General biological features of the South Atlantic
Coastal area between 20 and 34¡S (Benguela Current)

—Physical processes and seasonality
Wind-driven upwelling is the dominant forcing mechanism determining the physical characteristics of the Benguela shelf region. Strong southerly winds blow permanently in the Luderitz area (25°S), which, combined with the narrow shelf, make it probably the most active upwelling centre in the world. Southerly winds become more seasonal to the north and south, with peak winds in the austral winter in the northern Benguela, and in the austral summer in the southern parts. The shelf is relatively wide and deep, except in the extreme north and south and off Luderitz. A number of active upwelling sites exist along the coast, where upwelling is pulsed in response to changes in coastal winds and the passage of coastally trapped waves. The northern and southern boundaries are characterized by intensive mixing with warm water of tropical origin, and shifts in the boundary conditions can cause major perturbations to the productivity of the fish stocks (Shannon et al., 1992). The offshore boundary is a complex series of jet currents, eddies and frontal features such as filaments. Agulhas rings break off from the retroflection area and transport heat, salt and Indo-Pacific fauna into the South Atlantic. Occasionally, an Agulhas ring will move northwards close to the shelf, entraining Indian Ocean water into the South Atlantic and also drawing neritic water offshore into the subtropical gyre.