Purpose. This study aims to evaluate the vertical turbulent diffusion coefficient in the lower part of the main pycnocline in the areas of continental slope and deep waters of the Black Sea.

Methods and Results. Data were collected during the 87th cruise of R/V Professor Vodyanitsky in the central sector of the northern Black Sea from June 30 to July 18, 2016. Profiles of temperature, salinity, and current velocity were obtained using CTD/LADCP probes. A method applying the G03 parameterization to a layer ~ 200 m thick, spanning isopycnals with conditional densities between 15.5 and 16.8 kg/m3, is proposed. To suppress measurement noise, isopycnal averaging across the station ensemble and approximation of the resulting parameter profiles using power functions were employed. Differences in the transfer functions for CTD and LADCP data processing were accounted for when integrating the canonical spectrum of internal waves. Data from 20 deep-sea stations enabled the derivation of buoyancy frequency profile averaged over the isopycnals, revealing layers of its power and exponential dependences on depth. The methodological challenges of applying the G03 parameterization to the lower part of the Black Sea’s main pycnocline are discussed in detail, including graphical data presentation. The profiles of the vertical turbulent diffusion coefficient indicate a nearly constant value of ~ 2⋅10⁻⁶ m²/s in the continental slope region, while in the deep waters of the sea, it increases linearly with depth from 1⋅10⁻⁶ m²/s to 2⋅10⁻⁶ m²/s. The maximum calculated heat flux reaches 12 mW/m², confirming its negligible impact on the heating of the cold intermediate layer. The salt flux at the upper boundary of the layer is 6⋅10⁻⁵ g/(m²s) in the continental slope region and ~ 3⋅10⁻⁵ g/(m²s) in the deep waters. At the lower boundary of the layer, salt fluxes are nearly identical in both regions, approximately ~ 5⋅10⁻⁶ g/(m²s). The shear-to-strain ratio exhibits a pronounced increase with depth, highlighting significant differences in the characteristics of small-scale processes at the boundaries of the lower part of the main pycnocline.

Conclusions. The vertical turbulent diffusion coefficient estimated using the G03 parameterization agrees well with the values obtained from the microstructural sounding in other marine regions. However, the comparability of these estimates remains unresolved and requires synchronous measurements using microstructural and CTD/LADCP probes.