The near-surface circulation in the northeastern corner of the South Atlantic ocean

Northeast of a line joining approximately 35°W, 5°S to 15°E, 25°S in the South Atlantic is the locus of a large-scale cyclonic geostrophic gyre, masked by northwestward Ekman flow at the surface and coincident with a zone of cyclonic wind stress curl. According to some descriptions, the gyre is centered near 4°E, 13°S, has a northern limb of eastward-flowing South Equatorial Countercurrernt and an eastern limb of poleward-flowing coastal Angola Current. It therefore appears to be eastern-intensified, a curious situation in view of the dynamics thought to govern motion in large-scale gyres. The northeastern corner of this ocean is also where two other eastward currents, the Equatorial Undercurrent and the South Equatorial Undercurrent, terminate and possibly feed the coastal flow.

The apparent eastern intensification of the observed geostrophic circulation is likely to be caused by the superposition of different dynamical regimes: on the one hand, a relatively weak circulation in Sverdrup balance including the South Equatorial Countercurrent and closing cyclonically within the interior, and on the other, relatively strong near-equatorial and coastal flows which, though geostrophic in the cross-stream direction, have entirely separate dynamics. Previous observations in the northeastern corner of the South Atlantic and relevant model results are examined within the framework of this hypothesis. An analysis of unpublished current measurements off Gabon and Congo (8°E-12°E, 1°S-6°S) shows a highly variable poleward undercurrent along the continental break. We refer to this current as the Gabon-Congo Undercurrent and compare it to the Peru-Chile Undercurrent in the eastern South Pacific, discussing its interpretation as a branch of the terminating Equatorial Undercurrent.