Some features of the Algerian Current

Abstract

An analysis of infrared images leads to a new conception of the dynamics of the Atlantic Water in the Algerian Basin. In situ current measurements are not yet available, but coherent hypotheses are presented with the aim to initiate further experimental and theoretical works. The Algerian Current flows steadily along the coast near 0° and becomes unstable near 1–2°E. Eddies of both signs are generated that are advected by the mean current, but only the anticyclonic eddies increase in size (diameter about 100 km). As they extract more energy from the mean current they are advected more slowly, and they may detach from the coast and drift for several weeks in the Algerian Basin. They grow as deep as 1000m at least, and they are able to pull lenses of Intermediate Water seaward from the Sardinian continental slope. Coastal upwelling cells are generated near 1–2°E between the cyclonic and the anticyclonic eddies; they are also advected by the mean current and are definitively not wind induced. These turbulent structures create intense mixing between the Atlantic and Mediterranean waters, which account for the large eastward gradient of salinity encountered in the surface layer. Therefore the Algerian Basin is characterized by a large mesoscale variability mainly due to the instability of the Algerian Current. It appears to be a reservoir in which the water of Atlantic origin is amassed, and it forms a buffer zone which disconnects the flow of Atlantic Water coming in at Gibraltar from the flow of hygly modified Atlantic Water exiting the basin through the Strait of Sardinia and into the Ligurian Sea.