Dynamics of Separating Western Boundary Currents

In this work, Pratt and Stern's quasigeostrophic, 1 1/2-layer, infinite jet model is connected to a western boundary by a system of two converging boundary currents. The model has a piecewise constant potential vorticity structure and the departing jet has a zonal cusplike profile in the ocean interior. The relative strengths of the coastal jets can be varied and the coastline can be tilted relative to north. The coastline tilt and the coastal current asymmetry cause an alongshore momentum imbalance that creates a spatially damped, quasi-stationary wave pattern. The presence of the boundary favors the long waves in the model, which behave fairly linearly in all study cases. The effects of the coastline tilt and the coastal current asymmetry are varied to reinforce or cancel each other. In the former case, a retroflection type of boundary current separation, like the one observed in most Southern Hemisphere western boundary currents, is obtained. In the latter case, a much smoother separation results, as when the Gulf Stream leaves the North American coast. In order to comply with the piecewise constant potential vorticity constraint, the beta effect is included in the model only very crudely. The "beta" term in the potential vorticity relationship is totally compensated for by a steady flow pattern similar to the edge between two Fofonoff gyres. It is found that when beta is nonzero, the wavelengths are somewhat shorter than those of f-plane cases.