A parameter space for evaluating oceanic convection regimes

Oceanic convection, parameterized through vertical mixing schemes, is still not well captured by ocean general circulation models. A preliminary step necessary to improve these schemes is to evaluate and compare how the models behave for different forcing regimes. Literature often proposes single-case comparison (either on a specific location or a specific time or with a specific metrics). The goal of our work is to propose a more systematic framework allowing evaluations and comparisons over a larger range of forcing regimes. For doing so, we define a parameter space which has been derived thanks to a theoretical 1D model of the mixed-layer depth (MLD) evolution. This parameter space is formed by two dimensionless numbers : λ_s which describes the relative contribution of the buoyancy flux and the wind in the surface layer, and the Richardson number R_h which characterizes the stability of the water column at the mixed layer base. In this presentation, I will highlight the key features of this parameter space and I will illustrate its physical robustness with an ensemble of 1D simulations. These simulations were conducted by applying a 10 years JRA55-do 1.4.0 atmospheric forcing within a 1D standalone code making use of the NEMO Turbulent Kinetic Energy + Enhanced Vertical Diffusivity (TKE + EVD) scheme. Then, I will present a test case to study the impact of the horizontal resolution on the convection regimes for a TKE + EVD scheme in 1D, 1°, 1/12° and 1/60° realistic NEMO simulations. I will define convective regimes by sorting the values according to the normalized evolution of the mixed layer depth d_t MLD / MLD and I will show that these regimes are almost kept in the parameter space between 1D and 1° but become generally less convective / more restratifiying when increasing the resolution, highlighting the restratification processes by lateral fluxes. Moreover, I will show that the dynamics in the Mediterranean is much more affected by the increase of resolution than the Labrador sea, suggesting that it involves more intense lateral restratification processes.