Extended Abstract (168K)12.4
Inter-hemispheric comparison of the dynamics of rain-producing systems in tropical Africa: A study of climate change within the Kalahari and Sahelian transects
J. A. Adedoyin, University of Botswana, Gaborone, Botswana
ABSTRACT
It is well documented that since the late 60’s different parts of tropical Africa have experienced devastating droughts. The attendant inter-annual variability of rainfall and the shifts in the dynamics of the major rain-producing systems have also been the subject of much research work. This variability is closely linked with other global climatic conditions like the El-Niño (ENSO) phenomenon. The mechanisms for this inter-annual variability are discussed in relation to the modulating effects of continental rain-belt mode, large-scale atmospheric circulation and sea-surface temperature(SST) patterns. Observations show that above-normal (below-normal) Kalahari rainfall is usually accompanied by the southward (northward) displacement in the rain-belt mean axis, and there is a concurrent increase (decrease) in the total rainfall (Shinoda and Kawamura, 1994). Also, above-normal (below-normal) Kalahari rainfall is accompanied by dominant negative (positive) anomalies of 700 hPa heights over southern Africa. The correlation between tropical Africa rainfall patterns and global 700 hPa heights is therefore investigated with a two-layer model of the atmosphere. Results show that the most unstable perturbations occur when the interface between tropospheric air masses in tropical Africa is at 700 hPa whilst the locations where tropical Africa monsoon winds attain this critical precipitating depth of 700 hPa is determined by global SST anomalies. Another interesting solution was obtained for a wave-like disturbance which has a phase speed of 6.0 ms-1 in the East-West direction, a wavelength of 2000 km, a period of 3.49 days and growth rate of 3.6 per hour, when the surface of discontinuity between the North East monsoon and the South East trade winds, in southern Africa, is at 500 hPa (Mphale, 1999). It is shown that the axis of this wave is greatly influenced by the Indian Ocean SST. A look at the ECMWF analyses for MSL and vertical velocity at 400 hPa during a period of convective activities over Botswana (15-21 February 1995) confirmed the existence of this wave pattern. In Sahelian Africa, the same dynamic pattern is observed. This is shown to influence the southward shift in the axis of squally activities during dry ENSO events, thereby depriving the northern part of this sub-region of its main rain-producing mechanism. Implications of observed variabilities on water resources, agriculture, energy, health-related issues and the tourism industry are discussed.
Adedoyin, J.A. (1997) Instability of squall-inducing waves, global sea-surface temperature anomalies and climate change in tropical North Africa, Meteorol. Atmos. Phys., 62, 79-89
Mphale, K.M. (1999) Dynamics of rain-producing systems in southern Africa: A case study of Botswana, M.Sc Dissertation, University of Botswana
Shinoda, M. and Kawamura, R. (1994) Tropical African rainbelt and global sea-surface temperatures: Inter-hemispheric comparison, Proceedings of the International Conference on monsoon variability and prediction, I.C.T.P., Trieste, Italy, 9-13 May, 1994
Session 12, Regional Issues: Africa
Wednesday, 26 March 2003, 1:30 PM-5:15 PM
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