The Predictability of Rainfall over the Greater Horn of Africa. Part I: Prediction of Seasonal Rainfall

The predictability of each of the three rainy seasons affecting the Horn of Africa is examined using multiple linear regression and cross validation. In contrast to most other empirical forecast models, atmospheric dynamics are emphasized. Two geographical sectors are considered: the summer rainfall region with a single rainfall peak in the boreal summer and an equatorial rainfall region with rainy seasons in both the boreal spring and the boreal autumn. These two seasons are termed the “long rains” and “short rains,” respectively, in much of East Africa. Excellent predictability is noted 5 months in advance for both regions during the boreal autumn season and 2 months in advance for the summer season in the summer rainfall region. There is also excellent predictability for the short rains of the equatorial region 2 months in advance. Two notable findings are that atmospheric variables generally provide higher forecast skill than surface variables, such as sea surface temperatures and sea level pressure, and that ENSO and the Indian Ocean dipole provide less forecast skill than atmospheric variables associated with them. As in other studies, the results show that the spring predictability barrier limits the lead time for the forecasting of spring and summer rainfall.