A Coupled Modeling Study of the Seasonal Cycle of the Pacific Cold Tongue. Part I: Simulation and Sensitivity Experiments

Abstract

A coupled tropical ocean?atmosphere model that fills the gap between anomalous coupled models and fully coupled general circulation models is described. Both the atmosphere and ocean are represented by two and one-half layer primitive equation models, which accentuate the physical processes in the oceanic mixed layer and atmospheric boundary layer. The two media are coupled through both momentum and heat flux exchanges without explicit flux correction. The coupled model, driven by solar radiation, reproduces realistic seasonal cycles of the mixed layer temperature, currents, and depth, and the surface winds and rainfall in the tropical Pacific. The model results indicate that the equatorial westward phase propagation of the annual warming is primarily caused by zonal temperature advection and downward solar radiation modified by clouds, whereas the wind-evaporation-SST feedback plays a minor role. The meridional wind component appears to have a stronger impact than the zonal wind component on the seasonal cycle of the eastern Pacific cold tongue, because the cross equatorial winds have stronger annual variation, which is more effective in regulation of SST through changing surface evaporation and mixed layer entrainment. The annual variation of the solar forcing is shown to have a significant impact on the long-term mean state. Without the seasonal cycle forcing, the western Pacific warm pool would shift eastward and the latitudinal climate asymmetry in the eastern Pacific would be stronger.