Transport and bottom boundary layer observations of the North Atlantic Deep Western Boundary Current at the Blake Outer Ridge

The North Atlantic Deep Western Boundary Current (DWBC) was surveyed at the Blake Outer Ridge over 14 days in July and August 1992 to determine its volume transport and to investigate its bottom boundary layer (BBL). This site was chosen because previous investigations showed a strong and bottom-intensified DWBC on the ridge's flanks with a thick BBL. The primary instrument used was the Absolute Velocity Profiler, a free-falling velocity and conductivity-temperature-depth device. Two sections across the width of the current produced volume transports of 17 ± 1 Sv and 18 ± 1 Sv (1 Sv = 1 × 106 m3 s−1) for all water flowing equatorward below a potential temperature of 6°C. Transport values were derived using both absolute velocities and AVP-referenced geostrophic velocities and were the same within experimental uncertainty. Good agreement was found between these results and historical ones when both were similarly bounded and referenced. The mean of a nine-day time series of absolute velocity profiles was the same as the means of year-long current-meter records at three depths in the same location suggesting these observations reflect the mean DWBC. A turbulent planetary BBL was found everywhere under the current. Frictional bottom stress was mostly balanced by an along-stream change in the current's external potential energy evidenced by a change in depth of the velocity core along the ridge. The thickness of the bottom mixed layer (BML) (where density, nutrient, and suspended sediment concentrations are vertically uniform) was asymmetrical across the current and up to five times thicker than the BBL. There was little velocity shear in the BML above the BBL and the across-slope density gradient was minimal. Observations suggest a combination of processes maintains the thick BML including large-scale turbulence, downwelling Ekman transport in the BBL and up-slope return flow in the BML, and buoyant convection into the BML.