About the origin of the Mediterranean Waters warming during the twentieth century

Abstract. The Mediterranean Sea transforms Atlantic Waters inflowing through the Strait of Gibraltar into saltier, cooler and denser Mediterranean Waters that outflow into the Atlantic Ocean. A theoretical steady state functioning of the Mediterranean Sea would be the result of the balance between the net heat and volume transports through the Strait of Gibraltar and the heat loss to the atmosphere through the sea surface and the net evaporation. The salt transport for the inflow and outflow should be balanced. Changes in the heat content, temperature and salinity of the Mediterranean waters reveal that the present Mediterranean functioning is out of this equilibrium state. A new analysis for MEDAR data shows that the temperature and salinity averaged for the Mediterranean Waters in the whole basin increased at rates of 0.0020 ± 0.0018 ºC/yr and 0.0007 ± 0.0003 psu/yr. This temperature trend is equivalent to a 0.43 ± 0.38 W/m² heat absorption. This warming trend would have increased during the beginning of the twenty first century at the Western Mediterranean, maybe linked to the Western Mediterranean Transition. Results from a simple box model using heat, volume and salt conservation laws indicate that the observed changes cannot be attributed only to an increase of the net evaporation, nor to a salinity increase of the Atlantic Waters flowing through the Strait of Gibraltar as previous hypotheses suggested. An increase of the net evaporation ranging between 5 and 7 % combined with a reduction of the heat losses to the atmosphere ranging between 0.4 and 0.5 W/m² could explained the observed changes.