Modeling the Variability of the Climate System over Lake Victoria Basin

Abstract

The physical mechanisms associated with the diurnal to inter-annual variability of Lake Victoria Basin and regional climate are examined based on a three-tier modeling approach. First, diagnosis of eastern and the Horn of Africa climate variability, in which the Lake basin climate is intimately embedded, is performed based on two GCM ensemble simulations. The goal was to identify some of the systematic errors inherent in the GCMs before downscaling their output using regional climate model. The second part evaluates the downscaling ability of RegCM3 over eastern Africa based on multi-year ensemble simulations of the short rains season. Finally, the physical mechanisms associated with Lake Victoria Basin climate variability are investigated using a fully coupled RegCM3-3D lake modeling system.

Overall, the RegCM3 simulated monthly and seasonal rainfall climatology during the short rains season are consistent with the observed in both the simulations forced by GCM output and NCEP reanalysis over specific homogeneous climate sub-regions. However, over central Kenya highlands the model simulates drier than normal conditions throughout the season. The simulated latitude-time (north-south) rainfall evolution over East Africa is quite consistent with the observed during the entire season. This is also in tandem with the expected ITCZ-driven southward migration of regions of rainfall maxima as the season progresses. A particularly distinct feature is the persistent wet conditions simulated over the equatorial belt, between 1oS and 2oN, throughout the season. The wet conditions are apparently associated with local convection induced by topography and Lake Victoria, since there is substantial reduction in the simulated rainfall amount in the 'no-lake' simulations.

In the fully coupled RegCM3-POM simulations, it is evident that topography along the eastern border of Lake Victoria, large-scale moisture transported via the prevailing easterly trades and changes in the physical characteristics of the Lake are among the principal mechanisms associated with Lake Basin climate variability. In particular, one of the most unique evidence from our simulations is that changes in large-scale moisture transported via prevailing trades through the eastern boundary of our model domain results in significant enhancement/suppression of simulated rainfall amount over Lake Victoria Basin throughout the short rains season.