Variability of the diapycnal mixing coefficient in coastal oceans investigated with direct microstructure measurements

Mass and heat fluxes in the ocean are important for understanding global ocean dynamics and ecosystems. Estimates of the diapycnal diffusivity (K_ρ) are required for quantifying these fluxes. In this regard, one of the key parameters that is required to estimate the diapycnal eddy diffusivity is the “mixing coefficient” (Γ). The diapycnal diffusivity is estimated from a combination of the rate of dissipation of turbulent kinetic energy ε, the buoyancy frequency N (a measure of background density stratification) and Γ. This study investigates how the mixing coefficient (Γ) may be inferred from field measurable parameters using in-situ direct microstructure measurements in coastal oceans. Four microstructure data sets were analyzed to investigate the variability of Γ and associated parameters. While Γ is found to vary widely within a range of O(10⁻³−10¹), it can be parameterized using a ratio of relevant turbulent length scales: the Ellison scale (L_E, which is approximately equivalent to the Thorpe scale) and the Ozmidov scale (L_O). When L_E/L_O is less than unity, the results show that Γ is proportional to (L_E/L_O)^4/3, consistent with previous observations. On the other hand, when L_E/L_O exceeds unity, Γ is approximately a constant with no discernable dependence on L_E/L_O, consistent with a recent theoretical and numerical study.