A model study of oceanic mechanisms affecting equatorial Pacific sea surface temperature during the 1997-98 El Nino

In this study, the processes affecting sea surface temperature variability over the 1992-98 period, encompassing the very strong 1997-98 El Nino event , are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1-2 and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The m odel reproduces the main features of the tropical Pacific mean state, despi te a weaker than observed thermal stratification, a 0.1 m s(-1) too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a sl ight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 10 degree sN-10 degreesS band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during t he 1992-98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s(-1) for surface currents. The model therefore reproduces well the observed interann ual variability, with wind stress as the only interannually varying forcing . This good agreement with observations provides confidence in the comprehens ive three-dimensional circulation and thermal structure of the model. A clo se examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993-96 period and the 1997-98 El Nino. In t he eastern Pacific, cooling by exchanges with the subsurface (vertical adve ction, mixing, and entrainment), the atmospheric forcing, and the eddies (m ainly the tropical instability waves) are the three main contributors to th e heat budget. In the central-western Pacific, the zonal advection by low-f requency currents becomes the main contributor. Westerly wind bursts (in De cember 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997-98 El Nino. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward adve ction of the warm pool that they generated temperature anomalies in the cen tral Pacific. The end of El Nino can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from t he end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical proce sses. In the central Pacific, easterlies pushed the warm pool back to the w est. In the east, they led to a shallower thermocline, which ultimately all owed subsurface cooling to resume and to quickly cool the surface layer.