Variations of thermohaline structure and turbulent mixing on the Black Sea shelf at the beginning of autumn cooling

Abstract

Microstructure measurements taken prior and after some strong atmospheric events on the shallow Black Sea shelf allowed to track the formation and evolution of the thermohaline structure caused first by wind-induced mixing and local convection, later by a storm surge, and finally by intense heating during a short period of the ‘Indian summer’. It was found that even a day long mild surge can decrease the temperature by 2.5°C and increase salinity by 2 psu over the whole 20–25 m quasi-homogeneous upper layer, which was formed by previous intense vertical storm-induced mixing. During the following period of upwelling-favorable winds, the near-bottom temperature decreased by 8–9°C. The upwelling was accompanied by a series of thermohaline intrusions overlaying the inclined boundary of the sharp near-bottom thermocline. Restratification in the upper 10 m layer in the form of a series of quasi-homogeneous steps was successfully reproduced by a numerical model of wind-induced daytime mixing, followed by nighttime convection, using the measured sea-surface fluxes as background conditions. The vertical turbulence structure was depicted by the logarithm of the averaged kinetic energy dissipation, which showed a parabolic decrease from the boundary layers to the midpoint of internal weakly-stratified part of the water column. Intermittent turbulent patches were superimposed at this background profile, which closely coincided with mean structure of the vertical shear. A correlation between the averaged vertical profiles of the turbulent buoyancy Reynolds number R̂e b, mixing activity  G, and the gradient Richardson number R̂i was found. The  G cannot be solely determined by R̂e b, it also strongly depends on R̂i at all depths. Analytical relations betweeen these variables that may be used for mixing parameterization on the shelf are proposed.