Synoptic climatological analysis of 'wet' and 'dry' Red Sea Troughs over Israel

The Red Sea Trough is one of the most frequent surface atmospheric circulation patterns over the southeastern Mediterranean. This study presents results of a climatological analysis of the Red Sea Trough (RST) based on a new, objective technique for identifying RST situations. The new algorithm was applied to the GEOS-1 reanalysis gridded data set from NASA for the period 1985-1995 and displayed by the GrADS software. Synoptic differences between 'dry' and 'wet' (rainy) RSTs are displayed and also compared with non-RST rain situations. The results show that RSTs are most frequent in October and November with a secondary maximum in April. From June through August, the frequency of occurrence of RSTs is negligible. The longitudinal position of the RST shifts westward from September to January and back eastwards from February to May. Through all months except October, wet RSTs are positioned east of the dry RSTs. Pronounced diurnal variations of RST occurrence are also detected. During wet RSTs, the northward and westward (over the eastern Mediterranean (EM)) extension of the sea level trough is more pronounced than during dry RSTs, and through all months (except October) the sea level pressure is lower by about 3 hPa. A ridge or zonal flow at 500 hPa accompanies dry RSTs whereas a pronounced trough at 500 hPa extending from the Black Sea toward northern Egypt accompanies wet RSTs. Temperatures near the surface for both wet and dry RSTs show positive temperature deviations from the 11-years monthly mean. During wet RSTs it changes to negative deviations near 900-800 hPa, whereas during dry RSTs the positive anomaly extends through most of the troposphere. RST-associated rain is accompanied by a relatively narrow 500-hPa trough, located west of the Israeli coastline and characterized by a relatively deep southward penetration, to about 20 degrees N. Non-RST-associated rain is accompanied by a wider upper trough, located over the Israeli coastline with a shallower southward penetration, to about 25 degrees N. Copyright (c) 2005 Royal Meteorological Society.