Internal tides modelling in the Bay of Biscay. Comparisons with observations

An Isopycnic Coordinate Ocean Model is used to represent the three-dimensional propagation of internal tides in the Bay of Biscay. The model is forced by the semi-diurnal tidal waves M2, S2, N2 and K2. A high resolution topography of French and Iberian continental slopes preserves the shape of the main canyons, which are areas of internal tide generation. Hydrological and velocity data collected during a French Naval experiment in 1994 are compared with the model results. Near the generation area, the vertical distribution of the internal tide amplitude, associated with a vertical shear of baroclinic tidal currents, is in agreement with the observations. Moreover, at most locations, the increase in the baroclinic current modelled near the bottom depth matches the data. This increase is related to the distribution of the internal tide amplitude along the ray slope defined by c(z) = (w(2) - f(2) / N-2(z))(1/2) , where w is the tidal frequency, f the Coriolis parameter and N(z) the buoyancy frequency. In most of the data/model comparisons, the vertical density variation in the deep layers shows a difference in phase in agreement with the ray theory. Over the abyssal plain, high horizontal shears of surface current are modelled at mid-bay, i.e. in areas where the internal wave energy coming from the deep ocean encounters the seasonal thermocline. However, the location of these areas is very sensitive to the initial vertical density profile: the three-dimensional variation of the density must be introduced in the initial conditions to improve the model results.