BAROCLINIC EDDY GENERATION AT A SHARP CORNER IN A ROTATING SYSTEM

Laboratory experiments were conducted to investigate the generation of anticyclonic gyres by separation of a surface current from a coast in a rotating, two layer system. The experiments were motivated by the h ypothesis that the flow of coastal currents around capes can generate oceanic eddies, as well as by the observation of gyres at the mouths o f various straits. In the experiments the gyre is formed when the curr ent, which flows with the coast to its right if one is oriented in the downstream direction, encounters a sharp convex corner. The current o vershoots the corner, loops to the right, and reattaches to the coast downstream of the corner. Between the current loop and-the coast is an anticyclone whose width grows with time. If a countercurrent flows un der the surface current, a similar separation in the lower layer resul ts in the generation of a cyclone as well; under some circumstances th e cyclone and anticyclone advect each other away from the coast as a h eton. Previous studies on related systems found that the corner must b e sufficiently sharp for a gyre to form. I show that for a very sharp corner the angle made by the corner must be above a critical value of between 40-degrees and 45-degrees for a gyre to form. This is in contr ast to nonrotating flows of comparable Rayleigh number, which will sep arate from a sharp corner at virtually any angle. For angles below the critical value, the current profile downstream of the corner changes as a function of corner angle, indicating that it is the stagnation of the flow nearest the wall which causes the anticyclone to form. This stagnation is reminiscent of the two-dimensional, nonrotating picture of viscous boundary layer dynamics forcing separation of a boundary cu rrent. However, the gyre grows more slowly when the lower layer is muc h thicker than the upper layer, indicating that baroclinic processes a re at least quantitatively important in the generation of the gyre. By varying the initial condition of the current, it is shown that the gy re formation is not a product of the interaction of the nose of the cu rrent with the corner. In conclusion, the experiments indicate that th e basic mechanism of gyre formation may be viscous boundary effects as in nonrotating systems, but that rotation tends to inhibit eddy gener ation while baroclinic effects tend to enhance it.