THE BETA-INDUCED DRIFT OF SEPARATED BOUNDARY CURRENTS

Western boundary currents flow poleward from low latitudes until they ultimately separate from the coast and turn eastward into the ocean interior. The separation is mainly due to either: (i) the variation of the Coriolis parameter with latitude (P) which causes vanishing of the near-wall depth; (ii) vanishing wind stress curl over the ocean interior which forces zero meridional transport; or (iii) opposing currents that flow toward the equator and force the northward flowing currents to turn offshore (AGRA and Nor, Deep Sea Research 1, 40, 2259-2282). Here, we focus on the third kind of separated currents and show that, due to beta, such separated currents migrate along the wall. A nonlinear ''reduced gravity'' one-and-a-half layer model is used to compute the desired migration speed. Solutions of the primitive equations are constructed analytically assuming that the translation rate is steady. It is found that the migration rate along the wall is given by beta R(d)(2) cos alpha/2 siny, where R(d) is the Rossby radius, alpha an angle that measures the inclination of the joint offshore currents relative to the north, and gamma is the angle between the axis of the joint offshore currents and the wall. The migration meridional component can be either northward or southward (depending on the inclination of the wall) but the zonal component is always westward. When the separated joint offshore flow is in the east-west direction (i.e. alpha = pi/2 or 3 pi/2 so that the separated Bow is zonal) no migration is taking place. It turns out that the above migration formula is so robust that it also describes the migration rate in a two-and-a-half layer model where one current is allowed to, at least partially, dive under the other. For most separated currents the computed migration rate is a few centimeters per second. Possible application of this theory to the confluence zone in the South Atlantic (where significant seasonal movement of the separation latitude has been observed) is discussed.