Abstract

The influence of topography in a basin interior on the separation and time-dependence of strongly nonlinear western boundary currents is explored using a shallow water numerical model and scaling theory. In the linear limit, the western boundary current follows the western boundary to the latitude of the gap in the interior topography where it then separates from the coast and flows eastward in a narrow jet. As nonlinearity is increased, the flow initially remains steady but develops a series of stationary meanders extending off the western boundary at the separation latitude. For strongly nonlinear flows the solutions become time-dependent. The mean separation latitude continues to be tied to the interior topography even though the mean zonal flow far exceeds the baroclinic wave speed. In most cases, the variability is dominated by westward-propagating cyclonic and anticyclonic meanders of the separated western boundary current. The behavior on the western boundary alternates between an overshoot of the western boundary current with an anticyclonic meander and a premature separation of the western boundary current with the poleward formation of an anticyclonic eddy. The mean flow is consistent with the separation latitude of the North Atlatic Current to the west of the Charlie Gibbs Fracture Zone and the time-dependence shows many similarities with the observed variability of the East Australian Current to the west of New Zealand. The wavelength of the meanders and the frequency and amplitude of the oscillations are well predicted by a simple scaling that accounts for wave propagation, nonlinear advection, and a viscous sublayer along the western boundary.

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