Tim Palmer

Abstract:

Folland et al.1 have reported that persistently dry and wet periods of several years in the Sahel have been accompanied by global-scale patterns of sea-surface temperature (SST) anomaly. They also demonstrated that the response of a general circulation model (GCM) of the atmosphere to an observed composite SST difference field between a number of such dry and wet periods showed substantial reduction in Sahel rainfall compared with values from a simulation with climatological SSTs. I examine here the same model's response to the individual components of the composite SST difference field in the Atlantic, Pacific and Indian Oceans. It is found that over the western Sahel, the Atlantic and Pacific fields have a comparable effect in reducing rainfall whereas the Indian Ocean field produces a slight enhancement. Results suggest that, over the eastern Sahel, the Indian Ocean has the dominant role in reducing rainfall. © 1986 Nature Publishing Group.

Abstract:

Using the comprehensively quality-controlled Meteorological Office Historical Sea Surface Temperature data set (MOHSST)1,2 we show for the first time that persistently wet and dry periods in the Sahel region of Africa are strongly related to contrasting patterns of sea-surface temperature (SST) anomalies on a near-global scale. The anomalies include relative changes in SST between the hemispheres, on timescales of years to tens of years, which are most pronounced in the Atlantic. Experiments with an 11-level global atmospheric general circulation model (AGCM) support the idea that the worldwide SST anomalies modulate summer Sahel rainfall through changes in tropical atmospheric circulation3-6. El Nino events may also play a part. We do not discount the effects of soil moisture and albedo changes in the Sahel7,8, although Courel et al.9 have questioned the importance of albedo changes, but we do suggest that worldwide SST anomalies may have a more fundamental influence on Sahel rainfall. © 1986 Nature Publishing Group.