ON THE SLOW MODE MECHANISM IN ENSO-RELATED COUPLED OCEAN-ATMOSPHERE MODELS

A linear perturbation, coupled ocean-atmosphere model is revisited for further insights into the Fl Nino-Southern Oscillation phenomenon. Th e model oscillates as a slow, eastward propagating mode interpreted as a divergence mode, whose energetics are controlled by the ocean. Grow th requires that the work performed by the wind stress minus the work required to effect the ocean divergence exceeds the loss terms. The in trinsic scale of the atmosphere relative to the basin width is importa nt. For sustainable oscillations, the ocean basin must be large enough so that oppositely directed divergence can develop on opposite sides of the basin. The global aspect of the atmospheric pressure field sugg ests that continental heating may provide either a direct source affec ting adjacent oceans, or a connection between oceans. The important mo del parameters are the coupling and warming coefficients and the ocean Kelvin wave speed. The importance of the Kelvin wave speed derives fr om its specification of the background buoyancy state for the ocean. U pon further simplification, an analytical solution gives similar param eter dependence as found numerically and shows that growth requires bo th large zonal wavelength and a zonal phase lag between the anomalies of wind stress and SST.