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THE AGULHAS CURRENT. Funiversity Microfilms, A ........ , .....-.. _ .._...- _.- _.._-_ .... --- 71-12,218 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 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 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 calcu­ lations. The vertical distribution of velocity in the current is so constant, however, that geostrophic trans­ ports 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 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 Sub­ tropical 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 Surf­ ace Water and Subtropical Surface Water whose flow into the v system depends on seasonal variations in the South Equa­ torial Current. Tropical Surface Water flows mainly into the northern end of the Madagascar Channel, in greatest quantities in the southern winter, while Subtropical Surf­ ace 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 Madagas­ car 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. TABLE OF CONTENTS ABSTRACT iii LIST OF TABLES . viii LIST OF ILLUSTRATIONS . ix CHAPTER PAGE I. INTRODUCTION 1 A. Water Masses in the Region 1 B. Meteorology and Surface Currents of the Southwest Indian Ocean 2 C. Research Objectives ... 4 II. VERTICAL STRUCTURE OF THE CURRENT 6 A. Choice of a Primary Reference Level . 6 B. Vertical Structure Based on 2500 db as Reference Level . 7 Distribution of transport and velocity with depth 7 Reference levels to reveal surface flow patterns 8 C. Choice of a Secondary Reference Level . 9 Availability of data 9 Correlation between transports calculated from primary and secondary levels 10 D. Thermal Structure and Flow 11 III. DYNAMICS . 14 A. Surface Currents 14 ( i ) The inshore countercurrent 14 (ii) The rapid flow 15 (iii) The geost·rophic flow 16 B. Transport s .... 22 (i) The South Equatorial Current 23 (ii) The Zanzibar branch of the South Equatorial Current . 24 vi vii CHAPTER PAGE III. B. (Continued) (iii) The Mozambique branch ... 24 (iv) The East Madagascar Current. 25 (v) Westerly flow south of Madagas car ... 26 (vi) The Agulhas Current ... 26 (vii) The Agulhas Eddy . 28 (viii) The return Agulhas Current and the West Wind Drift 28 (ix) Northeasterly return flow 29 C. Variability .... 30 IV. WATER MASSES. 33 A. Tropical Surface Water 33 B. Subtropical Surface Water 34 C. Antarctic Intermediate Water 37 D. Red Sea Water 37 V. CONCLUSIONS 39 VI. APPENDIX. 42 VII. BIBLIOGRAPHY . 74 LIST OF TABLES TABLE PAGE 1 Geostrophic Transports in the Southwest Indian Ocean relative to 1000 decibars . 43 viii LIST OF ILLUSTRATIONS FIGURE PAGE Frontispiece. 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 1962 .. 44 2 . Dynamic topography of the 1500 decibar surface relative to 2500 decibars in dynamic centi­ meters during a cruise of S.A.S. NATAL, April 1962 . 3. Vertical distribution of velocity in percent of surface velocity and percentage contribu­ tion to total transport by 500 meter depth intervals ......... 46 4. Dynamic topography of the sea surface relative to 2500 decibars in dynamic centimeters during a cruise of S.A.S. NATAL, April 1962 . 47 5. Relationship between geostrophic transports from the surface to 2500 meters referred to 2500 decibars (M~500) and geostrophic trans­ ports from the surface to 1000 meters referred to 1000 decibars (M~OOO) for 11 station pairs across the core of the Agulhas Current . 48 6. Depth of the 15°C isotherm in meters and inferred circulation during a cruise of S.A.S. NATAL, April 1962 .... 49 7. Relationship between geostrophic transports from the surface to 2500 meters referred to O 2500 decibars (M ) and the factor F = ~z 2500 f ix x FIGURE PAGE where ~Z is the change in depth of the 15°C isotherm between two stations, and f is the Coriolis parameter . 50 8. Vertical section of velocities from current meter observations south-southeast of Durban, 5 November 1966. 51 9. Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Summer (December, January, February) 52 10. Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Autumn (March, April, May) 53 11. Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Winter (June, July, August) 54 12. Dynamic topography of the sea surface relative to 1000 decibars in dynamic centimeters during Spring (September, October, November) 55 13. Geostrophic transports from the surface to 1000 meters referred to 1000 decibars during a cruise of R. S. AFRICANA II, March 1964 56 14. Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Summer (December, January, February) 57 15. Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Autumn (March, April, May) 58 16. Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Winter (June, July, August) 59 17. Geostrophic transports from the surface to 1000 meters referred to 1000 decibars in Spring (September, October, November) 60 18. Distribution of geostrophic velocity and salin­ ity with depth at selected locations in the Southwest Indian Ocean . 61 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 1962 62 20. Two vertical sections of velocity from current meter observations along the same line south­ southeast of Durban, 28 June 1966. 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 63 21. Surface salinity (%0) November to March 64 22. Surface salinity (%0) May to September. 65 23. Salinity (%0) at the at = 25.8 surface in Summer (December, January, February) .. 66 24. 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) .. 68 26. 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) .. 70 28. Salinity (%0) at the at = 27.2 surface in Autumn (March, April, May) . 71 29. Salinity (%0) at the at = 27.2 surface in Winter (June, July, August) .. 72 30. Salinity (%0) at the at = 27.2 surface in Spring (September, October, November). 73 xii ~o 30 CM/SEC . 0 1000 ~ ~ ~ 1500 ~ " "-::;:::- . ...... ,'\;:~. - OISTANCE 2500 OFF SHORE IN KILOMETERS 3000 Frontispiece. 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. 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 influ­ ence its flow at 36°s.
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