AOML
NOAA

The fate of the Deep Western Boundary Current
in the South Atlantic

The pathways of recently ventilated North Atlantic Deep Water (NADW) are part of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In the South Atlantic these pathways have been the subject of discussion for years, mostly due to the lack of observations. Knowledge of the pathways of the AMOC in the South Atlantic is a first order prerequisite for understanding the fluxes of climatically important properties. Historical and new observations, including hydrographic and oxygen sections, Argo data, and chlorofluorocarbons (CFCs), are examined together with two different analyses of the Ocean general circulation model For the Earth Simulator (OFES) to trace the pathway of the recently ventilated NADW through the South Atlantic. CLIVAR-era CFCs, oxygen and salinity clearly show that the strongest NADW pathway in the South Atlantic is along the western boundary (similar to the North Atlantic)(Figure 1).

In addition to the western boundary pathway, tracers show an eastward spreading of NADW between ~17-25°S. Analyzed together with the results of earlier studies, Argo data and model output indicate that after crossing the equator, the Deep Western Boundary Current (DWBC) transports water with the characteristics of NADW and a total volume transport of approximately 14 Sv (1 Sv = 106 m3 s-1) (Figure 2). It crosses 5°S as a narrow western boundary current and becomes dominated by eddies further south. When this very energetic eddying flow reaches the Vitória-Trindade Ridge (~20°S), the flow follows two different pathways. The main portion of the NADW flow continues along the continental shelf of South America in the form of a strong reformed DWBC, while a smaller portion, about 22% of the initial transport, flows towards the interior of the basin. (Garzoli et al., 2015)


zonal_jet_fig2
Figure 1

Figure 1: The CLIVAR-era CFCs, oxygen and salinity demonstrate that the strongest NADW signal in the South Atlantic (similar to the North Atlantic) is along the western boundary and that there is eastward spreading of NADW between ~17°-25°S, and NADW is observed east of the Walvis Ridge at 30°S. Map of CFC-11 in pmol kg-1 (bottom) on neutral density level 27.9 kg m-3, the level of the upper NADW, generated from WOCE-era data from 1992 to 1997. Brown lines indicate highlighted WOCE lines: A10 zonal along 30°S, A16S meridional at 25°W and A13 meridional at 0°E. X Location where CFC-11 concentrations of 0.03 pmol kg-1 is observed from Clivar-era CFC' data (first two decades of the 2000s)


zonal_jet_fig1
Figure 2

Figure 2: Schmid (2014) developed a procedure to obtain three-dimensional fields of absolute geostrophic velocities from Argo and altimeter data. Top panel: Velocity field at 2000 dbar derived from the Argo data. Red vectors highlight the southward to southwestward flow along the western boundary; blue indicates the eastward velocity originating near the Vitoria Trindade ridge. Solid curves highlight the pathway of the DWBC along the South American coast. The meridional line indicates the location of the vertical section displayed in the lower panel. Lower panel: Meridional-vertical structure of the eastward pathway in the top panel showing the zonal velocity in cm sec‐1 along 25°W from 10° to 30°S.

Reference:
Garzoli, S.L., S. Dong, R. Fine, C. Meinen, R.C. Perez, C. Schmid, E. van Sebille, and Q. Yao, 2015: The fate of the Deep Western Boundary Current in the South Atlantic. Deep-Sea Res. I , (doi: 10.1016/j.dsr.2015.05.008) [PDF]