Physical Mechanisms Associated With the Variability of Lake Victoria Basin Climate

Lake Victoria Basin is one of the agriculturally productive areas in East Africa and thus a major breadbasket for the region. Besides having an agriculturally rich hinterland, the lake also supports and sustains important fisheries, energy supply (hydroelectric power) and is a potent source of both domestic and industrial water supply. The lake is also the primary source of river Nile (the longest river in the world) and as such forms the hydrological ‘placenta' that sustains the lifeline of semi-arid countries downstream, which include the Sudan, Ethiopia and Egypt. In the present study a fully coupled regional climate-3D lake modeling system is employed to investigate some of the physical mechanisms associated with diurnal to seasonal variability of Lake Victoria Basin (LVB) climate. LVB, situated within a shallow continental sag between the two arms of the East Africa Great Rift Valley system provides an enabling environment for complex interactions between regionally induced (lake-land breeze and orographic circulations) and large scale circulation systems. In order to examine the relative influence of different, and/or combinations of, forcings a suite of model experiments have been performed by systematically altering the terrain height, lake surface characteristics and the amount of large-scale moisture advected into the lake region through the four lateral boundaries of the model domain. In general, the mean monthly rainfall simulated over the Lake Basin is comparable to the satellite (TRMM) estimates. The model simulates higher rainfall amounts over the western/northwestern sector of the lake in agreement with the TRMM estimates. Also the least amount of rainfall is simulated over the southeastern quadrant, which reasonably agrees with satellite estimates. Furthermore, the simulated diurnal variability of rainfall over the four quadrants of the lake is also consistent with the satellite observations. The evidence adduced from the simulations with maximum terrain height around the lake smoothed to the lake surface elevation indicate that the topography along the eastern border of Lake Victoria play an important role in organizing and enhancing Lake Basin precipitation. However, negligible changes in simulated amount and spatial pattern of precipitation occur when the maximum terrain height over the western border of the lake is similarly smoothed. The interactions between the upslope/downslope flow generated by the mountains over the eastern border of the Lake and the land-lake breeze circulations are shown to influence the intensity, location of lake/land breeze fronts and the horizontal extent of the lake-land breeze circulation. This also determines the locations of flow convergence and regions of rainfall maximum within the Lake Basin. Results obtained from the simulations in which the lake surface is replaced with mash (as typified by the recent invasion of the lake by water hyacinth ( Echhornia crassipes)), show that the late night and early morning rainfall maximum located over the western sector of the lake shown in the control simulations dramatically decreases. This is also consistent with the simulated net subsidence of motion over the lake at night. A particularly peculiar result shown in our simulations is the impact of large-scale moisture over Lake Victoria Basin. In contrast to the notion advanced in some of the previous studies that Lake Victoria creates its own climate (rainfall) through precipitation-evaporation-reprecipitation processes, our simulations indicate that large-scale moisture transported via the prevailing easterly trades enhances Lake Basin precipitation significantly. In particular, our simulations show that a 50% reduction in the large-scale moisture entering the interior domain (Lake Basin) through the eastern boundary located over western Indian Ocean results in significant reduction (~40-50%) in the simulated rainfall over the lake surface and surrounding regions throughout the season, compared to the control.