How much Overturning is Numerical? Identifying Numerical Mixing in Idealized Ocean Model Experiments.

The representation of the Meridional Overturning Circulation (MOC) remains a major source of uncertainty in climate models, since different models show largely different upwelling pathways and magnitudes. The present paradigm is that the MOC consists of a quasi-adiabatic middepth overturning that is mainly Southern Ocean wind-driven with little interior diabatic transformation,  and a deep cell that is mixing driven. However, often the mixing and associated diapycnal transports are diagnosed without explicitly accounting for spurious numerical mixing, which may affect water mass transformation in models.

Here, we assess the role of such numerical mixing in shaping diapycnal transport and overturning circulation in models by using the unstructured-mesh ocean model FESOM2 in an idealized Neverworld2 configuration. We do this in two idealized configurations: one with parameterized eddies and one in which eddies are resolved (the latter being finer). By comparing vertical mixing profiles and their horizontal distributions, and by exploiting FESOM2's discrete variance decay and water-mass transformation diagnostics, we identify potential sources of spurious mixing and quantify its contribution to the diapycnal upwelling in the model.