An analysis of the ENSO signal in the tropical Atlantic and western Indian Oceans

This article examines the time-space evolution of the El Nino-Southern Oscillation (ENSO) signal in the tropical Atlantic and western Indian Oceans, using harmonic analysis. Composites of sea-surface temperatures (SSTs) and other variables are examined for a 24-month period beginning 6 months prior to the year of maximum warming in the Pacific (termed year 0). An ENSO signal is apparent in the Atlantic in six out of eight Pacific episodes and in the Indian Ocean in all eight episodes. Warming begins along the south-eastern Atlantic coast early in year 0, some months later elsewhere in the Atlantic and in the Indian Ocean. Maximum warming occurs in the Atlantic in October-December of year 0, but in the following January-March in the Indian Ocean. In these oceans a 'cold' phase occurs synchronously with the first half of the Pacific episode (July of year - 1 to June of year 0, in the Rasmusson-Carpenter terminology), a 'warm' phase with the second half. Maximum cooling is 1 year prior to maximum warming in both oceans. In the Atlantic the cold phase occurs most consistently; in the Indian Ocean the warm phase occurs most consistently. There is a season-by-season reversal of SST anomalies and, to a lesser extent, pressure anomalies between the cold and warm phases. This is the basis for the biennial component of the ENSO signal. Our results indicate that the ENSO signal in African rainfall variability is a manifestation of ENSO's influence on SSTs in the Atlantic and Indian Oceans and, in turn, their influence on rainfall. The cold and warm phases correspond roughly to enhanced and reduced rainfall over the African continent, respectively. A similar reversal of rainfall anomalies is apparent season-by-season during these phases. The timing of the warming and cooling is relatively constant in the Indian Ocean. However, the onset of the warming and cooling in the south and equatorial Atlantic occurs progressively later from south to north, thus the signal 'propagates' northward. A similar propagation, synchronous in timing and latitude, is evident in ENSO induced rainfall anomalies over southern and equatorial Africa during the 'cold' phase but not during the 'warm' phase. This may be indicative of a switch in the control of rainfall from the Atlantic Ocean to the Indian Ocean during these phases. (C) 1997 by the Royal Meteorological Society.