Anticyclonic eddies trapped on the continental shelf by topographic irregularities

Abstract

Nonlinear effects produced during the scattering of a barotropic shelf wave (BSW) by a spatially varying mean current are studied using a primitive equation numerical model. Both the BSW phase and the mean current propagate in the same (positive) direction along shelf/slope topography which is uniform everywhere except for a localized topographic irregularity, e.g., a submarine canyon. The mean current is specified at the upstream boundary and adjusts to the topography, closely following isobaths through the model domain. The incident BSW signal is then introduced at the upstream boundary either as a harmonic wave or as a pulse of finite duration. The BSW signal scatters its energy into other available wave modes when it encounters the topographic irregularity. The scattered wave field is dominated by evanescent modes which are trapped at the topographic irregularity and appear as intense mesoscale flows between the coast and the mean current. Nonlinear dynamics transform these large‐amplitude evanescent modes into persistent eddy‐like features on the shelf. The nonlinear interaction is much stronger when the current on the shelf associated with the BSW is opposite to the mean current direction (i.e., negative), so anticyclonic eddies are preferentially generated at the topographic irregularity. For a harmonic BSW, an anticyclonic eddy periodically appears when the negative current phase passes and disappears when the positive current phase passes. A BSW pulse with negative velocity at the coast produces a strong anticyclonic eddy which persists, after the pulse has passed, for a time period substantially longer than the pulse duration. A pulse with positive velocity at the coast does not generate any persistent features on the shelf. The anticyclonic eddies produce mass exchange between the shelf and the mean current and could contribute significantly to cross‐shelf exchange on continental shelves.