HR: 1340h
AN: A23B-0791    [Abstracts]
TI: Westerly Wind Bursts: ENSO's tail rather than the dog?
AU: * Eisenman, I
EM: eisenman@fas.harvard.edu
AF: Dept of Earth and Planetary Sciences, 20 Oxford St, Cambridge, MA 02138 United States
AU: Yu, L
EM: lyu@whoi.edu
AF: Woods Hole Oceanographic Institution, Physical Oceanography Dept., MS#21, Woods Hole, MA 02543 United States
AU: Tziperman, E
EM: eli@eps.harvard.edu
AF: Dept of Earth and Planetary Sciences, 20 Oxford St, Cambridge, MA 02138 United States
AU: Tziperman, E
EM: eli@eps.harvard.edu
AF: Division of Engineering and Applied Sciences, 20 Oxford St, Cambridge, MA 02138 United States
AB: Westerly wind bursts (WWBs) in the equatorial Pacific occur during the development of most El Ni{\~n}o events and are therefore 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 are modulated by ENSO itself. Satellite 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, we include an idealized representation of WWBs in an intermediate coupled ocean-atmosphere ENSO model, such that the occurrence of WWBs 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 increase in the slow frequency component of the WWBs. This, in turn, makes the amplitude of ENSO events forced by the modulated WWBs twice as large as the amplitude of ENSO events forced by stochastic WWBs. We therefore suggest 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. It is further shown that WWB modulation by the large scale equatorial SST field is equivalent to an increase in the ocean-atmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained.
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3374 Tropical meteorology
DE: 4504 Air/sea interactions (0312)
DE: 4522 El Ni¤o
DE: 3309 Climatology (1620)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting