Long Equatorial Waves in a High-Resolution OGCM Simulation of the Tropical Pacific Ocean during the 1985–94 TOGA Period

A high-resolution oceanic general circulation model (OGCM) of the three tropical oceans is used to investigate long equatorial wave activity in the Pacific Ocean during the 1985–94 TOGA period. The ARPEGE atmospheric general circulation model simulated zonal wind stress forcing and the OPA OGCM simulated dynamic height are interpreted using techniques previously applied to data. Long equatorial waves of the first baroclinic mode (Kelvin and first-mode Rossby waves) are detected propagating in the model outputs during the entire period. A seasonal cycle and interannual anomalies are computed for each long equatorial wave. In the east Pacific basin, long equatorial wave coefficients are dominated by seasonal variations, while west of the date line they display strong interannual anomalies. Interannual long-wave anomalies are then compared to wave coefficients simulated by a simple wind-forced model. The results presented here indicate the major role played by wind forcing on interannual timescales in generating long equatorial waves. Discrepancies between the simple wave model and the OPA first-mode Rossby coefficients allow one to draw limitations of interpreting sea surface variability in terms of waves of the first baroclinic mode alone. Finally, the simple wave model cannot fully explain the Kelvin wave amplitude near the western boundary, nor the first-mode Rossby wave amplitude near the eastern boundary. However, coherency between the Kelvin and first-mode Rossby wave coefficients at both boundaries lead to the conclusion that reflection occurs in this model and contributes to the wave amplitudes as they propagate away from the boundaries.