Modeling the inertial recirculation of a gyre

182A common feature of the “time-mean” flow in eddy-resolving models of the large-scale ocean circulation is the so-called “inertial recirculation” region. This region, found in the northwestern corner of the subtropical gyre (northern hemisphere), is usually associated with flow extending throughout the depth of the model ocean, a baroclinic eastward jet extending out from the western boundary into the ocean interior (corresponding to the Gulf Stream) and a much more barotropic westward recirculation. In this article, we begin by reviewing ideas concerning the physical nature of inertial recirculation and its connection with western boundary current transport. A potential vorticity-conserving model is then described. New work is presented in which the governing equations are solved numerically. The method of solution involves separating the equations into a set of vertical eigenmodes and solving the horizontal structure problem associated with each mode. The eigenmodes, which form a complete set, arise naturally from the governing equations and enable simple interpretations of the solutions to be given. The solutions exhibit a baroclinic eastward jet and a much more barotropic westward recirculation like that found in the eddy-resolving models and observed in the Gulf Stream system. It is shown that the vertical shear of the westward recirculation depends on the importance of the relative vorticity in its potential vorticity budget, the shear being greatest when its relative vorticity can be neglected.