The causes of drought ....

The causes of drought . . .

Scientists trying to understand what makes the land go dry in the African Sahel, a band of land stretching across the continent south of the Sahara, have been studying a variety of subjects ranging from agricultural practices to atmospheric weather patterns. Researchers have suggested that Sahelian droughts are linked to changes in the sea-surface temperatures (SSTs) in the tropical Atlantic Ocean (SN: 5/4/85, p. 282).

Now a group of scientists at the Meteorological Office in Bracknell, England, concludes in the April 17 NATURE that Sahelian rainfull patterns are related to SSTs on much larger-- nearly global--scales. Working with an improved SST data set, C.K. Folland, T.N. Palmer and D.E. Parker found, for example, that the driest years and epochs in the Sahel tend to occur when both the Southern Hemisphere and the North Indian Ocean are warm. Similarly, they found, the wettest times in the Sahel are associated with cold temperatures in these areas. The influence of global SST patterns is supported by numerical experiments using a computer model of atmospheric circulation.

In another NATURE paper, soon to be published, Palmer uses this model to explore further the individual roles that SSTs in specific areas play in the development of drought. He concludes that while the Atlantic Ocean strongly affects the amount of rainfall over the western Sahel, SSTs in the Pacific also play an important part, and that the worldwide pattern of SSTs exerts a stronger influence on rainfall than do SSTs in any of the ocean regions alone. In the April 17 paper, Palmer and his co-workers say they don't discount the role of soil moisture and other changes in the Sahelian land surface in affecting rainfall, but they "do suggest that the worldwide SST anomalies may have a more fundamental influence on Sahel rainfall.'