OCEANIC VERTICAL MIXING - A REVIEW AND A MODEL WITH A NONLOCAL BOUNDARY-LAYER PARAMETERIZATION

If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of t ime and space scales of importance to climate, they must be strongly p hysically based. Observations, theories, and models of oceanic vertica l mixing are surveyed, Two distinct regimes are identified: ocean mixi ng in the boundary layer near the surface under a variety of surface f orcing conditions (stabilizing, destabilizing, and wind driven), and m ixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion r ates of heat and salt). Mixing schemes commonly applied to the upper o cean are shown not to contain some potentially important boundary laye r physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulen t contribution to the vertical shear of a bulk Richardson number is pa rameterized. Expressions for diffusivity and nonlocal transport throug hout the boundary layer are given. The diffusivity is formulated to ag ree with similarity theory of turbulence in the surface layer and is s ubject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal ''K profile parameterization'' (KPP) is then verified and compared to alternatives, including its at mospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable therm ocline in both convective and wind-driven situations. The diffusivitie s of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of th e relative importance of stratification to destabilizing shear), and f unctions of the double-diffusion density ratio, R(rho). Oceanic simula tions of convective penetration, wind deepening, and diurnal cycling a re used to determine appropriate values for various model parameters a s weak functions of vertical resolution. Annual cycle simulations at o cean weather station Papa for 1961 and 1969-1974 are used to test the complete suite of parameterizations. Model and observed temperatures a t all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected difference s. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshw ater input. With these advections, good multiyear simulations of tempe rature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermo cline in a manner consistent with observations, and at least as well o r better than alternatives.