Interannual rainfall variability over western Tanzania

The evolution of the circulation anomalies associated with wet and dry austral summers (October-April) over western Tanzania (11.5-4.75 degrees S, 29-37 degrees E) are investigated. It is found that wet (dry) years are characterized by weaker (stronger) equatorial westerlies over the western Indian Ocean that lead to less (more) export of equatorial moisture away from East Africa. These anomalies arise from an anticyclonic (cyclonic) anomaly over the tropical western Indian Ocean during wet (dry) austral summers. In addition, enhanced (reduced) westerly moisture flux from the southern Congo basin occurs during the wet (dry) seasons. Large-scale modulation of the Indian Ocean Walker cell is also evident in both cases, but particularly for the dry years. During wet seasons, the rains typically begin earlier and end later, tend to be more evenly distributed and end more gradually. For dry seasons, the reverse tends to be the case. An inverse relationship between anomalies in Nino 3.4 sea-surface temperature and those in dry-spell frequency over the region was found. Copyright (c) 2005 Royal Meteorological Society.