Upper-layer circulation in the eastern Equatorial and South Atlantic Ocean in January–March 1995

Abstract

The upper-layer circulation in the eastern basin of the South Atlantic was studied from hydrographic and direct velocity measurements along WOCE lines A11, A13 and A14. A13 and A14 provide quasi-meridional samplings of the equatorial, subequatorial and subtropical circulation regimes. A13 was carried out along the African coast at about 600 km offshore from it, and A14 along the nominal longitude 9°W. A11 intersects the Cape Basin between 46°S in the west and 30°S in the east. Transport estimates were derived from direct velocity measurements and a box inverse model. In the equatorial eastern Atlantic, the Equatorial Undercurrent (EUC) transport decreases from 25×10 6 m 3 s −1 at 9°W to 13×10 6 m 3 s −1 at 5°E. Re-circulations of the EUC into the northern and equatorial branches of the South Equatorial Current (SEC) are evidenced at 5°E and quantified. In the tropical Atlantic, we estimate 7.5×10 6 and 4.2×10 6 m 3 s −1 for the transports at 9°W of the South Equatorial Undercurrent (SEUC) and South Equatorial Countercurrent (SECC), respectively. Both the SEUC and SECC extend vertically down to intermediate depths and contribute to the northern limb of the Angola Gyre. The Angola Current transport is estimated to be 16±5×10 6 m 3 s −1 for σ 1<32.1 at 13°S. South of the Angola Gyre, the transports show an apparent cyclonic circulation, developed mostly at the intermediate level. The water mass properties suggest that it is, at least partially, a re-circulation of the Benguela Current. Further south and for the subtropical gyre, we estimate 10±5×10 6 m 3 s −1 for the transport of the South Atlantic Current across 9°W and 28±4×10 6 m 3 s −1 for the transport of the Benguela Current at 10°E for σ 1<32.1.