Westerly Wind Bursts and Surface Heat Fluxes in the Equatorial Western Pacific in May 1982

Abstract

This paper presents a case study of westerly wind bursts in the equatorial western Pacific (EWP) during May 1982. Using 12-hourly island, atoll, and ship observations, the latitudinal extent of the 11-day long burst was determined to be within about five degrees from the equator, whereas its longitudinal domain varied considerably, raging from 1000 km when the burst was first recognized to more than 3500 km when the burst was fully developed. Consequently, Rossby numbers changed appreciably throughout the burst period, and the acceleration term, as expected, dominate the Coriolis force term at the initial stage of the burst.A pressure increase in the far western equatorial Pacific appeared to follow midlatitude forcing. The imposed zonal pressure distribution there greatly enhanced the otherwise slack background pressure gradient in the EWP and thus favored westerly wind bursts. Westerly winds averaged in the box bounded by 5*N-5*S and 135*-155*E peaked at 12-h and 2 days after the establishment of the equatorial zonal pressure gradient. In addition to the pressure rise in the extreme western Pacific, a pressure plunge in the core of the EWP (*150-160*E) resulted in a strong eastward-directed pressure gradient that acted to prolong the burst.Hovmoller diagrams of the meridionally averaged zonal wind and horizontal divergence in the equatorial waveguide indicate that the major axis of the surface divergence was located on the west of maximum westerlies throughout the burst period. The region of maximum surface convergence was on the eastern terminus of the burst, where convection and ascending motion were strong. Deep convective clouds were sometimes found east of strong westerlies. During the burst period, the zonal circulation in the EWP was marked by two cells, linked by strong convection on the eastern extreme of the burst. While the eastern cell resembled a local Walker circulation, the western cell resembled a reverse one.The mean SST in the aforementioned box fell by 1* during the burst period. The equatorial atmosphere in the same area gained a small amount of heat and a relatively large amount of moisture from the warm ocean during the burst period. These conditions were favorable for the enhancement of moist convection over the equatorial ocean.

Structure of the Mixed Layer and Estimates of Sea Surface Fluxes during TOGA-COARE IOP

“…For westerly wind regimes, the enhanced surface heat fluxes favors the growth of convective clouds, which in turn decreases the heights of mixed layers, as illustrated previously. This result also is in good agreement with estimates of surface heat fluxes in the equatorial western Pacific in May 1982 (Chu and Frederick, 1990). They found that the latent heat flux tripled from the pre-burst period to the burst period of westerly wind and plummeted afterward.…”

Analysis of a Reconstructed Oceanic Kelvin Wave Dynamic Height Dataset for the Period 1974–2005

“…This figure shows that the wave during June-July 1982 was followed by gradual eastward progression of anomalously active convection and the development of mature El Niño conditions. Chu and Frederick (1990) describe an intense westerly wind burst during mid- May 1982. The solid gray line in Fig.…”

Upscale Feedback of Tropical Synoptic Variability to Intraseasonal Oscillations through the Nonlinear Rectification of the Surface Latent Heat Flux*

“…The surface LHF may be calculated according to the following bulk formula (Weare et al 1981;Chu and Frederick 1990):…”