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地-副热带过渡带被认为是大西洋子午线翻转环流(AMOC)在十年时间尺度上变化的潜在关键。在AMOC的这个十字路口，南行的深西部边界流(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性质。事实证明，全球颠覆的环流--AMOC是其中的一个重要组成部分--比上世纪80年代首次被比作一艘巨大的海洋运输船时可能想象的要复杂得多。随着对其结构和可变性的直接连续观测在几个纬度地区持续多年甚至数十年，在发展对AMOC对各种强迫的反应的机械理解方面的进展正在加快。令人感兴趣的是，在北大西洋高纬度产生的水团异常沿西部边界向南发展，特别是对于横跨亚极点-亚热带过渡带的溢流水。最近观测到的26°N的AMOC强度下降部分归因于西部边界的溢流水密度异常。追踪这些异常的上游来源和路径，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的路径。浮标测量将填补模型评估所需观测中的一个关键空白，并有助于对溢出水从亚极地向亚热带北大西洋转移时影响其性质的路径和过程有新的了解。AMOC的可变性与一长串具有社会相关性的气候影响有关。美国十字路口的重点是一个地区，该地区已被提议“在十年时间尺度上设定AMOC的变化性”，因此其影响超出了地区进程研究的范围。美国十字路口与两个由欧洲牵头的项目高度协同，这两个项目的目标相似，工具相辅相成，即“解释和预测颠覆循环”(EPOC)和“法国十字路口”。与欧洲项目的合作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，该博士后将在世卫组织和FSU花费时间，以获得在观察和模型输出分析方面的经验。该项目还将扩展为盲人和视障学生提供的长期推广项目Ocean Insight，支持开发一种可访问的“远程实地考察工具包”，以帮助在STEM中代表性较低的群体培养对海洋科学的兴趣和兴奋。到目前为止，最受关注的是拉布拉多海水(LSW)出口及其对AMOC变异性的影响，揭示了跨越亚极-亚热带边界的弱连通性。模拟的粒子轨迹表明，与LSW相比，通过DWBC通过过渡区的更深的溢流水(较低的NADW)具有更好的连通性，但在溢流水中缺乏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，利用最先进的、高分辨率的、数十年的北大西洋模拟，对溢流水道进行新的浮动观测，并进行新的粒子模拟。总共80个声学跟踪的浮标将被释放在DWBC的溢流水中，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。数量级的更多模拟粒子轨迹将使用1/12°(6公里)和1/50°(1.5公里)配置的HYCOM(混合坐标海洋模式)北大西洋模拟来放大必要有限的浮动观测，测试溢出水路径对模型分辨率的敏感性，并调查这些路径中与NAC和AMOC变化相关的年代际变化。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.