Synoptic settings of westerly wind bursts

Abstract

The morphology of westerly wind bursts (WWBs) during the 1980s is investigated using 1000‐mbar winds analyzed by the European Center for Medium‐Range Weather Forecasts (ECMWF). The variety of synoptic situations in which WWBs occur are explored so that the dynamics of WWB generation and maintenance can be studied and so that those interested in short‐ and long‐term forecasts of conditions over the equatorial Pacific will have an increased knowledge of those elements associated with WWBs. WWBs are identified by searching for episodes in which the zonal wind was at least 5 m s−1 over at least 10° longitude for two or more days. This definition, based on surface winds rather than elevated winds or wind anomalies, reflects the impact WWBs have on air/sea interaction processes. The spatial and temporal criteria provide a synoptic‐scale envelope for such interactions and mitigate against erroneous analyses. According to these criteria, 131 distinct WWBs occurred during the 1980s. A new two‐dimensional classification scheme has been developed; the nine patterns in it describe the near‐surface flow during 90% of these WWBs. A single cyclone or a series of cyclones in one or both hemispheres and several different types of cross‐equatorial flow are the major components of the patterns. Only 8% of the bursts were associated with concurrent cyclones in both the northern and southern hemispheres. Four bursts involving cross‐equatorial flow are subjected to further study. The flow is decomposed into barotropic and baroclinic parts and into rotational and divergent baroclinic components. One of the bursts was predominantly barotropic and another predominantly baroclinic; the cross‐equatorial flow in all four cases was predominantly divergent in nature. The information from these cases, coupled with the typical vertical structures and small meridional length scales associated with the various synoptic patterns, suggests that simple baroclinic models of circulations driven by near‐equatorial heating are not adequate to describe the dynamics during WWBs.