Prediction of Surface M2 Tidal Currents by a Global Ocean Model and Evaluation Using Observed Drifter Trajectories

A global baroclinic ocean model with 1/12° horizontal resolution is first used to study surface currents associated with surface and internal M₂ tides. The model provides accurate predictions of tidal elevation (global RMS errors less than 6.5 cm) if allowance is made for internal wave drag and self attraction and loading. The surface tidal currents predicted by the model are altered significantly by internal tides in many regions of the global ocean. Overall the predicted surface tidal currents are in reasonable agreement with independent estimates based on statistical analysis of observed drifter trajectories. Phase locking of the surface and internal tides causes spatial changes in the amplitude of surface tidal currents that vary on the horizontal scale of the internal tide. Qualitatively similar, small-scale variations are also detected in the observed drifter trajectories. The seasonal variation of surface M₂ tidal currents is also predicted using a seasonal climatology as the background density stratification. Compared to surface elevation, the seasonal range of surface currents can be large (up to 50% of the predictions obtained with annual mean density). The largest seasonal changes in the deep ocean are restricted to regions with major bathymetric features such as mid-ocean ridges and island chains. The implictions of the above results for surge forecasting, including the effect of internal wave drag and self attraction and loading, is discussed.