Global modelling of surface and internal tides

Global prediction of oceanic tides is a classical problem in physical oceanography and was, probably, the first one to be formulated in a robust mathematical manner. A system of hydrodynamical equations that can describe tidal flows given astronomical forcing and global topography was formulated by Laplace almost 250 years ago. The quality and complexity of this model has increased considerably during the last 100 years as the importance of additional physical processes has been recognised, such as energy loss to a turbulent bottom boundary layer, oceanic self-attraction and Earth loading, and, more recently, energy loss due to generation of internal waves at tidal frequency. This so-called internal tide is now thought to play an important role in setting the global oceanic circulation, and has led to renewed interest in numerical modelling of ocean tides.

Here, a new dynamical numerical model of global tides is described, which incorporates and adequately resolves all the aforementioned effects. Although such dynamical models are less accurate than data-constrained models, they are more flexible and can be used to explore tidal regimes of the past and future and their climatological implications. Special emphasis is placed upon accounting for the energy loss to internal tides in a physically consistent way, which is achieved via explicit modelling of linear internal wave generation at specified tidal frequencies. The model resolution is sufficiently high to resolve a few low-mode internal tides which capture the majority of barotropic-to-baroclinic energy conversion. The advantages (and limitations) of this approach are examined, by comparison with the model results obtained with popular internal wave drag parametrization schemes, rather than explicit modelling of internal tide generation.