Pre- and Post-1997/98 Westerly Wind Events and Equatorial Pacific Cold Tongue Warming*

Abstract

Abstract Westerly wind events (WWEs) in the western equatorial Pacific have previously been shown to cause significant warming of sea surface temperature (SST) in the eastern equatorial Pacific. Observational statistics compiled during and prior to the large El Niño event of 1997/98 link WWEs to substantial (up to 3°C) warming in the eastern Pacific cold tongue region. Since 1998, however, relatively little WWE-related cold tongue warming has been observed, and warm equatorial Pacific SST anomalies (SSTAs) have tended to be trapped near the date line rather than extending to the American coast as in a classical El Niño–Southern Oscillation (ENSO) composite. Here, the relationship between WWEs and cold tongue warming is revisited using in situ and operational forecast winds and in situ and satellite-based SST. Significant differences are found in the basin-scale zonal wind anomalies associated with WWEs that occurred before and after 1997/98. Although the post-1997/98 composite WWE westerly anomalies are very similar to their predecessors within the WWE regions, conditions east of the WWE regions are different; there are enhanced equatorial easterlies in the post-1997/98 cases. General ocean circulation model experiments are conducted to explore the extent to which the observed changes in the character of post-1997/98 WWEs can explain the recent behavior of cold tongue SSTAs. It is found that the wind differences can account for the changes in the average cold tongue warming associated with pre- and post-1997/98 WWEs.