Westerly wind bursts (WWBs) in the equatorial Pacific occur during the development of most El
Niño events and are believed to be a major factor in ENSO's dynamics. Because of their short time
scale, WWBs are normally considered part of a stochastic forcing of ENSO, completely external to the
interannual ENSO variability. Recent observational studies, however, suggest that the occurrence and
characteristics of WWBs may depend to some extent on the state of ENSO components, implying that
WWBs, which force ENSO, are modulated by ENSO itself.
Satellite and in situ observations are used here to show that WWBs are significantly more likely to
occur when the warm pool is extended eastward. Based on these observations, WWBs are added to
an intermediate complexity coupled ocean-atmosphere ENSO model. The representation of WWBs is
idealized such that their occurrence is modulated by the warm pool extent. The resulting model run is
compared with a run in which the WWBs are stochastically applied. The modulation of WWBs by ENSO
results in an enhancement of the slow frequency component of the WWBs. This causes the amplitude of
ENSO events forced by modulated WWBs to be twice as large as the amplitude of ENSO events forced
by stochastic WWBs with the same amplitude and average frequency. Based on this result, it is suggested
that the modulation of WWBs by the equatorial Pacific SST is a critical element of ENSO's dynamics, and
that WWBs should not be regarded as purely stochastic forcing. In the paradigm proposed here, WWBs
are still an important aspect of ENSO's dynamics, but they are treated as being partially stochastic and
partially affected by the large-scale ENSO dynamics, rather than being completely external to ENSO.
It is further shown that WWB modulation by the large-scale equatorial SST field is roughly equivalent
to an increase in the ocean-atmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained.