Thermohaline Fields Research Articles

Variability of hydrophysical fields under the influence of tides in the river plume frontal zone of the Onega Bay of the White Sea according to a study conducted in June 2023

The results of a series of experiments conducted in June 2023 in the Onega Bay of the White Sea are presented. The study included quasi-temporal in situ and satellite observations in the vicinity of the river plume frontal area formed by the waters of the Onega River. The main objectives were to define the characteristics and dynamics of the area under the influence of tides, as well as to assess the causes of short-term spatial and temporal variability in the characteristics of thermohaline fields. The results show that tides have a significant effect on the structure of waters near the Lyamitsky shore. Here, a large contribution is made by fast-flowing water oscillations associated with short-term internal waves, which frequently occur near the frontal interface. The study demonstrates that the use of high-resolution optical and radar satellite imagery allows a more efficient analysis of frontal and wave dynamics, which is of great importance for understanding the peculiarities of local hydrodynamic processes in the water areas of tidal seas.

Sakawa River plume in Sagami Bay, Japan under weak wind condition: numerical simulation of coastal ocean dynamics and in situ observations for validation

AbstractThis study proposes a method for estimating river plume length from water levels and river discharge rates. A numerical model for coastal ocean dynamics was refined by comparing thermohaline fields calculated using the model with those measured off the mouth of the Sakawa River in Sagami Bay, Japan. The model successfully captured the reduction in salinity within the surface 1.0-m layer caused by riverine water transport. The simulated surface salinity maps revealed that the dynamic motions of the river plumes were primarily driven by one of the two diurnal occurrences of tidal current intensification. Regression analyses of the simulated results demonstrated that the river plume lengths were closely correlated with the water levels and river discharge rates, and that they could be accurately estimated from preceding river discharge rates under weak wind condition.

The in situ and satellite data blended for reconstruction of the surface salinity of the Sea of Azov

ABSTRACT In this paper, we propose a method for retrieving salinity in the Sea of Azov. The method is based on combining in situ and remote sensing measurements. The operational results of the salinity reconstruction are necessary for the assimilation of thermohaline fields, which is used to set initial conditions to the hydrodynamic model and achieve higher accuracy of its results. The proposed method consists of implementing a general regression between historical in situ measurements and L2 MODIS optical measurements. We used properly calibrated and validated satellite data taken from open internet services. The results show different approaches to obtain the linear regression in spring and summer. Particular emphasis is made on analysing the precision of the reconstructed salinity fields relative to in situ measurements. These estimates are based on the comparison of reconstructed salinity with the long-term seasonal trends on in situ data for the periods of 1990–2018 and 2000–2018. The final reconstructed salinity fields are computed from linear regressions for spring and summer. These reconstructed fields are stored as data sets on a regular grid of the Sea of Azov and they are ready for visualization and use as input data for the ocean circulation models.

Spreading of the Amazon River Plume

Results of a joint Russian-Brazilian expedition to study the dynamics of continental river runoff in the ocean associated with the Amazon plume are presented. The stations of the study region covered the seaward part of the Amazon plume. The work was carried out in the dry season (November). The data of in situ measurements and satellite data show that the most desalinated and rich in suspended particulate matter and chlorophyll-a waters were localized on the shallow inner shelf. The horizontal and vertical structure of the thermohaline fields indicates the presence of a well-pronounced river plume about 15 m thick. The decrease in salinity in the plume relative to the background values exceeded 6 PSU even at 300–400 km from the river mouth. The plume waters were characterized by increased concentrations of suspended matter. The best approximation to the in situ measurements is provided by the SMOS satellite salinity data and reanalysis GLORYS12. Chemical determinations in the surface layer in the area of the plume reveal elevated concentrations of silicates, phosphates, and nitrites compared to the seaward part.

Three-Dimensional Thermohaline Structure Reconstruction in the Northwest Pacific Ocean from HY-2 Satellite Data Based on a Variational Method

In this study, a variational method was used to reconstruct three-dimensional thermohaline fields in the Northwest Pacific Ocean. First, we evaluate the applicability of HY-2 satellite data in three-dimensional thermohaline fields reconstructions, and it confirms that the HY-2 data are reliable. A comparison with the forecast products of operational system shows the reconstructions can describe the structural characteristics of the ocean three-dimensional thermohaline fields and reflect the mesoscale change process of the ocean.

An inversion method of subsurface thermohaline field based on deep learning and remote sensing data

ABSTRACT Seawater temperature and salinity are basic marine environmental parameters, which can be used to calculate other marine environmental parameters. However, most of the on-site observation data have the problems of uneven spatial distribution and time discontinuity, and it is difficult for remote sensing observation methods to obtain subsurface information. In this study, we proposed a deep learning model with combining remote sensing temperature and salinity as well as in-situ measured data by Argo profiles, and the nonlinear relationship was revealed. An effective and direct inversion method was realized for underwater three-dimensional thermohaline structure based on remote sensing temperature and salinity at accurate points on global scale. The SST data were obtained from the FY3C-VIRR daily sea surface temperature product, the SSS data were acquired from the SMAP Level 3 8-day running averages sea surface salinity product, and the Argo scatter data were got from the ‘Global Ocean Argo Scatter Data Set’. Based on the temporal and spatial location information of the data, this paper matched the remote sensing data from 2016 to 2019 with the Argo data, 64751 valid pairing points were obtained. The deep learning model was constructed as a multilayer perceptron model with 5 hidden layers. The RMSE of temperature had a maximum value of 2.106°C in 130 m depth and a minimum value of 0.367°C in 1000 m with an average of 1.174°C for validation dataset. And the RMSE of salinity had a maximum value of 0.356 psu in 0 m and a minimum value of 0.045 psu in 1000 m with an average of 0.202 psu. Compared with other methods based on fixed mesh products, this study realized a direct inversion method at any location in the global ocean and improved the inversion accuracy, which provides a reliable data support for refined marine monitoring.

ПРОСТРАНСТВЕННО-ВРЕМЕННАЯ ИЗМЕНЧИВОСТЬ ГИДРОФИЗИЧЕСКИХ И ЭНЕРГЕТИЧЕСКИХ ХАРАКТЕРИСТИК ЦИРКУЛЯЦИИ ЧЕРНОГО МОРЯ ПРИ ДОМИНИРОВАНИИ ДВИЖЕНИЙ РАЗНЫХ МАСШТАБОВ

Simulation results of the Black Sea dynamics for two periods when the annual mean circulation corresponded to the basin-scale and eddy regimes (2011 and 2016) are considered in the paper. Numerical experiments are carried out using the MHI model and considering the realistic atmospheric forcing from SKIRON. The seasonal variability of dynamic and thermohaline fields, as well as the kinetic and available potential energy, and their conversion rates are estimated. According to the model data on the seasonal mean distribution of currents velocity, it is found that in 2011 the RIM Current is detected in all seasons, and the most intense mesoscale eddies developed on its periphery over the continental slope in the warm period of the year; in 2016, separate cyclonic jets in the area of the continental slope are observed in the northern and southwestern parts of the basin during cold seasons, and mesoscale eddies are propagated in the central part of the sea throughout the year. The change in the mean current kinetic energy is determined by the circulation regime: energy maxima are revealed in the spring of 2011 and in the winter of 2016, when the mean current was the most intense. The distribution of the mean available potential energy is predominantly seasonal, the time variability is qualitatively similar for both modes and is provided by an increase in the density anomaly due to seawater heating. The eddy kinetic energy characterizing the mesoscale variability depends both on the circulation regime and on the season. In the spring 2011, the mean current and eddy kinetic energies are comparable; in 2016, the maximum eddy energy exceeded the mean current kinetic energy. In autumn and winter, for both calculations, the increase in eddy energy occurs due to the energy transfer from the wind and the mean current through the barotropic instability mechanism. In summer when wind activity weakens, in the basin-scale circulation mode, mesoscale variability is supported by commensurate contributions from barotropic and baroclinic instability; in the eddy circulation mode – mainly due to the conversion of available potential energy through baroclinic instability.

Temperature and Salinity Inverted for a Mediterranean Eddy Captured With Seismic Data, Using a Spatially Iterative Markov Chain Monte Carlo Approach

Ocean submesoscale dynamics are thought to play a key role in both the climate system and ocean productivity, however, subsurface observations at these scales remain rare. Seismic oceanography, an established acoustic imaging method, provides a unique tool for capturing oceanic structure throughout the water column with spatial resolutions of tens of meters. A drawback to the seismic method is that temperature and salinity are not measured directly, limiting the quantitative interpretation of imaged features. The Markov Chain Monte Carlo (MCMC) inversion approach has been used to invert for temperature and salinity from seismic data, with spatially quantified uncertainties. However, the requisite prior model used in previous studies relied upon highly continuous acoustic reflection horizons rarely present in real oceanic environments due to instabilities and turbulence. Here we adapt the MCMC inversion approach with an iteratively updated prior model based on hydrographic data, sidestepping the necessity of continuous reflection horizons. Furthermore, uncertainties introduced by the starting model thermohaline fields as well as those from the MCMC inversion itself are accounted for. The impact on uncertainties of varying the resolution of hydrographic data used to produce the inversion starting model is also investigated. The inversion is applied to a mid-depth Mediterranean water eddy (or meddy) captured with seismic imaging in the Gulf of Cadiz in 2007. The meddy boundary exhibits regions of disrupted seismic reflectivity and rapid horizontal changes of temperature and salinity. Inverted temperature and salinity values typically have uncertainties of 0.16°C and 0.055 psu, respectively, and agree well with direct measurements. Uncertainties of inverted results are found to be highly dependent on the resolution of the hydrographic data used to produce the prior model, particularly in regions where background temperature and salinity vary rapidly, such as at the edge of the meddy. This further advancement of inversion techniques to extract temperature and salinity from seismic data will help expand the use of ocean acoustics for understanding the mesoscale to finescale structure of the interior ocean.

Structure of the Field of the Colored Dissolved Organic Matter Concentration in the Kerch Strait

Purpose. Using the data obtained in the expeditions, 2001–2014, the authors intend to identify the typical features of vertical structure of the colored dissolved organic matter (fDOM) concentration field in the Kerch Strait, to type the fDOM(z) profiles, to zone the region under consideration according to a given set of the qualitative features, and also to determine the features of statistical characteristics of the fDOM concentration distribution on the sea surface layer for the water areas with a typical structure. Methods and Results. The typing was carried out by the method of visual expert assessment of the curves of vertical distribution of concentration of the considered value. It was based on analyzing the fDOM(z) profile shapes. To reveal the boundaries of the areas with typical stratification of the fDOM content field, the thermohaline field structure was analyzed. Three types of water vertical structure were identified; they differed in the fDOM(z) profile shape and in statistical indices of empirical distribution of this substance concentration on the sea surface. These are the Azov Sea and the Black Sea types peculiar to the Azov and Black seas waters not contaminated by dissolved organic matter, and the type, the structure of which identifies the waters containing the anthropogenic component in the concentration field of the analyzed value. The latter type is characterized by a special intrusive shape of the fDOM(z) profile. For each of the identified stratification types, the histograms of the fDOM concentration distribution on the sea surface were calculated. Conclusions. The fDOM(z) profiles were typed. The Kerch Strait water area was zoned in accordance with a given set of the preliminary revealed qualitative features. The boundaries of the areas with typical stratification and their displacements were determined. It is shown that each structure type has its own statistical distribution of concentration of the considered value on the sea surface

Struktura polya kontsentratsii okrashennogo rastvorennogo organicheskogo veshchestva v Kerchenskom prolive

Purpose. Using the data obtained in the expeditions, 2001–2014, the authors intend to identify the typical features of vertical structure of the colored dissolved organic matter (fDOM) concentration field in the Kerch Strait, to type the fDOM(z) profiles, to zone the region under consideration according to a given set of the qualitative features, and also to determine the features of statistical characteristics of the fDOM concentration distribution on the sea surface layer for the water areas with a typical structure. Methods and Results. The typing was carried out by the method of visual expert assessment of the curves of vertical distribution of concentration of the considered value. It was based on analyzing the fDOM(z) profile shapes. To reveal the boundaries of the areas with typical stratification of the fDOM content field, the thermohaline field structure was analyzed. Three types of water vertical structure were identified; they differed in the fDOM(z) profile shape and in statistical indices of empirical distribution of this substance concentration on the sea surface. These are the Azov Sea and the Black Sea types peculiar to the Azov and Black seas waters not contaminated by dissolved organic matter, and the type, the structure of which identifies the waters containing the anthropogenic component in the concentration field of the analyzed value. The latter type is characterized by a special intrusive shape of the fDOM(z) profile. For each of the identified stratification types, the histograms of the fDOM concentration distribution on the sea surface were calculated. Conclusions. The fDOM(z) profiles were typed. The Kerch Strait water area was zoned in accordance with a given set of the preliminary revealed qualitative features. The boundaries of the areas with typical stratification and their displacements were determined. It is shown that each structure type has its own statistical distribution of concentration of the considered value on the sea surface.

Raschet gidrofizicheskikh poley s assimilyatsiyey dannykh nablyudeniy v severnoy chasti Chernogo morya v osenniy sezon 2016 goda

The paper analyzes the results of calculation of the circulation and thermohaline water structure based on a three-dimensional nonlinear hydrodynamic model and hydrological data from a survey, carried out in the northern part of the Black Sea in autumn 2016. We used a high spatial resolution (horizontal grid ~1.6 × 1.6 km and 27 vertical layers) model and realistic atmospheric forcing. The assimilation procedure of observational data was based on a modified Kalman filter taking into account the heterogeneity and anisotropy of estimation error of the temperature and salinity fields. The calculated fields of currents were characterized by eddies and jet currents. The following circulation features were observed in the northern part of the Black Sea: an intense jet of the Black Sea Rim Current along the Crimean coast, anticyclonic eddies with a radius of about 30 km near Sevastopol and in the western part of the region, a cyclonic eddy with a radius of about 40 km between 34.5 and 35.5 E, anticyclonic eddies with a radius of about 25 km along the Crimean coast. Eddy formations of different scales and different signs of rotation were obtained (in the Kalamitsky Bay, in the eastern part and along the eastern coast of the Crimea), when the current flowed around irregularities of the coastline and the bottom relief in the coastal zone. The correlation between the formation of zones of lower and higher temperature relative to adjacent waters and the location of features in the field of current (in particular, the correspondence of zones with warmer desalinated water to the anticyclonic formations) was noted from the analysis of the reconstructed thermohaline fields.

Assessment of the Black Sea Temperature and Salinity Climatic Fields for the Recent Climatological Period (1991–2020)

Purpose. The purpose of the study is to assess the coefficient of vertical turbulent exchange for different layers of the Black Sea basin based on the experimental data on microstructure of the physical fields obtained for the period 2004–2019 in the Black Sea and using the semi-empirical models. Methods and Results. New array of the temperature and salinity climatic fields was assessed by the results of numerical experiments. In the experiment, annual variation of the Black Sea hydrophysical parameters was reconstructed by the numerical model. Modeling included the scheme of assimilating the data of the climatic temperature and salinity array assessed. In contrast to the averaged data of the field observations, the modeled fields are compliant with equations of motion. Besides the temperature and salinity three-dimensional fields, the three-dimensional climatic fields of the Black Sea currents were also reconstructed for each day of a climatic year that is quite impossible using the observational data only. Spatial-temporal variability of the modeled three-dimensional fields was analyzed. The integral characteristics of the Black Sea water dynamics for the recent 30-year climatic period were studied and compared with the analogous ones for the previous century. Simulation was carried out by three-dimensional non-linear model of the Black Sea dynamics developed in Marine Hydrophysical Institute. The horizontal resolution of the model was 5 km, and the EMODNet bathymetry was used. The performed calculations showed that the increased spatial resolution of the temperature and salinity climatic array for the recent period made it possible to reconstruct the dynamics of the Black Sea in all layers in more detail. At the same time, significant small-scale variability of salinity fields was revealed. It was most pronounced at the deep-water horizons. Conclusions. Modeling using a new array of thermohaline fields revealed an increase in the integral temperature of the upper mixed layer in comparison with the experiment with assimilation of the previous version of the climatic array. At that, thinning and «break» of the cold intermediate layer found in the central part of the sea, indicates warming of the sea upper layer during the last 30 years. The highest noise detected at the deep-water horizons in the modeled salinity fields is related to quantity and quality of the salinity data resulted from the field observations. Taking into account insufficient calibration facilities for measuring seawater electrical conductivity, the next version of climatic TS-array requires a more strict procedure for verifying and processing the observation data obtained in the deep-sea layers.

Otsenka klimaticheskikh poley temperatury i solenosti vod Chernogo morya dlya sovremennogo perioda (1991–2020)

Purpose. The study is aimed at assessing a new climatic array of the Black Sea temperature and salinity calculated using hydrological observations for the standard (according to the World Meteorological Organization definition) 30-year climatological period 1991–2020. Methods and Results. New array of the temperature and salinity climatic fields was assessed based on analyzing the results of numerical experiments. In the experiment, annual variation of the Black Sea hydrophysical parameters was reproduced by the numerical model. Modeling included the scheme of assimilating the data of the climatic temperature and salinity array assessed. In contrast to the averaged data of the field observations, the model fields were matched from the viewpoint of the motion equations. Besides the temperature and salinity three-dimensional fields, the three-dimensional climatic fields of the Black Sea currents were also reproduced for each day of a climatic year that is quite impossible using the instrumental measurements data only. Spatial-temporal variability of the modeled three-dimensional fields was analyzed. The integral characteristics of the Black Sea water dynamics for the recent 30-year climatic period were studied and compared with the analogous ones for the previous century. Simulation was performed at the grid with the 5 km horizontal resolution using the EMODNet bathymetry by means of the three-dimensional non-linear model of the Black Sea dynamics developed in Marine Hydrophysical Institute. Having been analyzed, the performed calculations showed that the increased spatial resolution of the temperature and salinity climatic array for the recent period made it possible to reproduce dynamics in all the layers of the Black Sea waters in more details. At the same time, a significant small-scale variability, most pronounced at the deep-water horizons, was present in the salinity fields. Conclusions. As compared to the experiment with assimilation of the previous version of the climatic array, the modeling based on the new array of the thermohaline fields revealed increase in the integral temperature of the upper mixed layer. At that, thinning and «break» of the cold intermediate layer found in the central part of the sea, indicates warming of the sea upper layer during the last 30 years. The highest noisiness detected at the deep-water horizons in the modeled salinity fields is related to quantity and quality of the salinity data resulted from the field observations. Taking into account insufficient metrological facilities for measuring seawater electrical conductivity, the next version of climatic array requires a more strict procedure for verifying and processing the observation data obtained in the deep-sea layers.

Generation of Vertical Fine Structure by the Internal Waves with the Regard for Turbulent Viscosity and Diffusion

Purpose. The study is aimed at evaluating effectiveness of the procedure of the observational data assimilation using the Kalman filter algorithm as compared to sequential analysis of the hydrophysical fields based on the optimal interpolation method, and at analyzing the mesoscale features of coastal circulation near the western Crimea coast and in the Sevastopol region. Methods and Results. Based on the hydrodynamic model adapted to the Black Sea coastal zone conditions including the open boundary and on the temperature and salinity data from the hydrological survey in 2007, the dynamic and energy characteristics of the Black Sea coastal circulation were calculated with high spatial resolution (horizontal grid is ~ 1.6 × 1.6 km and 30 vertical horizons). The hydrophysical fields were reconstructed using two algorithms of data assimilation: the sequential optimal interpolation and the modified Kalman filter. The kinetic energy changed mainly due to the wind action, vertical friction and the work of pressure forces; the potential energy – due to the potential energy advection and the horizontal turbulent diffusion. The following circulation features were reconstructed: the anticyclonic eddy with the radius about 15 km in the Kalamita Bay in the water upper layer, the anticyclonic eddy with the radius about 15 km between 32.2 and 32.6° E in the whole water layer, the intense current near Sevastopol and along the Crimea western coast directed to the north and northwest, and the submesoscale eddies of different signs of rotation in the upper layer. Conclusions. It is shown that having been taken into account, heterogeneity and non-isotropy of the error estimates of the temperature and salinity fields relative to the correlation function lead to qualitative and quantitative differences in the hydrodynamic fields (amplification of currents, change of the currents’ direction and eddy formations were better pronounced). At the same time, the mean square errors of the thermohaline fields’ estimates decreased. Formation of the anticyclonic eddy with the radius about 15 km in the Kalamita Bay could be related to the current shear instability. Submesoscale eddies with the diameters less than 5 km were formed when the current flowed around the coastline and the bottom topography inhomogeneities.

Analiz dinamicheskikh i energeticheskikh kharakteristik tsirkulyatsii vod u beregov Zapadnogo Kryma na osnove assimilyatsii dannykh nablyudeniy v chislennoy modeli dinamiki Chernogo morya

Purpose. The study is aimed at evaluating effectiveness of the procedure of the observational data assimilation using the Kalman filter algorithm as compared to sequential analysis of the hydrophysical fields based on the optimal interpolation method, and at analyzing the mesoscale features of coastal circulation near the western Crimea coast and in the Sevastopol region. Methods and Results. Based on the hydrodynamic model adapted to the Black Sea coastal zone conditions including the open boundary and on the temperature and salinity data from the hydrological survey in 2007, the dynamic and energy characteristics of the Black Sea coastal circulation were calculated with high spatial resolution (horizontal grid is ~1.6×1.6 km and 30 vertical horizons). The hydrophysical fields were reconstructed using two algorithms of data assimilation: the sequential optimal interpolation and the modified Kalman filter. The kinetic energy changed mainly due to the wind action, vertical friction and the work of pressure forces; the potential energy – due to the potential energy advection and the horizontal turbulent diffusion. The following circulation features were reconstructed: the anticyclonic eddy with the radius about 15 km in the Kalamitsky Bay in the water upper layer, the anticyclonic eddy with the radius about 15 km between 32.2 and 32.6° E in the whole water layer, the intense current near Sevastopol and along the Crimea western coast directed to the north and northwest, and the submesoscale eddies of different signs of rotation in the upper layer. Conclusions. It is shown that having been taken into account, heterogeneity and non-isotropy of the error estimates of the temperature and salinity fields relative to the correlation function lead to qualitative and quantitative differences in the hydrodynamic fields (amplification of currents, change of the currents’ direction and eddy formations were better pronounced). At the same time, the mean square errors of the thermohaline fields’ estimates decreased. Formation of the anticyclonic eddy with the radius about 15 km in the Kalamitsky Bay could be related to the current shear instability. Submesoscale eddies with the diameters less than 5 km were formed when the current flowed around the coastline and the bottom topography inhomogeneities

The White Sea: Available Data and Numerical Models

The White Sea is a small shallow semi-closed sea in the North-West of Russia. It is strongly affected by induced tides, so the tidal motion dominates in the sea. Sea ice is seasonal and the water salinity is less than in the neighbouring Barents sea due to strong river discharge. We review the sources of in-situ and satellite data that are available for the sea, and describe those few numerical models, together with the challenges that are faced. We focus on the large-scale circulation and thermohaline fields, but also cover sea ice, river runoff, and pelagic biogeochemical data.

Исследование и контроль циркуляции вод и структуры термохалинного поля в бухте Омега (Крым) в летне-осенний период 2019 года

Based on the data of two expeditions carried out in August and November 2019, the water circulation and the structure of the thermohaline field in Omega Bay are analyzed. A well-known idea on the predominantly wind nature of currents in the Sevastopol bays is confirmed. In August, under the influence of the north wind, a south-west oriented coastal wind current was observed in the Sevastopol seaside adjacent to the bay. Its speed in the upper and middle layers varied from 15 to 40 cm /s and from 10 to 15 cm /sec – at the bottom. The branch of this stream directed to the bay, interacting with the bottom rising located in its central part, contributed to the formation of an anticyclone water circulation cell. A similar dynamic situation in Omega Bay, but under conditions of a weaker and oppositely directed alongshore current, was also observed in November. It is shown that the anticyclone topographic eddy, found in the central part of Omega Bay is quasi stationary and has a topographic nature. In the structure of the thermohaline field, signs have been revealed that confirm the anticyclone nature of the local water circulation in Omega Bay, as well as the typical coastal current system generated by the wind surge, which was recorded in the August survey. In November, under the influence of convective mixing, the thermohaline field in the studied water area was more uniform. It is shown that, in contrast to relatively homogeneous waters of the most part of the bay water area, vertical stratification and frontal horizontal structure of the thermohaline field are characteristic of its kut region.

МОДЕЛЬНОЕ ИССЛЕДОВАНИЕ МЕЖГОДОВОЙ ИЗМЕНЧИВОСТИ ВОДООБМЕНА ПОЛЯРНЫХ МОРЕЙ С АТЛАНТИЧЕСКИМ И СЕВЕРНЫМ ЛЕДОВИТЫМ ОКЕАНАМИ

The interannual variability of the mass transports through the Denmark and Fram Straits, and through the sections separating the Nordic Seas from the Atlantic Ocean and Barents Sea is studied on the base of numerical simulations performed with the use of the Argobased Model for Investigation of the Global Ocean (AMIGO). The model consists of a block for variational interpolation to a regular grid of Argo floats data and a block for model hydrodynamic adjustment of variationally interpolated fields. Additional numerical experiments were carried out in order to study the contribution of the wind forcing to the interannual variability of the transports: the real thermohaline fields corresponding to the particular time period were replaced by climatic ones (1) and by replacing the real wind forcing data with the climatic ones (2). Analysis of the numerical experiments results has shown that the variable wind stress forcing is the key factor determining the interannual variability of the water exchange between Atlantic, Nordic Seas, and Arctic Ocean.

Features of Thermohaline Structure and Circulation in Lake Issyk-Kul

Field measurements carried out over 4 years made it possible to establish previously unknown features of the thermohaline fields and circulation of Lake Issyk-Kul. The most detailed salinity distribution maps for the entire history of observations were constructed. An area of slightly increased salinity was found in the central part of the lake, the specific “dipole” shape of which indicates the existence of not only a general cyclonic circulation, but also two separate gyres of a smaller, subbasin scale (which is partially confirmed by direct current velocity measurements). It has been established that, generally speaking, the salinity fields in Lake Issyk-Kul are extraordinarily conservative: its interannual and seasonal changes, as well as spatial variability throughout the lake (with the exception of estuarine regions), are usually measured only in hundredths of g kg–1. An important result of the project is the discovery of a subsurface salinity maximum persisting from year to year at depths from 70 to 130 m. As shown on the basis of balance estimates and then confirmed by analysis of direct current velocity measurements, autumn–winter differential cooling leads to the fact that in canyons (i.e., paleoriver channels) in the eastern littoral region, a significant amount (up to 1 km3) of cold coastal waters freshened by river runoff enters the bottom layers of the central part of the lake. These waters are then mixed with the overlying more saline waters, which, in a situation where the upper layer of the lake is also desalinated by river runoff, leads to a salinity maximum at intermediate depths. Our measurements do not confirm manifestations of global warming in the form of an interdecadal temperature increase in the deeper layers of Lake Issyk-Kul, which was previously reported: the current (in 2018) temperature at a depth of 500 m exactly coincided with that noted in the 2003 measurements, namely, about 4.44°C. However, one can point to a very weak (about 0.03 g kg–1) increase in the salinity of the bottom layer over the past 40 years.

The thermohaline structure of the North Atlantic waters in different phases of the Atlantic multidecadal oscillation

The meridional structure of climatic trends and anomalies of potential temperature and salinity in the North Atlantic waters in different periods of the Atlantic Multidecadal Oscillation (AMO) in 19482017 are studied based on the EN4 and WOA2013 objective analyses data. An analysis of these different data sets allowed us to reveal almost identical patterns of variability of the thermohaline fields of the North Atlantic, which increases the reliability of the results. Long-term temperature and salinity trends simulated over the period 19482017 show that warming and salinization of water occur in the upper ~1 km layer of the North Atlantic. On the contrary, cooling and freshening of deep waters are observed, which is associated with the melting of the Greenland ice sheet, transport of fresher waters from the Arctic Ocean, and deepening of these cold and fresher waters into the deeper layers. Composite analysis of the zonally averaged temperature and salinity anomalies of the North Atlantic waters after removing the trends showed that in the warm AMO periods warming and salinization of waters are observed in the upper 1-km layer of the North Atlantic when compared to the cold periods based both on the EN4 and WOA2013 data. Below the 1-km layer, significant regions of cooling and freshening are observed; this distribution is more pronounced in the EN4 data. Analysis of the dynamics of zonally averaged temperature and salinity anomalies in the successive periods associated with the temporal variability of the AMO index revealed that these anomalies propagate along the zonally averaged meridional thermohaline circulation. To show this using the Institute of Numerical Mathematics Ocean Model (INMOM), the stream function of the Atlantic Meridional Overturning Circulation (AMOC) was simulated. It is shown that positive and negative anomalies of both temperature and salinity circulate along the water motion in the AMOC around its core, descending down into the deep ocean layers approximately at 60 N and ascending to the surface at 25 N, replacing each other with a period of about 60 years. It can be assumed that due to this process both the warm and cold phases of the AMO are formed.