LONG-RANGE PREDICTION OF THE ONSET AND END OF THE RAINY SEASON IN THE WEST AFRICAN SAHEL

Using upper-wind data for three stations across the West African Sahel, simple empirical schemes for predicting the onset and cessation of the rainy season for more than 2 months ahead are presented. Agriculturally reliable rainfall commences about 70 days after the first sudden changes in wind direction from westerly to easterly at the 400 hPa level and above. These sudden changes signal the withdrawal of the subtropical (westerly) jet, thus leading to the establishment of the African easterly jet at the 700 hPa level. Cessation of rainfall occurs about 80 days after the destruction, at the peak of the rainy season, of the mid-tropospheric (700-400 hPa) vertical wind shear. A method for estimating the length of the rainy or growing season also is proposed.