Idealized model simulations of barotropic flow on the Catalan shelf

Abstract

The effect of along shelf variation in shelf width on barotropic shelf edge flows and topographic shelf waves are investigated with a numerical model. The model topography represents a gradual transition from a narrow straight to a broader straight shelf region. It is designed primarily to model the Catalan shelf on the northeastern Mediterranean coast of Spain, but the results obtained here are of general validity and will also apply to shelf flow under similar conditions elsewhere. The numerical experiments are carried out with flow imposed, both steady and oscillatory, at the upstream end of the narrow shelf region. With a prescribed shelf edge flow there is a strong tendency for topographic steering in the transition zone. The adjustment is found to take place on a short time scale of 2–5 days set by propagating wave modes rather than by advection. Bottom friction effects do not lead to significant on-shelf leakage of the flow. Flow instability and eddy formation are found to occur, even in cases where the potential vorticity for the imposed flow does not have a local maximum on the shelf slope. The instability leads to development of anticyclonic eddies on the narrow shelf which undergo strong non-linear adjustment at the transition zone from a narrow to a broader shelf. On the broad section of the shelf near the transition zone there is a tendency for formation of a large anticyclonic eddy. Bottom friction is found to influence the flow instability and the formation of eddies in cases where the flow is marginal for instability and growth of eddies. Particle tracking has been performed in order to study cross shelf mixing and transport. Finally, we have studied the current response due to a concentrated wind stress jet acting perpendicular to the coastline in the transition zone between the narrow and the broad shelf regions. A dipolar eddy structure is found to develop with the axis of the dipole tilting about 30° with the axis of the wind jet and with strong current shear in the zone near the dipole axis.