The Agulhas current

Typescript.Bibliography: leaves 74-76.xii, 76 l charts, graphs, maps, tablesThe 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 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°s, 20° 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 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