Average condition and seasonal variability of the structure and dynamics of transitional waters in the Dnieper-Bug estuary region

Based on the data of long-term coastal and expeditionary observations, the structure and dynamics of transitional waters in the system of the Dnieper-Bug estuary (the DBE) before the destruction of the Kakhovka HPP are considered as the basis for the future description of changes in the hydrological regime. Average annual and seasonal maps and vertical sections of temperature and salinity are drafted. The article analyzes the main factors of formation and distribution of transitional waters such as river runoff, salinity (density) contrast and surface wind. A number of dimensional and dimensionless criteria and indicators of the transitional water dynamics within and outside the DBE are calculated and analyzed. The analysis of previous studies of river water transformation in the North-Western part of the Black Sea (NWBS) and dynamics of floating plumes in other areas of the World Ocean's coastal zone allows us to establish a 14 P.S.U. isohaline as the outer limit for transitional waters of the NWBS. According to the monitoring data of 1992-2020, the highest degree of desalination of the DBE and output of transition water from the Kinburn Strait was observed in spring, under the conditions of increasing volume of river runoff and predominant wind from the east. At the same time, the increasing repeatability of the south wind contributed to pushing the plume towards the coastal area outside the estuary. In summer, the influence of open sea waters on the DBE increased due to decreasing river runoff and increasing frequency of westerly winds. According to theoretical criteria, under low-wind conditions, the estuary produces a surface-advective plume affected by buoyancy and Coriolis force, with no effect of friction in the bottom boundary layer. The distance over which such plume spreads, even in summer, does not exceed half way from the Kinburn Strait to the Odesa Bay. The ingress thereto of transitional waters of the DBE in spring and autumn can be facilitated by accompanying wind currents. The main factor of the plume's summer dynamics includes both westerly wind and the currents it generates. The latter prevent the spread of transitional waters to the west along the coast, pushing them towards the estuary and to the south off it. This does not negate the possibility of a different plume behavior due to the increasing volume of river runoff and synoptic variability of the wind field during certain years and months.