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

气候尺度海洋模式一致强调海洋翻转环流在十年和更长时间尺度上对地球气候和地球化学循环的关键调节作用。然而，在寻求解决许多局部气候问题的过程中，模式的可信度受到了挑战，因为它们对南大洋混合过程的代表性极其敏感。模型行为的这种特性反映了混合在调节南极绕极流（ACC）中水团的垂直和水平输送方面的独特作用，这些水团通过各自对翻转率和海洋间交换的影响形成翻转环流。美国国家科学基金会（NSF）最近资助了南大洋纵贯和等密度混合实验（DIMES），以直接测量ACC中沿着密度面的涡动混合。涡动混合将通过释放化学示踪剂和在一个垂直高度上的150个浮子来测量。新的理论结果，当时还没有，表明涡流混合率强烈增强在垂直的临界水平。该项目的目标是扩大DIMES项目，并在较浅的水平面释放50个额外的浮子，以检验临界水位控制南大洋水团上升和下降速度的假设。全球纬向翻转的概念模型和未来气候的数值预测对用于代表沿沿着和横跨ACC的混合的方法非常敏感。理论表明，混合速率在水平和垂直方向上变化很大。气候海洋模型一致强调，模型技术是强烈敏感的这些变化。DIMES项目将提供南大洋混合的首次直接观测，并可能提供有关这一部分海洋涡流输送的大量新信息。然而，示踪剂和浮子的部署仅计划在一个高度上，不会提供有关涡流混合的垂直变化的信息。另外50个浮标将部署在较浅的水面上，将利用这个机会了解南大洋涡流混合的一个关键方面。这一提议具有潜在的广泛影响，因为它旨在加深我们对气候系统核心组成部分的理解。拟议的工作也将有助于改善ofmixing和搅拌的大尺度海洋模型，如MITgcm。最后，通过培训一名研究生和一名博士后，以及制定新的课程，向麻省理工学院/世界卫生组织联合方案的学生介绍南大洋在气候系统中的作用，有很强的教育成分。

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.