A New High-Resolution Black Sea Physics Reanalysis

Ocean reanalyses utilize state-of-the-art models that are constrained by atmospheric forcing and incorporate the best available observations through data assimilation techniques to reconstruct historical conditions. The Black Sea Physics Reanalysis (BLK-REA) product delivered within the Copernicus Marine Service provides a comprehensive dataset of oceanographic fields for the Black Sea basin, starting from January 1993. This high-resolution reanalysis is built using the NEMOv4.0 general circulation ocean model, implemented at a horizontal resolution of 1/40° and 121 vertical levels, delivering a detailed and accurate representation of ocean dynamics in the region. The BLK-REA is driven by atmospheric fluxes derived from ECMWF ERA5 fields with spatial and temporal resolutions of 1/4° and 1 hour, respectively. Sea surface temperature (SST) relaxation, based on the ESA-CCI SST-L4 product, is applied for heat flux corrections. A key advancement in this version is the incorporation of lateral open boundary conditions (LOBCs), enabling more accurate inflow and outflow dynamics at the Bosphorus Strait. The data assimilation system, OceanVar, utilizes a three-dimensional variational (3D-Var) assimilation algorithm. It integrates model outputs with along-track sea level anomaly (SLA) observations from Copernicus Marine, as well as in-situ temperature and salinity profiles sourced from SeaDataNet and Copernicus Marine datasets. Enhancements in the data assimilation include the adoption of an improved background error covariance matrix and an observation-based mean dynamic topography for SLA assimilation. The results of the reanalysis demonstrate a significant improvement in accuracy compared to previous versions, with better alignment to observed data. BLK-REA has proven to be an invaluable tool for generating Ocean Monitoring Indicators, essential for assessing climate change impacts in the Black Sea. For example, the analysis reveals ongoing warming within the 25–150 m depth range, corresponding to the Cold Intermediate Layer.

Looking ahead, future iterations of BLK-REA aim to expand the domain to include the Azov Sea and introduce enhanced Bosphorus LOBCs. Planned upgrades to the data assimilation system include the integration of a barotropic model for SLA assimilation and the first-guess-at-the-appropriate-time approach. Starting from 1980, the next reanalysis will provide a more comprehensive temporal scope, further enhancing the monitoring and assessment of climate-related changes in the Black Sea region.