Intraseasonal Precipitation Variability Over Tropical Africa
Berhane, Fisseha Gebremariam
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Intraseasonal spatiotemporal precipitation variability over tropical Africa has significant impacts on rainfed agriculture, human health, and food and water security, affecting the lives of tens of millions of people. However, there is a paucity of studies that focus on precipitation variability over the region on this time-scale, which is essential for more accurate forecasts and precipitation projections in a changing climate. The dissertation research tries to contribute to a better understanding of the dynamics of sub-seasonal precipitation variability over selected regions of tropical Africa. The work includes: (1) analysis of the drivers and mechanisms associated with summer (June-September) precipitation variability in the Blue Nile river basin, (2) assessment of intra-seasonal influences of the Madden-Julian Oscillation (MJO) on the East African long (March-May) and short (October-December) rains, and (3) examination of sub-seasonal impacts of the MJO on Equatorial West Africa’s springtime (March-June) precipitation. In the Blue Nile river basin, Atlantic Ocean influences dominate precipitation variability in the basin in the early rainy season. In the late rainy season, however, connections with the tropical Pacific Ocean and Indian Monsoon region are more prominent. The mid rainy season experiences influences from both the east and the west. Incorporating the intra-seasonal variability of drivers and mechanisms of precipitation variability in the basin is vital for accurate seasonal and sub-seasonal forecasts and for climate projections under a changing climate since the various drivers and mechanisms may respond differently to climate change. The Madden-Julian Oscillation (MJO) influences precipitation over East Africa through a combination of various mechanisms. Further, the impacts of the MJO on precipitation in East Africa shows intra-seasonal and intra-regional variability. Moreover, the MJO shows significant correlations, at reasonable lead times, with precipitation over the region. These results have implications for understanding mechanisms of precipitation variability, and for weather and short-term climate forecasting tools development. Equatorial West African spring rainy season rainfall is modulated significantly by the MJO. The MJO influences rainfall over the region though the direct propagation of the MJO to the region and though equatorial waves triggered by the MJO in the Indo-Pacific region. About three weeks after the start of the suppressed phase of the MJO in the Indian Ocean, the MJO results in enhanced convection and precipitation over Equatorial West Africa. This association between the MJO and precipitation over Equatorial West Africa can be used to improve the accuracy of forecasts over the region.