Circulation in the Mediterranean Sea

Abstract

The overall functioning of the Mediterranean Sea, which transforms Atlantic Water (AW) into Mediterranean Waters (MWs), has been comprehended for a while, and the process of dense water formation, which leads AW to sink offshore in specific northern zones of the Western and the Eastern Basins, has been studied in the world ocean. However, the circulation of the various waters from/to the basins openings to/from the zones of sinking is still debated in the Western Basin, while a similar debate is only being initiated in the Eastern Basin. The differences between the circulation schemas published up to now can be large and they have already been commented upon in papers published recently. To provide a coherent introductory chapter, only the authors’ analysis is presented hereafter.

Overall, and due to the Coriolis effect, all waters (AW and MWs) that circulate at basin scale tend to follow, in the counterclockwise sense, the isobaths at their own level. Hence they tend to describe, in both the Western and the Eastern Basins, quasi permanent gyres a few 10s km thick and a few 1000s km long along the continental slope. This simple schema is complicated by the fact that the southern parts of both gyres described by AW are markedly unstable, the AW inflow being hence identified with the so-called Algerian Current and Libyo-Egyptian Current and generating specific systems over the whole depth. Indeed, these currents (100–200 m deep) meander and generate, a few times per year, anticyclonic eddies that can reach diameters of 100–200 km (and even more), propagate downstream (i.e. eastward) at speeds up to a few km/day, and sometimes extend down to the bottom (2–3000 m). Hence, these eddies follow the deeper isobaths, separate from their parent current where these isobaths diverge from the upper continental slope, and drift for years (up to 3 at least) in the central part of the basins, possibly coming back shoreward where they interact with their parent current, sometimes in a dramatic way. These eddies entrain AW and MWs from the peripheral part of the basins towards their central part, together with eddies induced by the wind in the Eastern Basin only and having similar characteristics. In both basins, the northern parts of the gyres display specific features linking them to the zones of AW sinking, hence being identified (more easily in the Western Basin than in the Eastern one due to the topography) with the so-called Northern Currents. In addition to these circulation features, and because the various openings are only a few 100s m deep, the deeper MWs must be uplifted before outflowing. This is achieved permanently and more or less everywhere through continuous mixing with less dense waters and uplifting by newly formed denser waters. This is also achieved when and where intense mixing with other waters (cascading from sills or sinking) and sucking upward straits occur.