The Trident Pacific model. Part 2: role of long equatorial wave reflectionon sea surface temperature anomalies during the 1993-1998 TOPEX/POSEIDON period

The present study aims to give a quantitative description of the role playe d by long equatorial waves on sea surface temperature anomalies during the 1993-1998 TOPEX/POSEIDON period and more specifically during the 1997-1998 El Nino using a new Pacific Ocean model called Trident. In a companion pape r, the Trident dynamical component was described and validated with observa tions. Briefly, the model exhibits skill in simulating not only sea level b ut also surface zonal current variability in the equatorial wave guide. Her e the model thermodynamics are described and validated. The thermodynamics of Trident consist of one single equation for interannual sea surface tempe rature anomalies. Compared to other similar models, the introduction in the temperature equation of a term equivalent to a vertical mixing term improv es significantly the temperature simulations in the eastern Pacific. Thus t he model comparison to interannual sea level, zonal current and sea surface temperature anomalies is fairly good over the entire equatorial Pacific Oc ean. The model is found to be sensitive to both subsurface variability (cen tral and eastern Pacific) and to zonal current advection, especially near t he dateline where the data suggest that advection is a first order process for determining sea surface temperature changes. The role of long equatoria l wave reflection observed in TOPEX/POSEIDON data is studied by either canc elling the eastern boundary or western boundary reflection. First, at the e astern boundary, although the reflected Rossby waves were found to act agai nst the warming during the onset phase of the 1997-1998 El Nino through zon al advection, the major impact of these reflected Rossby waves was to reinf orce the deepening of the thermocline in the eastern Pacific initiated by t he impinging Kelvin waves. Therefore, all things considered, the Rossby wav es strongly contributed to the warming in 1997-1998 east of 120 degreesW. S econdly, 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 ot her two thirds of the amplitude were found to be actually wind-forced, stro ngly suggesting that during that event easterly wind anomalies in the weste rn Pacific played a significant role in the termination of the recent 1997- 1998 El Nino event and its switch to the La Nina period.