Abstract
An observational study has been made of the recent decreasing trend of the Sahelian rainfall from the 1950's to the 1980's and its relation to the tropical African rainbelt, tropical atmospheric circulation, and sea surface temperature (SST) pattern. This investigation incorporates the trend and correlation analyses of the rainfall, rainbelt, upper-air, and SST data, along with a rotated empirical orthogonal function (R-EOF) analysis of SSTs.
An analysis for the northern summer season shows that the reduction in total rainfall of the rainbelt is more responsible for the decreasing trend in the Sahelian rainfall than the equatorward retraction of the rainbelt. The decreasing trend in the rainbelt's total rainfall is related to increasing trends in 700hPa heights and 850-500hPa thicknesses throughout tropical Africa. The vertical profile of temperature for Niamey suggests that these trends in the lower troposphere may result from intensified large-scale subsidence.
The trends of the tropical African upper-air are accompanied by a tropical east-west contrasting pattern for the 700hPa heights having increasing trends between the Atlantic-Africa regions and between the Indonesia-central Pacific regions, and decreasing trends over the Indian Ocean.
The R-EOF analysis of SSTs indicates that the occurrence of the above east-west trend pattern is probably associated with the dominant warming trends of the Indian Ocean, while the SST anomaly pattern contrasting the tropical North and South Atlantic documented in previous studies plays a major role in the year-to-year latitudinal displacement of the rainbelt.
