Tidal hydrodynamics of the Hudson Bay and its impact in

the global ocean tides

L. Chevallier1,2, D. Greenberg3, F. Lyard1

1LEGOS, Toulouse, France2Université de Hte-Normandie, Rouen, France

3Bedford Institute of Oceanography, Halifax, Canada

IntroductionThanks 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.

a rms a G rms G e

Global ocean 2 13 0 5 13

North Atlantic 2 16 1 1 14

Tropical Atlantic 3 12 1 10 11

South Atlantic -1 10 3 3 8

M2 misfits, uniform Z0, previous depths

M2 misfits, Hudson low Z0, previous depths

a rms a G rms G e

Global ocean 4 16 0 8 17

North Atlantic 12 27 2 4 33

Tropical Atlantic 15 17 0 19 20

South Atlantic 3 11 4 2 15

ConclusionAgain, 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.

a rms a G rms G e

Global ocean 2 13 0 5 13

North Atlantic 2 16 1 1 14

Tropical Atlantic 3 12 1 10 11

South Atlantic -1 10 3 3 8

a rms a G rms G e

Global ocean 4 14 0 6 14

North Atlantic 9 20 1 3 22

Tropical Atlantic 8 14 0 14 15

South Atlantic 2 11 3 2 13

M2 misfits, uniform Z0, new depths

M2 misfits, Hudson low Z0, previous depths