Analysis of an interactive instability mechanism for the Antarctic Circumpolar Wave

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 Organi sation (CSIRO) Mark 2 coupled model is analyzed with a more complete treatm ent than that studied by others. It is suggested that this interaction mech anism 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 atmosphe re, and a two-layer ocean with an inactive lower layer, the stability of un iform zonal flow to small perturbations was analyzed. The perturbation equa tions describe the velocity and temperature fields in both the atmospheric and oceanic layers and include the exchange in momentum and heat between th em by surface fluxes. The interaction occurs between long (most notably wav enumbers 2 and 3) barotropic Rossby waves in the atmosphere forced by surfa ce 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 orde r of several decades-indicating that modes can be sustained by the interact ion process but that they may need to be energized by other mechanisms to r each realistic amplitudes in a reasonable time.