The Black Sea (BS) is the largest deep estuarine basin in the world. Its meridional overturning circulation (BS-MOC) is here described for the first time: in particular, the BS-MOC interannual variability in both depth and density space has been computed using a state-of-the-art ocean reanalysis provided in the framework of the Copernicus Marine Service for the Black Sea of 28 year-period (1993–2020) for the definition of a new climate index.
The mean meridional stream function defined for the
upper 150 m in the depth space reveals an anticlock-wise cell in the southern part of the Black Sea coming
from the injection of the Mediterranean waters
through the Bosphorus Strait into the Black Sea and
a clockwise cell in the northern part of the Black Sea
as a manifestation of the Cold Intermediate Layer
(CIL). The new index is then defined as maximum
overturning circulation in density space between
22.45 and 23.85 kg m−3 . For the overall period, the
BS-MOC index weakened between 1993 and 2010
from 0.13 Sv down to 0.07 Sv, then it recovered and
has equilibrated around 0.1 Sv. Additionally, it shows
a very strong correlation with the CIL from observational data, supporting the concept of using such an indicator to understand and monitor the water mass formation in the Black Sea basin.