Circulation mechanisms of climate anomalies in the equatorial Indian Ocean

Abstract

The circulation mechanisms of climatic anomalies in the equatorial Indian Ocean are diagnosed from longterm surface and upper-air data for the October-November core of the East African "Short Rains" and season of zonal circulation cell along the Indian Ocean Equator. Climatic variability is dominated by variations in the intensity of the zonal circulation cell, manifested in subsidence in the West, ascending motion in the East, divergent westward flow in the upper troposphere, and equatorial surface westerlies (UEQ). The UEQ requires steep zonal pressure gradient, with low pressure in the East. Disastrous floods in East Africa and drought in Indonesia occur in years with weak development of the zonal circulation cell, characterized by high surface pressure anomaly in the East, which is common but not limited to the low phase of the Southern Oscillation (SO). A deficient Indian summer monsoon, common in the low SO phase, leaves behind an anomalously warm western Indian Ocean. With high pressure anomaly in the East during the months preceding October-November the Indonesian waters tend to be anomalously cold. Accordingly, with weak zonal circulation cell and in the low SO phase, the westward gradient of sea surface temperature in the equatorial Indian Ocean tends to be reduced. Weak equatorial zonal circulation in October-November is preceded by flow departures of same sense in the near-equatorial atmosphere evolving since boreal summer, and this is evidenced in the 1958-97 record as a whole, as well as for the individual extreme years 1961, 1994 and 1997. Peculiar to the 1961 extreme event is that it was preceded by an extremely good Indian summer monsoon yet anomalously warm waters in the West of the basin, and that the functionally important surface high pressure anomaly in the East did not coincide with the low SO phase.