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Prediction and Numerical Simulation of the Regional Climate of Equatorial Eastern Africa

Matayo Indeje
July 28th, 2000

The objective in this investigation was to provide a better understanding of the mechanisms and physical processes responsible for climate variability over the equatorial eastern Africa, and explore potential for short-term climate prediction. Both statistical and numerical methods have been employed in this research. Application of cluster analysis yields 8 and 9 homogeneous climatic zones respectively for the variability of the annual and seasonal rainfall. Regions that are prone to drought or floods during the different phases of El NiƱo-Southern Oscillation (ENSO) are delineated. Positive rainfall anomalies occur in much of the region during March-May (MAM) and October-December (OND) of the ENSO(0) years and negative anomalies dominate during the following ENSO(+1) years. These rainfall patterns are useful for short-term climate monitoring over the region. The relationship between the Quasi-Biennial Oscillation (QBO) in the lower stratospheric zonal winds and the long-rains of MAM is more significant in lagged than in simultaneous, with the most distinct relationships occurring over the western parts of the region. The QBO-index explains about 36% of the seasonal rainfall variance. There is a 60/63 percent likelihood for the occurrence of above/below normal rainfall during the westerly/easterly phase of the QBO-index. The NCAR Regional Climate Model (RegCM2) simulations have demonstrated the added value of the nesting approach in improving regional climate simulations. The model reproduces the observed characteristics of the Turkana low-level jet. The study has shown the importance of orographic forcing, the large-scale background monsoon flow and depth of the channel in the development and maintenance of the jet. Thermal and frictional forcing play equivalent roles as that of the large-scale winds in the formation of the jet. The identified regions of strong winds associated with the jet are important to the safety in the aviation industry and are also potential for alternative renewable energy resources in the form of wind energy. Large-scale orography is the most important factor. Divergence and anticyclonic vorticity partly explains the observed split in the jet cores. The dynamics of the flow in the channel and the dry conditions observed over the wider part of the valley is in partly explained by the Bernoulli theorem as applied to barotropic steady and non-viscous flows. Air-sea interaction phenomenon over the Indian Ocean, and the latitudinal location and intensity of the large-scale Walker and Hadley circulations are the main physical mechanisms responsible for the climate variability over the region during the wet and dry years. Abundant rainfall is associated with the presence of a midtropospheric cyclonic wind shear across the equator, and a negative vertical wind shear. Likewise, dry conditions are associated with the presence a strong westerly/southerly wind anomaly that occurs throughout the troposphere. Short-term climate prediction models developed in this study are capable of skillfully reproducing the space-time evolutions and distribution of the seasonal rainfall over the region, and specifically the observed floods that occurred during the 1997 ENSO year. Preliminary application of the RegCM2 in the prognostic mode successfully produces a 3-months projection of the extreme seasonal anomalies associated with the 1997 ENSO event. These models can be exploited further in operational short-term climate prediction over equatorial eastern Africa.

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