Low‐frequency shelf/slope responses induced by localized offshore forcings

Abstract

Using a two‐layer β plane model with bottom friction, we investigated the shelf/slope responses induced by localized offshore forcing sources which (1) propagate in the along‐slope direction and (2) have oscillating amplitudes. The two cases represent typical forcing patterns of cutoff eddy propagations and unstable meanders in western boundary currents. When the localized forcing source propagates in the along‐slope direction, it tends to induce components of bottom‐trapped topographic Rossby waves (TRWs) with a similar cross‐slope structure, and the shelf/slope response depends strongly upon the magnitude of the propagating speed c. In the in viscid limit, maximum response occurs when c is such that the induced bottom‐trapped TRW components may freely cross the slope region. When bottom friction is considered, however, the shelf/slope response due to the bottom‐trapped TRWs is important only in large c value cases. In small c value cases, we found the shelf/slope response is determined by surface‐intensified wave components, whose existence is due to the planetary β effect. For observed warm eddies with c = 3 ∼ 5 cm s−1, the result of the present study suggests that both bottom‐trapped TRWs and surface‐intensified baroclinic waves are significant in determining the shelf/slope response. In cases when the localized forcing source oscillates in amplitude, we found that a localized response peak exists in the shelf/slope region. Along a fixed cross‐slope section this response peak in the frictional case tends to shift offshore as the oscillating period of the forcing source increases. This result is opposed to the inviscid result, which shows that the response peak is independent of the forcing's oscillating period. The result for the frictional case is qualitatively consistent with that observed across the continental rise south of Cape Cod, thus suggesting the importance of bottom friction in determining the shelf/slope response.