Tropical Pacific long waves for the 1997-1998 El Niño-La Niña event from an altimetric data assimilation experiment
Boris Dewitte(1) , Séréna Illig(2) , Yves duPenhoat(1) , Laurent Parent(3) , Lionel Gourdeau(1) , and Jacques Verron(3)
14 av. Edouard Belin,
(2) NASA/JPL, 4800 Oak grove Drive, 91109 Pasadena, United States
(3) LEGI, 1025 rue de la Piscine , 38041 Grenoble Cedex 9, France
Abstract: A forced Ocean General Circulation Model simulation of the tropical Pacific in which combined TOPEX/POSEIDON and ERS altimetric data over January 1994-July 1999 are assimilated, is used to investigate equatorial wave characteristics during the intense 1997-1998 El Niño-La Niña event. 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. Kelvin and first meridional Rossby waves are then derived for the first two more energetic baroclinic modes. 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 reflects as first meridional Rossby waves at the eastern boundary (reflection efficiency of ~ 95%) and contribute to push back the warm pool westward. From January 1998, 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. At the western boundary, reflection of Rossby waves takes place for both modes, but a ~50% reflection efficiency 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. This 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.
short description: The results presented in this paper illustrate the benefit in using altimetric data in ocean model to infer information on the vertical structure variability.