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

最近的研究表明，在人为气候变化的影响下，阿古拉斯泄漏可能会增加，这将加强大西洋翻转环流，而人为变暖和相关的冰融化预计会削弱它。 然而，耦合气候模式，包括IPCC模式，并没有解决控制厄古拉斯泄漏及其对大西洋翻转环流的影响的动力学问题，因此在预测未来气候的能力上可能存在缺陷。厄古拉斯系统及其从印度洋-太平洋向大西洋的暖盐水泄漏是全球温盐环流的一个瓶颈，通过厄古拉斯环、漩涡和细丝内温跃层沃茨的非线性交换，为强大而深层的大西洋翻转环流的上臂提供营养。 耦合气候模式不能正确模拟渗漏，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翻转的影响，因为未解决的涡旋。 这意味着风、泄漏和翻转环流之间的相互作用可能在这些模型中存在缺陷。 最后，几乎所有的耦合气候模式都表现出向北翻转的淡水输送，而水文观测则指向向南。 这影响了模式中翻转环流的敏感性和稳定性，并且与厄古拉斯和南极输入南大西洋之间的平衡有关。将使用CCSM 4运行具有十分之一海洋分辨率的20世纪耦合气候模拟。 会议将首次审查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）诊断在有涡旋存在的情况下，与较粗分辨率的IPCC模式相比，Agulhas泄漏和翻转环流对风场变化的敏感性：（2）分析Agulhas海流惯性、泄漏强度、副热带风应力旋度和西风最大值纬度之间的关系，以检验泄漏的约束条件以及它们是否与理论模式预测的一样;（3）在没有直接风效应的情况下，将大西洋翻转对Agulhas泄漏的响应分离出来;（4）考虑进入翻转表面臂的冷与暖水路径的平衡，它与泄漏的相关性，以及它对翻转稳定性的影响。 该项目的一个更广泛的影响是，在存在已解决的Agulhas泄漏的情况下，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）在倾覆强度和敏感性方面的预测能力提供定性不确定性估计。 该项目还将支持一名研究生和一名博士后，他们将由一名实地海洋学家和一名气候建模师指导。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.