Abstract:
Observations and inverse models suggest that small-scale turbulent mixing is enhanced
in the Southern Ocean in regions above rough topography. The enhancement
extends 1 km above the topography suggesting that mixing is supported by breaking
of gravity waves radiated from the ocean bottom. In other regions, gravity wave
radiation by bottom topography has been primarily associated with the barotropic
tide. In this study, we explore the alternative hypothesis that the enhanced mixing in
the Southern Ocean is sustained by internal waves generated by geostrophic motions
flowing over bottom topography. Weakly-nonlinear theory is used to describe the internal
wave generation and the feedback of the waves on the zonally averaged flow. A
major finding is that the waves generated at the ocean bottom at finite inverse Froude
numbers drive vigorous inertial oscillations. The wave radiation and dissipation at
equilibrium is therefore the result of both geostrophic flow and inertial oscillations and
differs substantially from the classical lee wave problem. The theoretical predictions
are tested versus two-dimensional and three-dimensional high resolution numerical
simulations with parameters representative of the Drake Passage region. Theory and
fully nonlinear numerical simulations are used to estimate internal wave radiation
from LADCP, CTD and topography data from two regions in the Southern Ocean:
Drake Passage and the Southeast Pacific. The results show that radiation and dissipation
of internal waves generated by geostrophic motions reproduce the magnitude
and distribution of dissipation measured in the region.
