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

2011年，日本北方的福岛第一核电站（FDNPP）发生事故，导致大量放射性同位素释放。 由于含有这些示踪剂的水遍布太平洋，它提供了一个独特的机会，以查看模式沃茨，一个厚厚的一层内的典型分层结构的内部海洋几乎均匀的属性的转变和演变。从7年的观测中收集到的放射性同位素测量数据将与其他水的性质相结合，这些水的性质要么与同位素测量数据同时测量，要么由Argo自主剖面浮标网络测量，并通过高分辨率数值模型的模拟进行增强，以调查2011年形成的同位素标记水团的运输途径和三维分布。这项研究超越了各种放射性水平扩散的距离和速度的问题，利用这种独特的示踪剂可以提供的关于潜在物理过程和北太平洋模式沃茨在运输中的作用的知识， 混合和储存他们从大气中获得的特性，并在周围的时间， 阵 得出的关于海盆尺度上层海洋混合的模式和时间的结论， 平流过程将与那些试图了解海洋热含量、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个与科学有关的影响：1）提高对四维海洋输运过程的理解; 2）澄清模式沃茨在物理、生物、化学循环以及最终气候变化中的作用; 3）评估高分辨率数据同化模式的可靠性; 4）为今后北太平洋放射性核素取样提供了参考。 该项目将继续支持三名PI，他们组成了一个团队，结合了他们不同的专业知识和背景，包括理论，观测和基于模型的海洋学研究。 在该项目的第二年，一名客座学生将被指导和鼓励参与铯观测和/或模拟实验的物理分析。 该小组将一如既往地继续为WHOI暑期学生研究员提供潜在的项目和指导， 指导科学展的学生该项目将利用一次性释放的放射性同位素提供的信息（Cs 134、Cs 137和Sr 90的半衰期分别为约2年、约30年和约29年），2011年至2018年在太平洋上测量，以调查北太平洋模态沃茨（NPMW）的演变从它们在西部的形成到它们在环流周围的循环。目标是对现在长达7年的 国际 全流域 FDNPP 观察到 记录 的 放射性核素， 组合 与 其他 同时观测水的性质（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TS），并使用基于最先进的 高分辨率 数据 同化 模型 到 探讨 的 4维 运输途径和FDNPP放射性核素信号标记的NPMW的演变。 特别是，分析将检查：a）这些沃茨保持其关键海洋学特性的程度，特别是其潜在的密度和潜在的涡度，在7年的过程中; B）NPMW的运动受到沿等密度平流的限制的程度; c）从表面到1000米的潜在密度范围内的底辟混合强度; d）各种模式沃茨的起源、停留时间和归宿; e）放射性核素排放的时间对最终流域尺度信号模式的重要性;以及f）可以信任高分辨率模型再现4-这些水团在年际到年代际时间尺度上的空间运动和演变。这些科学目标旨在扩展先前研究的结果，这些研究利用了2011年FDNPP意外注入海洋的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环流路径和混合。在这项新的研究中，在以前的努力中磨练出来的工具将被用来扩大重点，以包括北太平洋海盆的整个宽度，只要FDNPP的铯观测是可用的，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NSF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.