Equatorial Pacific Easterly Wind Surges and the Onset of La Niña Events

It has become well accepted that Westerly Wind Events (WWE) lasting for about a week play a fundamental role in the onset and maintenance of El Niño events in the tropical Pacific. In this paper we show that there are wind events of similar size and duration that appear to play a similar role in the onset and maintenance of La Niña events. We call these wind events Easterly Wind Surges (EWSs). They have been previously overlooked in studies of the El Niño-Southern Oscillation phenomenon because they do not stand out in wind (speed) records in the same way that WWEs do; only when looking at wind stress records do EWSs appear clearly. We identify the average characteristics of EWSs over several decades, describe the oceanic waveguide cooling associated with them, and show that the average wind event forces ocean model responses consistent with the observed sea surface temperature behavior. Because relatively small increases in easterly wind are sufficient to generate EWSs, it is necessary to observe tropical Pacific winds more accurately than is necessary to observe WWEs. Our analysis depended upon the accurate winds provided by the TAO/Triton moorings.