Importance of damping in comparison of internal tides in several global hydrodynamical models with altimetry

High-resolution global hydrodynamical models that contain both eddies and internal waves are becoming increasingly useful for the SWOT mission. It is therefore important to know how well such models compare to observations. Here we focus on the ability of four such models to reproduce the internal tide sea surface height signature seen in nadir altimeters. These include simulations from the HYbrid Coordinate Ocean Model (HYCOM), the Modular Ocean Model Version 6 (MOM6), the Nucleus for European Modelling of the Ocean (NEMO), and the Massachussetts Institude of Technology general circulation model (MITgcm). Consistent with a previous study that employed only the HYCOM model, we find that the absence of an explicit damping mechanism on tidal flows in global ocean models results in internal tides that are too energetic and propagate too far from their sources. We also find that the length of model output used in harmonic analysis impacts the comparison of internal tide amplitudes to those derived from along-track satellite altimetry, consistent with the previous investigation. Differences in internal tide strength in different models run at the same resolution, and without additional explicit damping, are likely due to differences in numerics amongst the models, although further study is needed to confirm this. This paper also documents, for the first time, some details of the tidal forcing in the particular MITgcm runs examined here, that adversely affect the accuracies of the modeled tides, and suggests correction factors using less expensive barotropic shallow-water simulations that roughly mimic the tidal forcing in the MITgcm runs.