WESTERLY WIND BURSTS AND SURFACE HEAT FLUXES IN THE EQUATORIAL WESTERN PACIFIC IN MAY 1982

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. © 1990, Meteorological Society of Japan.