFF2-interpolated original and degraded of drifter 15366. Chapter 3. Sampling Strategies and Interpolation Schemes 46 low frequency portion of the mesoscale band (2-8 day period), and its partition between the clockwise and counterclockwise components, is similar for the original and each of the degraded series. The 48-24h degradation has too much counterclockwise energy at the highest frequencies, while the 16-8h degradation has too much high-frequency energy in both rotary components. The over-estimated high frequency energies are also seen in the MFF2-interpolated spectra of drifter 1310 (Figures 3.8b,d). The 16-8h degradation also has too much counterclockwise energy in the high frequency portion of the mesoscale band (17h-1.9d period). In general, it appears that the 32-16h degradation reproduces the total energy of the original series, and its frequency partition, very well. Further comparison of the duty cycles can be made by computing the rotary coeffi cient, given by b(<jj]u ) + b{u);u+) for the Station P ensemble (Table 3.4). For clockwise (counterclockwise) rotary motions, r is positive (negative), and (—1 < r < 1) (Thomson et al., 1998). For each of the duty cycles, the rotary coefficient is too low in all frequency bands, indicating an excess (deficiency) of counterclockwise (clockwise) energy in the interpolations of the degraded series. Thus, effects of the duty cycle are prevalent even at relatively low frequencies, where clockwise energies are underestimated in the low mesoscale band and counterclock wise energies are overestimated in the high mesoscale band (see Table 3.3). The rotary coefficients derived from the 32-16h degradations are in closest agreement with those of the original series, particularly in the inertial frequency band, which contained nearly 80% of the total energy of the motions with periods less than 8 days. Since clockwise oscillatory motions dominate the Station P ensemble, the clockwise spectral amplitude ratios between the original and degraded series can be compared in Chapter 3. Sampling Strategies and Interpolation Schemes 47 ENSEMBLE Original 48 - 24h 32 - 16h 16- 8h S~{u>) {cm2/s2) (%) low mesoscale 23.1 (2.4) 13.4 (1.5) 14.4 (1.6) 13.1 (1.2) high mesoscale 63.9 (6.6) 34.6 (4.0) 43.3 (4.7) 56.4 (5.0) inertial 765.5 (79.0) 625.0 (71.8) 705.1 (75.9) 673.4 (59.5) high 64.7 (6.7) 60.8 (7.0) 47.7 (5.1) 125.0 (11.0) S+{w) (cm2/s2) (%) low mesoscale 11.1 (1.1) 13.1 (1.5) 10.4 (1.1) 10.0 (0.9) high mesoscale 7.8 (0.8) 27.7 (3.2) 27.5 (3.0) 48.3 (4.3) inertial 2.0 (0.2) 30.8 (3.5) 29.8 (3.2) 77.3 (6.8) high 15.8 (1.6) 48.8 (5.6) 34.9 (3.8) 112.5 (9.9) Stot(oj) [cm2Is2) (%) low mesoscale 34.2 (3.5) 26.5 (3.0) 24.8 (2.7) 23.1 (2.1) high mesoscale 71.7 (7.4) 62.3 (7.2) 70.8 (7.7) 104.7 (9.3) inertial 767.5 (79.2) 655.8 (75.3) 734.9 (79.1) 750.7 (66.3) high 80.5 (8.3) 109.6 (12.6) 82.6 (8.9) 237.5 (20.9) Table 3.3: Clockwise (S~(u>)), counterclockwise (S+(u;)) and total (Stot{<*>)) rotary vari ance 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). Chapter 3. Sampling Strategies and Interpolation Schemes 48 ENSEMBLE Original 48 - 24h 32 - 16h 16 - 8h r(uj) low mesoscale 0.351 0.011 0.161 0.134 high mesoscale 0.782 0.111 0.223 0.077 inertial 0.995 0.906 0.919 0.794 high 0.607 0.109 0.155 0.053 Table 3.4: The rotary coefficient, r(oj), in four frequency bands of the MFF2-interpolated series of the Station P drifter ensemble. The frequency bands are the same as in Table 3.3. each of the four frequency bands, and for all three interpolation algorithms (Table 3.5). The spline interpolation clearly does not reproduce the inertial motions. The MFF1 interpolation yields the inertial peak, but does not have nearly enough clockwise energy in the high mesoscale and high frequency bands. The MFF2 interpolation adequately reproduces the clockwise rotary energy distribution in all frequency bands, with the 32-16h degradation yielding the best comparison. The MFF2 interpolation does not resolve the inertial and semidiurnal peaks in the degraded series (e.g. Figure 3.8). Since inertial motions were nearly two orders of magni tude stronger than semidiurnal motions in the Station P drifter records, it was not deemed necessary to attempt this resolution. The main objective was simply to reproduce the dominant motions in the degraded segments. The MFF interpolation can readily be made more sophisticated, depending on the nature of the data set and the user's interests. For example, one could add oscillations of known frequency if particular tidal components are Chapter 3. Sampling Strategies and Interpolation Schemes 49 ENSEMBLE Original 48 - 24fc Original 32 - l%h Original 16 - 8h Spline low mesoscale 1:7 1.2 0.7 high mesoscale 0.8 0.9 1.0 inertial 12.8 16.4 29.8 high 1.7 2.0 4.2 total 3.8 4.4 5.8 MFF1 low mesoscale 4.6 4.4 5.5 high mesoscale 18.2 14.8 11.1 inertial 1.2 1.4 1.3 high 24.7 32.2 5.6 total 1.6 1.8 1.7 MFF2 low mesoscale 1.7 1.6 1.8 high mesoscale 1.8 1.5 1.1 inertial 1.2 1.1 1.1 high 1.1 1.4 0.5 total 1.2 1.1 1.1 Table 3.5: The clockwise (S~) spectral amplitude ratios in four frequency bands, and for total clockwise energy, of the original to the degraded series for the spline-interpolated and MFF-interpolated series of the Station P drifter ensemble. The frequency bands are the same as in Table 3.3. Chapter 3. Sampling Strategies and Interpolation Schemes 50 known to dominate the motions, as was done by Pease et al. (1995). However, the results shown here demonstrate that a relatively simple procedure can (and should) be applied to general drifter data sets, where high-frequency motions may or may not be strong (or may or may not be known to be strong), and the prime and spectral statistics can be confidently estimated for both continuous and duty-cycle segments. It is recommended that users "learn" from the continuous segments of their drifter trajectories (i.e., those not spoiled by the duty cycle), and subsequently customize their interpolation routine in order to adequately account for the dominant modes of variability over the duty cycle segments. Since all SVP drifters were drogued at a depth of 15 m, inertial oscillations are expected to be the dominant mode of variability at mid- to high latitudes. 3.5 Conclusions Chereskin et al. (1989) explored the effects of upper ocean vertical shear on drifter slippage, while D'Asaro (1992) analyzed the effects of inherent Service ARGOS sam pling errors on drifter velocity estimates. In this study, the effects of reduced sampling schedules (duty cycles) on velocity statistics derived from satellite-tracked drifters in the northeast Pacific Ocean have been examined. The findings show that both spline inter polation and more sophisticated multi-functional (MFF) interpolation of degraded (duty cycle) segments of continuous drifter trajectory records adequately reproduce the mean velocities of the original data series. This applies to the standard duty cycle used in most large-scale drifter deployments consisting of 48 hours of no ARGOS data transmission followed by 24 hours of ARGOS transmission received (48-24h), as well as to duty cycles having shorter but more frequent gaps (32-16h and 16-8h). However, the ability to repro duce the rotary spectral characteristics of the original time series, as well as the velocity variances, is strongly dependent on the interpolation scheme applied to the duty cycle Chapter 3. Sampling Strategies and Interpolation Schemes 51 record. The spline interpolation produced numerous spurious spectral peaks, with iner tial energy aliased to multiples of the duty cycle frequency. On the other hand, the MFF routine described here, which allows for an oscillatory component to the drifter motions, was able to adequately reproduce the rotary spectral features of the original data series for each of the three types of degraded series examined. Best reproduction of the rotary spectral amplitudes and associated frequency partitioning required that the best fitted segments in the interpolation routine be selected, rather than the average of overlapping segments. Nevertheless, even at subinertial frequencies, the duty cycle results in biased estimates of the rotary velocity variances. It is expected that a more sophisticated fitting routine, which would allow for oscillations of known frequency in regions where specific frequency components (such as semidiurnal tidal motions) are known to dominate (cf. Pease et al., 1995), would further improve estimates of the spectral characteristics. Results of the analysis also indicate that, for the three duty cycles investigated and for mid-to-high latitude drifter motions having strong inertial and/or semidiurnal mo tions, the spectral characteristics of the original continuous records are most consistently reproduced by the 32-16h duty cycle. This presumably is because the 16h transmission-received segments of drifter tracks are sufficiently long to define superinertial motions while the 32h transmission-blackout segments are short enough that the main features of the motions remain relatively unchanged. Unfortunately, this duty cycle is not a permissible option with Service ARGOS. The duty cycle with the shortest and most frequent gaps (16-8h) yielded the worst reproduction, with significantly overestimated superinertial energies. Drifter engineering has progressed rapidly in recent years. Modern drifters are ca pable of measuring velocities and sea surface temperature to accuracies of 5 cm/s and 0.1° (Hansen and Poulain, 1996), respectively, over a period greater than a year, and, in some circumstances, of obtaining useful information across entire basins well over a year Chapter 3. Sampling Strategies and Interpolation Schemes 52 after deployment (Thomson et al., 1997, Bograd et al., 1998b). Consequently, drifter deployments have become an integral component of many large-scale and mesoscale ob servational studies. The user has the main obligation to generate reliable statistical information from the drifter data. In this regard, users are strongly encouraged to take into account high-frequency motions during time series interpolation. In light of the results presented here, it is further recommended that Service ARGOS: (1) lower the costs of drifter tracking and data transmission; and (2) provide a selection of duty cycle options which depend on the latitude of the drifter deployments (specifically, equatorial versus non-equatorial deployments). Chapter 4 Mean Circulation and Energy Distribution in the North Pacific 4.1 Introduction The near-surface circulation of the Alaskan Gyre impacts the local distribution of non-passive (e.g., temperature, salt) and passive tracers (e.g., nutrients). This has profound effects on the distribution and magnitude of primary (Polovina et al., 1995) and secondary production (Brodeur and Ware, 1992), which in turn affects the rates and mechanisms of carbon export to the deep ocean (Knauer et al., 1990). Quantifying these rates is the primary goal of the Canadian Joint Global Ocean Flux Study (CJGOFS), whose field measurements in the northeast Pacific overlap the WOCE drifter measurements stud ied here. Regional circulation variability is also an important factor in the survivability of many northern fish populations and their eventual recruitment to economically vital fisheries (Beamish and Bouillon, 1993; Mantua et al., 1997; Walter et al, 1997). Clearly, sufficient knowledge of the northeast Pacific circulation variability is a prerequisite to a more complete understanding of the ecosystem dynamics, and its climatological signifi cance, in this biologically productive region (Gargett, 1991). In this chapter, the drifter ensembles are analyzed in order to provide a statistical realization of the spatially varying mean flow and energy distribution over a broad region of the northeast Pacific Ocean for the period 1990-1995, and to allow for a comparison of the circulation at two depths representing the mixed-layer and the underlying pycnocline. These results can subsequently be used in constraining parameterizations and verifying 53 Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 54 results in both general circulation and coupled biophysical models of the region. 4.2 Oceanographic Setting of the Northeast Pacific The Alaskan Gyre is a wind- and buoyancy-forced, partially closed circulation regime located in the Gulf of Alaska and extending westward to near the dateline (Van Scoy et al., 1991). It is bounded to the south by the slow (~ 10 cm/s), eastward-flowing Subarctic Current at 45°-50°N, to the east by the highly variable Alaska Current, and to the north and west by the strong (> 30 cm/s) southwestward-flowing Alaskan Stream along the continental slope. Some of the water from the Alaskan Stream recirculates into the Subarctic Current or enters the Bering Sea through various passes, although both scenarios are extremely variable (Thomson, 1972; Reed, 1984; Reed and Stabeno, 1994). Both the Alaskan Stream and the Subarctic Current have been shown to be continuous to the ocean bottom (Warren and Owens, 1988). The Subarctic Current bifurcates in the eastern North Pacific, with the bulk of its transport turning either northward in the Alaska Current or southward in the California Current of the subtropical gyre (Chelton and Davis, 1982). A schematic of the major surface currents in the North Pacific Ocean is shown in Figure 4.1. The region is dominated in winter by the presence of an intense low-pressure system, the Aleutian Low. Relatively high precipitation and low evaporation, along with nu merous freshwater sources along its perimeter, yield an upper layer marked by a strong, shallow halocline, with winter mixed-layer depths in the center of the gyre reaching only 75-100 m (Levitus, 1982; Tabata, 1975). The Aleutian Low represents the integrated effect of numerous winter cyclones, which result in upwelling and an exchange of sensible heat from the warmer ocean to the atmosphere (Van Scoy et al., 1991). It is this Ekman pumping in the Gulf of Alaska which maintains high nutrient concentrations, and which Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 55 Figure 4.1: Map of the North Pacific Ocean showing the major surface currents. From Tabata (1975). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 56 permits relatively high levels of bio-productivity in the near-surface waters (Gargett, 1991). The atmospheric circulation over the North Pacific has been observed to vary on a decadal timescale (e.g., Trenberth and Hurrell, 1994; Mantua et al., 1997). In particular, a southward displacement and intensification of the wintertime Aleutian Low began in the mid-1970's and continued through the 1980's, in what has been called a "regime shift" (Trenberth and Hurrell, 1994). Similar variability has been observed on shorter time scales, most notable being an intensification and southeastward displacement of the Aleutian Low from its mean position during El Nino years. This could have significant consequences for the upper-level circulation and heat flux in the region. Northward trans port in the eastern North Pacific (Alaska Current) appears to dominate over southward transport (California Current) during El Nino years (Chelton and Davis, 1982). Kelly et al. (1993) used satellite altimetry data to observe an intensification of the Alaskan Gyre during the moderate El Nifio of 1987, and a weakening of the Alaskan Gyre during the following non-El Nifio year. Royer and Emery (1987) observed a significant westward displacement of the Alaskan Gyre in the summer of 1981, a year preceding a strong El Nifio event, with virtually no northward flow east of 145°W. They attributed this to an anomalous wind stress curl combined with the shoaling bottom topography in the northeast Pacific. Knowledge of the regional circulation is based largely on a robust history of hydro-graphic measurements, particularly in the near-coastal regions (Dodimead et al., 1963; Favorite et al., 1976) and along the frequently occupied Line P between 50°N, 145°W and the southern tip of Vancouver Island (Tabata, 1991). Lagrangian observations in the North Pacific have largely been confined to the eastern boundary current regimes, partic ularly the California Current system (Davis, 1985a,b; Poulain and Niiler, 1989; Brink et al., 1991; Swenson and Niiler, 1996) and the bifurcation zone off the west coast of British Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 57 Columbia (Thomson et al., 1990; Paduan and Niiler, 1993; Niiler and Paduan, 1995; van Meurs and Niiler, 1997). The only drifter deployments designed to map a broad region of the North Pacific surface circulation were undertaken as part of the North Pacific Experiment (NORPAX) Anomaly Dynamics Study (ADS) in 1976-80, during which 75 drifters, ostensibly drogued at 30 m, were deployed throughout the midlatitude North Pacific, though primarily within the Subtropical Gyre (Kirwan et al., 1978; McNally, 1981; McNally et al., 1983). Most of these drifters were observed to move nearly parallel to the sea level pressure isobars, suggesting a flow to the right of the surface winds. The effectiveness of the drogues used in these early experiments is questionable, however (van Meurs and Niiler, 1997). High-quality drifter measurements are far more abundant in the North Atlantic and equatorial Pacific than in the northeast Pacific. It is this void which has been filled by the SVP drifter data analyzed here. 4.3 Lagrangian Decorrelation Scales As a preliminary step to obtaining an Eulerian description of the circulation, the decorre lation scales of the motions at both drogue depths need to be determined. The Lagrangian integral time scale, which is a measure of the length of time beyond which the velocity fluctuations can be considered to be statistically independent, is defined as /•oo Tk = / Rk{r)dT, (4.1) Jo • Mr) = ,,asr / uk(t)uk(t + r)dr, (4.2) )imax JO where Rk is the normalized autocorrelation function for motions in the fc-direction, uk are the velocity fluctuations, r is the time lag, Tmax is the length of the time series, and angle brackets refer to ensemble averages. Ideally, the autocorrelation function for a given time series will approach zero at the integral time scale. However, as has been Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 58 pointed out in earlier drifter studies (e.g., Krauss and Boning, 1987), the autocorrelation function often does not approach zero when the mean current is not well defined and/or the time series is too short. Krauss and Boning (1987) derived autocorrelation functions indirectly using the structure function, given by Dk{r) = {[uh{t + T)-uk{t)]2). (4.3) The structure function is related to the autocorrelation function through the non-normalized correlation function B(T), Dh(r) = 2[Bh(0) - Bk(r)] = 2[<u?> - 2fc(r)], (4.4) Bk(r) = (u'?)Rk{T). (4.5) The autocorrelation functions derived from all drifters in both ensembles are shown in Figure 4.2. The integral time scales were derived by integrating the autocorrelation function from zero to the time of the first zero crossing, yielding global integral time scales of 2.5 (2.8) and 1.5 (2.4) days in the zonal and meridional directions, respectively, for the shallow (deep) ensemble (Table 4.1). Derivations based on integrations of the structure and autocorrelation functions gave nearly identical results. The integral length scale, i.e. the distance traversed over one integral time scale period, was then found from, L„ = <ti?>*Tfcl (4.6) yielding global eddy length scales of 29 (27) and 17 (23) km in the zonal and meridional directions, respectively, for the shallow (deep) ensemble (Table 4.1). As noted by Thom son et al. (1990), the decorrelation length scales are shorter in the North Pacific, with its weaker eddy field, than in the North Atlantic (Krauss and Boning, 1987). The pre dominantly zonal (eastward) flow results in anisotropic decorrelation scales, particularly Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 59 at 15 m depth. Integral time scales are slightly longer below the mixed layer. It is interesting to note that there is spatial and temporal (not shown) variability in the decorrelation scales (Figures 4.3,4.4; Table 4.1). In most regions, and at both depths, the scales are slightly longer in the zonal direction. Near the bifurcation zone, stronger meridional flow at 15 m depth yields nearly isotropic decorrelation scales. The most significant anisotropy occurs in the northern Gulf of Alaska. At 15 m, this sub-ensemble consists primarily of drifters deployed in the Alaskan Stream, and the result is long time and length scales. At 120 m, these drifters were deployed near the head of the Gulf, and subsequently revealed a slow recirculation. Also shown in Figure 4.2 are the global eddy diffusivity components as a function of time lag, derived from Kkk = (uk2) / Rk(r)dr. (4.7) Jo Peak values occur at a lag of approximately 10 days and are on the order of 2 X 10r cm2Is, which is comparable to the estimates obtained from 100-m drogued drifters in the bifurcation region (Thomson et al., 1990). The diffusivities are larger in the zonal direction, particularly for the mixed-layer drifters. These values are based on the global mean velocity statistics, which in fact have significant spatial structure (see below). The spatial variability of the eddy diffusivities is addressed in Chapter 5. 4.4 Selection of Grid Geometries Drifter-derived mean velocity maps require an ensemble grid geometry which provides high spatial resolution, while everywhere maintaining a data population sufficient for robust statistical estimates. These criteria run counter to each other, since a minimum number of data points may require a box size considerably larger than the dominant Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 60 Figure 4.2: The global average autocorrelation functions and eddy diffusivities as a func tion of time lag derived from the (a) shallow and (b) deep drifter ensembles. 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) Figure 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 northern Gulf of Alaska (1018 days), and (d) the Subtropical Gyre (3842 days). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 62 DEEP DEPLOYMENTS 1—i—i—i—i—i—i—i—r i—r Lag (days) Figure 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 Gulf of Alaska (456 days), and (d) the Subtropical Gyre (2759 days). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 63 REGION Tx (days) Ty (days) Lx (km) Ly (km) SHALLOW Line P 3.0 2.7 24.5 23.5 Subarctic Current 2.1 1.2 20.1 10.8 Gulf of Alaska 5.1 1.2 89.3 17.6 Subtropical Gyre 2.8 2.2 38.9 31.2 Global 2.5 1.5 28.8 16.5 DEEP Line P 2.8 2.4 19.3 14.4 Subarctic Current 3.8 2.6 28.0 18.4 Gulf of Alaska 1.4 2.6 16.5 29.5 Subtropical Gyre 3.0 2.2 31.6 24.0 Global 2.8 2.4 27.4 22.7 Table 4.1: Decorrelation time and space scales derived from the global and regional autocorrelation functions of each ensemble. horizontal scales of the current variability. With sufficient data, one can satisfy a pre determined objective criterion for deriving minimum box sizes. This approach was re cently taken for drifter studies in the North Atlantic (Brugge, 1995) and South Atlantic (Schafer and Krauss, 1995), in which the number of data points required for the statistics in a given box to converge on the box mean was computed. These studies pointed out that the standard errors (at the 95% confidence level) in the mean velocity, (uk), and the root-mean-square velocity, (%2)2, given by (uk)± (4.8) (4.9) are dependent not only on the number of degrees of freedom, A^, (i.e., the number of independent estimates, assuming a decorrelation time scale of the circulation), but also Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 64 on the eddy kinetic energy, (EKE) = \((u?+u?)). (4.10) Thus, regions having different energy levels would have different requirements for achiev ing statistical stability, and there is no benefit in choosing grid boxes of equal dimension or size. Brugge (1995) found that this stability criterion could be met in a majority of 3° x 2° (A x 4>) boxes in the North Atlantic. However, applying Brugge's (1995) convergence criterion to the ensembles presented here often required boxes which were considerably larger than the horizontal scales of the circulation. As an example, consider the region near the terminus of the Alaskan Stream in the shallow ensemble (Figure 2.3). Requiring a box size large enough for stability would result in a mean velocity which is representative of neither the Alaskan Stream nor its observed recirculation near 170°W. Two boxes are required here, even though the resultant 95% confidence limits on the mean speeds are large. The approach taken, therefore, was to construct a satisfactory compromise between resolution and stability by choosing boxes which resolved but were not significantly larger than the horizontal scales of the dominant currents, while trying to maintain a minimum of 30 degrees of freedom in each box. This is an arbitrary minimum, but statistics in boxes with considerably fewer than 30 degrees of freedom tended to have non-convergent means and should be treated with caution. The grid geometries for both ensembles, along with annual data density histograms, are shown in Figures 4.5 and 4.6, respectively. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 65 60°N-50°hH 40°N 30°N 20°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~ 140°E 160°E 180° 160°W 140°W 120°W 60°N 50°N 40°N 30°N 20°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 II I I I I I r~ °E 160°E 180° 160°W 140°W 120°W Figure 4.5: The trajectories of the drogued segments of the (a) shallow and (b) deep drifter ensembles, with overlying grid geometries. The box marked "ESC Eddies" refers to the subject of Chapter 6 and the box marked "KKT Eddies" refers to the subject of Chapter 7. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 66 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 Figure 4.6: Maps showing annual histograms of number of drifter days in grid boxes for the (a) shallow and (b) deep drifter ensembles. The scale is given in upper right. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 67 4.5 Mean Circulation and Energy Distribution 4.5.1 The overview In computing Eulerian statistics, the integral time scale was assumed to be 5 days, which is larger (by a factor of ~ 2) than the global estimates for both the shallow and deep ensembles (Table 4.1). Following the method of Stabeno and Reed (1994), the number of independent samples (degrees of freedom) was defined as the number of integral-time-scale periods in which data, from any number of drifters, were available in a given box. This is a rather conservative definition, both because the assumed integral time scale is likely an overestimate and because all drifters active in a box over a 5-day period contribute a single independent observation. At 15 m depth, the number of degrees of freedom ranged from fewer than 20 in the north central Gulf of Alaska, where data was sparse and the scales of motion large, to greater than 60 in most of the boxes in the data-rich eastern Subarctic Current (Figure 4.7a). The mean circulation derived from the data contained in these boxes reveals the entire Alaskan Gyre, as well as portions of the northern and eastern branches of the Subtropical Gyre (Figure 4.7b). Mean speeds in the Subarctic Current are generally < 10 cm/s, and peak near 150°W. At 15 m depth, the bifurcation of the Subarctic Current occurs near 48°N, 130°-135°W, with accelerated northward flow in the Alaska Current (most years), or southward flow in the California Current (1990-91). The Alaskan Stream is clearly evident between 150°-170°W, with mean speeds exceeding 25 cm/s near 155°W, and a recirculation into the Subarctic Current near 170°W. South of 45°N, the Kuroshio Extension/North Pacific Current is well-sampled, with mean speeds of 10-15 cm/s. There is evidence of a weak southward recirculation of the western Subtropical Gyre at 160°-170°W, although north of 40°N the flow is still eastward. Mean kinetic energies are on the order of 20 cm2/s2 throughout the eastern North Pacific, but reach 60 cm2/s2 within Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 68 the Kuroshio Extension and 120 cm2/s2 in the Alaskan Stream. The data coverage at 120 m depth is more confined than at 15 m, but nonetheless samples the Alaskan Gyre quite well (Figure 4.8a). Again, the number of degrees of freedom generally exceeds 30, with the eastern Subarctic Current being most effectively sampled. A striking feature of the mean velocity map is the tight circulation of the Alaskan Gyre at this depth (Figure 4.8b). Although deep and shallow drifters were generally deployed at the same positions and times, only one deep drifter went east of ~ 140°W in the Subarctic Current. Instead, there is a northward velocity component near 52°N everywhere east of 155°W, and a broad, strong northward flow in the Alaska Current. Only three deep drifters sampled the core of the Alaskan Stream, near 165°W. Flow south of 45°N, in the North Pacific Current, is weak and eastward, as it is within the mixed layer. Mean kinetic energies are similar as well, with low values (~ 20 cm2/s2) in the entire Alaskan Gyre except within the Alaskan Stream (~ 120 cm2/s2). Velocity variance ellipses reveal nearly isotropic velocity fluctuations throughout most of the open ocean, but significant anisotropy along the perimeter of the Alaskan Gyre and in the Kuroshio Extension region (Figure 4.9). Variances at both depths are particularly small and nearly isotropic in the western Subarctic Current between 40°-50°N, where ratios of eddy to mean kinetic energy are near, or even less than, unity. This matches the mid-latitude hydrographic and XBT transect observations of Bernstein and White (1977), who reported an abrupt drop in near-surface eddy energy east of 170°W, as well as current meter observations at mid-latitude stations along 152°W (Niiler and Hall, 1988; Hall et al., 1997). This region has been termed an "eddy desert" (e.g., Hall et al., 1997), a moniker supported by the drifter measurements. Overall, the velocity fluctuations are slightly more anisotropic in the mixed layer (Figure 4.10). It should be kept in mind that the derivation of the eddy kinetic energies requires the mean flow to be known, which is not necessarily the case. In some grid boxes, the mean velocities have large accompanying Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 69 Figure 4.7: Maps showing (a) the number of degrees of freedom contained in the grid boxes of the shallow drifter ensemble, and (b) the derived mean velocity and mean kinetic .energy (MKE; cm2/s2). Boxes with fewer than 30 degrees of freedom are dashed, and marks in (b) refer to the box center-of-mass positions. MKE contours extrapolated beyond the data range in the northwest portion of the map (Bering Sea) are not valid. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 70 Figure 4.8: Maps showing (a) the number of degrees of freedom contained in the grid boxes of the deep drifter ensemble, and (b) the derived mean velocity and mean kinetic energy (MKE; cm2/s2). Boxes with fewer than 30 degrees of freedom are dashed, and marks in (b) refer to the box center-of-mass positions. The closed contour near 48°N, 150°W is a maximum. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 71 standard errors. The mixed-layer variances in the Subarctic Current increase and become more anisotropic with proximity to the coast. Highest variances are seen in the Kuroshio Extension region at 15 m depth, and in the northern Gulf of Alaska at both depths. The major axes are aligned with the Alaskan Stream, but are perpendicular to the mean flow of the Kuroshio Extension. The high energies in the shallow box centered at 47°N, 152°E and the deep box centered at 39°N, 172°E reflect the long-lived, topographically-generated eddies ob served over the Kuril-Kamchatka Trench (Chapter 7) and the Emperor Seamount Chain (Chapter 6; Bograd et al., 1997), respectively. A close-up of the mean circulation at both depths in the Alaskan Gyre is given in Figure 4.11. Eddy kinetic energies are relatively weak in this region, being on the order of 40-80 cm21s2 at both depths in the Subarctic Current. A minimum in eddy kinetic energy is seen in the northern Subtropical Gyre near 45°N, 160°W, the "eddy desert", while higher values are seen in the bifurcation region at 15 m depth and within the Alaskan Stream at both depths. This pattern is also evident in the variance ellipse maps. The higher eddy kinetic energies near the coastline may be a reflection of instabilities in the coastal currents, and the subsequent intrusion offshore of decaying eddies. Thomson and Gower (1998) have observed the formation and decay of mesoscale eddies in this region in satellite imagery, attributing it to wind-induced instabilities in the Northeast Pacific Coastal Current. Numerous eddies of similar dimension to those observed in the satellite imagery were also observed in the drifter tracks, particularly in the shallow ensemble. One must interpret the Eulerian mean velocity statistics with caution, as they are not a synoptic characterization of the flow field. The represented time intervals and the sampling density vary considerably from box to box (Figure 4.6). Davis (1985b) pointed out that non-uniform deployments (in time and space) can lead to a sampling Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 72 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 180°E' ' '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 (120m) 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 Figure 4.9: Maps showing velocity variance ellipses (given as (u'k2)1/2) in grid boxes derived from the (a) shallow and (b) deep drifter ensembles. The scale is given in upper right. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 73 0 5 10 15 20 25 Urms (cm/s) Figure 4.10: Zonal vs. meridional r.m.s. speeds derived from the shallow and deep drifter ensembles. The lines are least-squares fits. The deep Alaskan Stream box, which had fewer than 10 degrees of freedom, is not included in this plot. 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 Figure 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. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 75 bias, evident as errors on the mean velocities of up to a few cm/s. This is a reflection of the fact that the flow field in a given location (grid box) is temporally variable, and the sampling of that location by a drifter is biased by the time during which sampling occurs and the direction from which the drifter arrived. However, the integral time scales computed from the drifter velocities are quite short, so a drifter's history is not well-correlated with its present velocity and location. The mean residence times in the grid boxes (Figure 4.12) are generally much longer than the integral time scale of the motions, thus it can be assumed that the sampling biases are small on the chosen grids. It should also be kept in mind that the velocity statistics presented here are a single statistical realization. Different grid geometries could have been chosen, which would have resulted in different mean and eddy velocity fields. The statistics presented in the Data Report (Bograd and Eert, 1996) were derived using uniform 5° x 5° grid boxes, and the means along the perimeter of the Alaskan Gyre differ considerably from those on the non-uniform grid. This results primarily from a lack of sufficient spatial resolution using 5° boxes in the high eddy energy regions. Use of a different integral time scale could also affect the derived statistics. Tests were done (not shown) on the same grid geometries using integral time scales of 2 days, 5 days and 10 days, revealing slight variations in the mean velocity fields in each case. 4.5.2 The Alaskan Stream The axis of the Alaskan Stream is clearly visible in the "spaghetti" diagram, which includes data from undrogued (i.e., surface) trajectories (Figure 2.3). A narrow axis is seen offshore of Kodiak Island and extending to near 170°W, where drifters were observed to either recirculate southward into the Subarctic Current or enter the Bering Sea through one of several Passes along the Aleutian Island arc. The axis of the Alaskan Stream can also be seen from the locations of all high (e.g., > 40 cm/s; Stabeno and Reed, 1991) Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 76 Figure 4.12: Maps showing the mean residence times (days) in the grid boxes of the (a) shallow and (b) deep drifter enesmbles. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 77 daily-averaged drifter speeds (Figure 4.13). These locations generally Ue on the shelf break, which is further offshore east of 160°W. Although the two seasons had nearly the same number of total observations in the region, the winter months had twice as many high-speed observations as the summer months, with much of the difference coming from the undrogued drifters. The 15 m and undrogued drifters had a larger percentage of high speed days (11%) than those drogued at 120 m (7%). Reed and Stabeno (1994, 1997) reported an intensified Alaskan Stream in September 1993 and the summer of 1995, respectively. The drifter measurements confirm this, and suggest that the intensification extended into the faU periods of 1993 and 1995, when many of the undrogued high-speed observations were made. At 120 m depth, nearly all of the high-speed observations were from October 1991, a year in which there was an anomalous lack of inflow into the Bering Sea (Stabeno and Reed, 1992). Indeed, the only observations from this period show a recirculation at 167°W (Figure 4.13e). There was a strong surface inflow into the Bering Sea through Amutka Pass (near 172°W) in December 1992 to January 1993 (Figure 4.13a), and a subsequent entry into the Bering Slope Current (Stabeno and Reed, 1994). Undrogued drifters also entered the Bering Sea further west, through Amchitka and Near Passes (Figure 2.3). Recirculation events at 15 m depth were observed near 170°W in October 1992 and near 160°W in December 1992 (Figure 4.13c). None of the drogued .drifters recirculated out of the Stream in the summer months. One shallow drifter entered the Alaskan Stream from the southwest in the summer of 1992, reveaUng an anomalous circulation similar to that described for summer 1981 (Royer and Emery, 1987). 4.5.3 Variability in the Alaskan Gyre There were sufficient data in the shaUow ensemble to attempt to resolve seasonal vari-abiUty of the mixed-layer flow (Figure 4.14). The most pronounced seasonal difference Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 78 WINTER SUMMER Nl 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 Figure 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 April 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 lines) delineate the postion of the Aleutian Trench. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 79 is along 50°N east of 150°W, where the flow is weak and variable in summer. Mean kinetic energies in the Subarctic Current (not shown) are < 20 cm2/s2 in summer, but are 20-30 cm2/s2 in winter, peaking at 150°W (~ 40 cm2/s2). Larger seasonal differ ences are seen in the eddy kinetic energy field, where values near 90 cm2/s2 are seen near 150°W in winter. Eddy kinetic energies are nearly twice as high in winter than summer in the bifurcation region, but are comparable in the two seasons in the Alaskan Stream recirculation region. An interesting feature seen in the spaghetti diagrams is a zone located near 52°-53°N, 150°-155°W where no drifters entered at either drogue depth or at the surface (Figure 2.3). This is apparently a region of divergent near-surface flow, and can be taken as the center of the Alaskan Gyre for the time periods represented by these drifter trajectories. This area is located slightly further to the west at 120 m depth than at 15 m, which is consistent with a westward displacement of the gyre with depth previously observed in hydrographic data (Royer and Emery, 1987). Using monthly mean wind data from the Comprehensive Ocean-Atmosphere Data Set (COADS) (Woodruff et al., 1987), and the wind-speed-dependent drag coefficients of Large and Pond (1981), the Ekman vertical velocity at the base of the mixed layer was derived from, WB = 1^, (4.11) PJ where T*. is the wind stress, p is density, / is the local Coriolis parameter, and WE is positive upwards. At the gyre "center" (53°N, 155°W), the mean winter Ekman vertical velocity over the period 1990-95 is consistently positive, with an average rate of upwelling of 7 x 10-5 cm/s (~ 20 m/yr). Seasonal maps of mean Ekman vertical velocity for the entire North Pacific over the 1990-95 period reveal a double upwelling maximum in winter, with peaks in the western Subarctic Gyre and the eastern Gulf of Alaska (Figure 4.15a). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 80 175°E 165°W 145°W 125°W j i i J i i i i i i i i 175°E 165°W 145°W 125°W Figure 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. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 81 Upwelling occurs north of 50°N in summer, although it is considerably weaker in the eastern Gulf of Alaska (Figure 4.15b). The patterns in Figure 4.15 are consistent with climatological estimates of upwelling based on the curl of the wind stress (e.g., Talley, 1985), but they mask significant inter annual variability over the study period. A comparison of the sea level pressure patterns during the 1990-95 period with the maps prepared by Emery and Hamilton (1985) for the period 1947-83 suggests that the Aleutian Low was particularly strong during winter 1991-92, near normal during the winters of 1990-91 and 1993-94, and weaker than usual during winter 1992-93. The 1994-95 winter SLP field was near normal, but the Aleutian Low was shifted anomalously eastward. This variability, which may have been associated with El Nifio events in the tropical Pacific (Trenberth and Hoar, 1996), can also be seen in the time series of COADS monthly mean sea level pressure at the gyre "center", 53°N, 155°W (Figure 4.16). The PNA index, which is a measure of the strength of the Aleutian Low (Wallace and Gutzler, 1981), also reveals this interannual variability. The highest PNA index over the 5-year period occurred in the winter of 1991-92. These patterns resulted in divergence and strong Ekman pumping (up to 20 x 10-5 cm/s) in the eastern Gulf of Alaska during the winters of 1991-92 and 1992-93, and in the central Gulf of Alaska during winter 1993-94 (Figures 4.17, 4.18). Upwelling was weak throughout the Gulf during winter 1994-95, when the Aleutian Low appeared to be shifted eastward of its climatological position. The double upwelling maximum seen in the 5-year winter mean upwelling pattern (Figure 4.15a) is a reflection of this interannual variability. The drifters also revealed a near-surface response to the variable atmospheric forcing, evident from a perusal of the individual trajectories (e.g., Bograd and Eert, 1996). There was a strong northward flow throughout the eastern Gulf of Alaska at both depths during the winters of 1991-92 and 1992-93, whereas the Subarctic Current had a significant southerly component during winter 1994-95. These observations are consistent with Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 82 Figure 4.15: Maps showing the 1990-1995 (a) winter and (b) summer mean Ekman vertical velocity, WE (X IO-5 cm/s), derived from COADS winds. Seasons have the same definition as in Figure 4.13. WE is positive upwards. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 83 1991 1992 1993 1994 1995 Figure 4.16: Time series of COADS monthly mean sea level pressure (heavy Une) at 53°N, 155°W and its 3-month running mean (dashed Une). Also plotted are the monthly values of the Pacific North American (PNA) index (octagons), which is defined as a Unear combination of the normahzed 500 mb height anomaUes at four centers located near Hawaii, over the North Pacific, over Alberta and over the U.S. Gulf Coast. The PNA index is a measure of the strength of the Aleutian Low (high positive PNA corresponds to a stronger Aleutian Low). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 84 earlier studies which showed increased northward transport in the bifurcation region during El Nino years (Chelton and Davis, 1982; Kelly et al, 1993). Although there are not sufficient data to determine the representativeness of the drifter measurements analyzed here, there was significant year-to-year variability in the large-scale atmospheric forcing over the Gulf of Alaska during the early 1990's. The magnitude of this variability, and of the corresponding upper-ocean response, is of the same order of magnitude as the regime shifts which have been related to large-scale fluctuations in North Pacific fish stocks (Francis and Hare, 1994; Mantua et al., 1997). 4.6 High Frequency Energy Distribution The geographical distribution of mean and eddy kinetic energy given in the earlier sec tions represents a long-term mean, or "pseudo-Eulerian", pattern. Except for a seasonal separation of the shallow ensemble in the Gulf of Alaska, data were not sufficient to at tempt any kind of temporal resolution. However, variability at the high frequency end of the energy spectrum can be determined by computing the rotary spectra over the first 90 days (i.e., prior to the duty cycle) of the drifter trajectories (Figure 4.19). Figures 4.20 and 4.21 show composite rotary spectra over the first 90 days of individual deployments for the shallow and deep ensembles, respectively. Only those trajectories which were rela tively clustered, in time and space, were used in this analysis. The frequency-partitioned composite rotary variances for each of the deployments are given in Table 4.2. At the lower frequencies (periods of ~ 5-30 days), the highest mixed-layer energies are along the perimeter of the Alaskan Gyre (deployments 7 and 9, within the Alaskan Stream) and in the Subtropical Gyre (deployment 6). Energies in the low mesoscale frequency band are nearly an order of magnitude lower within the Subarctic Current (deployments 1, 5, 11, 12). Low mesoscale energy was nearly isotropic everywhere, Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 85 175°E 165°W 145°W 125°W 175°E 165°W 145°W 125°W Figure 4.17: Maps showing the winter mean Ekman vertical velocity, WE (x 10 5 cm/s), derived from COADS winds, for (a) 1991-1992 and (b) 1992-1993. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 86 175°E 165°W 145°W 125°W 175°E 165°W 145°W 125°W Figure 4.18: Maps showing the winter mean Ekman vertical velocity, WE (x 10 5 cm/s), derived from COADS winds, for (a) 1993-1994 and (b) 1994-1995. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 87 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 Figure 4.19: Map showing the first 90-day trajectories of the (a) shallow and (b) deep drifter deployments (indicated by numbers). Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 88 SHALLOW DRIFTERS x) sx o CQ I w f~, h-> o CD OH m u cO -t-> o PH 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 Mill) 1—1 1 1 1 1111in i ' '1 i r i i mil 1 = (c) 1 M W J953 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 mil ' I _5 ~\ September 1994 10 I-2 • • • • i • |—i i • • • • • • | 10" 10u 10' 10"2 10"1 10° 101 (•I.II 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 TI 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° IO"1 10"2 10~3 IO"4 10" Frequency (cpd) Figure 4.20: Composite rotary energy density spectra (clockwise, S~, is solid; counter clockwise, S+, is dashed), and 95% confidence limits, corresponding to each of the shallow drifter deployments. Deployment numbers and dates are given in lower left. All spectra are for the first 90-day period after initial deployment. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 89 DEEP DRIFTERS 10 10"*-J / 10 September 1990 —I—I I I Mil] rm 10-1 4 10" 10" 10-* 10-5 • 10° 10"1 10"2 10"3 IO"4' 10-5 IO-1 ^\ - ^\ - *\ t I95*r r - June 1992 ft f w r 1 i i 1 ! (e) I95^r II r V \ ^ \ 1 1 \ V i jjf 1 // • May 1994 1 1 1 T rill] 1 1 1 MINI i «L ^ r l I I IIIII 10" 10° 101 10" 10" 10° 101 (b) —,\ \ \ \ * I\A J953 3 ' TF December 1990 10° 1 1 1 1 IIIIJ IIII Mil) -1—l l lllll (d) \ 9 June 1993 1—1 I 1 IIII! 1—1 1 ITI III— —\—1 111III %- 10 10-I I I I llllj IIII I • 111 1—I I lllll| 10" 10' 10' k IO"* 10" . L i, I,UJHII ' 1 (f) -I • i i i"t J953 V 12 September 1994 —i i IIIIIII—iTrnui]—i—i 11 nu 10" 10"3 10"4 10"5 10- 10" 10u 10' Frequency (cpd) Figure 4.21: As in Figure 4.20, but for the deep drifter deployments. Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 90 except for the clockwise-polarized energy partition of deployment 11. At periods of ~ 1-5 days (high mesoscale), energy levels are comparable in all regions and represented time periods, but are highest within the Alaskan Stream (deployment 7). All composite spectra reveal a significant near-inertial clockwise peak, the strongest occurring for the Fall 1990 Line P drifters (deployment 1). Strong near-inertial peaks also occur for the Subtropical Gyre drifters and the Fall 1991 (deployment 5) and Fall 1994 (deployment 12) Subarctic Current deployments. The May 1994 Line P deployment has an order-of-magnitude less near-inertial energy than the deployment in the same region in Fall 1990, revealing an expected strong seasonal difference in atmospheric forcing in this region. The Alaskan Stream drifters have relatively weak near-inertial peaks, but contain the highest energies at the semidiurnal frequency. High frequency energy levels (periods < 12 hours) are of comparable magnitude for all deployments. For deployment 1, which had the highest total energy level at 15 m, 87 % of the total energy at periods less than ~ 30 days is contained in the near-inertial peak (periods of 13-24 hours). This implies that the wind influence is concentrated at high frequencies. (Note that the eddy kinetic energy fields presented in Section 4.5.1 were obtained from filtered - daily-averaged - time series, and thus represent the geostrophic eddy field.) The next highest total energy level (deployment 6) reveals the strong eddying motions of the North Pacific Current in that part of the Subtropical Gyre, while the lowest total energy levels were contained in the late Spring/early Summer Line P drifters (deployment 11). Although the energy levels are considerably lower, a similar frequency-partitioning of the high frequency energy is seen at 120 m as at 15 m (Figure 4.21 and Table 4.2). There is a wider range of energy levels at the lower frequencies (periods of ~ 5-30 days), with the low mesoscale rotary variances ranging from 160 cm2/s2 for deployment 6 to < 5 cm2 js2 for deployment 1. Except for the northern Gulf of Alaska drifters of deployment Chapter 4. Mean Circulation and Energy Distribution in the North Pacific (a) Shallow Deployment Frequency s-{«) Stot{u) low mesoscale 9.3 (0.9) 10.1 (1.0) 19.4 (2.0) m high mesoscale 43.3 (4.4) 11.9 (1.2) 55.2 (5.6) near-inertial 855.0 (87.1) 8.0 (0.8) 863.0 (87.9) semidiurnal 19.4 (2.0) 3.2 (0.3) 22.6 (2.3) high 11.4 (1.2) 9.9 (1.0) 21.3 (2.2) low mesoscale 28.2 (6.7) 26.1 (6.2) 54.4 (12.9) high mesoscale 39.1 (9.3) 28.3 (6.7) 67.4 (16.0) 5 near-inertial 249.6 (59.2) 15.7 (3.7) 265.3 (63.0) semidiurnal 8.0 (1.9) 1.7 (0.4) 9.7 (2.3) high 13.2 (3.1) 11.5 (2.7) 24.6 (5.8) low mesoscale 122.9 (17.6) 106.1 (15.2) 229.1 (32.8) high mesoscale 29.5 (4.2) 15.2 (2.2) 44.8 (6.4) 6 near-inertial 278.9 (39.9) 66.5 (9.5) 345.4 (49.5) semidiurnal 6.0 (0.9) 4.2 (0.6) 10.2 (1.5) high 35.7 (5.1) 33.4 (4.8) 69.1 (9.9) low mesoscale 85.8 (14.3) 100.5 (16.7) 186.3 (31.0) in high mesoscale 32.6 (5.4) 40.1 (6.7) 72.7 (12.1) near-inertial 220.1 (36.6) 7.4 (1.2) 227.5 (37.9) semidiurnal 79.5 (13.2) 4.3 (0.7) 83.8 (14.0) high 18.5 (3.1) 12.1 (2.0) 30.7 (5.1) low mesoscale 38.8 (10.7) 83.8 (23.2) 122.6 (34.0) high mesoscale 13.5 (3.7) 8.3 (2.3) 21.8 (6.0) 9 near-inertial 133.0 (36.8) 8.3 (2.3) 141.3 (39.1) semidiurnal 43.9 (12.2) 3.8 (1.1) 47.8 (13.2) high 18.3 (5.1) 9.4 (2.6) 27.7 (7.7) low mesoscale 22.2 (7.8) 5.0 (1.7) 27.1 (9.5) high mesoscale 7.6 (2.7) 6.0 (2.1) 13.6 (4.8) ES near-inertial 108.0 (37.8) 34.0 (11.9) 142.1 (49.7) semidiurnal 23.7 (8.3) 8.4 (2.9) 32.1 (11.2) high 37.3 (13.1) 33.7 (11.8) 71.0 (24.8) low mesoscale 15.3 (3.3) 14.7 (3.1) 29.9 (6.4) high mesoscale 20.2 (4.3) 9.1 (1.9) 29.3 (6.2) 12 near-inertial 302.6 (64.4) 39.0 (8.3) 341.5 (72.7) semidiurnal 13.9 (3.0) 4.6 (1.0) 18.5 (3.9) high 25.7 (5.5) 24.7 (5.3) 50.4 (10.7) Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 92 (b) Deep Deployment Frequency Stot{v) low mesoscale 1.9 (1.7) 1.7 (1.4) 3.6 (3.1) 0 high mesoscale 2.5 (2.2) 2.3 (2.0) 4.8 (4.2) near-inertial 45.2 (39.2) 5.7 (4.9) 50.9 (44.1) semidiurnal 37.0 (32.1) 3.2 (2.8) 40.2 (34.9) high 8.7 (7.5) 7.2 (6.2) 15.8 (13.7) low mesoscale 29.2 (9.1) 69.7 (21.8) 98.8 (30.9) high mesoscale 18.0 (5.6) 28.0 (8.7) 46.0 (14.4) 3 near-inertial 58.6 (18.3) 21.4 (6.7) 80.0 (25.0) semidiurnal 36.8 (11.5) 5.9 (1.8) 42.7 (13.3) high 22.4 (7.0) 30.4 (9.5) 52.9 (16.5) low mesoscale 93.5 (46.7) 66.4 (33.2) 160.0 (80.0) high mesoscale 4.0 (2.0) 2.1 (1.1) 6.2 (3.1) 6 near-inertial 18.0 (9.0) 3.8 (1.9) 21.8 (10.9) semidiurnal 2.3 (1.1) 0.7 (0.4) 3.0 (1.5) high 4.9 (2.5) 4.4 (2.2) 9.3 (4.7) low mesoscale 6.6 (8.4) 6.6 (8.4) 13.3 (16.8) high mesoscale 1.1 (1.4) 1.0 (1.3) 2.1 (2.7) 9 near-inertial 38.2 (48.4) 3.9 (4.9) 42.0 (53.3) semidiurnal 9.9 (12.6) 1.6 (2.0) 11.5 (14.7) high 5.0 (6.4) 4.9 (6.2) 9.9 (12.5) low mesoscale 5.0 (4.0) 3.3 (2.6) 8.3 (6.6) high mesoscale 3.5 (2.8) 2.6 (2.1) 6.2 (4.9) 11 near-inertial 37.1 (29.5) 7.6 (6.1) 44.8 (35.6) semidiurnal 43.7 (34.7) 5.6 (4.5) 49.3 (39.2) high 8.4 (6.7) 8.8 (7.0) 17.2 (13.7) low mesoscale 28.3 (5.5) 23.1 (4.5) 51.3 (10.0) high mesoscale 20.6 (4.0) 18.1 (3.5) 38.6 (7.5) 12 near-inertial 315.5 (61.6) 45.4 (8.9) 360.9 (70.4) semidiurnal 8.7 (1.7) 5.2 (1.0) 13.9 (2.7) high 26.0 (5.1) 21.8 (4.3) 47.8 (9.3) Table 4.2: Clockwise (S~ (w)), counterclockwise (S+(o;)) and total (Stot(w)) composite rotary variances (cm2/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), 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, there is a high mesoscale energy trough seen in all of the rotary spectra. The near-inertial clockwise peak is strongest for the Fall 1994 Subarctic Current deployment, and weakest in the Subtropical Gyre. Relatively strong semidiurnal peaks occur for each of the eastern Northeast Pacific deployments (1, 3, 9, 11). For five of the deployments, the frequency-partitioned spectral amplitudes can be compared for drifter motions within and below the mixed layer (Table 4.3). Shallow-to-deep total spectral amplitude ratios are highest at the near-inertial frequencies for deployments 1 and 6, which had very strong mixed-layer inertial motions. The shallow drifters within deployment 9 had significantly higher mesoscale energy levels, presumably due to the entrainment of one of these drifters into the Alaskan Stream. Mixed-layer en ergy levels were only slightly higher for the May 1994 Line P deployment, and were comparable to the pycnocline energy levels for the Fall 1994 Subarctic Current deploy ment. Energy at the semidiurnal frequency was slightly higher at 120 m for both Line P deployments. 4.7 Applications Finally, the drifter velocity statistics can be used to address some of the relevant issues raised during the CJGOFS field studies. For example, iron-rich near-surface water was observed at some offshore stations along Line P, and its origin is unclear. At Station P (50°N, 145°W), the near-surface mean flow is consistently from the west (e.g., Figure 4.11). Any water of recent coastal origin observed here, or along the offshore portion of Line P, is more likely to have arrived via a strong recirculation from the head of the Gulf, at least in the winter months, rather than from the coastal waters off Vancouver Island. However, it is possible that water of immediate coastal origin may be deposited offshore near Line P by westward-propagating eddies. This scenario would be more likely Chapter 4. Mean Circulation and Energy Distribution in the North Pacific 94 Deployment low mesoscale high mesoscale near-inertial semidiurnal high 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 Table 4.3: The total (Stot(u;)) composite spectral amplitude ratios (shallow/deep) in five frequency bands for each of the deployments containing shallow and deep drifters. The frequency bands are the same as in Table 4.2. Boxed numbers refer to the deployments (see Figure 4.19). in spring and summer, when the Alaskan Gyre circulation is weaker. The region to the southwest of the Queen Charlotte Islands (~ 51°N, 134°W) is a site where eddies, likely formed by tidal rectification (Thomson and Wilson, 1987), have frequently been observed in drifter trajectories (including from these data), satellite imagery, and hydrography. Eddies formed by coastal current instabilities have been observed all along the eastern perimeter of the Gulf of Alaska (Thomson and Gower, 1998). Thus, while the mean flow certainly advects open-ocean water to the offshore stations along Line P, it is possible that mesoscale eddies may be event-scale transporters of coastal waters to the region. Satellite altimetry, along with the regular seeding of surface drifters in the slope waters off the British Columbian coast and near the head of the Gulf, would be ideal tools for resolving this issue. Chapter 5 Eddy Statistics in the North Pacific 5.1 Introduction Lagrangian drifters are ideally suited for the analysis of dispersion behavior. From a dispersion analysis, and some simplifying assumptions, one can derive decorrelation time and length scales (i.e., "eddy mixing" scales) and horizontal eddy diffusivities, which are vital parameters for numerical ocean circulation models. Obtaining reliable values for these eddy statistics can lead to improved parameterizations in eddy-resolving circula tion models and ultimately to a better understanding of lateral mixing processes in the ocean. In this chapter, Taylor's (1921) theories of single particle dispersion are applied to the drifter ensembles in order to derive the magnitude of the eddy mixing scales and diffusivities over a broad region of the North Pacific Ocean and at both drogue depths. 5.2 Theory and Methods In the seminal work of Taylor (1921), the dispersion of a particle from its origin in homogeneous, isotropic turbulence was related to the velocity fluctuations. The mean square dispersion can be written as, (x'k2(t)) = 2(u*) (t-r)Rk(r)dr, (5.1) Jo where x'k(t) is the displacement of the particle due to u'k. The autocorrelation function has two limits, i.e. Rk = 1 at r = 0 and Rk —> 0 for r large. Correspondingly, there are 95 Chapter 5. Eddy Statistics in the North Pacific 96 two limits on the dispersion: (x'k2) = (u'2)t2 for *«Tfc, (5.2) (x'2) = 2(u2)Tkt for i»Tfc, (5.3) where Tk is the Lagrangian integral time scale. In the first limit (initial dispersion), dispersion is a linear function of time. For times greater than the integral time scale (random-walk regime), dispersion varies as t1!2. Equation (5.3) can be used to estimate the integral time scale independently from equation (4.1). The Lagrangian eddy diffusivity, 2 dt can then be written as, kk i f°° (u2) / Rk(r)dr, (5.5) Jo which yields Kkk = (u2)t for t<Th, (5.6) Kkk = (u'2)Tk for t^Tk. (5.7) The eddy diffusivity components can be found directly through equation 5.4 or indirectly through equation 5.7. The assumptions of homogeneity and stationarity are rarely if ever met in the oceans. However, Taylor's turbulent mixing theories have proven to be successful in predicting particle dispersion from drifter ensembles (Krauss and Boning, 1987; Thomson et al., 1990; Schafer and Krauss, 1995). The assumptions are certain to break down in regions Chapter 5. Eddy Statistics in the North Pacific 97 of strong eddy activity, regions of topographic steering, and boundary current regimes. Regions (boxes) which are representative of the more homogeneous (and quiescent) ocean interior were therefore selected to test Taylor's theories on the mixed layer and pycnocline drifter ensembles (Figure 5.1). Although the question of stationarity cannot be addressed with this data set, the assumption of isotropy is a safe one (Figures 4.9, 4.10). Since most of the earlier applications of Taylor's theory involved the use of deep-drogued (~ 100 m) surface drifters or sub-thermocline floats, their application to drifter trajectories from the same regions and at two depths, within the strongly wind-influenced mixed layer and below it, will be instructive. 5.3 Applicability to the drifter ensembles Decorrelation scales have been derived for the global ensemble and for regional sub-ensembles (Chapter 4). In this chapter, the decorrelation scales are re-derived based on Taylor's theory of single particle dispersion. The approach taken has become standard in particle dispersion studies using La grangian trajectories, and makes use of the assumptions of homogeneity and stationarity (Colin de Verdiere, 1983). Data were increased in each box by splitting each drifter's time series into separate series, each re-set to an origin point after a multiple of 10 days (i.e., longer than the integral time scales estimated in Chapter 4). Time series of less than 10 days were not included in the analysis. Drifters which left a box for longer than one day and re-entered were treated as separate trajectories. The resulting number of separate trajectories contained in the "Taylor" boxes ranged from 47 to 247. Figure 5.2 shows an example of the displacement "plumes" derived from this exercise. The net mean particle displacement in the shallow box centered at 49.3°N, 145.3°W was eastward (positive zonal displacement), which is consistent with the derived mean velocity (Table Chapter 5. Eddy Statistics in the North Pacific 98 Figure 5.1: Spaghetti diagrams for the (a) shallow and (b) deep drifter ensembles. "Tay lor" boxes are regions in which Taylor's theories of single particle dispersion are tested. Chapter 5. Eddy Statistics in the North Pacific 99 5.1). From these displacement plumes, the box-mean dispersions {(x'k2(t))) can be calcu lated. Time series of mean dispersion over the first 10 days for all of the shallow and deep Taylor boxes are shown in Figures 5.3 and 5.4, respectively. The theoretical initial dispersion curves (equation 5.2) are also given. In all of the boxes there was a rapid initial dispersion over the first 2-4 days, followed by a slower dispersion. After the first 10 days, the mean dispersions ranged from 30-90 km, with zonal dispersion exceeding meridional dispersion in all boxes except for those in the northeastern Gulf of Alaska and the northern Subtropical Gyre. Higher velocity fluctuations yielded larger disper sion in the boxes of the Subtropical Gyre (e.g., Figures 5.3g,h and Figure 5.4e). Taylor's prediction of an initial dispersion regime appears to hold quite well in the all of the "Tay lor" boxes, including the one with the fewest trajectory segments (centered at 55.2°N, 141.4°W, Figure 5.3a). Although the horizontal eddy diffusivity components can be found directly from the rate of change of mean square dispersion (equation 5.4), this calculation is very sensitive to small variations of dispersion (Schafer and Krauss, 1995). Figure 5.5 presents this calculation for one box within the Subarctic Current region from each ensemble. With increasing time, there are fewer data points in the calculation and random fluctuations may become dominant (Krauss and Boning, 1987). Thus, even smoothing with a 5-day running mean filter leaves a fairly noisy time series. The maximum values attained by the calculation can be assumed to be the upper limit of the diffusivities for these boxes (Thomson et al., 1990). At both depths, the zonal diffusivities are slightly larger than the meridional diffusivities. Peak values are on the order of 3-5 (1-2) x 107 cm2 / s in the zonal (meridional) direction. The eddy diffusivity components can be more readily obtained through application of Taylor's theory. According to equation (5.3), there should exist a time range over which Chapter 5. Eddy Statistics in the North Pacific 100 Figure 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. 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 55.2, 141.4 -5.1 ± 2.2 5.7 ± 2.5 13.8 ± 1.5 15.8 ± 1.8 48.0, 173.3 6.2 ± 2.2 0.3 ± 1.0 11.8 ± 1.5 9.9 ± 0.7 49.4, 159.6 6.3 ± 1.2 -0.1 ± 0.6 8.7 ± 0.8 8.1 ± 0.4 49.3, 145.3 4.6 ± 0.9 -0.1 ± 0.5 8.8 ± 0.7 8.8 ± 0.3 48.6, 134.3 5.3 ± 0.9 -0.3 ± 0.5 8.0 ± 0.6 8.2 ± 0.4 42.4, 154.4 5.3 ± 1.0 -0.4 ± 0.6 7.8 ± 0.7 7.7 ± 0.4 40.8, 193.1 10.7 ± 2.7 -0.8 ± 1.4 15.6 ± 1.9 16.5 ± 1.0 37.8, 175.1 7.6 ± 1.8 -2.4 ± 1.6 14.5 ± 1.3 17.6 ± 1.1 DEEP 54.9, 145.0 3.1 ± 1.3 2.3 ± 1.6 11.6 ± 0.9 11.8 ± 1.1 50.1, 155.9 5.9 ± 1.2 0.1 ± 0.7 9.1 ± 0.8 8.8 ± 0.5 50.0, 144.4 3.9 ± 1.0 1.0 ± 0.6 7.8 ± 0.7 7.0 ± 0.4 40.6, 148.9 5.0 ± 1.2 0.0 ± 0.7 8.8 ± 0.8 8.7 ± 0.5 37.8, 170.0 3.8 ± 2.3 -1.4 ± 1.8 12.6 ± 1.6 11.7 ± 1.3 Table 5.1: Mean and r.m.s. velocities in the "Taylor" boxes. Standard errors (at the 95% confidence level) are based on different integral time scales in the zonal and meridional directions. Mean positions refer to the center-of-mass locations. Chapter 5. Eddy Statistics in the North Pacific 102 SHALLOW DRIFTERS o •i—i SH 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 Time (days) Figure 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. Chapter 5. Eddy Statistics in the North Pacific 103 DEEP DRIFTERS 02468 10 02468 10 0 2 4 6 8 Time (days) Figure 5.4: As in Figure 5.3, except for the deep "Taylor" boxes. Chapter 5. Eddy Statistics in the North Pacific 104 Figure 5.5: Time series of eddy diffusivities, using a 5-day running mean filter, derived directly from the derivative of mean square dispersion for one (a) shallow and one (b) deep "Taylor" box from the Subarctic Current region. Chapter 5. Eddy Statistics in the North Pacific 105 the mean square dispersion can be represented as a straight line, i.e. dispersion varies as t1/2. This random walk regime appears to occur in all of the shallow and deep boxes after about 10 days (Figures 5.6, 5.7). Time ranges over which this approximation holds were subjectively determined (solid lines) and, following Schafer and Krauss (1995), the integral time scales were then estimated from the mean square dispersion over this time range, %=wrr^- (5-8) Equations (4.6) and (5.7) were then used to determine the integral length scales and eddy diffusivities, respectively, from the derived time scales (Table 5.2). The global mean time scales derived from dispersion are 2.5 (2.8) days in the zonal direction and 1.7 (2.3) days in the meridional direction, respectively, for the shallow (deep) ensembles. The longest zonal time scales at 15 m depth are in the central Subarctic Current region, while the meridional time scales are 1.5-2 days throughout the near-surface North Pacific (Table 5.2, Figure 5.8). These estimates compare very well with the estimates derived earlier from the global- and regional-mean autocorrelation functions (Table 4.1). Diffusivities are highest at both depths in the Subtropical Gyre, particularly in the region just east of the Emperor Seamount Chain. The eddy statistics are slightly anisotropic, with the zonal scales and diffusivities everywhere exceeding the meridional statistics, except in the northeastern Gulf of Alaska. Schafer and Krauss (1995) found the same anisotropy in the South Atlantic, and attributed the enhanced particle dispersion in the zonal direction to the /3-effect. As was found earlier, the decorrelation scales are slightly longer, and more nearly isotropic, below the mixed layer. The diffusivities derived here are comparable to those derived from 100-m drogued drifters in the eastern North Pacific (Thomson et al, 1990; roughly the region of the shallow Alaska Current Chapter 5. Eddy Statistics in the North Pacific 106 C\2 a o • 1—I tn SH CD OH OT • i—i Q CD SH cd GO cd SHALLOW DRIFTERS 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 IIIII 1—I—I IIIII -1 1 I I • ' • • 1(e) 48.6°N, 134.3°Wt n6 . -i—i—i i 11 ii| 1—i i i 11 II —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 IIIII 10° 101 102 Time 10° 101 i i ' i 11111 102 (b) 48.0°N, 173.3°Wf n—i—i IIIII) r—i—n~r !<*> i r r r 49.3°N, r 145.3°W : :(f) |—i niiinj'1 IIIIIII—I'TIIIIII \ i .X mui i iiiiiij :'" 42.4°N, 154.4°W : ' • 1 ' IIIIIII 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) Figure 5.6: Mean square dispersion (zonal-dashed, meridional-dotted) over the first 100 days for the shallow "Taylor" boxes. Solid curves are the theoretical values from equation 5.3. Coordinates refer to center-of-mass positions. Chapter 5. Eddy Statistics in the North Pacific 107 DEEP DRIFTERS 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' II III] 1 1—l l l 111 (e) 105^ 10* 103 102 10' 37.8°N, 170.0°W 10' 1—i—i i 11111 1—i—r r T i II ,o 10i 10z 10" 10' IO'' 1(b) 50.1°N, 155.9° -1 1—I I Mill 1 1—I I I I I I 10° 105 104 103 lr102 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 Time (days) Figure 5.7: As in Figure 5.6, except for the deep "Taylor" boxes. Chapter 5. Eddy Statistics in the North Pacific 108 box, centered at 48.6°N, 134.3°W), but, due to lower eddy kinetic energies in the North Pacific, are about half the magnitude of those derived from 100-m drogued drifters in the North Atlantic (Krauss and Boning, 1987) and South Atlantic (Schafer and Krauss, 1995). It should be kept in mind that there are inherent limitations of the analysis presented here. First, the diffusivity estimates are dependent on the complete removal of the mean flow ((«)) and its effects, which is subject to some uncertainty. Of course, the estimates are also dependent on the validity of the assumptions of homogeneity and stationarity. As pointed out by Davis (1991), Eulerian statistical inhomogeneity (which is apparent in these data) can lead to Lagrangian nonstationarity. Nonetheless, the method yielded results which matched Taylor's predictions quite well. Consistent with previous studies based on surface drifters, a scaling of integral time scale and eddy diffusivity with r.m.s. velocity is found (Figure 5.9). These relationships hold both within and below the mixed layer, and fit well with the relationships derived from drifter data in the Atlantic (e.g., Figures 8 and 12 of Schafer and Krauss, 1995). An exception is the meridional time scale, which appears to have a weaker dependence on vT.m.s.- As for the diffusivities, the integral length scales are slightly anisotropic, and are largest in the subtropical boxes. The global mean length scales are about 30% smaller than those derived from drifters in the Atlantic (Krauss and Boning, 1987; Schafer and Krauss, 1995). Thus, there appears to be little geographical variation in upper-ocean mixing lengths. These are the first Lagrangian estimates of particle dispersion over a broad region of the near-surface North Pacific, and the consistency of the results with previous stud ies from the Atlantic suggests that the simplifying assumptions of Taylor (1921) are reasonably valid throughout the upper ocean. Since velocity variance is readily and syn-optically measured by satellite altimetry (for low-frequency motions), use of the simple 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) SHALLOW 55.2, 141.4 1.5 ± 0.1 1.6 ± 0.2 17.8 ± 2.3 21.9 ± 3.7 2.4 ± 0.5 3.5 ± 0.9 48.0, 173.3 2.5 ± 0.1 1.4 ± 0.1 25.4 ± 3.4 12.1 ± 1.2 3.0 ± 0.7 1.2 ± 0.2 49.4, 159.6 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 49.3, 145.3 3.3 ± 0.1 1.9 ± 0.1 24.8 ± 2.1 14.1 ± 0.9 2.2 ± 0.4 1.2 ± 0.1 48.6, 134.3 2.1 ± 0.2 1.8 ± 0.1 14.2 ± 1.7 12.6 ± 0.9 1.1 ± 0.2 1.0 ± 0.1 42.4, 154.4 3.1 ± 0.2 1.8 ± 0.1 21.2 ± 2.3 11.8 ± 0.9 1.7 ± 0.3 0.9 ± 0.1 40.8, 193.1 2.4 ± 0.2 1.1 ± 0.1 32.4 ± 4.8 16.0 ± 1.7 5.1 ± 1.3 2.6 ± 0.4 37.8, 175.1 2.7 ± 0.1 2.0 ± 0.1 33.3 ± 3.2 30.9 ± 2.5 4.8 ± 0.9 5.5 ± 0.7 Global Mean 2.5 ± 0.1 1.7 ± 0.1 23.4 ± 2.7 16.6 ± 1.7 2.7 ± 0.6 2.1 ± 0.3 DEEP 54.9, 145.0 1.5 ± 0.1 2.5 db 0.3 14.8 ± 1.5 25.6 ± 3.9 1.7 ± 0.3 3.0 ± 0.7 50.1, 155.9 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 50.0, 144.4 3.5 ± 0.2 2.3 ± 0.1 23.4 ± 2.5 14.0 ± 1.0 1.8 ± 0.3 1.0 ± 0.1 40.6, 148.9 3.5 ± 0.2 2.1 ± 0.1 26.9 ± 2.9 15.4 ± 1.1 2.4 ± 0.5 1.3 ± 0.2 37.8, 170.0 3.3 ± 0.2 2.8 ± 0.4 36.4 ± 5.1 28.8 ± 5.2 4.6 ± 1.2 3.4 ± 0.9 Global Mean 2.8 ± 0.2 2.3 ± 0.2 23.8 ± 2.8 19.5 ± 2.5 2.4 ± 0.5 2.0 ± 0.4 Table 5.2: Integral time (Tk) and length (Lk) scales and eddy diffusivities (Kkk), with standard errors, derived for the "Taylor" boxes in the shallow and deep drifter ensembles. Mean positions refer to center-of-mass locations. Chapter 5. Eddy Statistics in the North Pacific 110 Figure 5.8: Maps showing the zonal/meridional integral time scales (days) and eddy diffusivities (x 107 cm2/s) for the "Taylor" boxes in the (a) shallow and (b) deep drifter ensembles. 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_TX vs. (15m) O—Ty vs. V^, (15m) A—Tx vs. (120m) + Ty VS. VJTBS (120m) O O r 8 12 16 20 r.m.s. speed (cm/s) Figure 5.9: (a) Integral time scale and (b) eddy diffusivity vs. r.m.s. speed derived for the "Taylor" boxes. Straight lines are least-squares fits. The fits are based only on the range of r.m.s. speeds sampled here, and should not be extrapolated to the y-intercept. Chapter 5. Eddy Statistics in the North Pacific 112 relationship, Kkk = (u'2)l*Lh, (5.9) may yield good estimates of near-surface eddy diffusivity for modeling applications (Schafer and Krauss, 1995). Chapter 6 Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 6.1 Introduction The previous chapters provided a statistical overview of the near-surface circulation and its variability in the North Pacific Ocean based on the trajectories of nearly 100 satellite-tracked drifters drogued either within the mixed layer or in the underlying pycnocline. A perusal of these trajectories (e.g., Figure 4.5) reveals some interesting mesoscale features which warrant further analysis. In this chapter, the focus is on a subset of drifters which sampled the region near the Emperor Seamount Chain (near 172°E) and were entrained into long-lived, topographically-generated eddies. 6.2 Theoretical Background Seamounts are ubiquitous features of the world's oceans which can profoundly affect physical and biological processes, generating mesoscale circulation features and increas ing primary production (Royer, 1978; Hogg, 1980; Boehlert and Genin, 1987; Lobel and Robinson, 1986; Roden, 1987; Roden, 1991; Freeland, 1994; Comeau et al., 1995.) Modeling the effects of submerged topography on ocean circulation and thermohaline structure has therefore garnered considerable attention in the oceanographic community. The dominant principle is the conservation of potential vorticity along a particle trajec tory: as a column of water moves over an obstacle, vortex compression requires a decrease in local vorticity, resulting in anticyclonic currents (a Taylor column) over the feature 113 Chapter 6. Eddies h Seamount-attached Eddies at the Emperor Seamount Chain 114 (Taylor (1923)). Vortex stretching upon moving away from the feature produces cyclonic rotation. Early analytical and numerical models by Hogg (1973) and Huppert and Bryan (1976), respectively, demonstrated the potential initialization of a Taylor column over submerged isolated topography. Numerical models describing quasi-geostrophic steady-state (Kozlov, 1981; Johnson, 1982) and time-variable (Verron and Le Provost, 1985; Verron, 1986) flow over an isolated submerged obstacle have shown that anticyclonic and cyclonic eddies can be formed, with dynamical characteristics dependent on the back ground flow and the obstacle geometry. More recently, Chapman and Haidvogel (1992) and Smith (1992), using primitive equation numerical models, have both shown that the formation of Taylor caps or eddies near the topography depends critically on seamount height. Laboratory models have also been developed to study the motion of rotating, stratified, steady and oscillatory currents past isolated topography (Boyer and Zhang, 1990a,b; Zhang and Boyer, 1993) and in the vicinity of multiple seamounts (Zhang and Boyer, 1991), each showing the development of mesoscale eddies on top of or in the lee of the obstacle(s). Brink (1990) used an analytical model to demonstrate the generation of seamount-trapped waves by the resonant excitation at certain frequencies of a hori zontally uniform ambient flow. Roden (1991) provides a comprehensive review of recent modeling and observational studies of flow-topography interaction, with an emphasis on the North Pacific. Although the models of flow over submerged topography have produced an intriguing list of predicted mesoscale circulation features, observations of these features have been less definitive. Owens and Hogg (1980) used hydrographic data from the Mid-Ocean Dynamics Experiment (MODE) in the North Atlantic to observe Taylor columns over seamounts, while Richardson (1980) observed anticyclonic eddies in the trajectories of satellite-tracked drifters in the lee of the Corner Rise Seamounts, describing them as Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 115 Taylor columns shed from the topography. Recently, intensive field investigations were undertaken at the Fieberling seamount group in the subtropical North Pacific. Direct current measurements revealed diurnal tidal amplification over the summit and a residual anticyclonic circulation around the seamount (Eriksen, 1991; Noble et al., 1994; Brink, 1995). Flow patterns in the vicinity of the seamounts were dominated by pairs of transient mesoscale (10-30 km diameter) cyclonic and anticyclonic eddies and accompanying jets with core speeds of 20-50 cm/s (Roden, 1994). In the North Pacific, one of the most prominent topographic features is the Emperor Seamount Chain (ESC), a meridionally-oriented underwater mountain range, extending approximately 2400 km near 170°E from the Aleutian Islands arc to the Hawaiian Islands arc and rising 3 to 5 km above the ocean floor (Figure 6.1), with some individual peaks near its southern end approaching 100 m of the sea surface (Roden et al., 1982). The ESC effectively splits the North Pacific into separate western and eastern basins. Using sparse historical hydrographic data, Roden et al. (1982) demonstrated that the Kuroshio Extension west of the ESC is a well-defined meandering jet, extending nearly to the bottom, which tends to turn northward upon its approach to the southern end of the chain and then turns southward in an anticyclonic loop as it crosses the axis of the seamounts, generally in the 6000 m deep Main Gap at 39°N, 171°E. East of the ESC, the Kuroshio Extension is weak and poorly defined. An extensive field survey of the region carried out in June and July, 1982 (Roden and Taft, 1985) revealed this deflection and weakening of the Kuroshio Extension over Kinmei Seamount (near 35°N), as can be seen in the dynamic topography of the 150 db surface relative to 800 db, near the depth of the seamount peaks (Figure 6.2). Mesoscale perturbations of large vertical amplitude, suggestive of eddies or Taylor columns, were also observed near several of the seamounts. A current meter mooring in the Main Gap between June 1982 and November 1983 demonstrated flow rectification with depth and a mean bottom-referenced northeastward transport of 14 Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 116 Sv (Hamann and Taft, 1987). Roden and Taft (1985) suggested that this flow through the Main Gap may provide one of the few avenues through which northeast Pacific abyssal waters communicate with the rest of the Pacific. Vastano et al. (1985) presented observations of two satellite-tracked drifters, drogued at 100 m, which crossed the ESC in 1977. Both drifters revealed upstream meanders, cyclonic eddy activity just west of the chain, and passage through the Main Gap. Due in part to its remoteness, no systematic fieldwork has been forthcoming at the ESC since the observations in the early 1980's, nor have there been any extensive Lagrangian observations. Although the summer 1982 field survey provides a characterization of the upper level flow pattern at the ESC (Figure 6.2), it represents only one realization and the persistence of the observed mesoscale features is unkown. In this chapter, data are presented and described from seven satellite-tracked drifters, five drogued within and two below the wind-mixed layer, which crossed the southern end of the ESC in the summers of 1991 and 1992 and the winter of 1993. In particular, one of the first observations of an extended attachment (>60 days) of a topographically-generated eddy to a seamount is presented. Thus, although limited, this data set further illustrates the pronounced eddy activity thought to be present at the ESC, and provides a measure of the characteristics and persistence of the mesoscale features. Comparisons are made between the observations and the flows predicted by laboratory and numerical models of current-topography interactions. The observations presented here reveal the ESC as a generator of quasi-stationary, long-lived upper-level mesoscale eddies, and pro vide an especially useful test of models of flow over tall seamounts, demonstrating both their general validity and specific limitations. Chapter 6. Eddies h Seamount-attached Eddies at the Emperor Seamount Chain 117 Figure 6.1: Map of the North Pacific Ocean showing location of the Emperor Seamount Chain. Light and dark shading in this and subsequent figures represents water depths of 2000-4000 m and shallower than 2000 m, respectively. The inset shows the bathymetric section along the boxed region, which is the crest of the southern portion of the Emperor Seamount Chain (adapted from Roden et al. (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* Figure 6.2: Map of dynamic topography (J/kg) of the 150 dbar surface relative to 800 dbar from cruises of the RV Thomas G. Thompson (dots) and RV Hokusei Maru (trian gles) in June/July 1982. From Roden (1987). Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 119 6.3 Data and Methods 6.3.1 The ESC ensemble Of the total of 102 drifters deployed, seven (live shallow-drogued and two deep-drogued) were operational in the vicinity of the ESC. Four shallow-drogued drifters deployed near the Kuril-Kamchatka Trench or within the Kuroshio Extension survived long enough to cross the ESC on their eastward journey across the mid-latitude North Pacific. Three other drifters were deployed just west of the ESC in June 1992 with the intention of observing the topographic effects on the near-surface circulation. The analysis presented below uses only this ensemble of seven drifters. 6.3.2 The rotary multiple filter technique As stated in chapter 3, the rotary analysis of velocity records is particularly useful when one or the other rotary component dominates the energy spectrum. This is certainly the case for Lagrangian instruments trapped within a mesoscale eddy. In the rotary spectral analysis of currents (Gonella, 1972; Mooers, 1973), velocity records are analyzed in terms of their frequency-dependent rotary components instead of the standard time-dependent Cartesian components (u(t),v(t)). A Fourier analysis was performed on the Cartesian-formatted drifter records to transform them into clockwise and counterclock wise (u~,u+) rotary components. Spectral estimates, 5(w,u±), were obtained for each rotary component. To improve the spectral estimates, each data segment was weighted using a Kaiser-Bessel window (Harris, 1978) prior to the Fourier transform. Half-window overlapping was performed to increase the number of degrees of freedom. The rotary variance of the clockwise and counterclockwise components was estimated as Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 120 Var{u-,u+)= f [%,tt+),%,tt-)]dw, (6.1) J Au Vartot = Var{u+) + Var{u~), (6.2) where the bandwidth Au; encompasses a specified range of frequencies. Rotary spectral components and rotary variances are invariant under coordinate transformation, allowing for a comparison of drifter motions from different regions and time periods. However, although they provide a frequency-dependent (5,(a>,ti±)) or general (V^ar(,u:t)) description of cyclonic and anticyclonic motions, they do not allow for an examination of eddy evolution in time or space. A recently elaborated method, the rotary multiple filter technique (RMFT) (Kulikov and Rabinovich, 1998), was used to investigate temporal variations of the clockwise and counterclockwise rotary components of the drifter motions and eddy activity in the vicinity of the ESC. The original multiple filter technique, introduced by Dziewonski et al. (1969) to examine scalar seismic signals, is a processing tool which uses a set of narrow-band digital filters to examine variations in an input signal's amplitude and phase as functions of frequency and time (Thomson et al., 1997; Kulikov and Rabinovich, 1998). The modification of the method for vector processes enables such an examination for the rotary components. The original multiple filter technique has been widely used to analyze tsunami waves (e.g. Kulikov et al., 1996), while the RMFT has recently been used effectively to quantify a wide range of oceanographic phenomena observed in a single drifter trajectory (Thomson et al., 1997). Narrow-band filters with a Gaussian window were applied to isolate a specific center frequency uin = 2irfn: Hn(u) = e-«[(«—«)/'-n]a) (6.3) Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 121 where the parameter a is used to control the width of the filter and depends on the dispersion characteristics of the original signal and the degree of frequency or time reso lution desired (Thomson et al., 1997). A higher value of a gives better resolution in the frequency domain, but poorer resolution in the time domain. For the present analysis, a value of a = 25 has been used. Demodulation of the vector time series, V(ujn, t) = u{u>n, t) + iv(ujn, t), (6.4) yields a matrix of amplitudes, with columns representing time and rows representing frequency, which can be contoured to give a three-dimensional plot of the clockwise and counterclockwise components of velocity as a function of frequency and time (the f — t diagram; Thomson et al., 1997). Further details of the RMFT can be found in Emery and Thomson (1997). 6.4 Observations 6.4.1 1991 drifters Two sets of satellite-tracked drifters deployed as part of the SVP in the western North Pacific crossed the ESC. Three SVP shallow-drogued (15 m) drifters, deployed near the Kuril Islands (45°N, 150°E) in the fall of 1990, approached the ESC in the summer of 1991 (Figure 6.3). On approach to the ESC from the west, each drifter was deflected northward near 166°E, approximately 350 km west of the seamounts. This matches the near-surface flow pattern derived from the hydrographic observations of Roden and Taft (1985). Each drifter then arrived at the ESC near 41°N-42°N and was deflected anticyclonically over the peak of Nintoku Seamount. Drifter 1314 crossed near the peak (170°E to 171°E) in 5 days (from June 30 to July 4, 1991), while drifters 1315 and 1316 took 10 days (August 4-14, 1991) and 37 days (June 22 to July 29, 1991), respectively, Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 122 to cross the seamount summit. Unfortunately, drifter 1316 ceased transmission soon after crossing the ESC. Drifters 1314 and 1315 continued an eastward trek, crossing the dateline in August 1991 and October 1991, respectively (Figure 6.3). None of the three drifters was retained in the vicinity of the ESC for an extended period of time, nor did they reveal any eddy activity in the lee of the seamounts. 6.4.2 1992 drifters In contrast to the 1991 drifters, three drifters which were deployed together just west of the ESC (40°N, 166°E) on June 23, 1992 were each retained in the vicinity of the ESC for several months. These drifters, one standard SVP shallow-drogued (8098) and two deep-drogued (120 m; 1417 and 4859), began an anticyclonic meander towards Ojin/Jingu Seamount (Figure 6.4,6.5,6.6). Drifter 8098 slowly drifted southwestward and was re tained in a cyclonic eddy near 33°N, 164°E before sweeping across Kinmei Seamount from the southwest in October 1992 (Figure 6.6). It then made one loop in a large an ticyclonic eddy and two loops in a smaller anticyclonic eddy in the gap between Kinmei and Ojin/Jingu Seamounts before continuing eastward in December. Drifters 1417 and 4859, drogued below the mixed layer, approached Ojin/Jingu Seamount in early July 1992, swept through the Main Gap in mid-July at an average speed of 39 cm/s, and were subsequently retained for four and two months, respectively, within eddies in the lee of Ojin/Jingu Seamount (Figs. 6.4,6.5). An examination of the rotary variance (for motions with periods of 2-64 days) derived from the 1992 drifter trajectories shows that there was considerably more energy near the ESC in the summer and fall of 1992 than in the summer of 1991 (Table 6.1). The shallow drifter (8098), like the 1991 drifters, had nearly equal contributions from both rotary components, but had more than twice as much energy as the most energetic 1991 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-30°N * 5/23/91 1071/91;'' ^1314 1315 tl EMPEROR SEAMOUNTS 1991 DRIFTERS (15m) 160°E 170°E 180° 1 170°W Figure 6.3: Trajectories of shallow-drogued drifters 1314 (dotted Une), 1315 (dashed line) and 1316 (dash-dot Une) which crossed the Emperor Seamount Chain in the summer of 1991. Selected dates are marked by crosses. 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 ESC, 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 Figure 6.5: Trajectory of deep-drogued drifter 4859 which crossed the Emperor Seamount Chain in the summer and fall of 1992. Selected dates are marked by crosses. Boxes outline the immediate vicinity of the ESC, which is shown in Figure 6.10. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 126 Figure 6.6: Trajectory of shallow-drogued drifter 8098 which crossed the Emperor Seamount Chain in the summer and fall of 1992. Selected dates are marked by crosses. Boxes outline the immediate vicinity of the ESC, which is shown in Figure 6.12. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 127 drifter (1314). Significant energy in both rotary components was present at frequencies less than 0.25 cpd. The deep drifters (1417 and 4859) were dominated by clockwise and counterclockwise energy, respectively, reflecting the dominance of the eddies sampled (see discussion below). Most of this energy was contributed at the lowest frequencies (by the mesoscale eddies at the ESC), with very little energy between 0.25-0.5 cpd. The distribution of energy derived from the 1992 drifter motions can also be seen in their rotary energy density spectra (Figure 6.7). The spectra of drifters 1417 and 4859 (Figs. 6.7a,b) have a similar pattern, with clockwise energy (representing the anticyclonic eddy in the lee of the Ojin/Jingu Seamount and the large-scale meanders immediately west of the ESC) dominating the lowest frequencies and a sharp energy drop-off with nearly equal clockwise and counterclockwise contributions at frequencies greater than 0.1 cpd. The dominant feature in the spectra of drifter 4859 is the large counterclockwise energy peak, contributed by the cyclonic eddy in the lee of Ojin/Jingu Seamount, near 0.08 cpd (12.5 day period). The counterclockwise contribution at this frequency is an order of magnitude greater than the clockwise contribution. The spectra of drifter 8098 (Figure 6.7c) show that there was significantly more energy in both rotary components at 15 m than at 120 m, with nearly an order of magnitude more energy at frequencies between 0.1-0.5 cpd. While the lowest frequencies were nearly rectilinear, the 0.2-0.3 cpd frequency range (period of 3-5 days) was dominated by counterclockwise energy, which represents the cyclonic eddy activity encountered by drifter 8098 west of the ESC. A significant portion of the total energy contained in the 1992 drifter trajectories was contributed by mesoscale eddies encountered over or on the leeside of the ESC. A closer examination of the 1992 drifters in the immediate vicinity of the ESC is therefore in order. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 128 Frequency (cpd) 1314 1316 1417 4859 8098 5"H (%) 0.015 - 0.08 69.3 (12.5) 64.3 (39.5) 410.2 (36.1) 257.8 (29.3) 462.5 (36.1) 0.08 - 0.25 228.4 (41.1) 23.1 (14.2) 32.5 (4.7) 28.1 (3.2) 153.2 (12.0) 0.25 - 0.5 12.5 (2.3) 3.4 (2.1) 0.6 (0.1) 0.5 (0.1) 6.2 (0.5) total 310.2 (55.8) 90.8 (55.8) 443.3 (64.2) 286.4 (32.5) 621.9 (48.6) s+H (%) 0.015 - 0.08 159.4 (28.7) 62.6 (38.4) 215.0 (31.1) 522.8 (59.3) 451.7 (35.3) 0.08 - 0.25 75.5 (13.6) 8.0 (4.9) 31.5 (4.6) 71.4 (8.1) 190.3 (14.9) 0.25 - 0.5 11.1 (2.0) 1.5 (0.9) 0.4 (0.1) 0.5 (0.1) 16.5 (1.3) total 246.0 (44.2) 72.1 (44.2) 246.9 (35.8) 594.7 (67.5) 658.5 (51.4) StoM (%) 0.015 - 0.08 228.7 (41.1) 126.9 (77.9) 625.2 (90.6) 780.6 (88.6) 914.2 (71.4) 0.08 - 0.25 303.9 (54.6) 31.1 (19.1) 64.0 (9.3) 99.5 (11.3) 343.5 (26.8) 0.25 - 0.5 23.6 (4.3) 4.9 (3.0) 1.0 (0.2) 1.0 (0.1) 22.7 (1.8) total 556.2 (100) 162.9 (100) 690.2 (100) 881.1 (100) 1280.4 (100) Table 6.1: Clockwise (S~(u)), counterclockwise (S+(u;)) and total (Stot(w)) rotary vari ance (cm21s2) in three frequency bands derived from the drifter trajectories in the vicinity of the Emperor Seamount Chain. Numbers in parentheses refer to percent of total vari ance. Estimates were derived from the periods May 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. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 129 Frequency (cpd) Figure 6.7: Total Stot (solid line), clockwise S (long-dashed line), and counterclockwise S+ (short-dashed line) rotary energy density spectra (cm2/s2/cpd) derived from the tra jectories of drifters (a) 1417, (b) 4859, and (c) 8098 for the period June 23 to December 23, 1992. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 130 Drifter 1417 After drifter 1417 meandered anticyclonically through the Main Gap, it was retained in a cyclonic eddy (eddy C) in the lee of Ojin/Jingu Seamount (to its right, facing downstream) for 31 days (Figure 6.8). It made 2 complete loops in eddy C (Table 6.2), the first in 11 days at a radius of nearly 50 km (mean rotational speed, of 28.5 cm/s) and the second in 20 days at a radius of approximately 40 km (mean rotational speed of 15 cm/s). Drifter 1417 was very close to the unnamed seamount at 36.3°N, 171.8°E as it slowed down during the second loop, and may have been tracing out the outer edge of the eddy. Drifter 1417 then exited eddy C and crossed over the top of Ojin/Jingu Seamount in September before being retained in a larger anticyclonic eddy (eddy A), again in the lee of Ojin/Jingu Seamount (to its left, facing downstream). After making one complete circuit of eddy A in 26 days, at a radius of approximately 100 km (mean rotational speed of 29 cm/s), it was retained in another, much smaller anticyclonic eddy at the southern tip of Nintoku Seamount (40°N, 170°E) before leaving the region in mid-December. Drifter 1417 made 2 loops over a 24-day period in the latter eddy, which had a diameter of approximately 35 km and mean rotational speed of 11 cm/s. The mesoscale eddy activity delineated by drifter 1417 is readily seen in its / — t diagrams, which allow for a more detailed examination of cyclonic and anticyclonic features in time and frequency space (Figs. 6.9a,b). The first 40 days (June 23 to August 2, 1992) are dominated by clockwise energy centered at a period of about 25 days, which represents the large anticyclonic meander just west of the ESC and through the Main Gap. Drifter 1417 then entered eddy C, and had strong counterclockwise energy centered first near a period of 11-12 days (near day 45; loop 1) and then centered near a period of 18-22 days (between days 60-70; loop 2). A broad band of low-frequency clockwise energy, centered at a period of 16-26 days, can be seen after drifter 1417 left Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 131 166°E 168°E 170°E 172°E 174°E Figure 6.8: Trajectory of drifter 1417 in the vicinity of Ojin/Jingu and Kinmei Seamounts in the summer and fall of 1992. Each mark represents the daily drifter position at 1200Z, and selected dates are labeled. The anticyclonic (A) and cyclonic (C) eddies are labeled. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 132 Loop Period (days) Diameter (km) Speed (cm/s) Drifter 1411 1 11 94.2 28.5 2 20 83.4 15.2 Drifter 4859 1 12 71.7 21.7 2 12 77.3 23.4 3 15 60.7 14.7 4 13 88.3 24.7 5 10 66.2 24.1 Table 6.2: Period, diameter, and mean rotational speed of the cyclonic eddy (eddy C) as delineated by the trajectories of drifters 1417 and 4859. eddy C and made a large loop in eddy A (days 80-100). The small anticyclonic eddy at the southern tip of Nintoku Seamount shows up as a weak clockwise energy peak centered at a period of 10-12 days (days 145-170). Drifter 1417 then looped southward out of the region, towards Hess Rise, with moderate energy in both rotary components. Drifter 4859 The trajectory of drifter 4859 was almost coincident with that of drifter 1417 as it sped through the Main Gap, made a large anticyclonic loop in the region where 1417 was later retained in eddy A, and finally was trapped within eddy C (Figure 6.10). Drifter 4859 made an initial loop in 12 days at a radius of approximately 36 km (mean rotational speed of 22 cm/s), some 10-15 km closer to the eddy center than drifter 1417 was. It was subsequently retained in eddy C for another 50 days, making 4 more complete circuits with periods ranging from 10 to 15 days at radii of 30 to 45 km (mean rotational speeds of 15-25 cm/s; Table 6.2). Figure 6.11a again shows the trajectory of drifter 4859 within eddy C, with each Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 133 time (days) 0 40 80 120 160 ' ' ' ' i i ' i time (days) 0 40 80 120 160 ' ' 40 80 120 160 time (days) 40 80 120 160 time (days) Figure 6.9: Amplitude evolution of the clockwise and countercockwise rotary velocity components for drifters (a,b) 1417, (c,d) 4859, and (e,f) 8098 for the period June 23 to December 23, 1992. Amplitude contours are given in cm/s. Rotary components for the low-frequency range are shown (log(-0.6) = period of 4 days, log(-l.O) = period of 10 days, log(-1.4) = period of 25 days). The dashed line near the bottom of each plot indicates the time period that the drifter was in the vicinity of the ESC (between 166°E and 174°E). Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 134 Figure 6.10: Trajectory of drifter 4859 in the vicinity of Ojin/Jingu and Kinmei Seamounts in the summer and fall of 1992. Each mark represents the daily drifter position at 1200Z, and selected dates are labeled. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 135 loop plotted uniquely and with the approximate eddy center (as determined from the drifter track) marked. A time series of the rotational speed of eddy C derived from the trajectory of drifter 4859 is given in Figure 6.11b. Speeds ranged from 25-60 cm/s during the first two loops, with faster speeds at the northern end of the eddy and slower speeds at the southern end. The same pattern is seen in loops 3 and 4, but peak speeds at the northern end were only 40 cm/s. Drifter 4859 then accelerated northward (speed > 50 cm/s) in early October, out of eddy C, after having slowed to less than 10 cm/s at the southern end of loop 5. When drifter 1417 was at the southern tip of its second loop in eddy C (August 29; Figure 6.8), drifter 4859 was at the northern tip of its second loop (Figure 6.10). If these two positions define the northern and southern boundaries of eddy C, its north-south extent was approximately 200 km, highly elongated parallel to the axis of the seamounts. After August 29, drifter 1417 accelerated northward and left eddy C as drifter 4859 moved southward in its third loop, on the western side of eddy C and presumably closer to the eddy center. Drifter 4859 was then retained within eddy C for another 23 days, through loops 4 and 5, before leaving the eddy in early October and looping over Ojin/Jingu Seamount (Figure 6.10). It then made a small, slow anticyclonic loop in the gap between the Ojin/Jingu and Kinmei Seamounts before again crossing over the top of Ojin/Jingu Seamount, looping anticyclonically in the previous location of eddy A, and finally leaving the region in late November. The / — t diagrams for drifter 4859 (Figs. 6.9c,d) are comparable to those of drifter 1417 for the first 40 days, when both drifters were nearly coincident. The rest of the six-month period, however, is dominated by very strong counterclockwise energy centered near a period of 12 days between days 35-100, which is eddy C (Figure 6.9d). As seen in the rotary spectra, eddy C contributed a significant counterclockwise energy peak at this period (Figure 6.7b). There was also energy in the clockwise component at a period 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 Figure 6.11: (a) Trajectory of drifter 4859 in the cyclonic eddy (eddy C) in the lee of Ojin/Jingu Seamount between August 4 and October 7, 1992. Each different line type represents a separate loop around the eddy. The approximate center of the eddy, as determined by the drifter track, is marked with an octagon for each loop, (b) Time series of rotational speed derived from drifter 4859 in eddy C. Each of the five loops of eddy C is marked at bottom. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 137 of about 12 days during this time (Figure 6.9c), demonstrating the elliptical structure of eddy C. Clockwise energy, centered at a period of about 10 days, then increased as drifter 4859 looped over Ojin/Jingu Seamount and out of the region (after day 150). During the period that drifter 4859 was retained in eddy C, the eddy moved approx imately 150 km in about 60 days to the west-southwest (240T), at a speed of 2.9 cm/s (Figure 6.11a). This is comparable to the motion of the anticyclonic eddies (Richard son, 1980) observed in the lee of the Corner Rise Seamounts, which translated to the southwest, in the direction of the prevailing background flow, at 4-5 cm/s. The observed motion is also consistent with self-propulsion due to the (3 -effect, which predicts a west ward propagation of u — —(3Qr2, where f30 is the rotation parameter (2 x 10-11 m-1s-1 at mid-latitudes) and r is the eddy radius (cf, Mc Williams and Flierl, 1979). For an eddy radius of 41.5 km (taking the second loop of drifter 1417 to be the maximum east-west extent of eddy C), the predicted speed is 3.4 cm/s. Alternatively, eddy C may have simply been advected westward by a convergent flow between eddies A and C. The shape of eddy C, as delineated by the trajectory of drifter 4859, changed con siderably as the eddy approached and crossed the southern end of Ojin/Jingu Seamount and the unnamed seamount at 36.8°N, 171.3°E (Figs. 6.10 and 6.11a). The trajectory traced out a more circular path through loop 4, suggesting that the bulk of eddy C was squeezing through the gap between the seamounts. This observation is consistent with previous observations of meddy interactions with seamounts in the North Atlantic. In one case, a meddy (tracked with SOFAR floats) was apparently destroyed upon colliding head-on with Hyeres Seamount (Richardson et al., 1989), while in another case, a meddy squeezed through a gap between Hyeres and Irving Seamounts, altering its shape but maintaining dynamical stability as a coherent structure (Shapiro et al., 1992). Schultz Tokos et al. (1994), tracking a meddy with RAFOS floats, speculated that the Josephine Seamount acted as a wedge to "break off the outer pieces" of the meddy as it side-swiped Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 138 the seamount. Eddy C had apparently dissipated by the end of October, when drifter 4859 made the small anticyclonic loop in the location (37°N, 170°E) where eddy C would have been expected from its observed translation. It is reasonable to assume that eddy C decayed from the bottom up by friction as it crossed over the axis of the ESC. The fast speeds of drifter 4859 (57 cm/s) on its northward transect of loop 5 may be an indication of this decay, as a northward jet-like squirt of fluid leaving eddy C would be expected as it runs into the seamount (Nof, 1988). Drifter 8098 Although released simultaneously with drifters 1417 and 4859, the shallow-drogued drifter 8098 diverged from their meandering path towards the ESC within several days (Figure 6.12). After a slow drift to the southwest and retention in two cyclonic eddies near 33°N, 164°E, drifter 8098 approached Kinmei Seamount from the southwest in early October, just as drifter 4859 was leaving eddy C (Figure 6.10). It slowed as it crossed over the top of Kinmei Seamount, staying over the summit for nearly a week. It then made one loop in a large anticyclonic eddy situated on top of the unnamed seamounts at 36.3°N, 171.8°E and 36.8°N, 171.3°E (October 18 to November 10), then two loops in a smaller anticyclonic eddy just south of Ojin/Jingu Seamount (November 11-30) before continuing eastward in early December. The eddies were approximately 175 km and 90 km in diameter, and both had rotational speeds of 27-28 cm/s. It is interesting to note that the smaller eddy was the same size and in the same location as eddy C approximately six weeks after eddy C had apparently dissipated. Thus, it appears that a cyclonic eddy (eddy C) existed in the vicinity of Ojin/Jingu Seamount for at least two months and, upon dissipation, was replaced by an eddy of similar dimensions but opposite rotation. It is unclear, however, whether the anticyclonic eddy revealed by drifter 8098 had a signature that extended below the mixed layer, where eddy C was observed, or when or where it Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 139 may have originated. The extensive eddy activity revealed by the trajectory of drifter 8098 can also be seen in its / — t diagrams (Figs. 6.9e,f). Strong counterclockwise (and weaker clockwise) energy centered first near a period of 8-10 days (near day 65) and then at less than 4 days (near day 75) reflect the cyclonic eddies encountered near 33°N, 164°E. This eddy activity shows up as small peaks in the counterclockwise energy density spectrum (Figure 6.7c). The two anticyclonic eddies located in the gap between Ojin/Jingu and Kinmei Seamounts can be seen as clockwise energy peaks with periods of 25-30 days (between days 90-120) and 10-13 days (between days 120-150). Drifter 8098 then made a cyclonic meander out of the region and towards Hess Rise, producing another counterclockwise energy peak centered near a period of 12 days. In general, drifter 8098, drogued at 15 m, revealed more low-frequency energy in both rotary components than did the deep-drogued drifters. 6.4.3 1993 drifter Another shallow-drogued drifter (4856) was deployed in August 1992 within the Kuroshio Extension, at 40°N, 153°E. It subsequently drifted eastward at speeds of 25-30 cm/s, ap proaching the region of the ESC along 39°N in late February 1993 (Figure 6.13). At about 100 km from the Main Gap, drifter 4856 slowed down and was deflected first southeast ward towards Ojin/Jingu Seamount then northward towards Nintoku Seamount. It made a slow anticyclonic deflection within the Main Gap and over the southern tip of Nintoku Seamount, staying over the ESC (170°E to 171°E) for 9 days. It then accelerated east ward, out of the region, in late March 1993. As with the shallow-drogued drifters in the summer of 1991, this drifter was decelerated and deflected anticyclonically over the southern tip of Nintoku Seamount, but showed no evidence of eddy activity in the wake of the ESC. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 140 166°E 168°E 170°E 172°E 174°E Figure 6.12: Trajectory of drifter 8098 in the vicinity of Ojin/Jingu and Kinmei Seamounts in the summer and fall of 1992. Each mark represents the daily drifter position at 1200Z, and selected dates are labeled. Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 141 Figure 6.13: Trajectory of shallow-drogued drifter 4856, which crossed the Emperor Seamount Chain in the winter of 1993. Each mark represents the daily drifter position at 1200Z, and selected dates are labeled. Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 142 6.4.4 Satellite altimetry Unfortunately, no hydrographic stations were occupied or XBTs cast in the vicinity of the ESC in the summer of 1992. However, the TOPEX satellite altimeter was launched in August 1992, and was providing reliable sea surface height data by October. These data were then blended with data from the ERS-1 altimeter, which were corrected using crossover difference detrending (D. Chart, pers. comm.) The absolute sea surface topog raphy was estimated relative to the Joint Gravity Model-3 (JGM-3) geoid (e.g. Desai and Wahr, 1995). Standard geophysical data record corrections were applied (wet and dry tropospheric range delays, ionospheric delay, sea state bias), and tidal corrections were computed using the tide model of Desai and Wahr (1995). Accuracy is better than 5 cm pointwise (J. Hendricks, pers. comm.). Figure 6.14a shows the TOPEX/ERS-1 blended sea surface height anomalies for the North Pacific basin (20°N-55°N, 150°E-130°W) for TOPEX cycle 2 (October 3-12, 1992). These are anomalies from the two-year 1993-94 mean sea surface height field, derived from TOPEX, and interpolated to a 1/4° resolution global grid. There was very little eddy energy anywhere in the basin except in the vicin ity of the Kuroshio Extension (150°E-170°E near 35°N) and in the region just east of the ESC. No evidence of the Kuroshio Extension or seamount wake effects can be discerned east of the dateline. A more extensive analysis of TOPEX data for the North Pacific has concluded that the ESC is a region of high eddy energy (Thurston, 1995). This year-long analysis (February 1993 - March 1994) found that the majority of mesoscale variability was contained in the region between the southern portion of the ESC and Hess Rise, where four anticyclonic eddies were identified and tracked for 80-120 days. The 10-day tracks (October 3-12) of the 1992 drifters are also plotted on the sea surface height anomaly field (Figure 6.14b). There was a pronounced "depression" of about 20 cm in the same location where drifter 4859 had been in eddy C (loop 5; see Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 143 Figure 6.10), and a "hill" of about 50 cm in the location where drifter 1417 had been in eddy A (southwest portion of loop; see Figure 6.8). It is clear that the eddies delineated by the drifters at 120 m had a surface manifestation as well, which may have been maintained after the decay of the eddies at depth. Both features were still discernable in the sea surface height anomaly field in early November, although the anomalies were considerably weaker. Drifter 8098, in fact, revealed an anticyclonic eddy at 15 m near the prior location of eddy C by mid-November. TOPEX cycles 17 (February 28 - March 10, 1993) and 18 (March 10-20, 1993) revealed only weak sea surface height anomalies near the ESC between 39°N-41°N (not shown), corroborating the lack of eddy activity encountered by drifter 4856. 6.5 Discussion 6.5.1 Comparison with models Each of the drifters deployed in 1992 were retained in the vicinity of the ESC for nearly 5 months, within eddies that were apparently attached to the lee side of Ojin/Jingu Seamount. Why were leeside eddies observed in 1992 but not in 1991 or 1993, and do the drifter observations match the predictions of laboratory and numerical models of flow past isolated topography or seamount chains? Zhang and Boyer (1991) demonstrated that for two seamounts separated by at least one obstacle diameter, the flow impinging on each can be considered to be interacting with isolated topography. This is approximately the case for the upper-level flow approaching Nintoku and Ojin/Jingu Seamounts (see Figure 6.1 inset). Abyssal currents incident upon the ESC, however, will be interacting with a seamount chain. Although nonlinear interactions resulting from flow around neighboring seamounts in the upper levels are likely important, it will be assumed that Nintoku and Ojin/Jingu are isolated seamounts 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 Figure 6.14: (a) Map of sea surface height anomalies (cm) from merged TOPEX/ERS-1 altimetry for TOPEX cycle 2 (October 3-12, 1992) for the North Pacific basin from 20°JV-55°iV and 150°E-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 ESC from TOPEX 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. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 145 for the sake of easy comparison with prior theoretical and modeling work. Laboratory and numerical models have determined that the primary mechanisms which determine the flow pattern past isolated topography are the strength and time dependence of the background flow, the stratification of the water column, the obstacle geometry and bottom friction. For example, the laboratory experiments of Boyer and Zhang (1990b) found three distinct flow regimes, (i) fully attached flow, (ii) attached leeside eddies, and (iii) eddy shedding, which depend on these mechanisms. The relative importance of each mechanism can be quantified with the non-dimensional parameters the Rossby number (R0), which is the ratio of the advective to the Coriolis terms in the equation of motion, the horizontal Ekman number (Ek), which is the ratio of the frictional to the Coriolis terms, and the Burger number (S), which is the ratio of a measurement of stratification to the Coriolis term: R °~ fD' E« = T£F. (6-5) NH fD' where U0 is the mean background flow, / = 2Q,sin<b is the Coriolis parameter, where Q, is the Earth's rotation rate and (b is latitude, D is the seamount length scale (diameter), v is the viscosity, TV is the Brunt-Vaisala frequency, and H is the mean water depth. In particular, it was found that the resulting flow regime was most sensitive to variation of the Rossby number (Boyer and Zhang, 1990a,b). For subinertial flow with Rossby numbers on the order 0.01-0.1, the laboratory models predict attached leeside eddies, with an anticyclonic eddy on the left and a cyclonic eddy on the right, facing downstream (e.g., Boyer and Zhang, 1990b, their Figure 10C). The numerical models of Kozlov (1981), Johnson (1982) and Verron and Le Provost Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 146 (1985) demonstrated that another relevant quantity for predicting the steady-state quasi-geostrophic flow pattern over seamounts is the blocking parameter, HU0 R0 where h is the obstacle height and a = h/H is the seamount fractional height. Both Nintoku and Ojin/Jingu Seamounts rise to about 1000 m from a water depth of approx imately 6000 m, thus a ~ 0.83. Kozlov (1981) obtained a trapped cyclonic eddy to the right of the obstacle (looking downstream) for B > 0(1), while no eddy was formed for B < 1. Verron and Le Provost (1985) obtained attached leeside eddies at a "moderate" blocking parameter of 15.6 (their Figure 5). To estimate the relevant nondimensional parameters for the observed flow, a seg ment of each drifter's trajectory which represented eastward flow incident upon the ESC was selected. The dates chosen to represent each drifter's mean background flow are given in Table 6.3, along with the computed mean speeds and values of the Coriolis parameter, the Rossby and horizontal Ekman numbers, and the blocking parameter. The seamount length scales were taken to be 50 km and 100 km for the Nintoku and Ojin/Jingu Seamounts, respectively. The viscosity was taken to be the horizontal eddy diffusivity, KH- Although KH can vary by several orders of magnitude in the ocean (Pond and Pickard, 1983), a value of 4 x 10r cm2/s has been used, which is consistent with the estimates presented in Chapter 5. The 15 m flow impinging on Nintoku Seamount had Rossby numbers ranging from 0.03 to 0.11 in the summer of 1991 and 0.06 in the winter of 1993 (Table 6.3). The Rossby number for the 120 m flow at Ojin/Jingu Seamount in the summer of 1992 was lower, 0.02. In all cases, the flow incident upon the ESC was weakly nonlinear. Thus, the observations of attached eddies in the lee of Ojin/Jingu Seamount in 1992 match the predictions of Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 147 Drifter Dates U0 (cm/s) f (10-4 s-1) D (km) R0 Ek B ta (days) 1991 1314 6/19-6/30 32.8 0.954 50 0.07 0.017 12.12 1.76 1315 7/27-8/3 50.9 0.964 50 0.11 0.017 7.89 1.14. 1316 5/31-6/21 14.8 0.973 50 0.03 0.016 27.39 3.91 1992 1417 7/9-7/13 22.0 0.895 100 0.02 0.004 33.90 5.26 4859 7/9-7/13 22.0 0.895 100 0.02 0.004 33.90 5.26 1993 4856 2/22-3/5 28.6 0.919 50 0.06 0.017 13.33 2.02 Table 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 10r cm2/s and a seamount fractional height of 0.83. laboratory and numerical models, which have attached leeside eddies for Rossby numbers between 0.01-0.1. Higher Rossby numbers generally corresponded to the eddy-shedding regime of the laboratory models. No eddies can be seen in the trajectories of drifters 1314, 1315 or 4856 downstream or in the vicinity of the ESC, however, suggesting that eddies having a near-surface manifestation were not being generated at the times of drifter passage in the summer of 1991 or late winter of 1993. This could be due to the different seamount geometry of the Nintoku and Ojin/Jingu Seamounts, or simply that topographically-generated eddy activity in this region may have been present only at greater depths, and not extending into the mixed layer. The mixed-layer depth in the North Pacific rarely exceeds 100 m, and in summer is much shallower (Pickard and Emery, 1990). It is expected that in a stratified ocean, seamount effects will be bottom-intensified (Roden, 1991). Addressing this question would require knowledge of the stratification of the water column above the seamount summits. The lack of hydrographic observations concurrent Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 148 with the drifter observations, however, precludes any calculations of the Burger number for the observed flows, or of the vertical trapping scale, hf = ^f, (6-7) which is a measure of the vertical extent above the bottom to which seamount effects can be felt (Freeland, 1994). However, an estimate of these parameters can be made using the values of Brunt-Vaisala frequency given in Emery et al. (1984), which were derived from historical hydrographic data in 5° squares for the North Atlantic and North Pacific. From these data, the mean depth-averaged (upper 1000 m) value of N in the vicinity of the ESC is approximately 0.005 s-1 in summer and 0.003 s_1 in winter, yielding a Burger number of 0(1) for the flow incident on the ESC and suggesting that stratification is an important effect here. Comparing the crossing latitudes of the 1991 and 1992 drifters, seamount effects would be expected to reach the 15 m level over Nintoku Seamount (summit depth approximately 1000 m, diameter 50 km) if N < 0.005 s-1 , while above Ojin/Jingu Seamount (summit depth approximately 800 m, diameter 100 km) seamount effects would be felt at 15 m if N < 0.011 s~\ and at 120 m if N < 0.013 a'1. Thus, although drifter 8098 demonstrated that eddy activity at the ESC can be present near the surface, it would not be entirely surprising if the 15 m flow was often decoupled from the topographic effects, at least over Nintoku Seamount. Drifter 8098 crossed over Kinmei Seamount, whose summit approaches 100 m of the ocean surface. The implication is that eddy formation within the mixed layer near the ESC may be confined to the region around the Ojin/Jingu and Kinmei Seamounts. Drifters 1314 and 1315 (in 1991) and drifter 4856 (in 1993), all of which crossed the summit of Nintoku Seamount relatively quickly, had blocking parameters on the order of 8-13 (Table 6.3). Drifter 1316, which took over a month to cross Nintoku Seamount, had Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 149 a blocking parameter of 27, while the two drifters retained in the leeside eddies (1417 and 4859) had a blocking parameter of 34. The drifter observations are again consistent with the models, although particle retention in the wake of the ESC appears to be limited to higher blocking parameters than suggested by Verron and Le Provost (1985). This discrepancy is probably due to stratification effects, which were ignored in their model. The low values of the horizontal Ekman number (0.017 for the 1991 and 1993 back ground flows and 0.004 in 1992) suggest that frictional effects in the upper layers are not significant. This is expected to be the case, considering the long lives of the observed eddies. The magnitude of the horizontal eddy diffusivity, however, was prescribed from the results of an earlier study in the northeast Pacific Ocean. Its true value in the vicinity of the ESC is unknown. 6.5.2 Eddy dimensions Comparison between observations and theory can also be made with regard to the tem poral and spatial scales of the attached eddies. The dominant time scale for mesoscale flow over topography is the advective time scale, «.=•£ (6.8) which is also given in Table 6.3 for the observed flows. Again, the longer advective time scale occurs for the drifters which were retained for longer than a month in the vicinity of the ESC. A recent numerical model of the ocean response over an isolated seamount induced by the combination of slow steady inflow and weak diurnal tides (Goldner and Chapman, 1997) has shown that particles located at mid-depth and shallower are retained for up to 5 advective time scales. Drifters 1417 and 4859 demonstrate that particles below the mixed layer near the ESC can be retained for at least up to 12 advective time scales Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 150 (62 days for drifter 4859). The evolution in time of the wake flow behind the seamounts, however, is related to the varying incident flow as well as nonlinear vorticity interaction (Verron and Le Provost, 1985), neither of which can be examined with the present data set. The dimensions of the attached leeside eddies observed in 1992 are on the same order as the seamount diameter (about 100 km). This is comparable to the topographically-generated eddies observed by Richardson (1980), Pattiaratchi et al. (1987), and Hey wood et al. (1990,1996). The eddies revealed by the drifters downstream of the Corner Rise Seamounts (which have a diameter of roughly 150 km and a fractional height of 0.8, similar to Ojin/Jingu Seamount) had diameters of approximately 200 km, slightly larger than the obstacle diameter (Richardson, 1980). The observed and modeled eddies shed from the tropical island of Aldabra are also roughly the same size as the island (Heywood et al., 1990,1996). These observations differ from the predictions of the laboratory model of Boyer and Zhang (1990b), which have the "leeside bubble region" (the length scale of the attached eddies) on the order of one-third the seamount diameter for a Rossby number on the order of 0.02. It should be kept in mind that the parameters used for the comparison between the observations and the model predictions (Table 6.3) were estimated from the drifter trajectories at their approach to the ESC. The 1992 drifters remained in the region for nearly six months, however, during which time the background flow incident upon the ESC was unmeasured. Thus, caution should be applied in making conclusions based on these comparisons. Chapter 6. Eddies I: Seamount-attached Eddies at the Emperor Seamount Chain 151 6.6 Conclusions Satellite-tracked drifters were used to investigate eddy activity in the vicinity of the Emperor Seamount Chain during 1991-1993. A pair of counter-rotating mesoscale ed dies were observed attached to the lee side of Ojin/Jingu Seamount in the summer of 1992. This is one of the first observations demonstrating an extended attachment (>60 days) of a topographically-generated eddy to a seamount. The flow incident on the seamount had a low Rossby number (0.02) and a high blocking parameter (34). Results of laboratory and numerical modeling predict counter-rotating, attached leeside eddies for flow over isolated topography with parameters enveloping those calculated for the 1992 flow. Thus, although the drifter observations are few and temporally limited, they match the predictions and suggest that the existing models are addressing the dominant dynamical constraints quite well. The lack of eddies observed in 1991 and 1993, even though the incident Rossby numbers were within the range of the attached leeside eddy or eddy-shedding regimes of the models, was most likely due to a partial decoupling, due to stratification, of the topographic influence of Nintoku Seamount (summit depth near 1000 m) on the 15 m flow. Mixed-layer eddy activity at the ESC was present over the taller Ojin/Jingu and Kinmei Seamounts. The flow depicted by the satellite-tracked drifters in the summer of 1992 - a meander ing current approaching the ESC at speeds of 20-25 cm/s - is consistent with historical observations from the region. Thus, it can be expected that eddies with sizes on the order 100 km, extending from the surface to well below the mixed layer, having lifetimes of weeks to months, and attached to the lee side of the seamounts may be a common phenomenon. The ESC, at least at its southern end, may indeed be an "eddy generator" (Roden, 1991), with important resulting effects on biological production and plankton re tention. Further, a significant fraction of the deep water exchanged between the western Chapter 6. Eddies L Seamount-attached Eddies at the Emperor Seamount Chain 152 and eastern basins of the North Pacific is through the Main Gap between the Ojin/Jingu and Nintoku Seamounts. Since this exchange may be affected by the presence of attached leeside eddies, with profound implications for property redistribution into the subarctic North Pacific, it is highly recommended that further hydrographic, current meter, altime-try and drifter studies be performed in the vicinity of the ESC to quantify the production frequency and dynamic characteristics of these eddies. Chapter 7 Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 7.1 Introduction The analysis of mesoscale circulation features is continued in this chapter, with the focus now shifted to two groups of drifters deployed in the far western North Pacific, near the Kuril-Kamchatka Trench (~ 45°N, 150°E), in fall 1990 and late summer 1993. In both years, the drifter trajectories revealed large anticyclonic eddies positioned over the deepest part of the trench (e.g., Figure 4.5). Some of the general characteristics of these eddies are presented below, along with a discussion of their potential influence on the spreading of North Pacific Intermediate Water. 7.2 Oceanographic Setting The Kuril-Kamchatka Trench (KKT), which lies just east of the Kamchatka Peninsula and the Kuril Island chain, is over 10,000 m at its deepest point and deeper than 6000 m over a 2200 km length (Figure 7.1). As with the other deep ocean trenches (e.g., the Aleutian Trench, just seaward of the axis of the Alaskan Stream, or the Mariana Trench), the KKT was formed at the zone of subduction of an oceanic plate under a continental lithospheric plate (Thurman, 1994). To the northwest of the Kuril Islands is the Okhotsk Sea, a semi-enclosed basin which forms the westernmost part of the Pacific subpolar gyre. (The Alaskan Gyre, Bering Sea Gyre, and Western Subarctic Gyre are the other components of the Pacific subpolar gyre.) 153 Figure 7.1: (top) Map of the North Pacific Ocean and surrounding area, showing the location of the Kuril-Kamchatka Trench (KKT). (bottom) Chart of the KKT region. From Kono (1996). Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 155 The Okhotsk Sea is generally ice-covered throughout winter, but ice-free in the summer months. There is a dominant cyclonic circulation within the basin, although significant eddy activity is also prevalent (Talley, 1996). Several passes connect the Okhotsk Sea to the North Pacific, the deepest being Kruzenshtern Strait in the north (sill depth of 1400 m) and Bussol' Strait in the center (sill depth of 2300 m). The net exchange between the Okhotsk Sea and the North Pacific has been estimated at 3-5 Sv, although adequate data for such estimations are scarce (Talley, 1996). Tidal currents are very strong (up to 250 cm/s) through the passes (Talley and Nagata, 1995). The East Kamchatka Current (EKC), which originates in the Bering Sea, flows south-westward along the axis of the KKT as the western boundary current of the Western Subarctic Gyre. Mean volume transports in the EKC are on the order of 10-20 Sv, and maximum speeds reach 80-100 cm/s (Talley and Nagata, 1995; Stabeno et al., 1994). A portion of the EKC is transported into the Okhotsk Sea, primarily through Kruzen shtern Strait. Another portion of the EKC remains east of the Kuril Islands and mixes with Okhotsk Sea outflow, primarily from the Bussol' Strait, to eventually form the Oy-ashio Current. The Oyashio continues southward towards Hokkaido, then separates from the western boundary and flows eastward into the open North Pacific, just north of the Kuroshio Extension. Satellite, hydrographic and drifter data have confirmed the prevalence of large eddies in the area east of the Kuril Islands, which modify the transport and properties of the EKC and Oyashio (e.g., Bulatov and Lobanov, 1983; Rogachev et al., 1993; Stabeno et al., 1994). At the southern end of the islands, the eddies have warm, saline cores, and most likely originate as Kuroshio warm-core rings (Lobanov et al., 1991; Lobanov and Bulatov, 1993). The eddies further north have cold, fresh cores, and may be warm-core rings which have migrated north and entrained sufficient amounts of EKC and Okhotsk Sea water to transform their core properties (Talley and Nagata, 1995). Lobanov et Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 156 al. (1991) and Lobanov and Bulatov (1993) have used hydrographic data and satellite AVHRR images to track eddies from their origin near the Kuroshio to the KKT region, demonstrating eddy lifetimes of up to several years (Figure 7.2). Their northeastward translation velocity is on the order of 2-6 cm/s. The region has attracted renewed attention in the oceanographic community in recent years (e.g., Talley and Nagata, 1995). It has been proposed that the Okhotsk Sea is the primary source region for North Pacific Intermediate Water (NPIW), a low-salinity, high-oxygen water mass found at mid-depths (corresponding to a$ = 26.7-26.9) throughout the subtropical North Pacific (Talley, 1991; Talley, 1993; Talley et al, 1995). The low salinity Okhotsk Sea water (identified as having a low potential vorticity, Q = (£)(§f)) which outflows through Bussol' Strait mixes with warmer, more saline EKC water to form the Oyashio, a portion of which subsequently mixes with water in the Kuroshio Extension to form new NPIW (Talley, 1993; TaUey et al., 1995; Yasuda, 1997). The ultimate source for NPIW is apparently newly ventilated Okhotsk Sea water. Although the studies reviewed here acknowledged the ubiquity of large eddies near the outflow region, the role these eddies play in the formation, modification or spreading of Oyashio and NPIW water masses was not addressed. Although the region contains rich fisheries resources which have been exploited by Japan and Russia, and an accompanying history of hydrographic sampling, much of the data are old and/or inaccessible. There have been relatively few descriptions of the physical oceanography of the region reported in the western literature. The North Pacific Marine Science Organization (PICES) has established a working group to promote data exchange between groups and nations and to establish priorities for future oceanographic research in the area (Talley and Nagata, 1995). Their recommendations focused on (a) improved understanding of water mass modifications and the formation of intermediate water within the Okhotsk Sea, (b) quantification of rates and processes of exchange Chapter 7. Eddies IE Anticyclonic Eddies at the Kuril-Kamchatka Trench 157 Figure 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 KKT region. From Lobanov and Bulatov (1993). Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 158 between the Okhostsk Sea and the North Pacific, including the role of eddies in the formation and spreading of NPIW, and (c) improved forecasting of sea ice cover in the Okhotsk Sea. The results presented below are a contribution to the efforts to better understand the characteristics and dynamics of persistent anticyclonic eddies at the KKT. 7.3 The Data In November 1990, three shallow-drogued drifters were deployed at the center and edges of an anticyclonic warm-core ring (here designated Eddy Al), which can be indentified in earlier hydrographic, XBT and remotely-sensed data sets (Rogachev et al., 1993; Kono, 1996). The drifter deployed at the eddy center (1315) executed five complete loops over a period of ~ 45 days, each loop at a successively greater distance from the eddy center, before exiting the eddy and heading first towards the Kuril Islands then seaward into the open North Pacific (Figure 7.3). Strong inertial oscillations were superimposed on the clockwise rotation over the first two loops (~ 11 days). Drifters 1314 and 1316 (not shown) were deployed at the western and eastern edges of Eddy Al, respectively, and promptly exited the eddy and meandered southward and eastward in the Oyashio. Both of these drifters also exhibited strong inertial oscillations, along the perimeter of Eddy Al, at the same time drifter 1315 made its initial two loops near the center. In September 1993, three more shallow-drogued drifters were deployed in the vicinity of the KKT. Drifters 15372 and 15374 (not shown) failed prematurely, providing only 17 and 70 days of data, respectively. The third drifter, 15371, was retained in an anticylonic eddy (A2), making two complete circuits and part of a third over approximately 40 days (Figure 7.3). Upon leaving Eddy A2, drifter 15371 went onto the Kuril Island shelf, through Friza Strait and into the Okhotsk Sea, and ultimately back through Bussol' Chapter 7. Eddies lh Anticyclonic Eddies at the Kuril-Kamchatka Trench 159 150°E 152°E 154°E 156°E Figure 7.3: Map of the region showing the trajectories of drifter 1315 in eddy Al and drifter 15371 in eddy A2. Hexagons mark the deployment locations. The bathymetry contours in this and subsequent maps are in meters. Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 160 Strait and into the open North Pacific, where it spent another 2.5 years making a trans-Pacific crossing (Thomson et al., 1997). The trajectory of this drifter in the vicinity of the Kuril Islands has been extensively analyzed in Thomson et al. (1997) and Rabinovich and Thomson (1998), who found evidence of diurnal shelf waves and very strong tidal currents in the passes. The analysis here is restricted to the 40-day period within Eddy A2. 7.4 The 1990 Eddy As drifter 1315 spiraled out of Eddy Al over a 43 day period, the eddy's center (as estimated from the mean drifter position within each loop) translated northward ap proximately 50 km, for a net translation velocity of ~ 1.5 cm/s (Figure 7.4). This is comparable to the translation velocities estimated from satellite imagery by Lobanov et al. (1991), although uncertainties in determining the eddy center would put large error bars on this estimate. Each of the five successive loops was larger than the previous, the first being 27 km and the fifth 115 km from the eddy center (Table 7.1). Time series of drifter longitude and latitude clearly demonstrate the increasing radial distance, partic ularly in the meridional direction (Figure 7.5). Assuming the fifth loop traced its outer edges, this would yield a horizontal dimension of approximately 230 km for Eddy Al at 15 m depth, considerably larger than the eddies observed at the Emperor Seamount Chain (previous chapter). Mean 3-hourly speeds decreased from 88 cm/s in loop 1 to < 50 cm/s in loops 3-5, while the mean rotational speeds (^p, where r is eddy radius and T is rotational period) increased outward from the eddy center from 30 cm/s at ~ 30 km to 47 cm/s at ~ 115 km. High-amplitude inertial oscillations are clearly evident in drifter 1315's trajectory over its first two loops, near the eddy center (Figure 7.6). Maximum 3-hourly speeds Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 161 Figure 7.4: Trajectory of drifter 1315 within Eddy Al 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.) Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 162 Figure 7.5: Time series of (a) longitude and (b) latitude from drifter 1315 in Eddy Al during November-December, 1990. The periods of each loop are marked at bottom. Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 163 Loop Period (days) Radius (km) Srot (cm/s) Smean (cm/s) Smax (cm/s) Drifter 1315 1 5.0 27.3 29.6 88.3 128.6 2 5.5 39.2 38.7 62.4 123.3 3 9.0 59.9 36.1 44.8 77.3 4 9.0 78.4 47.3 45.7 79.5 5 13.3 115.5 47.1 47.2 87.5 Drifter 15371 1 10.3 81.1 43.0 38.4 53.1 2 12.8 100.2 42.7 37.1 61.5 Table 7.1: Period, radius, mean rotational speed, mean 3-hourly speed, and maximum 3-hourly speed of the KKT anticyclonic eddies as delineated by the trajectories of drifters 1315 (Eddy Al) and 15371 (Eddy A2). during this period were greater than 120 cm/s, with most of the high amplitude inertial motions occuring within the first loop, just after deployment (Figures 7.6 and 7.7, Table 7.1). Without these inertial motions, the mean 3-hourly speeds of drifter 1315 would have remained nearly constant regardless of distance from the eddy center, even though the mean rotational speeds increased outward. Kawai (1992) pointed out that almost all previous observations have shown warm-core anticyclonic rings and eddies to be in solid-body rotation, with only one observation showing a warm-core ring with a static core. The rotary spectra for drifter 1315 and the two drifters deployed at the edges of Eddy Al (calculated for a 90-day period and thus including trajectory segments outside of the eddy) all show a strong near-inertial peak, significant low-frequency clockwise peaks (indicative of Eddy Al's rotation), and generally noisy spectra at periods less than one day (Figure 7.8). The largest spectral peak occurs for clockwise motions of drifter 1315 at a period of ~ 10 days. A frequency partitioning of the rotary variances reveals that Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 164 Figure 7.6: Close-up of the trajectory of drifter 1315 within Eddy Al over (a) loop 1 and (b) loop 2. A hexagon marks the loop's start position, 'x's denote 3-hourly positions, and stars denote positions where 3-hourly drifter speeds were greater than 100 cm/s. Chapter 7. Eddies IT. Anticyclonic Eddies at the Kuril-Kamchatka Trench 165 Figure 7.7: Time series of zonal (U) and meridional (V) 3-hourly speeds from drifter 1315 in Eddy Al over loops 1 and 2, November 8-19, 1990. Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 166 Drifter s~H Stot(u) 1315 low mesoscale high mesoscale near-inertial high 1050.8 (71.4) 22.3 (1.5) 182.7 (12.4) 38.1 (2.6) 157.9 (10.7) 6.1 (0.4) 2.9 (0.2) 11.8 (0.8) 1208.7 (82.1) 28.4 (1.9) 185.6 (12.6) 49.9 (3.4) 15371 low mesoscale high mesoscale near-inertial high 455.9 (65.1) 104.1 (14.9) 33.3 (4.8) 25.4 (3.6) 38.3 (5.5) 20.5 (2.9) 3.0 (0.4) 19.6 (2.8) 494.2 (70.6) 124.6 (17.8) 36.4 (5.2) 45.1 (6.4) Table 7.2: Clockwise (S~(u)), counterclockwise (5+(u;)) and total (5t<>t(<*>)) rotary vari ance (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). most of the energy (71%) in drifter 1315's trajectory was contained in low frequency clockwise motions (periods greater than 2 days, i.e., the eddy rotation), and that over the three month period, clockwise motions at all frequencies accounted for nearly 90% of the total energy (Table 7.2). Inertial motions in drifter 1315's trajectory comprised 12% of this total. One of the more interesting aspects of drifter 1315's trajectory is the relationship between the presumably wind-forced inertial oscillations and the rotation of Eddy Al. Kunze (1985) derived a dispersion relation for near-inertial internal waves propagating in geostrophic shear which took into account interaction terms involving both advection by the mean flow and straining by mean flow shear, terms which are ordinarily neglected. It was found that advection leads to a Doppler shift, and that straining by the mean Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 167 Frequency (cpd) Figure 7.8: Clockwise S~ (solid line) and counterclockwise S+ (dashed Une) rotary energy density spectra (m2/s2/cpd) derived from the trajectories of drifters (a) 1314, (b) 1315, and (c) 1316 over the period November 8, 1990 to February 6, 1991, and (d) 15371 over the period September 4 to December 3, 1993. The 95% confidence Umits are shown in (a), and the inertial (/) and semidiurnal (M2) peaks are shown in (b) and (d). Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 168 flow vorticity, £, shifts the lower bound of the inertial waveband from the local planetary vorticity (Coriolis frequency, /) to an effective Coriolis frequency, In other words, an internal wave experiences both the earth's and the fluid's rotation. With the broadening of the inertial waveband, more inertial energy is available for the generation of turbulence and mixing in the upper water column (Kunze, 1985; Rogachev and Carmack, 1998). In fact, trapping and amplification of inertial energy can occur in regions of negative vorticity (e.g., anticyclonic eddies), where the intrinsic inertial fre quency is less than the effective Coriolis frequency (Kunze, 1985). As near-inertial waves are intermittent, so too is the enhanced wave activity. Intense downward-propagating packets of near-inertial energy have been observed at the negative vorticity sides of fronts (Kunze and Sanford, 1984a), anticyclonic eddies (Kunze and Sanford, 1986; Kunze and Lueck, 1986), and warm-core rings (Kunze and Sanford, 1984b). Clear evidence of the broadening of the inertial waveband is found in the f — t diagram for the clockwise rotary velocity component of drifter 1315 in Eddy Al (Figure 7.9). Over the first few days after deployment, there is a significant clockwise spectral amplitude peak centered at ~ 1.28 cpd, approximately 0.13 cpd lower than the local planetary vorticity (Figure 7.9a). These high-amplitude inertial motions had dissipated by November 20, the start of loop 3. Inertial motions with peak amplitudes centered on / were observed again in late November (the end of loop 3 and beginning of loop 4; see Figure 7.4) and mid-December (loop 5). Except for a weak diurnal peak in loop 1, no other high frequency motions are evident from drifter 1315's trajectory in Eddy Al. At low frequencies (Figure 7.9b), the clockwise spectral amplitude peaks were centered at the rotation rates of Eddy Al, first at 0.2 - 0.24 cpd (4-5 days; loops 1 and 2) and then Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 169 near 0.1 cpd (10 days; loops 3, 4 and 5). Knowing the local planetary vorticity, and taking the clockwise velocity amplitude peak during loop 1 (Figure 7.9a) as the effective Coriolis frequency, the relative vorticity for Eddy Al at a radius of ~ 30 km is found to be £ = -1.9 x 10~5 s"1. This is approximately —0.2/, a smaller magnitude than the — |/ < ( < — |/ predicted from a l| layer model of an anticyclonic ring in solid-body rotation presented in Kawai (1992). This discrepancy with the model prediction is consistent with comparisons made from the estimated vorticities of other warm-core rings, which were generally less than 50% the magnitude of / (Joyce and Kennelly, 1985; Olson et al., 1985). It is also not surprising because Eddy Al does not appear to be in solid-body rotation. It is interesting to note that Rogachev and Carmack (1998) have suggested that the broadening of the inertial waveband by the eddy's rotation resulted in resonance with the diurnal tide, and subsequently an increased energy input to Eddy Al through wave trapping and amplification at the base of the eddy's core. This supply of energy, they argue, would counter frictional decay and increase the life of the eddy. Thus, diurnal tidal forcing of near-inertial waves within Eddy Al (or presumably any eddy in the central KKT region) would provide a regular supply of energy (principally at spring tides) to maintain dynamical coherence. The theory of Rogachev and Carmack (1998) is evidently contradicted by the spectral analysis presented here. The effective inertial frequency in Eddy Al is not diurnal, but is only ~ 9% lower than the local planetary vorticity (Figure 7.9a). Furthermore, there were no perceptible motions of diurnal period observed at any time during drifter 1315's entrapment in Eddy Al. Although diurnal tides and diurnal period shelf waves in the Kuril Islands region are of generally high amplitude (see Rabinovich and Thomson (1998), and section 7.5 below), there is no evidence from the drifter data to suggest that these motions had any impact on the near-surface environment of Eddy Al, situated over the Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 170 Nov Dec Jan 10 20 30 10 20 30 10 ' i • "i mini Drifter 1315 Clockwise (S~) IIIIIIIIIIIIMIII niim I'l M '11 ' 1111111 n 1111 11 Drifter /. Clockwise 0.05 ' fin i'l i II 111 i^i 1111111111111111111II11111 M 111II111111111111 ii 111 Ii t 10 20 30 10 20 30 10 Nov Dec Jan Figure 7.9: Amplitude evolution of the clockwise rotary velocity component for drifter 1315 in Eddy Al at (a) high frequencies (periods of 11 hours to 1.25 days) and (b) low frequencies (periods of 2 to 20 days). Contours are in cm/s. The start date is November 8, 1990, and the dotted line in (a) refers to the local value of planetary vorticity (/) at the drifter position. Chapter 7. Eddies lh Anticyclonic Eddies at the Kuril-Kamchatka Trench 171 deepest part of the trench. Instead, the inertial oscillations observed here were most likely wind-induced, as suggested by the intermittent inertial peaks in Figure 7.9a. Although wind data from the immediate vicinity of the KKT are not presently available, it is known that three storms passed through the Okhotsk Sea between November 6-12, 1990 (Mariners' Weather Log, 1991). In particular, the storm of November 6-8, 1990, just prior to the drifter deployments, was centered right along the axis of the Kuril Island chain. Storms also affected the region at the times of the two later inertial peaks. 7.5 The 1993 Eddy Drifters were again deployed at the KKT in September 1993, and again revealed a large anticyclonic eddy positioned over the deepest part of the trench (Figure 7.3). The eddy revealed by drifter 15371 in September 1993 (A2) had characteristics similar to those of Eddy Al (Table 7.1). As with Al, the period of rotation at 80-100 km from the eddy center was 10-13 days and mean 3-hourly speeds were on the order of 40 cm/s. The evidence suggests an eddy in solid-body rotation, although no observations are available from nearer the eddy center. The two-and-a-half loops made by drifter 15371 were relatively smooth, with no evidence of significant high-frequency energy until early October, when the drifter was about to leave the eddy and approach the Kuril Island shelf (Figure 7.10). The f — t diagram reveals the relative lack of high-frequency energy in drifter 15371's trajectory within Eddy A2, with the only clockwise spectral amplitude peaks occurring near 0.1 cpd (the eddy's rotation rate) and at the local inertial frequency (but not a lower effective Coriolis frequency; Figure 7.11). The inertial peak, which occurred on the drifter's final half-loop of Eddy A2, was followed by strong peaks at the diurnal tidal frequencies from late October to mid-November (Figure 7.11a). These diurnal-period motions yielded the Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 172 relatively strong clockwise variances observed for drifter 15371 in the high mesoscale band (Table 7.2). Short periods of superinertial motions, including at the semidiurnal tidal frequency, also occurred at these times. Further discussion of these high-frequency shelf motions can be found in Thomson et al. (1997) and Rabinovich and Thomson (1998). 7.6 Discussion How often can eddies be found at the KKT, and what is their origin and life expectancy? The drifter data discussed here are insufficient for answering these questions. However, from a perusal of the Japanese and Russian literature, it is clear that large, long-lived, anticyclonic eddies are commonly observed in the KKT region. Kono (1996) analyzed hydrographic data collected in the KKT area in successive summers between 1989 and 1992, and found anticyclonic eddies present in each year. In the August-September 1990 survey, Eddy Al clearly shows up in maps of acceleration potential anomaly (referred to 1500 db) and potential temperature (Figure 7.12), positioned in precisely the same location where it was sampled by the drifters three months later. Kono (1996) suggests that the eddy observed just southeast of Bussol' Strait in 1991 was the same eddy as the one observed there in 1990 (i.e., Eddy Al), implying that Eddy Al had been nearly stationary during that year, perhaps locked to the topography. Water in Eddy Al at the erg — 26.8 isopycnal (~ 150-180 m) was about 1° warmer than surrounding waters, although this signal was not apparent below about 350 m. An XBT transect through Eddy Al taken at the time of the drifter deployments (early November 1990) revealed a warm-core extending to ~ 300 m (Rogachev and Carmack, 1998). In each year of Kono's (1996) observations, the Oyashio was observed to meander around the eddies towards the southwest. Numerous eddies were also observed in drifter trajectories in the region between 1988 and 1993 (Stabeno et al., 1994), and hydrographic observations conducted Chapter 7. Eddies IT. Anticyclonic Eddies at the Kuril-Kamchatka Trench 173 Figure 7.10: Time series of (a) longitude and (b) latitude from drifter 15371 in Eddy A2 during September-October, 1993. Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 174 10 20 30 10 20 30 10 20 30 Sep Oct Nov Dec Figure 7.11: Amplitude evolution of the clockwise rotary velocity component for drifter 15371 in Eddy A2 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 tidal frequency (M2), the local value of planetary vorticity at the drifter position (/), and the two dominant diurnal tidal constituents (Ki and Oi). Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 175 in summer 1994 revealed three anticyclonic eddies along the KKT, including one just southeast of Bussol' Strait (Rogachev and Verkhunov, 1996). Eddy Al was apparently still present southeast of Bussol' Strait nearly a year after the drifter observations were made. In fact, there is evidence to suggest that Eddy Al originated as Kuroshio warm-core ring 86B in late summer 1986, and was sampled by drifter 1315 more than four years into its life (Figure 7.2; Lobanov et al., 1991; Yasuda et al., 1992; Rogachev and Carmack, 1998). If this were the case, Eddy Al existed for at least five years. What are the mechanisms responsible for such a long life? Rogachev and Carmack (1998) have suggested that direct tidal forcing (at the diurnal frequency, and in resonance with the effective Coriolis frequency) of near-inertial waves provides a constant supply of energy to the eddy, and, because tidal near-inertial oscillations do not extract energy from the mean flow, this would serve to prolong the eddy's life. However, as pointed out in Section 7.4, there does not appear to be resonance of inertial waves with the diurnal tide, so this mechanism can be ruled out. They also suggest that inertial wave trapping and amplification along the eddy's boundary would lead to mixing between waters with large temperature differences, which in turn would lead to the conversion of potential energy to kinetic energy due to volume contraction upon mixing. Even if this mechanism did occur, it is not clear if the temperature gradients observed here would be sufficient to maintain this energy conversion for five years or more, and Rogachev and Carmack (1998) do not offer any time-scale estimates. As there have been no continuous long-term observations within any KKT eddy, or measurements of fine-scale mixing and turbulent dissipation, theories on the mechanisms responsible for their longevity remain largely untested. Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 176 Figure 7.12: (top) Acceleration potential anomaly (10 m2/s2) on the erg = 26.8 surface referred to 1500 db, derived from CTD casts taken from the R/V Hokko Maru in Au gust-September, 1990. (bottom) Potential temperature (°C) on the erg = 26.8 surface, from the same CTD survey as (top). From Kono (1996). Chapter 7. Eddies II: Anticyclonic Eddies at the Kuril-Kamchatka Trench 177 7.7 Conclusion The characteristics of two large anticyclonic eddies located over the Kuril-Kamchatka Trench have been described using position data obtained from satellite-tracked drifters in fall 1990 and late summer 1993. In 1990, a drifter deployed near the eddy's center was retained for over 40 days, during which it made five loops at successively greater distances from the eddy center. High-amplitude inertial oscillations, which coincided with a storm passage, were observed during the first two loops. The relative vorticity of the eddy resulted in a lowering of the inertial waveband, as predicted by Kunze (1985) and others. In 1993, a drifter was again retained in an eddy for ~ 40 days. This eddy had characteristics similar to the 1990 eddy, but was relatively devoid of significant high-frequency motions until the drifter's final half-loop. The size and prevalence of KKT eddies implies that they could have a large impact on the transport dynamics of the East Kamchatka and Oyashio Currents. Although the Oyashio has been observed to meander around the relatively stationary eddies (Kono, 1996), it is likely that mixing events are occurring along the perimeters of the eddies. Thus, it seems reasonable to suppose that KKT eddies could affect the characteristics and spreading of Okhotsk Sea water, and thus modify the density and volume of newly-produced NPIW. Although the drifter data allowed for a description of some of the general character istics of KKT eddies, there are many questions which remain unanswered, including: • What is the origin and lifetime of KKT eddies? What mechanisms could be respon sible for the longevity implied from the available observations? There are two main ideas on the nature of the KKT eddies: (1) They are Kuroshio warm-core rings which have migrated northward to the region, and (2) they are formed and remain in the vicinity Chapter 7. Eddies IL Anticyclonic Eddies at the Kuril-Kamchatka Trench 178 of the central Kuril Islands. Although there is evidence to support the former hypoth esis (cf., Lobanov et al., 1991), it remains unclear why the eddies are most commonly observed along the trench axis and, in particular, near the deepest strait of the Kuril Islands (Bussol' Strait). Rabinovich (pers. comm.) has suggested that trench waves may play a role in the origin and behavior of KKT eddies, and that the KKT and Bussol' Strait may act to trap the eddies along the trench axis ("a ping pong ball on a fountain jet"). Long-term observations, as well as microscale measurements which could quantify turbulent dissipation rates within the eddies, are needed to address these questions. • What role does externally-forced near-inertial energy play in the internal dynamics of KKT eddies? Are near-inertial waves ever tidally-induced? Wind data from the period of drifter 1315's sampling could help to address this issue, as well as the question of why the drifter spiraled out of the eddy. • What is the precise role of KKT eddies in the mixing and spreading of Okhotsk Sea water? Do they modify the characteristics and transport of eventual NPIW? A longer record of high-quality hydrographic data could provide these answers. Subsurface floats, which are not affected by wind slippage, have been used with great success to explore the origin, movement, dynamics, and life-stage characteristics of Med-dies (Bower et al., 1995). If seeded at an appropriate depth near the center of a Kuroshio warm-core ring, a float could remain there for the extent of the eddy's life. It is rec ommended that floats be utilized in conjunction with microscale measurements, satellite AVHRR and altimetry data, surface drifter deployments, and continued hydrographic sampling in order to better understand the dynamics and life history of KKT eddies, and their relative importance in local biological production and the formation and spreading of NPIW. Chapter 8 Summary and Synthesis 8.1 Summary This thesis has presented an analysis and interpretation of the position time series, and accompanying derived velocities, obtained from a large set of satellite-tracked drifters deployed and tracked throughout the North Pacific Ocean over the period 1990-1995. A majority of the drifters were drogued within the mixed layer (15 m depth), and comprised the North Pacific component of the WOCE-SVP, while the rest were drogued within the underlying pycnocline (120 m depth). The overall 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. A summary of the principal results of each analysis is given below: (a) SAMPLING STRATEGIES • As a first step, the effects of reduced sampling schedules (duty cycles) on drifter-derived velocity statistics were investigated by degrading continuous segments of drifter records to match the standard duty cycle used in the WOCE-SVP (48 hours of no ARGOS data transmission followed by 24 hours of received transmission) and two other duty cycles (32-16h and 16-8h). 179 Chapter 8. Summary and Synthesis 180 • It was found that strong high-frequency (primarily inertial and semidiurnal) mo tions prevalent in the drifter records could result in significantly biased statistics derived from the degraded series. In particular, reproduction of the velocity variances and ro tary spectral characteristics of the original time series was strongly dependent on the interpolation scheme applied to the duty cycle record. • The commonly used spline interpolation produced numerous spurious spectral peaks, with high-frequency energy aliased to multiples of the duty cycle frequency. Reproduction of the original (undegraded) prime and spectral velocity statistics required an interpo lation which took into account the oscillatory component of the drifter motions. The multi-functional fit (MFF), which fits a fourth-order polynomial to the observations plus an oscillation whose frequency is determined by the interpolation, was able to repro duce the rotary spectral features of the original data series for each of the three types of degraded series examined. • The MFF interpolation most consistently reproduced the spectral characteristics of the original continuous records for the 32-16h duty cycle. This is presumably because the 16h transmission-received segments are long enough to capture the local inertial period, while the 32h transmission-blackout segments are short enough to leave the main features of the motions relatively unchanged. The duty cycle with the shortest and most frequent gaps (16-8h) yielded the worst reproduction, with significantly overestimated superinertial energies. • Recommendations were made for users to take into account the dominant high-frequency signals in their drifter records, and to customize their interpolation schemes to account for these motions. It was also recommended that Service ARGOS provide a selection of duty cycle options which depend on the latitude of the drifter deployments (specifically, equatorial versus non-equatorial). Chapter 8. Summary and Synthesis 181 (b) EULERIAN STATISTICS • The trajectories of all drifters were used to characterize the upper-ocean (15 m and 120 m) mean circulation and eddy variability in the North Pacific Ocean over the period 1990-1995. Estimates of integral time and length scales were derived from the ensemble-mean autocorrelation functions. Ensemble-mean velocities and (mean and eddy) kinetic energies were derived in grid boxes which were chosen to satisfy a compromise between horizontal resolution and statistical reliability. • All branches of the Alaskan Gyre were well-sampled at both drogue depths. At 15 m depth, a weak Subarctic Current was observed, along with strong, variable flow in the Alaska Current and Alaskan Stream. The bifurcation of the Subarctic Current was observed near 48°N, 130°W. At 120 m depth, northward flow in the Alaska Current occurred much further offshore than within the mixed layer, revealing a relatively small gyre. A region devoid of drifter data (near 52°N, 155°W) was identified as the "center" of the Alaskan Gyre during the observation period. An estimate of the winter mean Ekman pumping velocity at this location was ~ 7 x 10~5 cm/s. • Mean transit times around the Alaskan Gyre were on the order of 1-2 years, although a few drifters revealed much longer retention times (3-4 years) in the near-surface waters of the Gulf of Alaska. • 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 compared to below, and 2-3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. The high eddy kinetic energies observed along the perimeter of the Alaskan Gyre may be due to the offshore intrusion of eddies formed by coastal current instabilities. • There was evidence of interannual variability in the drifter trajectories, with an apparently intensified Alaskan Gyre during winter 1993-94 and more southerly transport Chapter 8. Summary and Synthesis 182 the following year. (c) EDDY STATISTICS • Taylor's theory of single particle dispersion was applied to the drifter ensembles in order to derive the magnitude of the eddy mixing scales and diffusivities over a broad region of the North Pacific Ocean and at both drogue depths. • Both the initial dispersion and random walk regimes predicted by Taylor's the ory were identified in the dispersion time series computed for several regions of both ensembles. • The integral time scales and eddy diffusivities computed from the dispersion scale linearly with r.m.s. velocity. An exception is the meridional time scale, which appears to have a weaker dependence on fr.m.«.-• The magnitudes of the derived eddy statistics are comparable to those derived from surface drifters in other parts of the world ocean. The consistency of the results with previous studies lends credence to the idea that the simplifying assumptions of Taylor (1921) are reasonably valid throughout the upper ocean, which bodes well for the effective parameterization of near-surface diffusivities in general circulation models. (d) SEAMOUNT-ATTACHED EDDIES • A subset of drifters were used to examine eddy activity in the vicinity of the Emperor Seamount Chain (~ 170°E) during summer and fall 1992. • The trajectories of two deep-drogued drifters revealed a pair of counterrotating mesoscale eddies attached to the leeside of Ojin/Jingu Seamount. The eddies had diam eters of 75-100 km and mean rotational speeds of 20-40 cm/s. One of the drifters made Chapter 8. Summary and Synthesis 183 five loops within the cyclonic eddy over a period of 62 days, during which time the eddy translated westward at ~ 2.9 cm/s until colliding with the seamounts. This is one of the first observations demonstrating an extended attachment of a topographically-generated eddy to a seamount. • Shallow-drogued drifters which crossed the ESC in the summer of 1991 and the winter of 1993 revealed no eddy activity, most likely because of a decoupling of the topographic influence to the 15 m flow at their crossing latitude over the shorter Nintoku Seamount (summit depth at 1000 m). The implication is that eddy formation within the mixed layer near the ESC is confined to the region around the taller Ojin/Jingu and Kinmei Seamounts. • The observations of attached leeside eddies at the ESC match the predictions of numerical and analytical models very well. (e) TRENCH-TRAPPED EDDIES • Subsets of drifters near the Kuril-Kamchatka Trench in the western North Pacific revealed the presence of large anticyclonic eddies positioned over the deepest part of the trench, both in fall 1990 and late summer 1993. • In fall 1990, a drifter released near the center of a previously identified eddy remained in the eddy for 43 days, during which it made five loops at successively greater distances from the eddy center. High-amplitude inertial oscillations, which coincided with a storm passage, were observed during the first two loops. The relative vorticity of the eddy resulted in a lowering of the inertial waveband, as predicted by Kunze (1985) and others. • It has been proposed that the broadening of the inertial waveband by the eddy's rotation resulted in resonance with the diurnal tide, and a subsequent energy input which increased the eddy's life. A spectral analysis of the drifter's record within the 1990 eddy Chapter 8. Summary and Synthesis 184 suggested that the proposed mechanism is invalid. • The 1990 and 1993 eddies had similar characteristics, with diameters of ~ 200 km and mean rotational speeds of ~ 45 cm/s at 80-100 km from the eddy center. • The drifter observations support the implication from historical data that large, long-lived anticyclonic eddies are common in the KKT region. 8.2 A Synthesis In order to describe individual circulation features or to define a mean circulation over a broad region, careful planning may go into the deployment of a set of satellite-tracked drifters. The acquired observations, however, will generally evolve in ways that are neither predictable nor necessarily representative of the circulation features intended to be sampled. What Lagrangian instruments do provide is a snapshot of the circulation, a glimpse of the state of the ocean over a period of time which is short compared to the timescales over which large-scale changes in the ocean typically take place. With the drifter deployments presented here, the primary objective was to describe and compare the mean circulation and its variability in the North Pacific Ocean at depths representing the mixed layer and the underlying pycnocline. The drifters obliged, and, in etching out a snapshot of the state of the ocean at the drogue levels, provided numerous surprises as well. Throughout the thesis, this snapshot has been viewed from various angles. In Chapter 3, refinements to the tools used to define the image (the position time series) were explored so that the snapshot produced would be a statistically reliable one. In Chapters 4 and 5, the "wide-angle" view was presented, in which all trajectories were used to derive ensemble-mean velocity statistics and a characterization of the near-surface circulation over the observation period. In Chapters 6 and 7, the snapshot was viewed with a Chapter 8. Summary and Synthesis 185 narrower focus, with some of its more interesting features, i.e., topographically-controlled mesoscale eddies, being examined in greater detail. The thesis has integrated these disparate views into one document, yielding a comprehensive description of the near-surface circulation in the North Pacific Ocean over the period 1990-1995. Although it will not be known for a long time just how "representative" this snapshot is, these observations can be used to put constraints on the planning of future drifter deployments in the region, and to provide improved parameterizations for, and truthing of, general circulation models. 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