Study of the Process of the Black Sea Upper Layer Mixing in the Zone of the Rim Current Activity in Winter Based on the Drifters Data
Purpose. The process of the upper layer mixing in the western Black Sea during the heightened heat transfer from its surface in the winter season is studied. The mechanism forming average temperature both in the upper mixed layer and in the layer below the lower boundary in the seasonal thermocline is analyzed. Methods and Results. Measurements of the thermistor chain drifters as well as the MERRA reanalysis data and the simulation results of the regional model RegCM4 which included the ERA-Interim reanalysis (2012-2014) data as the input information, were involved in the calculations. It is revealed that growth of the wind speed and the total heat transfer from the sea surface was accompanied by increase of the current velocity that resulted in intensification of turbulent mixing in the upper layer (40-50 m depth) and lowering of its average temperature. It is also found out that increase of the current velocity was followed by deepening of the seasonal thermocline and cold intermediate layer; whereas its decrease contributed to elevation of cold waters from the lower boundary of the upper mixed layer to the smaller depths. At the same time, diminution of the cold intermediate layer depth was observed. At lowering of the current velocity, this resulted in decrease of the upper mixed layer average temperature, and growth of the temperature in the layer below the seasonal thermocline. Conclusions. The current velocity intensification induces deepening of the seasonal thermocline and the cold intermediate layer; whereas its decrease promotes elevation of these layers to smaller depths. This process results in lowering of the upper mixed layer average temperature.