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