STRESS-DRIVEN MIXED-LAYER IN A STABLY STRATIFIED FLUID

We investigate numerically a two-dimensional flow where a shear layer is forced at the top of a linearly stratified fluid. As a consequence of the mechanical forcing, a statistically steady stress is exerted on the underlying fluid. The downwards transfer of momentum and buoyancy , characterized by the deepening of a mixed layer, is studied for thre e different values of the initial Brunt-Vaisala frequency N. We show that the flow reaches an asymptotic stage where the temporal evolution of the mixed layer becomes statistically self-similar. The spatial sc aling factor is the mixed layer depth il(t) whose evolution is proport ional to u/N*root tN*, where u* is a velocity associated to the stres s tau = rho(0)u(2). These results are compared to previous theoretica l and empirical models which have been proposed to describe the deepen ing of the oceanic mixed layer under the action of a wind stress. Astr ophysical applications are also mentioned.