Bifurcation analysis on e ows in a two-layer shallow-water model is used to clarify the dynamical origin of low-frequency variability of the double-gyre wind-driven ocean circulation. In many previous model studies, generic low-frequency variations appear to be associated with distinct regimes, characterized by the level of kinetic energy of the mean e ow. From these transient e ow computations, the current view is that these regimes, and transitions between them, arise through a complex nonlinear interaction between the mean e ow and its high-frequency instabilities (the eddies). On the contrary, we demonstrate here, for a particular (but relevant) case, that the origin of these high- and low-energy states is related to the existence of low-frequency instabilities of steady-state e ows. The low-frequency modes have distinct spatial patterns and introduce preferential patterns oscillating on interannual to decadal time scales into the e ow. In addition, these lowfrequency modes are shown to be robust to the presence of (idealized) topography; the latter may even have a destabilizing effect.
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Nauw et al. (2001) studied this question.