Transport across stable density interfaces in forced stratified turbulence*

Understanding how turbulence enhances irreversible scalar mixing in density-stratified fluids is a central problem in geophysical fluid dynamics. Unstable vertical density inversions are often assumed to be an indirect proxy for mixing when direct measurements of scalar diffusion are unavailable. We here investigate this assumption by using a forced direct numerical simulation of stratified turbulence to consider spatial correlations between the vertical density gradient ∂ρ/∂z and the dissipation rates of kinetic energy ε and scalar variance χ, the latter quantifying scalar mixing. The computational domain develops a vertical density staircase, characterized by relatively well-mixed layers separated by sharp, stable gradients that are correlated with sheared velocity interfaces. While density inversions are most prevalent within the mixed layers, much of the scalar mixing is localized to the intervening interfaces, a phenomenon not apparent if considering local static instability or ε alone. We highlight that while the majority of the domain is indeed characterized by the canonical flux coefficient Γ≡χ/ε=0.2, often assumed in ocean mixing parameterizations, extreme values of χ within the statically-stable interfaces, associated with elevated Γ, strongly skew the bulk statistics.