COLLISION AND SEPARATION OF BOUNDARY CURRENTS

The collision and separation of two opposing boundary currents is investigated with simple inviscid, barotropic and baroclinic models on an f-plane. The underlying hypothesis is that currents originally set in the oceanic interior turn into jets that flow along a coast until they ultimately collide. At the confluence zone both currents turn seaward; our aim is to compute the speed, width and veering angle (i.e. the angle between coast and the axis of offshore jet) of the merged offshore currents. Steady solutions are obtained analytically by considering the conservation of potential vorticity, the Bernoulli integral and the integrated constraints associated with conservation of mass and momentum. In the purely barotropic case the currents turn offshore when they reach the stagnation point, resulting from the collision. It is found that the velocities of the two approaching jets at the coast (y = O) need to be identical in order for a stationary steady state to be reached. When this ''balance'' condition is not met, the entire system of separated currents drifts along the wall toward the weak current. For the more complicated (and realistic) baroclinic cases, we consider a 11/2-layer (consisting of one active layer and an infinitely deep inactive layer) and 11/2-layer system (consisting of two active layers and an infinitely deep inactive layer). For simplicity, one jet is assumed to have zero potential vorticity, whereas for the other jet the potential vorticity is taken to be uniform. The relation between the depth of the approaching jets at the coast and the veering angle is computed for both cases, As in the barotropic case, both baroclinic cases show that stationary steady solutions exist only for specific currents. It is expected that when the particular computed relationship between the two currents does not exist (i.e. the currents are unbalanced) the entire separated system will again drift along the wall. An alternative possibility is that the upstream conditions will somehow be altered until the stationary balance is obtained. These results suggest that separation of the western boundary currents can occur even without the beta-effect or a vanishing wind stress curl over the ocean interior. The new separation process discussed above is due to the momentum imparted on the poleward flowing currents by the opposing hows. Possible application of this theory to the western boundary currents system in the South Atlantic is discussed.