The role of the South Atlantic in the upper branch of the global thermohaline circulation

Abstract

The global thermohaline ocean circulation moderates the temperatures over Northern Europe. The transport of heat, mass and salt between different ocean basins is an important part of this link.

The pathways of interocean exchange of thermocline and intermediate waters in the South Atlantic Ocean have been investigated with a Lagrangian particle following technique in the ocean model OCCAM. More than 90\% of the flow toward the North Atlantic originates from the Indian Ocean via leakage from the Agulhas Current system. Several observational studies have indicated a dominant role in the transport toward the North Atlantic for intermediate water or for direct inflow from Drake Passage. The section averaged water mass transports in OCCAM are largely in agreement with these observational estimates, in apparent contradiction with the Lagrangian path. The reason for this discrepancy is that south of Africa the net mass flux consists of opposing, and in the thermocline layer nearly compensating, east- and westward flows. In the thermocline layer, part of the westward flow connects with the cross-equatorial flow in the Atlantic, while the eastward flow is partly derived from upwelled intermediate and thermocline water that originates from Drake Passage. It is arguable to draw conclusions about the flow pathways from integrated mass fluxes across ocean sections, especially when these contain opposing flows in the same density classes.

Air-sea fluxes of heat and freshwater induce transformations between different water masses. These water mass transformations are calculated for the South Atlantic with OCCAM. The uncertainty of the estimates of water mass transformations from several observational datasets is at least 10 Sv, largely caused by the uncertainties in heat fluxes. Subduction generates teleconnections between the South Atlantic and remote areas where these water masses re-emerge in the mixed layer and has been analyzed with a Lagrangian trajectory analysis. 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. 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.

The leakage of water from three Agulhas rings has been studied in a high resolution global ocean model using a Lagrangian particle following technique. A bowl shaped ring boundary that reaches a radius of 140\,km and a depth of 800\,m separates regions of fast and slow leakage. A shallow secondary circulation, caused by subinertial motions due to surface cooling, enhances leakage in the upper 150\,m.

Most of the intermediate waters that cross the equatorial Atlantic as part of the return flow for North Atlantic Deep Water were subducted in the southeast Indian Ocean. Less than 20\% comes directly from Drake Passage without looping into the Indian Ocean; the majority being provided by Agulhas leakage. Most of the intermediate waters that form in the South Atlantic do not follow the South Atlantic/Indian Ocean supergyre, but remain within the Antarctic Circumpolar Current, and gradually transform into Circumpolar Deep Water by diapycnal mixing.