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 (*155*E). 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.

The role of the southward wind shift in both, the seasonal synchronization and duration of ENSO events

“…These wind events, known as westerly wind bursts (WWB), force downwelling Kelvin waves, which propagate to the eastern equatorial Pacific and ultimately act to warm SST there, potentially initiating the event (e.g., Harrison 1990, 1991). Equatorial westerly wind activity has been associated with tropical cyclones (Keen 1982), cold surges from midlatitudes (Chu 1988), the convectively active phase of the Madden-Julian oscillation (Chen et al 1996;Zhang 1996), or a combination of all three (Yu and Rienecker 1998). Although different definitions have been proposed to diagnose WWB from observations (e.g., Harrison and Vecchi 1997;Yu et al 2003;Eisenman et al 2005), there is a broad agreement that it can be represented roughly by a Gaussian shape in both space and time, where x 0 (160°) and y 0 (0°) are the central longitude and latitude of the wind event, T 0 (10 days) is the time of peak wind, A is the peak wind speed, T (10 days) represents the event duration, and L x (20°) and L y (9°) are the spatial scales.…”

Westerly Wind Bursts: ENSO’s Tail Rather than the Dog?

“…Harrison and Vecchi 1997), occur about 3 times per year on average with higher occurrence associated with El Niño events (Verbickas 1998). They have been linked to a variety of atmospheric phenomena including paired and individual tropical cyclones (Keen 1982), cold surges from midlatitudes (Chu 1988), the convectively active phase of the Madden-Julian oscillation (Chen et al 1996;Zhang 1996), or a combination of all three (Yu and Rienecker 1998).…”

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

“…Based on recent observations (Chu, 1988;Lau, Nakazawa and Sui, 1991;Delcroix et al, 1993), the relatively stable trade wind flow in this region is sometimes interrupted by the "westerly wind bursts", which appear to follow the movement of the super cloud cluster systems. When these super cloud cluster systems move, generally from west to east with a low-frequency mode, into the COARE domain, they greatly enhance the convective activity in this region, and episodically change the structure of regional wind field.…”