Eddy Formation in 2.5-layer Western Boundary Currents and their Extensions

Meanders and Rossby-wave-like motion are often found in regions where western boundary currents separate from continental margins. In some cases, the current separation is smooth and the meander amplitudes increase toward the ocean interior. In others, the current overshoots its separation latitude and retroflects, forming a large amplitude lobe near the coast and meanders which decay in the downstream direction. Eddy detachment may occur in both types of separation. When the separation is smooth, eddies are pinched off in the downstream region, where the western current extension may be regarded as a free jet. When a current retroflects, the eddy-shedding usually takes place close to the coast.

All the mid-latitude Southern Hemisphere western boundary currents, the Brazil Current in the Atlantic, the East Australian Current in the Pacific, and the Agulhas Current in the Indian Ocean, retroflect as they leave the western boundary. It has been widely reported the formation and pinch off of large anticyclonic rings by these three current systems.

In this work, we theoretically investigate the formation of nonlinear eddies in unstable 2.5-layer, quasi-geostrophic, inertial western boundary currents using a piecewise constant potential vorticity, ``contour dynamical'' model. We examine the eddy formation in regions several deformation radii away from the western boundary and close to it. In the first category, we perform numerical experiments analogous to those of Meacham [1991], who used a double-front, two-layer model. Unlike Meacham's [1991] model, ours has a two baroclinic mode structure in the background jet. The lower layer current transport ranges from westward to eastward (with the upper layer transport considered eastward in all cases) and a continuum of eddy shedding events is obtained. When dipolar vortex formation occurs, it develops very similarly to Meacham's [1991] two-layer experiments. It is found that apparently there are no bounds for dipole formation in terms of lower layer transport direction. Even weakly unstable jets are able to shed dipolar eddies when initial conditions target the most unstable wavenumbers. In the second category, motivated by the Brazil Current separation (which happens as it converges with the Malvinas Current around 38^oS), we build the 2.5-layer analog of the Silveira et al. [1999] equivalent-barotropic, separating western boundary current model. We then verify that coastal and retroflection eddies are shed when baroclinic instability mechanisms are included. However, it seems that the unstable waves must either propagate westward or slowly in the eastward direction to allow this process to occur.