f University Microfilms, A XEROX Company, Ann Arbor, Michigan [

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1 ..., _.._...- _.- _.._-_ ,218 DUNCAN, Charles Peter, 1940 THE AGULHAS CURRENT. University of Hawaii, Ph.D., 1970 Ocean.ography I! f University Microfilms, A XEROX Company, Ann Arbor, Michigan [ THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED

2 THE AGULHAS CURRENT A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN OCEANOGRAPHY SEPTEMBER 1970 By Charles Peter Duncan Dissertation Committee: Klaus Wyrtki, Chairman Gunter Dietrich Colin S. Ramage Keith E. Chave Brent Gallagher

3 ABSTRACT The Agulhas Current, which is the western boundary current of the South Indian Ocean, is here considered as an integral part of the subtropical gyre whose circulation and water masses are influenced by seasonal variations in the meteorology unique to the Indian Ocean. In this study 3400 hydrological stations in the southwestern Indian Ocean were used. The depth to which motion in the Agulhas Current may be traced is so great that 2500 decibars was chosen as a primary reference level for geostrophic calculations. The vertical distribution of velocity in the current is so constant, however, that geostrophic transports and velocities to 2500 decibars may be accurately estimated by reference to the 1000 decibar level, and accurate surface flow patterns may be obtained with any choice of reference level. Changes in the pressure field in the current are strongly reflected in changes in the temperature field. Accurate estimates of geostrophic transports may therefore be made from temperature observations. Maps of dynamic topography reveal the dependence of the Agulhas Current on the South Equatorial Current as affected by changing meteorological conditions, variations iii

4 iv in the width of the South Equatorial Current, and the existence of three large anticyclonic eddy systems in the southwestern Indian Ocean. The Agulhas Current appears as a rapid flow 1500 kilometers long and 300 kilometers wide between the South African coast and the topographic high of the largest of these eddies. The subtropical gyres of the Atlantic and Indian oceans are shown to be separate and distinct, little or no continuous flow taking place from east to west around the southern tip of Africa. The Agulhas Current turns east at about 40 0 s, 20 0 E and flows eastward with the West Wind Drift, the boundary between the two currents approximately coinciding with the Subtropical Convergence. Current profiles and maps of volume transport are used to trace the flow of water from the broad, shallow South Equatorial Current to the narrow, deep Agulhas Current where transports may be as high as 100 megatons/ sec during the southern winter and 80 megatons/sec in summer. Seasonal and shorter time-scale variations in the Agulhas Current indicate that cyclonic eddies inshore and anticyclonic eddies offshore are common, resulting in rapid changes in the temperature and velocity fields. The water masses in the Agulhas Current are dominantly Tropical Surface Water and Subtropical Surface Water whose flow into the

5 v system depends on seasonal variations in the South Equatorial Current. Tropical Surface Water flows mainly into the northern end of the Madagascar Channel, in greatest quantities in the southern winter, while Subtropical Surface Water enters the Agulhas Current system only past the southern tip of Madagascar, in greatest quantities in the same season. Antarctic Intermediate Water enters the system only from the east. water mass of Red Sea origin flows south down the Madagascar Channel, being observed as far as 38 s. The flow of high-salinity water may playa major role in the salt budget of the northern Indian Ocean.

6 TABLE OF CONTENTS ABSTRACT LIST OF TABLES iii. viii LIST OF ILLUSTRATIONS. ix CHAPTER I. INTRODUCTION A. B. C. Water Masses in the Region Meteorology and Surface Currents of the Southwest Indian Ocean Research Objectives... II. VERTICAL STRUCTURE OF THE CURRENT III. A. Choice of a Primary Reference Level. B. C. D. DYNAMICS. Vertical Structure Based on 2500 db as Reference Level. Distribution of transport and velocity with depth Reference levels to reveal surface flow patterns Choice of a Secondary Reference Level. Availability of data Correlation between transports calculated from primary and secondary levels Thermal Structure and Flow A. Surface Currents ( i ) The inshore countercurrent (ii) The rapid flow (iii) The geost rophic flow B. Transport s.... (i) The South Equatorial Current (ii) The Zanzibar branch of the South Equatorial Current. PAGE vi

7 vii CHAPTER PAGE III. B. (Continued) (iii) The Mozambique branch... (iv) The East Madagascar Current. (v) Westerly flow south of Madagas car... (vi) The Agulhas Current... (vii) The Agulhas Eddy (viii) The return Agulhas Current and the West Wind Drift (ix) Northeasterly return flow C. Variability... IV. WATER MASSES. A. Tropical Surface Water B. Subtropical Surface Water C. Antarctic Intermediate Water D. Red Sea Water V. CONCLUSIONS VI. VII. APPENDIX. BIBLIOGRAPHY

8 LIST OF TABLES TABLE PAGE 1 Geostrophic Transports in the Southwest Indian Ocean relative to 1000 decibars. 43 viii

9 LIST OF ILLUSTRATIONS FIGURE Frontispiece. PAGE Perspective view of the Agulhas Current south-southeast from Durban, from direct current measurements taken during a cruise of the R. K. FRAAY 22 October 1966 xii 1. Dynamic topography of the sea surface relative to 2500 decibars in dynamic centimeters during a cruise of S.A.S. NATAL, April Dynamic topography of the relative to 2500 decibars meters during a cruise of April decibar surface in dynamic centi S.A.S. NATAL, 3. Vertical distribution of velocity in percent of surface velocity and percentage contribution to total transport by 500 meter depth intervals Dynamic topography of the sea surface relative to 2500 decibars in dynamic centimeters during a cruise of S.A.S. NATAL, April Relationship between geostrophic transports from the surface to 2500 meters referred to 2500 decibars (M~500) and geostrophic transports from the surface to 1000 meters referred to 1000 decibars (M~OOO) for 11 station pairs across the core of the Agulhas Current Depth of the 15 C isotherm in meters and inferred circulation during a cruise of S.A.S. NATAL, April Relationship between geostrophic transports from the surface to 2500 meters referred to 2500 decibars (M O ) and the factor F = ~z 2500 f ix

10 x FIGURE where ~Z is the change in depth of the 15 C isotherm between two stations, and f is the Coriolis parameter. PAGE Vertical section of velocities from current meter observations south-southeast of Durban, 5 November Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Summer (December, January, February) Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Autumn (March, April, May) Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Winter (June, July, August) Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Spring (September, October, November) Geostrophic transports from the surface to 1000 meters referred to 1000 decibars during a cruise of R. S. AFRICANA II, March Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Summer (December, January, February) Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Autumn (March, April, May) Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Winter (June, July, August) Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Spring (September, October, November) Distribution of geostrophic velocity and salinity with depth at selected locations in the Southwest Indian Ocean. 61

11 xi FIGURE PAGE 19. Geostrophic transports from the surface to 1000 meters referred to 1000 decibars during a cruise of S.A.S. NATAL, July Two vertical sections of velocity from current meter observations along the same line southsoutheast of Durban, 28 June Stations 11 to 15 were observed on the outward leg and stations 16 to 20 were done on the homecoming leg on the same day Surface salinity (%0) November to March Surface salinity (%0) May to September Salinity (%0) at the at = 25.8 surface in Summer (December, January, February) Salinity (%0) at the at = 25.8 surface in Autumn (March, April, May) 25. Salinity (%0) at the at = 25.8 surface in Winter (June, July, August) Salinity (%0) at the at = 25.8 surface in Spring (September, October, November). 27. Salinity (%0) at the at = 27.2 surface in Summer (December, January, February) Salinity (%0) at the at = 27.2 surface in Autumn (March, April, May). 29. Salinity (%0) at the at = 27.2 surface in Winter (June, July, August) Salinity (%0) at the at = 27.2 surface in Spring (September, October, November)

12 xii ~o 30 CM/SEC ~ ~ ~ 1500 ~ OISTANCE OFF SHORE IN KILOMETERS " "-::;:::-....,'\;:~ Frontispiece. Perspective view of the Agulhas Current southsoutheast from Durban, from direct current measurements taken during a cruise of the R. K. FRAAY 22 October 1966.

13 CHAPTER I INTRODUCTION The Agulhas Current is the western boundary current of the Indian Ocean. It flows southwards along the east coast of South Africa until the land mass ceases to influence its flow at 36 s. The current has been studied piecemeal by various authors (Dietrich, 1935; Clowes, 1950; J. Darbyshire, 1964; Orren, 1966) but no comprehensive work has been done on the system as a whole, and the effects of the exceptional conditions prevailing in the Indian Ocean have largely been ignored. A brief description of these conditions follows to facilitate the reading of the text; but in neither the introduction nor the text itself will the general features of a western boundary current or the basic hydrology of the region be given because these topics are adequately covered by Stommel (1965) and the forthcoming Oceanographic Atlas of the International Indian Ocean Expedition (I.I.O.E. Atlas) which will be extensively used in support of arguments presented here. A. Water Masses in the Region In the southern Indian Ocean the Tropical Surface Water is characterized by high temperatures (greater than 1

14 2 23 C) and low salinities (less than 35.0% 0 ), SUbtropical Surface Water, having been formed at the surface by an excess of evaporation over precipitation, is usually identified by its characteristically high salinity (greater than 35.5% 0 ), often as a subsurface salinity maximum beneath the Tropical Surface Water. The Antarctic Intermediate Water spreads northwards in the Indian Ocean from the Polar Front at depths between 800 and 1200 meters until the characteristic salinity minimum of this water mass is no longer discernible at the equator (Wyrtki, 1968). In the northern Indian Ocean the place of the Antar<~tic Intermediate Water is taken by water of Red Sea origin having a similar density (crt = 27.2). The Red Sea water, typified by high s alinities and low oxygen content, may be traced by either property. Some Atlantic Deep Water enters the southern Indian Ocean around the southern tip of Africa but it lies too deep to flow northward through the Madagascar Channel and is trapped in the cul-desac of the Natal Basin. Antarctic Bottom Water extends throughout the whole ocean at depths greater than 4000 meters (I.I.O.E. Atlas). B. Meteorology and Surface Currents of the Southwest In di 8,n Ocean In the southern winter an atmospheric high extends from South Africa to Australia, and a low exists over Asia.

15 3 The resultant winds are strong southeasters from 30 0 S to the equator, and north of the equator they turn to become the southwest monsoon. The South Equatorial Current, under the influence of these winds, attains its greatest northward extent and provides a supply for the Somali Current and the Monsoon Drift which are driven by the southwest monsoon (Koninklijk Nederlands Meteorologisch Instituut, 1952). This is the dry season for southern Africa with the single exception of the Cape of Good Hope which falls under the Atlantic regime and has a Mediterranean climate (Thompson, 1965). In the southern summer the South Indian Ocean atmospheric high is not as well developed as it is during winter, and it is displaced to the southeast. A high pressure region exists over Asia and the broad equatorial trough of low pressure extends from Africa to Sumatra. The resulting winds are the northeast monsoon in the north, and easterlies in the south, providing southern Africa with its rainy season (Thompson, 1965). The South Equatorial Current is weakest during this season. There is a southerly flow down Equatorial Current. the Somali Coast, joined by the North The northern branch of the South Equatorial Current joins the southward flow in the vicinity of Mombasa. Some water enters the Madagascar Channel but most returns to the east as the Equatorial Countercurrent

16 4 (Koninklijk Nederlands Meteorologisch Instituut, 1952). To the south, the West Wind Drift flows steadily eastwards, under the influence of the westerly winds which prevail on the poleward side of the subtropical atmospheric high pressure belts of the South Atlantic and Indian oceans. Since the Agulhas Current is in the southern hemisphere, "summer" throughout this paper refers to the months of December, January and February, and "winter" refers to the months of JUly, August and September. C. Research Objectives Previous work on the Agulhas Current has been limited by the lack of available data, and conflicting opinions are reported in the literature. This study is based on 3400 hydrological stations in the southwestern Indian Ocean, not less than 650 stations being available in any season. The main data source has been the revised and corrected data from which the Oceanographic Atlas of the International Indian Ocean Expedition was prepared. Supplementary data in the Indian Ocean were obtained from the publications of the Division of Sea Fisheries, South Africa, and in the Atlantic Ocean from the National Oceanographic Data Center, Washington, D. C. This large amount of data makes it possible to consider the Agulhas Current system as a whole, so that this study will attempt to:

17 5 (i) Provide a definition of the Agulhas Current system, its structure and its vertical and horizontal extent. (ii) Determine the source waters of the (iii) Agulhas Current. Determine the mass transport of the Current and the variation of transport with time. (iv) Determine what proportion of the water brought south by the Agulhas Current rounds the Cape of Good Hope into the Atlantic Ocean, how much is contained in eddies, whether or not these eddies are permanent, and how much of the Current returns to the east and north.

18 CHAPTER II VERTICAL STRUCTURE OF THE CURRENT A. Choice of a Primary Reference Level Initially a reference level of 2500 meters was chosen for the calculation of geostrophic velocities and mass transports. This choice was somewhat arbitrary, but was based on the following: (i) The sources of supply for the Agulhas Current through the Madagascar Channel and south of Madagascar are cut off at this depth by the bottom topography (Dietrich and Ulrich, 1968); (ii) Unless large-scale vertical motions occur, the flow of North Atlantic Deep Water into the cul-de-sac of the Natal Basin at about 2500 meters must be slow, and the presence and distribution of this water mass in the Basin indicate that no great southerly movement exists (I.I.O.E. Atlas, map of salinity at 2500 m). The oxygen minimum layer at about 1500 meters was not chosen as a reference level because there is a plentiful supply of Red Sea water of low oxygen content in the northern Indian Ocean so that an oxygen minimum layer could 6

19 7 could indicate strong advection (I.I.O.E. Atlas, map of oxygen at 1500 m). Above the oxygen minimum lies the Antarctic Intermediate Water, which could not be chosen as a primary reference level because this water mass, in the Agulhas Current region, moves in the direction of the surface waters (Clowes, 1950; Wyrtki, 1968). Figures 1 and 2, which show the dynamic topography of the surface and of the 1500 dh level for a single cruise, both relative to 2500 db, indicate that there is substantial flow at 1500 meters, and that this flow is in the same direction as the surface flow. The primary reference level must be chosen deeper than 1500 db, and 2500 db is reasonable. B. Vertical Structure Based on 2500 db as (i) Reference Level Distribution of transport and velocity with depth Figure 3 gives the average distribution of mass transport and velocity with depth in the Agulhas Current, calculated from 11 pairs of stations. Current direction was uniform to 2500 meters in all cases. The paucity of data results from the conditions imposed that both stations of any pair should come from the same line to preserve the usefulness of fairly synoptic data, that the flow be in the main stream of the current, and that both stations reach

20 meters. Values are given in percentages because transports between station pairs ranged from 10 megatons/ sec to 82 megatons/sec. This wide range exists ma~nly because station pairs did ~ot always span the entire width of the current, and should not be taken to indicate the variability of the Agulhas Current itself. The geometric similarity of the velocity curves was tested by comparing, for each station pair, the transport above 1000 meters with the total transports. The regression analysis showed (r = 0.999) that the flow in the upper 1000 meters consistently amounted to 80% of the total flow. The velocity profile given in Figure 3, therefore, is the typical curve for the Agulhas Current. The choice of 2500 db is seen to be a good one. (ii) Reference levels to reveal surface flow patterns If the vertical distribution of velocity shown in Figure 3 is generally true for the Agulhas Current region as well as the main flow, that is, if the velocity, at a depth is in the same direction as the surface velocity and is some constant fraction of that velocity, then it should be possible to base maps of dynamic topography on any arbitrary level and to deduce true flow patterns and velocities. Figure 4, which is a map of dynamic topography at the surface relative to 500 db, reveals the same

21 9 features of circulation as Figure 1, which is based on 2500 db. The offshore topographic high of the Agulhas Current system, the southerly flow of the Agulhas Current inshore, the return of water to the east, and the recycling of water by the large anticyclonic eddy are evident in both. The close similarity of flow patterns in the two maps (Figs. 1 and 4) and the greater horizontal gradients in Figure 1 imply that the distribution of velocity with depth is similar throughout the region. Therefore, any arbitrary level may be chosen as a base for geostrophic calculations of direction of flow in the Agulhas Current. This result makes it possible to regard earlier work with greater insight. J. Darbyshire (1964) considers that "it would... be reasonable to assume a level of no motion at about 600 m.," and uses 400 db and 1000 db reference levels. The current speeds which he reports are therefore too low, but his maps of dynamic topography do reveal the direction of surface flow. Similarly, the transport values reported by Dietrich (1935), using 1000 db as reference level, are underestimated but the flow patterns are correct. C. Choice of a Secondary Reference Level (i) Availability of data One thousand decibars was chosen as a secondary reference level because many more 1000 meter stations

22 10 (1410) are available in the southwest Indian Ocean than are 2500 meter stations (781). (ii) Correlation between transports calculated from primary and secondary levels To test the validity of using 1000 db as a secondary reference level, mass transports in the upper 1000 meters were calculated from the 11 station pairs in the core of the current, mentioned above, with 1000 db as the reference level (M~OOO) and these were compared with total mass transports between the surface and 2500 db a (M 2 )OO)' There is a very good correlation (Fig. 5) of r = which is significant at the.001% level, and which not only provides a basis for estimating total transports if the transports to 1000 db are known: = a M IOOO x 2.66 The correlation also means that transports calculated to 1000 db may with confidence be compared to one another. Wooster and Taft (1958), using 1000 db as reference level, calculated tha.t " for a pair of stations at 30 of latitude separated by 100 km, the [method] error... in geostrophic current speed is :!:-1.5 em/sec." The error in mass transport (M~OOO) between such a pair of stations could be as much as 0.75 megatons/eec. Low mass transports should therefore be viewed with caution.

23 11 The multiplier of 2.66 found by the slope of the line in Figure 5 agrees with the value of 2.67 calculated from the velocity distribution (Fig. 3) by a comparison of the shaded area with the whole. A velocity factor of 1.5, deduced from the same figure, may be used to multiply velocities referred to 1000 db to obtain estimates of the true values. D. Thermal Structure and Flow In the Agulhas Current, changes in the temperature field strongly reflect changes in the pressure field. This occurs because the minimum temperature change in the upper 1000 meters of the Agulhas Current is about 12 degrees Centigrade, while the maximum salinity change over this depth interval is about 1.2% 0 from the Subtropical Surface Water to the Antarctic Intermediate Water. Moreover, a well defined relationship exists between temperature and salinity in the region, especially below 300 meters (Orren, 1966). A comparison of the surface topograph~ relative to 2500 db (Fig. 1) with the depth of the 15 C isotherm shows that a very good approximation of circulation patterns may be gained from temperature maps (Fig. 6). Similar patterns (not shown) were obtained by plotting the depth of the 20 C and 10 C isotherm, indicating that the result is quite general. In a two-layer system an exact. correspondence exists

24 12 between the mean horizontal pressure gradient and the slope of the interface between the layers, and transports can be calculated from the equation Transport, T = mean velocity x area through which flow takes place ~ = g p D tj.z f where g is the acceleration of gravity; tj.p is the density difference between upper and lower layers; p is the density of the upper layer; D is the depth to which the flow reaches; tj.z is the change in depth of the interface between stations; f is the Coriolis parameter. The close correspondence between the pattern shown by the 15 C isothermal surface (Fig. 6) and that of the dynamic topography (Fig. 1) prompted a comparison of the total geostrophic transports with the factor F tj.z = f by analogy with the two-layer model above, used by Wyrtki and Kendall (1967), where tj.z here is the change in depth of the 15 C isotherm between stations on either side of the current. The relationship

25 13 o M 2500 = F o where M 2500 is in megatons/second and F is in units of 10 5 meter seconds was found (Fig. 7) with a correlation coefficient of r = which is significant at the 0:001 level. The analogy with a two-layer model is inexact because density and velocity vary continuously with depth in the Agulhas Current. The existence of such an extremely good correlation as r = 0.965, however, does indicate that a practical, rapid and inexpensive method of estimating geostrophic transports in the Agulhas Current has been found, because airborne expendable bathythermographs could be used to determine the depth of the 15 C isotherm. The accuracy of this method comrared with geostrophic calculations referred to 2500 db is as good as that found in the previous section by a comparison of transports based on 1000 db with total transports. Considering the accuracy of the geostrophic method itself and the cost and labor involved in obtainin~ the data, the temperature gradient method has much to recommend it. These results indicate that the flow of the Agulhas Current is well approximated by a model in which the slope of the 15 C isotherm is taken as proportional to the mean horizontal pressure gradient, and that the thermal structure of the Current dominates the dynamic topography.

26 CHAPTER III DYNAMICS A. Surface Currents Surface flow along the east coast of South Africa may conveniently be divided into three parts: a weak countercurrent close inshore, a narrow band of water whose high surface speed over the continental rise falls off rapidly with depth, and, offshore, a broad, deep region of water flowing southwestwards. Figure 8, which illustrates typical conditions at 30 0 S off the South African coast, was drawn from direct current observations taken from the R. K. FRAAY out of Durban, 5 November 1966, utilizing the "free drift" techni~ue. The underwater instrument consisted of a Savonius rotor, a vane referred to a magnetic compass and an electrical pressure gauge. The ship's position was determined by a shore-based radio navigational aid enabling current velocities to be determined with an accuracy of +10 em/sec. (i) The inshore countercurrent The inshore countercurrent shown in Figure 8 extends, as a surface current, to a distance of 20 km offshore. Sailing Directions (U. S. Hydrographic Office, 1951) and unpublished data (National Physical Research Laboratory, 14

27 15 South Africa) confirm this distance as being an upper bound for the lateral extent of the northeasterly-flowing, inshore countercurrent when it exists, and corroborate the low speeds of the current. Harris (1961), analyzing 68 observations taken over five years, concluded that the frequency of occurrence of the northeasterly inshore flow is 51% at 30 0 S, and that there is a 1'90% correlation between current and wind direction" in the vicinity of Durban. Seasonal changes in the position of the South Indian Ocean atmospheric high result in seasonal changes in wind direction along the east coast of South Africa (Thompson, 1965). The countercurrent is therefore more common in winter under the influence of southwesterly winds coincident to the easterly passage of cyclonic depressions from the Atlantic to the Indian Ocean when the continental atmospheric low is weakly developed (Mallory, 1961; U. S. Hydrographic Office, 1951). (ii) The rapid flow The highest speeds of about 200 cm/sec at the surface occur at the edge of the continental shelf (U. S. Hydrographic Office, 1951) in depths too shallow to allow the geostrophic approximation to be used for accurate velocity determinations (Shannon, 1966). Direct current measurements indicate that velocities decrease rapidly with depth and that the rapid flow is about 10 km in width, lying 30 to 40 km offshore (Frontispiece; Fig. 8).

28 16 Stavropoulos (1966) compared direct current measurements with maps of surface temperature prepared from airborne radiation thermometer measurements and found that large horizontal temperature gradients correlated with surface currents. His very nearly synoptic maps of surface temperature in the Agulhas Current off Durban (not reproduced) indicate that the rapid flow is seldom more than 10 km wide and that its distance offshore corresponds roughly, but not exactly, with the width of the continental shelf. (iii) The geostrophic flow The offshore region is the most susceptible to study by the dynamic method; therefore seasonal maps of the surface topography relative to 1000 db have been drawn for the entire southwest Indian Ocean. The gross features of the surface flow (Figs. 9, 10, 11, and 12) show that the western half of the subtropical gyre in the southern Indian Ocean is composed of the South Equatorial Current at 15 S, the narrow, rapid Agulhas Current along the southeast coast of Africa, and a broad northeasterly return flow from the confluence of the Agulhas Current and the West Wind Drift in about 40 0 s. Three large anticyclonic eddies are permanent features of the system: one in the Madagascar Channel (Menache, 1963), one to the east of Madagascar, and one off the southeast

29 17 coast of South Africa, forming part of the Agulhas Current. The seasonal variation of wind direction and strength between 20 0 S and the equator, mentioned in the Introduction, produces marked changes in the width of the South Equatorial Current. At the 60 0 E meridian, the current extends in winter from 21 0 S to 7 0 S with surface speeds of about 16 em/sec, but in summer the width of the current decreases to a third of this, extending from 17 S to 12 S with surface speeds of about 50 em/sec (Deduced from Figs. 9 and 11). At all seasons the axis of flow is roughly at the same latitude, 15 S, and it is about this axis that the South Equatorial Current is split by Madagascar, the division in flow occuring some 300 miles offshore in the vicinity of the island of Mauritius at 55 E (Figs. 9, 10, 11, and 12; U. S. Hydrographic Office, 1951). The northern branch of the divided flow is again divided by the mainland of Africa at approximately 9 0 S in summer and 110S in winter, the southerly branch of the twicedivided flow entering the Madagascar Channel. It is convenient to assign arbitrary names to the three branches of the divided South Equatorial Current. The northernmost may be termed the Zanzibar branch because it flows northwards along the African coast past the island of Zanzibar in 6 0 s. The flow which enters the Madagascar Channel from the north may be termed the Mozambique branch. The most

30 .18 southerly division, which flows south along the east coast of Madagascar, will be referred to as the East Madagascar Current. at about 5 0 S on A topographic low exists in all seasons the east coast of Africa so that the resultant flow of the Zanzibar branch of the divided South Equatorial Current is always northwards (Figs. 9, 10, 11, and 12). No connection, therefore, exists between the southerly flow at the surface along the coast of Somalia during the northeast monsoon and the Agulhas Current. The surface flow which enters the Madagascar Channel from the north makes its way south along the African coast as the Mozambique Current. Defant (1961) is of the opinion that" the Mozambique Current, with a tributary current from the east coast of Madagascar... form the source for the Agulhas Current at about 30 0 S... ". The flow from the most southerly branch of the South Equatorial Current, however, after flowing past the southern tip of Madagascar, may enter the Madagascar Channel from the south and flow northwards along the island coast (Figs. 9 and 12). In only one case (Fig. 11) does the dynamic topography suggest a simple confluence of two currents in the manner proposed by Defant (loc. cit.). Indeed, the Mozambique and Agulhas currents may at times be discontinuous (Fig. 10; M. Darbyshire, 1966). The surface flow leaving the Channel at 25 S has its origin

31 19 in the large anticyclonic system which exists within the Channel in all seasons (Figs. 9, 10, 11, and 12). The Agulhas Current assumes its identity as a western boundary current between 25 S and 30 0 S, depending on the form of the eddy system within the Madagascar Channel. (Compare Fig. 9 with Fig. 10.) The topographic high of the Agulhas Current lies at most 550 km offshore during autumn, and half of this distance during the other seasons. The widths of the Kuroshio and the Gulf Stream, as reported by Stommel (1965), lie within this range, indicating that the width of a western boundary current is not related to the width of the ocean in which it is contained. The Agulhas Current is 5 to 10 percent of the width of the subtropical gyre in the South Indian Ocean. but the Kuroshio is only 2% of the width of the North Pacific. Geostrophic velocities in the offshore region of the Agulhas Current may be as high as 100 cm(sec (Deduced from Fig. 1), but the average surface velocity from the coast to the topographic high is much less, between 30 and 40 em/sec (Deduced from Figs. 9, 10, 11, and 12 and corrected according to Chapter II, Section C). A very distinct low in the dynamic topography extends from the southernmost tip of the African Continent to the West Wind Drift in all seasons, indicating that the circulation of the Indian Ocean gyre is separate and distinct

32 20 from the Atlantic Ocean gyre (Figs. 9, 10, 11, and 12). The only permanent flow of any magnitude between the oceans is the West Wind Drift which flows eastwards into the Indian Ocean from the Atlantic. Typically the flow of the West Wind Drift from the Atlantic Ocean into the Indian Ocean is south of 43 S at the Greenwich Meridian, and does not vary greatly in latitude with season. In all seasons the Agulhas Current turns abruptly eastwards once it has left the land mass at 36 s, penetrating farthest to 41 0 S, 17 E in autumn (Figs. 9, 10, 11, and 12). In the region south of Africa, bounded by the great subtropical gyres to the east and west, and the West Wind Drift in the south, some intermingling of Atlantic and Indian Ocean water takes place. Over the Agulhas Bank the currents are "of rotating kind with tidal periods" (Welsh, 1964) but a seasonal reversal of surface currents was found by Duncan and Nell (1969). From this work, and isentropic analysis, Shannon (1966) came to the conclusion that vigorous mixing takes place over the Bank, the mixed water penetrating in summer past Cape Point and up the West Coast as far north as 32 S. In the deep ocean of this region large anticyclonic eddies may separate themselves from the main flow (Dietrich, 1935) and mix with water of Atlantic origin (Visser, 1969). Duncan (1968) considered that the eddy shown in Figure 13 disrupted the subtropical

33 21 convergence so that north of 41 8 and west of 18 E the convergence was not observed as a strong front. To the southeast of the eddy, however, the subtropical convergence was very marked, with the surface temperature changing 10 C in as many miles at 43 8, 20 0 E where the Agulhas Current returned to the east. A map of geostrophic transports has been prepared from the data of the only cruise to cover this region in detail (Fig. 13). The usefulness of using the geostrophic method, even in such a strong eddy system, is borne out by the coincidence of flow in and flow out of the region considered, to within 4%. The large eddy centered at 40 8, lsoe is seen to be an en~ity separate from the Agulhas Current and the main flow of either the Atlantic gyre or the West Wind Drift, although tributaries from both enter the eddy system. A very small amount (at most, 3 megatons/sec) may enter the eddy from the Agulhas Current at 42 8 and so enter the Atlantic circulation, but there is no continuous flow from east to west. The dynamic topography'maps for SUillmer and autumn (Figs. 9 and 10) indicate such eddies contained in the Atlantic circulation system, but apart from this and the coastal drift mentioned above, there is no constant flow of water from the Agulhas Current system into the Atlantic Ocean. The distribution of water masses (Chapter IV) confirms these conclusions. The

34 22 absence of large eddies in winter (Fig. 11) may indicate that their formation and dissipation occur seasonally. B. Transports To obtain maps of transports the potential energy anomaly referred to 1000 decibars was calculated for each station and the resultant values were plotted for each season. Figures 14, 15, 16, and 17 present the results schematically, the width of a flow line being used to represent the magnitude of the flow. The general pattern of transport is that of the surface circulation shown in the maps of dynamic topography. Care should be taken in making comparisons between different areas, because transport values calculated relative to 1000 db will underestimate true values, depending on the depth to which the current reaches. The Mozambique Current and the current which flows northwards up the west coast of Madagascar have largely been omitted from the following discussion because exact flow paths in the Madagascar Channel are uncertain and variable. Certainly a large volume of water flows into the southeastern end of the Channel and joins with the southerly flow from the northern end, but there appears to be no single axis of an eddy system, but rather a succession of eddies. Where uncertainty about flow paths existed, the maps were drawn to show merely transports

35 23 into and out of the Channel. In the western half of the South Indian Ocean nine branches of the circulation which are pertinent to a study of the Agulhas Current may be perceived. Figure 18, which should be compared with Figures 14, 15, 16, and 17, was prepared from selected stations across the nine branches of circulation shown in the sketch map. Wherever possible, calculations of geostrophic velocity and transport were made from station pairs taken on a single cruise and were chosen to illustrate representative features of the flow. The figure illustrating the westerly flow past the southern tip of Madagascar, however, was derived from non-synoptic data. (i) The South E~uatorial Current The South E~uatorial Current is a surface current (Fig. 18i) with transports between 54 megatons/sec in winter and 40 megatons/sec in summer (Table 1). High winter transports are maintained despite low surface speeds in this season (Chapter III, Section A iii) because of the great width of the current--over 1500 km (Fig. 11). Throughout the year the major origins of the Agulhas Current clearly lie in the two southernmost branches of the South E~uatorial Current. Note that the sum of the transports of the Zanzibar, Mozambi~ue and East Madagascar branches (Table 1) exceeds the transport of the South E~uatorial Current. This discrepancy arises because

36 24 transports calculated relative to 1000 db underestimate the volume transport of the South E~uatorial Current which extends to 2000 meters (Fig. 18i). ( i i) The Zanzibar branch of the South E~uatorial Current Off the African coast between 100S and 4 s the Zanzibar branch of the South E~uatorial Current exists as a surface current with high velocities near the coast (Fig. 18ii) and a width between 500 kilometers in autumn and 1100 kilometers in spring (Figs. 10 and 12). Velocities decrease rapidly with depth,(fig. 18ii) and distance (c offs h 0 r e (Fi gs. 9, 10, 11, an d 12). Transports in the surface current vary between 16 megatons/sec in winter and 34 megatons/sec in spring (Figs. 16 and 17). Transports are least in winter, when the flow of the South E~uatorial Current is greatest, because a greater proportion and amount of the flow is diverted into the Mozambi~ue branch with the seasonal variation in the position at which the flow divides at the African coast. A subsurface flow to the south exists beneath the surface flow to the north, extending from 800 meters to 2500 meters (Fig. 18ii). (iii) The Mozambi~ue branch The Mozambi~ue branch of the South E~uatorial Current attains its greatest volume of 26 megatons/sec in winter l but does not vary as greatly as the Zanzibar

37 25 branch, its least volume being the high value of 20 megatons/sec in summer (Figs. 14 and 16). This branch, on reaching the African coast, turns south as the Mozambique Current which reaches effectively to 1300 meters and is thus some 500 meters deeper than the Zanzibar branch (Figs. 18ii and 18iii). That the Mozambique Current extends to 1300 meters implies that the values of transport reported for the Current in Figures 14, 15, 16, and 17 are underestimates, because the transport maps were based on a 1000 decibar reference level. Surface speeds in the two coastal currents are similar, about 30 centimeters/sec, but total (M~500) transport in the Mozambique Current is greater, both because the current is unidirectional and because of its greater depth (Figs. 18ii and 18iii). (iv) The East Madagascar Current The southernmost branch of the South Equatorial Current becomes a narrow, rapid flow along the east coast of Madagascar (Figs. 9, 10, 11, and 12). Insufficient coastal stations exist for detail to be seen (except in winter, Fig. 18iv), but it appears likely that a miniature western boundary current exists along the southeast coast of Madagascar, to judge by the high transports between the coast and the topographic high which is located less than 200 kilometers offshore (Figs. 14, 15, 16, and 17). High

38 26 surface velocities are characteristic of the current, a value of about 30 centimeters/sec being deduced from Figure 9, and the current extends to 1400 meters (Fig. 18iv). (v) Westerly flow south of Madagascar Some of the water brought south by the East Madagascar Current is recycled by the eddy to the east of the island, but the greater part flows west towards the African coast in quantities never less than 18 megatons/ sec (Fi gs. 15 an d 1 7 ). Surface speeds are low and the current is negligible below 1500 meters but its lateral extent may be as much as 500 kilometers (Fig. 12) so that o total (M 2500 ) transports of the current may be large. (vi) The Agulhas Current The flow of the subtropical gyre of the South Indian Ocean attains its greatest depth of 2500 meters in the Agulhas Current (Figs. 3 and 18vi), deepening from the surface flow of the South Equatorial Current (Fig. l8i) to 1400 meters in its flow south along the coasts of Africa and Madagascar. The combined flow of these branches, together with a substantial recycling of water by the Agulhas Eddy (whose transport is never less than 9 megatons/sec) becomes the Agulhas Current in about 25 S (Figs. 14, 15, 16, and 1 7 ). The deepening of the flow on its way

39 27 south causes transport values to be underestimated, because the maps of transport are based on the 1000 decibar level. In Figure 16 and Table 1, for example, 26 megatons/ sec are reported as the flow in the Mozambique branch of the South Equatorial Current in winter, 20 megatons/sec as the westerly flow south of Madagascar, and 19 megatons/ sec are shown as being recycled by the Agulhas Eddy, yielding a total input for the Agulhas Current of 65 megatons/sec. The value reported in the Agulhas Current in Figure 16 and Table 1 is 32 megatons/sec, which is an underestimate. This value, however, when multiplied by the factor 2.66 derived in Chapter 2, puts the total geostrophic transport of the Agulhas Current in winter at 85 megatons/sec. The flow between the 1000 meter contour and the coast, which is not reported in the transport maps, amounts to about 15 megatons/sec (estimated by numerical integration over the velocity field shown in Fig. 8). The difference between the resultant value of 100 megatons/sec in the Agulhas Current and 65 megatons/sec input may be attributed to flow close inshore and to the underestimate of input transports obtained by using a 1000 decibar reference level. The Agulhas Current, then, is seen to be one of the large western boundary currents of the world, the minimum transport (geostrophic and coastal) of the current being about 80 megatons/sec in summer (Deduced

40 28 from Figs. 8 and 14). The Gulf Stream, by way of comparison, has been observed to vary seasonally between 76 and 93 megatons/sec (Defant, 1961), and values as high as 137 megatons/sec have been reported (Stommel, 1965). (vii) The Agulhas Eddy There is a permanent anticyclonic eddy about the topographic high of the Agulhas Current, with geostrophic transports (M~OOO) in the eastern arm of the eddy between 9 megatons/sec in summer and 19 megatons/sec in winter. The flow is deep (greater than 1000 meters) but very much slower in the east than the Agulhas Current (Fig. 18vii). Total geostrophic transports (M~500) may nevertheless be high in this branch of the circulation because of its width--the transport of 41 megatons/sec reported in Figure 18vii occurred between stations 450 kilometers apart, three times the normal width of the Agulhas Current. The waters in the Agulhas Eddy system therefore contribute a large part of the total southward flow of the Agulhas Current itself. At times as much as one-third of the water flowing south along the coast is recycled by the eddy. (viii) The return Agulhas Current and the West Wind Drift The confluence of the return Agulhas Current as it turns eastwards and the West Wind Drift takes place

41 29 at approximately 40 0 s, 20 0 E although the position may vary considerably, depending on the formation of eddies in the area (Figs. 9 through 17). The waters of the return Agulhas Current are considerably warmer than those of the West Wind Drift, a sharp drop in temperature from 17 C to 11 C taking place from north to south across the Subtropical Convergence which marks the boundary between the two currents (J. Darbyshire, 1964). The approximate position of the Convergence as ~ boundary is shown as a dashed line in Figures 14, 15, 16, and 17, deduced from the dynamic topography and transports. Note that the transports given in Figures 14, 15, 16, and 17 have been calculated only for the region shown. The West Wind Drift south of 50 0 S is not represented. The combined currents between 35 S and 50 0 S are a wide, deep, eastward flow of moderate average surface velocities (Figs. 9, 10, 11, 12, and 18viii) with transports of about 48 megatons/sec at 40 0 E decreasing eastwards as water returns to the northeast in the South Indian Ocean subtropical gyre. (ix) Northeasterly return flow In the southeast of the region under consideration a very slow flow takes place to the northeast as the waters turned to the east by the return Agulhas Current complete the cycle of the subtropical gyre (Figs. 14, 15, 16, 17, and 18ix).

42 30 C. Variability A pronounced seasonal variation exists in the South Equatorial Current~ transports being higher in winter during the season of strong southeast trade winds than in summer (54 and 40 megatons/sec respectively). Similar seasonal variations exist in the transports of the Mozambique branch and the East Madagascar Current (Table 1) which derive their flow from the South Equatorial Current. The East Madagascar Current is supplemented by the northeasterly return flow from the return Agulhas Current (Figs. 14~ 15~ and 17). The flow recycled by the Agulhas Eddy is also a maximum in winter so that the total deep geostrophic flow of the Agulhas Current (supplied by the Mozambique Current~ the East Madagascar Current and recycled water) is 85 megatons/sec in winter and 64 megatons/sec in summer. (Values from Table 1 increased by a factor of 2.66 according to Chapter II B.) Variations from the seasonal average also occur. Figure 19~ a map of transports in the Agulhas Current during July 1962~ illustrates how the momentary flow differs from the seasonal average flow depicted in Figure 16. The strongly flowing current along the coast~ the easterly return flow and the anticyclonic eddy of the Agulhas Current may be discerned~ but many small eddies are contained within these dominant features. These small eddies~ which

43 31 are found in all seasons, have no permanent existence, as a comparison of the flow in April (Fig. 1) with the flow in July of the same year (Fig. 19) shows. The two eddies (to the north in these maps) which appear to coincide, were not present in January 1963 (Darbyshire, 1964). On a shorter time scale are the variations discussed by Stavropoulos (1966). His observations of surface temperatures by airborne radiation thermometer indicate that eddies as large as 30 kilometers in diameter can form and dissipate in three days. Large changes in the velocity field can take place even more rapidly than this, as Figure 20 shows. On June 28, 1966, inshore currents were faster in the morning than in the afternoon, but offshore currents were slower. In the morning no countercurrent existed inshore, but by the afternoon a countercurrent of 25 em/sec was flowing. Any tidal effects will be included in the direct current measurements, but there appear to be real and large differences in the flow of the Agulhas Current itself because typical tidal velocities in the deep ocean are less than 10 em/sec (Defant, 1961) in contrast to the observed changes in velocity which are as great as 90 em/sec. Anderson (1967) observed that the western edge of the Current may change its position relative to the coast by as much as 20 kilometers, and that a significant correlation exists between the atmospheric

44 32 pressure gradient along the coast and the position of the edge of the Agulhas Current. The inshore countercurrent between the coast and the western edge of the Agulhas Current has been observed to change from northeasterly to southwesterly flow as often as nine times over a period of thirteen days (Anderson, 1961), leading to the suggestion that such rapid alternation may be caused by small-scale cyclonic eddies (Mallory, 1961). This problem is at present being investigated by the National Physical Research Laboratory of South Africa (F. P. Anderson, personal communication).

45 CHAPTER IV WATER MASSES Since so much of the volume transport in the Agulhas Current takes place in the upper 1000 meters, and since the North Atlantic Deep Water is assumed to be essentially motionless, a discussion of the water masses of the Agulhas Current becomes a discussion of Tropical and Subtropical Surface waters, Antarctic Intermediate Water, and Red Sea water. A. Tropical Surface Water Transports and extent of the South Equatorial Current are least in the summer months so that the extent of lowsalinity (less than 35.0% 0 ) Tropical Surface Water associated with the Current (Fig. 18i) is least in this season, both to the north and east of Madagascar, despite dilution of surface waters by the heavy summer rainfall and run-off (Fig. 21; Thompson, 1965). The greatest penetration of Tropical Surface Water into the AgUlhas Current occurs in the winter months when the flow of the South Equatorial Current is greatest, so that this water mass reaches to 20 0 S in its southerly flow down the Madagascar Channel, and small quantities enter the Channel around the 33