Circulation in the Mediterranean Sea

The Mediterranean Sea is the prototype of a semi-enclosed sea with a limited connection to the ocean through a narrow and shallow strait. Evaporation exceeds the fresh water input through precipitation and river discharge and therefore the Mediterranean is a concentration basin in which lower salinity water from the ocean enters through the strait in the upper layer and the more saline water mass formed in the basin flows out in the lower layer, above the sill. The sea contains a variety of water masses, one of which enters from the Atlantic Ocean in the upper layers while all of the others, located at various depths, are formed in various locations by the action of the surface heat and fresh water fluxes. Superimposed on this basin wide thermohaline cell is a complex and energetic circulation containing sub basin scale gyres, meandering jets, and mesoscale eddies. Various studies conducted over the past 20-30 years have shown that in the second half of the twentieth century the deep water of the Mediterranean has warmed. The upper and intermediate layers have warmed in the Western Mediterranean but have cooled in the Eastern Mediterranean. Several abrupt changes in water properties have also been observed, some with far reaching consequences. Projections for the twenty-first century indicate that the regional drying will continue mainly due to reduced precipitation. Consequently, salinity will continue to increase with maximum changes projected to occur in the regions of deep-water formation.