Yann-Treden Tranchant: Unrealistic submesoscales and artificial mixing in ocean models
The growing use of kilometre-scale ocean models, together with new high-resolution observations from the Surface Water and Ocean Topography (SWOT) mission, provides new opportunities to study submesoscale dynamics and their role in vertical heat and tracer fluxes.
Here, we use a kilometre-scale regional model to investigate how vertical discretization and momentum closure control numerical mixing and the representation of interior ocean dynamics in a meander of the Antarctic Circumpolar Current. We show that the vertical resolution needed to suppress grid-scale noise depends strongly on stratification, in agreement with recently proposed criterion. We further show that, for a similar number of vertical levels, a stratification-aware vertical grid strongly reduces numerical instabilities compared with a conventional grid.
Because very fine vertical resolution can become too expensive at kilometre and sub-kilometre horizontal resolution, we also introduce and evaluate a wave-dissipation momentum closure. This scheme suppresses spurious divergent grid-scale modes while preserving the resolved submesoscale dynamics.
When these numerical instabilities are not controlled, enhanced diapycnal mixing erodes stratification, weakens mesoscale buoyancy gradients and fronts, and artificially increases small-scale variability and vertical fluxes. Combining stratification-aware vertical discretization with targeted wave dissipation therefore provides a practical way to set up submesoscale-resolving models that maintain realistic interior dynamics while limiting numerical diapycnal mixing.