The effects of the Southwest Indian Ocean tropical cyclones on Ethiopian drought

Tropical cyclones are one of the prominent weather systems that are generated over the tropical oceans. The cyclones that develop in the Southwest Indian Ocean (SWIO) usually travel west then southwest and finally recurve to southeast, generally before reaching the East African coast. However, it is shown in this study how SWIO-tropical cyclones/depressions can indirectly affect Ethiopian weather. Using correlation and composite analyses, interannual and intraseasonal rainfall variations in Ethiopia were compared with the frequency and time of occurrence of the tropical cyclones. Years showing the consecutive occurrence of several tropical depressions over the SWIO coincide with the drought years of Ethiopia. Reciprocally, years of abnormally low frequency of tropical cyclones are associated with heavy rainfall in Ethiopia. Belg rainfall ('small rains', February-May) is much more influenced by the cyclonic activity than Kiremt rainfall ('big rains', June-September), which occurs outside the cyclonic season of the SWIO. On a daily basis, rainfall activity during the Belg period is significantly reduced when a tropical depression is observed in the SWIO, before picking up again a few days later. No systematic time-lag was found between the cyclone occurrence and the decrease in daily rainfall amounts. However, at interannual time-scales, a higher (lower) frequency of tropical depressions during the months of November-January tends to be followed by abnormally low (high) Belg rainfall. One of the mechanisms suggested to explain this seasonal persistence in the frequency of tropical depressions involves sea-surface temperatures in the Southeast Indian Ocean, from where many of the depressions originate. The atmospheric processes accounting for the teleconnection between Ethiopia rainfall and tropical depressions were examined. Monthly anomalies of upper air winds for years showing frequent occurrence of tropical depressions proved significant not only over the SWIO, but also the Northern Hemisphere above Northeast Africa. In addition to a slightly enhanced cross-equatorial flow, diverting moisture advection towards the active centre of the system, much stronger than usual equatorial easterlies in the upper troposphere and a reduced southward excursion of the subtropical jet stream result in weaker convective activity over Ethiopia. (C) 1998 Royal Meteorological Society.