Mixing efficiency from microstructure measurements in the Sicily Channel

The dissipation flux coefficient, a measure of the mixing efficiency of a turbulent flow, was computed from microstructure measurements collected with a vertical microstructure profiler in the Sicily Channel. This hotspot for turbulence is characterised by strong shear in the transitional waters between the south-eastward surface flow and the north-westward deep flow. Observations from the two deep passages in the channel showed a contrast in turbulent kinetic energy dissipation rates, with higher dissipation rates at the location with the strongest deep currents. This study investigated the dissipation flux coefficient variability in the context of mechanically driven turbulence with a large range of turbulence intensities. The dissipation flux coefficient was shown to decrease on average with increasing turbulence intensity Re-b, with median values of 0.74 for low Re-b (<8.5), 0.48 for moderate Re-b (8.5 Re-b <400) and 0.30 for high Re-b (400). The dissipation flux coefficient inferred from the measurements was systematically higher on average than the parameterisation as a function of turbulence intensity suggested by Bouffard and Boegman (Dyn Atmos Oceans 61:14-34, 2013). A plateau at moderate turbulence intensities was observed, followed by a decrease in the dissipation flux coefficient with increasing turbulence intensity as predicted by the parameterisation, but at higher turbulence intensity. The dissipation flux coefficient showed a strong variability with the water column stability regime for the different water masses. In particular, high dissipation flux coefficient (median 0.40) was found at Re-b between 400 and 10(4) for the transitional waters at the northeastern passage, where dissipation rates were high, stratification and shear were strong but the Richardson number Ri was sub-critical. Vertical diapycnal diffusive fluxes were computed, and upward salinity sustained density fluxes of the order of 9 x 10(-6) and 4 x 10(-6) kg m(-2) s(-1) were found to be characteristic of the transitional (28 < sigma <29 kg m(-3)) and intermediate (sigma >29 kg m(-3)) waters, respectively. Turbulent mixing led to a lightening of the transitional and intermediate waters, which was consistent with previous estimates (Sparnocchia et al. J Mar Syst 20:301-317, 1999), but an order of magnitude lower when inferred from the (Bouffard and Boegman Dyn Atmos Oceans 61:14-34, 2013) parameterisation.