Towards Resolving the Role of Agulhas Leakage in 20th Century Global Climate Change

最近的研究表明，由于人为气候变化，阿古拉的渗漏可能会增加，这将加强大西洋的颠覆环流，而预计人为变暖和相关的冰川融化将削弱大西洋的颠覆环流。来自古海洋学指标、理论和模型的证据表明，阿古拉斯泄漏可能会在多个时间尺度上影响气候系统。然而，耦合的气候模式，包括IPCC模式，并不能解决控制Agulas泄漏及其对大西洋翻转环流的影响的动力学，因此它们对未来气候的预测能力可能存在缺陷。Agulas系统及其从印度-太平洋向大西洋泄漏的暖水和咸水是全球温盐环流的一个阻塞点，通过Agulhas环、涡旋和细丝内温跃层水的非线性交换，为强大而深的大西洋倾覆单元的上臂提供食物。耦合气候模型不能正确模拟泄漏，因为惯性机制和环的形成没有得到解决。气候模型也倾向于高估南半球西风对倾覆的影响，因为未解决的涡流。这意味着在这些模型中，风、泄漏和倾覆环流之间的相互作用可能是有缺陷的。最后，几乎所有的耦合气候模式都显示了向北的淡水输送，而水文观测则指向了向南的输送。这影响了模式中倾覆环流的敏感性和稳定性，并与南大西洋的阿古拉和南极输入之间的平衡有关。将使用CCSM4进行20世纪海洋分辨率为1/10的耦合气候模拟。我们将首次研究阿古拉斯系统及其渗漏如何对耦合气候系统中的人为强迫作出反应。该项目的目标是：(1)诊断在涡旋存在的情况下，与在较粗分辨率的IPCC模式中相比，阿古勒斯泄漏和颠覆环流对变化的风的敏感性，(2)分析阿古拉斯流惯性、泄漏强度、副热带风应力旋度和西风最大纬度之间的关系，以检验对泄漏的约束是否如理论模式所预测的那样；(3)在没有直风影响的情况下，分离大西洋倾覆对阿古拉斯渗漏的响应；(4)考虑进入倾覆表层臂的冷水和暖水路线的平衡及其与渗漏的关系，以及它对倾覆稳定性的影响。该项目的一个更广泛的影响是对粗分辨率耦合气候模型(特别是政府间气候变化专门委员会模型)在存在已解决的阿古拉泄漏时的倾覆强度和敏感性的预测能力提供定性的不确定性估计。该项目还将支持一名研究生和一名博士后，他们将得到一名实地考察的海洋学家和一名气候建模专家的指导。所有模拟数据将免费与社区共享，以供进一步分析。

Recent studies show that Agulhas leakage may be increasing in response to anthropogenic climate change, which would act to strengthen the Atlantic overturning circulation at a time when anthropogenic warming and associated ice melt are predicted to weaken it. Evidence from paleoceanographic proxies, theory, and models indicate that the Agulhas leakage may impact the climate system on a number of time scales. However, coupled climate models, including the IPCC models, do not resolve the dynamics that control Agulhas leakage and its impact on the Atlantic overturning circulation, and hence may be flawed in their ability to predict future climate.The Agulhas system and its leakage of warm and salty water from the Indo-Pacific into the Atlantic represents a choke point in the global thermohaline circulation, feeding the upper arm of the strong and deep Atlantic overturning cell via a non-linear exchange of thermocline waters within Agulhas Rings, eddies, and filaments. Coupled climate models do not simulate leakage correctly, because inertial mechanisms and Ring formation are not resolved. Climate models also tend to overestimate the effect of the Southern Hemisphere westerly winds on overturning, because of unresolved eddies. This means the interplay between winds, leakage, and the overturning circulation may be flawed in these models. Finally, almost all coupled climate models exhibit a northward overturning freshwater transport, while hydrographic observations point to a southward one. This effects the sensitivity and stability of the overturning circulation in the models, and is linked to the balance between Agulhas and Antarctic inputs into the South Atlantic.A coupled climate simulation of the 20th century with one tenth degree ocean resolution will be run using the CCSM4. For the first time, the Agulhas System and how its leakage responds to anthropogenic forcing in a coupled climate system will be examined. The project objectives are to: (1) diagnose the sensitivity of Agulhas leakage and the overturning circulation to changing winds in the presence of eddies versus in coarser resolution IPCC models; (2) analyze the relationship between Agulhas Current inertia, leakage strength, subtropical wind stress curl, and the latitude of the westerly wind maximum to test the constraints on leakage and whether they are as theoretical models predict; (3) isolate the response of Atlantic overturning to Agulhas leakage in the absence of a direct wind effect, and (4) consider the balance of the cold versus warm water routes into the surface arm of the overturning, its correlation with leakage, and its effect on the stability of the overturning. One of the broader impacts of this project is to provide qualitative uncertainty estimates of the predictive capability of coarse resolution coupled climate models (especially Intergovernmental Panel for Climate Change models) with respect to overturning strength and sensitivity in the presence of a resolved Agulhas leakage. This project will also support a graduate student and a postdoc who will be mentored by both a field-going oceanographer and a climate modeler. All simulation data will be freely shared with the community for further analyses.