Topographic inviscid dissipation of balanced flow

Recent analyses have argued that energy enters the mesoscale, balanced flow at a rate of roughly 1 TeraWatt (1TW = 10⁹Watts). As the mesoscale appears not to be accelerating, energy must be lost from the balanced flow at essentially the same rate. How this loss occurs is unclear, because the natural evolution of balanced flows is to large length scales where viscosity is ineffective. Additionally, frictional interactions with the boundaries are not obviously strong enough to provide the needed sink. Losses from the balanced flow to unbalanced flows (that can be viscously dissipated) are likely to be important although the rates and mechanisms by which this occurs are uncertain. The resolution of this issue is crucial however for the advancement of modeling parameterizations and our understanding of ocean climate. This paper outlines a new mechanism for the direct excitation of unbalanced flow by balanced dynamics, depending only on topography and stratification. Analytical arguments show that when both ingredients are present, balanced flow close to the topography can arrest and efficiently excite high mode internal boundary waves. Numerical simulations demonstrate that these dynamics can lead to features resembling hydraulically critical flow, in which local dissipation and mixing appear to be commonplace.