Dynamics of the semi-diurnal and quarter-diurnal internal tides in the Bay of Biscay. Part 1: Barotropic tides

The generation of internal tides in the ocean is due to the interaction of strong barotropic tidal currents with variable topography in stratified waters, transferring energy from the external to the deep ocean. The internal tides feed later the ocean mixing, playing a major role for the maintenance of the stratification of the global ocean. A remarkable region in terms of tidal energy is the European continental shelf. As a first step toward the study of internal tides in the Bay of Biscay, this paper aims at understanding the barotropic tides and associated energy budgets. On continental shelves and in coastal seas the use of regional models with fine grid resolution is preferred to the use of global tidal atlases derived from altimetry. The unstructured grid T-UGOm model is used to compute the NEA-2004 tidal solutions in the North-East Atlantic ocean, with errors greatly reduced in coastal areas compared with global models. Energy budgets are discussed based on the inclusion of nonlinearities in the tidal solutions. The sea surface height and depth-averaged currents are used to compute the tidal energy conversion from barotropic to baroclinic tides, tidal dissipation and energy flux. A total amount of energy of 250 GW is found for the M2 tide. The path of M4 energy from the Southern Atlantic ocean toward the Bay of Biscay is highlighted, advocating for nonzero boundary conditions in regional models. The 3D coastal ocean SYMPHONIE model has been implemented to simulate the surface tides in the Bay of Biscay. Solutions are validated by comparison with the NEA-2004 solutions and observations.