Zonal Variations of Eddy Diffusivities in an ACC-like Channel: Discrete Transport Corridors.

Abstract:

The meridional overturning circulation in a wind-driven re-entrant channel arises from a balance between an Eulerian mean overturning and an eddy overturning. These cancel to leading order in the Southern Ocean's Antarctic Circumpolar Current (ACC). An ACC-like flow, with realistic stratification, zonal transport and distributions of eddy kinetic energy, develops even when these two overturning components cancel completely. Many studies have noted that an enhancement of the Eulerian overturning circulation, which tends to steepen isopycnals, is balanced in part by an enhancement of the eddy circulation that relaxes isopycnal tilt. Thus the domain-averaged isopycnal slope and zonal transport are relatively insensitive to changes in wind forcing. However, the response of the system's mesoscale variability and eddy fluxes is not uniform throughout the domain. We present a process study of an idealized eddy-resolving ACC-like channel with negligible residual overturning to explore how the along-stream distribution of eddy characteristics establishes a balance between wind and eddy overturning circulations.

For each simulation, we decompose the overturning circulation into mean, standing and transient components. As the surface wind stress increases, the standing component balances a larger portion of the mean overturning. This in turn leads to an increasing departure from zonally-symmetric eddy characteristics. A zonal-mean, or net, eddy diffusivity Κ_net is defined as the eddy diffusivity required to exactly balance the mean overturning based on the zonal-mean isopycnal slope, s. This gives Κ_net=τ/ρ₀fs, where τ is the wind stress, ρ₀ is a reference density and f is the Coriolis parameter. Κ_net is compared to local eddy diffusivities, Κ_local, diagnosed directly from the divergent component of the eddy buoyancy flux divided by the local isopycnal slope. We find that with a simple topographic ridge and moderate wind forcing, along-stream averages of Κ_local diverge from Κ_net, and are typically smaller than Κ_net. Strong “transport corridors," where Κ_local≥Κ_net develop in the lee of topography. We explore the sensitivity of the Κ_local distribution and meander structure to wind forcing and to channel length, and we discuss the challenges this presents for eddy flux parameterizations.