Abstract
Most of the ocean’s kinetic energy is contained within the mesoscale
eddy field. Models that do not resolve these eddies tend to parameterize
their impacts through down-gradient transport of buoyancy and tracers,
aiming to reduce the large-scale available potential energy and spread
tracers. However, the parameterizations used in the ocean components of
current generation Earth System Models (ESMs) rely on an assumption of a
flat ocean floor even though observations and high-resolution modelling
show that eddy transport is sensitive to the potential vorticity
gradients associated with a sloping sea floor. We show that buoyancy
diffusivity diagnosed from idealized eddy-resolving simulations is
indeed reduced over both prograde and retrograde bottom slopes
(topographic wave propagation along or against the mean flow,
respectively) and that the reduction can be skilfully captured by mixing
length parameterization by introducing the topographic Rhines scale as a
length scale. This modified ‘GM’ parameterization enhances the strength
of thermal wind currents over the slopes in coarse-resolution,
non-eddying, simulations. We find that in realistic global
coarse-resolution simulations the impact of topography is most
pronounced at high latitudes, enhancing the mean flow strength and
reducing temperature and salinity biases. Reducing buoyancy
diffusivities further with a mean-flow dependent eddy efficiency factor
has notable effects also at lower latitudes and leads to reduction of
global mean biases.