Seasonality of East African Rainfall

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The seasonality of rainfall in East Africa from Ethiopia in the north to Malawi and Mozambique in the south is complex, including bimodal and unimodal rainy seasons. Regions with bimodal rainy seasons, with precipitation maxima and agricultural growing seasons in the spring and fall, occur generally within about 7 degrees of the equator in Kenya, southern Somalia, southern Ethiopia, and northern Tanzania. This rainfall distribution is traditionally explained as being related to having two overhead passes of the sun each year. However, this explanation is not complete since regions 10 or 15 degrees from the equator have one rainy season each year despite two overhead passes of the sun at these latitudes. In addition, equatorial central and western Africa experience humid climate with rainfall throughout the year. This suggests that a more complete understanding of the seasonality of East African rainfall should consider such factors as the roles of the Indian Ocean, topography, and near-equator atmospheric dynamics (with a small or absent Coriolis force) in addition to the solar forcing. This research project is aimed at advancing our understanding of how the seasonality of rainfall in East Africa is determined, and to apply that improved understanding to issues of climate variability and change, including impacts. Analysis of observations, comparing multiple datasets in this data-poor region, and output from high-resolution model simulations will be used to investigate these physical processes to produce a more comprehensive understanding of how the seasonality of rainfall is distributed across East Africa.It is crucial to understand the seasonality of rainfall to improve our ability to predict change and variability, especially in regions such as East Africa where the population is highly vulnerable. Variations in rainfall, including its seasonality, strongly impact the region's agriculture, economy, and water resources which, in turn, affect livelihoods and political stability. These issues will be directly addressed within the project by relating observed and simulated variations and trends in climate variables to variables more directly related to impacts. Changes in climate variables will be used to calculate changes in natural vegetation using a model that relates vegetation type to climate conditions. Changes and trends in growing season days and surface water availability will also be evaluated. These representations will provide direct input for impacts analysis. The research project benefits the climate modeling field more generally in contributing to the development of regional-scale climate simulation, including high-resolution models that can explicitly resolve convection within the governing equations. This is important since the parameterization of convection in models with coarser resolution, such as global climate models, is known to degrade simulations. Broader impacts of the research program also include the education of graduate students in STEM fields.