Mixing in the abyssal ocean is significantly influenced by the breaking of topographically forced internal waves. In the atmosphere this process is well understood in connection with the occurrence of the severe downslope windstorms that often develop in the lee of high topography (eg Peltier and Clark, 1979 and Scinocca & Peltier 1989). In the abyssal oceans the same phenomenology occurs in response to the barotropic tide which leads to the development of a field of internal waves that constitutes the so-called internal tide. When such topographically forced internal waves break, this often leads to the formation of a jet in the topographic lee which thereafter goes unstable via the Kelvin-Helmholtz mechanism. This in turn leads to intense diapycnal mixing.

This talk will focus upon the diapycnal diffusivity that is associated with the fully resolved three dimensional stratified turbulent flows that are expected to occur via this mechanism. Particular attention will be placed upon the impact of high Prandtl number on the mixing, expected at sufficiently high Reynolds number to be relevant to the abyssal ocean environment. The results will be compared to those recently obtained for fluid of Prandtl number unity in which the focus has been upon mixing "efficiency" and upon the utility of the Osborn (1980) formula which relates turbulent dissipation to diapycnal diffusivity (Mashayekhi and Peltier, 2013; Mashayekhi, Caulfield and Peltier, 2013). We are interested in the quantitative comparison of the inferred diapycnal diffusivities with those required by analysis of the large scale energy requirements of the global overturning circulation (eg Munk and Wunsch, 1998).