ANTICYCLONIC EDDIES TRAPPED ON THE CONTINENTAL-SHELF BY TOPOGRAPHIC IRREGULARITIES

Nonlinear effects produced during the scattering of a barotropic shelf wave (BSW) by a spatially varying mean current are studied using a pr imitive equation numerical model. Both the BSW phase and the mean curr ent propagate in the same (positive) direction along shelf/slope topog raphy which is uniform everywhere except for a localized topographic i rregularity, e.g., a submarine canyon. The mean current is specified a t the upstream boundary and adjusts to the topography, closely followi ng 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 othe r 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 mesoscal e hows between the coast and the mean current. Nonlinear dynamics tran sform these large-amplitude evanescent modes into persistent eddy-like features on the shelf. The nonlinear interaction is much stronger whe n the current on the shelf associated with the BSW is opposite to the mean current direction (i.e., negative), so anticyclonic eddies are pr eferentially generated at the topographic irregularity. For a harmonic BSW, an anticyclonic eddy periodically appears when the negative curr ent phase passes and disappears when the positive current phase passes . A BSW pulse with negative velocity at the coast produces a strong an ticyclonic eddy which persists, after the pulse has passed, for a time period substantially longer than the pulse duration. A pulse with pos itive velocity at the coast does not generate any persistent features on the shelf. The anticyclonic eddies produce mass exchange between th e shelf and the mean current and could contribute significantly to cro ss-shelf exchange on continental shelves.