The ocean circulation is the result of a balance between forcing at basin scales and dissipation at millimeter scales. While we have measurements of the climatological mechanical and thermal energy sources, and some limited coverage of small-scale dissipation, much less is known about the processes that transfer energy from the forcing to the dissipation scales. The goal of this project is to quantify from observations and numerical models the major pathways towards dissipation in the global oceans. The analysis will be global, but particular attention will be paid to the Southern Ocean, because most of the large scale wind forcing powers this part of the ocean and the resulting currents play a key role in driving the meridional overturning circulation. It is widely believed that most of the wind power input to the surface geostrophic flow drives the large reservoir of available potential energy of the large-scale mean flow, and that this energy is released by baroclinic instability, driving mesoscale eddy kinetic energy (EKE). How this energy is dissipated is much more controversial. The hypotheses to be tested are: 1) A majority of the EKE cascades to the bottom of the ocean through an inverse cascade into larger horizontal scale barotropic motions. 2) A large fraction of this energy flux is dissipated in the bottom boundary layer by quadratic drag. 3) A significant residual energy flux generates topographic waves over rough topography, which have the potential to propagate and upon breaking to drive mixing and dissipation higher in the water column. A combination of satellite and in situ observations, and two realistic, high-resolution ocean general circulation models (OGCMs) will be used.
Quantifying the relative importance of the many pathways to dissipation is a great unsolved problem of physical oceanography. Knowing the mechanisms of energy dissipation is a necessary first step in proper parameterization of diapycnal and along isopycnal mixing processes in more realistic OGCMs. Mixing is a critical oceanic process that must be parameterized in order to have the proper feedbacks operative in state-of-the-art models that simulate present and future climate. Together these results will be crucial for guiding future research on the variability and predictability of the global oceans and the climate that it influences.
