NONLINEAR TROPICAL AIR-SEA INTERACTION IN THE FAST-WAVE LIMIT

The fast-wave limit is an approximation useful for understanding many aspects of tropical air-sea interaction. It is obtained when the time scale of dynamical adjustment of the ocean by equatorial waves occurs fast compared to the time scale on which the system is evolving throug h coupled processes. The linear and nonlinear behavior of a simple cou pled model is examined for the Pacific basin. It consists of an SST eq uation for an equatorial band, shallow-water ocean dynamics in the fas t-wave limit governing the thermocline, and an embedded surface layer for equatorial Ekman pumping; it may be characterized as a simple fast -wave limit version of the Neelin model, which is in tum a stripped-do wn version of the Zebiak and Cane model. It offers a converse approxim ation to simple models that retain wave dynamics while eliminating SST time scales. This simple model produces a rich variety of flow regime s. The first bifurcation can give westward-propagating, stationary, or eastward-propagating variability according to the relative strength o f the surface-layer and thermocline processes and the atmospheric damp ing length. These parameter dependences can be largely explained by re ference to the simpler zonally periodic case, but the finite basin and zonally varying basic state introduce east basin trapping. These weak ly nonlinear regimes offer a simple analog of oscillations in a number of other models. Some of the oscillations show thermocline evolution that could be easily mistaken for wave-dependent behavior in other mod els. Over a substantial region of parameter space, two SST modes-one s tationary and one westward-propagating-have comparable growth rate in the linear problem. This introduces mode interaction in the nonlinear problem. Relaxation oscillations at strong nonlinearity prove to be a very robust feature of the model, showing strong parallels to behavior noted in a hybrid coupled general circulation model.