General biological features of the South Atlantic
East Africa coastal
—Physical processes
The east coast of South Africa is bounded by the fast, southward flowing Agulhas Current which is composed of warm, equatorial Indian Ocean water. The inner edge of the Agulhas Current tracks the edge of the continental shelf, which is close inshore north of East London, before moving offshore and westward along the edge of the Agulhas Bank to retroflect in the region of 16 to 20°E (Lutjeharms and van Ballegooyen, 1988). The Agulhas Bank is a roughly triangular extension of the continental shelf off southernmost South Africa. The western part of the region (West of Cape Agulhas) is considered to be part of the Benguela ecosystem (Shannon and Nelson, 1996) while the eastern part is directly affected by the Agulhas Current (Boyd and Shillington, 1994) ( Gb31). The structure of the interior of the Bank is strongly influenced by wind forcing and is deeply stratified in summer but well mixed in winter (Probyn et al., 1994). Easterly winds in summer result in upwelling at the Capes and headlands along the coast and a cool water ridge may develop across the shelf from the eastern to the central Agulhas Bank (Boyd and Shillington, 1994). This ridge serves to prevent the penetration of deep Aguhas Current features onto the inner shelf. Conditions over the outer region of the Agulhas Bank are forced by oceanic processes and an upliftment of the isotherms may be induced by the Agulhas Current at the shelf edge (east of Cape Agulhas) which can result in a permanently stratified water column there. Patterns of water flow at the edge of the Agulhas Bank are generally W and SW to the east of Cape Agulhas and NW to the west of Cape Agulhas. Water flow over the central Bank is sluggish and of variable direction for much of the year but there is pronounced eastward flow inshore along the coast. The Agulhas Current penetrates onto the Agulhas Bank directly only in the region of the Agulhas Bight (especially during summer), and this warm water may intensify the seasonal stratification of the water column. It also serves to seed shelf communities with Agulhas Current species. Filaments and cyclonic eddies develop from instabilities along the northern boundary of the Agulhas Current while rings may form in the region of the retroflection. The rings are deep (up to 1500 m), broad (up to 300 km diameter) features which are generally situated well offshore and whose only influence on coastal communities as they move off into the south Atlantic is thought to be surface entrainment (Duncombe Rae et al., 1992). Although it is possible that central water of Indian Ocean origin (it is physically similar to Atlantic central water) may get upwelled on to the west coast shelf during summer from the Agulhas rings, this is unknown. Filaments of Agulhas Current water are much shallower (50 m) than rings and they may penetrate to Cape Columbine in the southern Benguela ecosystem. They occur just beyond the shelf edge and if entrained in the northward moving shelf-edge jet serve to intensify the frontal zone. They may be advected shoreward onto the Benguela shelf during wind reversals. ( Gb31)