ON THE STABILITY OF THE WIND-DRIVEN CIRCULATION

This work examines the instabilities of steady circulations driven by stationary single-gyre wind forcing in closed rectangular basins with different aspect ratios. The stratified ocean is modeled with quasi-ge ostrophic 1.5-layer (equivalent-barotropic) and two-layer models. As f riction is reduced, a stability threshold is encountered. In the vicin ity of this threshold, unstable steady states and their unstable eigen modes are determined. The structures of the eigenmodes and their assoc iated energy conversion terms allow us to characterize the instabiliti es. In each case, the loss of stability is associated with an oscillat ory instability. Several different instability mechanisms are observed . Which of these is responsible for the onset of instability depends u pon the basin aspect ratio and the choice of stratification (1.5- or t wo-layer). The various mechanisms include instability of the western b oundary current, baroclinic instability of the main recirculation gyre , instability of a standing meander located downstream of the main rec irculation gyre and a complex instability involving several recirculat ions and the standing meander. The periods of the eigenmodes range fro m several months to several years depending upon the kind of instabili ty and type of model. Additional insight into the western boundary cur rent and baroclinic gyre instabilities is provided by an exploration o f the stability of (a) the Munk boundary layer flow in 1.5- and two-la yer models in an unbounded north-south channel, and (b) an isolated ba roclinic vortex on anf-plane.