Large-scale ocean circulation in the eddying regime: the effects of diffusivity, forcing, and geometry

Classical theories of the ocean's large-scale overturning circulation assume a balance between transport of heat by flows at the basin scale and the irreversible temperature mixing by microscale turbulence. All the dynamics between these two extremes of the spatial scales are considered inconsequential. In particular, the energetic mesoscale currents, i.e. motions on horizontal scales of the order of 100 km are ignored. This study seeks to understand the role of mesoscale flows in the maintenance of the oceanic main thermocline, of the deep stratification, and in the heat transport by examining the climatological response of an idealized eddy-resolving ocean model to variations in the external parameters of diffusivity, geometry, wind stress, and buoyancy forcing. The resulting stratification and circulation patterns are compared the predictions of classical, eddy-free theories.

The most significant departure from the classical theory comes in the diffusive scaling of the overturning circulation and stratification. For small diffusivities, the circulation seems to approach a zero-residual-flow state reminiscent of that observed in zonally-reentrant geometries. In this state, the heat flux by both the mean and eddy flows asymptotically approach finite values while their sum approaches zero. In contrast to eddy-free theories of the vertical stratification, which predict a collapse of the internal thermoclines to discontinuities at zero diffusivity, the vertical stratification in these eddy-resolving models appears to approach a smooth, diffusivity-independent profile for small diffusivities.