Abstract
An Ocean General Circulation Model (OGCM) of the tropical Pacific in which combined TOPEX/Poseidon and ERS sea level anomalies are assimilated over January 1994 through July 1999, is used 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 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 modes contribution is weakly impacted. Kelvin and first‐meridional Rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation. Kelvin waves of both modes constructively contribute to the strong warming observed in 1997, with the first (second) baroclinic mode being more energetic than the second (first) 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 westward. The reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28°C isotherm in the surface layer in the far eastern Pacific from January 1998. At the western boundary, reflections of Rossby waves take place for both modes with an estimated total efficiency of ∼50% at 165°E. This suggests that, in our model, the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 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 an assimilation experiment.
Highlights
[2] From recent modeling and observational studies, there is increasing evidence that the vertical structure of the low
In these regions where the variability associated with the equatorial waves is constrained by the structure and position of the thermocline, the assimilation scheme should help confine the isotherms at the correct position, which means respecting to the vertical gradients
The present paper focuses on an assimilation experiment using a technique which was recently shown to be successful in improving model simulations of the tropical Pacific [Gourdeau et al, 2000; Parent et al, 2002]
Summary
[2] From recent modeling and observational studies, there is increasing evidence that the vertical structure of the low-. In a coupled context, Dewitte [2000] and Yeh et al [2001] showed that the ocean’s vertical structure could determine the timescales of the variability in sea surface temperature anomalies (hereinafter referred to as SSTAs) in the tropical Pacific In their model, the relative contribution of the baroclinic modes (mostly mode 1 versus mode 2) controls the characteristics of the unstable coupled modes and the duration and amplitude of the El Nino events. The tropics, with a marked equatorial thermocline, present a highly linear physics, which may explain why the models are by themselves generally successful in simulating most features of the low-frequency (seasonal to interannual) variability when forced with realistic forcing In these regions where the variability associated with the equatorial waves is constrained by the structure and position of the thermocline, the assimilation scheme should help confine the isotherms at the correct position, which means respecting to the vertical gradients.
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