Sahel rainfall
Drought in the Sahelian zone of northern Africa has been a major cause of the sequence of famines that has affected that region in recent decades. The Sahel rainfall record provides a clear indication of the dessication of that area since the late 1960s.
The Sahel rainfall record can be viewed or downloaded from here. The data are presented as percentage departures from the 1961-90 mean for the months of June to September, the rainy season. Station data for the area 10-20°N and 15°W-30°E have been averaged to form this index. Note that significantly fewer stations were available during the earlier decades of the record.
Discussion
The record of Sahel rainfall shows a pronounced decline in rainfall level starting during the second half of the 1960s. The difference between the means for the periods before and after this date is of the order of 30 per cent. This is the most marked precipiation trend in any part of the world during the present century.
On the year-to-year timescale, drought in the Sahelian region is clearly associated with a difference in sea surface temperature between the northern and southern tropical Atlantic Ocean and, more widely, between the Northern and Southern Hemispheres. This relationship, which is used as a basis for a seasonal forecasting scheme, appears to be related to a shift in the atmospheric circulation over the Sahelian region. The rain-bearing winds fail when the northern tropics are relatively warm.
The causes of the long-term decline in rainfall are unclear. There is, again, a link with the temperature gradient across the tropics. Land degradation in the Sahel itself may also be a contributing factor, altering the heat and moisture budgets of the region.
Key references
Hulme, M. Recent climatic change in the world's drylands. Geophysical Research Letters 23:61-64, 1996.
Hulme, M., and Kelly, M. Exploring the links between desertification and climate change. Environment 35(6): 4-11, 39-45, 1993.
Rowell, D.P., Folland, C.K., Maskell, K., and Ward, M.N. Variability of summer rainfall over tropical North Africa (1906- 92) - observations and modeling. Quarterly Journal of the Royal Meteorological Society121:669-704, 1995.
Acknowledgements
The Sahel rainfall record is maintained by Mike Hulme, Climatic Research Unit, University of East Anglia.