A simplified energetics based planetary boundary layer (ePBL) approach for ocean climate simulations. Ocean Modelling

This paper presents a method to parameterize vertical turbulent mixing coefficients within the ocean surface boundary layer (OSBL) for climate applications. The new method is specifically constructed to satisfy two requirements. The first aspect is to explicitly consider the mechanical energy budget of the turbulence that drives mixing. This constraint ensures a realistic and robust simulation of the OSBL, which is critical for coupled climate simulations. The second aspect is that the model should be formulated so that it is not sensitive to the numerical limitations common to climate simulations, such as long time-steps and coarse vertical grids. This goal is achieved by combining an existing resolved shear-driven mixing parameterization (here Jackson et al., 2008) with a new method to avoid time step sensitivity. The new method is motivated by the Kraus-Turner-Niiler type bulk boundary layer parameterization, but relaxes the requirement for vertical homogeneity. The non-dimensional coefficients m* and n* from the Kraus-Turner-Niiler approach are parameterized for the new method based on results of simulations using a previously tested parameterization at high resolution. The resulting parameterization is evaluated by comparing simulations with the new parameterization to simulations with the k−ϵ parameterization over a wide range of combinations of surface wind stress, surface buoyancy flux, and latitudes. The new method for vertical turbulent OSBL mixing is therefore proposed as a computationally efficient, implicitly energetically constrained option appropriate for ocean climate modeling applications.