Vertical Kelvin Wave Propagation Produced by a Subsurface Ridge

Abstract The influence of a subsurface ridge on a subinertial coastal Kelvin wave is investigated using a numerical model and a simplified analytical framework. A first baroclinic mode Kelvin wave is made to impinge on progressively complex subsurface geometries. For an idealized submerged shelf that extends infinitely in both offshore and alongshore directions, the over‐shelf response includes low vertical modes of the shallower domain and an upward‐propagating beam from the shelf‐top corner, inducing alongshore variability in the shelf flow. When the alongshore extent is truncated to form a ridge between two deep basins, the downstream response strongly depends on ridge width. Phase patterns reveal upward and downward energy propagation from the downstream ridge corner, along with horizontal propagation of multiple vertical modes. As the ridge narrows, beam amplitudes decrease, though vertical propagation remains visible in phase patterns. If the ridge width is less than approximately twice the Rossby radius of deformation of the ridge‐top domain, evanescent modes along the ridge flanks begin to overlap and interact, promoting more horizontal energy transmission into the downstream basin. Thus, the Rossby radius of deformation sets a natural scale governing the balance between vertical scattering and horizontal transmission. In the most realistic configuration, a submerged ridge of finite offshore and alongshore extent, waves trapped to and propagating around its periphery affect the downstream basin through combination with the signals that propagate over the top of the ridge.