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Deep water exchange in Bute Inlet, British Columbia Lafond, Claude 1975

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DEEP WATER EXCHANGE IN BUTE INLET, BRITISH COLUMBIA by CLAUDE LAPOND B.Sc, Laval University, 1 9 ? 2 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Physics and the Institute of Oceanography We accept this thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA March, 1 9 7 5 In presenting th is thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree ly ava i lab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r ly purposes may be granted by the Head of my Department or by h is representat ives . It is understood that copying or pub l ica t ion of th is thes is fo r f inanc ia l gain sha l l not be allowed without my wri t ten permission. Department of U The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT Bute Inlet i s a s i l l e d embayment of the British Columbia mainland coast connected to the Strait of Georgia through Sutil Channel. The properties of the waters in the inlet have been observed during a series of cruises from June 1972 to June 197^ with the main objective of deter-mining the water circulation below the upper layer. The time-series of salinity, temperature and dis-solved oxygen distributions obtained from this two-year survey are presented and used to analyse the circulation pattern and renewal mechanism of the intermediate and deep inlet waters. Inflows of deep water from the Strait of Georgia into Bute Inlet occurred frequently during the study period and took place when the water from the Strait of Georgia appearing above the s i l l was denser than the water in the basin of the inlet. Volumes *tff inflows into Bute Inlet have been estimated and some calculations indicate that inflow speeds could be large enough to be recorded by existing current meters. The renewal of the deep water in Bute Inlet basin appears to be basically consistent with the annual cycle of deep water replacement in the Strait of Georgia and with i t s year to year ^ variations. i i i TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v LIST OF FIGURES v i ACKNOWLEDGEMENTS v i i i Chapter I. INTRODUCTION .1 1.1 General Introduction . . . . . 1 1.2 Geographical Description of Bute Inlet . 2 1.3 Oceanographic Data 3 II. PROPERTIES AND STRUCTURE OF THE WATER . . . . 5 11.1 Introduction 5 11.2 Salinity, Temperature and Dissolved oxygen 8 II. 3 Density and Stability . 24 II.k Temperature-Salinity Relations 27 11.5 Cross-sectional Variations 30 11.6 Short-Term Fluctuations 32 iv Page III. CHARACTERISTICS OF INFLOWS INTO BUTE ... . . 36 111.1 Introduction 36 111.2 Occurrence 36 III. 3 Volume Estimate 38 III. 3. A Method 1 39 III .3.B Method 2 44 III .3.C Results ^6 III.4 Estimates of Inflow Speeds 49 IV. ANNUAL CHANGES IN WATER PROPERTIES . . . . . 53 IV. 1 Description 53 IV. 2 Discussion 55 V. DEEP WATER REPLACEMENT CYCLE IN SUTIL AND BUTE .-: 60 VI. SUMMARY AND CONCLUSIONS 66 BIBLIOGRAPHY 68 APPENDIX 69 V LIST OF TABLES Table Page I. Bute Inlet and Sutil Channel cruise information 71 II. Stability of the water column below the upper layer at station 2 in Bute for the period June 1972 to June 197^ 72 III. Comparison between the amplitude of the short-term fluctuations and the amplitude of the month to month variations expressed in terms of standard deviations . . . . 73 IV. Summary of the inflows which occurred into Bute in the intermediate and deep zones during the period June 1972 to June 197^. The inflows are grouped according to the characteristics of the intruding waters 7k V. Mean values of salinity (&), temperature (°C) and dissolved oxygen (ml/l) for the volume of water contained below 100 m and below 350 m in Bute and for the water above the s i l l . . . . 75 VI. Volume estimates and related data of the water masses which intruded into Bute between May 1973 and February 197^ 76 v i LIST OF FIGURES Figure Page 1. Southern coastline of British Columbia, showing the location of Bute Inlet 77 2. Map and longitudinal section of Bute and i t s approaches showing the location of the oceanographic stations 78 3. Vertical profiles of salinity, temperature, density (o^ ) and dissolved oxygen at station 2 m Bute for (a) Winter and (b) Summer 79 4. a)-p) Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for the period June 1972 to June 1974. Significance of line typess for standard isopleths used on the entire series of sections to the extent permittrd by the individual property distributions. _ i f o r isopleths only used on a few sections to supplement the standard isopleths. - - - - - for isopleths or sections of isopleths whose position i s known with less certainty. Dashed lines are used between stations Geo 4747 and Sut 1 because of the big distance between these two stations 80 5. Relative effects of salinity and temperature variations with depth in establishing the sta b i l i t y of the water column at station 2 in Bute 97 6. Longitudinal sections of density (at ) for S u t i l and Bute in (a) June 1973 and (b) December 1973 98 v i i 7 . a)-p) Temperature-salinity characteristics of the waters below the upper layer at station 2 in Bute, at station 3 in Sut i l and at station k?k7 in Georgia for the period June 1 9 7 2 to June 1 9 7 ^ 99-: 8 . Transverse section of temperature (0qC) at station 2 in Bute in February 1 9 7 ^ . 102 9 . Short-term fluctuations of (a) Salinity and (b) Temperature below the upper layer at station 2 in Bute in June 1 9 7 4 1 0 3 1 0 . Monthly values of (i) Temperature and ( i i ) Salinity (a) at station 1 7 4 8 in Georgia, (b) at station k?^7 in Georgia, (c) at station 1 in Su t i l , (d) at station 3 in S u t i l and (e) at station 2 in Bute 104 1 1 . Comparison of monthly values of density (at) between^(a) Stations 17^8 and ^7^7 in Georgia and station 1 in Su t i l , (b) Stations 1 and 3 in Sutil and (c) Station 3 in Sutil and station 2 in Bute 1 0 9 v i i i ACKNOWLEDGEMENTS I am sincerely grateful to a l l who have assisted in the preparation of this thesis. In particular I would like to thank my supervisor, Dr. G.L. Pickard, for his guidance and suggestions. Thanks are also due to ' " ' v . . Dr. S. Pond who cr i t i c i z e d the manuscript. The assistance and co-operation of the crew and officers of the vessels C.S.S. Vector and CF. A. V. Laymore are gratefully acknowl-edged. I also thank the staff and fellow graduate students of the Institute of Oceanography for their assistance in the collection of the data. Finally, I thank the National Research Council of Canada who have supported me during this study. 1 CHAPTER I INTRODUCTION I.1 General Introduction The British Columbia mainland inlets have been for several years an object of study for the Institute of Ocean' ography, U.B.C.. A general description of the morphologi-cal and oceanographic features of these inlets has been given by Pickard ( 1 9 6 l ) . One of the features of interest of these inlets i s that i t has been found that very low values of dissolved oxygen (e.g. below 2 ml/l) were uncom-mon in the mainland inlets (Pickard, 1 9 6 1 ) . Since the dis-solved oxygen content indicates that stagnation does not occur at any time in the deep water of these inlets, /renew-al of the deep water must occur in some fashion to maintain this oxygen level. In his study of the optical turbidity of some mainland inlets, Giovando (1959) commented brie f l y the deep water renewal. He considered the possibility that the deep water turbidity could be u t i l i z e d as a tracer to confirm the presence of deep water renewal suggested by other oceanographic variables. From long term changes in water properties, Pickard ( 1 9 6 1 ) concluded that there was no annual cycle of change of the deep water in the inlets of the mainland coast and that the renewal was apparently 2 taking place at irregular intervals. So far l i t t l e was known about the renewal of the deep water in these inlets. Consequently a more detailed examination was desirable. It i s the purpose of this study to obtain a better description of the deep water circula-tion by describing the processes in a typical mainland inlet, Bute Inlet, which has been studied frequently and systematically since 1 9 5 1 . 1 . 2 Geographical Description of Bute Inlet Bute Inlet i s located about 100 nautical miles northwest of Vancouver. Figure 1 shows the general location of the inlet. Figure 2 i s a map of the in l e t and i t s ap-proaches. Both figures indicate the position of the ocean-ographic stations. Bute Inlet proper i s about 40 (nautical) miles in length? i t averages about 2 miles in width. It ranges from 2 0 0 m depth near the head, through 650 m half-way down inlet, to 350 ro near the mouth (the s i l l ) . Its greatest depth (about 6 6 0 m) i s located about 10 miles from the mouth. The inlet communicates with the waters of Cordero Channel through both Yuculta Rapids and Arran Rapids. It i s connec-ted to the Strait of Georgia by two approaches: Sutil Channel (the more direct) and Pryce-to-Homfray Channel. Communication at depth between Homfray and the Strait of Georgia i s restricted to Baker Passage (about 135 ro depth) and the passage between Savary Island and the mainland 3 (about 85 m depth). Between Sut i l Channel and the Strait of Georgia the shallowest depth which o&ssms in the outer part of the channel i s about 250 m. The Homathko and the Southgate rivers flow into the head of Bute Inlet. A secondary river, the Orford, also enters the inlet on the eastside midway along the inlet. For simplicity, Bute Inlet, S u t i l Channel and the Strait of Georgia w i l l be generally referred as Bute, Sutil and Georgia in the text. I.3 Oceanographic Data The oceanographic data used for this study have been obtained in a series of cruises conducted by the Institute of Oceanography, U.B.C. during the period June 1972 to June 197^. Table I contains a l i s t of these cruises. Gener-al l y the survey included the observations of temperature, salinity and dissolved oxygen at standard stations. Temperatures were obtained by Richter & Wiese and by Yoshino Keike reversing thermometers attached to the Atlas or N.I.O. water sampling bottles and read at sea with an accuracy of - 0.02 C°, by bathythermographs or recorded using a Plessey Model 9060 S.T.D. (Salinity, Temperature, Depth recorder). The salinity was determined ashore using an Autolab (inductively coupled) Salinometer and also using the S.T.D. records. For salinity above 28 to the salinometer has a claimed accuracy of 0.003 t o . The dissolved oxygen content was determined on shipboard by Winkler's method using 5 0 ml. samples, but modified by using reagents recom mended by Carritt and Carpenter ( 1 9 6 6 ) . Meteorological dat were also recorded at each station. Standard sampling depths below the surface layer were 5 0 , 7 5 , 1 0 0 , 1 5 0 , 2 0 0 , 3 0 0 , 400, 5 0 0 , 6 0 0 m to the extent permitted by the depth of station. 5 CHAPTER II PROPERTIES AND STRUCTURE OF THE WATER II.1 Introduction The physical oceanography of Bute has been described by many workers, notably Tabata and Pickard (1957). Pickard and Giovando ( I 9 6 0 ) , Pickard (1961) and MacNeill (1974). The general characteristics of salinity, temperature, density and dissolved oxygen content are illustrated graphically in Figure 3 and discussed below. For the purpose of this study of Bute, when re-ferring to layers of water in the water column, the term "upper layer" i s applied to the waters extending from the surface to 100 m depth, the term "intermediate layer" for those extending between 100 m and 3 5 0 m and the term "deep layer" for those extending from 3 5 0 m to the bottom of the inlet. Referring to Figure 3» the salinity distributions may be divided into two main groups, one occurring at periods of small river runoff (winter: October to April) and the other at periods of large runoff (summer: May to September). In both cases there i s a positive salinity gradient which i s most marked near the surface. The salinity gradients are smaller during the low runoff period than during the large runoff period. In contrast to the wide 6 variations which occur in the upper layer, the deep waters exhibit very small variations throughout the year. The temperature distribution may be divided into two classes, winter and summer. The temperature profiles are however less simple and less regular than those of salinity. In general during winter the temperature increases with depth down to a few hundred metres and sometimes to the bottom. During the summer the temperature decreases with depth to a well defined minimum at a depth of about 75 m and then increases. Throughout the year the variations of temperature are large at the surface but small, in the deep waters. In general the content of dissolved oxygen i s high in the top 100 m layer with values above k ml/l. The value decreases gradually in the deeper waters but the content in the deep water i s not low (above 1.8 ml/l). It may be presumed from this observation that inflows of water must occur in the deep basin to maintain this oxygen level. The density profiles generally follow those of salinity more closely than those of temperature. Consequent-ly the comments on salinity apply for the density except in particular cases which w i l l be diseussedAsubsequently. As in other British Columbia inlets, the distribution of mass and hence the movements of waters in Bute depend mainly on the salinity distribution (Pickard, 1961). On longitudinal sections of Bute, the isopycnals are generally horizontal and therefore rarely give information on water movements. 7 Consequently the longitudinal sections of density w i l l not be presented in this study. Oxygen consumption due to respiratory and oxidative processes occurring in the inlet affects the distribution of oxygen. But even i f the oxygen content i s not a conserva-tive property, i t can provide useful information on the circulation. The major factors which cause an increase in oxygen are production by phytoplankton activity, exchange from atmosphere to water at the surface, introduction through the fresh water sources and advection from outside the inlet of water with an oxygen content higher than that of inlet waters. Those processes which decrease the oxygen amount are oxygen consumption and outflow in the surface layer. Below the euphotic zone however, only a few of these processes can change the oxygen content. These are advec-tion, transfer through diffusion and oxygen consumption. A l l except one, advection, are slow processes and from these considerations i t i s believed that a sudden decrease or increase in the oxygen content may be attributed to inflow from outside. Thus examination of the oxygen distribution at a particular time in comparison to the distribution during the previous survey can provide at least a qualitative picture of the circulation pattern during this period. Advection of water from outside the inlet w i l l be shown to occur in Bute. Water must enter Bute either from Cordero Channel by way of Yuculta Rapids and Arran Rapids 8 (Figures 1 and 2) or from the Strait of Georgia. In the former case, horizontal movement of water i s restricted to depths less than about 50 m. Observations made during the second year of this study have indicated that the well mixed water from Cordero Channel possesses at a l l times a density comparable to that of the water at depths of less than 100 m in Bute and consequently markedly less than the density of the deep waters of Bute. Therefore i t would appear that the Strait of Georgia must supply deep waters to the approach channels and to Bute i t s e l f . From the topographic point of view the most l i k e l y avenue of renewal would appear to be Sut i l Channel rather than Homfray Channel since the shallowest depth in the former appears to be about 250 m while the horizontal move-ment of water at the entrance of Homfray i s restricted to depths less than about 150 m. Giovando (1959) reported that in Baker Passage the density was found to be considerably smaller than the density in the deepest parts of Bute. Thus advection into the deep basin of Bute through, Homfray Channel appears unlikely and i t i s believed that Su t i l i s the avenue of renewal from Georgia to Bute. II.2 Salinity, Temperature and Dissolved Oxygen To investigate the deep and intermediate water circulations in Bute, the observations of salinity, tempera-9 ture and dissolved oxygen content which have been made during the series of cruises in Bute (Table I) are presented in chronological order and plotted in longitudinal sections which run along the centre of the inlet. The distribution of these properties at one time i s indicative of the oceanographic state of the inl e t at that moment and such a series of property sections, distributed in time, can be used to follow time changes in the water masses of the inlet. In the following description of the data emphasis i s placed on the changes in distributions observed on succes-sive cruises for each property. June 1972 (Figure 4a) Looking f i r s t at the salinity section, i t can be seen that in Sutil the sal i n i t i e s in the intermediate and deep layers are significantly lower than at corresponding depths in Bute. The isohalines from the surface to the s i l l depth in Bute are continuous with those of Sut i l . The deep waters however, with sa l i n i t i e s ranging between 30.6 and 30.^5 tot appear to be isolated since the isohalines at these depths are not continuous over the s i l l . The longitudinal section;-for temperature indicates that in a l l parts of the system the temperature decreases with depth to a minimum value and then increases down to the bottom. The structure and range of temperatures are however different in Sutil and in Bute. In Sutil the temperature minimum centered at about 125 m i s a tongue- like structure 10 extending from the entrance. It i s believed to be the result of advection from Georgia of cold water of temperature of about 7.4 °C. In Bute the temperature minimum i s centered at about 75 m depth. It i s the result of surface winter cooling (Pickard, 196l; MacNeill, 1974). Below the minimum the temperature increases smoothly down to the bottom where i t s value reaches 8.8 °C. The isotherms above the s i l l are continuous with those in Sut i l but not below the s i l l and the deep water appears to be isolated. The longitudinal section for dissolved oxygen shows that the content in the intermediate and deep waters decreases with depth down to the bottom. The oxygen content is nowhere less than 2.8 ml/l. In the upper layer there i s a shallow maximum extending down inlet from the head at about 75 m. It i s probably associated with the winter cooling temperature minimum. July 1972 The distributions of the oceanographic properties in July are so similar to those of June 1972 that the same comments could be made and so no figures are presented. August 1972 (Figure 4b) The general pattern of the isopleths i n August i s similar to that for June. There has been however a slight increase of salinity at intermediate depths in Sutil and a decrease of about 0.1 X» at corresponding levels in Bute. The deep water isohalines are not continuous over the s i l l and the deep water appears to be isolated as in June. Its salinity i s less than in June by about 0.04 to . A core of water with an uniform temperature of about 7.7 °C occupies the intermediate layer in Sutil and extends into the mouth of Bute. It can be seen, by comparison of the temperature sections for June and August, that some water has been advected at the entrance of Bute at about 200 m. The temperature in the deep zone of Bute has decreased slightly since June. In the upper layer in Bute, the tem-perature minimum i s warming up; i t s 7.0 °C isotherm now extends to mid-inlet length only. The section for oxygen reveals a slight increase in the oxygen content at intermediate depths in the outer half part of Bute. Since this increase i s well below the euphotic zone, i t confirms the inflow conclusion deduced from the salinity and temperature variations. October 1972 (Figure 4c) The salinity and temperature distributions suggest that an inflow of water with uniform properties has occurred at intermediate depths (down to 300 m) in Sutil and has taken place also at the entrance of Bute. This i s suggested by the slight increase of salinity which has occurred at intermediate depths in Sutil between August and October 12 together with the appearance of a water mass of uniform temperature (8 °C) between 100 and 300 m in Sut i l . In Bute, the extension of the presumed inflow into:the inlet i s suggested by the sloping of the isotherms at the entrance caused by the temperature difference between the intruding and the resident waters. The occurrence of an inflow at intermediate depths in Sutil i s also suggested by a 0 . 0 3 increase in g t at 300 m. It i s however possible that some of the changes in salinity and temperature at intermediate depths in Sutil could be due to diffusive rather than advective processes. Unfortunately no information can be inferred from the oxygen distribution because data are lacking in Sutil. December 197? (Figure 4d) The longitudinal sections for December when compared with those for October reveal that an inflow of water with new characteristics has occurred at intermediate and deep levels in Sutil and at intermediate depth in Bute during this period. The sections show that water with salinity and temperature up to 30.7 % and 8 . 8 °C has replaced the colder and less saline water present in Sut i l during October. In Bute, the intrusion, as suggested by the temperature and salinity increases, extends in the outer half of the inlet at intermediate depths but down to about 500 m at the mouth. The upward sloping of the intermediate and deep isohalines and isotherms and of the 3.2 ml/l oxygen 1 3 isopleth toward the head of the inlet suggests that the intruding waters have pushed the mid-depth resident waters ahead and upward. The Qt distribution for December reveals that the a t lines are level but that the density has increased in S u t i l and at intermediate depths (down to about 5 0 0 m) in Bute. February 8 , 1 9 7 3 (Figure 4 e ) The changes in the property distributions between December and February suggest that important inflows of water have occurred into Sutil and Bute during this period. In Sutil the newly intruded water i s slightly less saline and significantly colder than the resident water observed in December. This new water of uniform properties (about 3 0 . 6 5 % and 8 . 5 °C) has also intruded into Bute but just at the mouth (station 1 ) at depths between 1 5 0 and 5 0 0 m where i t s properties, different from those of the water mass in the main part of Bute, make i t evident. There has also been a renewal of water in the main part of Bute. It appears that the water observed in Sut i l basin during December, presumingly displaced by the intru-ding water of new characteristics, has flowed into Bute basin and caused some renewal at intermediate and deep levels. This i s suggested by the increases in salinity, temperature and dissolved oxygen which occurred in Bute and particularly in the outer half of the inlet. The inflow seems to have reached the 6 0 0 m level at station 2 but to have extended to lesser depths further ahead. 14 February 27 , 1973 (Figure 4f) These sections show the state of Sutil and Bute only three weeks after the previous survey. In Sutil the salinity and the temperature at intermediate levels have decreased by about 0.1 f*> and 1.5 C° respectively during this period. In Bute the water mass with particular charac-te r i s t i c s present at the mouth at the beginning of the month i s no longer apparent. There have been apparently no further inflows into Bute during this three week period and the new distribution of properties at the end of the month seems to result from the mixing of the different water masses present in Bute at the beginning of February. March 1973 (Figure 4g) There have been no major changes- in the property distributions between February and March. In Sut i l there has been, as in February, a slight decrease in salinity and temperature below the upper layer. In Bute the isopleth patterns for salinity and temperature are similar to those for the end of February. The most significant change i s the slight decrease in the oxygen content below the upper layer which i s believed to be due to oxygen consumption. It i s interesting to note the emergence of the winter cooling temperature minimum in the upper layer at the head of Bute. May 2, 1973 (Figure k-h) The sections for this survey include observations made in the northern part of the Strait of Georgia. Surveys in this part of Georgia were conducted from May 1973 to June 1974, together with those in Sutil and Bute, to study the possible relation between the variations of properties in the deep and intermediate waters of Sut i l and Bute with those occurring in Georgia. Looking at the temperature distribution for May, i t can be seen that the isotherms define in Sutil a tongue-like structure enclosing cold water with temperature less than 8.0 °C. The core extends from Georgia through Sut i l into the mouth of Bute. It suggests that an inflow has taken place from Georgia into Sutil and into the mouth of Bute at intermediate depths. In the deep basin of Bute, the s a l i n i -ty, temperature and oxygen content have decrease slightly between March and May. There i s no evidence of advective transport of water into the deep basin of Bute for this period and i t i s believed that this slow decrease of s a l i n i -ty and temperature i s due to vertical turbulent diffusion. It i s acting a l l the time but the changes in properties caused by this diffusive process are so small that they can be apparent only when the advective processes which general-ly cause greater variations of properties are not acting. The decrease in the oxygen content in the deep water i s attributed to oxygen consumption. 16 May 7 , 1973 The distributions of the oceanographic properties for May 7 . 1 9 7 3 are so similar to those for May 2 , 1 9 7 3 that the same comments could be made and so no figures are presented. June 1 9 7 3 (Figure 4i) The distributions of salinity, temperature and oxygen reveal that an inflow has taken place into S u t i l and Bute between May and June. It can be seen that a core of water with a temperature of about 8 .0 °C and with an i n i t i a l oxygen content of more than 4 . 0 ml/l extends from Georgia, where i t has i t s source, into S u t i l and Bute. The intrusion appears to have flushed the Sut i l basin to at least 400 m since the temperature has markedly decreased at this depth while the oxygen content has increased. In Bute the new water from Georgia has been advected in the outer half of the inlet at depths between 150 and 500 m. The upward displacement of the isohalines, isotherms and oxygen isopleths at the head of the inlet (by comparison with May distributions) and the presence of low-oxygen waters at about 150 m in the upper reach of the inlet indicate that a part of the resident deep waters has been pushed toward the head and l i f t e d up by the intruding water. The oxygen minimum, located just below the upper layer, can be traced from the head down to station 1 in Sutil. It suggests that an outflow i s taking place at this level. 17 It i s believed to result from the seaward movement of r e s i -dent deep waters of Bute (of low oxygen content) which are being replaced by denser water of new characteristics. Indeed the density of the intruding water i s at least equal to or higher than the density of the old resident deep waters and this explain the upward displacement of those ones by the intruding water. In spite of this inflow situation, the density distribution exhibits, within the limits of the observations, level lines in Bute. August 1973 (Figure 4 j ) The longitudinal sections for August exhibit very convincing data in favor of the occurrence of a major inflow into Bute. The distributions of properties indicate that since June more water has been advected further ahead and deeper into Bute basin. The core of water extending from Georgia to mid-inlet length has a slightly higher salinity and temperature than in June. In the upper reach of the inlet, the isopleths slope upward and i t can be seen how the deep resident waters are being displaced by the intruding waters. As in June, the temperature but above a l l the oxygen isopleths suggest that the old waters are leaving the inlet in a layer centered at about 150 m and which can be traced down to Sut i l . October 1973 (Figure 4k) The longitudinal distributions of properties for 18 October indicate that there has been further advection of water into S u t i l and Bute. A water mass with markedly higher salinity and temperature and lower dissolved oxygen content has appeared in Georgia. Some of this water with new charac-te r i s t i c s has been advected into Sutil and has caused an important increase of salinity and temperature together with a decrease of oxygen in Sutil basin. In Bute the longitudinal sections reveal the presence of a complex water mass distribution. The continu-itycaver the s i l l of the deepest isohalines and isotherms between Sut i l and Bute together with the marked increase in salinity in Bute basin indicate that some high salinity water like that observed in Georgia and in Sutil has flowed over the s i l l into the deep basin of Bute. The temperature at the bottom of the inlet i s significantly lower than the temperature in Sutil basin. It i s however believed that the new water did mix with the resident colder water when i t flowed into the basin. The 8.20 °C water extending between about 200 and 400 m can be recognized as the water mass of similar charac-t e r i s t i c s which was intruding into Bute in August. At this time, this cold water mass extended down to the bottom in the outer half of the inlet. Presumably i t has been dis^ placed between August and October to i t s actual level by the intrusion of denser water into the deep basin. Above, at about 150 m, the remnant of the resident deep water, which occupied the inlet before the August 19 inflow, can be recognized by i t s typical low oxygen content. This old water mass extends across the length of the inlet and forms a core which appears to move seaward since the comparison with the oxygen section for August indicates that i t extends further down inlet in October than in August. A slight salinity maximum at the head of the inlet is asso-ciated with this low oxygen water. The salinity difference between this water and that below i s very small (of the order of 0.03 & ) but i t i s significant. Generally in the inlet, the salinity i s the most important factor controlling the density and such an inversion of salinity, even so small, i s unusual. The inversion occurs in this case because of the presence of warm and less dense water overlying cold and denser water. It i s interesting to note the occurrence of an inflow in the upper layer. The intrusion at this level i s indicated by the split t i n g of the winter cooling temperature minimum in the vi c i n i t y of station 4 and 5« December 1973 (Figure 41) The distributions of properties for this month indicate again that the replacement of the deep waters in Sutil and Bute has been going on between October and December. In Georgia the salinity and temperature below the upper layer have increased while the dissolved oxygen content decreased. This new water, denser than that in October, has flowed into Sutil and has caused an increase 2 0 in salinity and temperature and a decrease in oxygen in the basin. It has also intruded in Bute and has caused a major renewal in the basin from about 2 5 0 m to the bottom of the inlet. The occurrence of this water renewal in Bute i s suggested by the marked increase in salinity and temperature at these depths during this period. Above 2 5 0 m in Bute, the intermediate water masses can be most easily identified by their dissolved oxygen content. The oxygen minimum centered at about 1 5 0 m and extending a l l along the inlet isnbelieved to be the remnant of the more important minimum observed in October. The oxygen maximum just below the minimum and centered at a depth of about 2 0 0 m i s associated to the water mass of minimum temperature which was observed at a greater depth in October. While i t i s not so apparent on the salinity and temperature sections presented here, the continuous vertical profiles (not shown here) of salinity and temperature recorded with the S.T.D. show the presence of an uniform layer of water with a salinity of about ; 3 0 . 6 5 %° and a temperature of about 8 . 2 5 °G at depths around 2 0 0 m. As mentioned above, this water mass has the same characteristics as the water mass observed between 2 0 0 and kOQ m in October, the latter having presumably been l i f t e d to i t s actual level due to the intrusion of denser water below. January 1 9 7 ^ The longitudinal sections for January 1 9 7 ^ are 21 similar to those for December 1973 and they show that not much change occurred in the property distributions during this period. Thus they are not presented here. February 197^ (Figure km) The longitudinal sections of properties for February 1974 when compared with those for December 1973 indicate a decrease in salinity and temperature (and also a decrease in at ) in Georgia during this period. The same trend of variations i s also observed in the Sutil basin. In Bute there has not been much change in the bottom water and there i s no evidence of further inflows there. At intermediate depths however the displacements of the 30.7 f°° isohaline and of the 8.5 and 8.6 °'C isotherms suggest that an inflow of water with a temperature higher than 8.6 °C occurred in the outer half of the inlet at these depths. On the other hand the volume of 8.0 to 8 .3 °C waters associated with the oxygen minimum in the upper reach of the inlet has been markedly reduced during this period. This water mass was recognized in the discussion for December to be the remnant of the August inflow whose waters were partly replaced by the intrusion of denser waters between August and December. Its volume has been further reduced between December and February presumably under the action of the new water advected into the outer reach during this period and the mass occupies only a small volume in the v i c i n i t y of the head during February. Since these 8.2 °C waters which were 22 f i l l i n g a great part of Bute in August have almost complete-ly disappeared i t must be concluded that the waters below the upper layer in Bute have been almost completely renewed since August 1973. Some transverse sections taken across the width of Bute during February have shown the existence of significant differences in water properties across the inlet width in the outer reach at depths between 200 and 400 m. These observations w i l l be commented on further in section II.5 . March 1974 (Figure 4n) The longitudinal sections for March exhibit only few changes since February. Apparently there have been no further inflows into Sutil and Bute. The salinity, tempera-ture and dissolved oxygen content have however decreased slightly below the upper layer in both basins. The decreases in salinity and temperature are believed to be due to vertical turbulent diffusion. As mentioned previously this process i s acting continuously but i t s effects are apparent only when larger property variations caused by processes like advectiomare not occurring. The oxygen decrease i s believed to be due to oxygen consumption. Besides that, there has been at intermediate depths in Bute an apparent redistribution of the water masses observed in February. For instance the 8 .5 to 8 .6 °C waters appear to have moved toward the head of the inlet between February and March, thus overlaying the 8 .3 to I 23 8 .5 °C waters. It should however be remembered that the transverse sections taken in the outer reach of the in l e t during February showed that the water properties were not uniform acrassiithe width of the inlet. This makes the interpretation of the mid-depth temperature variations i ^ more questionable because no cross-sectional observations were made in March due to the lack of time. Finally i t i s interesting to note the formation of the temperature minimum in the upper layer resulting from the past winter cooling followed by the spring warming at the surface. May 1974 (Figure ko) The main feature of the property distributions in May i s the presence of a core of cold water centered at about 130 m and extending from Georgia through Sutil into the mouth of Bute. The emergence of this temperature minimum is related to the warming at the surface but i t i s mainly due to the advection of cold water at these depths. Indeed i t can be seen that the temperature at depths around 150 m in Sutil has actually dropped by 0.5 C° between March and May indicat-ing that cold water has been advected at these depths. Besides that, the vertical temperature profiles recorded by the S.T.D. for station 47^ 7 in Georgia and station 1 and 3 in Sut i l (not showed here) exhibit, at depths corre-sponding to the temperature minimum, a very definite layer of cold water delimited by abrupt changes of properties, whose structure i s characteristic of advective rather than 2k diffusive processes. Also only the advection of water of higher oxygen content can explain the oxygen increase at the depth of the temperature minimum which i s well below the euphotic depth. Further ahead, in the upper reach of Bute, the win-ter'eooling temperature * minimum" presents a well developed characteristic structure. Below in the deep basins of Sutil and Bute, only slight decreases of salinity, temperature and oxygen have occurred. There are attributed to vertical turbulent diffusion and oxygen consumption. June 1974 (Figure kp) The tongue of cold water which extended from Georgia to the mouth of Bute at depths around 150 m during May i s intruding well into Bute to about station k during June and consequently i t appears that more cold water has been advec-ted into Bute during this period. In the upper reach of the inlet, at intermediate depths, the 8 .3 °C isotherm has moved slightly upward. It i s possible that this apparent movement of water at the head has been induced by the inflow at the mouth. Otherwise there have been no significant changes in the intermediate and deep layers except for slight decreases of salinity, temperature and dissolved oxygen. II.3 Density and Stability Typical density profiles for winter and summer have 2 5 been presented in Figure 3 . Generally the density curves (expressed in terms of afc ) follow those for salinity more closely than those for temperature. In the upper layer the density i s determined almost entirely by the salinity < (Pickard, 1 9 6 1 ) . At intermediate depth the relative effect of salinity variations diminishes but i t s t i l l remains largely dominant over the temperature variations. In the deep layer the influence of temperature on the density i s not secondary to the effect of salinity. It must be stressed that, even i f the density profile conforms to the salinity one, the salinity i s not the only factor establishing the sta b i l i t y of the deep waters. Generally the temperature increases with depth and this positive gradient reduces the stability. In order to show the relative effect of salinity and temperature gradients on the density, their relative effectiveness in establishing s t a b i l i t y has been estimated and the results are presented graphically in Figure 5 . In the surface layer the salinity gradient i s much more important than the temperature^grad^en^t in deter-mining the stability. During the period of high runoff in summer, the salinity can contribute up to 9 8 ^ to stabil i t y . During winter, when the runoff i s low and when the water is cooling the temperature contribution reaches kOfo, However averaged over the year the temperature contribution to s t a b i l i t y i s less than 5 $ . At intermediate depths the relative effect of s a l i n i -26 ty gradient diminishes. Between 150 and 200 m, the average contribution of temperature to stability i s about 20 %. On one occasion however, in October 1973. the temperature contribution became more important than the salinity con-tribution. In the deep waters the average temperature con-tribution i s about 40 %. At these depths the temperature effect usually does not overcome the salinity effect but for short periods the temperature may be the most important determinant of stability. The actual st a b i l i t y (defined here as 10^(da t/dz); units m~*) has been also computed and the results for the waters below the upper layer at station 2 in Bute are presented in Table II for the period June 1972 to June 1974. Throughout this two years study the sta b i l i t y at depths below 200 m always has been less than 50 m""*. For the deep waters i t has generally been of the order of the accuracy of i t s determination (20 m ). A very low stability i s a characteristic of the deep waters in many British Columbia inlets but such a low stability i s usually encountered at much deeper levels in the ocean. This characteristic might be of importance in the process of deep water renewal in these inlets. For instance, because of this low st a b i l i t y i t is possible that a relatively minor density change occurring at about s i l l depth would have some effects at depths much greater than the level of the s i l l . As mentioned previously the longitudinal sections of density have not been presented because they yield l i t t l e 27 information in regard to the purpose of this study. On such density sections the isopleths are usually almost level and they are rarely indicative of the water movements. An example taken during an inflow situation w i l l i l l u s t r a t e this. On Figure 4 i , the distributions of salinity, tempera-ture and dissolved oxygen show much evidence that an inflow was taking place into Bute during June 1973. However, - ' besides the indication of the presence i n Georgia and in Sutil of waters denser than those at corresponding depths in Bute, the density distribution for this month, shown in Figure 6 a , adds l i t t l e to the description obtained from Figure 4i. The density distribution for December 1973 (Figure 6b) i s exceptional in showing an interesting fea - i turei a core of dense ( at - 2 3 . 9 0 ) deep water extends along the length of Bute at 500 m and l i e s over less dense water. The core i s associated with a salinity maximum at this depth (Figure 41). Observations and laboratory procedures, and calculations have been checked and no error which might account for this distribution of ot has been found. It must be remembered that a major intrusion was taking place in Bute basin during this period. II.k Temperature-Salinity Relations During the 2-years study in Sutil and Bute, there have been some periods during which intrusions and water renewal were occurring in the basins and other periods 28 during which l i t t l e exchange was apparently taking place (see section II.2 ) . In addition to the longitudinal sections of properties, T-S diagrams can be useful to follow the evolution of the water masses in Sutil and Bute basins. For this purpose the temperature-salinity characteristics of the waters below the upper layer at station 2 in Bute, at station 3 in Sutil and at station 4747 in Georgia for the period June 1972 to June 1974 are presented in Figure 7. In June 1972 (Figure 7a) the T-S characteristics of the deepest waters in Bute were markedly different from the water in the Sut i l basin. This suggests that there was no exchange at this time between the two basins and that the deep waters in Bute were isolated. The same comments could be made for August 1972 (Figure 7 b ) . Between August and October 1972 the T-S characteristics of the intermediate waters in Sut i l and Bute have changed significantly and this reveals that renewal has occurred at these depths (Figure 4 c ) , Between October and December (Figure 4 d ) , the T-S diagram reveals that warmer and more saline waters have intruded into the Sutil basin and at intermediate depths in Bute. There are no significant changes in the very deep waters of Bute. Between December 1972 and February 1973 (Figures 7e and 7 f ) . further changes have occurred in the intermediate and deep water T-S characteristics in Sutil and in Bute. In February, the water characteristics in the Su t i l and Bute basins are very similar and they are f a i r l y uniform between 150 m and the bottom of the basins. It 29 appears that new water has intruded into these basins during this period. In March and in May 1973 (Figures 7g and 7 h ) , the T-S diagrams reveal not much change in the water proper-ties except at intermediate depths in Su t i l . Between May and June 1973 (Figure 7 i ) » new water has intruded below the upper layer in Sutil and at intermediate depths in Bute. According to i t s characteristics, the intruding water originates from Georgia at depths around 150 m. The deep water characteristics in Bute have not changed significantly during this period. However in August (Figure 4 j ) , the T-S characteristics of the Bute deep waters have changed and i t i s obvious that the inflow from Georgia has extended into the deep basin of Bute. Between August and October 1973 (Figure 7k)» a marked increase of salinity and temperature has occurred in Georgia. Similar changes have taken place in Sutil and this indicates that Sutil basin has been flushed by the new Georgia water. In Bute basin the salinity and the temperature have increased slightly suggesting that the inflow into this basin has been much less important than into Su t i l . A further increase in sali n i t y and tempera-ture has taken place in Georgia and.in S u t i l between October and December 1973 (Figure 41) together with an increase in Bute also. It i s obvious that an important inflow has taken place into Bute. As noticed in section II.3 . some slightly denser water at 500 m i s overlaying less dense water at 600 m at that time. From February to June 1974 (Figures 7m td\7p) the salinity and temperature decrease slowly in 30 Georgia. The same trend i s observed in Sut i l but the changes are much smaller. In Bute the temperature-salinity charac-t e r i s t i c s change very slightly during this period with the result that in May and June the deep waters in Sutil basin are significantly different from the deepest waters i n Bute and the latters appear isolated at this time. T-S diagrams can provide some information on the amount of mixing which i s taking place when a mass of water intrudes into the deep zone of the inlet and encounters the resident water. The inflow of new water into the depths of the inlet results in an upward displacement of the water above the depth of the inflow and in an outflow of the same volume as the inflow. The intruding water then mixes with the resident water yielding the temperature-salinity characteristics of the resultant water observed after the inflow. An example of this can be seen in Figures ?k and 71. Warm and saline water observed in Georgia and in Sut i l during October and December 1973 intruded into Bute and mixed with the cold and less saline water (observed in Bute during October) yielding in December a deep water mass having intermediate characteristics. II.5 Cross-sectional Variations The longitudinal distributions of properties pre-sented in section II.2 have been drawn from observations taken along the approximate centerline of the inlet. Some observations reported by Tabata and Pickard (1957) and by 31 Johns (1968) have however shown small salinity and tempera-ture differences across the width of the inlet in the upper layer. In order to extend the observations below the upper layer, a limited number of transverse sections across Bute were taken in the course of this study of the intermediate and deep waters. In January 1974 a transverse section taken at sta-tion 4 in Bute showed no significant difference in proper-ties across the width of the inlet. In February 1974 another transverse section taken at., station 4 showed also no significant difference of properties across the inlet. However transverse sections taken at stations 1 and 2 in February 1974 showed small but significant salinity, temper-ature and dissolved oxygen content differences at depths between 200 and 400 m. At station 2, the differences in salinity, temperature and dissolved oxygen content across the inlet were of the order of 0.05 X , 0.15 *Q and 0.08 ml/l respectively, the lower salinity and temperature and the higher oxygen content being on the west side of the inlet. The apparent distribution of temperature across the inlet, shown in Figure 8 and which i s to be viewed in relation to the longitudinal temperature distribution for February 19?4 (Figure 4m), indicates that the mid-depth water mass of temperature less than 8.5 °C occupies a larger volume on the west side than on the east side of the inlet. An inflow of water with a temperature higher than 8 .6 °C did occur at these depths between December 1973 and February 32 1974 and this transverse inhomogeneity might be due to this new water having not yet mixed with the resident water. Another transverse section taken during March 1974 also indicated a temperature difference, slightly smaller than in February however, across the inlet in the same depth range. L i t t l e i s known about the occurrence and the significance of these transverse inhomogeneities in the water properties and further investigations w i l l be necessary to elucidate these phenomena. II.6 Short-Term Fluctuations The distribution of oceanographic variables i s usually mapped as through a l l observations had been taken simultaneously when, in reality, they are generally the results of a series of stations occupied successively by a single ship. For instance the time taken to complete eight oceanographic stations along the 40-mile length of Bute was usually about 10 hours. When patterns of water movements are inferred from the distribution of oceanographic variables observed in this manner, i t is important to ensure that spatial variations are not being confused with short period temporal variations. Short-term fluctuations do occur in Bute. In his 196l paper , Pickard reported the observations done in B.C. mainland inlets of shallow and mid-depth oscillations interpreted as internal waves. He pointed out that shallow and deeper internal waves were common features in some 33 inlets, among others Bute and Knight. Because of ship-time limitations few attempts have been made during this 2-year study, in Bute to extend the observations of short-term fluctuations to the deep waters. It was however f e l t that i t would be of interest to investigate the short-term variations in the deep layer and thus anchor stations were carried out on a number of occasions (Table I). Sample records of salinity and temperature short-term fluctuations below the upper layer at station 2 in Bute during June 197 4 are shown in Figure 9. The gross fluctua-tions in the f i r s t few hundred metres have periods similar to that of the surface tide at Waddington Harbour (head of Bute - Figure 2). The fine structure of the fluctuations may be real or may be only the random fluctuations resulting from the accuracy of the method of observation or may be both. For instance most of the fluctuations of temperature in the deep water are within the accuracy of i t s determina-tion (see section 1.3) and are not considered to be s i g n i f i -cant. However some of the fluctuations, like those of salinity in the deep and intermediate waters and those of temperature at intermediate depths, are believed to have some real i t y since they are larger than the random fluctua-tions which could be expected from the techniques of observations. The characteristics and the origin of these fluctua-tions present interesting problems in themselves. Up to now only a l i t t l e information has been gathered about them and 3k further properly designed observations w i l l be necessary to investigate them. An important point for this study i s however the comparison between the amplitude of the short-term fluctuations and the amplitude of the month to month variations since care must be taken not to confuse the latter with the former. A summary of the range of fluctua-tions in salinity and temperature recorded below the upper layer during the few anchor stations carried on during the two-year study i s presented in Table III. The table gives the standard deviation of the short-term fluctuations about the mean for each cruise. For comparison, the lower row gives the standard deviation about the two-year mean of the month to month variations in salinity and temperature recorded at station 2 in Bute during the two-year study. The short-term fluctuations in temperature appear to be much smaller than the month to month temperature variations. The same comment may also be made for salinity except for the short-term salinity fluctuations recorded during June 1974 for which the standard deviation of these fluctuations represents an appreciable percentage of the standard deviation about the mean of the month to month variations, In spite of this case, the month to month variations of properties may be used with confidence to infer water movements. Indeed in some cases, the amplitude of the month to month variations i s much larger than the standard deviation about the two-year average and in other cases, successive month to month variations, even smaller than the 35 standard deviation, have the same trend for a long period such that in both cases they can not be confused with short-term fluctuations. Besides that, the month to month varia-tions for temperature, as mentioned above, and those for oxygen (not presented here) are much larger than the short-term fluctuations. This and the fact that any f i n a l picture of the water circulation i s drawn from the study of a l l salinity, temperature and dissolved oxygen distributions help to prevent long-term variations being confused with short-term fluctuations and spurious movements inferred. 36 CHAPTER III CHARACTERISTICS OF INFLOWS INTO BUTE 111.1 Introduction As has been seen in the previous chapter, the observations made in Bute during the 2-year study indicate that at certain times the waters in the intermediate and deep zones of the inlet tend to remain unchanged for rela-tively long periods of time, though a gradual modification is observed due to a slow exchange with the upper layer causing dilution of the deep and intermediate sea waters, while at some other times replacements of some portion of the deep and intermediate waters took place through inflows of new water from outside. These inflows are of major importance in the inlet water circulation below the upper layer and a more detailed examination of these events w i l l be undertaken in this chapter. 111.2 Occurrence Table IV gives a l i s t of the inflows which occurred into Bute, in the deep and intermediate zones, during the period June 1972 to June 1974. For each entry, the table gives the northward extent of the flow in the inlet, i t s depth range and the properties which indicate i t s occurrence. 37 The table l i s t s the inflows that were observed to take place during every period between two successive cruises. It i s however possible that individual entries represent only a part of an event which extended over a longer period. In view of this possibility, the inflow records have been assembled in distinct groups according to the characteris-tics of the intruding waters. Under this classification, five different events can be recognized. The f i r s t intrusion observed occurred during summer and f a l l 1972. It caused the renewal of the intermediate water and of a part of the deep water and extended inward up to two thirds of the inlet length. Aisecond inflow was recorded between December and February 1973 and resulted in the renewal of the intermediate and deep waters in the outer half section of the inlet. Another intrusion occurred during the summer 1973 and caused the renewal of a great portion of the deep and intermediate waters, particularly in the vi c i n i t y of the mouth. Another massive intrusion also took place between October 1973 and February 1974. This was the largest inflow recorded during the 2-year period and i t resulted in the replacement of almost a l l the deep and intermediate waters. Finally a small intrusion occurred at depths of about 100 m during May and June 1974. The table makes conspicuous the fact that no inflows took place during the period March to May for both years. The occurrence of this 'quiet' period w i l l be discussed in Chapter V. 38 III.3 Volume Estimate The evaluation of the volume of water entering Bute during an intrusion presents interest in i t s e l f and may be useful to estimate the extent of replacement in the basin, the average speed of inflow, etc... Two methods have been used to estimate such volume. Both methods are based on a water property budget which takes account of the changes in the inlet water characteristics which might take place when new water intrudes into the inlet. In the f i r s t method, the budget i s done over the entire volume of the inlet below a certain depth. In the second method, the budget i s computed only on the part of the inlet volume to which the inflow extended, as suggested by the longitudinal sections of properties. These methods w i l l be detailed below. The volume of inflow has been estimated only for those intrusions which occurred between May 1973 and February 1974 (Table IV). This period has been chosen instead of any other because two major inflows occurred during this interval and both caused marked changes in water characteristics rendering possible the application of the budget methods. The volumes of the other inflows which took place during the 2-year period have not been calculated because they were less important (May-June 1974) or because they resulted in a much less marked changes in water proper-ties (Fall 1972, winter 1973). The water volume in Bute has been estimated from the Canadian Hydrographic Service Chart of Bute Inlet (No 3524). 39 For that purpose the whole volume of the inlet was divided into layers 50 m thick and further longitudinally subdi-vided into smaller divisions or unit-volumes defined by the position of the oceanographic stations. The volume of each so defined c e l l was then evaluated. The total volume of the inlet was estimated to be ( 134 - 10 ) x 1 0 9 m3, the volume of the intermediate and deep waters below 100 m to be 105 x 1 0 9 m3 and the volume of the deep water alone below 350 m to be 43 x 1 0 9 m3. A. Method 1. The f i r s t method used to estimate the volume of inflows i s based, on the computation of a water property budget applied over the whole volume of the inl e t (below a certain depth) for the period between two succes-sive cruises. The water property budget involves the following considerations. A basin of volume V-t i s f i l l e d with water having a conservative property of mean charac-t e r i s t i c PQ . A water mass of volume Vi and mean charac-t e r i s t i c Pi intrudes in the basin and replaces a part of the resident water. After the inflow the resultant water mass has a mean characteristic P a . The budget for the conservative property can be stated by the equation: Vt Pa * V t Pb ? + Vi Pi - Vi Pb or rearranged slightly by: v i = V t . ( P a - Pb ) / ( Pi - Pb ) If P a , Pb and Pi are known, the volume of new water entering the basin can be estimated. 40 In Bute, for this study, we are concerned only with the intermediate and deep waters and consequently V-^  , the volume considered, w i l l be restricted to the volume of water below 100 m. The procedure to estimate the volume of an inflow w i l l be the following. When significant changes in the water properties indicate that an inflow has occurred during the period between two successive cruises, P D and P a for a particular property are evaluated by calculating the weighted mean value of this property over the entire volume defined above using the distribution of this property for the surveys done before and after the inflow. The characteristics of the water above the s i l l before or after the inflow (or an intermediate value, according to the situation) give P^  , the characteristic of the intruding water. Then the volume of the inflow may be computed from the relation above. However some problems arise. /V The f i r s t problem i s the fact that the difference between the mean values of a property over the volume V-fc of the inlet before and after the inflow, i.e. the di f f e r -ence between P D and P a , i s sometimes very small. To illustra t e this, the weighted mean values of salinity, temperature and dissolvrd oxygen for the volume of water below 100 m and below 350 m in Bute for each survey between May 1973 to February 1974 together with the corresponding values for the water above the s i l l are shown in Table V. It i s obvious from this table that the changes in the mean values of the properties between two successive cruises are 41 very small. Since the mean i s taken over the entire volume, an important change of property in a part of the volume due to an inflow i s sometimes counterbalanced in the mean by an opposite change in an other part of the volume due for instance to displacement of resident waters. Indeed the new water entering the inlet and sinking to i t s own density level within the deep zone of the inlet displaces the resident water at this level and this results in an upward displacement of the water above the depth of the inflow. For example between June and August 1973 (Figures 4i and 4j) a deep inflow of 8.1 °C water displaced upward the deep resident water of temperature higher than 8 . 3 °C with the result that the mean temperature did not change during this period (Table V) in spite of the cold water inflow. In some cases the difference in the mean for two successive cruises i s of the order of the accuracy of the-individual measure-ments used to calculate the mean, in which case the budget method 1 used to estimate the volume of the inflow can not be applied. Besides that, other processes like vertical turbu-lent diffusion cause changes in water properties by exchanges between the upper layer and the deep and interme-diate layers. These changes over a month period are very small but when the variations in the mean values of properties between two successive cruises are very small, a non-negligible part of the latter variations may be possibly be attributed to diffusion. Since diffusive effects 42 can hardly be taken into account, the f i r s t budget method can not be used in such a case. The difference between the mean value P D and P a must be significant and be clearly due to the change of properties resulting from an inflow for the f i r s t budget method to be applicable. The deep waters below the s i l l in Bute are contained in a f a i r l y closed basin. Changes when no inflow takes place are very slow since these waters are less subject to vertical turbulent diffusion. In these waters, the variations of properties caused by inflows of water with new character-i s t i c s between-May-1973 and February 197^ (Table V) have been generally larger than the corresponding variations for the deep and intermediate waters altogether. Consequently i t turns out that the f i r s t budget method has been found suitable for the deep water below 350 m only. Salinity and temperature characteristics have been used in the property budget and in some cases the dissolved oxygen content, which i s of course not a conservative property. The rate of oxygen consumption has been estimated using the decrease in the oxygen content during periods when-no inflows where occurring to be about 0.08 ml/l/month in the deep water of Bute. When a decrease or an increase in the oxygen content was observed during a period when an inflow of water with a new oxygen content was taking place, the rate of oxygen consumption should have been taken into account to "correct" this decrease or increase attributable to the inflow. However i t was impossible to determine at 43 which time during the period between two successive cruises the inflow occurred and moreover the oxygen content of the intruding water was known only approximatively (see discus-sion below). Consequently i t was f e l t in this study that i t would be a better policy to use the oxygen budget method (with no oxygen consumption correction) only i f the varia-tions of the oxygen content during an inflow period were much larger than the rate of oxygen consumption for this period rather than attempting to introduce an uncertain correction to take account of the oxygen consumption. The values of the intruding water characteristics P i are necessary to calculate the property budget. Those are not the values for the resultant water mass found after the inflow since some mixing takes place between the intruding water and the resident waters. The best estimates of the characteristics Pi are given by the characteristics of the waters above the s i l l before and after the inflow or a combination of both. In the case where an inflow started during the period between two cruises, as between May and June 1973i the characteristics of the water above the s i l l at the end of the period, in June in this case, appeared to be the best estimate of P^  . In the other case where the inflow was apparently continuous over the period, as between June and August 1973. the best estimate was given by the mean characteristics of the waters above the s i l l at the beginning and at the end of the period. Thus the estimate of the characteristics P< has been only 44 approximate and involved a f a i r amount of personal judgement. B. Method 2. The major d i f f i c u l t y encountered in the application of the property budget taken over the entire volume of the inlet was to deal with the very small difference in the mean values of the water properties observed before and after the inflow. Owing to this problem, a second budget method was developed where the budget was applied only to the portion of the inlet volume over which the inflow extended. The latter information was drawn from the longitudinal sections of properties presented in section II.2. It has been seen that the longitudinal sections give, through the isopleth patterns, some indications about the apparent extent of the inflow into the inlet. However their use i s not without d i f f i c u l t i e s . As a matter of fact, the apparent limits of an inflow suggested individually by the salinity, temperature or oxygen distributions are not unique. This i s due to the fact that these properties are themselves independent and of different character. This problem may be illustrated with the property d i s t r i -butions for June and August 1973 (Figures 4i and 4j). In both cases a l l three sections of salinity, temperature and oxygen suggest that an inflow has occurred into Bute. However the apparent limits of the inflow suggested by each of these properties are significantly different. In view of this problem, i t has been necessary, in order 45 to estimate the presumed "true" volume of the inflow, to consider each property individually and to compare i t s distribution before and after the inflow with the charac-t e r i s t i c s of the intruding water. In some cases, as in June and August 1973 (Figures 4i and 4j), the temperature and the oxygen distributions were the clearest indicators of the inflow extent. In other cases, as in December 1973 (Figure 41), the salinity distribution was a clearer indicator. In spite of a careful analysis, the choice of these limits was s t i l l subject to a considerable amount of personal judgement. Once the presumed area of extension of an inflow on the longitudinal sections was delimited;•fithe-apoption-©.f the in l e t volume Ve (for volume of inflow extent) associated with the area was calculated, assuming that the inflow extended to the f u l l width of the inlet. Since the water in the volume Ve i s a mixture of resident and newly intruded waters, a property budget was applied to this restricted volume Ve to estimate the volume Vi of newly intruded water, using the relations Vi = V e . ( Pa,e ~ Pb,e >/( *>i - Pb,e ) where Pj_ i s the characteristic of the intruding water as defined previously, P D | 6 i s the characteristic of the resident water in the region of the inflow during the survey before the inflow, P a # e i s the characteristic of the water in the volume V e of inflow extent. 46 This budget method used over the volume Ve turns out to be easier to use and to give, I believe, more reliable results than method 1. The volumes of the inflows occurring in the intermediate and deep layers have been estimated with method 2. This method has also been used to evaluate the volume of the inflows taking place below 350 m in order to make a comparison with the results given by the f i r s t method and also for the interest in the inflows occurring below the s i l l depth. C. Results. Before presenting the results of the calculations of the inflow volumes made with the f i r s t and second methods, a few comments must be made about the amount of computation involved in these estimates. Besides the evaluation of Bute volume done section by section, the application of methods 1 and 2 necessitated in both cases numerous calculations of weighted mean values of salinity, temperature and oxygen. Method 2 implied also a careful analysis of the property distributions to determine the extent of the inflows in the inlet, the evaluation of areas on the longitudinal sections, the calculations of volumes from these areas, etc... In order to become familiar with:• the necessary computations, a l l the calculations were done by hand in this study and they have taken an enormous amount of time. It has been found that some parts of the work necessitated by this type of estimate could be made with the help of computers. However some other partsi_dn-volved decision processes which could not be programmed. 4? Thus even with the help of computers, this type of estimate might s t i l l represent a f a i r amount of work. The results of calculation of inflow volumes are presented in Table VI. The table gives the volume of inflows and the percentage of renewal resulting from these inflows for the inlet volumes below 350 m (methods 1 and 2) and below 100 m (method 2 ) . The last two columns on the right of the table, giving.the inflow speeds, w i l l be discussed in section III.4 . Each value of inflow volume Vi recorded in the table i s the mean value estimated, whenever possible, with individual budgets of salinity, temperature and oxygen. In spite of the crudeness of these methods the estimates of inflow volumes obtained independently by method 1 and by method 2 are f a i r l y consistent. Two major inflows occurred into Bute between May 1973 and February 1974 (Table IV) and Table VI gives a quantitative estimate of the extensive renewal which resulted from these inflows. Considering the deep waters below 350 m and summing up the percentage of renewal between May and October 1973» i t can bg^seen that the inflow during this period virtually flushed the deep basin. The next inflow which took place between October and February also resulted in an extensive renewal of the deep waters with most of the replacement taking place between October and December. For the entire volume below 100 m, the percentages of renewal are less than in the deep waters alone. 48 Depending on the mixing between the intruding and the resident waters, between 45 and 7,0 % of the waters below the upper layer have been replaced during the period extending from May to October 1973- The extent of renewal during this period may also be qualitatively evaluated by comparing the temperature and oxygen distributions for May and October 1973 (Figures 4h and 4 k ) . It i s obvious from these sections that the relatively warm and poorly oxygenated waters observed during May 1973 were largely replaced by October by colder and more oxygenated waters, the remnant of the old water mass being at this date only observed at depths around 150 m. The second inflow between October and February also caused an important replacement of the deep and intermediate waters, especially between October and December. Considering the successive percentages*of-renewal for the water below the upper layer for the period May 1973 to February 1974, the successive products of the percentage of renewal during a cruise interval by the percentage of water not replaced during the previous cruise interval give a total replacement of 80 % for the entire period from May to February (and possibly higher, depending on the mixing between the intruding and the resident waters). This result agrees with the comments made in section II.2 while discussing the longitudinal distributions of properties for February 1974. 49 III.4 Estimates of Inflow Speeds The estimates of the order of magnitude of the speeds of the inflows occurring into Bute might be of interest. For instance such estimates could indicate the f e a s i b i l i t y of using existing current meters to monitor inflows of this nature. In the previous study of month to month longitudinal property distributions made in section II.2, the progression of water masses in the inlet system have occasionnally been followed. For instance, between May and October 1973 (Figures 4h, 4 i , 4j and 4k), a water mass observed in Georgia and Su t i l during May intruded into Bute to station 4 between May and June arid further ahead (station 5) and deeper between June and August and then extended a l l along the inlet length between August and October. The cruise to cruise observations give some indications on the time taken for the movement of a water mass from Georgia to Bute. This time appears to be of the order of few months. However the intervals between two successive cruises are too long to give more than general information. Like the inflow of high-salinity water between August and December (Figures 4j, 4k and 4 l ) . inflows into Bute appear to take place within very few cruise intervals and closer 1 surveys would be desir-able for a better description of these events. Some estimates of the speeds of inflows can be derived from the inflow volume estimates made in section III.3. Water masses from Georgia and Su t i l intruding into 50 the intermediate and deep zones of Bute must flow over the s i l l between Sut i l and Bute. If i s the volume of water intruding below the upper layer into Bute during the period t between two successive cruises, the average speed v of the flow over the s i l l during the period t i s given by the relations v = Vi / ( t.A ) where A i s the cross-sectional area of the flow over the s i l l . For the period May 1973 to February 1974, the longi-tudinal sections of properties suggest that the waters intruding below the upper layer into Bute flowed over the s i l l at depths below 100 m. Using the inflow volume Vi below 100 m given in Table VI and assuming that the cross-sectional area Alofithe flow over the s i l l i s equal to the cross-sectional area of the inlet at the s i l l at depths 5 2 below 100 m (estimated to be 2.1 x 10^ m ), the average speed v during a period t can be easily computed with the relation above. The values for v are l i s t e d in Table VI. They are given in cm/sec for determining the instrument sensitivity required to measure this speed and in km/day to help the reader to mentally visualize the event. The speed estimate v i s a time average for the entire period between two successive cruises. During this period the flow may have taken place during a shorter interval and thus with a higher speed. The speed; vr is^also 51 a cross-sectional average and higher speeds than the average could be expected in the middle of the channel. Besides that, the cross-sectional area of the flow has been taken above as the cross-section of the inlet at the s i l l below 100 m. However inflows must be compensated by outflows of the same volume which must take place somewhere at the s i l l above the inflows. For the period under consideration, some obser-vations indicate that outflows had occurred at depths below 100 m in the region of the s i l l . For instance compensating outflows taking place at least partly below 100 m can be observed on the longitudinal properties sections for August, October and December (Figures 4j, 4k and 41). Compensating outflows taking place below 100 m reduce accordingly the area of the inflows assumed in the previous calculations and thus higher speeds of inflows perhaps up to two may be expected. To sum up,;,the-estimated values of inflow speeds presented in Table VI, which range between 1 and 5 cm/sec, are close to minimum values and actual speeds could be several times these estimates at times. Attempts have also been made to estimate the average speeds of compensating outflows. Those have been occasional-ly observed on the ^property::: sections presented in section 3HE?2. For instance the temperature and oxygen longitudinal sections for August and October 1973 (Figures 4j and 4k) suggest that the old resident Bute waters leave the inlet at depths around 150 m. In particular in August, the volume of water with an oxygen content less than 3.2 ml/l has been 52 estimated to be 45 km^ . In October, this water mass reduced 3 to 21 km . Neglecting the oxygen consumption and the possible mixing between the old water mass and the new one which intruded during the period, i t can be assumed that 3 about 24 km -of low oxygen content water has flowed out of the inlet in the layer centered at about 150 m. A convenient place to estimate the speed of the outflow appears to be at station 3 in Bute during October where the outflow seems to range between 120 and 170 m. Assuming that the flow extends the entire width*of the inlet at this station, i t s cross-sectional area i s found to be 5 2 about 1.5 x 10 m . It i s f i n a l l y deduced that the average outflow speed for the period between August and October i s about ;<2.8 cm/sec, this value being subject to the same considerations made about the significance of the average in the inflow speed discussion. The speeds of the advected water masses moving "en masse" into and out of Bute appear from the previous estimations to be appreciable. The movement of waters inside these masses might of course be quite different from the average movement since they could be subject to other forces like internal waves, tides, etc... However since larger inflow speeds than the values l i s t e d in Table VI could be expected, i t i s believed that the movements of the water masses which flow in and out of the in l e t below the upper layer could be recorded by existing current meters. •IP CHAPTER IV ANNUAL CHANGES IN WATER PROPERTIES IV.I Description In order to describe their characters, the annual variations of salinity and temperature over the 2-year study period which occurred in the central (station 1748, see location on Figure l) and northern (station 4747) parts of Georgia, in Sutil (stations 1 and 3) and in Bute (station 2) are shown in Figures 10a to lOe. Station 2 has been chosen in Bute because i t i s believed to be quite representative of the entire basin. The data presented concern the intermediate and deep waters. The scales for temperature and salinity have been selected so that a change of temperature of a given length represents the same change in at as does a change of salinity of the same length. This f a c i l i t a t e s the comparison of the significance of fluctua-tions of these two quantities which together determine the density of sea water at constant pressure. Georgia (Figures 10a and 10b) Salinity and temperature measurements in the inter-mediate and deep waters at station 1748 in the central part of Georgia show that the salinity and temperature are lowest in early spring and highest in autumn. At station 4747 in 54 the northern part of Georgia, no data are available for the f i r s t year of study. The observations made during the second year indicate however that the annual variations in the northern part of Georgia are similar to those in the central part. The annual maxima in temperature and salinity occur however about two months later at station 4747 than at station 1748. Sutil (Figures 10c and lOd) At station 1 in Sutil the salinity and temperature below the upper layer are lowest in the late spring and early summer and highest in the early winter. At station 3 located further ahead in Sut i l , the salinity and temperature fluctuations are similar to those observed at station 1 but some maxima and minima occur about a month later. Bute (Figure lOe) The intermediate waters in Bute are coldest and i -least saline during summer and warmest and most saline during winter. At these depths the general pattern of property fluctuations i s similar to the corresponding patterns for Georgia and Sutil. The maxima and minima in these fluctuations occur however about one month later than in Sutil and the ranges in property fluctuations are smaller than in Sutil and Georgia. The deep waters below the s i l l in Bute do not apparently undergo a regular cycle of fluctuation. For 55 instance the salinity and temperature have decreased during the summer and autumn 1972 and after that showed few changes for few months. Then there has been a decrease of tempera-ture and salinity down to a minimum in summer 1973 with a subsequent rise to a maximum in late autumn and early winter. Thereafter there has been a slight decrease. It must be pointed out however that the occurrence of a minimum in summer and a maximum in winter, as observed during the second year of the study, corresponds nevertheless to the general annual pattern of fluctuation observed above in Georgia, in Sutil and in the intermediate layer of the inlet. IV.2 Discussion The variations of the water characteristics in Georgia have been described by Waldichuck (1957). He' deter-mined that deep waters in Georgia undergo replacement twice annually. Deep waters are formed below the surface in the southern part of Georgia during winter. The increased salinity of the inflow at the southern end of the s t r a i t and the surface cooling form a water mass of temperature between 7 and 8 °C. This dense water flows toward the north along the bottom, gradually diffusing upward. The flow starts from the south in late autumn and reaches the northern end of Georgia in early summer. During some cold winters, formation of an intermediate water may also occur in the northern part of Georgia. 56 Adeep water mass of different characteristics from the water mass occurring in winter i s formed during late summer. At this time the intrusion of high-salinity water from Juan de Fuca Strait at the southern end of Georgia and the mixing-in of this saline water with the warm surface water forms a warm intermediate water mass. This water has a slightly higher salinity and higher temperature (about 9 °tl) than those of the existing deep waters. Diffusion and/or advection of this water occur both upward and downward u n t i l most of the colder water i s displaced sometimen in late autumn. The conditions encountered in Georgia during the 2-year study agree with this description. Referring to Figures 10a and 10b, the advection of a warm and high-salinity water mass i s clearly shown to occur during early f a l l at station 1?48 (for both years) and about two months later at station 47*47. The cold water mass formed during winter i s also apparent through the evidence of a marked decrease of temperature observed in the deep water during the spring. Besides that the vertical distribution of density, linked to salinity and temperature variations, also undergoes seasonal changes in Georgia (Waldichuk, 1957). The maximum seasonal density i s associated with the interme-diate water mass formed during late summer and which displaces during f a l l the cold deep water present in Georgia throughout spring and summer. The annual variations in the intermediate and deep 57 water properties in Sutil and Bute are basically consistent with the gross annual features of the deep water circula-tion in Georgia. In the description of the annual changes made in the previous section, i t has been observed that the annual pattern of property variations in Sut i l and in Bute were similar to the annual pattern for Georgia waters (with the exception of the Bute deep water which case i s discussed below). This similarity in the annual variations of the intermediate and deep water properties between Georgia, Sutil and Bute i s not, of course, due to the fact that these water bodies are subject to similar conditions at the surface but indeed to the frequent replacement of the deep and intermediate waters of Sut i l and Bute by inflows above the s i l l s of deep water from Georgia. These inflows occur when the Georgia deep waters appearing above the entrance s i l l between Georgia and Sut i l happen to be denser than the Sutil and Bute basin waters. In this case the dense Georgia water sinks into the basins of Sutil and Bute and settles to i t s own density level in these basins, causing the renewal of their waters. Thus the inflows from Georgia into Sutil and Bute are gravity flows which follow the appearance in Georgia and above the entrance s i l l s of a water mass denser than the Sut i l and Bute basin waters. The occurrence of this situation during the 2-year study period i s shown in Figure 11 which presents the time-variations of a t at selected depths in Georgia, in Sutil and in Bute during this period. 58 Referring to Figure 11a, i t can be seen that an increase in density at station 1748 in the central part of Georgia i s followed a few months later by an increase at station 4747 located in the northern part of Georgia and at station 1 in Su t i l above the entrance s i l l . This increase in density above the s i l l is followed about one month later by an increase in density in the deep basin of Su t i l (station 3 at 400 m, Figure l i b ) . During the 2-year study period, the density of the water above the s i l l between Georgia and Sutil was larger than in the deep basin of Su t i l between August 1972 and February 1973 and between May and December 1973* and inflows from Georgia into S u t i l basin have been observed during these periods. Inflows of dense water from Georgia into Sutil may also, i f the inflows are large enough, be followed by > inflows into Bute. Figure 11c shows the time-variations of the density above the s i l l between Sutil and Bute (repre-sented by the density at 300 m at station 3 in Sutil) in comparison with the density at 300 and 600 m at station 2 in Bute. The density above the s i l l was higher than the density at corresponding depth (300 m) in Bute between October 1972 and February 1973 and then between May 1973 and February 1974. Inflows in the intermediate zone of Bute occurred during these periods (Table IV). The density above the s i l l was higher than the density at 600 m in Bute during February 1973 and during the period June 1973 to January 197^ and inflows into the deep basin of Bute took place 5 9 during these periods (Table IV). It i s interesting to note that the deep inflows of. February 1 9 7 3 and June-August 1 9 7 3 into Bute occurred at times when the water above the s i l l was only slightly denser than at 6 0 0 m in Bute. As mentioned in section I I . 3 i i t i s possible that a slight increase in density occurring at the s i l l would result in an advective intrusion into Bute extending far below the s i l l because of the remarkably low st a b i l i t y of the deep water in Bute. The similarities i n the annual pattern of deep and intermediate water property variations between Georgia, and S u t i l and Bute during the 2-year study are explainable by the inflows of Georgia deep water into Su t i l and Bute basins. These inflows produce in the Sut i l and Bute basins the annual pattern of variations of the deep water proper-ties in Georgia. The inflows occur when the density condi-tion for exchange i s f u l f i l l e d . The occurrence of this las t condition and thereby the renewal cycle in Su t i l and Bute are related to both the cycle of density variations in Georgia and to the density reduction in Sutil and Bute deep waters caused by vertical turbulent transfer of heat and salt between the upper layer and the intermediate and deep layers. The discussion of the renewal cycle w i l l be the subject of the next chapter. to CHAPTER V DEEP WATER REPLACEMENT CYCLE IN SUTIL AND BUTE The density difference between the deep water in Georgia and the deep basin waters of Sutil and 3ute, which is the principal factor governing the exchange across the s i l l s between Georgia and the deep basins of Sutil and Bute, is linked with two processes: the seasonal cycle of density in Georgia and the sustained density reduction through vertical eddy diffusion in the resident waters of Sutil and Bute. To illustrate this process i t i s expedient to analyse the situation that might occur i f the oceanographic conditions in Georgia were constant. In this case the distribution of conservative properties in Su t i l and Bute would be substantially uniform and constant from s i l l depth to the bottom of these basins, and similar to those existing at s i l l depth in Georgia. Above the s i l l depth, the water in S u t i l and Bute would be continuous with that in Georgia. Because of the vertical gradient of density always present in the surface layer, there would be eddy diffusive exchange between the zones above and below the s i l l depth tending to reduce the density in the latter zone (see Appendix). Advective flows of Georgia water above the s i l l into the basins would however continuously compensate the density 61 reduction and on the whole the density in Sutil and Bute would remain constant and equal to the density of Georgia water at s i l l depth. Departures from this basic pattern occur primarly with the seasonal variations in Georgia waters. The density of these waters reaches a maximum once a year. If the density variations of the deep Georgia waters followed year after year a constant annual cycle both in phase and in amplitude, inflows from Georgia into Sutil and Bute basins would occur only at the time when the density at and above the s i l l depth in Georgia reached i t s seasonal maximum. During the remaining part of the year no advective exchange would be possible and the density of the deep waters in the inlets would decrease slightly through vertical eddy di f f u -sion exchange with the upper layer. When the next year the water in Georgia reached i t s seasonal density maximum, the density condition-for exchanges would be restored and flushing of the inlet basins would occur once again. Thus i f the annual density cycles in Georgia were constant and identical year after year, renewal of the deep waters in Sutil and Bute would occur every year at the time when the density in the northern part of Georgia reached a maximum. Year to year variations do exist in Georgia (Waldichuk, 1957). Due to year to year variations in the annual cycle of density in Georgia and due to the relatively isolated state of the deep waters in Sutil and Bute basins, the renewal in Sutil and Bute basins may take place 62 irregularly, even i f the density in Georgia reaches a maximum every year. For instance i f some year, the water in Georgia reached an annual density maximum of extreme value, a very dense water mass would intrude that year into S u t i l and Bute basins. Then i f the next year the annual density maximum in Georgia happened to be significantly lower than the maximum of the previous year, vertical eddy diffusive transfer during the year period between the two successive density maxima might not have been sufficient to reduce the density of the deep waters in the inlet basins to the level for which inflows from Georgia would be possible. Con-sequently there would be no renewal in the deep basins of Sutil and Bute for that year. For instance, the density maximum at station 1?48 in Georgia and above the entrance s i l l at station 1 in Sutil (Figure 11a) was smaller in 1972 than in 1973. These variations, from one year to another, in the amplitude of the density maximum could be the principal reason for which the intrusion into Bute during late f a l l 1972 and early winter had a smaller extent than the intrusion during f a l l 1973 (Table IV, Figure 11c) . It i s presumed that renewal might occur less regularly in Bute basin than in Sut i l basin because Bute i s further away from Georgia than Sutil and because Bute basin is much deeper than Sutil basin. Inflows from Georgia into Sutil must be large enough to also carry water over the second s i l l into Bute. It also appears that eddy diffusive transfers are slower in Bute basin than in Sut i l basin. This 6 3 i s particularly apparent when comparing the decrease in density in Sutil (at 400 m) after December 1 9 7 3 with the corresponding decrease in Bute ( 6 0 0 m) after January 197^ (Figures l i b and 11c). Because the density of the deep water in Georgia and thus the density of the water above the entrance s i l l reach i t s annual maximum during f a l l , inflows from Georgia into S u t i l and Bute are most l i k e l y to occur during f a l l (presumably few months later than the time of occurrence of the density maximum in Georgia because of the delay in the occurrence of events in Georgia, Sutil and Bute (see section III.4)). However the condition for exchange between these water bodies, namely that the Georgia water appearing above the s i l l s must be denser than the waters in Sut i l and Bute basins, may possibly be f u l f i l l e d at some other times than f a l l and early winter. As discussed in chapter IV, Georgia deep waters undergo replacement twice a year. The intermediate water mass characterized by a warm temperature and a high salinity i s formed during late summer and early f a l l . It i s thereafter subject to dilution through turbulent diffusion u n t i l i t i s displaced later on in winter by the cold water mass which i s present in Georgia during spring and summer. Then, starting a new cycle, the latter mass i s replaced by the next intermediate water mass and so on. If the density condition for exchange happens to be f u l f i l l e d when the cold water mass i s present in Georgia, an intrusion of cold water would occur into Sutil and Bute during this 6k period and would be most probably followed later on during f a l l by an intrusion of warm and more saline water (associated with the density maximum in Georgia). Thus the double annual replacement of the deep waters in Georgia may possibly be reflected in Su t i l and Bute. Actually, in Sut i l , deep inflows of cold water occurred between July and October 1972 and between May and August 1973 (Figures lOd and lib) and were followed by inflows of warmer and more saline water (and also denser) which took place during the f a l l and early winter of both years. In Bute, a cold water inflow occurred at intermediate depths during late summer and f a l l 1972 and another took place during summer 1973 and extended to the bottom of the inlet. They were followed by intrusions of warmer and more saline water during the period December to February 1973 (not very apparent) and during f a l l 1973 (Figures lOe and 11c, Table IV). While the cold inflow in summer 1973 flushed Bute basin, the cold inflow of summer 1972 extended only to intermediate depths. This explains why the pattern of property variations for the deep water of Bute during the f i r s t year did not exhibit the characteristic pattern of minima and maxima of properties observed in Georgia and Sutil for that year. • >\: >•;,-} "< <• • No inflow occurred into Bute during the period March to May for both years of study and the occurrence of this "quiet" period i s related to the cycle of density variations in Georgia. After the annual density maximum, the water 65 density in Georgia decreases u n t i l replacement of the deep water takes place with the occurrence of the cold water mass during spring and summer. Considering the delay in the occurrence of events between the southern and the northern parts of Georgia, the corresponding density decrease for the water above the s i l l takes place between about February and May (Figure 11c, station 3 in Suti l , 300 m). During this period however, the deep waters in Bute are most l i k e l y to be denser than the water above the s i l l because the renewal of the deep waters in Bute has presumably occurred a few months previously (at the time of the density maximum in Georgia) and because the deep waters in Bute basin are less subject to dilution than the water above the s i l l . Thus, due to the density cycle in Georgia, inflows into Bute are un?,-li k e l y to occur during the late winter and early spring. In summary, the cycle of deep water renewal in Sut i l and Bute, even i f possibly irregular, appears to be basicalr ly consistent with the annual deep water cycle in Georgia . and with i t s year to year variations. 66 CHAPTER VI SUMMARY AND CONCLUSIONS A series of cruises in Bute Inlet system has been conducted during a 2-year period to study the circulation of the intermediate and deep waters. The distributions of salinity, temperature and dissolved oxygen based on the oceanographic surveys have been examined and have revealed that frequent inflows of deep water from the Strait of Georgia occurred into S u t i l and Bute basins during this 2-year period. The amount of water which entered Bute during the major inflows which took place during the second year of the study period has been estimated by using water property budget methods. These calculations gave reasonable results. Average speeds of inflows have been derived from the inflow volume estimates. It appears from these evaluations that deep inflow speeds are significant in the sense that they could be recorded by existing current meters. Inflows from Georgia into Sutil and Bute occurred when the density of the deep Georgia water which appeared above the entrance s i l l s of Sut i l and Bute was larger than the density of the deep waters in the inlet basins. The deep water replacement cycle in Sutil and Bute appears from this 2-year study to be basically consistent with the annual 67 cycle of deep water replacement i n Georgia and with i t s year to year variations. In order to obtain a more detailed description of the water c i r c u l a t i o n below the upper layer i n S u t i l and Bute, moored i n t e r n a l l y recording instruments w i l l be required since cruises at i n t e r v a l s of one to two months are not alone s u f f i c i e n t . Future studies should include both regular cruises at two month i n t e r v a l s (and preferably shorter i n t e r v a l s during periods of inflow a c t i v -i t i e s ) i n Bute, i n S u t i l and i n the northern part of Georgia, and the extensive use of continuously recording instruments, e s p e c i a l l y current meters. I t would be i n t e r e s t i n g to do more investigations on the deep inflows to determine t h e i r actual c h a r a c t e r i s t i c s such as speed, duration, the r e l a -tions, i f any, between the deep water c i r c u l a t i o n and the c i r c u l a t i o n i n the upper layer, the possible e f f e c t s of tides,LmeIte:or,o:lojgicial conditions at the surface, etc... 68 BIBLIOGRAPHY Carritt, D.E., and J.H. Carpenter. 1966. J. Mar. Res. 24: 286-318. Giovando, L.F. 1959. Some aspects of the optical turbidity of British Columbia inlet waters. Ph.D. thesis, Institute of Oceanography, University of British Columbia. Vancouver, B.C. Johns, R.E. 1968. A study of the density structure and water flow in the upper 10 m of a selected region in Bute Inlet B.C. M.Sc. thesis, Institute of Oceanography, University of British Columbia, .Vancouver, B.C. MacNeill, M.R. 1974. The mid-depth temperature minimum in B.C. inlets. M.Sc. thesis, Institute of Oceanography, University of British Columbia, Vancouver, B.C. Pickard, G.L. 1961. Oceanographic features of inlets in the British Columbia mainland coast. J.Fish. Res. Board Canada, 18(6)s 907-999. Pickard, G.L., and L.F. Giovando. i960. Some observations of turbidity in British Columbia inlets. Limn, and Oceanog., 5(2): 162-170. Tabata, S., and G.L. Pickard. 1957. The physical oceanography of Bute Inlet, British Columbia. J. Fish. Res. Board Canada, 14(4): 487-520. Waldichuk, M. 1957. Physical oceanography of the Strait of Georgia, British Columbia. J. Fish. Res. Board Canada, 14(3): 321-486. 6 9 APPENDIX EDDY DIFFUSION COEFFICIENT ESTIMATE Pickard (1961) estimated an eddy diffusion c o e f f i -2 - 1 cient of the order of 0 . 0 2 cm sec for the deep waters in Bute, assuming no advection from 1 9 5 3 to 1 9 5 7 when year-to-year variations of the deep water properties were very small (according to the few surveys done in Bute during this period). However i f the 1 9 7 2 - 7 4 period i s typical, i t is unlikely that no advection occurred for several years and Pickard's assumption i s unlikely to be correct. An estimate of the eddy diffusion coefficient for shallower waters has also been made by Tabata and Pickard 2 - 1 ( 1 9 5 7 ) giving values of 0 . 2 to 1 . 2 cm sec at depths of 9 0 to 1 3 0 m just below the temperature minimum where stab i l i t y i s higher (and eddy diffusion coefficient i s expected to be lower) than in the deep water. Using the more detailed observations available xft c 1 (3? through this study of Bute, an attempt has been made to e s t i -mate the magnitude of the eddy diffusion for the deep waters by considering the rate of vertical transfer of a property at times when the advective transfers of this property were undetectable. Throughout the 2-year study period, no apparent inflows occurred into the deep basin of Bute between June and October 1 9 7 2 . between March and May 1 9 7 3 70 and after February 1974 (Table IV). During these periods, slight decreases of salinity and temperature (Figure lOe) were observed in the deep waters. An eddy coefficient has been estimated by assuming that these changes were only due to vertical turbulent diffusion. The evaluation of the vertical transfer of properties necessitated for this estimate was subject to great inaccuracy because the varia-tions of temperature and salinity considered were not much larger than the accuray of determination of these properties. Nevertheless the eddy diffusion coefficient was found to be of the order of 10- to 100 cm2sec~1. This value i s larger by about an order of magnitude than the generally accepted value (T.R. Osborn, personal communication).The most l i k e l y explanation for this very large coefficient i s that the assumption made for the estimate was^not f u l f i l l e d , namely that the vertical transfer of properties was not only due to vertical turbulent diffusion. 71 Table I. Bute Inlet and Sutil Channel cruise information Year Month Date No. of Stations Remarks Bottle* Total** casts Jun 2 0 - 2 1 8 Jul 2 5 - 2 6 4 Aug 2 3 5 2 9 - 3 0 4 Oct 17-18 4 Dec 19 4 Feb 7-8 5 27 4 Mar 27 4 May 1-2 8 7 - 1 0 8 Jun 14-15 5 2 5 - 2 6 9 Aug 17-18 9 Oct 2 2 - 2 3 10 Dec 13-14 10 Jan 1 6 - 1 7 9 Feb 13-14 10 Mar 18-19 9 May 6 - 7 10 Jun 20 10 2 6 - 2 7 1 10 10 5 10 10 10 10 10 10 10 10 5 9 >9 10 10 10 11 10 10 10 1 Anchor station at Bu 4 (11 hours) Anchor stations at Bu 4 ( 2 3 hours) and Bu 6 (30 hours) Transverse section at Bu 4 Transverse sections.at Bu 1 , 2 , 4 Transverse section at Bu 2 Anchor station at Bu 3 ( 1 3 hours) Anchor station at Bu 2 (32 hours) *Number of bottle stations in Bute and Sutil. **Total number of stations in Bute and in Sutil for which at least either bottle or S.T.D. observations are available. ***The observations made during the cruises on August 2 3 , 1972 and June 14 to 1 5 , 1 9 7 3 are not presented because there are no data available for Sutil and no S.T.D. records to supplement the bottle information. 72 Table II. Stability of the water column below the upper layer at station 2 in Bute for the period June 1972 to June 1974 Date Depth Range (m) Year Month 100-150 150-200 200-300 300^400 400-500 500-600 1'972 Jun 440 120 30 10 30 20 Jul 300 140 40 40 20 20 Aug 300 120 30 40 30 10 Oct 380 — 40 10 10 Dec 520 — 30 10 *.o 1973 Feb 220 60 10 10 0 0 Feb 480 80 30 -10 10 -10 Mar 700 40 20 10 0 20 May 320 100 30 20 10 0 Jun 200 40 20 20 0 10 Aug 320 40 20 0 0 0 Oct 340 60 20 0 20 0 Dec 500 0 30 0 10 -30 19?4 Jan 500 40 30 10 0 0 Feb 480 60 30 10 0 0 Mar 1000 280 60 50 0 0 May 400 280 70 30 20 0 Jun 360 180 100 60 30 0 Stability = 10 5 ; units m"1 dz Table III. Comparison between the amplitude of the short-term fluctuations and the amplitude of the month to month variations expressed in terms of standard deviations Observations Standard Deviation (CT) Date Station Duration Number SALINITY & TEMPERATURE °C of Position (hour) Casts Depth (m) Depth (m) 100 200 400 600 100 200 400 600 8-9 May 1973 Bu 6 30 5 .013 .019 .017 - — .086 . 0 0 9 .008 9-10 May 1973 Bu 4 23 5 .022 .012 .005 .011 , o 8 l . 0 2 9 .01 .008 7 May 1974 Bu 3 13 5 .017 .009 .007 .024 0 . 0 0 5 26-27 Jun 1974 Bu 2 32 17 .088 .033 .024 .021 .044 . 0 3 8 .012 .012 Jun 1972-Jun 74 Bu 2 18 cruises .195 .085 .055 .047 . 2 9 . 2 3 .13 .12 "N3 74 Table IV. Summary of the inflows which occurred into Bute in the intermediate and deep zones during the period June 1972 to June 197^. The inflows are grouped according to the characteristics of the intruding waters Date Characteristics Year Month Group Northward Extent Depth Range (m) Properties 1972 J,un I Bu 1 100-200 S T Jul Bu 2 100-300 s T 02 Aug Bu 4 100-350 s T 0 2 Oct Bu 4 150-400 s T 02 Dec Bu 6 150-500 s T 02 1973 Feb II Bu 4 I5O-6OO ' s T o2_ Mar May Jun III Bu 4 100-400 s T 02 Aug Bu 5 150-600 s T 02 Oct* Bu 8 200-400 s T Oct* IV Bu 5 bottom s T Dec Bu 8 200-600 s T 1974 Jan** Bu 1 150-300 s T Feb** Bu 3 1^0-350 s T Mar May V BU 2 75-150 T Jun Bu 4 75-200 T *Inflow III ended between August and October while inflow IV started during this period. **These inflows at intermediate depths are grouped with the October-December inflow because they appear to be the continuation of the latter event rather than a new one in regard to the characteristics of the intruding waters. Table V. Mean values of salinity (%>), temperature ( °G) and dissolved oxygen (ml/1) for the volume of water contained below 100 m and below 350 m in Bute and for the water above the s i l l Date Salinity (&) Temperature ° C Dissolved Oxygen (ml/l) Below 100 m Below 350 m Above the s i l l Below 100 m Below 350 m Above the s i l l Below 100 m Below 350 m Above the s i l l May 1973 3 0 . 5 5 3 0 . 6 3 : 3 0 . 5 8 8 . 3 ^ 8.50 7.90 3.26 3.00 4 . 0 0 Jun 1973 30.57 3 0 . 6 3 30.58 8 . 2 9 8.42 8.00 3.22 3.01 3.90 Aug 1973 3 0 . 5 8 30.62 3 0 . 5 9 8.29 8.33 8.10 3.21 3.20 3.90 Oct 1973 3 0 . 6 3 3 0 . 6 5 30.70 8.26 8.30 8.50 3.^7 3.65 3.65 Dec 1973 3 0 . 6 6 3 0 . 7 ^ 30.75 8.42 8.57 8.65 3.^6 3.44 3.^0 Feb 1974 30.62 30.73 3 0 . 7 4 8.47 8.58 8.70 3.40 3.36 3.40 Table VI. Volume estimates and related data of the water masses which intruded into Bute between May 1973 and February 1974 i-o Length Below 350 m Below 100 m Date of the Method 1 Method 2 Method 2 period Vi P Vi P Vi P V (days) (10 9 m3) (*) (10 9 m3) (*) (IO 9 m3) {%) (cm/sec) (km/day) 2 May to 2 6 Jun 1973 55 8.2 19 6 14 21 20 2.1 1.8 2 6 Jun to 18 Aug 1973 53 11.2 2 6 8 . 6 20 14 14 1.4 1.2 18 Aug to 2 3 Oct 1973 6 6 3 2 75 — — 3 6 35 3.0 2 . 6 2 3 Oct to 1 3 Dec 1973 5 1 34 80 3k 80 47 k5 5.1 4.4 1 3 Dec 1973 14 Feb to 197^ 6 3 5.2 12 4.7 11 10 10 0.9 0.8 Vi i s the volume of inflow P i s the percentage of resident water (below 1 0 0 m or below 3 5 0 m, according to the case considered) replaced by new water v i s the average speed of inflow above the s i l l 77 Figure 1. Southern coastline of British Columbia, showing the location of Bute Inlet 7 8 Figure 2. Map and longitudinal section of Bute and i t s approaches showing the location of the oceanographic stations E JZ Q. Q 0 0 I o l 0 I 0 50 100 200 400 600 a) Winter 5 I 10 20 I I 5 10 I I 10 20 b) Summer 10 I 30 I 15 I 30 ////////////////// / ////////iff////////////// Diss. Oxygen-• Density • • • 0 I 0 I 0 I 0 5 I 10 I 5 I 10 "IT 10 ml/l 20 1 l 30 1 Ot 10 15 °C 1 20 1 30 °/ loo Temperature Salinity — • Figure 3. Vertical profiles of salinity, temperature, density (a t) and dissolved oxygen at station 2 in Bute for (a) Winter and (b) Summer 80 Figure 4. a)-p) Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for the period June 1972 to June 1974. Significance of line types: for standard isopleths used on the entire series of sections to the extent permitted by the* individual property distributions. for isopleths only used on a few sections to supplement the standard isopleths. for isopleths or sections of isopleths whose position i s known with less certainty. Dashed lines are used between stations Geo 4747 and Sut 1 because of the big distance between these two stations. 81 Figure 4a. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for June 1972. (See the preceding page for the significance the line types) 82 Geo 4747 0-Sul I 600H Figure 4b. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for August 1972 Figure 4d. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for December 1972 85 Geo 4747 Sut I iii) OXYGEN (mi/I) Figure 4e. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for February 8, 1973 Figure Jff. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for February 2 7 , 1 9 7 3 Figure 4g. Longitudinal sections of (i) Salinity, ( i iT Temperature and ( i i i ) Oxygen for March 1973 88 20CH 300 400 500 600H 200H 300H 400 500 600 200 300 400 500 600 Figure 4h. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for May 2, 1973 Figure 4i. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for June 1973 Geo 4747 Sut I 4 5 6 7 8 200 300 400 500 600 200 300 400H 500H 600 200 300 4CXH 500H 600- iii) OXYGEN (ml/l) Figure 4j. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for August 1973 91 Geo 4747 Sut I 300 600H Figure 4k. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for October 1973 Figure 41. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for December 1973 Figure 4m. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for February 19?4 94 Geo 4747 O-r1-Sut I 200-1 300 400H 500-600 200 300 400 500 600 200 300H 400-1 500 600 Figure 4n. Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for March 1974 95 Geo 4747 Sut I 3 Bu I 2 3 4 5 6 7 8 Figure 4o. Longitudinal sections of (i) Salinity, ( i i i Temperature and ( i i i ) Oxygen for May 197^ 96 Geo 4747 Sut I 3 Bu I 2 3 4 5 6 7 8 Figure 4p, Longitudinal sections of (i) Salinity, ( i i ) Temperature and ( i i i ) Oxygen for June 1974 0 to IO m -50,000 L 400 «1« i n O II 1x1 J O O cn Contribution of dS/dz to stability Contribution of dT/dz to stability 150 to 200 m Figure 5. Relative effects of salinity and temperature variations with depth in establishing the stability of the water column at station 2 in Bute 98 Figure 6. Longitudinal sections of density ( afe) for Sut i l and Bute in (a) June 1973 and (b) December 1973 3020 30.40 30.60 30.80 7.5 9.0 8.5 8.0 7.5 9.0 8.5 8.0 7.5 T i r c) October 1972 » r cf sOtOO &• 0,=,'24 Salinity (%o) 30.20 1 r 30.40 30.60 30.80 -1 1 1 d) December 1972 1 , , _, , 400 -'50 _ 400 , ' „ - ' ' ^ O+V'24 -1 1 b) August 1972 I I I , , J" / / -150 -150 a , - ' 24 f) February 27, 1973 200 ^ " ^ 0 4 0 0 150 jS* . - a - —^oo OH'24 Figure 7. a)-f) Temperature-salinity characteristics of the waters below the upper layer at station 2 in Bute and at station 3 in Sutil for the period June 1972 to February 1973 vo VO Salinity (%») Figure 7. g)-l) Temperature-salinity characteristics of the waters below the upper layer at station 2 in Bute, at station 3 in Sutil and at station 4747 in Georgia for the period March to December 1973 o o Salinity (%o) a. E 90 3020 30.40 T 1 1— m) February 1974 30.60 8.5 8.0 2 7.5 1 90 30.80 a»v24 8.5 8.0 7.5 n 1 r r n) M a r c h 1974 Q, =,'24 30.20 30.40 30.60 30.80 _,„._ — , .— p) June 1974 i 1— I '1 1 *' 200_o. * * .200 _ - -JZ^-—\ ~o- 300 ^_-^-r^2oo *G0 / * / / / Ot =/24 Figure 7. m)-p) Temperature-salinity characteristics of the waters'below'-therupper layer at station 2 in Bute, at station 3 in Sutil and at station in Georgia for the period February to June 1974 102 West Center Bu 2W 2WC 2C East Q. Q 100 200 A 300 400 500 A 600 A 2E i i i i i i Nautical Miles •//' //////////////////////////////// Figure 8. Transverse section of temperature (5C) at station 2 in Bute in February 19?4 a) Salinity (%») b) Temperature (°C) Figure.9. Short-term fluctuations of (a) Salinity and (b) Temperature below the upper layer at station 2 in Bute in June 19?4 o o o Q. E o-o-. J J A S 0 N D J F M A M J J A S O N D J F M A M J Georgia 1748 o—-o 200 m o—© 300 m A--A 350 m J 1 1 1 I ' I J 1 i L 1 I I I A J . . A - I J I I • ' J J A S O N D J F M A M J J A S O N D J F M A M J 1972 1973 1974 Figure 10a. Monthly values of (i) Temperature and ( i i ) Salinity at station 1748 in Georgia o o <D i _ Z3 O v. 0> Q. E 8.0 8.5 9.0 30.3 30.4 30.5 30.6 -5 30.7 >• 30.8 w 30.9 31.0 31.1 31.2 31.3 J L J L J 1 t , J J A S O N D J F M A M J J A S O N D J J 1 i i _ F M A M \ , o O r-d 1 b + i II II b" <J ' AT o ii) Georgia 4747 0---0 150 m «—© 200 m A ••••A 300 m J L J L i 1 1 1 1 1 1 i ' I • • i i i J J A S O N D J F M A M J J A S 0 N D J F M A M J 1972 1973 1974 Figure 10b. Monthly values of (i) Temperature and ( i i ) Salinity at station k-7k? in Georgia 5 d < < o t—I 1 1 1 1 1 1 1 1 I I I I 1 I I I I I I I I I I J J A S O N D J F M A M J J A S O N D J F M A M J 1972 1973 1974 Figure 10c. Monthly values of (i) Temperature and ( i i ) Salinity at station 1 in Sutil £ ON ' ' 1 ' — — I 1 1 1 1 1 1 I I • ' ' ' I ' I I I I I • J J A S O N D J F M A M J J A S O N D J F M A M J 1972 ^ 1973 1974 Figure lOd. Monthly values of (i) Temperature and ( i i ) Salinity at ^ station 3 in Sut i l o o £ 8 . 0 3 o. £ 8.5 9 . 0 3 0 . 3 3 0 . 4 3 0 . 5 3 0 . 6 3 3 0 7 £ 3 0 . 8 w 3 0 . 9 •31.0 31.1 31 .2 r-3 1 . 3 * -A r- o e ©-A--i) •A A-_J [_ J 1 L , J J ^ A S O N D J F M A M J J A S O N D J F M A M J i » b to ii) J I L J L B u t e 2 J 1 1 i •» ' <5 O 150 m 0---0 2 0 0 m ® © 4 0 0 m A---A 6 0 0 m J i i ' J J A S O N D J F M A M J J A S O N D J ' F M A M J 1972 1 9 7 3 1974 'l Figure lOe. Monthly values of (i) Temperature and ( i i ) Salinity at station 2 in Bute o co 109 23.7 238 23.9 24.0 24.1 23.7 g 238 § 239 o 24.0 o- -o Geo 1748- 200 m • o Geo 4747- 200 m Sut I - 200 m b) . . A A" * ••••A Sut 1- 200 m » — • Sut 3 - 400 m ' 'A~ * A.. .A.. A' - ...a' • -1 1 , 1 i 1 ' i i i i i i 1 I . 1 1 23.7 23.8 239 24.0 24.1 A-- - -A Sut 3 - 300 m o — o Bu 2 - 300 m o a Bu2 - 600m * A • *A. C T -1 I I I ' - 1 1 1 1 1 1 U I I I i I i _l I 1_ J J A S O N O J F M A M J J A S O N D J F M A M J 1972 1973 1974 Figure 11. Comparison of monthly values of density ( at) between (a) Stations 1748 and 4-747 in Georgia and station 1 in S u t i l , (b) Stations 1 and 3 in Sutil and (c) Station.3 in Su t i l and station 2 in Bute 

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