Diapycnal mixing in the Early Eocene: insights from the DeepMIP intercomparison project phase 1

Tides are the main energy source for diapycnal mixing in the ocean interior. However, energy-constrained tidal mixing parameterizations are not routinely included in ocean models applied to the deep-time past of the Earth. Instead, diapycnal mixing is usually parameterised by a constant vertical diffusivity or a prescribed vertical profile of vertical diffusivity.

Here, by leveraging outputs from the DeepMIP project, we compute the power effectively consumed by parameterized diapycnal mixing in each DeepMIP model and for different CO₂ concentrations. We show that this power slightly increases with increasing CO₂ in simulations integrated to quasi-equilibrium but skyrockets in warming, out-of-equilibrium, simulations. This reflects the increased stratification in a warming ocean, even though in principle the same amount of tidal energy is available for mixing. We find no evident relationships between the intensity of the overturning circulation and the power consumed by diapycnal mixing across the DeepMIP models. Finally, we use coupled climate-biogeochemistry simulations performed with the IPSL-CM5A2 model to show that the marine biogeochemistry is largely impacted by the vertical mixing scheme employed, even if the total power consumed by diapycnal mixing remains similar.