A Study of Westerly Wind Bursts Preceding the 1991-1992 El Nino

Anomalous westerly wind events in the western equatorial Pacific are of interest because of their potential to generate equatorially trapped ocean Kelvin waves that may be related to the development or enhancement of the El Nino. While a number of ideas have been proposed about the nature and origin of the equatorial westerlies, detection and description of these events is hindered by the lack of observational data in the area of interest 3 deg S-3 deg N, 130 deg E-180 deg E. Under such circumstances, the best tool for obtaining a realistic representation of the atmosphere is a four-dimensional global data assimilation system which continually updates a short-term model forecast using synoptic data to produce meteorological fields that are consistent with known dynamical and physical relationships. Using low-level wind fields from the U. S. Navys operational global analysisforecast system for the period October 1990- February 1992, we observed a clear association between significant westerly wind events and tropical cyclone development. In general, there is an eastward and equatorward shift in cyclone activity that is associated with the concentration of westerlies along the equator in the western and central Pacific. The strongest westerlies are associated with the development of dual-hemisphere cyclone pairs, and these events preceded the maximum warming in the eastern Pacific by a couple of months, thus supporting the hypothesis for ocean Kelvin wave contributions to the El Nino. Similar patterns in tropical cyclone activity are observed during other years marking the beginning of the El Nino. El Nino, Southern oscillation ENSO, Westerly wind bursts, NOGAPS.