When forced only with the observed record of sea surface temperature (SST) over 1930-2000, NSIPP1 - version 1 of the atmospheric model developed at NASA's Goddard Space Flight Center in the framework of the Seasonal to Interannual Prediction Project - exhibits unparalleled skill at reproducing variability of northern summer precipitation over tropical Africa at interannual to interdecadal time scales.

A Principal Component Analysis (PCA) of July-September precipitation reveals that the two leading tropical African patterns represent variability in two distinct regions, the Gulf of Guinea and the Sahel. The two patterns appear to be separate not only statistically, but also dynamically. While Gulf of Guinea rainfall is controlled by eastern equatorial Atlantic SSTs, Sahel rainfall is sensitive to global, tropical SSTs. The drying trend of the 1970s and 1980s is shown to be related to a warming of the oceans around Africa, Indian as well as Atlantic, while tropical Pacific SSTs, e.g. the El Nino/Southern Oscillation phenomenon, play the dominant role on the interannual time scale.