Is the exchange between the Black Sea and the Mediterranean changing over time?

Abstract The Black Sea Waters (BSW) that are exported into the Aegean Sea largely control its overturning circulation, as well as its regional biogeochemical characteristics. Numerical modelling studies demonstrate the importance of the BSW outflow and its potential impact if it were to be disrupted, albeit only in a theoretical sense. Recent observational studies suggest a reduction in the exchange of water between the Black Sea and the Aegean Sea. However, all of these studies focus primarily on the impact on the Aegean Sea and do not examine the potential causes of the reduction in Black Sea water outflow. Using datasets spanning four decades, this study investigates the Black Sea’s freshwater surplus decline and reveal a persistent decline that is driven by increased evaporation and reduced riverine inflows. The result is a gradual salinification of the Black Sea, weakening of the surface outflow towards the Mediterranean, and the respective diminishing of the and the Northern Aegean Sea’s characteristic low-salinity layer due to the reduced inflow. The Northern Aegean’s higher salinity and weaker buoyant surface layer reflect these changes, impacting regional circulation and productivity. While sea level rises in both Black Sea and Northern Aegean due to thermal expansion, the increase is stronger in the Northern Aegean. This has led to a diminished sea level difference between the Bosporus and the Dardanelles, confirming the reduced sea level gradient driving the export to the Aegean. The decline in freshwater supply and reduced pressure difference indicate a long-term reduction in export from the Black Sea to the Northern Aegean. This suggests a long-term transition in the Black Sea and Northern Aegean systems, impacting stratification, dense-water formation, and ecosystem functioning in the Eastern Mediterranean.