From interannual to decadal: 17 years of boundary current transports at the exit of the Labrador Sea

Over the past 17 years, the western boundary current system of the Labrador Sea has been closely observed by maintaining the 53 degrees N observatory (moorings and shipboard station data) measuring the top-to-bottom flow field offshore from the Labrador shelf break. Volume transports for the North Atlantic Deep Water (NADW) components were calculated using different methods, including gap filling procedures for deployment periods with suboptimal instrument coverage. On average the Deep Western Boundary Current (DWBC) carries 30.2 +/- 6.6 Sv of NADW southward, which are almost equally partitioned between Labrador Sea Water (LSW, 14.9 +/- 3.9 Sv) and Lower North Atlantic Deep Water (LNADW, 15.3 +/- 3.8 Sv). The transport variability ranges from days to decades, with the most prominent multiyear fluctuations at interannual to near decadal time scales (+/- 5 Sv) in the LNADW overflow water mass. These long-term fluctuations appear to be in phase with the NAO-modulated wind fluctuations. The boundary current system off Labrador occurs as a conglomerate of nearly independent components, namely, the shallow Labrador Current, the weakly sheared LSW range, and the deep baroclinic, bottom-intensified current core of the LNADW, all of which are part of the cyclonic Labrador Sea circulation. This structure is relatively stable over time, and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the deep water export by 10-15%.