On the currents along the Northwest African coast South of 22° North

From January to March, 1972, the coastal upwelling experiment ldquoAuftrieb '72rdquo took place off Northwest Africa between Cap Blanc and Cap Timiris (18°N to 21°N). A current meter array was moored across the continental slope during 3 weeks. Hydrographical data were sampled at the position of the moorings. This paper describes briefly the general hydrography and presents the situation of the mean currents in the working area. The cross-circulation of the coastal upwelling regime has a horizontal diameter of 50 to 100 kilometres. The thickness of the near-surface Ekman layer with offshore flow, ranges from 15 metres just offshore from the Banc d'Arguin down to about 50 m at 20 km further seaward. Below the Ekman layer, the flow is onshore, being maximum at intermediate depths between 30 m and 150 m depending upon the water depths. Within this compensation layer, the waters upwelling are coming from the North at shallower depths, and from the South at lower depths, due to the poleward-going undercurrent. The undercurrent is a large-scale eastern boundary phenomenon. North of 15° N (Cap Verde) it is fed by two different subsurface currents. One current flows northward along the African continental slope from the Gulf of Guinea. The other current branches off the Equatorial Countercurrent in the central Atlantic, and flows towards the Northeast. Similar to earlier observations, the data suggest northward motions in the near-surface layer offshore. Historical data give clear evidence, as well, of a countercurrent at the surface inshore during summer and autumn. These separate northward-flowing currents offshore and on the shelf, are partly sustained by a baroclinic pressure gradient. Primarily, however, the meridional large-scale barotropic pressure gradient maintains the countercurrents at the surface and the undercurrent. There is some evidence that, in general, the equatorial circulation is an important clue to the dynamics of the poleward-going undercurrents along the continental slopes, as observed in the major coastal upwelling areas.