Moored observations of the Deep Western Boundary Current in the NW Atlantic: 2004-2014

A moored array spanning the continental slope southeast of Cape Cod sampled the equatorward-flowing Deep Western Boundary Current (DWBC) for a 10 year period: May 2004 to May 2014. Daily profiles of subinertial velocity, temperature, salinity, and neutral density are constructed for each mooring site and cross-line DWBC transport time series are derived for specified water mass layers. Time-averaged transports based on daily estimates of the flow and density fields in Stream coordinates are contrasted with those derived from the Eulerian-mean flow field, modes of DWBC transport variability are investigated through compositing, and comparisons are made to transport estimates for other latitudes. Integrating the daily velocity estimates over the neutral density range of 27.8-28.125 kg/m(3) (encompassing Labrador Sea and Overflow Water layers), a mean equatorward DWBC transport of 22.8 x 10(6) +/- 1.9 x 10(6) m(3)/s is obtained. Notably, a statistically significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the current as a whole. The largest linear change (a 4% decrease per year) is seen in the layer of Labrador Sea Water that was renewed by deep convection in the early 1990s whose transport fell from 9.0 x 10(6) m(3)/s at the beginning of the field program to 5.8 x 10(6) m(3)/s at its end. The corresponding linear fit to the combined Labrador Sea and Overflow Water DWBC transport decreases from 26.4 x 10(6) to 19.1 x 10(6) m(3)/s. In contrast, no long-term trend is observed in upper ocean Slope Water transport. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation. Plain Language Summary A sustained measurement program located southeast of Cape Cod observed the equatorward limb of the Atlantic Ocean overturning circulation for the period 2004-2014. The data are analyzed to document the time-averaged structure and volume transport of the Deep Western Boundary Current and explore its modes of variability. Integrating the daily velocity estimates over the Labrador Sea and Overflow Water layers, a mean equatorward DWBC transport of 22.8 x 10(6) +/- 1.9 x 10(6) m(3)/s is obtained. A statistically significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the current as a whole. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation.