Effects of isopycnal and diapycnal mixing on the stability of oceanic currents

This paper considers several influences of mixing on the stability of oceanic boundary currents, analyzed primarily in terms of the effects of mixing on the distribution of potential vorticity. This perspective relates both to the Charney-Stern criterion for the stability of currents and the Haynes-McIntyre constraints on the evolution of potential vorticity when mixing occurs. It is shown that diapycnal mixing can alter the structure of a stable current so that it becomes potentially unstable according to the Charney-Stern criterion. The effects of isopycnal diffusion and a viscous lateral boundary layer are also considered. Occurrences of instability are then illustrated with both computational solutions and experiments in a rotating tank. Finally, subsurface float trajectories are shown near the coast of Portugal that are suggestive of the instability of the boundary current downstream from the Mediterranean outflow, known to be subject to strong mixing.