A Framework for Assessing Ocean Mixed Layer Depth Evolution

The ocean surface mixed layer plays a crucial role as an entry or exit point for heat, salt, momentum, and nutrients from the surface to the deep ocean. In this study, we introduce a framework to assess the evolution of the mixed layer depth (MLD) for realistic forcings and preconditioning conditions. Our approach involves a physically-based parameter space defined by three dimensionless numbers: λ_s representing the relative contribution of the buoyancy flux and the wind stress at the air-sea interface, R_h the Richardson number which characterizes the stability of the water column relative to the wind shear, and f/N_h which characterizes the importance of the Earth's rotation (ratio of the Coriolis frequency f and the pycnocline stratification N_h). Four MLD evolution regimes (“restratification,” “stable,” “deepening,” and “strong deepening”) are defined based on the values of the normalized temporal evolution of the MLD. We evaluate the 3D parameter space in the context of 1D simulations and we find that considering only the two dimensions (λ_s, R_h) is the best choice of 2D projection of this 3D parameter space. We then demonstrate the utility of this two-dimensional λ_s − R_h parameter space to compare 3D realistic ocean simulations: we discuss the impact of the horizontal resolution (1°, 1/12°, or 1/60°) and the Gent-McWilliams parameterization on MLD evolution regimes. Finally, a proof of concept of using observational data as a truth indicates how the parameter space could be used for model calibration.