On the influence of bottom topography and the Deep Western Boundary Current on Gulf Stream separation

The Gulf Stream separates abruptly from the North American coastline at Cape Hatteras. The absence of significant seasonal and interannual variability in the separation point, compared with that of other separating boundary currents, suggests that Gulf Stream separation is locally controlled. In this paper we consider the possible influence of bottom topography and the Deep Western Boundary Current (DWBC), which descends underneath the Gulf Stream at Cape Hatteras. The path of the DWBC is strongly constrained by bottom topography. At Cape Hatteras, the continental shelf widens and the DWBC is forced to swing offshore and pass beneath the Gulf Stream. Three possible mechanisms by which bottom topography and the DWBC can affect the separation of the Gulf Stream are proposed and investigated: (i) topography modifies the background potential vorticity contours; (ii) the DWBC advects the Gulf Stream separation point southward; (iii) intense downwelling as the DWBC passes beneath the Gulf Stream induces an adverse pressure gradient in the Gulf Stream, leading to its separation. Results from a series of idealized numerical experiments with a geostrophic vorticity model are presented to investigate these mechanisms. Topography alone does have an impact on the separation point, broadly consistent with modification of the background potential vorticity. We also show that the presence of a DWBC does, indeed, push the time average separation of the Gulf Stream farther southward, consistent with both the advection and adverse pressure gradient mechanisms. However, the time-dependent boundary current separation is more nonlinear than suggested by each of the above mechanisms, undergoing a series of abrupt transitions between northern and southern separation states. As the DWBC transport is increased, the southern separation state is occupied more and more frequently