13C.4
The seasonal cycle and El Niņo termination
D.E. Harrison, NOAA/ERL/PMEL, Seattle, WA; and G. A. Vecchi
The processes that control the termination of El Niņo are of forecast and scientific importance. We suggest that the seasonal cycle plays a fundamental role in the termination of El Niņo, presenting a complement to 'Delayed-Oscillator' type mechanisms.
A prominent aspect of El Niņo events (observed since the TAO array has been deployed) is thermocline shallowing that begins in the central equatorial pacific sometime between November(0) and January(+1) and propagates eastward in subsequent months. We suggest that this shallowing involves direct atmospheric forcing, in addition to 'Delayed-Oscillator' processes. In this process the equatorially centered surface westerly wind anomalies are shifted by the seasonal solar-driven heating of water south of the Equator, driving thermocline shallowing to the east. This thermocline shallowing preconditions the termination of El Niņo.
During moderate and weak El Niņo events, easterly trades persist to some degree throughout the event. Once shallowing has been forced by the southward shift of equatorial westerly wind anomalies, SST quickly cools through upwelling of cold sub-surface water.
However, during strong El Niņo events the cold tongue equatorial easterlies are weak or non-existent and there is insufficient upwelling to bring cool subsurface water to the surface to reduce SST as the thermocline shallows. Some mechanism is required to bring equatorial easterly winds back, to return SST to normal in these events. We suggest that, again, the seasonal cycle of solar heating and the associated large-scale surface wind changes are the likely mechanism.
Thus, the seasonal cycle appears to be fundamental to the termination of El Niņo.
Session 13C, Ocean Atmosphere Interaction II (Parallel with Sessions 13A, 13B, and 13D)
Thursday, 2 May 2002, 11:00 AM-12:30 PM
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