The Structure and Transport of the Agulhas Return Current Isabelle Jane Ansorge Submitted in fulfillment of the requirements of the Master of Science degree Department of Oceanography University of Cape Town 1996 University of Cape Town The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town I ABSTRACT The Agulhas Current flows along the eastern coast of southern Africa as one of the largest western boundary currents in the world's ocean. On overshooting the southern tip of Africa at approximately 20°E, a retroflection loop is formed that causes the current to double back on itself and to fonn the easterly flowing Agulhas Return Current. No focussed investigation to establish the hydrography of this imponant component of global ocean circulation has to date been carried out. To trace the Agulhas Return Current's passage eastwards into the South Indian Ocean, data collected from six CTD hydrographic cruises; AJAX, ARC, SCARC, Marathon, Discovery 164 and SUZIL, as well as from three XBT temperature cruises,· Marion 83, Gallieni 72 and FIBEX, have been analysed. It is shown that the Agulhas Return Current initially flows zonally from 20°-25°E at a latitude approximately 39°30'-40°30'S before forming a planetary wave over the Agulhas Plateau between 25°-28°E. This wave results in a nonhward shift of the · Current to 37°S. Funher eastward the Current continues it's zonal flow more or less at 40°S, except that over the Mozambique, Madagascar and Southwest Indian Ridges similar waves are evident. Geostrophic speeds (referenced to the bottom) of the Agulhas Return Current show a gradual decrease from an average of 65 cmls in the retroflection region to 17 cmls at 76°E. Volume transpons at the same depth, are similarly reduced from an average of 60 Sv in the retroflection region, to 23 Sv at 76°E. II Temperature-salinity properties show water masses characteristic of the Agulhas Current, > 16 ° C and > 35, 4 psu, to extend as far east as 61 °E. East of this longitude, at approximately 66°E, Subantarctic Swface Water is shown to "cap" the swface waters, as the Agulhas Return Current weakens beyond recognition. Based on these hydrographic results, it is clear that the Agulhas Return Current may be considered to extend across the width of the Southwest Indian Ocean, terminating between 66°E and 76°E. I therefore propose that the name "South Indian Ocean Current", given by Lutjeharms and Van Ballegooyen (1984) and later by Stramma (1992) for the flow just north of the Subtropical Convergence in the Indian Ocean, be retained for the flow east of 70°E only. III ACKNOWLEDGEMENTS I would like to thank Professor J. R. E. Lutjeharms and Mr Henry Valentine for their constructive criticism, guidance and support. A special thank you is extended to Mr Roy van Ballegooyen for his invaluable advice. I thank my fellow students Stefan Kohrs, Craig Matthysen and Andrew Lee-Thorp for their office antics and the many jokes we shared. I also thank Mrs Lesley Staegemann for help and patience with my administrative problems. The financial support of the Foundation for Research and Development and the Department of Environmental Affairs is gratefully acknowledged. CONTENTS IV PAGE Abstract I Acknowledgements ill Contents IV List of Figures V Chapter 1: Chapter 2: Present knowledge of the Agulhas Return Current 4 Chapter 3: Knowledge gaps on the Agulhas Return Current 18 Chapter 4: Data and methodology 22 Chapter 5: Geographic location and general hydrography of the Agulhas Return Current 3 6 Chapter 6: Geostrophic velocities and the volume transported by the Agulhas Return Current 67 Chapter 7: Water masses of the Agulhas Return Current 116 Conclusion 150 References 152 Addenda 162 V LIST OF FIGURES FIGURE PAGE 1.1 A conceptual diagram of the Agulhas Current System 1 2.1 Kerhallet' s 1851 chart of the Agulhas Current System 4 2.2 Petermann's 1865 chart of the Agulhas Current System 5 2.3 Merz' s 1925 chart of the Agulhas Current System 6 2.4 Dietrich's 193 5 chart of the Agulhas Current System 7 2.5 Track showing the path taken by buoy 1210 10 2.6 The l 5°C isotherm and the track of the torpedo-shaped drifter 11 2.7 1976 GOSSTCOMP charts of the Sub-Tropical Convergence 12 2.8 Flow field in the South Indian Ocean 14 4.1 Distribution of CTD and XBT stations 23 4.2 Distribution of CTD stations during AJAX 24 4.3 Distribution of CTD and XBT stations during SCARC 26 4.4 Distribution of CTD stations during ARC and XBT stations during Marion 1983 28 4.5 Distribution of CTD stations during Marathon 29 4.6 Distribution of CTD stations during Discovery 164 30 4.7 Distribution of CTD stations during SUZIL 31 5.1 Location of the AJAX transect 37 5.2a/b Potential temperature and salinity section during AJAX 38 5.3 Location of the three transects during ARC 40 5.4 Dynamic height anomaly of the sea surface during ARC 40 5.5a/b Potential temperature and salinity sections during ARC transect 1 41 5.6a/b Potential temperature and salinity sections during ARC transect 2 42 5.7 Temperature section during XBT cruise Marion 83 43 5.8a/b Potential temperature and salinity sections during ARC transect 3 45 5.9 Dynamic height anomaly during SCARC 46 5.10 Location of the two transects during Marathon 47 VI 5.1 la/bPotential temperature and salinity sections during Marathon transect 1 49 5.12a/bPotential temperature and salinity sections during Marathon transect 2 50 5 .13 Location ofMarion 83 and FIBEX XB T tracks 51 5.14 Temperature section during XBT line Marion 83 52 5.15 Temperature section during XBT lineFJBEX 52 5 .16a/b General location and passage of the Agulhas Return Current from isotherms and isohalines 53 5 .17 Location of the Discovery 164 transect 54 5 .18a/b Potential temperature and salinity sections during Discovery 55 5.19 Location of the Gallieni XBT line 57 5.20 Temperature section between XBT 1-25 57 5.21 Location of the four transects during SUZIL 58 5.22a/b Potential temperature and salinity sections during SUZJL transect 1 60 5.23a/b Potential temperature and salinity sections during SUZIL transect 2 61 5.24a/b Potential temperature and salinity sections during SUZIL transect 3 63 5.25a/b Potential temperature and salinity sections during SUZIL transect 4 64 6.1 Geostrophic velocities during AJAX 71 6.2a-c Location of AJAX(a), geostrophic velocities (b) and volume transport (c) 72 6.3 Geostrophic velocities during ARC transect 1 76 6.4a-c Location of ARC transect 1 (a), geostrophic velocities (b) and volume transport ( c) 77 6.5 Geostrophic velocities during ARC transect 2 78 6.6a-c Location of ARC transect 2 (a), geostrophic velocities (b) and volume transport ( c) 79 6. 7 Geostrophic velocities during ARC transect 3 81 6.8a-c Location of ARC transect 3 (a), geostrophic velocities (b) and volume transport ( c) 82 6. 9 Geostrophic velocities during Marathon transect 1 86 6. lOa-c Location ofMarathon transect 1 (a), geostrophic velocities (b) and volume transport ( c) 87 VII 6.11 Geostrophic. velocities during Marathon transect 2 89 6.12a-c Location of Marathon transect 2 (a), geostrophic velocities (b) and volume transport ( c) 90 6.13 Geostrophic velocities during Discovery 164 92 6.14a-c Location of Discovery (a), geostrophic velocities (b) and volume transport ( c) 93 6.15 Geostrophic velocities during SUZIL transect 1 96 6. l 6a-c Location of SUZIL transect 1 (a), geostrophic velocities (b) and volume transport ( c) 97 6.17 Geostrophic velocities during SUZIL transect 2 99 6.18a-c Location of SUZIL transect 2 (a), geostrophic velocities (b) and volume transport ( c) 100 6.19 Geostrophic velocities during SUZIL transport 3 102 6.20a-c Location of SUZIL transect 3 (a), geostrophic velocities (b) and volume transport ( c) 103 6.21 Geostrophic velocities during SUZIL transect 4 106 6.22a-c Location of SUZIL transect 4 (a), geostrophic velocities (b) and volume transport ( c) 107 6.23 Geostrophic velocities during SUZIL transect 5 109 6.24a-c Location of SUZIL transect 5 (a), geostrophic velocities (b) and volume transport ( c) 110 6.25a/b Calculated surface velocities (a) and volume transport (b )for all stations within the Agulhas Return Current 112 6.26 Volume transports for all stations within the Agulhas Return Current 114 7. la/b TIS (a) and TIO plots for the western Indian Ocean 117 7.2 Distribution of Agulhas Current (inflow) CTD stations 121 7.3a TIS plot of the Agulhas inflow CTD stations 122 7.3b T10 plot of the Agulhas inflow CTD stations 123 7.4 Distribution of AJAX CTD stations 125 7.5a TIS plot of the AJAX CID stations 128 VIII 7. 5b T10 plot of the AJAX CTD stations 129 7. 6 Distribution of Agulhas Return Current (outflow) CTD stations 131 7. 7 a TIS plot comparing Agulhas Current (inflow) CTD stations to Agulhas Return Current (outflow) CTD stations 133 7. 7b TIO plot comparing Agulhas Current (inflow) CTD stations to Agulhas Return Current (outflow) CTD stations 134 7.8 Distribution of Discovery 164 and SUZJL CTD stations 137 7.9a TIS plot comparing Agulhas Return Current (outflow) CTD stations to Discovery CTD stations 17-18 139 7.9b TIO plot comparing Agulhas Return Current (outflow) CTD stations to Discovery CTD stations 17-18 140 7.10a TIS plot comparing Agulhas Return Current (outflow) CTD stations to Discovery frontal stations 141 7.10b TIO plot comparing Agulhas Return Current (outflow) CTD stations to Discovery frontal stations 142 7 .11 a TIS plot comparing Agulhas Return Current (outflow) CTD stations to SUZJL CTD stations 145 7 .11 b T10 plot comparing Agulhas Return Current (outflow) CTD stations to SUZIL CTD stations 146 A.la Potential temperature section of AJAX 162 A.