Laboratory experiments of mixing at oceanographic scales

A quantitative description of the efficiency of mixing in stratified flows has remained an elusive goal. A recent review (Gregg et al., 2018) suggests that, in oceanographic situations, a constant value of 0.2 (the canonical value) is a reasonable choice. Deviations from the canonical value have been suggested to depend on the buoyancy Reynolds number (Shih et al., 2005). Monismith et al. (2018) considering a large collection of efficiency measured in the field show that, despite a large scatter, the canonical value is probably an appropriate value, though deviations may be expected at very large values of the buoyancy Reynolds number. In this talk, we revisit the issue with the aid of an experimental dataset acquired in the UNC Joint Fluid Lab. In a large tank, we generate shear-driven turbulent mixing that spans a significant range of the parameter space encountered in the ocean, including values of the buoyancy Reynolds number that are much larger than what can be obtained in numerical simulations. Rather than focusing on the buoyancy Reynolds number, we show that that an appropriately defined Richardson number is a much better predictor for the efficiency. We discuss the implication for the interpretation of the field data.