Том 39, № 6 (2023)

Purpose. The paper is purposed at summarizing the main results of experimental and theoretical studies of vertical turbulent exchange in the upper mixed and stratified layers of the Black Sea carried out in recent years.

Methods and Results. The equations for semi-empirical dependences of turbulence intensity on the governing parameters are proposed, based on a large amount of experimental data on the turbulent structure obtained at the research vessels and the stationary oceanographic platform using modern high-frequency equipment combined with the conventional measurements of main hydrophysical characteristics. The experimental data obtained were used to verify the theoretical models and to specify the empirical coefficients in the proposed equations. A multiscale model was applied additionally to the Kraus – Turner model to forecast reliably the upper mixed lay deepening after a storm has passed. The turbulent energy dissipation rate and the turbulent diffusion coefficient in the stratified layers were found using the data on microstructure of the hydrophysical fields. The coefficient dependences on buoyancy frequency in different layers are expressed by a power function with different degree indices.

Conclusions. Having been examined in detail, the stratification conditions as well as the large array of sounding data made it possible to identify five layers with different density gradients and different mechanisms dominant in generating turbulence. Such a differentiation specifies the expressions describing the turbulent diffusion intensity depending on the layer depth and the physical and geographical conditions affecting vertical exchange. On the whole, the resulting power-law dependences agree well with the earlier developed 1.5D model of vertical turbulent exchange for the Black Sea. The proposed way of considering the effect of the turbulence generation mechanisms in the upper mixed layer improves the correspondence between the model calculations and the experimental data. The Kraus – Turner model supplemented with the multiscale turbulence model permits to forecast deepening of the mixed layer resulting from storm conditions.

Purpose. When discussing the Sea of Azov salinization both in the 1970s and in the current period, the reduction of river runoff (primarily in the Don River) is indicated as the main reason for this phenomenon that results in the increased advection of the Black Sea waters. However, this factor alone is not enough to explain salinity decrease in the last quarter of the 20th century as well as its rapid growth in 2007–2020. The paper is purposed at assessing the contribution of water balance components to the change in the Azov Sea salinity for more than 50 years (1966–2020).

Methods and Results. A mathematical model of water-salt balance is used to assess the annual average salinity of the Sea of Azov. The variability of all the water balance components and the sources of its uncertainty are considered. It is shown that the model applied is resistant to variation of the input data. Based on the correlation analysis, it is found that during the salinization period, evaporation plays an important role along with the river runoff. At that in the 1970s, a decisive impact upon evaporation was exerted by the wind speed, whereas at the beginning of the 21st century – by the temperature and humidity regimes.

Conclusions. At the beginning of the 21st century, the reason for rapid salinity increase in the Azov Sea waters consisted in the prolonged period of low-water in rivers coinciding with the period of high water and air temperatures that resulted in an increase of evaporation from the sea surface. The obtained results permit to conclude that at the changed atmospheric processes in the region, the only factor consisting in transition from the low river inflows to the Azov Sea basin to the high ones may be not enough to provide significant decrease of seawater salinity. This circumstance should be taken into account when preparing the plans for adapting the economic activities to the climate changes.

Purpose. The study is purposed at systematizing the regional features of coastal upwelling in the southeastern part of the Baltic Sea (SEB) during the summer seasons of 2000–2019 in a changing climate.

Methods and Results. The results of the identification of upwelling events in the southeastern Baltic Sea are presented as a negative anomaly of the coastal sea surface temperature relative to the selected area of the open sea. The estimates of influence of the local and global atmospheric processes (based on the data on wind characteristics and SCAND and NAO teleconnection indices) upon the upwelling event characteristics, (frequency, duration and area) were obtained. On average, 4 upwelling events with a total duration exceeding 20 days were observed, at the upwelling zone average area of about 620 km². The upwelling frequency in the area under study was the highest along the northern part of the Curonian Spit and the Sambia Peninsula western coast. In comparison to 2000–2009, the upwelling durations in the summer seasons of 2010–2019 were recorded to decrease by about 8 days at the decline of frequency of the winds favorable for their development by 4–5%. The correlation between the number of upwelling days in southeastern Baltic Sea and the SCAND index is established (the correlation coefficient was 0.65), that allows further attempts on obtaining the long-term forecasts of the probability of coastal upwelling development in the southeastern Baltic Sea.

Conclusions. It is shown that a steady decrease in the upwelling frequency in SEB during the recent decades is related to a declining frequency of the upwelling-favorable winds largely conditioned by the anticyclogenesis development over the Scandinavian Peninsula.

Purpose. The purpose of the work is to summarize information on the features of spatial variability of the ²²⁶Ra and ²²⁸Ra concentration fields and the factors influencing these features in the surface water layer of the Black Sea.

Methods and Results. The data on spatial variability of the ²²⁸Ra and ²²⁶Ra concentrations in the surface (0.3–3.0 m) layer of the Black Sea obtained during four expeditions were used. The ²²⁸Ra and ²²⁶Ra isotopes were extracted from the seawater samples using the MnO₂-based fiber. Their activity was measured by a UMF-2000 alpha-beta radiometer. The data on the content of main elements of the basic biogenic cycle were obtained photometrically.

Conclusions. The concentrations of ²²⁸Ra and ²²⁶Ra varied in a range of 17.2 to 172.2 dpm/m³ and from 38.0 to 270.1 dpm/m³, respectively. It is shown that in the region under study, the influence of submarine sources and, presumably, sewage is of a local character and is manifested in an increase of the concentrations of these radionuclides or one of them by 1.5–2.3 times. The mesoscale eddies observed in the region of the Southern Coast of Crimea are assumed to affect spatial variability of the radium isotope concentration fields that results in a local decrease or increase in their concentrations by 2.3–2.8 times. It is shown that propagation of the Azov Sea waters in the Black Sea is traced by the ²²⁸Ra and ²²⁶Ra concentration fields: the increased (by 2.3–2.6 times) values of the contents of both isotopes are observed in these areas. It is established that in the areas subjected to the affect of river runoff, the concentration of long-lived radium isotopes is observed to increase with distance from the coast. The spatial scales, on which the influence of a particular source is manifested, are expected to be proportional to its power (flow rate and radionuclides concentration): the higher the power, the greater the distance at which its influence is monitored.

Purpose. The purpose of the study is to analyze the features of spatial-temporal and vertical distribution of oxygen, hydrogen sulfide and main nutrients (phosphates, nitrates, ammonium and silica acid), as well as the characteristics of carbonate system in the Black Sea in the modern period.

Methods and Results. The data used in the study were obtained by the scientists of Marine Hydrophysical Institute in the scientific cruises in the Black Sea in 2013–2021 within the economic zone of Russia. During these cruises, more than 200 deep-sea stations were carried out, samples were taken using a cassette of 12 bathometers of the Sea-Bird 911 plus CTD Seabird-Electronics INC device at certain isopycnic surfaces. At the coastal shallow-water stations, samples were taken from the surface and near-bottom horizons. Precipitations were sampled by the automatic precipitation collectors at the meteorological stations located on the Pavlovsky Cape in Sevastopol and at the Black Sea hydrophysical subsatellite polygon (the Southern coast of Crimea).

Conclusions. The location of upper boundary of the suboxic zone varies from s_t = 15.7 kg/m³ to s_t = = 15.9 kg/m³, that in the depth scale corresponds to the interval of ⁓ 40 m. The vertical distribution of hydrogen sulfide is more of isopycnic character, the boundary of isosulfide 3 µM appearing is located within s_t = 16.10–16.15 kg/m³. On the vertical profile of nitrates, their maximum concentration not exceeding 4 µM, is observed within the range s_t = 15.2–15.5 kg/m³. It is shown that the content of oxidized nitrogen forms has almost returned to the pre-eutrophication level. The concentration of ammonium ions in the aerobic and suboxic zones predominantly does not exceed 0.5 µM, the ammonium concentration starts to increase at the depth of isopycne s_t = 16.10–16.15 kg/m³, at which hydrogen sulfide appears. The maximum concentrations of ammonium ions (96 ± 5 µM) were noted at the depth 1800 m and below that corresponded to the earlier obtained data. The phosphates vertical distribution is characterized by their minimum (˂ 0.5 µM) content at s_t = 15.8 kg/m³, and by their maximum (not more than 8 µM) one at s_t = 16.2 kg /m³. The hydrogen sulfide content at the depths exceeding 1750 m in the Black Sea is currently 383 ± 2 µM. The pH value of surface waters was 8.29–8.38 decreasing to 7.67 below ~ 50 m. Total alkalinity varied within the range 3268–3335 µM, below s_t = 16 kg/m³ it increased sharply reaching its maximum value ~ 4364 µM in the bottom layer. The results obtained confirm the immutability of the previously established features of the vertical distribution of hydrochemical components and the ranges of their concentration variations.

The spatial distribution of nutrients clearly shows a decrease in their concentrations from the coastal areas to the deep-sea ones. One of the largest external sources of nutrients in the Black Sea waters is the atmospheric precipitations. On the synoptic spatial-temporal scales, they can become the main source of inorganic nitrogen, phosphate and silica entering the sea surface layer.

Purpose. The study is purposed at analyzing the physical mechanisms of formation of the Black and Marmara seas circulation structures based on the numerical experiments with climatic boundary conditions.

Methods and Results. To investigate the reasons for formation of the circulation features, the energetic approach was applied that permitted to calculate the work of the forces affecting the marine environment. Location in the same geographical region determines similarity of the atmospheric conditions for the Black and Marmara seas, and the clearly pronounced two-layer water stratification in both basins is related to a significant difference in salinity of the Black Sea and Mediterranean waters. To analyze the mechanisms of circulation variability, the mean and eddy fields formed under the impact of climatic atmospheric forcing and calculated using a numerical model of sea dynamics were considered. Wind influence, thermohaline fluxes on the sea surface, buoyancy work, friction, and diffusion were quantitatively assessed based on calculation of the Lorenz energy cycle components. The common features were found in the mechanisms of mesoscale variability, and the differences – in the mechanisms of large-scale circulation variability.

Conclusions. It is shown that the main source of energy for the Black Sea mean circulation is the wind stress work, and as for the Marmara Sea, the dominant factor is the buoyancy work. For both basins, variability of the eddy kinetic energy characterizing the mesoscale dynamics is conditioned by baroclinic instability. At that, about a quarter of the available potential energy in the Black Sea, and about a half of it in the Marmara Sea is transformed into the eddy kinetic energy.