This study investigated long-term changes and seasonality of phytoplankton carbon biomass and algae blooms in the open Black Sea surface layer within the post-eutrophication period (1998–2022). Phytoplankton carbon biomass was calculated from satellite-derived chlorophyll a concentration using carbon-to-chlorophyll a ratio from in situ measurements. This allowed us to identify the months experiencing algal blooms within this period (August – November). We show that phytoplankton biomass and bloom frequency were negatively coupled to the vertical density gradient, suggesting that these features were driven possibly by large diatoms harvesting nutrients from deeper depths. Our results also suggest that photoinhibition, together with high vertical density gradients, could reduce phytoplankton biomass and blooms in the surface layer during summer, with the first signs of alleviation in August. Our study also discusses the large differences in drivers of the phytoplankton community between three distinct periods: the pre-eutrophication (before the early 1970 s), intense phase of eutrophication (mid-1980 s – early 1990 s) and post-eutrophication, and conclude that decadal changes in climate and nutrient inputs caused substantial changes in phytoplankton biomass and algae blooms.

• • Emiliania huxleyi blooms in the open Black Sea during late spring-summer and winter • Large interannual variability in late spring-summer bloom frequency and duration • Shallow upper mixed layer promotes E. huxleyi blooms during late spring-summer • Winter bloom frequency has declined from 5–28% before 2007 to 1–3% in recent years • Weakened convective mixing and deep location of the nutricline reduce winter blooms

The thermohaline regime trends of the Black Sea basin in the period 2000–2021 were studied based on the data from three retrospective analyses (reanalyses) of the hydrophysical fields of the Black Sea. It is shown that the cumulative effect of an increase in the average sea surface temperature (SST) and increasingly milder winter conditions in the Black Sea over the last seven to eight years led to a record-breaking warming and then to the disappearance of the ventilated cold intermediate layer (CIL) of the Black Sea in its traditional meaning by 2020. In addition, in the past seven to eight years, accelerated warming of seawaters has been observed within the permanent (main) pycnocline. Changes in the thermal regime of the upper layer of the sea are accompanied by a continuing increase in salinity in the main pycnocline.

This study investigated temporal changes of phytoplankton biomass with particular focus on blooms and their underlying mechanisms within different shelf regions in the western part of the Black Sea for the poorly studied post-eutrophication period. We combined satellite chlorophyll with carbon-to-Chl a ratios to obtain a better proxy of phytoplankton biomass, and applied an algorithm to partition these data into a base seasonal variation and blooms. The base phytoplankton biomass was characterized by a seasonal maximum in October–November, highest values in the Romanian inner shelf region that receives inputs directly from the Danube River, and downward time trends in all shelf regions, coinciding with reduced nitrate inputs from the Danube. The majority of phytoplankton blooms were short-lived events (lasting up to 5 days), which accounted for over 60% of total bloom events. The initiation of spring blooms coincided with the peak riverine input for the Romanian inner shelf and with the formation of the thermocline in other regions. Autumn blooms were observed with the breakdown of the thermocline in October–November. These were most likely sustained by nutrient accumulation in bottom waters originating from riverine inputs in spring and regenerated nutrients from sedimenting organic matter and sediments. Large diatoms may contribute to autumn blooms in the outer shelf regions through harvesting “new” nitrate from deeper waters and hosting endosymbiotic cyanobacteria to extract “new” nitrogen from the atmosphere. Finally, high intensity of upwelling processes could support winter blooms in the Bulgarian outer shelf region.

Global warming is increasing the frequency and severity of the marine heat waves, which poses a serious threat to the marine ecosystem. This study analyzes seasonal and interannual dynamics in the abundance and structure of the mesozooplankton community in Sevastopol Bay based on bi-monthly routine observations over 2003–2014. The focus is on the impact of the summer 2010 marine heat wave (MHW2010) on crustaceans belonging to different ecological groups. As a response to the MHW2010, three warm-water species (O. davisae, A. tonsa and P. avirostris) exhibiting the maximum seasonal density in latter summer showed a sharp increase in the annual abundance and their share in the mesozooplankton community. The increase in the annual abundance in 2010 of the eurythermal species P. parvus and P. polyphemoides exhibiting seasonal peaks in spring and autumn is not related to the MHW2010 but can be explained by a rise of temperature in the first part of the year. O. davisae and A. tonsa showed the most pronounced response among the species to the MHW2010, confirming that non-native species exhibited great flexibility as an adaptive response to environmental changes, especially in the case of climate warming. Among crustaceans observed in this study, O. davisae can be considered as an indicator of the environmental conditions associated with the warming of the Black Sea and the Mediterranean basin as a whole.

Abstract Chlorophyll a (Chl a) observations from satellites exhibit a unimodal seasonal variation in the open Black Sea that peaks during winter, which has led to the hypothesis that phytoplankton production is sustained by convective mixing of nitrate from deeper layers. We compiled in situ carbon‐to‐Chl a ratios for the entire year, displaying an expected seasonal pattern ranging from 46 in February to 195 mg C mg Chl−1 in September. We combined monthly ratios with satellite Chl a to obtain a comprehensive proxy data set for phytoplankton carbon biomass, and used this to examine the seasonal variation in phytoplankton biomass and bloom occurrences. Contrary to current understanding, our results showed that phytoplankton accumulation is predominantly taking place from July to September, when biomass increased threefold despite nitrate transport from below being negligible. We hypothesize that nitrate harvesting at depths and endosymbiont nitrogen fixation by large diatoms could be important, albeit unexplored mechanisms.

Abstract. High-resolution modelling of a large ocean domain requires significant computational resources. The main purpose of this study is to develop an efficient tool for downscaling the lower resolution data such as available from Copernicus Marine Environment Monitoring Service (CMEMS). Common methods of downscaling CMEMS ocean models utilize their lower resolution output as boundary conditions for local, higher resolution hydrodynamic ocean models. Such methods reveal greater details of spatial distribution of ocean variables; however, they increase the cost of computations, and often reduce the model skill due to the so called double penalty effect. This effect is a common problem for many high-resolution models where predicted features are displaced in space or time. This paper presents a Stochastic Deterministic Downscaling (SDD) method, which is an efficient tool for downscaling of ocean models based on the combination of deterministic and stochastic approaches. The ability of the SDD method is first demonstrated in an idealised case when the true solution is known a priori. Then the method is applied to create an operational eddy-resolving Stochastic Model of the Red Sea (SMORS) with the parent model being the eddy-permitting Mercator Global Ocean Analysis and Forecast System. The stochastic component is data-driven rather than equation-driven and applied to the areas smaller than the Rossby radius, where distributions of ocean variables are more coherent. The method, based on objective analysis, is similar to what is used for data assimilation in ocean models, and stems from the philosophy of 2D turbulence. The SMORS model produces higher resolution (1/24th degree latitude mesh) oceanographic data using the output from a coarser resolution (1/12th degree mesh) parent model available from CMEMS. The values on the high-resolution mesh are computed under condition of minimisation of the cost function which represents the error between the model and true solution. The SMORS model has been validated against Sea Surface Temperature and ARGO floats observations. Comparisons show that the model and observations are in good agreement and SMORS is not subject to the ‘double penalty’ effect. SMORS is very fast to run on a typical desktop PC and can be relocated to another area of the ocean.

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.

The paper is aimed at reviewing the studies of mesoscale eddies in the open ocean carried out by the Soviet and foreign institutions during the experiments in the specified geographical regions. The eddy-current effects are known since the 1930ies, and just the experiments “Polygon-67” in the Arabian Sea, “Polygon-70”, MODE , POLYMODE, Tourbillon , “Mesopolygon” in the tropical Atlantic, “Megapolygon” in the northwestern Pacific Ocean have advanced our knowledge about physical nature of the ocean eddy dynamics. Long-term complex measurements including current meters at the moored buoy arrays accompanied by a series of hydrographic surveys have provided the required experimental data for developing the theory of mesoscale variability. It explained arising and evolution of the eddies in the ocean as a result of interaction between various physical processes: baroclinic instability of a large-scale current, transformation of geostrophic turbulence in the Rossby wave field, barotropization of eddies and others. Later on, the studies of the ocean mesoscale variability evolved from the oceanographic surveys and special-purpose experiments at the polygons performed due to the research vessel cruises to the satellite altimetry methods and the drifter technologies. All the modern global estimates of eddy formations published in the last decade confirm the previous summaries, particularly those describing spatial distribution of the eddy kinetic energy in the World Ocean. They also represent new or the improved assessments of the eddies’ various characteristics: their size, drift velocity and direction, sign of rotation, nonlinearity, trajectory shear and others. Despite of the progress in remote sensing and regular launching of great amount of drifters and profiling floats, it is still difficult to obtain a comprehensive pattern of eddy dynamics within the whole World Ocean. It seems perspective to resume in future special-purpose experiments in the key regions of the World Ocean based on modern technologies.

The paper is devoted to the Black sea marine information systems. FSBSI (Marine Hydrophysical Institute of RAS) has gained a great experience in their development. A special feature of marine GISs is that in many cases the multidisciplinary and multi-component character of scientific oceanology leads to creating narrower specialized software tools to operate oceanographic data. At the same time, the accuracy of the information system representation of the processes taking place in the marine environment and in the sea – land interactive zone is determined in a considerable degree by the comprehensiveness and quality of databases in use. While creating the GISs for the Black sea, MHI uses as an information basis the Black sea database containing more than 160,000 oceanographic stations made so far since 1890. More than 80 per cent of the data have passed the quality check procedure. However, a number of GISs, for their successful operating, demand more parameters than the Black Sea database includes. This can be exemplified by the structure of the software used in the Geoinformation system of the Russian Black sea coastal zone. The database providing operation of the Specialized GIS for beach cadastral evaluation also has its peculiarities. The specific aspects of structure and functionality of the above mentioned and some other GISs created in MHI are described. The immediate objectives are identified both to create a comprehensive universal system for a wide range of researchers, with an optimized query system while addressing the integrated database, and to develop methods of spatial data sharing which allow gaining access to shared databases of the existing applied specialized systems.

Introduction.Tracking of spread of various contaminations and elaboration of the operational systems to control wrecking discharges are among the important tasks of marine environment monitoring.The processes of transport of the contaminating impurity inflowing from different sewers were modeled based on the diagnostic calculations of water circulation in the Sevastopol Bay.Data and Methods.The currents field was calculated using the sigma-coordinate version of the Princeton Ocean Model adapted for the regional conditions in the Sevastopol Bay.To calculate the polluting impurity transport, the model of the matter transfer and diffusion was incorporated into the circulation model.The data on the wind speed and direction obtained at the Sevastopol met office, the temperature, salinity and density climatic fields calculated using the information of 2.7 thousands hydrological stations in the Sevastopol Bay, average seasonal variations of the River Chernaya water discharge and the digital bottom relief with spatial resolution 68 m were used in the model.Analysis of Results.Numerical experiments on the contaminant propagation from the point of possible discharge in the Gollandiya Bay reveal that dependence of the pollutant movement trajectory (direction) upon the pattern of water circulation is most evident in the Yuzhnaya Bay and less manifested in the central part of the Sevastopol Bay.In case of a wrecking discharge in the River Chernaya mouth, a contaminant spot, regardless of wind conditions, moves to the northwest and reaches the Gollandiya Bay.Further evolution of the polluted water volume is similar to the process developing after a sewage discharge directly within the Gollandiya Bay. Discussion and Conclusions.The carried out numerical calculations confirm operatioinal capability of the model and its adequate reproduction of the physical processes under study.It permits both to model the circulation seasonal variation and the thermohaline structure of the Sevastopol Bay waters, and to describe more accurately trajectories of the contaminants' spread.

Purpose. Spatio-temporal variability of the mixed layer depth (MLD) in different areas of the Black Sea in 1985-2017, its relationship with basin dynamics and atmospheric forcing are studied. Methods and Results. The study is based on the hydrological data archive for 1985-2017 including the measurements of the ship expeditions, the Argo buoys and the moored buoy "Aqalog". Seasonal and interannual variability of the mixed layer depth was determined using the density criterion (dr = = 0.07kg/m(3)) between the surface layers and the base of the upper mixed layer. Conclusions. In January - March, the large-scale and mesoscale dynamics significantly affects the mixed layer depth variability Minimum monthly average values of the mixed layer depth in winter are observed in the mesoscale cyclonic eddies and in the center of the sea (20-30 m), the moderate values - on the periphery of the basin (40-45 m) and the maximum ones - in the mesoscale anticyclones (60-70 m). Several times the mixed layer depth values exceeding 150 m were detected in the downwelling areas of the basin. Analysis of the whole period (1985-2017) shows that the mixed layer density was never more than 1015 kg/m(3). This isopycnal limits the maximum possible depth of the upper mixed layer. The impact of wind velocity on the spatial and temporal variability of the mixed layer thickness is the largest in spring and autumn when the seasonal thermocline is weak. It is less important in summer when solar heating stabilizes the upper layer, and in winter when the mixed layer depth is large. Rise of the mixed layer depth in summer is observed in recent years that is associated with rise of the wind speed in a warm period of a year.

Mathematical method combining optimal interpolation and expansion into the empirical orthogonal functions is developed to implement a retrospective analysis of the Black Sea thermohaline structure using incomplete archival oceanographic data. In order to increase spatial consistency of the resulted hydrologic structure, the earlier applied reconstruction method based on the horizontal empirical orthogonal functions was transformed to the combined one in which the vertical empirical orthogonal functions were the basic elements. The results of computing experiments make it possible to limit the number of the modes by 5 both for horizontal and vertical empirical orthogonal functions. Such a combination significantly reduces the calculation time and lowers the error level. This method was applied to reconstruct the monthly fields (spatial resolution is 10′ latitude × 15′ longitude) for almost a hundred-year period from 1923–2015. The relative part of the monthly average fields’ successful reconstruction constitutes about 70 %. Based on the reanalysis data, the temperature and salinity climatic fields were calculated by various methods both for the entire observational period and for certain decades. It is revealed that in the XX century the gain-phase climatic characteristics of the Black Sea remain very stable whereas general tendencies in the long-term variations of the temperature and salinity seasonal cycles are opposite: when the sea temperature seasonal range rises the phase of annual harmonic of seasonal oscillations diminishes, and in the case of salinity, it increases, i. e. the salt content maximum shifts for the later period. The reanalysis data were used to study various aspects of the inter-annual and inter-decadal variability of the Black Sea thermohaline structure, density stratification, geostrophic circulation etc. The future trends imply application of the thermohaline fields’ reanalysis array for studying long-term changes in the Black Sea basin as well as for assimilating observational data in the hydrophysical fields’ reconstructions by the hydrodanamic models.

Combination of altimetry-based method of eddy identification and historical hydrological measurements for 1992-2015 is used to analyze the thermohaline and dynamic structure of the Black Sea eddies and its relation with eddy intensity, eddy age and season of a year. Anticyclonic eddies (AEs) are characterized by negative salinity anomalies, which can reach -1.7 psu at the depth of the main halocline. The temperature anomalies are positive in their upper layers, and negative in the deeper layers, because of the vertical displacement of the waters of the Cold Intermediate Layer (CIL). Cyclonic eddies (CEs) have the opposite structure with increased salinity, colder upper layers and warmer deeper layers. Thermohaline anomalies in the eddies of both signs are maximal in summer, while in winter they are shallowest and minimal. The displacement of pycnocline in eddies causes the decrease/increase of stratification in the upper layer of AEs/CEs and opposite increase/decrease in their deeper layers. It also causes the deepening/uplift of the layer of maximum geostrophic vertical shear in AEs/CEs. The latter is the probable reason of the observed higher intensity and deeper penetration of orbital velocities in AEs than in CEs. The changes of isopycnals positions during the eddies' lifetime are used to quantify the evolution of vertical velocity in AEs and CEs. In the beginning of AEs life during intensification phase, vertical velocity is directed downward, while during the decaying phase it change its sign and is directed upward. The opposite is observed in CEs. Vertical velocity is maximal at the pycnocline depth of 100-110 m with values changing from (- 8 to 8)* 10(-6) m/s in AEs, and from (+ 5 to -25)10(-6) m/s in CEs. Eddies thermohaline structure and altimetry-derived orbital velocity is tightly related. This relation obtained in the study and altimetry-derived data an the distribution of eddy frequency, translational speed and orbital velocity is used to quantify eddies salt, heat content and transport in the basin. The transport velocity of water in the eddies core (2-4 cm/s) is significantly smaller than the average velocity of the large-scale currents (similar to 10-40 cm/s). Such slowing causes the "relative" transport of eddies against the mean flow direction. This effect leads to the accumulation of brackish and cold water in the deep layers of east Black Sea and maintain the observed east-west asymmetry of the basin thermohaline fields.

Reliability of the forecasted fields in the Arctic Basin is limited by a number of problems resulting, in the first turn, from lack of operational information. Due to the ice cover, satellite data on the sea level and the sea surface temperature is either completely not available or partially accessible in summer. The amount of CTD measuring systems functioning in the operational mode (3-5 probes) is not sufficient. The number of the temperature-profiling buoys the probing depth of which is limited to 60 m, is not enough for the Arctic as well. Lack of spatial resolution of the available altimetry information (14 km), as compared to the Rossby radius in the Arctic Ocean (2-12 km), requires a thorough analysis of the forecasting system practical goals. The basic factor enhancing reliability of the oceanographic forecast consists in the fact that the key oceanographic regions, namely the eastern parts of the Norwegian and Greenland seas, the Barents Sea and the Chukchi Sea including the Bering Strait (where the Atlantic and Pacific waters flow in and transform, and the halocline structure is formed) are partially or completely free of ice and significantly better provided with operational information.

Certain results of investigating dangerous phenomena and potential natural disasters in the Azov-Black Sea basin are represented. The decrease of storm activity in the Black Sea observed in the end of the XX century is due to diminution of total amount and intensity of the passing cyclones. According to long-term tendencies of the North Atlantic Oscillation and the East Atlantic Oscillation atmosphere indices, future increase of the storm amount in the Black Sea would be expected. The effective sources of storm surges in the Sea of Azov are the atmospheric cyclones spreading with the 20-40 km/h velocity. The decrease of a cyclone movement velocity results in a storm surge intensification in the Gulf of Taganrog and increase of the flooded area in the Don delta. When the Don discharge becomes lower than the threshold value similar to 1600 m(3)/s, the wind surge exerts a blocking impact upon the river water that promotes the sea level rise in the branches and the delta lowland. The highest potential tsunami hazard for the Black Sea northern coast is represented by the earthquake epicenters located in the Crimea - Caucasus seismic zone. Noticeable sea level oscillations can arise in some locations of the Crimea Southern Coast as a result of the trapped waves propagating to the northwest, north and northeast from the seismic centers nearby the southern coast of the sea.

The Desktop Oceanographic Data Processing Module was developed for visual analysis of interdisciplinary cruise measurements. The program provides the possibility of data selection based on different criteria, map plotting, sea horizontal sections, and sea depth vertical profiles. The data selection in the area of interest can be specified according to a set of different physical and chemical parameters complimented by additional parameters, such as the cruise number, ship name, and time period. The visual analysis of a set of vertical profiles in the selected area allows to determine the quality of the data, their location and the time of the in-situ measurements and to exclude any questionable data from the statistical analysis. For each selected set of profiles, the average vertical profile, the minimal and maximal values of the parameter under examination and the root mean square (r.m.s.) are estimated. These estimates are compared with the parameter ranges, set for each sub-region by MEDAR/MEDATLAS-II1 and SeaDataNet2(2) projects. In the framework of the PERSEUS project, certain parameters which lacked a range were calculated from scratch, while some of the previously used ranges were re-defined using more comprehensive data sets based on SeaDataNet2, SESAME(3) and PERSEUS4 projects. In some cases we have used additional sub-regions to redefine the ranges more precisely. The recalculated ranges are used to improve the PERSEUS Data Quality Control.