Current oscillations in the diurnal–inertial band on the Catalonian shelf in spring

Abstract

Measurements with acoustic Doppler current profilers and conventional current meters on the Catalonian shelf indicate that the energy spectrum is dominated by oscillations in the diurnal–inertial band. The oscillations take the form of strong clockwise near circular motions with near surface speeds of up to 15 cm s −1. Motion of comparable magnitude is found to penetrate down through the water column to the sea bed at all depths across the shelf and to the shelf edge (200 m). Analysis of the vertical structure reveals a persistent phase shift of close to 180° between surface and near bed velocities at both inertial and diurnal frequencies, which are observed to increase as one moves off-shore. The putative source of forcing for these motions is diurnal by on-shore–off-shore windstress with a strong response to the diurnal clockwise component of stress being favoured by the proximity of the site (latitude 40°N) to the critical latitude for diurnal–inertial resonance (30°N). A frictionless, two-layer, analytical model is used to demonstrate this feature of the flow and to explain both the depth penetration and phase reversal of the oscillations. The model, which is forced only by a diurnal on-shore–off-shore windstress, incorporates a condition from the vertically integrated solution of Craig (Continental Shelf Res. 9 (1989) 965) which relates the cross-slope pressure gradient to the forcing stress. This condition results in a component of forcing in the lower layers which is 180° out of phase with the surface stress. The model is extended in a numerical solution to a multilayer structure with frictional coupling between the layers via an eddy viscosity. The results confirm the central role of the pressure gradient forcing even when upper and lower layers are not de-coupled by an intervening pycnocline.