Response of the N.W. European Shelf Sea at subtidal frequencies

Abstract

Studies involving subtidal circulation of the N.W. European Shelf seas have sometimes suggested “anomalous” flow components of unknown origin (e.g. Jeffereis et al., 1982, Deep-Sea Research, 29, 713–738). The present study aims to assess how low-frequency barotropic forcing existent in the adjoining ocean might influence shelf circulation and hence contribute to such “flow anomalies”. A linearized two-dimensional numerical model is used to examine the shelf response. Imposed boundary conditions include (i) a persistent north-south sea level gradient, (ii) a plane wave propagating eastwards and (iii) a plane wave propagating northwards; wave periods for (ii) and (iii) were varied in discrete steps from a quarter to 16 lunar days. For periods longer than a day or two, the elevation response is generally close to unity as expected from simple theory. However both the persistent north-south gradient and the northwards propagating wave can generate significant “residual” flows in shelf regions to the west of the U.K. Further understanding of such phenomena is limited by the difficulties in measuring both the forcing gradients and the induced response. The complex relationship indicated by this study between coastal elevation recordings and the associated oceanic gradients emphasizes the problems which may arise in using such recordings to infer long-term ocean circulation.