Westerly Wind Events in the Tropical Pacific and their influence on the coupled ocean-atmosphere system: A review

Observational and modeling aspects about Westerly Wind Events (WWEs) and their influence on the tropical Pacific ocean-atmosphere system are reviewed. WWEs are a large part of the intraseasonal zonal wind activity over the warm pool. They have typical amplitudes of 7 m s -1 , zonal width of 20° longitude and duration of about 8 days. Their root causes are often a combination of various factors including the Madden-Julian Oscillation, cold surges from mid-latitudes, tropical cyclones and other mesoscale phenomena. The relationship between WWEs and the ENSO cycle is complex, involving among others the equatorial characteristics of the WWEs, the oceanic background state and the internal atmospheric variability. Both observational and modeling studies demonstrate that WWEs tend to cool the far western Pacific, shift the warm pool eastward and warm the central-eastern Pacific through the generation of Kelvin waves. They are therefore important processes for the central and eastern Pacific warming during the onset and development phase of El Nino. The strong atmospheric feedbacks that are likely to be generated by the ocean response to WWEs even suggest that a single WWE is capable of establishing the conditions under which El Nino can occur. The important role played by WWEs in the evolution and amplitude of recent El Nino events may therefore strongly limit the predictability of El Nino.