Synoptic disturbances over the Nile valley north of the intertropical convergence zone



The synoptic climatology of disturbances crossing the Nile valley of Egypt and Sudan during 1967 was studied using 1200 GMT horizontal time sections, at surface and upper levels, for thirteen stations roughly along a north-south line from Nicosia (35°N) to Juba (5°N). Disturbances were defined as prescribed departures from dominant surface wind direction or speed at two or more adjacent stations. There were 43 disturbances during the year, most frequent in winter and spring. Almost all were windshift lines associated either with cyclonic centres over or north of the Mediterranean, or with desert depressions. The lines were accompanied by changes in temperature tendency and were therefore advection discontinuities. Because they were followed by falling temperatures they were also cold fronts. In winter, cool air at the rear of the disturbances penetrated deep into Sudan while in late spring, khamsin weather with southerly winds and interdiurnal temperature rises of up to 13° C preceded fronts over northern Egypt. Most disturbances were associated with troughs in the upper tropospheric westerlies.