Formation and Evolution Patterns of the Upper Mixed Layer in the Black Sea during the Warm Period at the Gelendzhik Research Site

The article studies patterns and dynamics of the upper mixed layer (UML), its temperature and thickness during the warm period (April–November). The initial CTD data were collected during regular measurements in the active layer (AL) at the Gelendzhik research site, in the shelf–slope area of the northeastern Black Sea, in 2010–2023. The heat, buoyancy, and turbulent energy fluxes were calculated for the same period using the NCEP CFSv2 reanalysis dataset. All data were averaged on a ten-day basis, where ten-day grouping was performed for all studied years simultaneously, with only day and month being taken into account, to remove synoptic variability and concentrate on general seasonal dynamics. The UML in April, when surface heating has just started, is often absent, and only in May does it become a regular part of the AL. From May to August inclusive, the wind-wave turbulent energy coming from the surface is partially spent against the buoyancy force, which in turn is caused by heat transfer into the UML through air–water boundary. The thickness of the UML in this period is proportional to the modified Monin–Obukhov scale (1954), calculated using the positive buoyancy that comes into the sea through air–water boundary, minus the buoyancy flux that passes through the UML into seasonal thermocline (ST), while taking into account vertical velocity (seasonal upwelling). During the second half of the warm period (September–November), when the buoyancy flux becomes negative, the thickness of the UML increases due to convection and turbulent entrainment of water from the ST, the thickness of which decreases with time.