The general circulation in the Mediterranean Sea: a climatological approach

A significant amount of hydrographic data has been collected in the Mediterranean during this century, providing the basic ingredients needed to construct a climatological picture of the general circulation. A pooled hydrographic data set has been prepared, which currently contains over 100,000 station profiles (CTD, Nansen bottle, XBT and MET) taken in the Eastern and Western Mediterranean Sea between 1900 and 1993. Climatological analyses of temperature and salinity data are performed at seasonal and monthly scales, using a variational method and a finite element numerical technique. The variational formulation had been demonstrated to be equivalent to objective analysis, and a hybridation of the statistical and variational methods allows to compute error fields associated to the climatology. The free parameters of the scheme are determined using a cross-validation algorithm to extract the best seasonal statistics from the observed data sets. Compared to earlier climatologies, the recent analyses show significant improvements with respect to the regional scales of the general circulation gyres. In addition, several features of the general circulation are described with a better accuracy. Finally, the barotropic equations (external mode) of a free-surface, primitive equation model are integrated in time to diagnostically adjust the sea-surface elevation to the thermohaline structures. Geostrophic velocities are then computed, integrating the thermal-wind equations with the sea-surface pressure as a reference. Results of fully prognostic, three-dimensional, primitive equation models are compared with these diagnostic computations in order to identify which features of the general circulation can be captured by observations. (C) 1998 Elsevier Science B.V. All rights reserved.