Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon

Abstract

In light of the droughts and subsequent food crises that have plagued the Ethiopia?Sudan region in the course of its history, and especially during the last 3 decades, the author examines both the interannual and intraseasonal variabilities of the July?September rains and compares them to the Indian summer monsoon. Regional rainfall indexes for the region stretching from Eritrea to Lake Victoria are computed using seasonal totals for the period 1901?88. Daily data for 1982?88 are also considered. Though all these regions are only partly affected by the Indian monsoon cross-equatorial flow and although they are separated from India by an extensive dry belt (Red Sea, Somalia, west Arabian Sea), there exists a close association between summer rainfall variations in India and in the western parts of East Africa. An even stronger relationship is revealed between these latter regions and Bombay surface pressure, with as much as 79% of the variance in common during 1953?88. This relationship has been virtually stable throughout the twentieth century. Although there also exists a statistical connection between East African rainfall and the Southern Oscillation index (SOI), partial correlation coefficients show that the India?East Africa teleconnection is to a large extent independent of SOI. At intraseasonal timescales, this connection with India still holds, although less consistently. Abnormally low pressure along the western coast of India is associated with above normal daily rainfall and westerly wind anomalies over western Kenya. The most significant relationship is found with a lag of 2?6 days after the occurrence of a low pressure anomaly in Bombay. It is suggested that monsoon activity over India is a major trigger for July?September rainfall variability in the East African highlands. Active/strong monsoon conditions correspond to an enhanced west?east pressure gradient near the equator and, therefore, to abnormally strong westerly winds, advecting moisture from the Congo Basin to Ethiopia, Uganda, and western Kenya. This advection is reduced for weak/break monsoon conditions in India. This teleconnection bears some potential for the improvement of intraseasonal and seasonal rainfall forecasting in the Ethiopia/western Kenya area.