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Project titleMECHANISMS AND HYDROPHYSICAL FEATURES OF VERTICAL MIXING IN HOMOGENEOUS AND STRATIFIED SEA LAYERS

AffiliationFederal State Budget Scientific Institution Federal Research Centre "Marine Hydrophysical Institute of RAS",

Research area 07 - EARTH SCIENCES, 07-505 - Turbulence and small-scale processes

Keywordsmarine turbulence, turbulent exchange coefficients, heat and matter fluxes, in situ experiments, numerical simulations, turbulence generation mechanisms, homogeneous and stratified sea layers, turbulent exchange models, vertical mixing

Annotation
The main goal of the project is to establish physical laws of vertical and horizontal exchange in homogeneous and stratified sea layers, to create numerical and semiempirical models to calculate and forecast currents and other hydrophysical fields. The relevance of the problem consists in the need, under conditions of changing climate, anthropogenic and technogenic load, to objectively assess the ecological state of coastal and deep waters, to confidently develop operational and long-term forecasts of variability and trends in hydrophysical and hydrochemical fields. For this purpose it is necessary to describe the physical mechanisms of vertical and horizontal exchange as precisely as possible, taking into account the typical features caused by physical-geographical conditions, seasonal and regional factors. The most important component of the planned research is carrying out in-situ measurements with modern high-resolution equipment. At the same time, a comprehensive approach is used to identify the physical mechanisms influencing the processes of mixing in layers with different density stratification, which includes both experimental and theoretical studies. Original turbulence characterization techniques combined with new instrumentation will allow us to develop our understanding of the physical processes in the upper active layer and in the deep sea layers. An essential factor in the high quality of the fine structure and microstructure planned field measurements of the water column in a wide range of depths will be the use of the new MSS-90L microstructure probe. Together with the available data on the currents nature at different depths, this will significantly improve our understanding of heat and matter fluxes in different seasons and take into account both background hydrological characteristics and regional features. It is planned to use, first of all, the Black Sea basin, which is available for expedition work and model verification, as an object of research. The team scientific experience in the Black Sea study, proven models and a database of field measurements will allow obtaining new knowledge at a higher level. Research of the Black Sea water circulation, reproduced by a numerical three-dimensional model of the sea environment dynamics with the boundary conditions zoned on the basis of assessments of intrusion and geothermal flows, will allow to specify ideas about the features of the Black Sea hydro-physical fields, to obtain new information about physical mechanisms of formation of the deep thermal and dynamic structures, to establish dependence of the flow characteristics on the stratification features. The results of experimental studies and numerical modeling of the system of currents in deep layers on the example of the Black Sea will be the basis for similar experimental and theoretical works in other basins. To study the processes of turbulent exchange in the upper boundary layer, a unique infrastructure object will be used - the stationary oceanographic platform on the Black Sea Hydrophysical Sub-satellite Polygon of the FRC MGI in Katsiveli village. The intensity of vertical fluxes of momentum, heat and matter in the upper quasi-homogeneous layer depends on a large number of factors and is the subject of numerous studies. The multiplicity of physical processes near the boundary of two media and complex interrelations make it very difficult to obtain objective dependences and require improvement of approaches to theoretical modeling. The existing models of turbulent exchange in this case do not always give sufficient correspondence between calculations and in-situ measurements and need qualitative improvement. Theoretically, the emphasis is expected to be on a promising approach to modeling turbulent exchange, taking into account different scales of turbulence generation mechanisms near the sea surface. The multiscale model developed so far gives a better agreement with in-situ measurements. In development of this direction it is planned to improve the existing models, to take into account non-stationary character of running processes and presence of sub-mesoscale coherent structures in turbulent flow, in particular Langmuir circulations. The set of in situ instrumentation available in the team is unique for turbulence studies near the sea surface. The original patented system of data collection, the main part of which is a Sigma-1 measuring complex, allows to measure basic hydro-physical quantities (including pulse components) from the surface to a depth of ~20 m. Accompanying measurements of a wide range of parameters (meteorological data, current velocity, wave characteristics, etc.) provide a complete record of background hydrometeorological conditions. Scientific novelty of the project is the use of modern high-precision measuring equipment and extensive use of field data for creating and improving semiempirical models. The development of a multiscale model with the inclusion of additional Langmuir and Stokes drift circulations will improve the objectivity of the calculations. A new approach is to distinguish different layers in stratified basin area taking into account hydrophysical features and influence of mechanisms determining vertical exchange. Parameterization of turbulent exchange coefficients will allow receiving physically reasonable semiempirical relations necessary for practical estimations of flows of heat, momentum, and dissolved substances.

Expected results
Expected results and their significance (the results, their scientific and social significance are indicated (compliance of the expected results with the world level of research, the possibility of practical use of the expected results of the project in the economy and social sphere, contribution to the solution of specific tasks of the chosen scientific direction from the Strategy of the scientific-technological development of the Russian Federation)) This information can be published on the Foundation's website on the Internet information and telecommunications network. As a result of the expedition research with the help of a modern microstructural probe, new information will be obtained about the fine and microstructure of hydrophysical fields (velocity fluctuations, temperature, turbidity, oxygen content) in stratified layers of the sea to a depth of 1000 meters. Together with the data on the background hydrophysical characteristics of various layers, on the vertical distribution of the turbulent energy dissipation rate and comparison with the developed models, this will allow: 1) to establish objective regularities of vertical mixing processes in the water column of the Black Sea; 2) make estimates about the intensity of exchange at the boundaries of the hydrogen sulfide zone in the deep-water part and in the interface zone of the shelf and the continental slope; 3) to obtain the zoned semi-empirical dependences of the vertical turbulent exchange coefficients as a function on the buoyancy frequency. Numerical modeling of three-dimensional hydrophysical fields of the Black Sea, taking into account the observational data, will give an idea of the spatial scales of the dynamic and thermohaline structures formed in various areas of the basin and the intensity of horizontal heat transfer and dissolved substances. Numerical modeling of three-dimensional hydrophysical fields of the Black Sea, taking into account the observational data, will give an idea of the spatial scales of the dynamic and thermohaline structures formed in various areas of the basin and the intensity of horizontal heat transfer and dissolved substances. Full-scale measurements with a wide range of measuring equipment (positional turbulimeter Sigma-1, vector-averaging flow velocity meter Vostok-M, acoustic flow velocity profile meter, mini CTD probe) in the upper boundary layer of the sea on a stationary oceanographic platform will allow to establish statistically significant dependences of the intensity of turbulent exchange on determining hydrometeorological parameters. Special attention in these experiments at this stage is planned to focus on the study of individual mechanisms of turbulence generation in storm conditions, as well as coherent structures and submesoscale processes (with periods from several hours to several tens of hours), which are not taken into account, as a rule, in models. Their adequate parametrization of new physical concepts about the turbulence characteristics in the upper layer will help to significantly improve the multiscale turbulent exchange model, as well as modeling in large-scale problems of the interaction of the atmosphere and the ocean. The scientific level of the results achieved during the implementation of the project will correspond to the world level of research. The new results will be published in peer-reviewed journals and presented in scientific reports at conferences and seminars.

Annotation of the results obtained in 2022
To study the turbulent mixing processes in the sea-surface layer, the instrument set was expanded and modernized, which includes the measuring probes "Sigma-1", "Vostok-M", ADCP, CTD-probe, which makes it possible to carry out synchronous hydrophysical measurements in a wide range of parameters. An experimental technique for near-surface layer turbulent regime studies has been developed in order to regiser the Langmuir circulations characteristics and main hydrophysical fields background characteristics. Meteorological factors affecting the Langmuir circulations parameters are revealed. The circulations contribution to turbulent exchange is estimated. Turbulent exchange stationary models verification for the sea-surface layer was carried out based on the supplemented results of the turbulence characteristics measurements. The models and turbulent energy dissipation rate depth distribution data comparison allows us to formulate a number of conclusions about the models usability in various hydrometeorological conditions. In full-scale experiments, the upper quasi-homogeneous layer thickness dynamics study was made under the storm mixing conditions. The one-dimensional Kraus-Turner seasonal thermocline model, supplemented by the multiscale turbulence model, generally gives satisfactory agreement with the observed values. The obtained model verification was carried out using measurement data collected at the Black Sea hydrophysical sub-satellite test site of the MHI RAS. On the R/V “Professor Vodyanitsky” 122nd cruise, for the first time, detailed information was obtained not only on the main hydrological values, but also on the flow fluctuation characteristics in stratified layers of the Black Sea, together with data on the vertical distribution of oxygen, chlorophyll A and turbidity down to a depth of 970 m. Data acquisition was carried out by a new microstructural probe MSS-90L. A preliminary data analysis showed the presence of several layers with characteristic features of mixing in them. Further studies will improve the vertical turbulent exchange model parametrization in stratified layers and achieve a better vertical exchange process understanding of heat, salts, biogenic elements and their dependence on the turbulence intensity. Based on the Black Sea climatic fields calculation analysis with a refined bottom topography, main features of the deep-water currents structure and their intra-annual variability were established. The velocity field main features include the absence of a single cyclonic basin circulation at horizons deeper than 250–300 m, the predominance of mesoscale dynamic formations (eddies and currents), and a significant decrease in the average velocity compared to the surface layer. In the autumn-winter season, the currents intensify; the Rim Current western branch intensification leads to its deepening to the horizons of 700–800 m during this period. The presence of non-stationary deep-water countercurrents, directed anticyclonically and characteristic of the narrow northeastern continental slope, has been verified. To improve the vertical turbulent exchange model calculations objectivity in the near-surface layer of the sea, a non-stationary model is developed with a more detailed consideration of the two media interaction processes consideration within this project framework: at small and submesoscales. Test calculations show an important difference from stationary models: the turbulence intensity in the non-stationary case is higher than in the case of constant wind. Scilicet, strong averaging commonly used in large-scale models leads to underestimations of the turbulent mixing intensity. This indicates the need to include data on the hydrophysical fields variability at small and submesoscales in the interaction models between atmosphere and sea.

Publications

1. Markova N.V. The Black Sea Deep-Water Circulation: Recent Findings and Prospects for Research Processes in GeoMedia – Volume VI, Springer Geology, In: Chaplina T. (eds) Processes in GeoMedia – Volume VI, Springer Geology. Chapter 49 (year - 2022)

2. Pavlov M.I., Chukharev A.M. Effect of Langmuir circulations on the intensity of turbulent mixing in the near-surface sea layer Processes in GeoMedia–Volume VII, Springer Geology, - (year - 2023)

Annotation of the results obtained in 2023
Based on the field experiments analysis results conducted in 2019-2023 at the hydrophysical sub-satellite polygon of the MHI RAS, data on the main Langmuir circulations (LC) dynamic characteristics were obtained. Using the developed specialized computer program, a quantitative and statistical analysis of the measured characteristics was carried out. It was found that the contribution of LC to the sea surface layer turbulization depends on both the speed and the variability of wind direction. The most intense turbulization was observed at wind direction variation in the range of 10-20° and wind speeds of 3-12 m/s. At wind speeds of more than 13-15 m/s, the bands break up rather quickly and the role of LC in mixing decreases. Taking into account the tangential wind stress quasi-periodicity on the sea surface at small and submesoscales leads to a noticeable increase in the intensity of turbulent momentum and energy transfer, thus increasing the intensity of vertical mixing in the marine medium. During the development of the upper mixed sea layer turbulent exchange non-stationary model, Langmuir circulations were included in addition to the previously considered two main mechanisms of turbulence generation (current velocity shear and surface waves). It is shown that turbulent transfer of wave kinetic energy plays an important role in the vertical energy redistribution. Verification of the model calculations on in-situ data obtained in the Black Sea coastal zone demonstrates the objectivity of model proposed. Further model development implies parameterization of submesoscale structures with scales from several hundred meters to several kilometers with their energy contribution consideration. According to the in-depth analysis of experimental data it was confirmed that there are seven regions of the Black Sea with characteristic features of vertical turbulent diffusion coefficient K distribution, which are determined by local hydrological-dynamic conditions depending on the season. High values of Pearson correlation coefficients are observed: for K and flow velocity and direction - up to -0.86 in certain ranges of latitude and longitude, while the correlation coefficient for K with density was lower (0.69), indicating predominantly shear instability in such regions. At the same time, there are regions with higher correlation of the K coefficient with thermohaline characteristics. The obtained results will be used in further studies with a differentiated approach to each region in order to identify additional features of K and to create a refined vertical turbulent exchange spatial model using statistical methods and machine learning techniques. Based on the Black Sea dynamics numerical model solutions, it is established that the atmospheric fluxes of heat, momentum and moisture can influence the circulation of both surface and deepwater layers, and the choice of atmospheric forcing data can be crucial for correct modeling of hydrophysical fields in the whole basin. It is shown that in spite of the blocking vertical exchange influence of the permanent pycnocline, not only the extremely strong atmospheric forcing can influence the structure of the Black Sea circulation, but also in the case of regular atmospheric conditions there is a dependence of model fields on the fluxes set on the atmosphere-sea surface boundary. We present model estimates of the Black Sea circulation at different depths obtained using in the MHI numerical model as atmospheric forcing two widely used available meteorological data sets - SKIRON and ERA5, which included wind fields and heat and moisture fluxes across on the sea surface for the same period. The calculations were tested on a sufficiently large volume of in-situ observations of the state of the marine environment obtained in 2016 by measurements from the R/V "Professor Vodyanitsky" and ARGO floats. It was found that the standard deviation of temperature and salinity between model and measured data decreases significantly when using ERA5 forcing (by 27% for temperature and 20% for salinity on average over the 0-1500 m layer). Also, the unsteady alongshore undercurrent below the main pycnocline detected along the real trajectory of float ARGO ID6901833 is modeled using ERA5 only. Similar unsteady undercurrents were identified during the previous phase of the project in the climatic velocity fields. Therefore, further studies are envisioned using the atmospheric forcing of ERA5. For the vertical exchange processes parameterization improvement during turbulent mixing in the upper quasi-homogeneous layer of the ocean, we considered the application of a multiscale turbulence model together with the Niiler-Kraus model to quantify the processes of mixed layer thickness change. The obtained modernized model is of main interest because of the additional parameters (Coriolis force, buoyancy frequency, mixing energy decrease in the process of thermocline deepening). It showed better results compared to the previously used Kraus-Turner model, which gives incorrect results at high wind speeds.

Publications

1. Dymova O.A., Markova N.V. Numerical Estimation of the Black Sea Circulation near the Continental Slope Using SKIRON and ERA5 Atmospheric Forcing MDPI Environmental Sciences Proceedings, Environmental Sciences Proceedings. 2023, 25(1), 61 (year - 2023) https://doi.org/10.3390/ECWS-7-14305

2. Kazakov D.A., Pavlov M.I., Chukharev A.M. "1.5D" vertical turbulent exchange model verification based on microstructural probe data on cruise 122 of the R/V Professor Vodyanitsky Springer Proceedings in Earth and Environmental Sciences, CIWO 2023. Springer Proceedings in Earth and Environmental Sciences. pp 48–58. Springer, Cham. https://doi.org/10.1007/978-3-031-47851-2_7 (year - 2023) https://doi.org/10.1007/978-3-031-47851-2_7

3. Korzhuev V.A. Turbulence Influence on the Thickness of the Mixed Layer in the Coastal Zone of the Black Sea CIWO 2023. Springer Proceedings in Earth and Environmental Sciences. Springer, Cham., Complex Investigation of the World Ocean (CIWO-2023), pp 69–79. (year - 2023) https://doi.org/10.1007/978-3-031-47851-2_9

4. Markova N.V. Глубоководные течения Чёрного моря в условиях регулярного и аномального атмосферного воздействия (по результатам численного моделирования) материалы Международного научного форума, посвященного 20-летию ЮНЦ РАН. Ростов- на-Дону: Издательство ЮНЦ РАН, 2023, Фундаментальные исследования, инновационные технологии и передовые разработки в интересах долгосрочного развития Юга России : материалы Международного научного форума, посвященного 20-летию ЮНЦ РАН, Ростов-на-Дону, 08–10 февраля 2023 года. 2023. С.30-34. (year - 2023)

5. Markova N.V., Demyshev S.G. The Black Sea deep-water circulation under climatic and anomalous atmospheric forcing Springer Geology. Processes in GeoMedia—Volume VII. Springer: Springer Nature Singapore Pte Ltd., In: Chaplina T. (eds) Processes in GeoMedia – Volume VII, 2023. P. 71–77. Springer Geology The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 (year - 2023) https://doi.org/10.1007/978-981-99-6575-5_6

6. Markova, N.V., Dymova, O.A. Conditions of Deep-Water Undercurrent Generation in the North-Eastern Black Sea Pleiades Publishing, Ltd., 2023. Russian Text © The Author(s), 2023, published in Izvestiya RAN. Mekhanika Zhidkosti i Gaza, 2023, Fluid Dynamics, 2023, Vol. 58, No. 5, pp. 852–863. © Pleiades Publishing, Ltd., 2023. Russian Text © The Author(s), 2023, published in Izvestiya RAN. Mekhanika Zhidkosti i Gaza, 2023, No. 5, pp. 25–36. (year - 2023) https://doi.org/10.1134/S0015462823600591

7. Samodurov A.S., Chukharev A.M., Kazakov D.A., Pavlov M.I., Korzhuev V.A. Вертикальный турбулентный обмен в Черном море: экспериментальные исследования и моделирование Морской гидрофизический журнал, - (year - 2023)