Marine data and surface changes

  The western basin of the Mediterranean comprises three water masses: a surface layer of about 150 m in depth, a Levantine intermediate water layer (situated between about 150-600 m in depth) issuing from the eastern basin, and deep waters reaching to the seabed (which is at a depth of more than 2800 m in the central part of the Algero-Provençal basin). Over the 1959-1997 period, mean values of trends for temperature and salinity in the deep waters are increases of 3.47 and 1.07 psu , respectively [Béthoux and Gentili, 1997]. Curiously, concomitant changes in temperature and salinity have had no evident effect on water density, which allows the hypothesis of a constant deep outflow over the sill of Gibraltar. According to spatial and temporal changes of water characteristics in the Levantine intermediate waters, estimated trends are less accurate but nevertheless greater than those in the deep water. These are comprised between 6.8 and 9.1 for temperature and 1.8 and 1.9 psu for salinity [Béthoux and Gentili, 1996, Sparnocchia et al., 1994]. The characteristics of water in the surface layer are strongly influenced both by Atlantic inflow and with changes in water circulation and in year to year variations in local climatology. Despite such variability, a warming phenomenon has been apparent since 1973 for waters of between 0 and 80 m in depth off the Catalan coast (Spain) [Pascual et al., 1995] and since 1977 over the Mediterranean from a synthesis of meteorological data [Metaxas et al., 1991]. Unfortunately, these surface data are not sufficiently robust to allow, in combination with data acquired from deep and intermediate waters, quantification of changes throughout the whole water column. As a rough estimate, [Béthoux et al., 1990] calculated that d amounted to 1 in the whole water column of the Algero-Provençal basin. Accurate quantification on the scale of the entire sea requires an estimate of changes likely to have occurred in the other inner basins of the Mediterranean, i.e. in the Tyrrhenian Sea and in the eastern basin.

A 20-box model was used to summarize the main hydrological features in different areas and water masses of the Mediterranean [Béthoux and Gentili, 1996, Béthoux and Gentili, 1997]. The heat and water budgets across the sea surface were constrained in order to simulate most of the geographical variations in temperature and salinity, and to verify a heat inbalance of -5 (balanced by thermal advection across the Strait of Gibraltar) and a freshwater deficit (E - P - R) of 0.95 m . This was assumed to correctly summarize Mediterranean functioning before the 1940s. Thereafter, the 20-box model was used to quantify the effects of continuous changes in heat and water budget across the sea surface on temperature and salinity in western deep water. The observed trend of temperature increase was simulated with changes in the surface heat budget over the entire sea reaching 1.5 in 1995. The shifts represented either an increase in heat inputs or a decrease in losses.

Over the 1940-1989 period, no continuous increase in solar radiation was recorded [Kelly and Wigley, 1990]. In the Atlantic ocean, anomalies are occurring on decadal scales [Hansen and Bezdek, 1996, e.g.,] but no continuous increase in surface temperature was found. Consequently the change of 1.5 in the heat budget may be indicative of a decrease of air-sea heat exchanges. The measured salinity trend was simulated with an increase in water deficit that reached 0.1 m in 1995. Considering that there are increasing trends in both temperature and salinity, the latter cannot be derived solely from an increase in evaporation, the large latent heat flux of evaporation resulting in water temperature decrease. Consequently the two changes are concomitant but more or less independent.