Advances in monitoring the Black Sea: a new regional multidecadal ocean reanalysis at 1∕40° resolution

Abstract. The Black Sea regional reanalysis serves as an essential tool for understanding the Black Sea's response to climate variability and advancing regional ocean monitoring efforts. In particular, the Black Sea reanalysis (BLK-REA) is built with high spatial resolution, 1/40° horizontal grid and incorporating 121 vertical levels. The model implementation includes lateral open boundary conditions (LOBC) at the Marmara Sea, allowing more accurate inflow/outflow dynamics through the Bosphorus Strait. BLK-REA assimilates sea level anomaly (SLA) and in-situ observations and applies a heat flux correction via sea surface temperature relaxation. The data assimilation system uses a background error covariance matrix evaluated through monthly EOFs over decadal periods to capture seasonal and decadal variability, and an observation-based mean dynamic topography is used for SLA assimilation. When compared to available observations, the numerical results show high accuracy, with the largest temperature errors observed in the upper layers, primarily linked to the formation of the seasonal thermocline during the summer months. The SLA anomaly error is consistently around 0.02 m from the year 2000 onwards, and regions with elevated SLA errors are closely associated with the Rim Current and its mesoscale variability. These results highlight the added value of a dedicated high-resolution regional reanalysis, as BLK-REA shows superior skill compared to a state-of-the-art global reanalysis in representing sea level, temperature, and salinity, particularly in the upper and intermediate layers of the Black Sea. Furthermore, BLK-REA plays a crucial role in generating Ocean Monitoring Indicators, which are essential for tracking and assessing long-term physical changes in the Black Sea. For example, temperature data indicate ongoing warming in the 25 to 150 m layer, where the Cold Intermediate Layer is located. The system is regularly updated, with the next version expected to improve both the model and DA components. For a future perspective, the next BLK-REA will expand the domain to include the Azov Sea and will feature an enhanced Bosphorus LOBC.