The Gulf Stream Separation and Topographic Wave Arrest

The separation of western boundary currents, such as the Gulf Stream, have been notoriously difficult to model accurately in coarse resolution ( O(100 km) ) models and requires some finesse in eddy resolving (O(10 km)) models. Numerous separation hypotheses have been spurred from the use of numerical ocean models, yet a clear recipe for separation is unknown. Establishing a clear recipe for separation will help guide future modelling practices. Competing theories can be divided into those which rely on global vorticity constraints and those which rely on local processes. Evidence will be given from a model comparison study which indicates that the separation of the Gulf Stream is insensitive to gyre-scale barotropic vorticity balances, which suggests that the separation is controlled by local processes. Currently, literature on this topic indicates a common agreement amongst all ``local-process'' theories, namely that separation relies on the production of a counterflow between the current and the boundary. For the Gulf Stream, this corresponds to the production of cyclonic vorticity. A series of artificial terraforming experiments investigate the sensitivity of the separation to the presence of the Deep Western Boundary Current and the local topographic structure. It is found that the separation is linked to the location where the continental shelf steepens, as it does between Charleston, SC and Cape Hatteras, NC. These results are consistent with an idealized model which illustrates that cyclonic vorticity is generated when a jet encounters steepening topography. The generation and subsequent evolution of the vorticity is found to depend on the criticality of the upstream flow with respect to local topographic waves.