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

Abstract Some aspects on 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 73 5‐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 on 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 as blocking activity and/or cut‐off lows, seem to play an important role too. Copyright © 2007 Royal Meteorological Society