DYNAMICS OF DIURNAL THERMOCLINE FORMATION IN THE OCEANIC MIXED-LAYER

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