How Does the Deep Western Boundary Current Cross the Gulf Stream?

Abstract

The manner in which the deep western boundary current (DWBC) crosses the Gulf Stream is investigated using data from a hydrographic survey conducted in 1990. Absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level. Three density layers are considered in detail: two mid-depth layers, which together make up the shallowest water mass component of the DWBC (500–1200 m), and a deep layer consisting of the Norwegian–Greenland overflow water (2500–3500 m). The shallowest layer does not make it through the crossover and is completely entrained by the Gulf Stream; however, the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover. In the intermediate layer, which is denser than the Gulf Stream coming off the shelf, part of the DWBC recirculates to the northeast while the onshoremost portion continues equatorward. In the deep layer only a small amount of recirculation occurs. The lateral fields of potential vorticity (Q) reveal a Q barrier associated with the Gulf Stream in the two mid-depth layers, which is partially lessened in the intermediate one allowing the equatorward continuation of flow. In the deep layer, the DWBC maintains its potential vorticity through the crossover.