On the effect of topographically enhanced mixing on the global ocean circulation

The strong influence of enhanced diapycnal mixing over rough topography on bottom-water circulation is illustrated using results from two global ocean model experiments. In the first, diapycnal diffusivity is set to the observed background level of 10(-5) m(2) s(-1) in regions not subject to shear instability, convection, or surface-driven mixing. In the second experiment, mixing is enhanced above rough bottom topography to represent the dissipation of internal tides. Three important results are obtained. First, without the enhanced mixing in the abyssal ocean, the deep North Pacific Ocean becomes essentially a stagnant basin, with little bottom-water circulation and very weak deep stratification. Allowing for the enhanced diapycnal mixing above rough bottom topography leads to increased bottom-water circulation and deep stratification and a potential vorticity distribution in the North Pacific that is much more realistic. Second, the enhanced diapycnal mixing above rough topography results in a significant intensification and deepening of the Antarctic Circumpolar Current, as well as in stronger bottom-water formation around Antarctica. Last, our experiments suggest that dissipation of internal tides and the associated enhanced diapycnal mixing in the abyssal ocean play no part in the circulation of deep water forming in the North Atlantic Ocean and in the associated transport of heat in the ocean.