The Eddy-Diffusivity Mass-Flux parameterization: improved representation of convective mixing, global evaluations and implications for Ocean Heat Content

The Eddy-Diffusivity Mass-Flux (EDMF) parameterization (Giordani et al., 2020) offers a new, coherent way to simultaneously parameterize local (diffusivity) and non-local (convective thermal) vertical mixing. This second component parametrizes sub-grid-scale convective plumes propagating through the water column which, through energy conservation, can propagate counter to the stratification gradient. The EDMF scheme is assessed in a 13-year global ¼° coupled NEMO4.2-SI3 simulation, forced by ERA5 atmospheric reanalysis. Its performance in representing observed ocean temperatures is compared to that of a twin simulation using the commonly applied Enhanced Vertical Diffusivity (EVD) parameterization.

The EDMF simulation shows globally reduced temperature biases relative to in-situ observations (0–700 m) compared to the EVD simulation, with similar RMSD (Root Mean Square Deviation) values between the two. By better representing tropical night-time shallow convection, EDMF reduces the cold bias typically observed in EVD simulations within the tropical ocean. We show that the horizontal scales (convective areas), penetration depths and vertical velocities of the simulated plumes agree with measurements of deep convective plumes in the Labrador Sea, and with diurnal convection in the equatorial Pacific Ocean. Additionally, first estimates of convection's contribution to Ocean Heat Content are proposed.