Shear coincidence: implications of the statistics of ocean turbulence microphysics for global diapycnal mixing

It is well established that small scale turbulent mixing induced by breaking of waves in the interior of the ocean plays a significant role in sustaining the deep ocean circulation and in regulation of tracer budgets such as those of heat, carbon and nutrients. There has been significant progress in fluid mechanical understanding of the physics of breaking internal waves. Connection of the microphysics of such turbulence to the global ocean, however, is significantly underdeveloped. We offer a theoretical-statistical approach, heavily informed by observations, to make such a link and then by employing climatological information show that in the global ocean, regions of optimal turbulent mixing coincide with regions that have a desirable balance of stratification and velocity shear. This optimality depends critically on the statistics of turbulent patches. Energetic mixing zones exhibit efficient bulk mixing that induces significant vertical density fluxes, while quiet zones (with small background turbulence levels), while efficient in mixing, exhibit minimal vertical fluxes. The transition between the less energetic to more energetic zones, quantifications of which we argue depends critically on turbulence statistics, implies upwelling and downwelling of deep waters may be stronger than previously estimated, which in turn has direct implications for the ocean overturning circulation as well as for the global budgets of heat, carbon, nutrients, and other tracers. Impact Statement Waves similar to those observed at the beach exist throughout the ocean interior and are induced by tides, winds, currents, eddies, and other processes. Similar to beach waves, internal waves can also roll up and break. Widespread internal-wave breaking helps drive the ocean circulation by upwelling the densest waters that form in polar regions and sink to the ocean abyss. They also play an important role in transport and storage of heat, carbon, and nutrients. In this work we show how well-understood concepts in wave physics can be used in conjunction with statistics of observed ocean turbulence to improve significantly our understanding of the impact of small-scale mixing on the global ocean, and thereby on the climate system.