Thermohaline variability of Mediterranean Water in the Gulf of Cadiz, 1948-1999

One of the key processes of the thermohaline circulation is the water mass transformation caused by the external forcing and the continuing renewal and stratification of the water column. In this context the marginal seas, such as the Mediterranean Sea, the Arctic Sea, the Greenland Sea, etc., play a fundamental role by supplying the global ocean with dense water. In particular, the Mediterranean Water (MW) is one of the intermediate-type water masses observed in the North Atlantic. It consists of a mixture of Levantine Intermediate Water (LIW) and deep waters produced within the Mediterranean basin and flows into the Atlantic Ocean through the narrow Strait of Gibraltar. The Gulf of Cadiz is the transition sub-basin where the pure MW undergoes, for the first time, a strong mixing and entrainment with the Atlantic water. The MW lies between 700 and 1400 m depth characterized, albeit density compensated, by significant salt and temperature anomalies, with Delta S similar to 1-2 and Delta T similar to 2-3 degrees C with respect to the overlying North Atlantic Central water. In the present work new temperature and salinity observations have shown seasonal, interannual and decadal variations. Changes in water properties of MW outflow were found, with an average value of 0.16 degrees C/decade and 0.05 in salinity per decade over the last 50 years (1948-1999). Even though the data set is not regularly distributed in time, it seems to be clear that the layer thickness ventilated by MW increases almost regularly in time, but with an evident acceleration in the last three decades: about 1000 m thickness and with salinity ranging between 36.0 and 37.0. In order to explain these variability and trends, some possible mechanisms are taken into consideration, in particular those related to the variability of Mediterranean thermohaline circulation and the physics of the Gibraltar Strait. (C) 2008 Elsevier Ltd. All rights reserved.