Seasonal and interannual variability of vertical turbulent exchange coefficient in the Black Sea pycnocline in 2013–2016 and its relation to variability of mean kinetic energy of surface currents

In this paper, we analyze seasonal and interannual variability of vertical turbulent diffusion coefficient (K_t). Our calculations are based on the time series from 2013 to 2016, obtained in the north-eastern Black Sea within the depth range of 30–220 m by using an Aqualog profiler. The synchronized measurements included vertical profiles of sea temperature, conductivity, density, and current velocity. The calculations were performed according to a well-known parameterization of vertical turbulent exchange coefficient based on the Richardson number, originally proposed by Munk and Anderson (1948). We found that strongest vertical mixing occurred from October to March and was located above the isopycnals of 14.0–14.2, reaching the maximum of 8.8 × 10^‑4 m²/s in December. In this period, K_t values exceeding 10^‑4 m²/s were observed in the water column down to the isopycnal of 15.0 and even below. Mixing intensity during the cold seasons of the year grew to 1.2 × 10^‑4 m²/s in average, which was twice as high as that in the warm seasons. Analysis of temporal variability of the mean surface kinetic energy (MKE), derived from altimetry data, showed a high correlation between the MKE and column-averaged K_t values on both seasonal and interannual time scales. The K_t variations lagged those of MKE by about a month, probably owing to a gradual deepening of current jets and an increase of mixed layer depth from October to January.