The factors governing the summer regime of the eastern Mediterranean

摘要

The synoptic scale features over the eastern Mediterranean (EM) for July-August are examined using National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data. The region is subjected to two primary factors: mid-upper level subsidence and lower level cool advection, associated with the Etesian winds.The interdiurnal variations of these factors were found to be correlated with each other, with a maximum of r = 0.76, found between the 700 hPa subsidence and the 925 hPa wind speed. The impact of these factors on the temperature regime was examined through their contributions in the temperature tendency equation at 32.5 degreesN, 35 degreesE. A significant correlation was found between them at the 850 hPa level, indicating that they tend to balance each other. This explains the low interdiurnal temperature variations there in summer.Zonal-vertical and isentropic cross-sections indicate the existence of a closed circulation connecting the EM with the Asian monsoon, and a meridional-vertical cross-section indicates a signature of the Hadley cell across eastern North Africa. Air back-trajectories demonstrate that the EM is connected at the lower troposphere with Europe, at the midtroposphere with eastern North Africa and at the higher troposphere with the Asian monsoon. Significant correlation was found between the interdiurnal variations in the upward motion over the Asian monsoon and the subsidence over the Levant, with a 1 day lag, implying that the Asian monsoon controls the interdiurnal variations over the Levant.A detailed analysis shows that the correlation between the two dynamic factors governing the EM results from a linkage existing between each one of them and the Asian monsoon. An intensification of the Asian monsoon enhances both the subsidence over the Levant, via the circulation connecting them, and the Etesian winds, due to the enhanced pressure gradient between the two regions. Copyright (C) 2004 Royal Meteorological Society.