Claude Millot

Circulation in the Western Mediterranean Sea

Abstract During the last decade, a considerable amount of work has been made and definite results obtained about the circulation in the Western Mediterranean Sea. The diagrams presented 10 years ago [Millot, C., 1987a. Circulation in the Western Mediterranean. Oceanol. Acta, 10, 2, 143–149.] have been confirmed and complemented, mainly in the south where all water masses appear to flow anticlockwise along the continental slope, as they do everywhere else in the sea. Definitive results have also been obtained about the mesoscale phenomena in the Algerian Basin which induce a dramatic variability of the circulation of all water masses, as far as around the Balearic Islands and through the Channel of Sardinia. Extremely interesting observations have been collected between Sicily, Tunisia and Sardinia about the hydrological and dynamical characteristics of the waters entering the Tyrrhenian Sea. Finally, the dense water formation processes have been specified, mainly since some uncommon situations have been encountered. As a whole, the relatively large importance of the seasonal (resp. mesoscale) variability in the north (resp. south) has been documented. Together, the observations in nature, the laboratory experiments and the numerical models have thus provided a more thorough understanding of the sea dynamics. Nevertheless, uncertainties remain about the amount of the waters formed in the sea mainly at intermediate depths, for these waters are often not easily distinguished from the surrounding ones. This is of major importance for, a priori, intermediate waters can flow out of the sea more easily than deep waters. In any case, the southern Tyrrhenian Sea appears to be a key place for the functionning of the whole sea.

Mesoscale and seasonal variabilities of the circulation in the western Mediterranean

Based on satellite information and current measurements, it has recently been possible to detect intense mesoscale phenomena in the western Mediterranean Sea which have important consequences on the distribution of the water masses. We emphasize the key role played by the Algerian and Northern Currents and by the instability processes affecting these major components of the circulation. Some implications for the seasonal variability of the circulation are analyzed.

Algerian Eddies lifetime can near 3 years

The Algerian Current (AC) is unstable and generates mesoscale meanders and eddies. Only anticyclonic eddies can develop and reach diameters over 200 km with vertical extents down to the bottom (f3000 m). Algerian Eddies (AEs) first propagate eastward along the Algerian slope at few kilometers per day. In the vicinity of the Channel of Sardinia, a few AEs detach from the Algerian slope and propagate along the Sardinian one. It was hypothesized that AEs then followed a counter-clockwise circuit in the eastern part of the basin. Maximum recorded lifetimes were known to exceed 9 months. Within the framework of the 1-year Eddies and Leddies Interdisciplinary Study off Algeria (ELISA) experiment (1997–1998), we exhaustively tracked two AEs, using mainly an f3-year time series of NOAA/AVHRR satellite images. We show that AEs lifetimes can near 3 years, exceeding 33 months at least. We also confirm the long-lived AEs preferential circuit in the eastern part of the Algerian Basin, and specify that it may include several loops (at least three). D 2002 Elsevier Science B.V. All rights reserved.

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.