This project will address the mechanisms and pathways of North Atlantic Deep Water (NADW) export from the South Atlantic to Indian Ocean. Two eddy-resolving ocean general circulation models will be analyzed to determine the quantity and characteristics of deep flow eddies in the NADW and the possible roles they may play in the inter-basin exchange. The project will also address interannual changes in NADW transport and its relationship to the subtropical front, if any. The results will be examined in relation to the Atlantic Meridional Overturning Circulation (AMOC).
The research bears on the AMOC and global thermohaline circulation which are crucial links in the global climate system. Improved understanding of the deep circulation links between the Atlantic and Indian Oceans should translate to improved certainty in climate modeling and prediction.
North Atlantic Deep Water (NADW) forms the deep water limb of the Meridional Overturning Circulation (MOC), ventilating the deep sea by transporting cold saline properties out of the Atlantic basin. We use high resolution model output to show that the outflow of NADW is strongly influenced by the Agulhas Current System as it flows into the Indian Ocean around South Africa. As a result, we propose that changes in the Agulhas Current, its retroflection and leakage into the Atlantic may have a direct influence on the lower arm of the MOC as well as its upper arm, possibly affecting the sensitivity of the MOC to climate change. The Meridional Overturning Circulation is the primary way by which the world's oceans redistribute heat from warm regions to cold regions. Warm waters are transported towards the poles at the surface, grow colder and denser and sink at high latitudes, before returning back towards the equator as cold deep waters. In this way, the MOC moderates climate, warming the poles and cooling the tropics, and hence its variability is linked to climate and climate change. As the lower arm of the MOC exits the Atlantic basin there is a high degree of mixing and dynamical blocking below the turbulent Agulhas system, which deflects the NADW southwards and dilutes its salinity rapidly as it enters the Indian Ocean. In particular, the Agulhas retroflection squeezes the NADW layer and stirs it vigorously. By releasing thousands of numerical drifters into the NADW layer of the model, we find extensive recirculations of deep water within the southeast Atlantic and an indirect pathway into the Indian Ocean that favors the area beneath the Agulhas ring corridor, retroflection, and a southern route around the Agulhas Plateau. Of the total amount of NADW that approaches the tip of South Africa an estimated 62% eventually ends up in the Indian Ocean. Spin-up of eddies (cyclogenesis) within the NADW layer results from instabilities in the surface flow and is favored in three distinct areas: In the lee of the Agulhas Bank, where cyclones are generated as a result of an anomalously strong northwestward extension of the Current; beneath the retroflection where a dipole forms when the retroflection broadens; and to the east of the Agulhas Plateau where cyclones spin up as the Return Current meander deepens. Cyclogenesis is most frequent beneath the retroflection, but eddies dissipate within weeks throughout all regions and hence do not play a significant role in transporting NADW around South Africa.
