A model study of “bump” induced western boundary current variabilities

Abstract

A time-dependent, three-dimensional numerical model is used to study the effects of a bottom irregularity or “bump” on western boundary current (WBC) variabilities along a simplified shelf and slope. Numerical experiments with (i) no bottom bump, (ii) a small bump and (iii) a large bump have been conducted. Case (i) produces low variabilities and cases (ii) and (iii) show significant increase in slope and shelf energetics both downstream and upstream of the bump. Disturbances generated at the bump are well correlated with flow variabilities upstream. Downstream variabilities are caused by meander development following the WBC deflection by the bump, while topographic waves excite upstream variabilities. The model also indicates two modes of deflection paths, small- and large-amplitude pahs, downstream of the bump. These findings are further supported by results obtained from a Gulf Stream simulation which incorporates the bathymetry of the U.S. South Atlantic Bight, and which has a more realistic boundary forcing. The simulated eddy kinetic energy distribution shows three regions of variability which are of interest: one inshore (and slightly downstream) and one offshore of the Charleston Bump, and a third region over the shelfbreak some 150–200 km upstream of the Bump. The inshore and offshore maxima are due to the small and large amplitude deflection paths of the model Gulf Stream, respectively, while the upstream maximum is presumably due to topographic wave activity.