A Numerical Simulation Study of Influence of Tropical Instability Waves on Air－Sea Interaction

Satellite observations of sea surface temperature (SST) have recently revealed an existence of unstable oscillations in the equatorial eastern Pacific and Atlantic. These oscillations appear as 20－40-day periodical fluctuations with westward phase speed of 0.4－0.6 m/s and wave length of 1000－2000 km during the boreal summer and fall. They are generally called tropical instability waves (TIWs). They are believed to play an active role in the oceanic heat fluxes in the tropical ocean, locally inducing strong air－sea coupling.In this study, TIWs simulated by a high-resolution atmosphere－ocean coupled general circulation model are investigated. The horizontal resolution of the model is 120 km in the atmosphere and 30 km by 20 km in the ocean. Model simulations show good agreement with observed main features associated with TIWs. In the numerical simulations, it is estimated that the strong cooling effect induced by the equatorial upwelling is partly (about 30%－40%) offset by the equatorward heat flux due to TIWs eddies in the eastern tropical Pacific. Furthermore, the TIWs cast a positive feedback on the atmosphere through changing the vertical mixing just above the TIWs. Though the eddy oscillations induced by TIWs south of equator are much weaker than those north of equator, a similar atmospheric response to TIWs is simulated in the atmospheric mixed layer.