Incidence and Reflection of Offshore Subinertial and Barotropic Pressure Signals in Wide Shelf Seas

Abstract

Abstract The response of a wide shelf to subinertial and barotropic offshore pressure signals from the shelf edge was investigated. By relaxing the semigeostrophic approximation, an elliptical wave structure equation was formulated and solved with the integral transform method. It was found that when the imposed offshore signal has an along-shelf length scale similar to the shelf width, it can efficiently break the potential vorticity barrier and propagate toward the coast, producing a significant coastal sea level setup. Thereafter, the pressure signal reflects from the coast or the sloping topography, producing a transient eddy and propagates to the downshelf. The intensities of the coastal setup and the eddy increase as the along-shelf scale of the subinertial signal decreases or when its time scale is close to the inertial period. For a signal with longer time scale, the eddy is insignificant. The nature of the shelf response is controlled by the shelf conductivity κ ≡ r/(fsB), in which r is the Rayleigh friction coefficient, f is the Coriolis parameter, s is the shelf slope, and B is the shelf width, respectively. For a given offshore signal, coastal setup increases with κ. For large κ, the eddy energy is concentrated at low modes, producing a large eddy, whereas a small κ produces a small eddy. The proposed theory can explain coastal sea level fluctuations under eddy impingement in the Mid-Atlantic Bight or other similar areas. Significance Statement Coastal sea level and shelf circulation are greatly affected by offshore pressure signals, e.g., mesoscale eddy impingements or boundary current fluctuations. It is often assumed that the along-shelf length scale of the forcing is much larger than the shelf width, i.e., the semigeostrophic approximation. Here in this study, we found this approximation significantly underestimates the shelf–ocean interaction. A general shelf wave equation was developed that relaxed the semigeostrophic approximation and was solved analytically with a novel mathematical method. The solution can characterize the shelf response to subinertial offshore forcing at arbitrary spatiotemporal scales. It was found that for a subinertial signal with scale close to or smaller than the shelf width, significant coastal sea level setup and transient eddy can be formed, which was consistent with realistic phenomena. The new theory could promote the understanding of coastal sea level variations and along-/cross-shelf transports at synoptic and intermediate scales.