Monitoring the Black Sea climate: recent advancements for building ocean indicators

Ocean reanalyses reconstruct the ocean state with a long integration of an ocean model constrained by atmospheric surface forcing and observations via data assimilation. Since their results are more accurate in comparison to those derived from a model alone, they are a powerful tool to provide ocean monitoring indicators (OMIs) as well as to better understand the physical properties and dynamics of the Black Sea. In the scope of the Copernicus Marine Service (CMEMS), the Black Sea physical reanalysis (BS-REA) system has been used to support the implementation of new indicators in the Black Sea. The system is built upon the hydrodynamic model NEMO v3.6 at 1/27° x 1/36°, and 31 unevenly distributed vertical levels, coupled to OceanVar for the assimilation of the best available observations (both in situ and satellite ones). In this contribution, we present the current operational indicators for the monitoring of temperature and salinity anomalies. BS-REA system provides temperature trends that indicate a warming of the basin: 0.0829±0.01069 ^oC year^-1, 0.0380±0.0005 ^oC year^-1, 0.0041±0.0001 ^oC year^-1, respectively, in 0-25 m, 25-150 m and 150-300 m. Since 2007, the warming signal has been very clear in such a way that the Black Sea cold intermediate layer (CIL) almost disappeared in recent years. However, this continuous warming is interrupted in 2012 and less explicitly in 2017, years in which a replenishment of the CIL is verified. Similar analyses for salinity reveal that salinity trends reduce in depth and are larger from 2005, especially in surface layers. The system is very suitable for understanding the physical state of the Black Sea in recent years and allows to obtain more accurate OMIs for the sea, which are important to understand its response to climate change. 

Following recent CMEMS Ocean State Report contributions, new OMIs based on the ocean heat content anomaly and freshwater content anomaly will be produced this year, whereas indexes on the basis of the interannual variations of the Black Sea Rim Current intensity, the Black Sea overturning circulation and the coastal upwelling along the Turkish coast will be provided as OMIs in the near future. BS-REA is under continuous update in order that new versions will bring improvements in both the numerical model and data assimilation components.