5.3.2.4 Mediterranean Sea

Marked changes in thermohaline properties have been observed throughout the Mediterranean (Manca et al., 2002). In the western basin, the Western Mediterranean Deep Water (WMDW), formed in the Gulf of Lions, warmed during the last 50 years, interrupted by a short period of cooling in the early 1980s, the latter reflected in cooling of the Levantine Intermediate Water between the late 1970s and mid-1980s (Brankart and Pinardi, 2001). The WMDW warming is in agreement with recent atmospheric temperature changes over the Mediterranean (Luterbacher et al., 2004). The salt content of the WMDW has also been steadily increasing during the last 50 years, mainly attributed to decreasing precipitation over the region since the 1940s (Krahmann and Schott, 1998; Mariotti et al., 2002) and to anthropogenic reduction in the freshwater inflow (Rohling and Bryden, 1992). These changes in water properties and circulation are linked to the long-term variability of surface fluxes (Krahmann and Schott, 1998) with contributions from the NAO (Vignudelli et al., 1999) that produce consistent changes in surface heat fluxes and a net warming of the Mediterranean Sea (Rixen et al., 2005).

These changes in the temperature and salinity within the Mediterranean have affected the outflow of water into the North Atlantic at Gibraltar (see also Section 5.3.2.3). Part of this shift in Mediterranean outflow properties has been traced to the Eastern Mediterranean. During 1987 to 1991, the Eastern Mediterranean Deep Water became warmer and saltier due to the switch of its source water from the Adriatic to the Aegean (Klein et al., 2000; Gertman et al., 2006), most likely related to changes in the heat and freshwater flux anomalies in the Aegean Sea (Tsimplis and Rixen, 2002; Josey, 2003; Rupolo et al., 2003). This 1987 to 1991 switch of source waters has continued and increased its impact, with density of the westward outflow in Sicily Strait now denser (Gasparini et al., 2005). While there are strong natural variations in the Mediterranean, overall there is a discernible trend of increased salinity and warmer temperature in key water masses over the last 50 years and this signal is observable in the North Atlantic.