Mixing on a Shallow Shelf of the Black Sea

Abstract

Microstructure measurements were carried out on the shallow shelf of the Black Sea, from the sea surface to the bottom, using a free-falling BAKLAN-S profiler released from an anchored ship. A northeast to southwest transect consisting of eight measurement stations, with several casts made at each station, enabled the evaluation of microstructure statistics across the shelf. The eddy and scalar diffusivities as well as the mixing efficiencies were evaluated for distinct layers that were identified based on mean stratification and the ?state? of turbulence. These include five main layers with persistent features (upper and bottom boundary layers, diurnal and main pycnoclines, and a stratified weakly turbulent inner layer) and several transient (patchy) features embedded within such layers (quasi-homogeneous, active turbulent, stratified dissipative, and microstructure displacement patches). The Thorpe displacement scale LTh measurements of this study, together with those reported in Dillon and Gibson et al. indicated that the normalized (by the patch thickness hp) Thorpe scale LTh/hp is a function of the mixing Reynolds number Rm and the patch Richardson number Rip, but approaches a constant value for high Rm. Layer-averaged diffusivities, which are of direct utility in computing vertical transports at various depths in shelf waters, were evaluated and from which the weighted column-averaged diffusivity ?K? ≈ 10?4 m2 s?1 in the Black Sea shallow shelf waters under moderate winds in the beginning of the autumn transition season was estimated. This latter value, however, may be an underestimation given the neglect of near-surface (<3 m) turbulent mixing in the calculations.