Countergradient vorticity Flux Generated in Continental Boundary Currents

Abstract

It is suggested that the inshore shear of continental boundary flows like the Florida Current can be accounted for by a countergradient vorticity flux, rather than by lateral diffusion to the shore. Two simple barotropic models with cross-stream and downstream topographic variations illustrate the point. In the first case, a broad jet with piecewise uniform vorticity accelerates through a slowly converging strait, eventually becoming locally critical (in the hydraulic sense) and then undergoing a transition to a different downstream state in which the maximum inshore vorticity is increased. The topographic conditions for this to occur are determined by a nonlinear long-wave theory. In the second model, the computed flow around an idealized cape illustrates the role of lee waves in generating mean downstream vorticity. For large-amplitude capes a nonlinear long-wave theory shows that a downstream transition (similar to the strait problem) can occur as well as upstream ?blocking.?