The Black Sea Mixed Layer Depth Variability and Its Relation to the Basin Dynamics and Atmospheric Forcing

Purpose. Spatio-temporal variability of the mixed layer depth (MLD) in different areas of the Black Sea in 1985-2017, its relationship with basin dynamics and atmospheric forcing are studied. Methods and Results. The study is based on the hydrological data archive for 1985-2017 including the measurements of the ship expeditions, the Argo buoys and the moored buoy Aqalog. Seasonal and interannual variability of the mixed layer depth was determined using the density criterion (dr = = 0.07kg/m(3)) between the surface layers and the base of the upper mixed layer. Conclusions. In January - March, the large-scale and mesoscale dynamics significantly affects the mixed layer depth variability Minimum monthly average values of the mixed layer depth in winter are observed in the mesoscale cyclonic eddies and in the center of the sea (20-30 m), the moderate values - on the periphery of the basin (40-45 m) and the maximum ones - in the mesoscale anticyclones (60-70 m). Several times the mixed layer depth values exceeding 150 m were detected in the downwelling areas of the basin. Analysis of the whole period (1985-2017) shows that the mixed layer density was never more than 1015 kg/m(3). This isopycnal limits the maximum possible depth of the upper mixed layer. The impact of wind velocity on the spatial and temporal variability of the mixed layer thickness is the largest in spring and autumn when the seasonal thermocline is weak. It is less important in summer when solar heating stabilizes the upper layer, and in winter when the mixed layer depth is large. Rise of the mixed layer depth in summer is observed in recent years that is associated with rise of the wind speed in a warm period of a year.