Following the Trail of Fukushima Tracers: Examining 7 Years of North Pacific Mode Water Evolution using Radionuclide Observations and High-Resolution Simulations

2011年，日本北部福岛第一核电站（FDNPP）发生事故，导致大量放射性同位素泄漏。当含有这些示踪剂的水在太平洋上扩散时，它提供了一个独特的机会来观察模态水的转变和演变，模态水是内部海洋典型分层结构中几乎均匀性质的厚层。通过对北太平洋盆地7年的观测收集到的放射性同位素测量数据，将与同位素测量同时测量的其他水特性相结合，或者通过Argo自主剖面浮标网络进行测量，并辅以高分辨率数值模型的模拟，以研究2011年形成的同位素标记水团的输送路径和三维分布。这项研究超越了不同程度的放射性传播的距离和速度的问题，利用这种独特的示踪剂可以提供的知识，了解潜在的物理过程，以及北太平洋模式的水在运输、混合和储存它们在形成时从大气和周围环境中获得的特征方面的作用。关于盆地尺度上层海洋混合和平流过程的模式和时间的结论将与那些寻求了解海洋热含量、营养物质、生物生产、二氧化碳吸收和海洋酸化的人有关。该研究将产生4个与科学相关的影响：1)提高对四维海洋运输过程的理解；2)阐明模式水在物理、生物、化学循环以及最终的气候变化中的作用；3)评估高分辨率数据同化模型的可靠性；4)为北太平洋未来的放射性核素取样提供见解。该项目将继续支持三名项目负责人，他们将各自不同的专业知识和背景，包括理论、观测和基于模式的海洋学研究，组成一个团队。在项目的第二年，将指导并鼓励一名客座学生参与铯观测的物理分析和/或模拟实验。与过去一样，该小组将继续向WHOI暑期学生研究员提供潜在项目和指导，并指导科学博览会的学生。该项目将利用2011年至2018年期间在太平洋测量的放射性同位素（Cs 134、Cs 137和Sr 90，半衰期分别为~2、~30和~29年）的一次性释放所提供的信息，研究北太平洋模态水（NPMW）从西部形成到环流周围的演变。目标是对已有7年之久的国际全流域FDNPP观测到的放射性核素记录进行有针对性的分析，并结合其他同时观测到的水性质(例如温度(T)，盐度(S)，氧气，其他示踪剂，速度等，以及来自剖面浮标的T &amp； S)，并辅以基于最先进的高分辨率数据同化模型的数值模拟，以研究带有FDNPP放射性核素信号标记的NPMW的四维运输途径和演变。具体而言，分析将检查：a)在7年的过程中，这些水域保持其关键海洋学特性的程度，特别是其潜在密度和潜在涡量；b) NPMW的移动受沿等压线平流约束的程度；C)在从地表到1000 m的一系列潜在密度范围内的潜流混合强度；D)各种模式水的起源、停留时间和命运；E)放射性核素释放的时间对信号的最终流域尺度格局的重要性；f)高分辨率模式在多大程度上可以在年际到年代际的时间尺度上再现太平洋这些水团的四维运动和演变。这些科学目标旨在扩展先前研究的发现，这些研究利用了2011年FDNPP意外向海洋注入放射性核素的独特铯特征，以更好地了解北太平洋西部和东部的循环途径和混合。在这项新研究中，在以前的努力中磨光的工具将用于扩大重点，包括北太平洋盆地的全部范围，只要有FDNPP铯观测数据，并通过包括最近公开的许多数据集，及时推进到事故发生后的7年。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。

An accident at the Fukushima Dai-ichi nuclear power plants (FDNPP) in northern Japan resulted in the release of large quantities of radio-isotopes in 2011. As the water containing these tracers spreads across the Pacific Ocean, it presents a unique opportunity to view the transformation and evolution of mode waters, a thick layer of nearly homogenous properties within the typically layered structure of the interior ocean. The collected radio-isotope measurements from 7-years of observation across the N Pacific basin will be combined with other water properties either measured simultaneously with the isotope measurements or by the Argo network of autonomous profiling floats and augmented with simulations from a high-resolution numerical model to investigate the transport pathways and 3-D distribution of the isotope-tagged water masses formed in 2011. The study moves beyond the question of how far and fast various levels of radioactivity have spread, to employing the knowledge this unique tracer can provide about underlying physical processes and the role of North Pacific mode waters in transporting, mixing and storing the characteristics they gain from the atmosphere and surrounds at the time of formation. Conclusions drawn about the patterns and timing of basin-scale upper-ocean mixing and advective processes will be relevant to those seeking to understand ocean heat content, nutrients, biological production, CO2 uptake and ocean acidification. The study will have 4 science-related impacts: 1) improved understanding of ocean transport processes in 4-D; 2) clarifying the role of mode waters in physical, biological, chemical cycles, and ultimately, climate change; 3) assessing the reliability of a high-resolution data-assimilating model; and 4) providing insight for future radionuclide sampling in the North Pacific. This project will continue to support three PIs who have formed a team combining their varied expertise and backgrounds including theoretical, observational and model-based oceanographic research. During the 2nd year of the project, a guest student will be mentored and encouraged to participate in the physical analysis of cesium observations and/or a simulation experiment. The group will continue, as in the past, to offer potential projects and mentoring to WHOI Summer Student Fellows and mentor science fair students. This project will use the information provided by the one-time release of radio-isotopes (Cs 134, Cs 137, and Sr 90 with ~2, ~30 and ~29 year half-lives, respectively), as measured across the Pacific between 2011 and 2018 to investigate the evolution of North Pacific mode waters (NPMW) from their formation in the west to their circulation around the gyres. The goal is to carry out a targeted analysis of the now 7-year-long international basin-wide FDNPP observed record of radionuclides, combined with other simultaneously observed water properties (e.g. temperature (T), salinity (S), oxygen, other tracers, velocity etc. as well as T & S from profiling floats) and augmented with numerical simulations based on a state-of-the-art high-resolution data assimilating model to investigate the 4-dimensional transport pathways and evolution of NPMW tagged with the FDNPP radionuclide signal. In particular, the analysis will examine: a) the extent to which these waters keep their key oceanographic properties, specifically their potential density and potential vorticity, over the course of 7 years; b) the extent to which the movement of NPMW are constrained by along-isopycnal advection; c) the intensity of diapycnal mixing across a range of potential densities from the surface to 1000 m; d) the origins, residence times, and fate of the various varieties of mode waters; e) the importance of the timing of the radionuclide discharge to the eventual basin scale patterns of the signal; and f) the extent to which the high resolution model can be trusted to reproduce the 4-dimensional movement and evolution of these water masses on interannual to decadal time scales in the Pacific Ocean. These scientific goals seek to extend the findings from prior research that has employed the unique cesium signature from the 2011 FDNPP accidental injection of radionuclides to the ocean to better understand circulation pathways and mixing in the western and eastern North Pacific. In this new study, the tools honed in previous efforts will be used to expand the focus to include the full breadth of the North Pacific Basin wherever FDNPP cesium observations are available and to press forward in time to 7 years after the accident through the inclusion of the many data sets recently made publicly available.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.