General biological features of the South Atlantic
Coastal area between 0 and 20¡S (chiefly Guinea Current)

—Physical processes and seasonality
The main surface currents in the region include the northward flowing Benguela Current, and the westward or north-westward flowing South Equatorial Current ( Gb17). These currents are influenced by seasonal fluctuations in the St. Helena anticyclone which also cause alternating periods of upwelling and stability at the coast.

The basic seasonal pattern is a pronounced warm period between October and May, followed by a cold season from June to September.

During the warm season, the anticyclone is the weakest and furthest south (Peterson and Stramma, 1991; Gouriou, 1993). The SW monsoon winds penetrate the African continent and give rise to significant precipitation: the Zaire (or Congo) River has the second largest discharge after the Amazon. As its basin straddles the meteorological equator, differences in the rainy seasons between northern and southern tributaries determine a four season regime at its mouth. Highest outflow is observed in December and a secondary maximum takes place in May, while minimum discharges are observed in July-August, with a second minimum in March. Its considerable discharge, about 45,000 msö-1 on average, fluctuates between 23,000 and 80,000 mö3 sö-1 (Eisma and van Bennekom, 1978). Its plume is directed to the W or NW and can be detected 1000 km offshore (Piton, 1988).
( Gb21)

According to Berrit (1966) the warm, low salinity (<35) surface waters which spread south along the coast ( Gb21) to Pointe-Noire (4°42’S) during the warm season originate in the Gulf of Guinea where they would be formed in the eastern part of the gulf. Berritt (1966) called them “Guinean waters”, but for Wauthy (1977) these waters result from a mixture of tropical waters with the outflow of the Congo River. Indeed, a low salinity, swiftly flowing (0.5 to 2 knots), northward directed current (the Congo Current) is present in the upper 10 m ( Gb21), between the mouth of the Congo river (6°S) and Cape Lopez (1°S) along the coast and beyond the edge of the shelf.

Water temperatures across the region are generally high during the warm season. They decrease to the south owing to the cold waters of the Benguela Current which are advected NW. Near the coast, north of Cape Frio (about 18°S), a thermal front separates the warm Angolan waters from the recently upwelled Benguela water (Angola-Benguela Front). North of this front, the water column is strongly stratified over most of the area and displays the characteristic typical tropical structure. The thermocline is pronounced and its lower point (18°C) can be found at between 30-100 m depth. The upper mixed layer is generally poor in nutrients and in plankton, but seasonal shoalings of the thermocline induce periodic enrichment processes.

During the austral winter the St. Helena anticyclone is reinforced and the SE winds extend along the coast to Cape Lopez (1°S). Because the winds are parallel to the coast, extensive coastal upwelling of waters from the Equatorial Undercurrent is observed in the region from Cape Lopez to Cape Frio. The acceleration of the surface current and the shallowing of the Equatorial Undercurrent reinforce the shear and increase the vertical mixing which leads to the production of a fairly deep upper mixed layer. Along the coast, the surface circulation is directed N from Cape Frio to Cape Lopez ( Gb21). Due to the continuity of upwelling north and south of Cape Frio, the thermal gradient at the Angola-Benguela Front weakens.

A four season regime can be observed in the coastal areas from Cape Lopez to Cape Frio: the Great Warm Season (January-May), the Great Cold Season (June-September), the Little Warm Season (October-November) and the Little Cold Season (December). The minor upwelling season that takes place in December is possibly a result of the secondary maximum observed at this time of year in the velocity of the South Equatorial Current. This Minor Cold Season, which is highly variable in intensity, is not detectable offshore and decreases in importance in the northern part of the area (near the seasonal front at Cape Lopez). The Little Warm Season is less important in the south (near the Cape Frio front). Offshore, the four seasons are replaced by a two seasons cycle.