Decline of the river Don runoff to its historic minima, as well as intensive cyclonic activity and abnormal advection of the Black Sea waters led to the fact that in 2014-2016, very high salinity values (up to 12 psu) were observed in the Taganrog Bay. Under certain hydrometeorological conditions, salt water can penetrate deep into the river Don mouth. Therefore, study of changes in the Azov Sea hydrothermodynamics is rather an actual problem, which is proposed to be solved by numerical modeling. Methods and Results. The paper represents the methodology for carrying out long-term model runs for joint dynamics of the Black, Azov and Marmara seas based on the eddy-resolving configuration of the NEMO modeling framework. A new-generation ERA5 reanalysis with a sufficiently high spatial resolution was used for the first time as a atmospheric forcing for the region. New information on the rivers Don and Kuban' runoffs were used and adjustment simulations were done to obtain the initial conditions. The results were verified based on the data from coastal hydrometeorological stations in the Sea of Azov. Some results of model simulations for the period from mid-2007 to 2016 are represented. A positive salinity trend in the basin of the Azov Sea is well pronounced. Surface boundary conditions for the heat flux were corrected for the purpose of carrying out simulations without ice modeling and reproducing adequate temperature values of the Azov Sea waters. Conclusions. The performed numerical experiments showed applicability for the developed model regional configuration to further investigations. However, more detailed analysis of the results obtained for the Black Sea basin is required. Consideration of the basic external conditions in modeling made it possible to reproduce positive tendency of salinity in the Sea of Azov. The temperature simulation results indirectly agree with the sea ice data.

Purpose. The study is aimed at investigating seasonal variability and vertical distribution of the submesoscale currents energy (scales L = 1 ... 10 km, T = 1 ... 10 days) in the deep and shelf zones of the Black Sea. Methods and Results. The study is based on the spectral analysis of the results obtained from the NEMO model numerical calculations performed with high spatial resolution 1 km. The analysis shows that in the areas under investigation, seasonal variability of the sub-mesoscale currents energy is significantly different. At that, in both regions, seasonal variation of energy of the sub-mesoscale currents whose scale is less than 10 km (Esp) is in good agreement with that of the density fluctuations on the same scales. In the central part of the sea, the high values of (Esp) are concentrated in the upper mixed layer throughout the whole year. The (Esp) peak is observed in winter at the depths 0–40 m, which indicates the important role of baroclinic instability (induced by the inhomogeneous distribution of the upper mixed layer during this period) in generation of sub-mesoscale processes in the Black Sea. At the same time, in February in the central part of the northwestern shelf, an absolute minimum of (Esp) is observed due to complete mixing and barotropization of the water column. The (Esp) maximum values are noted in September – October, that is related to intensification of the desalinated water cross-shelf transport from the river mouths being affected by the synoptic eddies. At the same time, in the autumn period in this region, the (Esp) high values are observed in the layer, the thickness of which is higher than that in summer (as well as in the central part of the sea). Dynamics of the (Esp) values distribution corresponds to the time variation of the upper mixed layer thickness. Variability of the sub-mesoscale currents energy is of a pulsating character with the short-term intensifications and weakenings. Such variability is significantly related to passing of the synoptic fronts and the cross-shelf water transport being influenced by the eddies and upwellings, which lead to baroclinic instability of waters. Conclusions. Seasonal and vertical variability of the spectral energy in the Black Sea deep and shelf zones testifies in favor of the decisive role of the water baroclinic instability arising due to heterogeneity of the upper mixed layer.

A new method of three-dimensional identification of mesoscale eddies based on the results of the Black Sea hydrodynamics modeling is applied. It is based on identification of the closed streamlines in the velocity fields. This method and the data resulted from the NEMO calculations for [2005][2006][2007][2008] are used to identify more than 1000 mesoscale eddies in the basin. The results permit to define eddy characteristic trajectories, calculate spatial variability of their velocities and radii and frequency of detection of cyclones and anticyclones. The obtained results are in good agreement with the earlier published studies of the eddy characteristics derived from satellite and in situ data. The modeling results permit to investigate the features of vertical distribution of eddy characteristics. The eddies are most often detected in the 0-150 m layer (the highest detection frequency F is within 20-50 m). In the 150-300 m layer this value is two times smaller, but still is substantial. Below 300 meters F quickly decreases with depth. The analogous distribution is characteristic of the eddy radii and orbital velocity: the highest values are observed in the upper 0-150 m layer, and in the lower layers they sharply decrease with depth. The analysis of seasonal variability of the eddy characteristics shows that the anticyclonic dynamics intensifies in summer, whereas the cyclonic one -in winter, that is consistent with the previous studies. The developed method provides additional opportunities for investigating the features of the eddy generation and evolution in the Black Sea.

Decline of the river Don runoff to its historic minima, as well as intensive cyclonic activity and abnormal advection of the Black Sea waters led to the fact that in 2014-2016, very high salinity values (up to 12 psu) were observed in the Taganrog Bay. Under certain hydrometeorological conditions, salt water can penetrate deep into the river Don mouth. Therefore, study of changes in the Azov Sea hydrothermodynamics is rather an actual problem, which is proposed to be solved by numerical modeling. Methods and Results. The paper represents the methodology for carrying out long-term model runs for joint dynamics of the Black, Azov and Marmara seas based on the eddy-resolving configuration of the NEMO model framework. A new-generation ERA5 reanalysis with a sufficiently high spatial resolution was used for the first time as a weather forcing for the region. New information on the rivers Don and Kuban' runoffs was used and adjustment simulations were done to obtain the initial conditions. The results were verified based on the data from coastal hydrometeorological stations in the Sea of Azov. Some results of model simulations for the period from mid 2007 to 2016 are represented. A positive salinity trend in the basin of the Azov Sea is well pronounced. Surface boundary conditions for the heat flux were corrected for the purpose of carrying out simulations with no ice modeling and reproducing adequate temperature values of the Azov Sea waters. Conclusions. The performed numerical experiments showed applicability for the developed model regional configuration to further investigations. However, more detailed analysis of the results obtained for the Black Sea basin is required. Consideration of the basic external conditions in modeling made it possible to reproduce positive tendency of salinity in the Sea of Azov. The temperature simulation results indirectly agree with the sea ice data.

Purpose. The study is aimed at investigating seasonal variability and vertical distribution of the sub-mesoscale currents energy (scales L = 1 … 10 km, T = 1 … 10 days) in the deep and shelf zones of the Black Sea. Methods and Results. The study is based on the spectral analysis of the results obtained from the NEMO model numerical calculations performed with high spatial resolution 1 km. The analysis shows that the seasonal variability of the submesoscale energy is significantly different in deep and shelf zones of the basin. At the same time, in both regions, seasonal variation of energy of the sub-mesoscale currents with scales L < 10 rm (Esp) is in good agreement with that of the density fluctuations on the same scales. In the central part of the sea, the high values of Esp are concentrated in the upper mixed layer throughout the whole year. The Esp peak is observed in winter at the depths 0–40 m, which indicates the important role of baroclinic instability induced by the inhomogeneous distribution of the mixed layer depth (MLD) in the generation of sub-mesoscale processes. At the same time, in February in the central part of the northwestern shelf, an absolute minimum of (Esp) is observed. This minimum is caused by the complete mixing and barotropization of the water column. The Esp maximum values are observed in the shelf in September – October. This is related to the intensification of the brackish water transport from the river mouths by mesoscale eddies. In the autumn period high values of Esp in the shelf and deep part of the basin are observed in the deeper layer, compare to summer months .Variability of the Esp vertical distribution coincides to the time variation of MLD. Variability of the submesoscale energy is of a pulsating character with the short-term intensifications and weakenings. Such variability is significantly related to the passing of the mesoscale fronts and the cross-shelf water transport caused by the eddies and upwellings, which lead to the increase of the baroclinic instability. Conclusions. Analysis of the seasonal and vertical variability of the submesoscale currents in the Black Sea deep and shelf zones evidences about the decisive role of the baroclinic instability triggered mainly by the heterogeneity of MLD on their dynamics.