Diapycnal Mixing

Diapycnal mixing is turbulent mixing across surfaces of constant density (isopycnals), transferring heat, salt, and tracers between water masses of different density. Open-ocean interior rates are small, with diapycnal diffusivity near 10^-5 square meters per second in the thermocline, rising to 10^-3 or higher near rough topography where breaking internal tides and lee waves dissipate. This mixing supplies the buoyancy that lifts dense bottom water and closes the lower limb of the meridional overturning circulation. The Osborn relation links the diffusivity to the turbulent dissipation rate through a mixing efficiency near 0.2.