Emergence of Inertial Gyres in a Two-Layer Quasigeostrophic Ocean Model

The emergence of Fofonoff-like flows over a wide range of dissipative parameter regimes is explored in a wind-driven two-layer quasigeostrophic model. Two regimes are found in which Fofonoff-like circulations emerge as a direct consequence of the baroclinic nature of the system, since the wind forcing used in these experiments has been shown to inhibit the formation of Fofonoff flows in the barotropic case. The first regime is one in which the magnitudes of the frictional coefficients (viscosity and bottom dissipation) are extremely small. The experiments clearly illustrate the transition of the numerical solution from a conventional wind-driven circulation to an inertial Fofonoff-like regime. The latter circulation first appears in the lower layer and then spreads throughout the water column via barotropization. The second regime, surprisingly, is obtained with very high bottom friction. This result indicates that entropy can be maximized independently in each layer, depending on the distribution of forcing and dissipation. This sheds a new perspective on the common assumption that forcing and dissipation are disruptive effects that prevent the system from displaying a Fofonoff state.