A diagnostic study of the eastern Africa monsoon circulation during the northern hemisphere spring season

The eastern Africa monsoons during the Northern Hemisphere spring (NHS) season are described based on composites derived from various rainfall anomaly scenarios. The years 1981/1984 were delineated as some of the recent wettest/driest years over this region during the NHS season. Wet/dry spells within these anomalous years were further selected using PCA T-mode analysis. Both the anomalous years and the wet/dry spells identified from these years were used to create wind composites that were used to describe the eastern Africa Monsoon circulation. Composite wind analyses showed the dominance of the westerlies/easterlies in the lower/upper troposphere and their migration from the Southern Hemisphere to the Northern Hemisphere. Analysis of the individual wettest/driest years showed that before the start of the equatorial eastern Africa (EEA) long-rains season, easterlies are dominant near the Equator and westerlies near 15 degrees S in the lower troposphere, while at the end of the rainy season the westerlies were located to the north of the Equator, near 5 degrees N, with easterlies on their lateral sides. However, during the EEA long-rains season westerly/easterly wind events occurred in alternation over the region. These westerly/easterly episodes were associated with wet/dry rainfall spells. Vertical sections of zonal wind component showed that the wind alignment during wet spells was similar to that generally observed in other monsoonal regions, with lower tropospheric westerlies overlain by upper tropospheric easterlies. But the reduced frequency of lower tropospheric westerlies is suggested to be due to the barrier effect of the north-south mountain chains which allow only the most intense westerlies to cross the mountains into the EEA region. Copyright (C) 1999 Royal Meteorological Society.