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Bulgarian National Operational Marine Observing System (NOMOS) is a module of MASRI - Infrastructure for Sustainable Development of Marine Research and Participation in the European Infrastructure Euro-Argo, a project of the National roadmap for scientific Infrastructure (2020 – 2027) of Republic of Bulgaria. NOMOS consist of six components, one of which is a waves and currents monitoring system managed in collaboration by the Institute of Oceanology, Bulgarian Academy of Sciences (IO-BAS) and the National Institute of Meteorology and Hydrology (NIMH). The development of the waves observing system started in 2020 with the deployment of six moored wave buoys, three by IO-BAS and three by NIMH. Next year another three wave buoys were deployed by NIMH. The deployment positions were chosen to provide optimal coverage of the Bulgarian Black Sea coast. The buoy measurements are transmitted using GPRS or satellite communications and are stored in databases both at the Bulgarian National Oceanographic Data Center and at the NIMH data center. WEB sites were developed to deliver real time wave data to stakeholders. The wave observing system has been in operation for over a year and sufficient data has been collected for an initial analysis. During the operation period, experience was gained in maintaining the system in order to provide reliable sea waves data. Biofouling and vandalism are assessed as the main factors influencing system performance. The wave observing system is a unique source of in-situ wave data in the Black Sea that provides real-time wave data and long series of data for science and marine industry. In-situ wave data are also distributed through CMEMS and used for the assimilation in wave models and quality assessment of forecasts and reanalyzes in Copernicus marine services.

Special issue “Advances in interdisciplinary studies at multiple scales in the Mediterranean Sea”. It is a result of the 8th MONGOOS Meeting & Workshop, Trieste, Italy, 3–5 December 2019.-- 57 pages, 9 figures, 1 table, 1 appendix, supplement https://doi.org/10.5194/os-18-997-2022-supplement.-- Data availability: Access to sea level data presented in this review is provided through national and international data portals and repositories quoted in Sects. 2 and 3 and in the Supplement, Employed for over a century, the traditional way of monitoring sea level variability by tide gauges – in combination with modern observational techniques like satellite altimetry – is an inevitable ingredient in sea level studies over the climate scales and in coastal seas. The development of the instrumentation, remote data acquisition, processing, and archiving in the last decades has allowed the extension of the applications to a variety of users and coastal hazard managers. The Mediterranean and Black seas are examples of such a transition – while having a long tradition of sea level observations with several records spanning over a century, the number of modern tide gauge stations is growing rapidly, with data available both in real time and as a research product at different time resolutions. As no comprehensive survey of the tide gauge networks has been carried out recently in these basins, the aim of this paper is to map the existing coastal sea level monitoring infrastructures and the respective data availability. The survey encompasses a description of major monitoring networks in the Mediterranean and Black seas and their characteristics, including the type of sea level sensors, measuring resolutions, data availability, and existence of ancillary measurements, altogether collecting information about 240 presently operational tide gauge stations. The availability of the Mediterranean and Black seas sea level data in the global and European sea level repositories has been also screened and classified following their sampling interval and level of quality check, pointing to the necessity of harmonization of the data available with different metadata and series in different repositories. Finally, an assessment of the networks' capabilities for their use in different sea level applications has been done, with recommendations that might mitigate the bottlenecks and ensure further development of the networks in a coordinated way, a critical need in the era of human-induced climate changes and sea level rise, This research has been partially supported by Horizon 2020 (EuroSea (grant no. 862626) and JERICO-S3 (grant no. 871153)), With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

International audience; The recent technological advances of the autonomous oceanographic monitoring platforms has allowed observations of unknown regions and dynamic processes that play a key role in the physical environment and marine ecosystem functioning. Under this context Euro-Argo RISE project aims towards the investigation of Argo extension in shallow coastal areas of European marginal seas (Black Sea, the Baltic Sea and the Mediterranean Sea) as well as the seasonally ice-covered areas of the high latitudes (Arctic Ocean). Different fl oat confi gurations are tested whilst the development and implementation of operational systems to control the platforms are performed. Test deployments under Euro-Argo RISE have provided indications for the best settings, the optimization of fl oat missions and the improvement of the life expectancy and the sampling effi ciency in shallow coastal areas. In addition, guidelines and best practices for operating fl oats in seasonally ice-covered areas are provided. The reinforcement and expansion of the Argo infrastructure at a regional scale is performed by approaching the regional research community (also through the organization of workshops and political events) with the aim of: promoting Argo data, cooperation at sea, collaboration in technical activities, sharing best practice, knowledge and expertise.

International audience; Copernicus Marine Service (CMEMS) includes the Black Sea Monitoring and Forecasting Center (BS-MFC) for the provisioning of high-quality forecast products and past reconstruction of the ocean state in the Black Sea. The BS-MFC is implemented by an international consortium since 2016, led by IO-BAS (Bulgaria) and including CMCC (Italy), ULiege (Belgium), HZ Hereon (Germany), USOF (Bulgaria), NIHWM (Romania), together with expertise from University of Bologna (Italy), ITU (Turkey) and DEU (Turkey). For each Production Unit-Physics, Biogeochemistry and Waves-an operational service and scientifi c evolution plans have been implemented. Every day, BS-MFC delivers analysis and 10-days forecast products for essential variables, including biogeochemistry and waves, at the spatial resolution of about 3 km. NRT systems are forced by ECMWF IFS atmospheric forcing, at 12.5 km horizontal resolution and 3-6 hours frequency in time. Twice per year, BS-MFC delivers MY products (reanalysis) starting from 1992 (1979 for BS-WAV). Systems are forced by ECMWF ERA5 atmospheric forcing, at 30 km horizontal resolution and 1-hour frequency in time. Scientifi c plans for the next generation of the BS-MFC systems put in top priority coupling strategy, ensemble forecasting, data assimilation capability and river runoff representation to support downstream services and applications, climate monitoring and coastal community.

International audience; This study will present the evolution of the Black Sea model physics in the framework of the Copernicus Marine Environment Monitoring Service, from EAS3 (current operational system) to EAS4 (operational from May 2021). EAS4 has been greatly improved thanks to new core model (based on NEMO v4.0), improved bathymetry and representation of rivers, revision of the spatial domain at higher resolution (2.5 km and 121 z-star-levels) to include the Marmara Sea box for the provision of the lateral open boundary conditions. For the optimal interface between Mediterranean and Black Sea though boundary conditions, a new high-resolution unstructured grid-based model has been developed for the Marmara Sea. Evolution in terms of modelling framework and prediction capacity will be detailed, describing the main results and skill score metrics.

The recently upgrated CMEMS product Black Sea Physical Reanalysis (BLKSEA_MULTIYEAR_PHY_007_004) covers the period 1993-2019 presenting a base for reliable long-term estimates on different aspects of the Black Sea physical processes. The data archive provides monthly and daily fields for the Black Sea basin including 3D variables (temperature, salinity, zonal and meridional velocity components) and 2D variables (mixed layer depth, bottom temperature and sea surface height). The good spatial and temporal resolution of the reanalysis gives possibility to evaluate the trend and variability of the subsurface temperature and salinity, as well as the general circulation changes. In the last two decades significant tendency for warming is observed at the surface and in deeper layers, reaching down ~100 m depth. This trend is associated with a slight positive salinity trend seen down to ~200 m depth, which is present almost in the entire Black Sea except for the north-western shelf close to the Danube and Dnestr river delta. Both temperature and salinity show strong interannual variability. The calculated Ocean Heat Content (OHC) in the Black Sea basin over the last ~30 year period suggests that the Black Sea water had experienced a general heating tendency after 2013. The increase of OHC is mostly due to the layer 0-200 m and the deeper layers are rather conservative in time. Nevertheless, the cold winter conditions in 2006, 2012 and 2017 led to significant surface water cooling and replenishment of the Cold Intermediate Layer. The variation in the main dynamic feature of the basin, the Black Sea Rim current, is studied using the reanalysis data. It shows that the surface current speed varies within ~30% in the period 1993-2019 with a slight positive tendency. The main factor which triggers the inter-annual variability of the Rim current is found to be the atmospheric forcing. Comparison with the surface wind curl from the ERA5 reanalysis data shows significant correlation, predominantly positive (cyclonic) curl for both sea and atmosphere circulation and similar positive trend of the wind/current speed. This proves that the Black Sea Rim Current could be considered a Sverdrup balanced flow and thus strongly related to the regional air circulation.

The BS-MFC (Black Sea Monitoring and Forecasting Centre) since 2016 is guaranteeing production and delivery of high quality ocean analysis, forecast and reanalysis fields for essential variables, biogeochemical quantities and waves in the Black Sea region within the Copernicus Marine Service. A reliable and robust service infrastructure serves both the production systems and data delivery, through ad hoc technical interfaces, for an efficient update of the catalogue, which includes 22 datasets for physical variables, 22 for biogeochemical variables and 4 for waves. Additionally, a Local Service Desk is in charge for ensuring connections among BS-MFC, CMEMS and Users with the scope to support end-users in using BS-MFC data for downstream applications from the technical and scientific perspectives. The production centres are the core of the BS-MFC: Physics, Biogeochemistry and Waves units implemented, over the Copernicus 2 Programme, state-of-the-art and advanced numerical approaches to improve the quality of the near real time and multi year products. In the 2020, in particular, the BS-Physics team proposed a new reanalysis product, based on new version of the hydrodynamical core model, based on NEMO v3.6, with assimilation of CMEMS observations (e.g. insitu temperature and salinity profiles, including also historical dataset provided by SeaDataNet, and sea level anomaly satellite data) and forced by ECMWF ERA5. The BS-Physics team is working also on preparing the new version of the near real time system, that will provide spatial high resolution analysis and forecast products, using a new version based on NEMO v4.0, online coupled to data assimilation scheme, with optimal interface with the Mediterranean Sea. BS-Biogeochemistry team updated the overall catalogue, with new near real time system, based on NEMO v3.6 online coupled to BAHMBI model, with new carbonate model, able to assimilate new chlorophyll satellite data provided by the CMEMS OC TAC; regarding multi year product, the BS-Biogeochemistry team delivered new datasets, generated by the new NEMO-BAHMBI coupled system forced by ECMWF ERA5 – totally aligned with the near real time system, without data assimilation – for reconstructing the past biogeochemical sea state in the Black Sea. BS-Waves team updated the overall catalogue as well, with new near real time system based on state-of-the-art WAM Cycle 6.0, one-way coupled with hourly currents fields provided by the BS-Physics near real time system; a new reanalysis, from 1979 to 2019, has been also delivered, based on same core model as the near real time system, forced by ECMWF ERA5 atmospheric forcing, and able to assimilate the significant wave height provided by CMEMS SL TAC. Systems are monitored through a product quality dashboard, based on standards inherited from GODAE/Oceanpredict and MERSEA/MyOcean (which includes CLASS 1, 2 and 4 metrics).

Ocean reanalyses are becoming increasingly important to reconstruct and provide an overview of the ocean state from the past to the present-day. These products require advanced scientific methods and techniques to produce a more accurate ocean representation. In the scope of the Copernicus Marine Environment Monitoring Service (CMEMS), a new Black Sea (BS) reanalysis, BS-REA (BSE3R1 system), has been produced by using an advanced variational data assimilation method to combine the best available observations with a state-of-the-art ocean general circulation model. The hydrodynamical model is based on Nucleus for European Modeling of the Ocean (NEMO, v3.6), implemented for the BS domain with horizontal resolution of 1/27° x 1/36°, and 31 unevenly distributed vertical levels. NEMO is forced by atmospheric surface fluxes computed via bulk formulation and forced by ECMWF ERA5 atmospheric reanalysis product. At the surface, the model temperature is relaxed to daily objective analysis fields of sea surface temperature from CMEMS SST TAC. The exchange with Mediterranean Sea is simulated through relaxation of the temperature and salinity near Bosporus toward a monthly climatology computed from a high-resolution multi-year simulation, and the barotropic Bosporus Strait transport is corrected to balance the variations of the freshwater flux and the sea surface height measured by multi-satellite altimetry observations. A 3D-Var ocean data assimilation scheme (OceanVar) is used to assimilate sea level anomaly along-track observations from CMEMS SL TAC and available in situ vertical profiles of temperature and salinity from both SeaDataNet and CMEMS INS TAC products. Comparisons against the previous Black Sea reanalysis (BSE2R2 system) show important improvements for temperature and salinity, such that errors have significantly decreased (about 50%). Temperature fields present a continuous warming in the layer between 25-150 m, within which there is the presence of the Black Sea Cold Intermediate Layer (CIL). SST exhibits a positive bias and relatively higher root mean square error (RMSE) values are present in the summer season. Spatial maps of sea level anomaly reveal the largest RMSE close to the shelf areas, which are related to the mesoscale activity along the Rim current. The BS-REA catalogue includes daily and monthly means for 3D temperature, salinity, and currents and 2D sea surface height, bottom temperature, mixed layer fields, from Jan 1993 to Dec 2019. The BSE3R1 system has produced very accurate estimates which makes it very suitable for assessing more realistic climate trends and indicators for important ocean properties.

Measuring historical shoreline change and cliff retreat is an essential aspect of understanding the long-term geomorphic evolution of any coastal system. These data are also critical for predicting future changes and incorporating that data into proper coastal management. In recent years, Marine Spatial Planning (MSP) has increasingly been recognized as an important tool in the sustainable management of marine space across Europe and worldwide, including also issues of coastal zone management and land-sea interactions (Directive 2014/89/EU for MSP). This paper deals with investigation of the shoreline changes and cliff retreat along the 34 km section of the Black Sea coast at Shabla Municipality (Northeast Bulgaria). The study area has a low density of development and comprises large sand beaches, dunes and rapidly retreating loess cliffs. The present research was formulated under the European Union (EU) Directive for MSP in the Black Sea Basin (MARSPLAN-BS) Project. The goal was to provide reliable data and useful information in support of the development of a pilot marine spatial plan for Shabla Municipality. The study was focused on the analysis of shoreline movement (at sand beaches – erosion/accretion) and cliff retreat (at rocky coasts) utilizing a Geographic Information System (GIS). Primary data sources include topographic maps in scale 1:5,000, tachometry survey and modern Very High Resolution (VHR) orthophoto images. Change rates were determined using the Digital Shoreline Analysis System (DSAS), an ArcGIS extension for calculating shoreline change (Version 4.3), and rate-of-change statistical method. Management of coastal erosion, hard engineering methods and lack of required setbacks were discussed. Results for rates of shoreline change and cliff retreat were verified with preliminary field measurement studies and cross-checked with existing hydrodynamics, geology and geomorphology of the coast of Shabla Municipality.

MASRI – Infrastructure for Sustainable Development of Marine Research Including the Participation of Bulgaria in the European Infrastructure Euro-Argo is a project of the National roadmap for scientific Infrastructure (2017-2023) of Bulgaria. The mission of MASRI is to build and utilize a modern research infrastructure which will provide the basis for highly efficient marine and maritime research to expand our knowledge of the marine environment and to support blue growth and implementation of marine policy and maritime spatial planning in order to achieve UN Sustainable Development Goal 14: Conservation and sustainable use of oceans, seas and marine resources for sustainable development.MASRI activities include the modernization of existing unique resources and equipment and the establishment of new facilities. The research infrastructure consists of four main modules: Research fleet; National Operational Marine Observing System – NOMOS; Data and information center and Research laboratory complex, each representing a distinct on functional basis part of the scientific infrastructure, and consists of separate components distributed physically in different scientific organizations, in the city of Varna. Thus, MASRI is intended to be a large-scale, interdisciplinary multifunctional (physics, chemistry, biology, geology, aquacultures, medicine, energy, underwater, and offshore technologies) marine research infrastructure of scientific significance and will provide unique facilities (including databases and computer network) which will be widely accessible on national, regional and international level for multidisciplinary researches.Research vessels are intended to provide access to the investigated medium – the sea and they are providing a working platform for conducting research. NOMOS is a system of systems to measure in situ parameters of the marine environment and the surrounding atmosphere. It is designed to provide information on the state of the marine environment for scientific research, forecasting and marine industry. Data and information center provide a computing environment, communication environment and environment for quality control and reliable storage of data and information within the scientific infrastructure. Research laboratory Complex represents a system of research laboratories for chemical, biological and geological analyzes and for relevant research on marine medicine as well as of laboratories for marine resources and technologies research.As an important module of MASRI, NOMOS includes several components: BulArgo – a system of profiling floats to measure the profiles of the characteristics of the marine environment in the depth up to 2000m; waves and currents monitoring system; national sea level observing system; moorings network; coast research bases and metrological control laboratory.MASRI is also intended to support the participation of Bulgaria in European research infrastructure consortia Euro-Argo ERIC. Al least three floats are provided and launched in the Black sea every year in the frame of the BulArgo project. Thus, BulArgo gives an important contribution to the Argo program in particular in the Black sea, providing a significant volume of very important in-situ data both for climatic research, for assimilation into the models and verification of the forecasts.

The ultimate goal of modern operational oceanography are end user oriented products with high scientific quality. Beneficiaries are the governmental services, coast and offshore based enterprises and research institutions that make use of the products generated by operational oceanography. Direct users are coastal managers, shipping, search and rescue, oil spill combat, offshore industry, ports, fishing, tourism, and recreation industry. Indirect beneficiaries, through climate forecasting based on ocean observations, are food, energy, water and medical suppliers. Availability of updated information on the actual state as well as forecast of marine environment is essential for the success and safety of maritime operations in the offshore industry. Various systems for the collection and presentation of marine data for the needs of different users have been developed and put in operation in the Black Sea. The systems are located both along the coast and in the open sea and the information they provide is used by both the maritime industry and the widest range of users. The Black Sea Monitoring and Forecasting Center in the frame of the Copernicus Marine Service is providing regular and systematic information about the physical state of the ocean, marine ecosystem and wave conditions in the Black Sea area, assimilating observations, keeping efficient operations, advanced technology and high quality modeling products. Combining and optimizing in situ, remote sensing, modeling and forecasting into a Black Sea observing system is a task that has to be solved, and that will allow to get a more complete and comprehensive picture of the state of the marine environment as well as to forecast future changes of physical and biogeochemical state of the Black Sea and the Black Sea ecosystem.

The geographical status of coastal zone as a boundary between sea and land makes it especially attractive, both for settlement and the implementation of various human activities. This in turn leads to accelerated urbanization of coastal areas. Generally, coastal areas cover only a fraction of the surface of the Earth, but they concentrate around 60 % of the world’s population. Bulgarian Black Sea coast is also no exception to this trend. According to data from the last national census, Black Sea municipalities comprise 5.21 % of the country’s territory and 8.85 % of the population. At the same time, as a consequence of global climate change and the trend towards increasing sea levels, natural risks of extreme weather events are becoming more important and more dangerous. This study assesses the risks of rising sea level for the population defined in different scenarios of increase in the level. It uses official data on the population in coastal towns and topographic maps to determine low-laying coastal areas. Analysis was conducted using a GIS ArcInfo 9.1. The most vulnerable coastal areas are identified and assessed on preliminary given scenarios of sea level rise from 1 to 5 meters. As a case study, the risk for the coastal population of the biggest Bulgarian Black Sea resort Sunny Beach is analysed in detail. Applying 3D GIS modeling, visualisation and accurate assessment of flooded coastal areas are performed, and numbers of population potentially endangered in case of different scenarios of sea level rise are estimated. The obtained results have significant practical importance and could serve as a primary base in coastal decision-making, for the needs of civil protection authorities and other prevention/mitigation measures in hazard prone coastal zones.