Physical Oceanography

Surface and Subsurface Currents

Spreading and Variability of the Antarctic Intermediate Water in the Atlantic

PIs: Claudia Schmid

The Antarctic Intermediate Water is formed in the Southern Ocean and is characterized by a salinity minimum below the thermocline. It is found north of the Subantarctic Front. This water mass plays an important role in the South Atlantic and in the global ocean circulation since it contributes significantly to the northward flow of the upper limb of the Meridional Overturning Circulation.

There are two source water masses that feed into the Antarctic Intermediate Water found in the Atlantic Ocean. One of them is the Subantarctic Mode Water, which is formed in the Pacific Ocean and enters the southwest Atlantic through the Drake Passage. This water mass is found south of the Subantarctic Front. The second one is Antarctic Intermediate Water of Indian Ocean origin that flows into the eastern South Atlantic via the Agulhas/Benguela Current system.

Climatological mean of the pressure and salinity at the salinity minimum of the Antarctic Intermediate Water.

A portion of the Subantarctic Mode Water from the Pacific Ocean flows northward in the Malvinas Current and subducts to become Antarctic Intermediate Water, while most of the Subantarctic Mode Water continues eastward in the Antarctic Circumpolar Current. Another area of subduction of Subantarctic Mode Water into the Antarctic Intermediate Water layer is the region just south of the Subantarctic Front (in the Polar Frpnt Zone) throughout the whole Atlantic Basin. The northward Malvinas Current meets the southward Brazil Current at approximately 38S in the Confluence Region. This region is characterized by a strong thermohaline front at the surface, because the Malvinas Current transports fresh and cool water, whereas the Brazil Current transports warmer and saltier water. From the Confluence Region the Antarctic Intermediate Water flows eastward in the South Atlantic/Antarctic Circumpolar Current system and about 86% continues into the Indian Ocean. The rest of it mixes with water of Indian Ocean origin in the Cape Basin, feeds into the Benguela Current, and continues westward around 30oS in the Benguela Current Extension which bifurcates at the western boundary (Santos Bifurcation). Between two thirds and three quarters of the water reaching the bifurcation at intermediate depth recirculates southward to the Confluence Region. This anticyclonic basin-wide flow pattern is part of the subtropical gyre of the South Atlantic.

Annual mean velocity between 800 and 1100~dbar. The velocities are derived from Lagrangian and quasi-Lagrangian floats (covering 1989 to 2009). Red to the east, blue to the west. Different scales are used for the vectors in the two panels.

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 degrees farther offshore;
  3. the core of the South Atlantic Current is located farther north than was thought earlier (at 35o to 38oS instead of south at about 40oS);
  4. significant flow of water from the South Atlantic Current to the Antarctic Circumpolar Current occurs east of the Zappiola Eddy (at about 45oS, 35oW);
  5. a quite robust eastward current exists at about 20oS; and (6) there are indications, only in the salinity distribution, for southward spreading of Antarctic Intermediate Water from the equator near the eastern boundary.

Schematic of the flow field at intermediate depth based on the velocity field at 800 to 1100~dbar and the water properties at the core of the AAIW layer. Hatching from the lower left to upper right (lower right to upper left) is used for broad eastward (westward) currents The names of currents and other features are, in alphabetical order: AC = Agulhas Current, ACC = Antarctic Circumpolar Current, BC = Brazil Current, BEC = Benguela Current, BECE = Benguela Current Extension, IWBC = Intermediate Western Boundary Current, MC = Malvinas Current, NICC = Northern Intermediate Countercurrent, S = stagnant region (poorly ventilated, cross-hatched), SAC = South Atlantic Current, SICC = Southern Intermediate Countercurrent, ZE = Zappiola Eddy.

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 45oW and only about 1 Sv near 14oE. It is also found that at the bifurcation of the Benguela Current Extension (at 28.5oS) 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 35oS this transport is 2.8 Sv, which amounts to 16% of the total northward transport of the MOC (18 Sv).

Annual mean transports between 800 and 1100~dbar. Left: Zonal Currents and integration limits. Right: Meridional flow. The red error bars display the total error (standard deviation plus shear-based error) and the blue error bars represent the shear-based error. (a) South Atlantic Current, (b) Benguela Current Extension, (c) averaged over western basin, (d) averaged over eastern basin, (e) averaged over whole basin.

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 (36oS) in July-September. On average the confluence is at 38oS. 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.

Quarterly velocity vectors between 800 and 1100~dbar. Red to the north, blue to the south. The thick grey lines indicate the extents of the Brazil Current and Malvinas Current. From left to right, top to bottom: Jan-Mar, Apr-Jun, Jul-Sep, Oct-Dec.