On the Deep Western Boundary Current south of Cape Cod

Abstract

Using CTD/oxygen data from eight cruises in the decade from 1994-2003, we have constructed a mean ‘section’ of properties across the Deep Western Boundary Current south of Cape Cod near 70°W. Since all sections included direct velocity measurements, our composite section enables us to portray the mean water mass structure as well as the flow field. Inshore of the Gulf Stream and between the 2500 and 4000m isobaths, the flow is to the southwest along the bathymetry and is remarkably barotropic. The equatorward flowing Labrador Sea Water is shown to have high dissolved oxygen, low salinity, and low potential vorticity, while the underlying Overflow Water is high in oxygen. Because the flow is largely barotropic, there is little vertical flow structure coincident with water mass changes associated with the deep high latitude source waters for the thermohaline circulation (THC): the North Atlantic Deep Water (NADW). Transport estimates for the cold limb of the THC give a range of –14 to –19 Sv for the N. Atlantic source waters found on the section. The greatest uncertainty is due to the presence of a Warm-Core Ring on one of the sections, which apparently completely reversed the flow in the DWBC. Offshore of the DWBC, some of the deep source waters are returned to the north in the deep Gulf Stream. The section is compared to two other, widely separated locations (Abaco and 55°W) which have markedly different characteristics of the DWBC.