The structure and stability of transport and mixing barriers within the Antarctic Circumpolar Current

南极绕极流(ACC)是气候系统的重要海洋组成部分，连接着所有主要的海洋盆地。顾名思义，ACC环绕地球，在穿越南极洲周围的南大洋时不受陆地干扰。在许多方面，ACC类似于同样环绕地球的大气急流，人们经常将这些大气和海洋中流体的东西运动进行比较。奇怪的是，尽管南大洋的风越来越大，但ACC输送的水量在过去30年里一直保持不变。有一种机制被称为“涡流饱和”，它与海洋涡流有关，可以解释这种奇怪的行为。海洋涡旋相当于大气天气系统，但它们要小得多。整个地球系统气候的计算机模型无法模拟这些小的海洋涡流，因为它们受到目前超级计算机能力的限制。因此，它们不包括涡旋饱和，并给出了与观测不一致的ACC预测。最近来自卫星的观测表明，南大洋涡旋和行政协调委员会的喷气式分支具有有趣和复杂的行为。我们需要了解如何将这一行为纳入气候模型，以改进全球气候预测。尽管超级计算机的能力正在增强，但气候模型在未来5-10年内仍无法直接模拟海洋涡流和喷流，因此我们需要找到间接模拟的方法。除了“涡旋饱和”，还有另一种机制，它是最近才观察到的，与气候模型高度相关。这是在ACC内部存在的“动力屏障”，它控制着涡流对水性质的输送和混合。在某些时候和某些地方，涡流可以将含有热量、盐、生物营养物质和溶解的二氧化碳的水通过ACC输送到南极洲或从南极洲离开。在其他时间和地点，涡旋无法进行这种转移。了解海洋中这种动力屏障的性质是海洋学的前沿，将对我们理解全球气候和气候变化产生巨大影响。幸运的是，ACC和大气急流之间的类比可以用来帮助理解ACC中的动力学势垒的性质。在1980年代，当人们对大气臭氧空洞感兴趣时，流体动力学专家发展了理论来描述为什么大气涡流(天气系统)有时能够将臭氧混合到臭氧层空洞的喷流边界上，而在另一些时候却不能。使用一个称为位涡(PV)的量来提供动力学洞察，建立了与臭氧空洞相关的动力屏障的存在和性质的理论。直到2008年，才发展出一个完整的理论来描述地球和木星大气中的动力学障碍和多个喷流的持久性。这一理论告诉你可能发生输送和混合的时间，但不包含可能发生的地方的信息，因为大气在东西方向上被认为是相当均匀的。我建议将势涡阶梯理论应用于南极绕极洋流，对其进行修改，以提供有关涡旋可以在洋流中输送和混合的时间和地点的信息。我将扩展我们的初步分析，支持PV楼梯模型，使用最先进的海洋建模和观测的补充组合。我将用PV和PV阶梯来量化南大洋风、涡旋饱和机制和ACC的树枝状结构之间的关系。我将确定动力障碍是否会影响涡旋饱和和影响全球气候。

The Antarctic Circumpolar Current (ACC) is a crucial ocean component of the climate system, linking all the major ocean basins. As its name suggests, the ACC circles the Earth and is uninterrupted by land as it crosses the Southern Ocean surrounding Antarctica. In many ways the ACC is similar to the atmospheric jet streams which also circle the Earth, and comparisons are often made between these east-west movements of fluid in the atmosphere and in the ocean. Strangely, the amount of water transported by the ACC has remained the same over the last 30 years, despite the winds in the Southern Ocean becoming stronger. There is a mechanism, known as 'eddy saturation', linked to ocean eddies, which can explain this curious behaviour. Ocean eddies are equivalent to atmospheric weather systems, but they are much smaller. Computer models of the full Earth System climate are unable to simulate these small ocean eddies because they are limited by the present power of supercomputers. Therefore they do not include eddy saturation and give predictions of the ACC which do not agree with observations. Recent observations from satellites show that Southern Ocean eddies and jet-like branches of the ACC have interesting and complex behaviour. We need to understand how to include this behaviour in climate models in order to improve global climate forecasting. Although supercomputers are increasing in power, climate models will remain unable to directly simulate ocean eddies and jets for the next 5-10 years, so we need to find indirect methods of simulation. As well as 'eddy saturation', there is another mechanism which is only recently observed and of high relevance to climate modelling. This is the presence of 'dynamical barriers' within the ACC which control the transport and mixing of water properties by eddies. At some times and in some places, eddies can transfer water containing heat, salt, biological nutrients and dissolved carbon dioxide across the ACC, either to or from Antarctica. At other times and places, the eddies are unable to make this transfer. Understanding the nature of such dynamical barriers in the ocean is at the forefront of oceanography and will have huge implications for our understanding of global climate and climate change. Fortunately, the analogy between the ACC and atmospheric jet streams may be used to help to understand properties of dynamical barriers in the ACC. In the 1980s, when there was much interest in the atmospheric ozone hole, experts in fluid dynamics developed theory to describe why at some times, atmospheric eddies (weather systems) were able to mix ozone across the jet stream boundary of the ozone hole, and at other times they could not. Using a quantity called Potential Vorticity (PV) to provide dynamical insight, theories for the existence and nature of the dynamical barrier associated with the ozone hole were formulated. Only in 2008, was a full theory developed to describe dynamical barriers and the persistence of multiple jets in the atmospheres of Earth and Jupiter. This theory tells you about the times when transport and mixing might happen, but does not contain information about the place it might happen, since the atmosphere is considered to be quite uniform in the east-west direction. I propose to apply this theory, the Potential Vorticity Staircase, to the Antarctic Circumpolar Current, adapting it to give information about both the time and the place that eddies can transport and mix across the current. I will extend our preliminary analysis, which supports the PV Staircase model, to use a complementary combination of state-of-the-art ocean modelling and observations. I will quantify the relationship between Southern Ocean winds, the eddy saturation mechanism and the branch-like structure of the ACC in terms of PV and the PV staircase. I will determine whether dynamical barriers can affect eddy saturation and influence global climate.