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-rho) 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 (Gamma). The diapycnal diffusivity is estimated from a combination of the rate of dissipation of turbulent kinetic energy epsilon, the buoyancy frequency N (a measure of background density stratification) and Gamma. This study investigates how the mixing coefficient (Gamma) 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 Gamma and associated parameters. While Gamma is found to vary widely within a range of O(10(-3)-10(1)), 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 Gamma is proportional to (L-E/L-O)(4/3), consistent with previous observations. On the other hand, when L-E/L-O exceeds unity, Gamma is approximately a constant with no discernable dependence on L-E/L-O, consistent with a recent theoretical and numerical study.

Automated summary

This study investigates how the mixing coefficient Gamma can be inferred from field measurable parameters using in-situ direct microstructure measurements in coastal oceans. The results show that Gamma can be parameterized using the ratio of relevant turbulent length scales.