BALWOIS 2004 Ohrid, FY Republic of Macedonia, 25-29 May 2004 Remote Climate Processes Underlying Summer Drought Events In The Mediterranean Marina Baldi, Francesco Meneguzzo, Giovanni A. Dalu, Toufic El Asmar Istituto di Biometeorologia Roma, Italy; Alfonso Crisci, Lorenzo Genesio, Francesco Piani, Fondazione Meteorologia Applicat Firenze, Italy Abstract An analysis of the actual aridity index and climate vulnerability is performed showing a possible desertification risk in the Mediterranean region. Results can be used to evaluate the impact of possible prolonged droughts in the Mediterranean Region on the basis of a vulnerability context. In this respect, the role of the Asian and the West Africa monsoons on the Mediterranean summer climate is investigated on the basis of multiple gridded data sets. Composite differences of few atmospheric parameters are evaluated in years when the monsoons were stronger and weaker than the climatic average. Results show a link between the monsoons and the summer Mediterranean climate, which depends on the monsoon system considered. This connection is more evident when years of strong or weak monsoons are considered, and shows a time evolution with the summer season. Keywords: climate change; drought; monsoons; reanalyses; teleconnections; Mediterranean Introduction The climate system, driven by the solar energy, evolves according to its internal dynamics and feedbacks, with the contribution of natural factors (biosystems, volcanic ash and aerosols) and important anthropogenic factors (greenhouse gases and aerosols, and land use changes, Houghton et al., 2001). While there is general agreement at least on the sign (positive) of the thermal response of the climate system. i.e. an average increase of the surface and atmospheric temperature, yet the quantification of the warming rate on a regional scale is very difficult (e.g. Houghton et al., 2001; Pielke, 2003; Rial et al., 2003). The balance between radiative cooling and the release of latent and sensible heat is the leading mechanism in determining the temperature of the atmosphere and its stability. However, the response of the hydrological cycles to a warmer climate is rather complicate, because of strong nonlinear feedbacks. Yang et al. (2003) recently showed that the average global annual precipitation change is almost linearly dependent upon surface warming, where small positive variations of the sea surface temperature (SSTs) are related to a small increased precipitation. They also find that the hydrological cycle shows small sensitivity to a small increase of CO 2 . Changes of the surface temperature over continental areas, of the heat stored in the ocean, and of the hydrological cycle contribute to the regional climate variability, with a dynamical contribution of the large scale climate systems. Namely, the Hadley meridional cell and Walker zonal cell have strengthened in the 1990s (Chen et al., 2002), with stronger upwelling in the tropics and downwelling in the subtropics. The summer monsoons are more vigorous with enhanced precipitations over specific areas (e.g. over the West Africa: Maynard et al., 2002). The tropopause height has increased (Santer et al., 2003). The pressure has increased at low to mid-latitudes and decreased at higher latitudes (decreasing NAO index) due to a possible anthropogenic contribution (Gillett et al., 2003). In the mid-latitude regions, the intensity and the track of the Pacific and Atlantic storms, and, in summer, the weakening of the storms and the northward displacement of their track, are the major factors affecting the daily weather as well as the seasonal and multi-annual climate, with a relevant impact on the level of the water resources on a regional scale (Chang et al., 2002). Many of the studies on the regional climate of Europe and of the Mediterranean basin are devoted to the winter season, when the leading role of NAO is rather well established (Hurrell, 1995; Hurrel et al, 2001). Few Authors also studied the variability and trends of the hydrological cycle in the Mediterranean region, focusing on precipitation climatology, and on the hydrological cycle in the basin, and on a possible desertification process (Geeson, 2001), and to the climatic sensitivity to Droughts and Floods 1