Water Masses in the Black Sea as Seen by Profiling Floats. A revisit of the role of winter, slope and geothermal convectio.

More than 6000 profiles from profiling floats in the Black Sea over the 2005-2020 period were used to study the ventilation of this basin from the top to the very bottom. In the upper layers and in the main pycnocline, water masses show a strong interannual variability following intermittent events of cold water formation. The density ratio decreased three times during the last 15 years, revealing the decreasing role of temperature in the vertical layering of the Black Sea halocline. The deep transition layer (DTL) between 700 and 1700 m acts as an interface between the baroclinic layer and the largest bottom convective layer (BCL) of the world oceans. On top of DTL are the warm intermediate layer (WIL) and deep cold intermediate layer (DCIL). They both showed strong trends in the last fifteen years due to warmer climate and intensification of warmer intrusions from Bosporus. A “salinity wave” was detected in 2005-2009 below ~1700 m, which evidenced for the first time the penetration of gravity flow from Bosporus down to the bottom. The layering of water masses was explained as resulting from the different distribution of sources of heat and salt, double duffusion and balances between the geothermal and salinity flows in the BCL.