Reducing spurious numerical mixing in simulations under strong tidal forcing : a case study

Tides, and more specifically the internal gravity wave field generated by the tidal current flowing over bathymetric features, play a significant role in the energy cascade and in setting the global state of the ocean by intensifying mixing. Accounting for these dynamics in numerical models is important for accurately depicting energy transfer across scales, and an increasing number of models use explicit tidal forcing. Yet, in fixed coordinates models, the strong vertical motions generated by internal waves trigger the tendency of advection to produce numerical errors, leading on the vertical to spurious numerical mixing that can easily be as high as the physical one. This can severely bias model output and impact the overall simulated dynamics by spuriously mixing water masses and modifying the stratification, which in turn impacts the propagation of internal waves. We propose a fresh way to look at advection schemes, along with a method to make their diffusivity more selective, and eventually protect the relevant vertical scales from being spuriously eroded. A case study on the South East Asian Seas using the Symphonie model is presented, since these seas are known for being the generation site of exceptionally strong internal tides.