New observations of the spreading and variability of the Antarctic Intermediate Water in the Atlantic
Abstract:The new and unique Argo data set currently available, in conjunction with other data previously collected, increases our understanding of the spreading of the Antarctic Intermediate Water in the southern and tropical Atlantic Ocean and to verify previous results. The combination of velocity and salinity data collected with Argo floats verified the main patterns of circulation at intermediate (800 to 1100 dbar) depths. Interesting new features in the pathways are found: (1) the existence of a new, third, branch of westward to northwestward flow that is fed by the Benguela Current; (2) two pathways through which the water from the Benguela Current Extension feeds into the Intermediate Western Boundary Current, one turns north at the western boundary while the other one turns north about 10° farther offshore; (3) the core of the South Atlantic Current is located farther north than was thought earlier (at 35 to 38°S instead of south at about 40°S); (4) significant flow of water from the South Atlantic Current to the Antarctic Circumpolar Current occurs east of the Zapiola Eddy (at about 45°S, 35°W); (5) a quite robust eastward current exists at about 20°S; and (6) there are indications, only in the salinity distribution, for southward spreading of Antarctic Intermediate Water from the equator near the eastern boundary.
Transport estimates for the 800 to 1100 dbar layer show that the transports of the zonal currents in the subtropical gyre at intermediate depth increase from east to west, and that this trend is nearly linear. The transport of the South Atlantic Current near the western boundary is between 5 and 10 Sv, while it is close to 1 Sv near the eastern boundary of the Atlantic. The transport of the Benguela Current Extension is about 8 Sv near 45°W and only about 1 Sv near 14°E. It is also found that at the bifurcation of the Benguela Current Extension (at 28.5°S) about two thirds of the Antarctic Intermediate Water recirculate in the subtropical gyre, which is a smaller portion than the three quarters reported previously. Zonally integrated transports in the Antarctic Intermediate Water layer show that, as a meridional average, about 3 Sv are transported northward in the 800 to 1100 dbar layer. At 35°S this transport is 2.8 Sv, which amounts to 16% of the total northward transport of the Meridional Overturning Circulation (18 Sv).
An analysis of the variability shows that the confluence of the Malvinas Current and the Brazil Current undergo seasonal variations at intermediate depth. The confluence is at its northernmost location (36°S) in July-September. On average the confluence is at 38°S. Both, the variability and the mean location of the confluence at the depth of Antarctic Intermediate Water is similar to what has been observed at the surface.
Document Type: Research Article
Publication date: 2009-11-01
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