Boundary Current Separation in a Quasigeostrophic, Eddy-resolving Ocean Circulation Model

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Abstract
The response of a rectangular, flat-bottom, eddy-resolving, quasigeostrophic ocean to a steady, double-gyre wind stress is studied to assess the sensitivity of the solutions to a partial-slip lateral boundary condition in which tangential stress is proportional to tangential velocity. The constant of proportionality (α) has limiting values of zero and infinity, corresponding to free-slip (no-stress) and no-slip conditions, respectively. Seven numerical solutions—corresponding to the α values 0.0, 2.0, 3.5, 5.0, 6.5, 8.0, and 100.0—are obtained, which span the free-slip and no-slip limits. Significant qualitative changes in the time-mean behavior of the solutions are observed to occur with increasing α. These changes include a gradual retreat of the separation points of the western boundary currents in the subtropical and subpolar gyres, a dramatic reduction in the basin-integrated reservoirs of mean and eddy kinetic energy, a weakening of bottom dissipation and its replacement by lateral dissipation as the dominant sink of kinetic energy, and the emergence of secondary pools of homogenized potential vorticity within the interiors of the time-mean gyres. Similar dependencies on α are found to apply across a broad dynamical regime encompassing alternate type and strengths of lateral friction, asymmetric wind forcing, and dynamically more complete governing equations. Despite the considerable complexity of the solutions as a function of α, a single dynamical interpretation of the process of boundary current separation is found to apply equally well in the no-stress and no-slip limits. In particular, irrespective of the value of α, we find separation to be associated with the occurrence of an adverse value of the higher-order pressure gradient term in the time-mean momentum budget just upstream of the point of separation. The results, therefore, strongly indicate that separation in this model is most easily understood diagnostically as the consequence of boundary current deceleration due to an adverse, along-boundary pressure gradient.

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