Analysis of an Interactive Instability Mechanism for the Antarctic Circumpolar Wave

Abstract

An interactive atmosphere?ocean instability mechanism that reproduces some salient properties of the observed Antarctic Circumpolar Wave and also its manifestation in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 2 coupled model is analyzed with a more complete treatment than that studied by others. It is suggested that this interaction mechanism is important in maintaining this phenomenon in both the model and the real atmosphere?ocean but is not strong enough to initiate it. Through use of a simple model consisting of a zonally periodic midlatitude beta plane with a uniform mean north?south temperature gradient, a barotropic atmosphere, and a two-layer ocean with an inactive lower layer, the stability of uniform zonal flow to small perturbations was analyzed. The perturbation equations describe the velocity and temperature fields in both the atmospheric and oceanic layers and include the exchange in momentum and heat between them by surface fluxes. The interaction occurs between long (most notably wavenumbers 2 and 3) barotropic Rossby waves in the atmosphere forced by surface heat flux from the ocean and similarly long waves in the upper layer of the ocean forced by the wind stress curl. Growth times are long?on the order of several decades?indicating that modes can be sustained by the interaction process but that they may need to be energized by other mechanisms to reach realistic amplitudes in a reasonable time.