Intraseasonal and Interannual Oscillations in Coupled Ocean-Atmosphere Models

Abstract

This study investigates the behavior of coupled ocean-atmosphere models in an environment where atmospheric wave speeds are considerably reduced from their dry atmospheric values, by, for example, condensation-moisture convergence. Preliminary coupled-model results presented earlier are verified and extended using more complete models and for a range of ocean thermodynamics. The models consist of linear coupled shallow-water equations for the equatorial ?-plane and are distinguished by the form of the equation for sea-surface temperature. The ocean is forced by wind stress, and the atmosphere by latent heating resulting from (i) moisture convergence and (ii) evaporation. Modes are calculated for zonally periodic, unbounded, ocean-atmosphere systems. The results in this paper emphasize the importance of including prognostic atmospheric equations in simple, coupled ocean-atmosphere models, especially for the simulation of intraseasonal variability and its possible interaction with interannual variability. Only low frequency modes (period ≥ 1.5 years) are destabilized by the coupling when atmospheric wave speeds are near dry atmospheric values. However, when atmospheric waves are slowed to a few meters per second and atmospheric dissipation rates are low [less than about (10 days)?1], coupling destabilizes both high frequency (period 30?70 days) and lower frequency modes. The dynamics of the two classes of modes are compared. The high frequency modes are essentially coupled oceanic and atmospheric Kelvin waves, and they rely on fundamentally nonequilibrium atmospheric dynamics for their growth. Their growth rates diminish rapidly with increased atmospheric wave speed or dissipation rate. The motion fields for the low frequency modes feature strong contributions from Rossby, as well as Kelvin, components; the associated atmospheric fields are generally in quasi-equilibrium with the sea surface temperature field, and the modes are not nearly so sensitive to changes in atmospheric wave speed or dissipation rate. Both classes of modes are sensitive to the degree to which surface wind anomalies are able to affect the evaporation rate. The possible relation of the high and low frequency modes to tropical intraseasonal and interannual variability is also discussed. The rapid diminution of growth rate with increased atmospheric dissipation makes it unlikely that the high-frequency coupled Kelvin instability can be by itself a major source of intraseasonal variability. More likely, ocean-atmosphere interaction may amplify intraseasonal variability of internal atmospheric origin; present results suggest that direct wind-evaporation feedback would be more important than the coupled Kelvin instability in such a case.