Purpose. The work is aimed at studying the changes in the thermal structure of the Black Sea upper layer during seasonal winter cooling in 2009−2010.
Methods and Results. The NEMO-OASIS-WRF (NOW) coupled sea-atmosphere mesoscale model with the 2 km horizontal resolution is used. The changes in the sea upper layer during the 01.12.2009--28.02.2010 period are reproduced, and the temporal variability of water temperature at different depths is considered. For a characteristic point in the deep-sea part, it has been shown that the upper mixed layer thickness increased with time, whereas the cold intermediate layer upper boundary lowered as a result of the entrainment of colder water from below to the warmer upper mixed layer. It is also indicated that lowering of the cold intermediate layer upper boundary is accompanied by an increase of its temperature. In order to describe the cold intermediate layer evolution during winter cooling, two criteria are proposed: minimum water temperature in the 0−120 m layer, and difference between this value and the sea surface temperature. Vertical temperature profiles at different stages of winter cooling are obtained, and the main changes in thermal structure of the sea upper layer are considered. It is particularly shown that in course of winter cooling, the cold but less salty water at the northwestern shelf does not mix with the open sea waters due to a large horizontal density gradient.
Conclusions. When describing the seasonal winter changes in the upper mixed layer, it is necessary to take into account not only heat transfer to the atmosphere through its upper boundary, but also the vertical turbulent exchange through its lower boundary. Heat accumulated over summer in the upper mixed layer is transferred not only to the atmosphere; its small part also goes to the lower levels, which leads to an increase of the cold intermediate layer temperature. The influence of boundary conditions, namely the inflow of waters with different features from the Marmara Sea, can lead to the formation of areas where the cold intermediate layer, though formally absent as a layer between two 8 °С isotherms, exists as an intermediate layer of colder (by 3−4 °С) water as compared to the upper mixed layer. During the 2009−2010 winter, vertical mixing including the transfer of warmer and less salty waters from the upper mixed layer to the lower ones was most intensive in the western part of the sea. This fact is assumed to be a result of the inhomogeneous sea cooling: heat flux directed from the sea surface to the atmosphere decreases from 200 W/m2 in the northwestern part of the sea up to 50 W/m2 in its southeastern part.