Remote Topographic Forcing of a Baroclinic Western Boundary Current: An Explanation for the Southland Current and the Pathway of the Subtropical Front East of New Zealand

Abstract

The Southland Current is a western boundary current adjacent to the South Island of New Zealand and flows along a segment of the Southern Hemisphere subtropical front (STF). The physical mechanisms that govern the behavior of this current and other portions of the STF and subantarctic front (SAF) are investigated using one regional and three global ocean simulations. The three global ocean simulations used in this study are a 1½-layer reduced-gravity linear simulation, a six-layer nonlinear flat-bottom simulation, and a six-layer nonlinear simulation that incorporates vertically compressed, but otherwise realistic, bottom topography confined to the abyssal layer. All three simulations have horizontal spacings of ?° and are forced with climatological-mean monthly wind stress data. The regional simulation has a horizontal spacing of ½° and contains two layers, with an idealized bottom topography. The only forcing is supplied by inflow and outflow ports. The pathway of the SAF is shown to be strongly influenced by a barotropic response of the associated flow to bottom topography. Currents associated with the SAF flow along the southern edge of the Campbell Plateau, a large submarine platform southeast of New Zealand. In contrast, the location of the Southland Current and the pathway of the STF east of New Zealand are due to remote forcing of upper-ocean currents by topographically constrained abyssal currents. Whereas most western boundary currents can be described as responses of the ocean to interior Sverdrup flow with some modification, analysis of the numerical simulations within this study shows that the Southland Current is due to a completely different formation mechanism. The presence of the Southland Current and the STF east of New Zealand are instead due to a combination of northward topographic steering of surface-layer thickness gradients approximately 12° east of New Zealand and westward propagation of these perturbed gradients. A portion of the abyssal flow associated with the SAF is shown to form a topographically constrained deep western boundary current (DWBC). This DWBC follows the edge of the Campbell Plateau, eventually flowing generally northward. When the surface zonal flow associated with the STF southeast of New Zealand encounters this meridional abyssal flow, it is advected northward, creating an imbalance in the potential vorticity within the flow. In an attempt to conserve potential vorticity, the perturbed surface-layer thickness gradients propagate westward until they encounter New Zealand's South Island, at which point they form the observed Southland Current. Transport associated with the STF thus flows northward along South Island, eastward along the Chatham Rise, and southward once east of the rise. This formation mechanism and the downstream behavior of the STF are illustrated in a simplified regional simulation that reproduces the pathway of the STF and the location of the western boundary current extremely well.