The Trident Pacific model. Part 2: role of long equatorial wave reflection on sea surface temperature anomalies during the 1993-1998 TOPEX/POSEIDON period
Résumé
The present study aims to give a quantitative description of the role played by long equatorial waves on sea surface temperature anomalies during the 1993-1998 TOPEX/POSEIDON period and more specifically during the 1997-1998 El Niño using a new Pacific Ocean model called Trident. In a companion paper, the Trident dynamical component was described and validated with observations. Briefly, the model exhibits skill in simulating not only sea level but also surface zonal current variability in the equatorial wave guide. Here the model thermodynamics are described and validated. The thermodynamics of Trident consist of one single equation for interannual sea surface temperature anomalies. Compared to other similar models, the introduction in the temperature equation of a term equivalent to a vertical mixing term improves significantly the temperature simulations in the eastern Pacific. Thus the model comparison to interannual sea level, zonal current and sea surface temperature anomalies is fairly good over the entire equatorial Pacific Ocean. The model is found to be sensitive to both subsurface variability (central and eastern Pacific) and to zonal current advection, especially near the dateline where the data suggest that advection is a first order process for determining sea surface temperature changes. The role of long equatorial wave reflection observed in TOPEX/POSEIDON data is studied by either cancelling the eastern boundary or western boundary reflection. First, at the eastern boundary, although the reflected Rossby waves were found to act against the warming during the onset phase of the 1997-1998 El Niño through zonal advection, the major impact of these reflected Rossby waves was to reinforce the deepening of the thermocline in the eastern Pacific initiated by the impinging Kelvin waves. Therefore, all things considered, the Rossby waves strongly contributed to the warming in 1997-1998 east of 120°W. Secondly, at the western boundary, Rossby wave reflection acted as suggested by the delayed action oscillator theory: the reflected Kelvin waves caused shoalling of the thermocline in the central and eastern Pacific where they weakened the warm or cold conditions observed in the Pacific Ocean. During the 1997-1998 event specifically, they contributed to about a third of the upwelling Kelvin wave amplitude propagating in the central Pacific. The other two thirds of the amplitude were found to be actually wind-forced, strongly suggesting that during that event easterly wind anomalies in the western Pacific played a significant role in the termination of the recent 1997-1998 El Niño event and its switch to the La Niña period.