The Retroflection Paradox

Abstract

Abstract The classical question of what happens; when a warm western boundary current, such as the North Brazil Current (NBC), retroflects is addressed analytically using a reduced-gravity nonlinear model. The traditional view is that the northwestward flowing current separates from the wall, turns to the right (looking offshore), and forms a zonal boundary current that flows eastward. Integration of the steady inviscid momentum equation along the boundary gives the longshore momentum flux (or flow force) and shows that such a scenario leads to a paradox. To resolve the paradox the separated current must constantly shed anticyclones, which propagate to the northwest due to β and an interaction with the boundary. This new eddy shedding mechanism, which is not related to the traditional instability of a zonal jet, may explain why the NBC must produce rings. A nonlinear analytical solution to the problem is constructed with the aid of a powerful theoretical approach based on the idea that nonlinear periodic ...