Diapycnal mixing is an integral factor in the meridional overturning circulation of the ocean. The slow upwelling in the ocean interior across density surfaces requires counteracting diapycnal diffusion. Diapycnal diffusion in a stably stratified fluid requires mechanical energy and the availability of this energy might well be the controlling factor in setting the strength of the overturning circulation as, e.g., discussed by Munk and Wunsch (1997, 1998). The conventional wisdom is that

• Diapycnal mixing in the ocean interior is driven by intermittent patches of small scale turbulence.
• The turbulent patches have a vertical extent of up to a few meters and are caused by breaking internal gravity waves.
• Internal waves break by either shear or convective instabilities that are caused by chance superpositions or encounters with critical layers.

Müller, P., and M. Briscoe. 2000. Diapycnal mixing and internal waves. Oceanography 13(2):98–103, https://doi.org/10.5670/oceanog.2000.40.

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