Vorticity and Instability during Flow Reversals on the Continental Shelf

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 forcing changes from upwelling-favorable to near-zero. Upwelling vorticity dynamics are considered using a simple equation, where also the importance of stress curls is discussed. During the wind relaxation events, the usually equatorward flow reveres 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 dynamics of the flow reversals and instability, a contour dynamics model is developed for barotropic flow on an exponentially sloping bottom, in which potential vorticity i...