Extratropical forcing and the burst of equatorial westerlies in the western Pacific: A synoptic study

Abstract

Based on surface winds at 6-h intervals for four Northern Winters and Springs (1970-73), two cases of strong westerly wind bursts were identified in the core of the equatorial western Pacific (~155ºE). One case occurred in early April and another in early May 1972, both prior to the maximum sea surface temperature anomalies along the Peruvian coast during the 1972 ENSO event. During the Northern Spring, as an anomalously strong anticyclone moves rapidly from north-central China to its east coast, the surface wind fields to the southeast of the Philippines respond swiftly, turning from an easterly background to northerly. In the meantime, surface pressure in the far western equatorial Pacific tends to rise. These rapid equatorial resposes are probably due to gravity wave-like motions induced by the pressure-wind imbalance in the midlatitudes. The local pressure increase in the extreme western Pacific enhances the west-to-east pressure gradient in the equatorial trough zone and results in a strong westerly wind acceleration in the core of the equatorial western Pacific. This acceleration is also preceded by a west-to-east displacement of the pressure surge in the equatorial trough zone. The enhanced zonal pressure-gradient force and the associated eastward displacement of the equatorial pressure surge are two critical factors for initiating westerly wind bursts. Westerly wind surges detected primarily from fixed-station data compare favorably with those calculated from ship records of an independent source.