The Southern Oscillation. Part IV: The Precursors South of 15°S 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 45°S during the southern fall and winter. There is therefore an anomalous northerly wind in these months over the Pacific Ocean between 15°S and 45°S, west of 140°W. 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 15°S and 45°S, from Australia to 140°W. 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. We demonstrate furthermore that a Cold Event, which is the opposite extreme of the Southern Oscillation, develops in a manner opposite to that of a Warm Event with an enhanced trough and weak trades in the year before the Cold Event, and a depressed trough and strong trades in the year of the event. The surface water over the area of interest south of 15°S therefore becomes colder than normal in the southern winter and spring of the year before the Cold Event. The colder water presumably depresses convection in the South Pacific Convergence Zone, and thus contributes to raising the pressure over large parts of the tropical and subtropical South Pacific Ocean.