**1. Introduction**

Understanding, predicting, and reconstructing the ocean state is globally one of the most challenging objectives for operational oceanography, especially to support user needs as well as down-stream services and applications for what is termed the Blue Growth and the Blue Economy for a Blue Society. In the European panorama, the Black Sea, together with the Azov Sea, the Marmara Sea, and the Mediterranean Sea, is part of a peculiar complex system of marginal seas that is functional to ocean monitoring and climate. Due to particular physical and biogeochemical processes, the Black Sea deserves a more dedicated focus so as to be able to set up a reliable modelling system and observing network for the next generation of operational ocean forecasting systems.

This baseline is one of the main priorities pursued within the Copernicus Marine Environment and Monitoring Service (CMEMS, [**?** ]). The Black Sea Monitoring and Forecasting Center (BS-MFC) is the CMEMS operational service dedicated to the regional needs of the Black Sea, which has been operational since the end of 2016 and serves about 60 "active users" (i.e., users that regularly download CMEMS data) from academia (~30%), public sectors and organizations (~20%), and the business and private sectors (~50%). The main objective of the BS-MFC is to serve the specific needs of the Black Sea countries in decision-making by developing new operational services that are able to protect the marine ecosystem, contrasting pollution and environmental emergencies, supporting maritime safety and routing, and promoting a science-driven cooperation for user-driven applications from the regional to the coastal scale. Access to high-quality operational data related to forecasting and reconstruction of past ocean states in the Black Sea is a pre-requisite for achieving these targets. The BS-MFC is a joint venture between the Institute of Oceanology-Bulgarian Academy of Sciences (IO-BAS, Bulgaria) in the role of coordinator, and the following institutes and universities: the Euro-Mediterranean Center on Climate Change Foundation (CMCC, Italy), the University of Liege (ULiege, Belgium), the Helmholtz-Zentrum Hereon (HEREON, Germany), the Sofia University "St. Kliment Ohridski" (USOF, Bulgaria), and the National Institute of Hydrology and Water Management (NIHWM, Romania).

BS-MFC delivers products for analysis, simulation, forecast, and reanalysis through the CMEMS and maintains a level of efficiency and robustness in operations, ensuring new advances in science for the development of specific products. The BS-MFC catalogue offers products for the Blue Ocean (physics and waves) and the Green Ocean (biogeochemistry) at the regional scale. These include: (a) near-real-time (NRT) datasets—analysis and forecast fields at different frequencies for supporting daily services and down-stream applications; and (b) multi-year (MY) datasets—reanalysis and long-term simulations—at different frequencies to provide an estimate of climate variability and trends.

The present paper provides a complete overview of the current BS modelling systems together with a description of the main operational products delivered through CMEMS interfaces. A detailed description of the BS-MFC components (PHY, BIO, and WAV) is also provided, together with a description of the models and data assimilation capabilities used to improve the accuracy of multi-year and forecasting products (Section **??**). Product quality and monitoring capacity are described in Section **??** by showing the operational dashboard used to evaluate the accuracy of the BS-MFC products. The last part of this paper summarizes the main scientific challenges and ongoing developments for the next generation of BS-MFC operational systems (Section **??**) together with the conclusions (Section **??**).

#### **2. The Black Sea MFC High-Level Architecture and Components**

The BS-MFC systems provide NRT and MY products over the Black Sea domain (Figure **??**), excluding the Azov Sea and the Bosporus Strait, at a resolution of 1/27◦ in the zonal direction and 1/36◦ in the meridional direction.

**Figure 1.** Black Sea spatial domain and bathymetry (in meters).

The BS-MFC uses an operational framework based on a high-level architecture, represented in Figure **??**. Three production units (PU) are responsible for the system and operational services, including evolutions of the physics (BS-PHY PU), biogeochemistry (BS-BIO PU), and wave (BS-WAV PU) components (run by CMCC, ULiege, and HEREON, respectively). Each PU is connected to its own archiving unit (AU) for the long-term storage of the BS-MFC products, and to a backup unit (BU) in the case of nominal operational failures and recoveries in order to guarantee the continuity of operational services. Additionally, each PU implements dedicated interfaces: (a) with the BS-MFC technical group, for the technical implementation of the product catalogue (BS TEC), (b) with the dissemination unit (DU) through the delivery buffer zone, for the operational delivery of the products (CMEMS CIS), and (c) with the BS-MFC service desk (BS LSD), for user support through the CMEMS service desk. Service evolution activities, aimed at R&D activities, are implemented at the PU level, with the collaboration of USOF and NIHWM (BS-EVO). Observations used by BS-MFC systems for assimilation and validation purposes are summarized in Table **??**.

**Table 1.** Upstream data dependency for the BS-MFC: for each type of observation, provider and product ID are given as well as the BS component—PHY, BIO, WAV—and product category—NRT and MY—which up-take it for assimilation (A) and/or validation (V).


**Figure 2.** BS-MFC high-level architecture operationally implemented within the CMEMS framework since April 2018.
