On the nature of cross‐isobath energy fluxes in topographically modified barotropic semidiurnal Kelvin waves

Abstract

AbstractContinental shelf topography modifies a Kelvin wave into a hybrid Kelvin‐edge wave with a nonzero across‐isobath velocity and a phase speed that decreases with increasing wave number while the group velocity reaches a minimum at intermediate wave numbers. We model the modified semidiurnal Kelvin wave adjustment to alongshore changes in the shelf width. The model domain consists of two alongshore‐uniform continental shelves of different widths adjoined through a 150 km long transition zone. The continental shelf and slope are adjacent to an ocean of a constant depth, allowing radiation of Poincaré waves. We consider three shelf widths of 150, 250, and 300 km, where properties of a zero mode at semidiurnal frequency change from Kelvin wave like to edge wave like. For each shelf width, a zero wave mode has its distinctive alongshore energy flux structure on the shelf. As the incident wave encounters a variable shelf width, the alongshore energy flux converges (diverges) on the shelf resulting in an offshore (onshore) energy flux over the continental slope. Furthermore, for a strongly convergent alongshore energy flux, the incident wave mode scatters into radiating Poincaré waves. On sufficiently wide shelves, a strong across‐isobath energy flux comparable with the incident wave energy flux can be triggered even by relatively modest changes of shelf width. An energy flux divergence parameter De is defined, which scales with magnitude and direction of the energy flux across the continental slope. More than 50% of the incident energy flux scatters into modes radiating offshore when De is −1 or less.