The Impact of Barotropic Nudging on Surface and Internal Tides in Realistically-forced Global Ocean Simulations

Accurately predicting surface and internal tides in 3D global ocean simulations is crucial for understanding ocean mixing, acoustics, and tracer dispersal. Despite advances, numerical model-derived tidal estimates and energy budgets still fall short of those obtained from altimetry-constrained tidal models. This study investigates the effects of nudging barotropic sea surface height towards the Ocean Topography Experiment/Poseidon (TPXO) tidal inversion model on tidal sea level and energetics in 1/12.5◦ global Hybrid Coordinate Ocean Model (HYCOM) simulations. We explore two nudging scenarios: (1) Deep Nudging below 500 m seafloor depth and (2) Shelf-toe Nudging along a 3◦ wide strip offshore the continental shelf. Our objectives are to (1) reduce barotropic tidal errors and (2) improve internal tide (IT) predictability and energetics, while accurately representing the subtidal and mesoscale flows. The Deep Nudging simulation shows significant improvements in surface tides, particularly for M2, with a root-mean-square error of 0.6 cm, representing a 77% reduction relative to our best non-assimilative simulation. A 48% reduction in error (to 0.4 cm) is also observed for K1. However, this method adversely affects barotropic subtidal waves and mesoscale flows. In contrast, the Shelf-toe Nudging method preserves subtidal flows while yielding smaller surface-tide error reductions. The largest regional improvements in the M2 tide occur in the Atlantic Ocean relative to historical HYCOM simulations. Area-averaged IT amplitudes remain broadly consistent with estimates from the empirical internal tide model HRET22 in some hotspot regions. However, an increase in M2 surface tidal amplitude in the northwest Atlantic increases barotropic-to-baroclinic energy conversion, thereby increasing IT amplitudes in this region. While nudging causes small regional changes in M2 internal tide variance, the global, depth-integrated modal energy estimates remain consistent with historical HYCOM simulation. Finally, we implement a correction for thermobaric instabilities, which have adversely affected historical HYCOM simulations.