Abstract
Abstract. 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), and therefore its variability affects the meridional heat transport and consequently the regional and global climate. Nearly 6 years of observations from a line of pressure-equipped inverted echo sounders (PIESs) have yielded an unprecedented data set for studying the characteristics of the time-varying DWBC volume transport at 34.5° S. Furthermore, the horizontal resolution of the observing array was greatly improved in December 2012 with the addition of two current-and-pressure-equipped inverted echo sounders (CPIESs) at the midpoints of the two westernmost pairs of PIES moorings. Regular hydrographic sections along the PIES/CPIES line confirm the presence of recently ventilated North Atlantic Deep Water carried by the DWBC. The time-mean absolute geostrophic transport integrated within the DWBC layer, defined between 800–4800 dbar and within longitude bounds of 51.5 to 44.5° W, is −15 Sv (1 Sv = 106 m3 s−1; negative indicates southward flow). The observed peak-to-peak range in volume transport using these integration limits is from −89 to +50 Sv, and the temporal standard deviation is 23 Sv. Testing different vertical integration limits based on time-mean water-mass property levels yields small changes to these values, but no significant alteration to the character of the transport time series. The time-mean southward DWBC flow at this latitude is confined west of 49.5° W, with recirculations dominating the flow further offshore. As with other latitudes where the DWBC has been observed for multiple years, the time variability greatly exceeds the time mean, suggesting the presence of strong coherent vortices and/or Rossby Wave-like signals propagating to the boundary from the interior.
Highlights
In the South Atlantic at 34.5◦ S the Deep Western Boundary Current (DWBC) is thought to carry the majority of the cold deep limb of the Meridional Overturning Circulation (MOC) southward toward the Southern Ocean
The time-mean absolute-velocity section calculated from the pressure-equipped inverted echo sounders (PIESs) data during 2009–2014 via the methods described above shows the Brazil Current flowing southward between PIES sites A and B between the surface and roughly 800 dbar, with the DWBC flowing southward below it (Fig. 2a)
These flows appear weak and smooth horizontally; keep in mind that because these velocities are calculated via the geostrophic method they represent a horizontal average between each pair of PIES sites – i.e., horizontal averages over 2–3◦ of longitude
Summary
In the South Atlantic at 34.5◦ S the Deep Western Boundary Current (DWBC) is thought to carry the majority of the cold deep limb of the Meridional Overturning Circulation (MOC) southward toward the Southern Ocean. Previous work has suggested that when this very energetic eddying flow reaches the VitóriaTrindade Ridge at about 20◦ S, the mean flow appears to follow two different pathways, with a significant fraction (estimates ranging from 3 to 12 Sv) flowing eastward across the Mid-Atlantic Ridge toward the African continent (e.g., Zangenberg and Siedler, 1998; Arhan et al, 2003; Hogg and Thurnherr, 2005; van Sebille et al, 2012) and the remainder flowing southward along the western boundary hugging the South American continental slope. The Garzoli et al (2015) study; focused primarily on the time-mean circulation pattern and provides little information about the time variability of the DWBC flow, which is the focus of the present study
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