The accuracy of barotropic and baroclinic tides in global non-assimilative ocean general circulation models

Abstract. We investigate the accuracy of barotropic tides and phase-locked baroclinic (internal) tides in forward (non-data-assimilative) simulations of four different high-resolution global ocean models: the HYbrid Coordinate Ocean Model (HYCOM), the Modular Ocean Model Version 6 (MOM6), the Nucleus for European Modelling of the Ocean (NEMO), and the Massachusetts Institute of Technology general circulation model (MITgcm). Consistent with a previous study that employed only HYCOM, we find that the phase-locked internal tides in our HYCOM, MOM6 and MITgcm simulations are large relative to altimeter observations unless an explicit parameterized topographic internal wave drag, which damps barotropic and baroclinic tides, is included. We also find that harmonic analysis of long records of model output results in estimates of phase-locked internal tides that compare more favorably with observed baroclinic surface elevations from along-track satellite-altimetry. Increasing the resolution within the same dynamical core reduces amplitudes of phase-locked internal tides, possibly due to a more efficient energy cascade to higher-frequency internal wave continuum motions. We also document details of the astronomical tidal forcing in the MITgcm runs, which have been widely used by the wideswath altimetry community, that adversely affect the accuracy of modeled barotropic and baroclinic tides. We verify the impacts of the erroneous tidal forcing with results of less expensive shallow-water simulations that also employ the erroneous forcing.