Abstract
Abstract. The first direct estimates of the temporal variability of the absolute transport in the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm, based on neutral density, temperature, salinity, and oxygen values, the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation. Both data sets indicate that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed in the North Atlantic, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ∼ 40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observational records are too short to be definitive. A Monte Carlo-style analysis using 27 yr of model output from the same location as the observations indicates that about 48–60 months of data will be required to fully assess the deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however its statistical significance with even 27 yr of model output is low, suggesting that seasonal variations in the model are weak.
Highlights
The role of the Deep Western Boundary Current (DWBC) as a primary pathway for the cold, lower limb of the meridional overturning circulation (MOC) has been well documented in the North Atlantic Ocean (e.g. Molinari et al, 1998; Schott et al, 2004; Johns et al, 2008; Meinen et al, 2012); the pathways and variability of the DWBC in the South Atlantic Ocean are less well known
Just under a year of data from a pilot array of pressureequipped inverted echo sounder (PIES) moorings have been used to provide a first glimpse at the time-varying absolute flow of the Deep Western Boundary Current (DWBC) at 34.5◦ S
The 10.5 months of PIES data illustrate a high degree of DWBC variability in this short time span, with a transport standard deviation of 25 Sv when integrated from 800–4800 dbar and from 51.5 to 44.5◦ W
Summary
The role of the Deep Western Boundary Current (DWBC) as a primary pathway for the cold, lower limb of the meridional overturning circulation (MOC) has been well documented in the North Atlantic Ocean (e.g. Molinari et al, 1998; Schott et al, 2004; Johns et al, 2008; Meinen et al, 2012); the pathways and variability of the DWBC in the South Atlantic Ocean are less well known. Warren and Speer, 1991; Speer et al, 1995; Zangenberg and Siedler, 1998; Van Sebille et al, 2012) Most of this southeastward flow appears to continue along the African coast in the Cape Basin to enter the Indian Ocean (Speer et al, 1995; Van Aken et al, 2004). The only direct current meter estimates available in the region are from off Cabo Frio (22◦ S) and Cabo Santa Marta (28◦ S) These 23-month records indicate a very weak southward volume transport by the DWBC: −0.5 ± 1.6 and −2.8 ± 4.9 Sv (1 Sv = 106 m3 s−1), respectively (Muller et al, 1998). The variability observed during the first year of deployment will be compared to 27 yr of output from a high-resolution model to evaluate the statistical information contained in a one-year record
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