A model is formulated to study the underlying dynamics of observed asymmetries in laboratory experiments on sidewall boundary layer separation and vertical shear layer instability in rapidly rotating fluids. These experiments show differences in transition behavior depending on the nature, cyclonic or anticyclonic, of the gyre forcing. Such qualitative signatures cannot be explained by the standardf-plane quasi-geostrophic model that is unaffected, apart from a switching of the flow direction, under changes of the sign of the forcing. An important ingredient of the intermediate model derived here is a nonlinear Ekman suction condition, which includes products of the interior velocity and its derivatives. The potential vorticity equation is closed using this nonlinear suction condition and other ageostrophic advections. For the case of a cylindrical,f-plane, barotropic ocean bounded by a vertical wall atr = L, and with a weak bottom slope, the nonlinear Ekman layers inhibit sidewall boundary layer separation in cyclonically forced gyres. However, this effect is more than countered by the stretching of the total vorticity in the interior, leading to a net enhancement of separation in such gyres, relative to what happens for anticyclonic forcing. In vertical shear layer instability, anticyclonic shear zones are destabilized relative to cyclonic shear lines. Both these results are in qualitative agreement with laboratory observations.
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