1.7 Mediterranean circulation
Marine waters are perpetually moving steadily with currents, in tidal phases, and suddenly with waves. They rise and fall or move sideways due to winds, pressure, temperature, salinity, solar irradiation, earth and moon movements. Through these movements, organisms and nutrients move, for example, with upwelling (FIG.1_SES1.7), water movements of deep water to surface and downwelling (FIG.1_SES1.7), water movements to bottom which allow mixing of the substances.
When the upwelling occurs, the water that rises to the surface is colder and rich in nutrients. The nutrient-rich upwelled water means that these waters have high biological productivity. In these waters phytoplankton and zooplankton are present in such a large amount that they can support the main populations of cetaceans in the Mediterranean Sea.
The circulation in the Mediterranean (VIDEO1_SES1.7) is conditioned by its morphology: a half closed basin in connection with the Atlantic Ocean through the Strait of Gibraltar and the Black Sea with the Dardanelles Strait. The geographic location also means that surface evaporation is not balanced by precipitation but by the entry of Atlantic water, it results in a current-conditioned by salinity values; in fact, ocean waters are less salty than the Mediterranean ones, and therefore enter the basin at a surface level and propagate along the Algerian coast. Through the channel of Sardinia, the mass of water divides, and while one part enters the Tyrrhenian Sea and proceeds along the western coast of Italy, through the Sicilian channel, the other part reaches the Levantine basin. In this basin the water still undergoes a transformation due to the strong evaporation, increasing density and dropping down to about 200-500 m, then carrying the reverse route through the Sicilian canal and the western basin to the Gibraltar stretch. Other transformations also occur in the Adriatic basin and in the Gulf of Leone; in these two zones, the interactions with the atmosphere during the intense cold period are so strong that it results in convection phenomena that give rise to density increase and consequent sinking of water to deeper levels, forming another two water masses. This thermohaline circulation affecting the entire basin determines on the surface a predominantly anti-clock circulation with an intensified current at the main channels.
These Mediterranean currents are like highways on which for example large marine vertebrates such as cetaceans, sharks and turtles travel and orient themselves; for example the sea turtles, follow the current for movement as to swim easier and save energy.