The Antarctic Circumpolar Current: a Fractured Transport Barrier

近年来，残差平均理论在南大洋的应用极大地提高了我们对涡旋输送如何控制南大洋南极环极流（ACC）分层和经向翻转的认识。然而，该模型牢牢扎根于二维或纬向整合的框架，这与最近南大洋研究的一个主要主题背道而驰：ACC动力特性的很大程度的纬向不对称性。最近的研究结果表明，涡旋-平均流相互作用是形成和维持ACC的特征性非均质锋面结构的主要因素，在ACC的路径上存在显著差异，不同区域之间的过渡很大程度上受地形控制。因此，越来越多的证据表明，南大洋的纬向平均模式不足以解决对运输和倾覆率的控制。这个项目有两个目标。一是量化和动态描述南大洋涡旋热和位涡通量的区域变率。特别令人感兴趣的是ACC锋面垂直结构的转变及其在地形附近作为运输障碍的有效性。在这个项目中，假设南大洋的经向输送发生在由流动与地形相互作用决定的离散位置，这一假设将得到检验。这种关于ACC的离散观点的一个主要分支是全球输运性质对局部强迫变化的潜在敏感性。第二个目标是通过进行一套过程研究模型，提供更好的流动-地形相互作用的动态描述。从涡旋通量分布中获得的见解将用于开发和测试预测这些“运输走廊”空间范围的尺度论证。这项研究还将考虑这种局部行为如何响应强迫条件的变化。知识价值：遥感技术和计算能力的进步意味着，在过去十年中，模拟和观测南大洋的能力取得了重大进展。从模式和观测中获得的见解都强调了ACC内部水文结构和动力行为的异质性。然而，目前对控制这种区域变异的机制的理解仍然不发达。由于南大洋是全球环流系统中水团交换和水团改变的主要场所，记录南大洋环流中输送和混合过程的空间分布对于理解其在气候系统中的作用至关重要。该项目试图超越南大洋翻转的纬向平均观点，目标是将我们对ACC的基本动力学理解与观测数据和海洋全球气候模型相一致。更广泛的影响：这个项目将有助于我们了解南大洋环流。由于该区域通常是海洋环流模式约束最差的方面，从系统过程研究建模方法中获得的见解将有助于指导涡旋海洋环流模式的改进和这些模式结果的解释，这些模式可能对垂直分辨率和地形特征的表示很敏感。该项目将为一名博士后研究员提供培训，他将有机会在加州理工学院和夏威夷大学两个令人兴奋的环境中工作。在分析高分辨率海洋模型结果的过程中，与日本JAMSTEC的同事进行国际合作也是该项目的一个重要方面。

In recent years, the application of residual mean theory to the Southern Ocean has greatly improved our understanding of how eddy transport controls the stratification and meridional overturning of the Southern Ocean's Antarctic Circumpolar Current (ACC). Yet, this model is firmly rooted in a two-dimensional, or zonally-integrated, framework, which runs counter to a major theme of recent Southern Ocean research: the large degree of zonal asymmetry in dynamical properties of the ACC. Recent results have shown that eddy-mean flow interactions, which are responsible for generating and sustaining the characteristic heterogeneous frontal structure of the ACC, vary significantly along the path of the ACC with transitions between different regions largely controlled by topography. Thus there is increasing evidence that zonally-averaged models of the Southern Ocean are insufficient to resolve controls on transport and overturning rates. The goals of this project are two-fold. The first is to quantify and dynamically describe the regional variability of eddy heat and potential vorticity fluxes in the Southern Ocean. Of particular interest are transitions in the vertical structure of the ACC fronts and their effectiveness as transport barriers near topography. In this project, the hypothesis, that meridional transport in the Southern Ocean occurs in discrete locations determined by flow interactions with topography will be tested. A major ramification of this discrete view of the ACC is the potential sensitivity of global transport properties to local forcing changes. The second goal is to provide a better dynamical description of flow-topography interactions by conducting a suite of process study models. Insight gained from the eddy flux distributions will be used to develop and test scaling arguments that predict the spatial extent of these "transport corridors." This study will also consider how this localized behavior responds to changes in forcing conditions. Intellectual Merit: Advances in remote sensing techniques and in computational power have meant that the ability to model and observe the Southern Ocean has progressed significantly over the past decade. Insight gained from both models and observations have emphasized the heterogeneity of the hydrographic structure and dynamical behavior within the ACC. Yet, the current understanding of the mechanisms that control this regional variability remains underdeveloped. As the Southern Ocean is the primary site of water mass exchange and water mass modification in the global circulation system, documenting the spatial distribution of transport and mixing processes in the ACC is essential for understanding its role in the climate system. This project attempts to move beyond the zonally-averaged view of Southern Ocean overturning with the goal of bringing our fundamental dynamical understanding of the ACC in line with both observational data and ocean global climate models.Broader Impacts: This project will contribute to our understanding of Southern Ocean circulation. As this region is typically the most poorly constrained aspect of ocean general circulation models, insight gained from a systematic process-study modeling approach will help to guide improvements in eddying ocean circulation models and interpretation of results from such models, which may be sensitive to vertical resolution and representation of topographic features. The project will provide training to a post-doctoral researcher who will have the opportunity to work in two stimulating environments at Caltech and the University of Hawaii. The international collaboration with colleagues at JAMSTEC in Japan during analysis of their high-resolution ocean model results is also an important aspect of the project.