Solomon, A. and F.-F. Jin, 2005: A study of the impact of off-equatorial Warm-Pool SST anomalies on ENSO cycles. Journal of Climate, 18, 274-286.


Concurrent with most large El Nino events, there are observed to be cold sea surface temperature (SST) anomalies over the western Pacific warm pool region (WPWP). We present observational evidence that SST anomalies that form in the off-equatorial western Pacific during El Nino-Southern Oscillation (ENSO) cycles are forced by subsurface ocean processes equatorward of 12N and air-sea fluxes poleward of 12N. It is demonstrated that diurnal mixing in the ocean equatorward of 12N plays a significant role in bringing subsurface temperature anomalies to the sea surface during an El Nino event.

The role of SST anomalies equatorward of 12N in ENSO cycles is tested in a Zebiak-Cane coupled model, modified to allow for the impact of subsurface temperatures on SSTs. This coupled model successfully simulates cold SST anomalies in the off-equatorial northwestern Pacific that are observed to occur during the warm phase of ENSO, and the atmospheric response to these anomalies, which is composed of both westerlies in the central Pacific and easterlies in the far western equatorial Pacific. It is found that there is little net change in the zonal mean wind stress at the equator, suggesting that the westerlies cancel the impact of the easterlies on the basin scale tilt of the equatorial zonal mean thermocline depth. The anomalous winds in the central equatorial Pacific are found to increase the amplitude of an El Nino event directly by increasing anomalous warm zonal advection and reducing upwelling. Moreover, the off-equatorial anticyclonic wind stress associated with the cold SST anomalies during the warm phase of ENSO tends to reduce the discharge of the equatorial heat content. Thus the coupled processes over the western Pacific warm pool, can serve as a positive feedback to amplify ENSO cycles.
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