STABILITY OF EQUATORIAL MODES IN A SIMPLIFIED COUPLED OCEAN-ATMOSPHERE MODEL

The stability, periodicity, and horizontal structure of equatorial mod es in a coupled ocean-atmosphere model, simplified by the assumption t hat zonal wind stress anomalies are proportional to sea surface temper ature anomalies lagged by a zonal phase difference, are examined analy tically in an unbounded basin. The gravest coupled Rossby and Kelvin m odes coexist with additional westward and eastward slow modes whose ph ase speeds are smaller than the former. Two of these four modes, one p ropagating westward and the other eastward, are destabilized in each c ase depending upon the model parameters. For some particular parameter choices, coupled Rossby and Kelvin modes merge with westward and east ward slow modes, respectively. For other parameters, however, they sep arate and remain distinct from the slow modes. For all of these modes the primary modifications by coupling relative to uncoupled oceanic eq uatorial waves are a decrease in phase speed and an increase in meridi onal scale. Among the model parameter effects, those of the zonal phas e lag between the wind stress and SST anomalies and the coefficients o f thermal and mechanical damping are the most interesting. Positive an d negative phase lags represent the wind stress anomalies located to t he west and east of the SST anomalies, respectively. The frequency of all modes is symmetric about zero phase lag, whereas the growth rate i s antisymmetric about zero phase lag relative to the uncoupled damping rate. Wind stress anomalies to the west of SST anomalies favor slow m ode growth and coupled Rossby and Kelvin mode decay. Dissipation for t he slow modes and the coupled Rossby and Kelvin modes is controlled di fferently. For the slow modes the dissipation is mainly thermal, where as for coupled Rossby and Kelvin modes the dissipation is mainly mecha nical.