Turbulent mixing efficiency at an energetic ocean site

Abstract

The unsteady and inhomogeneous forcing of the stratified ocean leads to highly variable turbulent temperature and velocity fields, both in space and time, complicating the characterization of mixing from field measurements. Mixing rates are often inferred indirectly from measurements of turbulent kinetic energy dissipation ϵ via Osborn's model (N is the background stratification) by assuming the flux Richardson number Rf = 0.17 is constant, despite continued debate on its relevance for environmental flows. From high‐frequency velocity measurements, we estimate ϵ within meters of the continental slope and compute Kρ with three different models, which we compare to the turbulent diffusivity for heat KT derived from colocated temperature measurements. We also infer Rf by equating Osborn's relationship for Kρ to our estimated KT. Applying Osborn's model with a fixed Rf = 0.17 overpredicted mixing rates by more than an order of magnitude. The other two models, which reduce the mixing efficiency with increasing magnitude of the ratio of the Ozmidov length scale LO to the Kolmogorov length scale η, fared better than Osborn's fixed model. The best agreement (within a factor of 2) was with a model derived from laboratory experiments, at much lower turbulent Reynolds numbers ReT than our measurements, but covering a wide range of the ratio of LO/η. However, our observations correlated more strongly with the turbulent ReT than with LO/η, likely because the ReT measures the “true” separation between the largest and smallest turbulent overturns. Mixing rate predictions improved when the ReT was used instead of LO/η.