Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies

• Published in: Nature (London) Vol. 462; no. 7272; pp. 495 - 498
• Main Authors: Biastoch, A., Böning, C. W., Schwarzkopf, F. U., Lutjeharms, J. R. E.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 26.11.2009; Nature Publishing Group
• Subjects: Africa; Analysis; Anthropogenic factors; Atlantic Ocean; Causes of; Changes; Circulation; Climatic changes; Computer Simulation; Earth, ocean, space; Environmental aspects; Evolution; Exact sciences and technology; Experiments; External geophysics; Foraminifera; Humanities and Social Sciences; Indian Ocean; Leakage; letter; Marine; Migration; multidisciplinary; Observations; Ocean circulation; Ocean-atmosphere interaction; Physics of the oceans; Salinity; Science; Science (multidisciplinary); Sea-air exchange processes; Seawater - analysis; Seawater - chemistry; Southern Hemisphere; Southern oscillation; Temperature; Thermocline; Thermohaline circulation; Thermohaline structure and circulation. Turbulence and diffusion; Transport; Trends; Water Movements; Wind; South Atlantic; Indian Ocean; Atlantic Ocean; Agulhas current; Africa; AN, North Atlantic; ASW, South America; ISW, Indian Ocean; ISW, West Indian Ocean, Agulhas Current; AS, South Atlantic; ocean currents; Hindcast; Westerly wind; General circulation models; Latitudinal variation; thermohaline circulation; Wind effect; digital simulation; ocean-atmosphere interaction; ocean circulation; Ocean model
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature08519

Clash of currents The possibility of future decline in the Atlantic meridional overturning circulation (AMOC) is central to discussions of climate change. Attention focuses on the effects of freshening of the subpolar North Atlantic, for example via increases in high-latitude precipitation and melting ice. A new high-resolution ocean model study shows that the North Atlantic is already experiencing an opposite effect from the south. In response to a southward shift of the westerly winds — probably anthropogenically influenced — the transport of warm and salty Indian Ocean waters around the tip of Africa is strengthening. Known as the Agulhas leakage, this flow may offset possible reductions in Atlantic deep-water circulation from freshwater inputs in the North Atlantic, possibly contributing to the stability of the AMOC system, including the Gulf Stream. The Agulhas leakage allows the transport of warm and salty Indian Ocean waters into the Atlantic Ocean and provides the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation. The results of a high-resolution ocean general circulation model now show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to a change in wind forcing. The transport of warm and salty Indian Ocean waters into the Atlantic Ocean—the Agulhas leakage—has a crucial role in the global oceanic circulation 1 and thus the evolution of future climate. At present these waters provide the main source of heat and salt for the surface branch of the Atlantic meridional overturning circulation (MOC) 2 . There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages 3 and model studies 4 that the amount of Agulhas leakage and its corresponding effect on the MOC has been subject to substantial change, potentially linked to latitudinal shifts in the Southern Hemisphere westerlies 5 . A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing 6 , but because of the sparse observational records it has not been possible to determine whether there has been a concomitant response of Agulhas leakage. Here we present the results of a high-resolution ocean general circulation model 7 , 8 to show that the transport of Indian Ocean waters into the South Atlantic via the Agulhas leakage has increased during the past decades in response to the change in wind forcing. The increased leakage has contributed to the observed salinification 9 of South Atlantic thermocline waters. Both model and historic measurements off South America suggest that the additional Indian Ocean waters have begun to invade the North Atlantic, with potential implications for the future evolution of the MOC.