ATMOSPHERIC-HYDROSPHERIC MECHANISMS OF CLIMATE ANOMALIES IN THE WESTERN EQUATORIAL INDIAN-OCEAN

Atmosphere-ocean mechanisms of rainfall anomalies at the coast of eastern Africa are studied using long-term ship observations in the Indian Ocean, surface current measurements, subsurface casts, upper air analyses by the European Centre for Medium Range Weather Forecasts, rain gauge series in eastern Africa and India, and an index of the Southern Oscillation (SO), the high-SO phase being defined by anomalously high/low pressure at Tahiti/Darwin. The causalities of precipitation anomalies at the coast of eastern Africa differ for the two rainy seasons centered on April-May and October-November, and only the latter is strongly related to the SO. In the high-SO phase, April-May pressure is low over the entire Indian Ocean domain, whereas in October-November, pressure is high in the west and low in the east. Concomitantly, surface waters are anomalously cold in the west, and strong westerlies sweep the equatorial zone of the Indian Ocean. Eastern African rainfall anomalies are related to the SO through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern Africa. (1) Equatorial westerly winds are conducive to lower tropospheric divergence over equatorial East Africa, and in the high-SO phase these are accelerated, especially in October-November, owing to the anomalous eastward pressure gradient. (2) The equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere, which entails cold-water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds. (3) In addition, cold-water anomalies in the western Indian Ocean, most pronounced in October-November during the high-SO phase, also suppress convection. (4) In the high-SO phase, the Indian summer monsoon tends to be strong, leaving behind an anomalously cold western Indian Ocean, which in turn feeds into mechanisms 1 to 3. The eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern Africa.