Deep Western Boundary Current Variability at 34.5°S During 2009-2015

The Deep Western Boundary Current (DWBC) at 34.5°S in the South Atlantic carries a significant fraction of the cold deep limb of the Meridional Overturning Circulation (MOC). Continuous observation of this key flow has been ongoing for nearly seven years from March 2009 to the present using a line of four pressure-equipped inverted echo sounders (PIES). The horizontal resolution of this array was greatly improved in December 2012 with the additional of two current-and-pressure-equipped inverted echo sounders (CPIES) at the midpoints of three of the existing sites. Regular hydrographic sections are also collected along the PIES/CPIES line as well as up onto the continental shelf – providing key information about the water masses and shelf flows. The data from the PIES and CPIES instruments are combined with local hydrographic information to produce daily estimates of the full-water-column profiles of temperature, salinity, and density at each site. Gradients in the density and bottom pressure between sites are used to produce profiles of the meridional component of the absolute geostrophic velocity. The mean absolute geostrophic transport integrated within the DWBC layer, initially defined between 800-4800 dbar, and from 51.5°W to 44.5°W is -15 Sv (negative indicating southward), with a peak-to-peak range from -89 Sv to +50 Sv and a standard deviation of 23 Sv. Results on the how these variations compare, both in amplitude and spectral character, to variations observed by a North Atlantic DWBC array at 26.5°N will be presented. Furthermore, insights into how the DWBC variability relates to the offshore recirculations will be discussed based on high quality numerical model comparisons, satellite data comparisons, and on comparisons to the data from the 2012-present time period when the array has enhanced horizontal resolution.