THE DYNAMICS OF AN EQUIVALENT-BAROTROPIC MODEL OF THE WIND-DRIVEN CIRCULATION

Various steady and time-dependent regimes of a quasi-geostrophic 1.5 l ayer model of an oceanic circulation driven by a steady wind stress ar e studied. After being discretized as a numerical model, the quasi-geo strophic equations of motion become a dynamical system with a large di mensional phase space. We find that, for a wide range of parameters, t he large-time asymptotic regimes of the model correspond to low-dimens ional attractors in this phase space. Motion on these attractors is si gnificant in determining the intrinsic time scales of the system. In t wo sets of experiments, we explore the dependence of solutions on the viscosity coefficient and the deformation radius. Both experiments yie lded a succession of solutions with different forms of time dependence including chaotic solutions. The transition to chaos in this model oc curs through a modified classical Ruelle-Takens scenario. We computed some unstable steady regimes of the circulation and the associated fas test growing linear eigenmodes. The structure of the eigenmodes and th e details of the energy conversion terms allow us to characterize the primary instability of the steady circulation. It is a complex instabi lity of the western boundary intensification, the western gyre and the meander between the western and central gyres. The model exhibits ran ges of parameters in which multiple, stable, time-dependent solutions exist. Further, we note that some bifurcations involve the appearance of variability at climatological time scales, purely as a result of th e intrinsic dynamics of the wind-driven circulation.