Collaborative Research: Critical Layers and Isopycnal Mixing in the Southern Ocean

气候尺度的海洋模型一致强调海洋颠覆环流在十年和更长时间尺度上对地球气候和生物地球化学循环所起的关键调节作用。然而，在寻求解决许多地区性气候问题的过程中，这些模型的可信度受到了挑战，因为它们对南大洋混合过程的表现极为敏感。模型行为的这种特殊性反映了混合在调解南极绕极流(ACC)水团垂直和水平输送中的独特作用，这些水团通过各自对倾覆率和洋际交换的影响来塑造倾覆环流。美国国家科学基金会最近资助了南大洋的双水和等水混合实验(DIMES)，以直接测量ACC中密度面上的涡旋混合。涡旋混合将通过在一个垂直水平释放一个化学示踪剂和150个漂浮物来测量。当时还没有的新的理论结果表明，在垂直方向的临界水平上，涡旋混合率得到了强烈的增强。该项目的目标是扩大DIMES项目，并在较浅的水平额外释放50个浮标，以检验临界水平控制南大洋水团上升和下降速度的假设。全球经向翻转的概念模型和对未来气候的数值预测对用于表示沿ACC和横跨ACC的混合的方法非常敏感。理论表明，混合率在水平和垂直方向上变化很大。气候海洋模型一致强调，模型技能对这些变化非常敏感。DIMES项目将提供对南大洋混合的首次直接观测，并可能提供关于该海洋这一部分涡旋输送的大量新信息。然而，示踪剂和漂浮物的部署只计划在一个层面上进行，不会提供有关涡旋混合的垂直可变性的信息。将在浅水层部署的另外50个花车将利用这一机会了解南大洋涡旋混合的一个关键方面。这一提议具有潜在的广泛影响，因为它旨在促进我们对气候系统的一个核心组成部分的理解。拟议的工作还将有助于改进MITgcm等大型海洋模型中的混合和搅拌。最后，通过培训一名研究生和一名博士后，以及开发新的课程，向麻省理工学院/世界卫生组织联合项目的学生介绍南大洋在气候系统中的作用，具有很强的教育作用。

Climate-scale ocean models unanimously stress the key regulatory function played by the oceanic overturning circulation in the Earth's climate and biogeochemical cycles over decadal and longer time scales. Yet in their quest to resolve many topical climate problems, the models credibility is challenged by their extreme sensitivity to the representation of mixing processes in the Southern Ocean. This peculiarity of model behavior reflects the unique role of mixing in mediating the vertical and horizontal transports of water masses in the Antarctic Circumpolar Current (ACC), which shape the overturning circulation through their respective impacts on the overturning rate and inter-ocean exchange. The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was recently funded by NSF to measure directly eddy mixing along density surfaces in the ACC. Eddy mixing will be measured by releasing a chemical tracer and 150 floats at one vertical level. New theoretical results, not available at the time, suggest that eddy mixing rates are strongly enhanced at critical levels in the vertical. The goal of this project is to extend the DIMES project and release 50 additional floats at a shallower level to test the hypothesis that critical levels control the rate of upwelling and downwelling of water masses in the Southern Ocean.Intellectual Merit. Conceptual models of global meridional overturning and numerical predictions for future climate are strongly sensitive to the methods used to represent mixing along and across the ACC. Theory suggest that mixing rates vary greatly in the horizontal and in the vertical. Climate ocean models unanimously stress that model skill is strongly sensitive to these variations. The DIMES project will provide the first direct observations of mixing in the Southern Ocean and will likely deliver a wealth of new information about eddy transport in this part of the ocean. However tracer and float deployments are planned only at one level and will not provide information about the vertical variability of eddy mixing. The additional 50 floats to be deployed at ashallower level will capitalize on the opportunity to learn about a key aspect of eddy mixing in the Southern Ocean.Broader Impacts. This proposal has potentially wide impact because it is designed to further our understanding of a central component of the climate system. The proposed work will also contribute to the improvement ofmixing and stirring in large-scale ocean models such as the MITgcm. Finally, there is strong educational component through the training of a graduate student and a post-doc, and the development of new curricula to introduce students in the MIT/WHOI Joint Program to the role of the Southern Ocean in the climate system.