Vorticity and Instability during Flow Reversals on the Continental Shelf

Abstract

Flow reversals during relaxation of the equatorward wind on the northern California shelf are studied with observations and a simple numerical model. Data from the CODE experiment are used to document the changes in the cross-shelf profiles of alongshelf currents and potential vorticity when the wind focing changes from upwelling-favorable to near-zero. Upwelling vorticity dynamics are considered using a simple equation, where also the importance of stress curl is discussed. During the wind relaxation events, the usually equatorward flow reverses to poleward on the inner shelf, resulting in a region of high shear and occasionally a localized maximum in potential vorticity on the shelf. In the course of such an episode, a single eddylike structure on the shelf was observed with satellite imagery. In order to simulate the basic dynamcis of the flow reversals and instability, a contour dynamics model is developed for barotropic flow on an exponentially sloping bottom, in which potential vorticity is advected and relative vorticity changes only by movement across topography or advection. Thus it is well suited for modeling the character of nonlinear shelf waves, instabilities and related processes. The model is applied to flow reversals and to local instability qualitatively similar to the observations. It is shown how an intrusion of low potential vorticity along the coast can generate a single "eddy" on the unstable flow behind it. This inshore potential vorticity minimum is likely to originate on the sheltered wide shelf north of Point Reyes. An extra tracer has been added to some simulations, and an example shows entrainment in the absence of any turbulence.