Abstract
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 M 2 tide. The path of M 4 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.
Highlights
The history of tidal dynamical science has recently been driven by two major revolutions: numerical modeling and satellite altimetry
Shallow-water barotropic models have so far been extremely successful at simulating the barotropic tides, and until the late 1990s the internal tides were generally seen as a marginal process with little influence on the first order, barotropic component
The examination of the tidal energy balance in global barotropic models has repeatedly shown a lack of tidal energy dissipation that could not be compensated by classical bottom friction
Summary
The history of tidal dynamical science has recently been driven by two major revolutions: numerical modeling and satellite altimetry. Altimetric and in situ data are available for observations of barotropic and internal tides and model validations (Lyard et al, 2006; Pichon and Correard, 2006) For those reasons, the Bay of Biscay is an ideal location to study the tidal dynamics, and to assess numerical model skills. The present NEA-2004 tidal atlas includes the nonlinear tidal dynamics It is more accurate than the FES2004 atlas from Lyard et al (2006) over continental shelves, especially in coastal regions, which is of particular interest for internal tide studies. The assimilated tidal current solution relies directly on the model error description, since only elevation data are assimilated Given that such a description remains extremely empirical and rough in the global tidal models, the question of our confidence in such derivation remains controversial, especially in the shelf and coastal waters. The North-East Atlantic (NEA-2004) tidal atlas has been obtained in order to provide a regional tidal atlas with high accuracy, high resolution and extended tidal spectrum
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