Representing eddy diffusion in the surface boundary layer of ocean
models with general vertical coordinates
Abstract
The mixing of tracers by mesoscale eddies, parameterized in many ocean
general circulation models (OGCMs) as a diffusive-advective process,
contributes significantly to the distribution of tracers in the ocean.
In the ocean interior, diffusive contribution occurs mostly along the
direction parallel to local neutral density surfaces. However, near the
surface of the ocean, small-scale turbulence and the presence of the
boundary itself break this constraint and the mesoscale transport occurs
mostly along a plane parallel to the ocean surface (horizontal).
Although this process is easily represented in OGCMs with geopotential
vertical coordinates, the representation is more challenging in OGCMs
that use a general vertical coordinate, where surfaces can be tilted
with respect to the horizontal. We propose a method for representing the
diffusive horizontal mesoscale fluxes within the surface boundary layer
of general vertical coordinate OGCMs. The method relies on
regridding/remapping techniques to represent tracers in a geopotential
grid. Horizontal fluxes are calculated on this grid and then remapped
back to the native grid, where fluxes are applied. The algorithm is
implemented in an ocean model and tested in idealized and realistic
settings. Horizontal diffusion can account for up to
10\% of the total northward heat transport in the
Southern Ocean and Western boundary current regions of the Northern
Hemisphere. It also reduces the vertical stratification of the upper
ocean, which results in an overall deepening of the surface boundary
layer depth. Lastly, enabling horizontal diffusion leads to meaningful
reductions in the near-surface global bias of potential temperature and
salinity.