Vertical Structure of Current Velocity Shears in the Main Pycnocline of the Black Sea Based on the in situ Data in 2016

Abstract

The characteristic features of the averaged vertical structure of the current velocity shears are discussed based on the analysis of the LADCP/CTD data collected in three expeditions of the R/V “Professor Vodyanitsky” in the northern Black Sea in 2016: the 87 th cruise (June, 30 – July, 18); the 89 th cruise (September, 30 – October, 20) and the 91 st cruise (November, 16 – December, 5). The maximum of the shear average profile is noted in the main pycnocline layer in the vicinity of the buoyancy frequency maximum. The ratio of the shear mean square to the buoyancy frequency mean square increases almost monotonously with depth from 0.1 to 0.4 in the layer 50–350 m that can indicate (at a qualitative level) relative intensification of turbulent vertical mixing in the lower part of the main pycnocline. The mean profiles of the shear vector rotation angle relative to its direction on the horizon 15 m are given. On the average, the shear vector rotates clockwise and makes about two rotations in the main pycnocline layer. The revealed rotation of the shear vector is due to influence of the internal waves propagating downward at the close to inertial frequency. The hodographs of the current velocity shear exhibiting a well-pronounced rotation with depth are shown. Considered is the example of calculating the parameters of a near-inertial internal wave using the current velocity shear and deformation based on the data of one of the stations.