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

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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.