Introduction

In this article, we investigate the low-dimensional dynamics of an idealized double-gyre ocean circulation as modelled by the equivalent- barotropic Quasi-Geostrophic (QG) equation. We expect such a study to be useful in better understanding the intrinsic low-frequency variability that has been recognized in the recent past in such models. Thus while the time scales of centuries that we consider are like those associated with long-range climate variability on earth, the relevance to more realistic ocean circulation is, however, purely paradigmatic: Various important physical processes including vertical stratification and topography are absent in the QG model we consider.

Besides their characteristic prevalence in the various ocean basins, we consider a double-gyre form of circulation because of the possibility of the internal compensation mechanism in dissipating vorticity that is input in one of the gyres, simply by transporting it to the other. Thus presumably, the lesser understood processes of subgrid-scale parametrization and the associated boundary conditions would be of lesser importance than in a case where such an internal compensation mechanism is absent, e.g. a single gyre circulation. However, in no ocean basin is the wind forcing perfectly antisymmetric as we presently consider.