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

南极环极流（ACC）是气候系统中至关重要的海洋组成部分，连接着所有主要的海洋盆地。正如它的名字所暗示的那样，ACC绕着地球旋转，当它穿过南极洲周围的南大洋时，它不受陆地的干扰。在许多方面，ACC与同样围绕地球运行的大气喷射流相似，人们经常将大气和海洋中这种东西向的流体运动进行比较。奇怪的是，尽管南大洋的风越来越强，但在过去的30年里，太平洋环流输送的水量一直保持不变。有一种机制，被称为“涡流饱和”，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，他们不包括涡旋饱和度，并给出与观测不符的ACC预测。最近的卫星观测表明，南大洋的漩涡和ACC的射流分支具有有趣而复杂的行为。我们需要了解如何将这种行为纳入气候模型，以改善全球气候预测。尽管超级计算机的能力越来越强，但在未来5-10年里，气候模型仍无法直接模拟海洋涡旋和喷流，因此我们需要寻找间接的模拟方法。除了“涡旋饱和”之外，还有另一种机制是最近才观测到的，与气候模拟高度相关。这是在ACC中存在的“动力屏障”，它通过涡流控制水的传输和混合。在某些时候和某些地方，漩涡可以将含有热量、盐、生物营养物质和溶解的二氧化碳的水从南极带输送到南极或从南极输送到南极。在其他时间和地点，涡流无法进行这种转移。了解海洋中这种动力障碍的本质是海洋学的前沿，将对我们对全球气候和气候变化的理解产生巨大影响。幸运的是，ACC和大气急流之间的类比可以用来帮助理解ACC中动力障碍的特性。在20世纪80年代，当人们对大气臭氧空洞非常感兴趣时，流体动力学专家发展了理论来解释为什么有时大气涡旋（天气系统）能够在臭氧空洞的急流边界上混合臭氧，而在其他时候却不能。利用位涡量（PV）提供动力学洞察力，制定了与臭氧空洞相关的动力屏障存在和性质的理论。直到2008年，才有了一个完整的理论来描述地球和木星大气中动力障碍和多重喷流的持久性。这个理论告诉你传输和混合可能发生的时间，但不包含可能发生的地点的信息，因为大气被认为是在东西方向上相当均匀的。我建议将势涡阶梯理论应用于南极绕极流，对其进行调整，以提供有关涡旋可以在洋流中运输和混合的时间和地点的信息。我将扩展我们支持PV阶梯模型的初步分析，使用最先进的海洋模型和观测的互补组合。我将根据PV和PV阶梯来量化南大洋风、涡旋饱和机制和ACC的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并影响全球气候。

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.