Deep Penetrating Cooling in the Black Sea As a Response to Cold Air Intrusions in Winter

The response of the Black Sea upper layer and, in particular, of the cold intermediate layer (CIL) to intense wind forcing during cold air intrusions (CAIs) in winter is considered. Using data of the ERA5 atmospheric reanalysis and Copernicus marine reanalysis, joint distributions of surface wind speed and water temperature differences at different depths for the period of 2000–2020 have been obtained. It is shown that the time scale of the sea response to such extreme phenomena is about two days and that the CAI influence extends to rather great depths, down to 60–70 m. Using the coupled mesoscale sea–atmosphere model, the cooling mechanisms in the sea upper layer during the CAI case in January 23–25, 2010, are investigated. Two numerical experiments with suppressed air–sea interaction have been performed. In the first experiment, sensible and latent heat fluxes from the sea surface into the atmosphere were turned off; in the second experiment, wind shear stress at the sea surface was turned off. It is shown that the main cause of the temperature decrease in the upper mixed layer is sea surface cooling due to sensible and latent heat fluxes. And the mechanism of deep cooling that penetrates into the pycnocline is vertical turbulent mixing caused by breaking of wind waves and shear instability. In the first experiment, the temperature decrease was insignificant; it was caused mainly by the entrainment of colder water from the CIL through the lower boundary of the mixed layer. In the second experiment, the temperature decrease was almost the same as in the control run. It is shown that turning off the wind shear stress changes the character of turbulent mixing in the upper quasihomogeneous layer: in order to compensate the considerable decrease in the intensity of turbulent eddies and provide the same vertical heat flux as in the main calculation, the vertical scale of the turbulent eddies increases.