Across-isobath Energy Fluxes in Semidiurnal Kelvin Waves Encountering Variations in Shelf Geometry

Continental shelf and slope topography modifies a Kelvin wave mode into a Hybrid Kelvin-Edge Wave (HKEW) mode with a non-zero across-isobath velocity component whose phase speed decreases with increasing wave number while the group velocity reaches a minimum at intermediate wavenumbers. We investigate how the modified semidiurnal Kelvin wave adjusts to alongshore variations of the shelf width by conducting a set of numerical experiments using ROMS. The model domain consists of two uniform-alongshore continental shelves of different widths adjoined through a 200 km-long transition zone. Continental shelf and slope topography is adjacent to the deep ocean of a constant depth, allowing radiation of Poincare waves. We consider three shelf widths of 150, 250, and 300 km, where a zero trapped mode at semidiurnal frequency has the structure of a Kelvin wave, HKEW, and edge wave, respectively. Each type of zero wave mode has its distinctive alongshore energy flux structure on the shelf. As the incident wave encounters a shelf width variation, the alongshore energy flux converges (diverges) on the shelf resulting in the offshore (onshore) energy flux over the continental slope. Furthermore, if the group velocity approaches zero in the area of the variable shelf width, the incident trapped wave mode scatters into radiating Poincare waves. On sufficiently wide shelves, strong across-isobath energy flux comparable with the incident wave energy flux can be triggered even by relatively modest shelf width variations (e.g., 300 to 250 km transition). The results yield a simple estimate for the energy flux magnitude across the continental slope in the modified semidiurnal Kelvin wave based on the theoretical mode structure and dispersion properties, and can be used for identifying areas of internal wave generation by semidiurnal tides.