Shelf and slope circulation induced by fluctuating offshore forcing

Abstract

A linear model which includes continuous vertical stratification, arbitrary cross‐shelf bottom topography, and bottom friction is used to examine the response of shelf and slope waters to fluctuating offshore forcing in the form of a specified pressure field. For forcing which is periodic in the alongshore direction and in time, the response varies dramatically with frequency. For periods of less than about 10 days, the response is dominated by near resonances with free coastally trapped waves. These are not pure resonances because of the mismatch between the forcing structure and the free wave structure. For periods of greater than about 10 days, the velocity response decays away from the forcing with a scale determined by the projection of the forcing onto the flat‐bottom baroclinic modes, typically the first baroclinic Rossby radius. If the continental slope is encountered within this scale distance, then the flow is altered and creates a bottom‐trapped, alongshore velocity maximum seaward of the shelf break. Increased stratification enhances bottom trapping and inhibits flow across the slope, moving the maximum in alongshore velocity seaward, thus reducing the alongshore flow near the shelf break. The shelf response is always weak and barotropic. Furthermore, the slope response is largely independent of the shelf geometry, suggesting that narrow shelves appear to be more easily influenced by offshore forcing only because their coasts are closer to the forcing. The model is used to simulate the response to a Gaussian‐shaped anticyclonic eddy translating uniformly in the alongshore direction. The eddy flow is blocked by the topography and becomes squashed at the shelf break. Some shelf water is entrained creating a weak shelf circulation cell. The shoreward flow in the eddy cannot move onto the shelf and instead forms an alongshore jet near the shelf break. The jet extends away from the eddy in the direction toward which free coastally trapped waves propagate, but the alongshore velocity within the jet is opposite to the alongshore velocity within the eddy.