Midtropospheric circulation patterns associated with extreme dry and wet episodes over equatorial Eastern Africa during the Northern Hemisphere spring

Abstract

The anomaly circulation patterns of contrasting wet and dry episodes over the equatorial eastern Africa (EEA) region are studied using the European Centre for Medium-Range Weather Forecasts analyses. The study focuses on the Northern Hemisphere spring (NHS). Examination of composite synoptic-scale features indicates that cyclonic circulation over southern Madagascar in the midtroposphere forces southerly flow through the Mozambique Channel. The southerlies turn eastward on approaching latitude 10°S giving broad westerlies in the latitude belt 10°S to the equator. These southerlies followed by westerlies were associated with wet spells over the EEA region. Dry spells exhibit stronger than normal easterly wind flow over the EEA region. These dry spells are associated with a cyclonic circulation with a center near the point 25°S, 25°E and an anticyclonic circulation over Madagascar that causes northerly winds to blow through the Mozambique Channel. Thus, the southerlies act as a trigger for active convection over EEA during the NHS by providing a surface leaning poleward with height over which the equatorial air mass rises.