Some aspects of climate variability in the North East Ethiopian Highlands - Wollo and Tigray

This paper presents a review of climate variability in the northeast Ethiopian Highlands, particularly Wollo and Tigray, during the last 10000 years (the Holocene) and an analysis of rainfall variability during the historical period. To date little work has been done on climate reconstruction in Tigray and Wollo, however, a number of ongoing studies in the area using palaeosoil analysis, archaeological techniques and lake level reconstruction will help reveal the magnitude of past climate variability in the region. On recent time scales, rainfall over Wollo is characterised by a distinctive bi-modal pattern with Belg rains in April-May preceding the main wet season Krempt, July to September, which is typical of the study region along much of the eastern escarpment. Further north over Tigray the Belg rains are less marked and the main Krempt rains dominate the seasonal pattern. Only three stations in the region, Dessie, Combolcha and Mekele, possess relatively unbroken records back to the 1950s. In all three, 1984 stands out as the driest year on record due to very low rainfall during the Krempt season, whilst 1999's Belg season was the driest on record. Although the succession of dry years between the late 1970s and late 1980s produced the driest decade in the Ethiopian Highlands this century there is no evidence for a long-term trend or change in the region's annual rainfall regime. Rainfall during the 1990s has seen a return to more humid conditions since the dry 1980s. The paper ends with a discussion of the major influences on rainfall variability over the Ethiopian Highlands: the El Niño-Southern Oscillation - El Niño events tend to be associated with lower than average rainfall; Atlantic, Indian and Pacific Ocean sea surface temperatures play a varying role in different seasons; the strength of the Indian Summer Monsoon; and the frequency of several tropical depressions over the Southwest Indian Ocean.