Correlations between summer Sahel rainfall and Southern Oscillation Index has increased during the last thirty years. At high frequency time scale (periods lower than 8 years), an intertropical Atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature (SST) anomalies between the eastern equatorial parts of Pacific and Atlantic. This zonal connection worked well during most of the El Nino/Southern Oscillation (ENSO) events occurring after 1970; positive/negative SST anomalies in the eastern Pacific/Atlantic led to rainfall deficits over the whole West Africa. At low frequency time scale (periods greater than 8 years), positive SST anomalies in the Indian ocean and in equatorial Pacific existing after 1970 have been associated with decreasing rainfall intensity over West Africa through another zonal divergent circulation. These different time scales remote SST forcings are combined to provide a global zonal divergent circulation anomaly pattern which could explain the strong association between Sahel drought and ENSO dynamics after 1970.
Using a high-resolution regional climate model-the Abdus Salam International Centre for Theoretical Physics Regional Climate Model version 3 (RegCM3)-and station and satellite observations, the authors have studied the spatial heterogeneity of climate variability over Java Island, Indonesia. Besides the wellknown anomalous dry conditions that characterize the dry and transition seasons during an El Niñ o year, analysis of regional model output reveals a wet mountainous south versus dry northern plains in precipitation anomalies associated with El Niñ o over Java during the peak rainy season. Modeling experiments indicate that this mountains/plains contrast is caused by the interaction of the El Niñ o-induced monsoonal wind anomalies and the island/mountain-induced local diurnal cycle of winds and precipitation. During the wet season of El Niñ o years, anomalous southeasterly winds over the Indonesian region oppose the climatological northwesterly monsoon, thus reducing the strength of the monsoon winds over Java. This weakening is found to amplify the local diurnal cycle of land-sea breezes and mountain-valley winds, producing more rainfall over the mountains, which are located closer to the southern coast than to the northern coast. Therefore, the variability of the diurnal cycle associated with this local spatial asymmetry of topography is the underlying cause for the heterogeneous pattern of wet south/dry north rainfall anomalies during El Niñ o years. It is further shown that the mean southeasterly wind anomalies during December-February of El Niñ o years result from more frequent occurrence of a quiescent monsoon weather type, during which the strengthened sea-breeze and valley-breeze convergence leads to above normal rainfall over the mountains.
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