Stable and Unstable Air-Sea Interactions in the Equatorial Region

Abstract

The linear stability of two coupled shallow water models of the equatorial atmosphere and ocean is investigated analytically. The ocean-to-atmosphere coupling is parameterized in terms of the sea surface temperature anomaly,T. In one model T is generated by zonal advection, while in the other model it is parameterized in terms of the thermocline depth anomaly, h. In both models, the behavior of a small amplitude wave disturbance is found to be extremely sensitive to the atmospheric first baroclinic mode Kelvin wave speed, Ca and mean zonal wind, ? Most importantly, the growth rates, phase speeds and meridional structures of the disturbances (and their dependence on the above basic state parameters) are sensitive to the form of the atmosphere-ocean coupling. This sensitivity is due to the fact that, for an oceanic Kelvin wave, the two methods of computing T result in different feedback effects. According to the simple analytic models used in this study, in which the meridional component of motion is neglected in both the atmosphere and ocean, equatorially trapped unstable (growing) modes occur only when the ocean-to-atmosphere coupling is parameterized in terms of the advectively produced sea surface temperature anomaly. The resulting growth rates and phase speeds of the growing modes can perhaps account for the onset of an ENSO (El Niño-Southern Oscillation) event, but only in the unusual case in which Ca?|?|.