Collaborative Research: Enhancing our Understanding of North Atlantic Deep Water Pathways using Nonlinear Dynamics Techniques

在上个世纪的大部分时间里，预计来自拉布拉多海和北欧海高纬度地区的冷水团向赤道的扩散将主要沿着北大西洋亚极地和亚热带的深西部边界流进行。自本世纪初以来，在这些水体中发射的观测浮标打破了这一预期。相反，北大西洋已经发现了无数的内部通道。与此同时，一份人为二氧化碳的清单显示，亚极地北大西洋是所有海洋盆地中（每单位面积）最强烈的碳汇，这一特征可归因于该盆地中新形成的水团的深度渗透。虽然大西洋经向翻转环流的深层分支长期以来一直被认为是一个热量和淡水储存库，但它作为碳储存库的作用现在是显而易见的。因此，在对这些深水团的传统认识被颠覆的时候，比以往任何时候都更有理由了解这些水团的分布和命运。在过去的十年中，模拟的漂浮物轨迹增加了北大西洋相对较少的观测漂浮物，以便对水团路径有更广泛的了解。然而，对观测轨迹和模拟轨迹的分析主要使用常规方法来确定有限数量的流动特性。本研究将利用非线性动力学的最新进展，以便对这些有限观测的水质量和流量提供更全面的描述和理解。因此，通过使用这些工具来解开深水通道，该项目将有助于我们了解北大西洋是一个深层碳库，从而产生重大的社会影响。这项拟议工作的更广泛影响还包括培养一名博士后研究员，以促进独立的研究事业，以及培养一名物理海洋学学生使用非线性动力学工具。本项目将通过使用非线性动力学和流体动力学界面上出现的两个互补工具来解决现代物理海洋学中的一个基本问题。概率工具将用于通过拉格朗日地理的构建来揭示深水的转变和命运，拉格朗日地理限制了连通性、在水团省内的停留时间、首选循环路径、沿着这些路径的运输时间以及跨省边界的运输。确定性工具最近被证明在提取持久的运输途径方面是有效的，也将用于描绘首选的运输途径，从而将用于构建深水路线。这为传统的物理海洋学研究领域提供了一个有前途的非线性动力学工具。具体来说，这项工作将应用动力系统工具来：1)确定深水团的长期命运并阐明优选路径；2)确定势涡守恒或其他动力学对这些路径的限制程度；3)确定界定水团居住的空间省（域），并评价它们之间的交换。该分析将使用少量但数量不断增加的观测浮标，包括最近从亚极地北大西洋翻转计划（opsnap）中恢复的浮标，以及Argo数据和海洋环流模型模拟的浮标轨迹。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

For most of the last century, the equatorward spread of cold water masses from high latitudes in the Labrador and Nordic Seas was expected to be largely contained along the Deep Western Boundary Current in the subpolar and subtropical North Atlantic. Since the turn of this century, observational floats launched within these water masses have defied this expectation. Instead, myriad interior pathways have been revealed across the North Atlantic. Contemporaneously, an inventory of anthropogenic carbon dioxide has shown the subpolar North Atlantic to be the most intense (per unit area) sink of all ocean basins, a characteristic attributable to the deep penetration of newly-formed water masses in that basin. While the deep limb of the Atlantic Meridional Overturning Circulation has long been appreciated as a heat and freshwater reservoir, its role as a carbon reservoir is now apparent. Thus, at a time when the conventional understanding of these deep water mass pathways has been upended, there is a stronger reason than ever to understand the spread and fate of these water masses. Over the past decade, simulated float trajectories have augmented the relatively small number of observational floats in the North Atlantic in order to gain a broader understanding of water mass pathways. However, analyses of the observed and modeled trajectories have largely used conventional methods to ascertain a limited number of flow characteristics. This study will capitalize on recent advances in nonlinear dynamics in order to provide a more comprehensive description and understanding of these water masses and flows limited observations. Thus, by using these tools to unravel deep water pathways, this project will aid our understanding of the North Atlantic as a deep carbon reservoir, and thereby have a significant societal impact. Broader impacts with this proposed work also include the training of a postdoctoral researcher to facilitate an independent research career and the training of a physical oceanography student in the use of nonlinear dynamical tools. This project will address a fundamental question in modern physical oceanography through the use of two complementary tools emerging at the interface of nonlinear dynamics and fluid dynamics. Probabilistic tools will be used to cast new light on the transformation and fate of the deep waters through the construction of Lagrangian geographies that constrain connectivity, residence times within water mass provinces, preferred circulation pathways, transit times along these pathways, and transport across province boundaries. Deterministic tools that have recently proven efficient at extracting persistent transport pathways will also be used to delineate preferred transport pathways, and thus will be used to frame deep water routes. This offers a promising fertilization of nonlinear dynamical tools into a traditional physical oceanographic area of study. Specifically, this work will apply dynamical systems tools to: 1) determine the long-term fate and elucidate the preferred pathways of the deep water masses; 2) ascertain the extent to which potential vorticity conservation or other dynamics constrains these pathways; and 3) identify spatial provinces (domains) that define water mass residence and evaluate exchanges among them. The small, but growing number of observational floats, including those recently recovered from the Overturning in the Subpolar North Atlantic Program (OSNAP), as well as Argo data and simulated float trajectories from ocean general circulation models, will be used in this analysis.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.