The Southern Oscillation. Part IV: the precursors south of 15oS to the extremes of the oscillation.

Abstract

The year before a Warm Event takes place in the Southern Oscillation the trough in the westerlies at the surface over the South Pacific Ocean fails to amplify to its normal size in the latitudes north of 45oS during the southern fall and winter. There is therefore an anomalous northerly wind in these months over the Pacific Ocean between 15oS and 45oS, west of 140oW. In contrast, the trough's amplitude is above normal in the fall and winter of the following year when the Warm Event takes place, and one therefore observes an anomalous southerly wind where a northerly anomaly occurred the previous year. Consistent with the different wind anomalies, the temperature of the surface water is higher in the year before the Warm Event than in the year of the event between 15oS and 45oS, from Australia to 140oW. We propose that when the South Pacific Convergence Zone expands toward the south as usual in the southern spring of the year before a Warm Event, the convection in the Convergence Zone is enhanced over the warmer water, and that this contributes to lowering the pressure over large parts of the tropical and subtropical South Pacific Ocean. -from Authors