Submesoscale Mixing on Initial Dilution of Radionuclides Released From the Fukushima Daiichi Nuclear Power Plant

Abstract

This study developed a submesoscale eddy‐resolving oceanic dispersal modeling system comprising a double‐nested oceanic downscaling model and an offline oceanic radionuclide dispersion model. This was used to investigate the influences of submesoscale coherent structures (SCSs) and associated ageostrophic secondary circulations (ASCs) on the three‐dimensional (3‐D) dispersal of dissolved cesium‐137 (137Cs) released from the Fukushima Daiichi Nuclear Power Plant (FNPP1). Extensive model‐data comparison demonstrated that the innermost high‐resolution model, with a lateral grid resolution of 1 km, could successfully reproduce transient mesoscale oceanic structures, the Kuroshio path and stratification, and spatiotemporal variations of 137Cs concentrations. Using an accompanying mesoscale eddy‐resolving model (grid resolution: 10 km) as a guide, we showed that submesoscale dynamics are important for improved representation of both the eddy field and the resultant 3‐D dispersal of 137Cs, with the temporal variability of surface 137Cs near the FNPP1 being equivalent to that in the coarse‐resolution model. According to energy conversion and spectral analyses, SCSs and ASCs occur most intensively on the submesoscale, primarily because of shear instability. However, baroclinic instability serves as a secondary mechanism. SCSs have prominent seasonality, reflected by intensification in the colder months, which is when the FNPP1 accident occurred. Analysis of the vertical flux of 137Cs was performed by decomposition of the variables into eddy, mesoscale, and submesoscale components using frequency and wave number filters. It revealed that 42.7% of the FNPP1‐derived 137Cs was transported downward below the mixed layer by eddies with the major contribution being from ASCs induced by submesoscale eddies.

Influences of the Kuroshio on Interisland Remote Connectivity of Corals Across the Nansei Archipelago in the East China Sea

“…We evidently presented insights into such processes affected not only by the Kuroshio but also by local dynamics accounting for the entrainment rate of pelagic particles in the Kuroshio upon the release, secondary processes such as those by the KCC responsible for transverse expelling transport from the Kuroshio to islands, and seasonally varying submesoscale dynamics. As have pointed out by Werner et al (), Siegel et al (), and White et al (), we emphasize here that numerical connectivity studies require high‐resolution models, preferably down to submesoscale eddy‐resolving resolutions with grid spacing of ~1 km (e.g., Kamidaira et al, ), to properly account for advective effects to minimize uncertainty of overly simplified diffusive transport assumption in genetic approaches.…”

“…More intensive eddy activity in the east of the archipelago is apparently caused by influences of westward propagating mesoscale eddies as Rossby waves headed toward the eastern side of the chain of the islands (e.g., Liu et al, ; Qin et al, ). In contrast, the other high SSH variances south of Kyushu and in the Kuroshio Extension Region (cf., Kamidaira et al, ; Uchiyama, Suzue, et al, ) correspond to the areas where the Kuroshio path largely meanders, reflecting intrinsic synoptic variability of the Kuroshio.…”

“…This result indicates that the KCC is likely accompanied by mesoscale eddies that develop in warmer seasons and propagate southwestward as Rossby waves. In turn, down‐front, northerly seasonal wind and surface cooling in winter induce symmetric and baroclinic instability, leading to the emergence of submesoscale eddies (e.g., Kamidaira et al, ; Sasaki et al, ), which act to break up mesoscale eddies into smaller eddies. These results suggest that development of the KCC is more stable and persistent in spring, than other times of the year, when submesoscale activities in the upper ocean are lower.…”

“…The JCOPE2 product is provided as daily-averaged sea surface height (SSH), temperature, salinity, and meridional and zonal horizontal current velocities, which are spatiotemporally projected onto the perimeter of the ROMS-L1 model for the open boundary conditions. The one-way off-line nesting approach described in Mason et al (2010), Uchiyama et al (2014), Uchiyama, McWilliams, et al (2017), Uchiyama, Suzue, et al (2017), Uchiyama, Kanki, et al (2017), Uchiyama et al (2018), Kamidaira et al (2017Kamidaira et al ( , 2018, and Tada et al (2018) is applied to successively decreasing grid spacing from~10 km (JCOPE2) to 3 km (ROMS-L1), and further down to 1 km (ROMS-L2). The parent ROMS-L1 model is designed to encompass a wide area to account for the Kuroshio flowing from the Taiwan Strait, consisting of 768 × 768 horizontal grid cells and 32 vertically stretched terrain-and surface-following s layers (Shchepetkin & McWilliams, 2005).…”

“…A high-resolution model is required for accurate evaluation of detachment and settlement of larvae in shallow areas, referred to as "nearshore retention," as has been argued in the previous modeling studies (e.g., Wolanski & Kingsford, 2014). An Eulerian passive tracer transport model in our own previous work (Kamidaira et al, 2018) has also demonstrated that high-resolution models are inevitable to obtain a plausible reproducibility of concentration field confined near the coasts, and that a 1-km lateral grid spacing is a marginally sufficient resolution. Moreover, a massive amount of Lagrangian particles representing planktonic coral larvae by considering their spawning timing and depth and survival rate before settlement was released from nearshore sites across the archipelago to properly quantify long-distance larval transport that scarcely occurs.…”