Topographic generation of internal waves by nonlinear superposition of tidal harmonics

Abstract

A numerical study of topographic generation of internal waves by superposition of the semidiurnal and the diurnal tidal harmonics was motivated by the experimental data collected in the area of the Malin Shelf. In situ measurements and synthetic aperture radar (SAR) images revealed the evidence of the packets of internal waves propagating on-shelf. The tidal periodicity of these waves and their location (between the isobaths 160 m and 1000 m, i.e. far from the shelf break) as well as the spatial orientation of wave fronts suggest that they were not locally generated but came from some distant source. A detailed analysis performed with the help of a two-dimensional nonlinear nonhydrostatic numerical model shows that the most probable place of wave generation is the eastern flank of the Rockall Bank located 150 km off the Malin Shelf. The result demonstrates that a nonlinear superposition of diurnal and semidiurnal barotropic tidal harmonics was capable of generating the waves even though each of harmonics separately was too weak to produce a substantial baroclinic response. Such a case of efficient superposition of two tidal forcings takes place only over relatively steep bottom topography when fundamentally unsteady lee waves are excited. The same mechanism of generation can take place in some other parts of the World Ocean where the barotropic tides comprise several harmonics.