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North Carolina State University (2001)

A numerical modeling study of the coupled variability of Lake Victoria in Eastern Africa and the regional climate

Yi Son

Titre : A numerical modeling study of the coupled variability of Lake Victoria in Eastern Africa and the regional climate

Auteur : Yi Son

Université de soutenance : North Carolina State University

Grade : Doctor of Philosophy (PhD) 2001

Résumé partiel
The objective of this investigation was to investigate and study the coupled atmosphere-lake climate system over the Lake Victoria basin, and determine the corresponding physical mechanisms that are involved. The primary research vehicle for the investigation is a fully coupled model of the regional climate of Eastern Africa and Lake Victoria which has been developed and applied in this study. The atmospheric component of the model is the NCAR Regional Climate Model (RegCM2). The lake component of the model is based on the Princeton Ocean Model (POM) configured for Lake Victoria by replacing the open boundaries in the standard version of the model with a closed coastline and adopting the bythemetry of Lake Victoria. The horizontal resolution is 20 km for both the atmosphere and lake model components.The results show that the bythemetry and geometry of the lake play a fundamental role in determining the climatology of Lake Victoria. There exists Kelvin-like waves in the thermocline trapped along the coast and they propagate clockwise around Lake Victoria with periodicity of about 30 days. The oscillations entirely disappear in the case of the isothermal conditions. The 3-dimensional model produces a surface temperature pattern indicative of horizontal lake water mixing associated with the horizontal spiral pattern that is not present in the 1-dimensional model. Preliminary comparison of the coupled RegCM2-POM model simulation results with the observations indicates that the model produces more realistic lake surface temperatures (LST) and rainfall over and around the lake than the standard version of RegCM2 in which a simple one dimensional thermal diffusion lake model is used. Over Eastern Africa, the regional climate variability is significantly influenced by the circulation over the Lake Victoria basin. Moisture advection contribution is important but secondary to evaporation in explaining the heavy rainfall over the lake. The interaction between the lake-land breeze and the prevailing northeasterly flow accounts for the asymmetry in the distribution of the diurnal rainfall variations and the southwestward movement of the dominant bands of divergence/convergence. During the 1982 El Nino when the averaged LST over the lake was higher than that during the normal year, the LST gradient was weakened along the SW-NE axis over the lake by the strong lake circulation. This results in LST distribution whereby the southwestern region of the lake is cooled while the region of maximum LST moves to the central-eastern region of the lake from the southwestern region of the lake. The net change in rainfall distribution over the lake during the 1982 El Nino is a combination of the effect associated with the large-scale convergence pattern and the meso-scale climate changes associated with the shift of the region of maximum rainfall toward the central-eastern part of the lake from the western part of the lake in response to the LST redistribution.


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