Seasonal and inter-annual changes of structure and dynamics in the buoyant plume generated by the Dnipro-Buh estuary, Black Sea

Based on long-term coastal hydrometeorological observations and oceanographic surveys, as well as satellite images, the average conditions, seasonal and interannual variability of integral water exchange and buoyant plume generation were studied for the Dnipro-Buh estuary (DBE).

Analysis of publications and available information showed that a simple Knudsen's box model can be adapted for the conditions of shallow non-tidal estuaries. Hydrodynamic dimensional and non-dimensional criteria that determine the nature and further behavior of plumes after their exit from the estuary into the open sea were considered. The application of the criteria for determining the nature of the plume for outflows from the DBE showed that on the exit of estuary plume of transitional waters is produced. It has a surface-advective nature, without the effect of friction in the bottom boundary layer, being driven by buoyancy and Coriolis forces, and is influenced by wind-wave mixing.

The main factor in the plume dynamics on the shallow waters are the wind currents, which contribute or hinder the spread of transitional waters along the coast to the right of the estuary mouth, or pushing them into the open sea and even turning them to the opposite. During the low-wind weather, the initial impulse of the river runoff plays the main role. This was especially evident in the situation of an abnormally large runoff volume after the explosion of the Kakhovka HPP on 06.06.2023. The consequences of its impact on the marine environment are shown according to a number of satellite images.

Using the mixture analysis method, the statistical structure was investigated and one-dimensional clustering of empirical salinity histograms at 3 shore stations in the vicinity of the DBE was performed.  It has been found that salinity probability distributions can be approximated by a set of 2-3 Gaussian functions. These functions, as a rule, correspond to waters of river origin, marine origin, and intermediate waters as a result of the interaction of the first two. Seasonal changes of these water masses' mean values and standard deviations were obtained for each shore station.