Regional and global atmospheric patterns governing rainfall in the southern Levant

Abstract

This study attempts to find a linkage between the interannual variations of the rainfall measured in 12 stations spread over the northern half of Israel (the southern Levant) and atmospheric circulations ranging from regional to global scale. The analysis was done for the midwinter months, December-February, in which two-thirds of the annual rainfall occurs, during the years 1950-2002. The study is based on composite maps for extremely dry/wet seasons and on maps of correlation between atmospheric variables and the rainfall time series. Our results show th at an upper-level trough extending from Eastern Europe toward the Eastern Mediterranean (EM) is closely linked with the seasonal rainfall over the study area. This is expressed by a correlation of -0.74 between the 500-hPa geopotential height at 323°N, 35°E and the rainfall. This trough has two effects on the southern Levant rainfall: one is the dynamics implied by the upper trough and the other is the cool advection over the EM imparted by the northwesterly flows induced by the trough. The latter presumably enhances the atmospheric instability when it sweeps over the warmer waters of the EM. The upper trough was found to be associated with three global factors: the polar stratospheric jets, in both the Northern and Southern Hemispheres, and the SST variations over the western tropical Pacific Ocean, represented by the 'Pacific Warm Pool' index. The EM trough is a ccompanied by a ridge covering western Europe, so that cold and wet winters in the southern Levant coincide with warm and dry winters over western Europe and vice versa. Copyright © 2005 Royal Meteorological Society.