Physical processes determining the rapid reestablishment of the equatorial Pacific cold tongue

This paper describes a coupled tropical ocean-atmosphere model that fills the gap between anomalous coupled models and fully coupled general circulation models. Both the atmosphere and ocean are described by two and 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, realistically reproduces annual cycles of the tropical Pacific. The outputs of this coupled model is used to diagnose the physical processe determining the rapid annual cooling in the COld tongue and the simultaneous warming in the ITCZ from March to May. The rapid reestablishment of the cold tongue is caused not only by the increases in entrainment, evaporation, and meridional cold advection but also by the annually varying mixed layer (ML) depth. The contribution of the shallow ML in this period to the rapid e`reestablishment of the COld Tongue is neraly as important as the increase of entrainment. It is also found that the deepening of mixed layer considerably increases the efficiency of reduced surface heat flux in cooling the mixed layer. On the other hand, it reduces the cooling efficiencies of the enhanced entrainment heat flux. Without taking into account of the mixed layer depth variations, one can not account for the annual variation of SST in the cold tongue regions reasonably. The ITCZ warming from February to May is primarily caused by the incerase of solar radiation. The northward progression of annual warming in the ITCZ/Cold Tongue Complex is due to the cloud variations in the ITCZ and the oceanic dynamics near the equator. The annual variation of the clouds in the ITCZ changes the annual cycle of solar radiation in the Northern Hemisphere, which combines with latent heat flux resulting in a norhtward march of net surface heat flux. The oceanic dynamics, which are active near rhe equator, amplify the northward progression by quickly changing the equatorial SST annual perturbations. This norhtward progression is thus resulted from the ITCZ/COld Tongue coupling.