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

摘要

“大西洋经向翻转环流”(AMOC)今后减弱的rn可能性在关于气候变化的讨论中居中心地位。rn人们的关注点集中在亚北极北大西洋海水淡化rn所产生的效应上，比如说通过高纬度降水的增rn加及冰的融化所造成的海水淡化。一项新的高rn分辨率海洋模型研究表明，北大西洋已经在经rn受来自南方的相反效应。作为对“西风南偏”现rn象(很可能是人类活动造成的影响)的响应，水rn温较高、咸度较大的印度洋海水绕非洲之角的rn输送正在增强。这股被称为Agulhas Leakagern的洋流会抵消掉北大西洋中淡水输入造成的rn大西洋深水环流的可能减少，从而有可能对rnAMOC系统、包括“墨西哥湾流”的稳定性做rn出贡献。本期封面图片所示为一个高分辨率模rn型中400米深处的温度和水流，该模型嵌入在rn一个全球海洋／海冰模型中。%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 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). There is evidence from past glacial-to-interglacial variations in foraminiferal assemblages and model studies 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. A progressive poleward migration of the westerlies has been observed during the past two to three decades and linked to anthropogenic forcing, 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 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 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.