Circulation mechanisms of climate anomalies in East Africa and the equatorial Indian Ocean

Abstract The “Short Rains” at the coast of equatorial East Africa, concentrated around October–November, are highly variable and have an extremely tight concurrent correlation with the westerlies over the central equatorial Indian Ocean. The equatorial westerlies drive the Wyrtki Jet in the upper ocean and enhance the westward temperature gradient, and they are the surface manifestation of a powerful zonal-vertical circulation cell along the Indian Ocean Equator at this time of the year. With a strong equatorial circulation cell, the enhanced subsidence and cool surface waters in the West serve to reduce the rainfall over East Africa, and conversely the enhanced ascending motion and warm waters in the East favor precipitation in Indonesia. In the interannual variability over the second half of the 20th century, the years 1961, 1994 and 1997 stand out with particularly weak westerlies over the central equatorial Indian Ocean and disastrous floods in East Africa. At longer time scales, there is evidence of an enhancement of the equatorial westerlies accompanying the drop of lake levels and onset of glacier recession in East Africa in the last two decades of the 19th century. Despite the tight concurrent correlations, the prospects for seasonal forecasting are modest. Predictability decreased from the 1958–1977 to the 1978–1997 period, although the boreal autumn equatorial zonal circulation cell remained strong throughout.