Internal gravity waves and turbulence in the abyssal Southern Ocean

Direct observations in the Southern Ocean report enhanced internal wave activity and turbulence in a kilometer above rough bottom topography collocated with the deep-reaching fronts of the Antarctic Circumpolar Current. Recent two-dimensional, idealized numerical simulations show that balanced oceanic flows impinging on rough small-scale topography are very effective generators of unbalanced internal gravity waves and predict vigorous wave radiation, breaking, and turbulence within a kilometer above bottom. However, idealized two-dimensional simulations tend to significantly overestimate the observed levels of internal wave kinetic energy and turbulent energy dissipation. Here we present three-dimensional, realistic-topography simulations with parameters typical of Drake Passage that better agree with the recent direct observations from Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean. The results show that the internal wave generation at the three-dimensional, finite bottom topography is suppressed compared to the two-dimensional case. The suppression is primarily associated with nonlinear flow blocking and splitting effects which reduce the amplitude of the internal waves radiated from topography and the associated wave breaking and turbulence away from topography. The implications of these results for ocean mixing and the global overturning circulation will be discussed.