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Episodic westerly wind events are a common feature in the western equatorial Pacific between the months of November and April. When these winds attain amplitude of 5m/s and greater, duration of 5-20 days, and spatial scales of 500-4000 km in longitude and 400-1000 km in latitude, they are called as westerly wind bursts (WWB). The role of WWB in ENSO has gained an increasing attention particularly after the major 1997-98 El Nino event. Studies have suggested that WWB play a major role in the timing and amplitude of the El Niño event and are a very viable candidate for ENSO irregularity.

WWB are usually viewed as stochastic forcing external to or independent of the low frequency ENSO dynamics. However, recent data analysis of Yu et al. (2003) suggested that the occurrence and characteristics of WWB are regulated or controlled by the structure of the large-scale sea surface temperature (SST) of the tropical Pacific Ocean and hence, by ENSO. If this is the case, WWB are an inherent part of ENSO dynamics and their activity can be parameterized as a function of the state of ENSO. This project is to investigate such possible regulation effect of ENSO on WWB and the role of regulated WWB on ENO dynamics, irregularity, and predictability.

I. Hypothesized ENSO regulation effect on WWB

The data analysis study of Yu et al. (2003) hypothesized that WWB are regulated by ENSO through the location of the warm water pool of the western equatorial Pacific. In the tropics high SSTs collocate with low sea level pressures (SLPs). When the warm pool is displaced eastward/westward in association with ENSO phases, the low SLP center moves eastward/westward accordingly. Such movement brings change to the equatorial SLP gradient in the western Pacific. For instance, an eastward (westward) displaced low SLP center generates large eastward (small eastward or westward) SLP gradient. As the SLP gradient is a precondition for the generation of WWB (e.g., the cyclone formation with which WWB are commonly associated), the activity of WWB is promoted when the background SLP gradient is large eastward. The schematic diagram given below summaries this mechanism.

II. Variations of WWB with ENSO phases

It is known that WWB become more frequent, more energetic, and extend farther eastward along the equator prior to and during El Niño and vice versa during La Niña. Such WWB-ENSO relationship is best observed during 1996-2000 by high-resolution, high-accuracy satellite scatterometers (see Fig.2). The regulation effect of the location of the warm pool on characteristics of WWB can be deduced from the fact that (1) WWB occur only over SST higher than 29ºC and (2) the east-west migration of WWB is in conjunction with the east-west displacement of the 29ºC SST isotherm during both El Niño and La Niña phases.

Publications in refereed journals

Contact the following PIs for questions regarding the project research.

Dr. Lisan Yu
Email: lyu@whoi.edu
Phone: 508 289 2504 Fax: 508 457 2181

Mailing address:
Drs. Lisan Yu
Department of Physical Oceanography
Mail Stop 21
Woods Hole Oceanographic Institution
Woods Hole, MA 02543