The main near-surface ocean circulations near South Africa are shown in Fig 1. The Mozambique/Agulhas current is relatively warm, whereas the Benguela current is cooled by coastal upwelling along the west coast of S. Africa and Namibia.
Fig 1. Ocean currents around South Africa (from (1)).
The upwelling is evidenced by the high phytoplankton concentration in the sea water off western S. Africa (Fig 2). Upwelling water is nutrient-rich, hence the high bio-productivity.
Fig 2: Combined cloud and ocean colour image of the South Atlantic ocean off Africa on 16 September 1997, by the SeaWiFS satellite (courtesy NASA/GSFC). The visible cloud image suggests stratus and stratocumulus over much of the South Atlantic, except just west of South Africa where the clearing allows a sampling of phytoplankton concentration. The highest concentrations are in red, the lowest in blue.
(1) Walker, N.D. 1989. Sea-surface temperature-rainfall relationships and associated ocean-atmosphere coupling mechanisms in the southern African region. Ph.D. thesis of the University of Cape Town.