Researchers from Marine Hydrophysical Institute of RAS and Shirshov Institute of Oceanology have uncovered the mechanisms behind the significant increase of the Black Sea salinity in recent years. The results of the study were published in the journal Deep Sea Research Part I: Oceanographic Research Papers.
The Black Sea is known for its unique structure: due to the large influx of fresh water from rivers and limited exchange with the Mediterranean Sea, it has a distinct layer of fresh water at the surface and very saline water at depth. This structure, called the halocline, has remained stable for a long time, but now is undergoing significant changes.
"In recent years, the Black Sea has seen a sharp increase in salinity, especially at depths of 50-150 m. Such changes significantly affect the sea ecosystem and its hydrological regime. Increased salinity leads to a decrease in the density stratification of water, as a result of which water masses are more easily mixed under effect of storms or winter cooling. More nutrients enter the surface layers from the depths of the sea, which contribute to the phytoplankton bloom. Our work was purposed at studying the causes of increased salinity and determine their relationship with various hydrodynamic factors," explains the first author of the article, Deputy Director for Research of FSBSI FRC MHI, Doctor of Physical and Mathematical Sciences Arseniy Kubryakov.
The scientists analyzed data from 40 years of observations carried out at Marine Hydrophysical Institute, as well as information obtained using Argo floats. In addition, an analysis of data obtained via the probing hydrological lift Aqualog was performed. "Our study has shown that an important cause for the salinity and temperature rise in the deep layers is the increase in storm activity and velocity of currents in the Black Sea. Strong winds lead to penetration of warm surface waters into the deep layers and entrainment of salt waters from the depths into the upper layers. Thus, the winds lead to the penetration of the global warming signal into the ocean depths and equalization of the ocean properties by depth. Previously, this effect was compensated in winter, when cold dense waters arose from above; they carried fresh waters to the depths. Due to the long-term Black Sea warming, the freshening process of the deep layers is now observed only in rare cold years. At the same time, the increase in wind speed, vorticity, and geostrophic currents contribute to the halocline erosion and an increase in salinity of the upper Black Sea layers," says Arseniy Kubryakov.
The researchers note that a significant increase in salinity began in the upper part of the Black Sea halocline (50-100 m depths) after 2007 and by 2020 was already traced down to 250 m depth. This indicates a rapid change occurring in the sea.
"Restructuring of the basin thermohaline structure affects many processes — gas exchange, functioning of marine ecosystems, and much more," emphasizes Kubryakov. "Understanding these processes is critically important for forecasting the future state of the Black Sea." The scientists subsequently plan to study in detail the physical mechanisms of mixing in areas of intense currents and strong storms, as well as their impact on the mixing of surface and deep waters of the Black Sea. This will help to create more accurate models for forecasting the marine ecosystem changes.
The study was carried out within the framework of state assignment on theme FNNN-2024-0012 “Analysis, diagnosis, and operational forecast of state of hydrophysical and hydrochemical fields of marine waters based on mathematical modeling using data from remote and contact measurement methods” and the RSF project 23-17-00056 using equipment purchased with subsidies from the Ministry of Science and Higher Education of the Russian Federation.