Parameterization of mixing in the ocean boundary layer

Abstract

A method for estimating eddy viscosity and diffusivity based on a similarity theory for the vertical exchange scale of dominant turbulent eddies in a surface forced flow is used to predict flux profiles for momentum, heat, and salt in the ocean boundary layer (OBL). By neglecting time dependence in solving for the turbulence properties, the technique requires specifying only the mean density profile along with surface friction velocity and surface buoyancy flux. Momentum and scalar fluxes in stratified fluid below the well-mixed layer are determined by iteration. The method is illustrated in two ways. First, measurements of turbulent stress and heat flux from an intense storm during the 1994 Antarctic Zone Flux Experiment are simulated with reasonable accuracy. The model is forced by Reynolds stress measured near the surface, and by buoyancy flux estimated from the thermodynamic mass balance at the ice undersurface. It is then verified by comparing predicted friction velocity with measurements at other levels in the boundary layer, and by comparing predicted and measured turbulent heat flux. The latter requires accurate simulation of eddy diffusivity in the boundary layer. In the second demonstration, a prognostic model for the evolution of upper ocean temperature and salinity structure is formulated for an idealized storm scenario in which freezing or melting at the surface alternately induces destabilizing and stabilizing surface buoyancy flux. Since closure is local, prognostic equations for temperature and salinity only are carried. The model simulations are compared with a second-moment closure model (Mellor–Yamada level 2 1 2 ) forced in the same way. For surface heat flux (the main diagnostic), the two approaches agree quite well. In other respects, significant differences are noted, and it is suggested how data may be used to evaluate (and discriminate among) ocean turbulence models.