A Laboratory Study of Nonlinear Western Boundary Currents, with Application to the Gulf Stream Separation due to Inertial Overshooting*

Various dynamical aspects of nonlinear western boundary currents (WBCs) have been investigated experimentally\udthrough physical modeling in a 5-m-diameter rotating basin. The motion of a piston with a velocity\udup that can be as low as up 5 0.5 mm s21 induces a horizontally unsheared current of homogeneous\udwater that, flowing over a topographic beta slope, experiences westward intensification. First, the character of\udWBCs for various degrees of nonlinearity is investigated. By varying up, flows ranging from the highly\udnonlinear inertial Charney regime down to a weakly nonlinear regime can be simulated. In the first case, the\uddependence of zonal length scales on up is found to be in agreement with Charney’s theory; for weaker flows,\uda markedly different functional dependence emerges describing the initial transition toward the linear, viscous\udcase. This provides an unprecedented coverage of nonlinear WBC dependence on an amplitude\udparameter in terms of experimental data. WBC separation from a wedge-shaped continent past a cape\ud(simulating Cape Hatteras) due to inertial overshooting is then analyzed. By increasing current speed,\uda critical behavior is identified according to which a very small change of up marks the transition from a WBC\udthat follows the coast past the cape to a WBC (nearly dynamically similar to a full-scale Gulf Stream) that\udseparates from the cape without any substantial deflection, as with the Gulf Stream Extension. The important\udeffect of the deflection angle of the continent is analyzed as well. Finally, the qualitative effect of a sloping\udsidewall along a straight coast is considered: the deflection of the flow away from the western wall due to the\udtendency to preserve potential vorticity clearly emerges