On the intra-annual variability of atmospheric circulation in the Mediterranean region

Some aspects oil the intra-annual variability and the seasonal peculiarities of the atmospheric circulation in the Mediterranean region are examined. T-mode, factor analysis is applied on the long-term (1948-2003) 5-day mean Values of relative geostrophic vorticity for the 735-day periods of the year. The analysis is applied for sea-surface and 500-hPa levels. For sea level, two Circulation types are found to be dominant: the type of the warm period and the type of the cold period, characterizing the so-called Mediterranean climate; they are strongly dependent oil the thermal and humidity characteristics of the earth's surface and the position and strength of the sub-tropical (Azores) anti-cyclone. For 500-hPa level, two extra but weaker circulation types are added, prevailing mainly during the transitional seasons and modifying the smooth intra-annual variation of the atmospheric circulation. In this case, the thermal influence of the earth's surface appears to be weaker, while other circulation features, Such Lis blocking activity and/or cut-off lows, seem to play an important role too. Copyright (C) 2007 Royal Meteorological Society. International Journal of Climatology