The efficiency of mixing in turbulent patches: inferences from direct simulations and microstructure observations

The time evolution of mixing in turbulent overturns is investigated using a combination of direct numerical simulations (DNS) and microstructure profi les obtained during two field experiments. The focus is on the flux coeffic ient Gamma, the ratio of the turbulent buoyancy flux to the turbulent kinet ic energy dissipation rate epsilon. In observational oceanography, a consta nt value Gamma = 0.2 is often used to infer the buoyancy flux and the turbu lent diffusivity from measured epsilon. In the simulations, the value of Ga mma changes by more than an order of magnitude over the life of a turbulent overturn, suggesting that the use of a constant value for Gamma is an over simplification. To account for the time dependence of Gamma in the interpre tation 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 Ozm idov scale L-O to the Thorpe scale L-T is found to increase monotonically w ith time in the simulated flows, and therefore may provide the needed time indicator. From the DNS results, a simple parameterization of Gamma in term s 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.