Transport of the North Atlantic Deep Western Boundary Current about 39°N, 70°W: 2004–2008

Begun in spring 2004, a sustained measurement program – Line W – is returning high-resolution observations of the North Atlantic's Deep Western Boundary Current (DWBC) southeast of New England. The study focuses on the cold limb of the Atlantic Meridional Overturning Circulation near the boundary between the subpolar and subtropical gyres. The field study consists of a 6-element, continental-slope-spanning moored array on a line underlying an altimeter satellite ground track, and periodic reoccupations of a full-depth hydrographic section along the line extending from the continental shelf towards Bermuda. Here, data from the first 4 years of the array (May 10, 2004–April 9, 2008) are analyzed along with 9 realizations of the section. The array, a mix of Moored Profiler and discrete, fixed-depth instrument moorings, returned temperature, salinity and horizontal velocity data with various temporal and depth resolutions. After averaging to filter inertial, tidal and other high-frequency motions, the combined moored data set was binned to the lowest common temporal resolution of 5-days (the nominal burst sample interval of the Moored Profilers) and interpolated to 2-dbar vertical resolution. Temperature, salinity, dissolved oxygen, tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises. The present work focuses on the 4-year-mean and time-varying meridional transport in 4 layers bounded by neutral density surfaces: Upper and Classical Labrador Sea Waters, Iceland-Scotland Overflow Waters and Denmark Strait Overflow Waters. The 5-d, 4-layer-summed meridional transport estimates range between −3.5 and −79.9 Sv with a record mean average transport of −25.1 Sv and standard deviation of 12.5 Sv. Bias adjustment to account for the finite width of the moored array increases the 4-layer mean transport estimate to −28.7 Sv. At time scales longer than about 1 month, the variations in equatorward DWBC transport appear correlated with meridional position of the Gulf Stream North Wall with stronger transport observed when the Stream is displaced south.