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 Cap e Hatteras. The absence of significant seasonal and interannual variability in the separation point, compared with that of other separating boundary c urrents, suggests that Gulf Stream separation is locally controlled. In thi s paper we consider the possible influence of bottom topography and the Dee p Western Boundary Current (DWBC), which descends underneath the Gulf Strea m 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 offs hore and pass beneath the Gulf Stream. Three possible mechanisms by which b ottom topography and the DWBC can affect the separation of the Gulf Stream are proposed and investigated: (i) topography modifies the background poten tial 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, leadin g to its separation. Results from a series of idealized numerical experiments with a "geostrophi c vorticity" model are presented to investigate these mechanisms. Topograph y alone does have an impact on the separation point, broadly consistent wit h modification of the background potential vorticity. We also show that the presence of a DWBC does, indeed, push the time average separation of the G ulf Stream farther southward, consistent with both the advection and advers e pressure gradient mechanisms. However, the time-dependent boundary curren t separation is more nonlinear than suggested by each of the above mechanis ms, undergoing a series of abrupt transitions between northern and southern separation states. As the DWBC transport is increased, the southern separa tion state is occupied more and more frequently.