Tropical Pacific baroclinic mode contribution and associated long waves for the 1994-1999 period from an assimilation experiment with TOPEX/POSEIDON data
Boris Dewitte, Séréna Illig, Laurent Parent, Yves duPenhoat, Lionel Gourdeau and Jacques Verron
We use a forced Ocean General Circulation Model (OGCM) simulation of the tropical Pacific in which combined TOPEX/POSEIDON and ERS altimetric data over January 1994- July 1999 are assimilated, to investigate equatorial wave characteristics during the intense 1997-1999 El Niño-La Niña event. Near the equator, the linear vertical modes are estimated at each grid point and time step of the OGCM simulation with and without assimilation. Consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin, the assimilation results in an increased contribution of the higher-order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western Pacific for the zonal current variability. For pressure, the first baroclinic mode contribution is reduced whereas the higher-order mode contribution is weakly impacted.
Kelvin and first-meridional Rossby waves are then derived for the first two more energetic baroclinic modes in the simulation where TOPEX/POSEIDON and ERS data were inserted. The results indicate that, during the 1997-1998 El Niño, the Kelvin waves of both modes contribute constructively to the strong warming observed in 1997, with the first (second) baroclinic mode being more energetic than the (first) second baroclinic mode in the early (mature) stage of the warming. Kelvin waves of both modes reflect as first meridional Rossby waves at the eastern boundary (reflection efficiency of ~95%) and contribute to push back the warm pool eastward. The reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28°C isotherm at the surface in the far eastern Pacific from January 1998. At the western boundary, reflection of Rossby wave takes place for both modes. Reflection efficiency is larger for the second mode but, because the first baroclinic mode is largely dominant in the west, a ~50% value is derived at 165°E. This suggests that the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions during the 1997-1998 El Niño-La Niña while the zonal advective feedback was at work. More generally, the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data, which can be done through assimilation experiment.