Water mass transformation and subduction in the South Atlantic

The transformation of water masses induced by air-sea
fluxes in the South Atlantic is calculated with a global ocean model
(OCCAM) and has been compared with several observational datasets.
Air-sea interaction supplies buoyancy to the ocean at almost all
density levels. The uncertainty of the estimates of water mass
transformations is at least 10 Sv, largely caused by the uncertainties
in heat fluxes. Further analysis of the buoyancy budget of the mixed
layer in the OCCAM model shows that diffusion extracts buoyancy from
the water column at all densities. In agreement with observations,
water mass formation of surface water due to air-sea interaction is
completely balanced by consumption due to diffusion. There is a
large interocean exchange with the Indian and Pacific Oceans.
Intermediate water is imported from the Pacific and light surface water
is imported from the Indian Ocean. South Atlantic Central Water and
denser water masses are exported to the Indian Ocean. The air-sea
formation rate is only a qualitative estimate of the sum of subduction
and interocean exchange. Subduction generates teleconnections between
the South Atlantic and remote areas where these water masses re-emerge
in the mixed layer. Therefore, the subduction is analyzed with a
Lagrangian trajectory analysis. Surface water obducts in the South
Atlantic, while all other water masses experience net subduction. The
subducted Antarctic Intermediate Water and Subantarctic Mode Water
re-emerge mainly in the Antarctic Circumpolar Current further
downstream. Lighter waters re-emerge in the eastern Tropical Atlantic.
As a result, the extratropical South Atlantic has a strong link with
the Tropical Atlantic basin, and only a weak direct link with the
extratropical North Atlantic. The impact of the South Atlantic on the
upper branch of the thermohaline circulation is rather indirect: water
is significantly transformed by air-sea fluxes and mixing in the South
Atlantic, but most of it re-emerges and subducts again further
downstream.