Annotation of the results obtained in 2021
The 1st year of the project “ Impact of physical factors on the evolution of meso- and submesoscale eddies in the marine environment” was devoted to: - Study of the characteristics of submesoscale eddies in the Black Sea, their spatiotemporal variability and formation mechanisms based on numerical modeling data, satellite and in-situ measurements - Study of the variability of the translational speed of eddies in the Black Sea , the reasons for their stationarity and separation from the place of stationarity - The study of the features of the dissipation of barotropic and baroclinic eddies in a laboratory experiments - Study of eddy dynamics in the Fram Strait and the Norwegian Sea, determination of their structure, impact on thermal characteristics, and causes of their spatio-temporal variability on the base of satellite measurements and ocean reanalyses . The following main results have been obtained : 1) On the basis of modeling data, the features of the spatiotemporal variability of the submesoscale dynamics of the Black Sea waters are studied. The areas and periods of their intense formation are revealed. It is shown that in winter one of the important mechanisms of their formation is the frontogenesis at the boundary of the cooling zone, caused by the Ekman transport under the action of storm winds. The same mechanism plays an important role in the formation of eddies at the front of intense upwellings in summer. The structure and evolution of such vortices have been studied based on simulation data and satellite measurements. The dynamic structure and mechanism of the formation of a chain of submesoscale anticyclones with a diameter of less than 500 m and an orbital velocity of up to 25 cm/s has been studied using data from unmanned aerial vehicles. The reason for their formation was the interaction of an intense current from the bay, caused by the action of strong upwelling winds, with the topographic boundary of the bay, the pier, accompanied by a sharp increase in the shear of the velocity and vorticity of the currents. 2) On the basis of numerical simulation data, the vertical distribution of geometric (radius, thickness, inclination of the vertical axis) and dynamic characteristics of eddies in various areas of the Black Sea, their change during eddies evolution, their spatial, seasonal and interannual variability, the vertical distribution of geometric (radius, thickness, vertical axis tilt) and dynamic characteristics of eddies in various areas of the Black Sea was determined. For the first time, a large statistical array was obtained about the translational speed of eddies, their spatial variability and relation with other characteristics of eddies. The analysis showed that eddy translational speed is inversely proportional to its, which itself is closely related to its vertical extent. Small eddies, which usually occupy the upper layer (30–150 m), move with relatively high velocities of 8–14 cm/s. The reason for this, apparently, is the drift of eddies under the action of the large-scale Rim Current, which is most intense in the upper 0-150 m layer. At the same time, large extended eddies with a lower boundary at depths of 250 m are only partially affected by background currents. They move at lower velocities under the action of the beta effect, corresponding to the baroclinic velocities of Rossby waves (2-6 cm/s), or become quasi-stationary with velocities of less than 4 cm/s. The study of the variability of the angle of inclination of the vertical axis of the eddies, obtained on the basis of automatic algorithms, showed that the effect of the background flow on the upper part of the Black Sea eddies leads to the fact that, on average, the vertical axis of the eddies is inclined in the cyclonic direction. On the basis of modeling data and satellite altimetry measurements, the reasons for the stationarity and detachment of the Sevastopol eddies are investigated. The reason for their stationarity is the meridional ridge, which acts as a vertical wall when the vortex moves to the west. As a result, the movement of the Sevastopol vortex is blocked - it becomes stationary. Under the imagery - effect ( Nof , 1999 ), the vortex initially moves to the north and rests against the northern wall. Here, to the north of the eddy, as a result of the topographic draf and intense velocity shear, an attached subsurface cyclone is formed . An increase in the cyclone radius causes the initial pushing of the anticyclone to the south. As a result, a vortex dipole arises. Under the action of the dipole momemtum , the resulting pair is displaced to the south ( Sutyrin et al ., 2009). After some time, the anticyclonic part of the dipole goes to the south beyond the meridional wall (Fig. 1-d) and the vortex separate from the obstacle and move further to the west. 3) A new method for creating baroclinic vortices in a laboratory experiment in a rotating two-layer fluid has been developed and tested. Unlike the vast majority of baroclinic eddies created in the laboratory, which are eddies with a core containing water of a different density - frontal eddies ( Griffits , Hopfinger , 1984; Cushman-Rosen , Beckers , 2011), this method is designed to create "open ocean" eddies , as well as most of the Black Sea eddies containing liquid of the same density as the surrounding liquid. A series of experiments were carried out to study the attenuation of barotropic (aqueous medium of uniform density) and baroclinic (aquatic medium of two layers in density). Barotropic eddies, both cyclones and anticyclones, decay rather quickly due to friction against the smooth bottom of the basin. Their decay time scale (a decrease in the orbital rotation velocity by a factor of “ e” ) is in good agreement with the spinup time scale τ ( Greenspan , 1975): τ = H/( fν )1/2. In experiments, this scale was 10–20 platform rotation periods (laboratory days). Baroclinic eddies, both cyclones and anticyclones of the upper layer, decayed by an order of magnitude longer (100–200 laboratory days) than barotropic ones due to: a) a decrease in effective viscosity at the density interface and a weak transfer of vorticity from the lower layer to the upper one; b) gradual transition of the available potential energy of stratification into the kinetic energy of the vortex. In the experiments, the evolution of the vortex was studied depending on the value of the Burger number Bu = ( Rd /R0)2 in the range Bu = 0.4 – 4. In this case, eddies for which Bu < 0.8 were baroclinically unstable and split into two eddies of the same sign as the initial eddy, while for eddies with Bu > 0.8, viscous relaxation of a vortex. 4) The characteristics of submesoscale eddies in the area of the Fram Strait have been studied . Their relationship with the dynamics of the ice bait zone based on satellite radar images were investigated. The registered eddies are elongated along the entire ice edge of the Greenland Sea with a length of about 1000 km, as well as over the southern slope of the Ermak Plateau, around the Spitsbergen archipelago, over the Spitsbergen Bank and near Nadezhda Island (Fig. 2a). The width of the area of regular observation of eddies reaches 200-250 km, which is determined primarily by the intra -seasonal variability of the position of the marginal ice zone. A high frequency of eddies this month is observed against the background of a moderate northeast wind, which contributes to the generation of eddies with this configuration of the marginal ice zone. Satellite data and ocean reanalysis was used to study the causes and seasonal dynamics of the formation of a cold spot over the Lofoten anticyclone . It is shown that the cold anomaly is associated with the lenticular structure of the vortex, which is formed in the warm period of the year, and increases in August-September during the destruction of the seasonal thermocline, which masks the effect of the lens in the summer. Based on the results of the first year of the project, 5 publications were prepared and sent to different journal. 1 of them was published in Q1, one was accepted for publication in RSCI (Study of the Earth from Space), other are under review.

 

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Annotation of the results obtained in 2022
The work for the 2nd year of the project “Influence of physical factors on the evolution of meso- and submesoscale eddies in the marine environment” is devoted to: - The study of the features of the evolution of barotropic and baroclinic eddies according to laboratory and numerical modeling data - Study of the mechanisms of influence of cross-shelf buoyancy flows on changes in the structure, kinetic and potential energy of mesoscale and submesoscale eddies - Study of the characteristics of eddies in the South and North Atlantic, the Arctic seas, the Sea of Japan, their spatio-temporal variability based on numerical modeling data and satellite measurements The following main results have been obtained: 1) Laboratory and numerical experiments was made to study the influence of various factors on the propagation of baroclinic vortices. The presence of a topographic obstacle leads to the following effects: 1) The speed of movement of the vortex is significantly reduced in the presence of an obstacle; 2) After crossing the obstacle, the potential energy of the vortex drops sharply and its thickness decreases. In this case, the strongest effects are observed with a gentle slope. The results of laboratory experiments have shown that the interaction of an anticyclone with an obstacle leads to the formation of an attached cyclonic vortex. The dipole moment leads to a displacement of the initial vortex from the wall and separation from the obstacle. After separation, the cyclonic vortex dissipates, and the anticyclone weakens considerably. - It was found that the bottom roughness increases the stability of both cyclonic and anticyclonic eddies. The same vortex-stabilizing effect was also found in the case of an inclined bottom. For stable vortices, a parametrization of the time scale of their viscous damping is proposed. - On the basis of idealized numerical calculations of the POM model, the influence of various factors on the free evolution of mesoscale eddies, namely, sizes, intensity, stratification, latitude, bottom slope, horizontal diffusion coefficients, horizontal density gradients, was studied. It is noted that of all the listed factors, horizontal gradients of buoyancy forces play a particularly important role, the presence or absence of which significantly changes the mode of vortex evolution. Depending on the configuration of the background density gradients, the vortex dynamics can be significantly different. This is due to the fact that the eddy draws waters of a different buoyancy into its orbital motion, causing the generation of attached eddies of a different sign. The configuration of the dipole determines the further direction of the vortex movement, its displacement to the north or south, and can cause its strengthening or dissipation. This process is periodic and can cause the formation of Rossby waves. In this case, the vortex extracts a large amount of energy from the background buoyancy gradients and the kinetic energy of the system increases strongly in the presence of intense density gradients - Based on idealized numerical calculations of the POM model and automatic identification methods, the influence of storms on the evolution of mesoscale eddies has been investigated. It is shown that storms cause a sharp displacement of the eddy, directed to the right from the action of the wind. If the wind is north, then the vortex moves to the east and loses more energy than during the reverse movement. After the action of the wind, the vortex shifts in the opposite direction, practically restoring its initial position. The influence of the wind was maximum for near-surface eddies and less pronounced for extended or barotropic eddies. 2) The influence of the horizontal transfer of buoyancy on the generation of synoptic and submesoscale eddies was studied. - Based on numerical simulation data and satellite measurements, it has been shown that the formation of submesoscale eddies is significantly intensified in the presence of cross-shelf exchange processes that cause a sharp increase in density gradients. Fresh waters of the shelf enter the central part of the sea as a result of their involvement by mesoscale anticyclones. The areas of entrainment of these waters are the zones of the most intense generation of submesoscale eddies on the continental slope. . The emergence of baroclinic instability causes the generation of periodic submesoscale structures, among which cyclonic eddies have the highest vorticity. - the entrainment of shelf waters is the important source of dynamic potential energy for Black Sea anticyclones. The accumulation of these waters leads to an increase in horizontal density gradients in the upper layer of the vortex, which causes its intensification in the surface layers. The intensification of downward motions causes the seasonal thermocline to lower, which during the warm period contributes to a significant increase in pressure gradients between the eddy and the surrounding waters. As a result of the intensification of downward movements, the main halocline descends. At these depths, the difference in density with the surrounding waters increases significantly, which leads to an increase in the available potential energy in the eddy, which is able to maintain the existence of the eddy for quite a long time after it has been separated from the source of desalinated waters. - On the basis of a simplified model of a point eddy, the evolution of various characteristics of anticyclones during the entrainemnt of shelf waters was studied, estimates were made of the influence of the size of the eddy, the density of shelf waters on their variability. - Based on the analysis of temporal variability, vertical structure and evolution of eddies, a mechanism for seasonal generation of anticyclones in the Black Sea is proposed: relaxation of the density field with weakening of downwelling, caused by a decrease in Ekman pumping, leads to an outflow of freshened waters of the shelf into the central part of the sea, which causes an increase in potential energy water and the formation of anticyclones. These eddies capture shelf desalinated waters in their orbital movements, which leads to an increase in their potential energy and their further strengthening. 3) On the basis of numerical modeling data and satellite measurements, the characteristics of eddies in the South and North Atlantic, the Arctic seas, the Sea of Japan, the features of their vertical structure, spatial and temporal variability were studied - Based on satellite altimetry data for 1992-2021 and data from Argo floats, the spatiotemporal variability of eddies and features of the thermohaline structure of eddies in the southern part of the western Atlantic (30°-55°S; 20 65 E) were studied. Validation of altimetry data based on comparison with ADCP measurements carried out in a series of Russian Antarctic expeditions was carried out. The spatial variability of the dynamic, geometric and thermohaline characteristics (anomalies of salinity and temperature at different depths) of the eddies has been determined, and data on the seasonal variability of these characteristics have been obtained. It is shown that the most intense eddies are observed in the region of the confluence of the Brazilian and Falkland currents. These eddies cause level anomalies up to 3m and have velocities up to 1.5m/s. The effect of vortex intensity on their thermohaline structure has been studied. In the most powerful eddies, temperature anomalies of more than 1° can be traced to depths of 1500 m. At depths of 0-300 m, temperature anomalies can exceed 5°, and salinity 1°. The strongest eddies are located at the edge of the subantarctic front, where temperature gradients are very sharp. - Based on the data of hydrological soundings Aqualog in the Sea of Japan and the data of satellite optical measurements MODIS, the detailed dynamic structure of the Primorsky current eddies was studied. Obtained data on the size, speed of movement of these vortices - Based on the methods of automatic identification of eddies, analysis of satellite altimetry data and the results of numerical modeling, the interannual variability of the large-scale and eddy dynamics of the Norwegian Sea, and its relationship with the temporal variability of the Lofoten eddy, was studied. It is shown that, on a seasonal scale, the weakening of the Norwegian Current leads to its meandering and the generation of a larger number of synoptic eddies. However, these eddies are most intense in years with a strong Norwegian current. Such intense eddies are able to reach the center of the Lofoten Basin and merge with the Lofoten eddy. As a result, the interannual variability of the energy of the Loftoen eddy correlates with the intensity of the Norwegian Current with a significant lag of ~1 year. This lag is related to the time required: 1) for the seasonal generation of eddies of the Norwegian Current during the weakening of the large-scale circulation (1/2 year); 2) to move eddies to the center of the Lofoten Basin (~ 1000 km) - Based on the processing of measurements from satellite synthetic aperture radars (SAR), an analysis was made of the spatiotemporal variability of the eddy field in the ice-free areas of the Norwegian and Greenland Seas for the summer period of 2007 and a study of its dependence on background wind conditions and the field of surface currents. It is shown that the most intense areas of eddy formation are the areas near the Voring Plateau, in the Danish Strait, above the Faroe-Iceland threshold, on the periphery of the quasi-stationary Lofoten eddy, and along the branches of the Norwegian Current. A large number of eddies were also observed in the region of the Mona and Knipovich ridges, along the main jet of the West Spitsbergen Current in the Fram Strait and on the western shelf of Arch. Svalbard. Max

 

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