1. Tidal hydrodynamics of the Hudson Bay and its impact in the global ocean tides L. Chevallier 1,2, D. Greenberg 3, F. Lyard 1 1 LEGOS, Toulouse, France 2 Université de Hte-Normandie, Rouen, France 3 Bedford Institute of Oceanography, Halifax, Canada

2. Introduction Thanks to the support of the french national space agency CNES, the Toulouse global tides atlas has been recently renewed (FES2012 atlas). It is based on spectral (i.e. harmonic) hydrodynamic modeling using T-UGOm unstructured model and ensemble data assimilation (SpEnOI code). Compared to the previous FES release (FES2004), the grid resolution has been increased and a special care have been put on creating the best possible model bathymetry. Prior to data assimilation, it is a good practice to reach the best possible accuracy from the hydrodynamic model alone, and a systematic tuning exercise (by varying model resolution, bottom friction and internal tide drag coefficients) has been carried out to optimize the model configuration. Surprisingly, the hydrodynamic solutions showed a robust tendency of lower accuracy in the whole Atlantic basin compared to the other oceans. In parallel to the FES2012 atlas construction, and in preparation of the FES2013 atlas, additional developments led to the conclusion that tidal energy dissipation in the Hudson Bay and Fox basin region was too high, and the consequence being tidal amplitude to be too weak in the Atlantic Ocean. Reducing the friction coefficient to very low values helped to artificially correct the model bias, and the resulting simulation reached the unprecedented accuracy of 1,3 cm RMS (when comparing to altimetry derived harmonic constants, TP/Jason-1/Jason-2 cross-overs in deep ocean). Albeit successful, this approach could not be considered as physically acceptable, and a dedicated study of the Hudson Bay tidal system has been carried out to understand the true reasons for its role in the global simulations accuracy.

4. arms aGrms Ge Global ocean21305 North Atlantic2161114 Tropical Atlantic31211011 South Atlantic10338 M 2 misfits, uniform Z 0, previous depths M 2 misfits, Hudson low Z 0, previous depths arms aGrms Ge Global ocean4160817 North Atlantic12272433 Tropical Atlantic151701920 South Atlantic3114215

5. Conclusion Again, bathymetry is the most crucial parameter for tidal simulations. However, as the overall accuracy of the available bathymetry datasets improve with time, and tidal simulation error budget reduces in accordance, more subtle ingredients become significant. A good example is the recent use of latitude varying g (gravitational acceleration). Compared to a uniform g, it now improves the simulations by a few millimeters, that have been totally hidden in earlier attempts (FES2004 for instance). Albeit difficult to prioritize, it seems clear that locally tuned friction coefficients (especially in dissipation regions) or 3D effects will need to be taken into account to reduce further the tidal simulation errors.

7. arms aGrms Ge Global ocean21305 North Atlantic2161114 Tropical Atlantic31211011 South Atlantic10338 arms aGrms Ge Global ocean41406 North Atlantic9201322 Tropical Atlantic8140 15 South Atlantic2113213 M 2 misfits, uniform Z 0, new depths M 2 misfits, Hudson low Z 0, previous depths