Dynamics of diurnal thermocline formation in the oceanic mixed layer

Diurnal thermocline formation in the oceanic mixed layer under a stabilizing buoyancy flux is studied by numerical simulation of a turbulence model in which the interaction between turbulence structure and density stratification is taken into consideration, and the mechanism for its formation is clarified based on the results. From the simulations, the flux of turbulent kinetic energy is a dominant source of turbulence in the upper mixed layer and plays an indispensable role for the formation of a diurnal thermocline; below the diurnal thermocline, turbulence is maintained by shear production, which causes the growth of diurnal thermocline thickness. The flux Richardson number at the diurnal thermocline maintains a constant value (about unity), regardless of the shear stress and buoyancy flux at the sea surface, and the diurnal thermocline depth grows more slowly than predicted by the Monin-Obukhov length scale. The model results are compared with the observational data, and the assumptions introduced in various mixed layer models are reexamined in view of these results.