Tropical Pacific sea surface temperature anomalies, El Nino, and equatorial westerly wind events

The authors examine global statistical relationships between westerly wind events (WWEs) and sea surface temperature anomaly (SSTA) variability, using a compositing technique for the period 1986-98. The authors describe the extent to which equatorial WWEs are associated with central and eastern equatorial Pacific waveguide warming and with local SSTA changes under the WWE. Their goal is to quantify the extent to which equatorial WWEs are fundamental to the onset and maintenance of warm El Nino-Southern Oscillation conditions. In order to understand the effect of WWEs on SSTA evolution, they begin by examining how SSTA changes in the absence of equatorial WWEs. They find that SSTA tends toward mean climate values in the absence of equatorial WWEs, whether the eastern equatorial Pacific has close to normal SSTA or warmer than normal SSTA. The two equatorial WWE types whose main surface wind anomalies are west of the date line are associated with weak local surface cooling. The equatorial WWE type that has equatorial westerly wind anomalies east of the date line is associated with weak warming under those anomalies, when the eastern equatorial Pacific SSTA is close to normal. When the tropical Pacific has near-normal eastern equatorial Pacific SST, each of the equatorial WWE types is followed by substantial equatorial waveguide warming in the central and eastern Pacific (composite warming as large as 1.0°C); also more than 50% of the large-amplitude WWEs were followed by Nino-3 SSTA warming in excess of 0.5°C. These changes are of similar amplitude and spatial structure as those seen in the onset of El Nino and are consistent with the predicted oceanic response to WWE forcing. When the eastern equatorial Pacific is initially warmer than usual, the two westernmost equatorial WWE types are associated with the maintenance of warm El Nino eastern and central Pacific SSTA; these warm anomalies tend to disappear in the absence of those WWE types. WWEs, or some mechanism strongly correlated with WWEs, represent a fundamental process for waveguide warming in the onset of El Nino and for eastern and central Pacific warm SSTA maintenance during El Nino.