The Leaky Thermocline

Abstract

A model for the vertical structure of ocean gyres is presented that extends the ideal thermocline theories of Rhines and Young and Luyten et al. to include a cross-layer volume flux associated with geostrophic eddy transfer. A two-and-one-half-layer model is considered that assumes that the intensity of the eddy transfer depends on the local strength of the current. The ideal thermocline models emerge in the limit where the parameter characterizing the cross-layer volume flux is asymptotically small. Inclusion of the eddy-induced volume flux resolves the nonuniqueness of the Sverdrup dynamics in the unventilated pool attached to the western boundary layer. In the ventilated region, solutions of the model equations converge to their ideal counterparts. The circulation is closed explicitly by developing a western boundary layer theory based entirely on the effects of the cross-isopycnal volume flux due to eddies. Unlike most models of western intensification, the leaky boundary layer here is active, and its dynamics are essential for determining the structure of the interior field.