Features of the processes of convection and water intrusion at the boundary of the hydrogen sulfide contamination layer in the Black Sea in the climatic period 1991—2020

The comparison of modern hydrophysical conditions in the Black Sea with the previously analyzed period of 1903—1982 was carried out based on the materials of the recently created (1991—2020) climate array of average monthly thermohaline characteristics of waters. The dynamic of processes in the upper 150-meter layer (halocline) of the Black Sea was evaluated based on the analysis of climatic materials on the vertical distribution of the depth of the maximum water salinity gradient, which is interpreted as the upper boundary of the hydrogen sulfide layer in the Black Sea. The comparison of state and seasonal fluctuations of this characteristic during the analysis of average monthly maps was carried out by their averaging. According to modern data(1991—2020), in the open part of the sea, a more intense intrusion of these waters up to a horizon of 40 m was found, as well as single cases of natural release of hydrogen sulfide from the depth to the surface. There is a clearly pronounced semi-annual cyclicity in temporal variability; however, in the modern period, the shift was for the whole season: in 1903—1982, the convection (the downwelling of these waters) occurred during autumn-winter, and intrusion (upwelling) — during spring-summer; in 1991—2020 according to the same climatic data, convection occurs during winter-spring, and intrusion occurs during summer-autumn, with the maximum depth of occurrence during the period of winter-spring convection and the minimum during the period of summer-autumn water intrusion. The axes of the northern and southern convergence zones laying near the corresponding coasts of the sea, and the axis of the zone of central divergence is close to open areas in its middle part. By these axes, the differences in the depth of occurrence between the period of convection and intrusion in the northern and southern convergence zones were 21.5 and 17.5 m, respectively; for the central divergence zone, it was 7.5 m.