Atmospheric and surface variations during westerly wind bursts in the tropical western Pacific

An analysis is made of variations in both the surface energy balance and th e regional atmospheric dynamic and thermal structure during 44 westerly win d bursts (WWBs) in the western equatorial Pacific Ocean from 1979 to 1995. The study assesses winds, convective available potential energy, cloud prop erties, precipitation, surface temperature, and surface heat flux while dis tinguishing between brief (5-25 day periodicity) and sustained (30-90 day) WWBs. Datasets used in the study include fields from the NCEP/NCAR and ECMW F re-analyses, and satellite retrievals of clouds (ISCPP), precipitation (M SU), moisture (TOVS), and surface solar flux. Both brief and sustained WWBs, by definition, experience strong low-level w esterly winds that typically induce an increased surface latent-heat flux o f approximately 30 W m(-2). Enhanced cloud thickness, precipitation, and up per tropospheric easterly wind anomalies accompany surface westerly winds, though maxima in winds lag those in clouds and precipitation by about one d ay for brief WWBs and four days for sustained events. WWBs of both types ex perience strong seasonality, occurring frequently in all seasons except bor eal summer. Important distinctions between brief and sustained WWBs can also be made. W esterly anomalies typically extend above 200 hPa during brief WWBs but are generally confined to the lower troposphere (below 400 hPa) for sustained e vents. Sustained WWBs are also preceded by a quiescent period of reduced cl oud thickness and surface winds that is accompanied by strong incident sola r flux. Convective instability, as judged by a Variety of techniques, incre ases by approximately 30% during this quiescent period. Brief WWBs do not i nclude the precursory surface warming or convective destabilization of sust ained WWBs. Notwithstanding the warming episodes before the events, sustain ed WWBs are associated with a net surface cooling approximately 40% larger than brief WWBs. The relationship between brief and sustained WWBs and the phase of the Madd en-Julian Oscillation (MJO) (as judged from outgoing long-wave radiation) i s also examined. Results support the classification of events as 'brief' an d 'sustained' as used in this study, with brief WWBs occurring frequently d uring both wet and dry phases of the MJO, while sustained WWBs occur unique ly during the MJO wet phase. The association of brief and sustained WWBs wi th the MJO is shown to be independent of the El Nino Southern Oscillation p hase. It is therefore proposed that some, but not all, WWBs may be viewed a s the surface signature of the MJO and that the mechanisms responsible for the MJO play an integral role in the formation and sustenance of sustained WWBs.