Zheng Hao, J. David Neelin and Fei-Fei Jin
Geophys. Res. Lett., 1993.
Abstract. The fast-wave limit is an approximation useful for understanding many aspects of tropical air-sea interaction. It obtains 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 through coupled processes. We examine the linear and nonlinear behavior of a simple coupled model for the Pacific basin. It consists of an SST equation for an equatorial band, shallow water ocean dynamics in the fast-wave limit governing the thermocline and an imbedded surface layer for equatorial Ekman-pumping; it may be characterized as a simple fast-wave limit version of the Neelin (1991) model which is in turn a stripped-down version of the Zebiak and Cane (1987) model. It offers a converse approximation to simple models which retain wave dynamics while eliminating SST time scales.
This simple model produces a rich variety of flow regimes. The first bifurcation can give westward-propagating, stationary or eastward-propagating variability according to the relative strength of the surface-layer and thermocline processes and the atmospheric damping length. These parameter dependences can be largely explained by reference to the simpler zonally-periodic case but the finite basin and zonally-varying basic state introduces east-basin trapping. These weakly nonlinear regimes offer a simple analog of oscillations in a number of other models. Some of the oscillations show thermocline evolution which could be easily mistaken for wave-dependent behavior in other models. Over a substantial region of parameter space, two SST modes---one stationary 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.
Citation. Yang, J.-Y., and J. D. Neelin, 1993: Sea-ice interactions with the thermohaline circulation. Geophys. Res. Lett., 20, 217-220.
Acknowledgments. This work was supported in part by NSF grants ATM-9215090 and ATM-9158294, NOAA grant NA26GP0114-01 and ONR grant N00014-89-1845.