Tracer subduction and energy cycles in an idealized ACC model, and the
potential for measuring energy transfers from space
Abstract
Fronts, at both mesoscale and submesoscales, are generally hypothesized
to play a significant role in mediating the transfer of tracers from the
surface boundary layer into the interior. With the advent of
computational capabilities numerous high resolution modeling studies
have shown the enhancement of of vertical velocities with increasing
horizontal resolution. In a carefully designed setup of an idealized
channel partially blocked by meridional topography and forced by steady
forcing, idealization of the Antarctic Circumpolar Current, we vary the
horizontal resolution as the control parameter, and analyze the impact
of enhanced vertical velocities on tracer subduction. It is found that
the submesoscale-permitting simulations flux far more tracer downward
than the lower resolution simulations, the 1km simulation takes up
50\% more tracer compared to the 20km simulation,
despite the increased restratifying influence of the resolved
submesoscale processes. A spectral decomposition of the flow and fluxes
illuminated the relative importance of scales, and the inefficiency of
inertia-gravity waves in influencing tracer transport. To further
understand the physical dynamics in these simulations we diagnosed how
energy was being transferred between the mean and eddy kinetic and
potential energy reservoirs (Lorenz energy cycles), and if changing the
resolution influenced this exchange. In particular we focussed on
separating the dynamics of the energy cycles that are active in the
interior of the water column and those that are trapped near the
surface. We also analyzed the inter-lengthscale exchange of energy to
understand the detailed spectral dynamics of the turbulence that is
resolved. Lastly, and probably most relevant to SWOT, we looked at the
energy budgets in terms of velocity and pressure structure functions, to
assess the potential for the future SWOT mission to directly measure the
inter-scale energy transfers at the ocean surface.