Case analysis of a role of ENSO in regulating the generation of westerly wind bursts in the Western Equatorial Pacific

Abstract

This study speculates that the low-frequency ENSO might have a regulating effect on the activity of episodic westerly wind bursts (WWB) in the western equatorial Pacific (WEP) based upon the analysis of two contrasting El Niño-Southern Oscillation (ENSO) phases during the 1996-2000 period and the onset of the 1982 El Niño. It suggests that ∇PWEP, the equatorial sea level pressure (SLP) gradient between 130°E and 160°E, could be a key parameter in relating ENSO to the WWB generation. ∇PWEP is a preconditioning index for the tropical cyclone formation associated with midlatitude atmospheric transient forcing (cold surges). The cyclonic circulation near the equatorial latitudes contributes to prolonged WWB events. ∇PWEP can be regulated by ENSO because the location of the low SLP center, which determines the strength of the ∇PWEP, correlates with the location of the warm water pool, and has an east-west migration in response to the ENSO phases. Through ∇PWEP, the variability of WWB was linked to the ENSO phases. The study finds that ∇PWEP was promoted prior to the 1997 El Niño when the warm pool was large and extended to near the dateline, whereas it was suppressed during the follow on La Niña in 1999/2000 when the warm pool was small and displaced far westward. However, only westerly winds generated in the east of New Guinea could have potential importance to ENSO as the islands over the Indonesian Seas represent a significant blocking to the propagation of a Kelvin wave. It appears that the active WWB in the winter of 1996 were generated on a favorable background conditioned by pre-El Niño state; they facilitated the development of the 1997 El Niño but may not be a sufficient onset mechanism. Further statistical analysis is required to test the veracity of the hypothesis derived from the case analysis of this study.