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
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
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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