SOUTH-ATLANTIC CIRCULATION IN A WORLD OCEAN MODEL

The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agre ement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather ba rotropic Antarctic Circumpolar Current (ACC) through the Drake Passage , and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates o f the mass transport shed westward at the Agulhas retroflection. Topog raphic steering of the ACC dominates the structure of flow in the circ umpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and f resher Subantarctic surface water (originating in the ACC). The Bengue la Current is seen to modify its flow and fate with depth; near the su rface it flows north-westwards bifurcating most of its transport north ward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the wate r column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the B enguela Current extension encounters the coast of South America. At in termediate levels, these northward currents trace the flow of Antarcti c Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the mode l's rather coarse resolution, some subtle features of the Brazil-Malvi nas Confluence are simulated rather well, including the latitude at wh ich the two currents meet. Conceptual diagrams of the recirculation an d interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and is obaric surfaces. This analysis shows how the return path of NADW is pa rtitioned between a cold water route through the Drake Passage (6.5 Sv ), a warm water route involving the Agulhas Current shedding thermocli ne water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).