Symmetry-Breaking Multiple Equilibria in Quasigeostrophic, Wind-Driven Flows

Abstract

The classical Munk problem of barotropic flow driven by an antisymmetric wind stress exhibits multiple steady solutions in the range of moderate to high forcing and moderate to low dissipation. Everywhere in the parameter space a perfectly antisymmetric solution exists in which the strength of the cyclonic gyre is equal and opposite to that of the anticyclonic gyre. This kind of solution has been well documented in the literature.In a subset of the parameter range a pair of nonsymmetric stationary solutions coexists with the antisymmetric solution. For one member of the pair the amplitude of the cyclonic circulation exceeds that of the anticyclonic flow. The other member of the pair is obtained from the quasigeostrophic symmetry y --> -y and psi --> -psi. As a result, the point at which the western boundary current separates from the coast can be either south or north of the latitude at which the antisymmetric Ekman pumping changes sign. This is the first oceanographic example of spontaneous breaking of the quasigeostrophic symmetry.Within the region of parameter space where three solutions are found, a second pair of nonsymmetric stationary solutions emerges; bringing the total number of stationary solutions to five. This last pair of nonsymmetric solutions is characterized by basin-filling gyres with amplitudes much above the Sverdrup prediction. Once again, the separation point is displaced from the latitude of vanishing wind stress curl.The existence of nonsymmetric double gyres in an antisymmetrically forced basin shows that there can be no general rule for determining the point of separation of the boundary current in terms of the relative strength of the subtropical and subpolar forcings.