A Numerical Study of Turbulent Processes in the Marine Upper Layers

Abstract

A second-order turbulence closure model, similar to Mellor and Yamada's (1974) level-3 model, is implemented. This is used to investigate the role of the different turbulent processes and the resulting dynamical and thermal structures, for oceanic upper layers subjected to a variety of unsteady atmospheric forcings. From the distributions obtained for current and temperature, turbulence levels, the production, transfer and dissipation terms in the turbulent kinetic energy and temperature variance budgets, the physics of the oceanic mixed layer and thermocline can be discussed and comparisons made with the models of Mellor and Durbin (1975), Warn-Varnas and Piacsek (1979) and Kundu (1980) and some experimental data. Most often, shear production and viscous dissipation of turbulent kinetic energy dominate within the mixed layer. Gravitational production or destruction and turbulent diffusion, however, play a crucial role in thermocline evolution. Under strong winds, current shear and turbulence diffusion at the base of the mixed zone control the erosion mechanism, while the effect of surface wave breaking seems rather limited. After a significant wind-speed reduction, turbulence regresses upward, leaving behind quasi-free inertial current oscillations. Surface cooling enhanced turbulence and mixing, whereas solar heating results in a diminished turbulent zone and a diurnal secondary thermocline. The time variability of atmospheric forcings appears as a determinant factor for the oceanic mixed layer.