**5. Conclusions**

Within the framework of the CMEMS and the BS-MFC, the Black Sea physical analysis and forecasting system provides essential variables for understanding the physical processes and dynamics of the Black Sea basin. The BSFS has been operational since the end of 2016 and has been developed and maintained at CMCC in collaboration with the USOF (University of Sofia, Bulgaria, scientific partner in the BS-MFC consortium).

We have presented the BSFS ocean model based on NEMO v3.4 able to assimilate near real time, in-situ and satellite observational products using the OceanVar scheme. The Black Sea hydrodynamic model has about 3 km horizontal resolution and uses 31 levels with partial steps. It implements a closed boundary condition at the Bosporus Strait and is forced by ECMWF analysis and forecasting atmospheric fields.

The BSFS implements two production cycles, one daily (which includes 3 days analysis) and one weekly (based on 14 days analysis), the latter to assimilate a higher number of collected observations to provide the best quality initial conditions for the forecasting cycle: the processing system is completed every day by 1-day simulation and 10-day forecasts.

We have also described an operational dashboard for product quality monitoring which assesses the skill scores of the analysis products for 2018–2019 and gives an accuracy of around 1 ◦C in the sea surface temperature. Errors increase at the sub-surface (10–30 m layer): in particular, the thermocline experiences a maximum error of 2 ◦C during the summer period, while salinity reaches an error of about 0.5 PSU. Considering the sea level, the assimilation of the along-track satellite SLA guarantees an average error of about 2.3 cm. Such skills put the BSFS on the same track of quality and robustness as state-of-the-art regional configurations in the CMEMS framework. A regional website is operationally maintained at CMCC to provide daily bulletins and metrics for monitoring the lifecycle and performances of the system.

The future forecasting system for the Black Sea will include at least four new main components which will significantly improve the quality of the BSFS analysis fields and forecasting skill scores by means of improved hydrodynamical core model: (1) increased vertical resolution for a better representation of open ocean dynamics and mixing processes, combined with a revised bathymetry and coastline and data assimilation upgrades; (2) the Bosporus Strait will work as an open boundary, in order to improve the connection with the Mediterranean Sea. This will be achieved thanks to a novel implementation of the Marmara Sea model, based on an unstructured grid method that provides open boundary conditions to the Black Sea through the Bosporus and the Mediterranean Sea through the Dardanelles; (3) improved representation of the land forcing: in particular, the representation of the Danube River using historical discharge datasets provided by the NIHWM (National

Institute of Hydrology and Water Management, Romania—scientific partner in the BS-MFC consortium); (4) an online wave-current model to improve the small scales dynamics at the surface.

**Author Contributions:** S.A.C. coordinated the work in collaboration with G.C. and N.P. The ocean circulation model has been designed and improved by S.A.C., E.P., N.P., G.C., M.I.; the data assimilation scheme has been designed by E.J. with the collaboration of L.L.; R.L. contributed on upstream data access; Funding acquisition, A.P.; S.C. (Sergio Creti') and L.S. contributed on operational chain design and operational maintenance of the system. Validation has been developed in collaboration with E.J., D.A. and S.C. (Salvatore Causio). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Copernicus Marine Environment Monitoring Service for the Black Sea Monitoring and Forecasting Center (Contract No. 72-CMEMS-MFS-BS).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This work has been used CMEMS data from the BS-MFC, in particular Black Sea physical analysis and forecast (https://marine.copernicus.eu/ (accessed on 1 April 2021).

**Conflicts of Interest:** The authors declare no conflict of interest.
