The Efficiency of Mixing in Turbulent Patches: Inferences from Direct Simulations and Microstructure Observations

Abstract

The time evolution of mixing in turbulent overturns is investigated using a combination of direct numerical simulations (DNS) and microstructure profiles obtained during two field experiments. The focus is on the flux coefficient , the ratio of the turbulent buoyancy flux to the turbulent kinetic energy dissipation rate . In observational oceanography, a constant value 0.2 is often used to infer the buoyancy flux and the turbulent diffusivity from measured . In the simulations, the value of changes by more than an order of magnitude over the life of a turbulent overturn, suggesting that the use of a constant value for is an oversimplification. To account for the time dependence of in the interpretation of ocean turbulence data, a way to assess the evolutionary stage at which a given turbulent event was sampled is required. The ratio of the Ozmidov scale L O to the Thorpe scale L T is found to increase monotonically with time in the simulated flows, and therefore may provide the needed time indicator. From the DNS results, a simple parameterization of in terms of L O / L T is found. Applied to observational data, this parameterization leads to a 50%-60% increase in median estimates of turbulent diffusivity, suggesting a potential reassessment of turbulent diffusivity in weakly and intermittently turbulent regimes such as the ocean interior.