Temporal evolution of the deep western boundary current where it enters the sub-tropical domain

Four repeat hydrographic sections across the Deep Western Boundary Current (DWBC) at 55°W, occupied between 1983–1995, are used to investigate the inter-annual variability of the deep flow. The sections include measurement of tracers (oxygen, CFCs) and absolute geostrophic velocity. All properties are interpolated onto a regular grid, both in depth space and density space. The analysis focuses on two of the water masses of the DWBC: the Denmark Strait overflow water (DSOW) and classical Labrador Sea water (CLSW), both of which are clearly revealed in the property sections. The mean volume flux of water denser than σθ=27.8 kg/m3 is 13.3(±4.2) Sv, comparable to that measured south of Greenland in the DWBC, suggesting that this is an accurate measure of the deep throughput into the sub-tropical North Atlantic. The largest property variability over the 12 yr period occurs in the CLSW, which in the 1990s became markedly colder, fresher, more weakly stratified, and higher in oxygen and CFCs – all indicative of new ventilation. By contrast, over this same period the deeper DSOW became less well-ventilated. Such opposite behavior of the two water masses is consistent with the large-scale atmospheric forcing in the North Atlantic. The deep absolute velocities were decomposed into a barotropic (CLSW) and baroclinic (DSOW) contribution. The DSOW flow intensified with the appearance of the new CLSW in 1991. An attempt is made to explain this, but it remains unclear to what extent the change could be due to local versus remote forcing.