Abstract
Many studies suggest that the climate of the Mediterranean region is shifting toward a more tropical regime. In order to verify this supposed modification, an analysis is performed using the water vapour density at the sea surface. Previous experimental results in the region show that this parameter can evidence when the tropical features of the Mediterranean climate predominate. Compared to the other common indicators like the air temperature, it gives a more comprehensive picture of any possible variation by analysing the combined effect of two distinct parameters, the air temperature and the humidity, providing at the same time additional information on the surface marine thermal stratification. The work was performed using data from the European Centre for Medium‐range Weather Forecast (ECMWF) ERA‐40 project covering the period ranging from 1958 to 2001. Yearly averages and maxima of the water vapour density were obtained from the daily mean values. The number of days of the year in which these daily means were greater/equal to 15 gm^−3^, and the first and the last day of the year when they crossed this threshold were also evaluated. The investigation is carried out for the whole basin and its three main subbasins, namely, the western, the central and the eastern Mediterranean, separately. The results confirm that the occurrence of tropical features in the Mediterranean climate has been increasing in the last two decades, with a widening of the period affected by the condition and enhancement of the extreme values. This trend is detectable over the whole basin notwithstanding the very high interannual variability. The good agreement with the results obtained in other works where different variables or proxy data were used confirms that the surface water vapour density is a good indicator for detecting long term climate variability. Copyright © 2005 Royal Meteorological Society.