Monthly Mean Island Surface Winds in the Central Tropical Pacific and El Niño Events

Abstract

The monthly mean surface wind changes during recent ENSO events, as observed from 11 islands in the tropical Pacific, are described. Two different composite ENSO wind fields are evaluated and compared. The month-to-month wind changes during each event are also discussed. The wind changes for each event between 1953 and 1980 except 1969 show several common features: (i) Westerly anomalies appear first west of the date line and then at the date line sometime in summer (0) to fall (0), then intensify over the following several months. The anomalies are confined to within ±3° of the equator during this stage. (ii) In either November (0), December (0), or January (+1) there is an abrupt southward shift of the narrow band of westerly anomalies, so that the maximum anomaly is then at ?5°S latitude at the date line, and nearly normal conditions prevail north of the equator. (iii) Westerly anomalies are gone or greatly reduced one to two months after the southward shift. The event-to-event variations are considerable, particularly prior to July (0) and after February (+1), so that composites show much reduced anomaly amplitude and much smaller month-to-month anomaly changes than are typical of any given event. The large amplitude months of the composites show similarities with a composite by Rasmusson and Carpenter, but a number of significant differences are identified. These findings, and their relationship to existing simple ideas concerning tropical Pacific coupled ocean-atmosphere interactions, are discussed.