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Project titleThe impact of physical factors on the evolution of meso- and submesoscale eddies in the ocean
Project LeadKubryakov Arseny
AffiliationFederal State Budget Scientific Institution Federal Research Centre "Marine Hydrophysical Institute of RAS",
|Implementation period||07.2021 - 06.2024|
Research area 07 - EARTH SCIENCES, 07-502 - Large-scale and synoptic processes in the ocean
Keywordsocean dynamics, mesoscale eddies, submesoscale eddies, laboratory modeling, numerical modeling, remote sensing, horizontal and vertical exchange, thermohaline structure, instability
This project is devoted to the study of the impact of physical factors on the evolution and dissipation of meso- and sub-mesoscale eddies in the ocean. The work will be based both on the results of laboratory and idealized numerical experiments, as well as on the results of satellite, contact measurements and realistic modeling in specific water areas, including the Black Sea and some regions of the Arctic Ocean. It is planned to study the impact of various processes on the strengthening / weakening of vortices and changes of their thermohaline and dynamic structure: the structure of large-scale flows; changes in ocean stratification; convective mixing; wind impact; buoyancy fluxes associated with the entrainment of surrounding waters into the eddy. Also, the impact of these processes on the seasonal and interannual variability of the characteristics and the number of eddies will be investigated. Special attention will be paid to the study of the influence of large-scale background currents, topographic obstacles (ridge) and bottom inclination (topographic beta-effect) on the change in the translational speed of movement of vortices. In particular, on the basis of altimeters data and numerical models, the reasons for the quasi-stationarity of the Sevastopol anticyclone and its subsequent separation will be studied. The project will allow to obtain new fundamental results on the mechanisms of the influence of external factors and large-scale flows on vortex dynamics. The results obtained will make it possible to deepen the existing knowledge about the processes of horizontal and vertical exchange in the World Ocean, necessary for the creation of high-quality systems for monitoring, modeling and forecasting the state of the ocean, its physical and biochemical structure.
The main aim of the project is to obtain new fundamental results on the evolution, formation and dissipation of eddies in the marine environment. The following results will be obtained: 1. Methods of automatic identification of eddies from the data of numerical modeling and satellite altimeters; hydrological data; satellite optical and IR measurements will be used to study the characteristics of eddies of the Black and Norwegian Seas, including the spatial-temporal variability of their dynamic and morphometric characteristics, thermohaline structure, trajectories and places of education. 2. Satellite radar measurements in the Arctic are used to study the characteristics of eddies in the areas of open water and the near-edge ice zone, including the main places of their formation and dissipation, trajectories, spatio-temporal variability of their kinematic and geometric characteristics. Analysis of the consecutive radar images will be used to obtain information on the dynamic characteristics of submesoscale eddies (orbital velocities, movement velocities) in the near-edge ice zone. 3. The features of the evolution of thermohaline, dynamic and morphometric characteristics of eddies in the Black Sea and the regions of the Arctic Ocean have been investigated; events of strengthening / weakening of eddies have been identified and studied. 4. The reasons for the change in the translational speed of vortices, including the reasons for their stationing, have been investigated: - on the basis of numerical modeling, the impact of large-scale currents and their vertical distribution on the velocity and shape of eddies of various sizes in the Black Sea is estimated - on the basis of numerical modeling and satellite measurements, the impact of a topographic obstacles on the movement and stationarity of vortices of various sizes and intensities (on the example of the Sevastopol eddy), the conditions for detachment / deceleration of a vortex near an obstacle (flow regime, vertical length of a vortex, intensity of a vortex) are investigated - the impact of the topographic beta effect on the movement of the barotropic and baroclinic vortex was estimated on the basis of laboratory experiments with an inclined bottom at various angles of inclination of the bottom to the horizontal Estimates are given and schemes of the impact of these processes on the movement of vortices are presented. 5. The physical reasons for the intensification / weakening of vortices in the process of their existence have been investigated. - on the basis of a laboratory experiments, the free damping of meso- and sub-mesoscale eddies in a homogeneous and two-layer stratified fluid is studied and parameterized - on the basis of numerical modeling and satellite measurements, the impact of buoyancy flows associated with the trapping of surrounding (e.g. shelf) waters by a vortex on its intensification, structure and evolution has been investigated. - on the basis of numerical modeling and satellite data, the impact of Ekman pumping and wind mixing on changes in the dynamic characteristics and vertical structure of eddies of different signs after intense storms is investigated. - based on the analysis of satellite data, the influence of ice and wind conditions on the intensity of formation and evolution of eddies in the near-edge ice zone was analyzed. Estimates of the influence of these factors on the dynamic characteristics and structure of vortices are given. Based on the analysis of satellite data and a high-resolution FESOM 1 km model, the influence of eddies on the characteristics of the ice cover in the near-edge zone is estimated - on the basis of a laboratory experiment, the impact of convection on the evolution of vortices of different signs, as well as the impact of vortices on convective mixing, was investigated. 6. The impact of physical processes on the seasonal and interannual variability of the generation and characteristics of eddies has been studied. - on the basis of numerical modeling, the impact of the weakening / intensification of currents on the number and intensity of the formed eddies in the Black and Norwegian Seas was investigated. Mechanisms of the impact of amplification / attenuation of large-scale circulation on the generation and dissipation of vortices are proposed. - on the basis of numerical experiments, the impact of changes in stratification on differences in the dynamic characteristics and structure of eddies formed in winter and summer is investigated - on the basis of numerical modeling and satellite measurements, the impact of inhomogeneity of vertical mixing (convection and wind action) on the variability of the formation of submesoscale eddies in the cold season has been investigated. - based on the analysis of the currents obtained from satellite altimetry measurements and the high-resolution FESOM 1 km regional model, satellite products on the sea surface temperature and wind fields, the influence of large-scale currents, fronts and wind on the temporal variability of eddy formation in open water areas was investigated in the Fram Strait Study of the formation of the structure of eddies and their dynamics is important for describing the processes of horizontal and vertical exchange in the ocean, which have a significant impact on both the physical and biological characteristics of ocean waters. Eddy dynamics has a significant effect on cross-shelf exchange, ventilation of the coastal zone, transport of suspended matter, pollution, and coastal erosion. Investigation of eddy dynamics is necessary for the creation of the systems for monitoring and forecasting the state of the ocean, which are important for ensuring the safety of navigation and coastal economic infrastructure, for investigation of the processes responsible for the coast formation. Eddies significantly impact on the horizontal and vertical transport of nutrients, which are necessary for the growth of phytoplankton. Its study will help to improve understanding about the influence of dynamic processes on the functioning of marine ecosystems.
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.
1. Kozlov, I. E., & Atadzhanova, O. A. Eddies in the Marginal Ice Zone of Fram Strait and Svalbard from Spaceborne SAR Observations in Winter Remote Sensing, Eddies in the Marginal Ice Zone of Fram Strait and Svalbard from Spaceborne SAR Observations in Winter. Remote Sensing, 14(1), 134. (year - 2022).
2. V.S. Travkin, T.V. Belonenko, A.A. Kubryakov Холодное пятно над Лофотенским вихрем Исследование Земли из Космоса, - (year - 2022).