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

北大西洋西部的副极-亚热带过渡带被认为是大西洋经向翻转环流（AMOC）年代际变率的潜在关键。在AMOC的这个十字路口，南行的深层西部边界流（DWBC）与北行的北大西洋流（NAC）相遇，它们的相互作用决定了输出到大西洋其他地区和更远地区的深水的特性。全球翻转环流——其中AMOC是一个重要的组成部分——已被证明是一个远比20世纪80年代它第一次被比作一个巨大的海洋传送带时想象的要复杂得多的系统。随着对AMOC结构和变率的直接连续观测在几个纬度持续了数年甚至数十年，对AMOC对各种强迫的响应机制理解的进展正在加快。这里令人感兴趣的是在北大西洋高纬度地区产生的水团异常向南的进展，沿着西部边界，特别是跨越亚极地-亚热带过渡区的溢流水。最近观测到的26°N AMOC强度下降部分归因于西边界的溢流水密度异常。追踪这些异常的上游起源和路径，并在模型中真实地表示它们，是能够预测未来AMOC变化的关键。该项目将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出水的路径。浮子测量将填补模式评估所需观测的关键空白，并有助于对影响从亚极地到亚热带北大西洋的溢流水特性的途径和过程有新的认识。AMOC变异性与一系列具有社会意义的气候影响有关。美国十字路口研究的重点是一个地区，该地区已被提议“在十年时间尺度上设置AMOC的变化”，因此具有超出区域过程研究的意义。美国十字路口与两个同时由欧洲主导的项目高度协同，它们具有相似的目标和互补的工具，即“解释和预测推翻循环”（EPOC）和“法国十字路口”。与欧洲项目的伙伴关系将促进国际资源共享和思想交流。该项目还将支持一名博士后的培训和职业发展，他们将在世界卫生研究所和FSU学习，以获得分析观测结果和模型输出的经验。该项目还将扩展OceanInsight，这是一项针对盲人和视障学生的长期外展计划，支持开发无障碍的“远程实地考察工具包”，以帮助在STEM中代表性不足的群体培养对海洋科学的兴趣和兴奋。迄今为止，大部分注意力都集中在拉布拉多海水（LSW）出口及其对AMOC变率的影响上，揭示了亚极地-亚热带边界的弱连通性。模拟的粒子轨迹表明，与LSW相比，较深的溢出水（较低的NADW）通过DWBC通过过渡区具有更强的连通性，但溢出水缺乏等效的拉格朗日观测。该项目将建立在先前的观测和建模工作的基础上，采用新的浮子观测溢出水的路径，并使用最先进的、高分辨率的、数十年的北大西洋模拟进行新的粒子模拟。在DWBC的溢流水域中，将总共释放80个声学跟踪浮标，以测量它们通过过渡区的路径，并确定导致边界-内部交换的过程。使用HYCOM（混合坐标海洋模式）北大西洋模拟的1/12°（6公里）和1/50°（1.5公里）配置，将生成更多的模拟粒子轨迹，以扩大必要的有限浮子观测，测试溢出水路径对模型分辨率的敏感性，并研究与NAC和AMOC变化相关的这些路径的年代际变化。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。

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.