Vertical temperature profiles are analysed for the case of cooling of brackish sea water around the temperature of maximum density. Field measurements in the Bay of Bothnia cover a period of 52 days, during which the surface water was cooled from 6°C to 1.5°C. A mathematical model, which is based on the conservation equations for momentum, heat and salt in their one-dimensional forms, is presented. The equation of state is linear with respect to salinity and quadratic with respect to temperature. Turbulent exchange coefficients are calculated with a two-equation model of turbulence.

The measured temperature profiles clearly demonstrate the importance of both temperature and salinity gradients in the mixed-layer dynamics. The mathematical model describes these and the general development of the temperature profile in a most satisfactory way.