| dc.description.abstract | In this study the NCAR regional climate model version 2 (RegCM2) was
successfully employed to investigate the impact of the variations in the Lake Victoria
surface temperature on the seasonal climatic conditions of the Lake Victoria basin of East
Africa. Case studies were conducted for the short rains seasons of 1988 and 1997
representing near normal and anomalous (EI Nino) years, respectively. The mam
objective of the modeling investigations was to test the hypothesis that lake surface
temperature (LST) plays a significant role in explaining the persistent memory exhibited
by the Lake-levels during the post-heavy rainfall event (season). For example, it has been
observed that during the years that followed the extremely wet conditions of 1961162,
although rainfall over the surrounding areas returned to normal, the Lake-level persisted
for long.
To test the hypothesis, we adopted the RegCM2 coupled with I-D lake model to
investigate the role of the LST anomalies on the variability of the climatic conditions over
the Lake basin during the short rains in East Africa. The model simulations comprised of
four month-runs, starting from September through December. The first month was
neglected to allow for the model spin-up. The LST was the key variable used to test how
the thermodynamic conditions of the Lake modulate the atmospheric flow pattern within
the basin. In the control experiment, the LST was fixed at the climatological value of
24°C during the entire period of model integration. However, in the anomaly experiments,
the equilibrium LST was either reduced or increased by a constant anomaly of I.SoC.
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It has been observed that the surface temperature over Lake Victoria is warmer by I.SoC
now than it was in 1960's.
The results showed that over the Lake Victoria Basin positive rainfall anomalies
were simulated in the experiments with a warmer Lake and the converse was observed in
the cases when the Lake was cooler. Thus high LST caused enhanced convective activity
over and around the Lake as a result of increased rate of evaporation and horizontal
moisture flux convergence over the Lake. The model simulations also indicated that as
the LST became warmer, regions of high rainfall shifted away from the southern parts of
the Lake. It was evident from the results that the lake-induced precipitation was localized
and the effect of the changes in LST on the rainfall pattern did not extend significantly
beyond the Lake perimeter.
Further to the investigation of the primary hypothesis outlined above, comparison
between 1988 and 1997 simulations clearly illustrated the anomalous (excess) rainfall
associated with the 1997 EI Nino episode over the entire domain. Results of the rainfall
difference showed that the regions of rainfall maxima were simulated over the eastern
part of the domain and along the East African coast. Rainfall maximum of 200mm or
more was simulated during 1997 case than in 1988 case, especially over the Lake Victoria
region, over the East African coast and over the Eastern Highlands. These results showed
that the LST changes could be used as a signal, in addition to other forcing mechanisms,
to predict the seasonal climate variability and hydrological pattern of the Lake Victoria
watershed. | en |
