Low frequency variation of westerly wind events regulated by ENSO SST

Strong westerly wind events, typically called westerly wind bursts (WWB), have been suggested as a major contributor to the irregularity of the ENSO cycle. Because these wind forcing have spatial and temporal scales far shorter than the interannual ENSO events, they are usually viewed as stochastic forcing external to or independent of the ENSO dynamics. However, recent high-resolution satellite observations indicate that there exists large-scale coherence between ENSO and the low-frequency variation of WWB. Our analysis suggested that such large-scale coherence is due to the regulation effect of ENSO. Central to the regulation mechanism is the coupling between high SST and low sea level pressure (SLP) over the western equatorial Pacific warm pool. When the warm pool is displaced east/westward on ENSO timescales, the low SLP center moves east/westward accordingly and such movement modulates the local SLP gradient on the equator. On the other hand, the SLP gradient provides precondition for the generation of WWB. The activity of WWB is promoted when the background SLP gradient is large eastward during the period such as before and during El Niño. It appears that WWB forcing may be stochastic in temporal domain, but their occurrence and characteristics are regulated by ENSO and so they have a low-frequency variation associated with ENSO. Implications of the ENSO-regulated WWB are investigated with a model.