The Algerian eddies

Abstract

This paper synthetizes various previous analyses of the Algerian eddies. These mesoscale anticyclonic eddies are of primary importance to the circulation of all the water masses in the Western Mediterranean. They are generated by instability processes disturbing the flow of Modified Atlantic Water (MAW) shortly after it reaches the African coast near 0°E, namely the Algerian Current. Young eddies, which have diameters of 50–100 km, and the upwelling generally attached to their southwestern edges, drift eastward along the coast at a few km per day, sometimes during several months; as they grow older and larger, the most vigorous ones leave the coastal zone and drift seaward. Some of them may reach the Sardinian continental slope where a well-defined vein of Levantine Intermediate Water (LIW) flows; we suspect them to be capable to pull fragments of LIW seaward, and in fact, the least modified LIW we encountered in the middle of the Algerian Basin had the form of rings or filaments trapped by one eddy. Eddies several months old may have diameters as large as ≈ 200km; they also entrain the Mediterranean Deep Water (MDW), and they probably extend down to the bottom. In 1984, two such eddies occupied most of the Algerian Basin; the MAW was deflected seaward by the westernmost one, from the Algerian coast at≈ 3°E as far as the Balearic Islands, and was then dispatched by other eddies through the whole basin as though by a set of paddle-wheels. Such interactions are perhaps not as exceptional as was previously expected. These mesoscale phenomena have strong biological implications. The Western Mediterranean Sea thus appears to be a very suitable place for the observation and modelling of such mesoscale coherent structures.