ONSET OF STRATIFICATION IN A MIXED-LAYER SUBJECTED TO A STABILIZING BUOYANCY FLUX

The formation of a thermocline in a water column, in which shear-free turbulence is generated both at the surface and bottom, and a stabiliz ing buoyancy flux is imposed at the surface, is studied using a labora tory experiment and a numerical model with the aim of understanding th e formation of a tidal front in coastal seas. The results show that th e formation of a thermocline, which always occurs in the absence of bo ttom mixing, is inhibited and the water column maintains a vertically mixed state, when bottom mixing becomes sufficiently strong. It is fou nd from both experimental and numerical results that the criterion for the formation of a thermocline is determined by the balance between t he rate of work that is necessary to maintain a mixed state against th e formation of stratification by the buoyancy flux and the turbulent k inetic energy flux from the bottom supplied to the depth of thermoclin e formation. The depth of the thermocline, when it is formed, is found to decrease with bottom mixing.