Along-slope generation as an explanation for some unusually large internal tides.

Why are the internal waves observed on the Portuguese shelf at 41N (which have thermocline displacements of up to 45 m) many times larger than expected from 2D shelf edge internal tide generation theory? Barotropic tidal forcing is too small to create them, either at the local shelf edge or from the nearby Oporto seamount. Using wave refraction techniques it is demonstrated that they must be created by the interaction between tidal currents and a major westward projection of the shelf edge about 50 km to the south. The off-shelf propagating internal tidal energy thus generated is subsequently refracted back onto the shelf in the form of non-linear internal wave packets. Refraction explains not only how the energy reaches the shelf, but also the orientation of the waves relative to the shelf edge and details of their appearance in a synthetic aperture radar image. The result demonstrates that shelf edge internal tide generation can be more complex than is suggested by the 2D approach, and that global shelf edge internal tide energy must be larger than previously thought.