Collaborative Research: U.S. Crossroads—Connectivity of the North Atlantic Deep Western Boundary Current through the Subpolar-Subtropical Transition Zone

合作研究：美国十字路口——北大西洋深西边界流通过副极地-副热带过渡区的连通性

• 批准号: 2318948
• 负责人: Xiaobiao Xu
• 金额: $ 46.96万
• 依托单位: Florida State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-10-01 至 2028-09-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2318948&HistoricalAwards=false
• 关键词: Collaborative; Research; U.S.; Crossroads; Connectivity

The subpolar-subtropical transition zone in the western North Atlantic has been identified as potentially key to Atlantic Meridional Overturning Circulation (AMOC) variability on decadal time scales. At this crossroads of the AMOC, the southbound Deep Western Boundary Current (DWBC) meets the northbound North Atlantic Current (NAC), and their interaction sets the properties of the deep waters exported to the rest of the Atlantic Ocean and beyond. The global overturning circulation—of which the AMOC is an important component—has proven to be a far more complex system than perhaps imagined when it was first likened to a great ocean conveyer in the 1980s. Progress in developing a mechanistic understanding of the response of the AMOC to various forcings is accelerating as direct continuous observations of its structure and variability are being sustained at a few latitudes for multiple years and even decades. Of interest here is the southward progression of water mass anomalies, generated at high North Atlantic latitudes, along the western boundary, particularly for overflow waters crossing the subpolar-subtropical transition zone. Recent observations of a decline in AMOC strength at 26°N have been attributed in part to overflow water density anomalies at the western boundary. Tracking down the upstream origin and pathways of these anomalies, and having them realistically represented in models, is key to being able to predict future AMOC changes. This project will measure the pathways of overflow waters with the deployment of 80 acoustically tracked floats and virtual floats in a high-resolution model. The float measurements will fill a critical gap in observations needed for model evaluation and contribute new understanding of the pathways and processes impacting the properties of overflow waters as they transit from the subpolar to subtropical North Atlantic. AMOC variability has been associated with a long list of climate impacts with societal relevance. U.S. Crossroads is focused on a region that has been proposed to "set the variability of the AMOC on decadal time scales," and therefore has implications beyond a regional process study. U.S. Crossroads is highly synergistic with two concurrent European-led programs with similar aims and complementary tools, namely "Explaining and Predicting the Overturning Circulation" (EPOC) and "French Crossroads." The partnerships with European programs will foster international resource sharing as well as exchange of ideas. The project will also support training and career development for a post-doc, who will spend time at both WHOI and FSU to gain experience in the analysis of both observations and model output. The project will also extend OceanInsight, a long-standing outreach program for blind and visually impaired students, with support for development of an accessible "remote field trip kit" to help foster interest and excitement in ocean science for a group underrepresented in STEM.Most attention has to date been focused on Labrador Sea Water (LSW) export and its impact on AMOC variability, revealing weak connectivity across the subpolar-subtropical boundary. Modeled particle trajectories suggest a greater connectivity for the deeper overflow waters (lower NADW) passing through the transition zone via the DWBC compared to LSW, but equivalent Lagrangian observations in overflow waters are lacking. This project will build on previous observational and modeling work with new float observations of overflow water pathways and new particle simulations using state-of-the-art, high-resolution, multi-decade North Atlantic simulations. A total of 80 acoustically tracked floats will be released in overflow waters in the DWBC to measure their pathways through the transition zone and identify processes that lead to boundary-interior exchange. Orders of magnitude more modeled particle trajectories will be generated using 1/12° (6 km) and 1/50° (1.5 km) configurations of HYCOM (Hybrid Coordinate Ocean Model) North Atlantic simulations to amplify the necessarily limited float observations, test the sensitivity of overflow water pathways to model resolution and investigate decadal variability in those pathways related to NAC and AMOC variability.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.

北大西洋西部的副极地-副热带过渡区已被认为是十年时间尺度上大西洋经向翻转环流（AMOC）变化的潜在关键。在 AMOC 的这个十字路口，南行的深西边界流（DWBC）与北行的交汇。北大西洋洋流 (NAC) 及其相互作用决定了输送到大西洋其他地区及更远地区的深水的特性。全球翻转环流（AMOC 是其中的一个）。事实证明，它是一个比 20 世纪 80 年代首次被比作大型海洋运输机时想象的复杂得多的系统，随着直接的连续观测，对 AMOC 对各种力的响应的机械理解的进展正在加速。其结构和变异性在几个纬度持续了数年甚至数十年，这里令人感兴趣的是北大西洋高纬度地区沿西部边界产生的水团异常向南推进，特别是溢流水域。最近观察到的 26°N AMOC 强度下降的部分原因是西部边界的溢流水密度异常，并追踪这些异常的上游来源和路径。该项目将通过在高分辨率模型中部署 80 个声学跟踪浮标和虚拟浮标来测量​​溢流水域的路径。测量将填补模型评估所需观测的一个关键空白，并有助于对影响溢流水从副极地过渡到亚热带北大西洋的路径和过程提供新的认识，AMOC的变化与一系列气候影响有关。具有社会相关性。美国十字路口的重点是“在十年时间尺度上设定 AMOC 的变化”的地区，因此其影响超出了美国十字路口的区域进程研究的高度协同性。两个由欧洲主导的项目，即“解释和预测翻转循环”（EPOC）和“法国十字路口”，具有相似的目标和互补的工具。与欧洲项目的合作将促进国际资源共享和思想交流。还将支持博士后的培训和职业发展，博士后将在 WHOI 和 FSU 获得分析观测和模型输出的经验。该项目还将扩展 OceanInsight，这是一项长期的外展计划。盲人和视障学生，支持开发易于使用的“远程实地考察套件”，以帮助培养 STEM 中代表性不足的群体对海洋科学的兴趣和兴奋。迄今为止，大多数注意力都集中在拉布拉多海水 (LSW) 出口上及其对 AMOC 变化的影响，揭示了副极地-副热带边界的弱连通性。模拟粒子轨迹表明，与 DWBC 相比，通过过渡区的更深溢流水域（较低的 NADW）具有更大的连通性。 LSW，但缺乏溢流水域中等效的拉格朗日观测。该项目将建立在以前的观测和建模工作的基础上，使用最先进的、高分辨率的、数十年的新的浮子观测和新的粒子模拟。北大西洋模拟将在 DWBC 的溢流水域中释放总共 80 个声学跟踪浮标，以测量它们通过过渡区的路径并识别导致边界内部交换的过程。将使用 HYCOM（混合坐标海洋模型）北大西洋模拟的 1/12°（6 公里）和 1/50°（1.5 公里）配置生成轨迹，以放大必然有限的浮标观测结果，测试溢流水路径对模型分辨率并调查与 NAC 和 AMOC 变异相关的路径的十年变异性。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响进行评估，被认为值得支持审查标准。