FIG. 1. – Western Mediterranean Sea sub-basins (deg. North and East are positive).
m to nearly 1800 m depth; the WMDW is caused by cell convection during severe winters in the northern part of the western basin.
The marine circulation along the western Mediterranean continental shelves and slopes is far from simple and well understood. During late spring and summer, the whole western Mediterranean is strongly stratified. The seasonal thermocline is 20- 50 m deep. In winter, the water column is rather homogeneous, especially in the open sea. The iso- lated and combined action of several factors like strong winds, the presence of shelf-slope density fronts and a complex bottom topography due to the existence of a sloping bottom and submarine canyons lead to complicated circulation patterns. Mesoscale variability, defined as the ensemble of flow fluctuations whose periods range from the order of a few days to a few tenths of days, is responsible for small gyres or vortices (i.e. mesoscale eddies) which are observed to modify the general circulation in the western Mediterranean basin. Among other characteristics, they are able to block the general circulation, they are supposed to contribute to the exchange between the continental shelf waters and slope waters, and they are associat- ed with vertical motions that allow water and nutri- ents to rise or sink, thus having strong biological, geological and chemical implications.
A review of the circulation of the different water masses present in the western Mediterranean basin
reveals that major contributions have focused on the study of surface currents. Deep and bottom currents, especially those outside the geostrophic assump- tions (like bottom current intensifications), are more unknown. A similar conclusion is reached concern- ing the origin of the different water masses. Nowa- days, a special effort is devoted to studying the processes and mechanisms of deep and bottom water formation and revealing new insights. Fur- thermore, the variability of the deep currents is of major interest, since it may be relevant for the ven- tilation of the deep western Mediterranean Sea. Therefore, further studies about the formation mech- anisms and behaviour of the large and mesoscale variability of the deep currents are needed. In this review we outline the main characteristics of the ori- gin and circulation of the above-mentioned water masses.
Surface water masses
The origin of the circulation in the Mediter- ranean Sea relies on its sole connection to the open ocean, the Strait of Gibraltar, a shallow and narrow aperture that connects the Mediterranean to the Atlantic Ocean. The Strait of Gibraltar allows a selective renewal of Mediterranean Sea waters. Low-density Atlantic waters intrude into the
MEDITERRANEAN DEEP-SEA BIOLOGY 15