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Submesoscale Mixing on Initial Dilution of Radionuclides Released From the Fukushima Daiichi Nuclear Power Plant
Author(s) -
Kamidaira Yuki,
Uchiyama Yusuke,
Kawamura Hideyuki,
Kobayashi Takuya,
Furuno Akiko
Publication year - 2018
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2017jc013359
Subject(s) - baroclinity , mesoscale meteorology , geology , eddy , climatology , atmospheric sciences , environmental science , mixed layer , stratification (seeds) , meteorology , turbulence , physics , botany , dormancy , biology , seed dormancy , germination
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 ( 137 Cs) 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 137 Cs 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 137 Cs, with the temporal variability of surface 137 Cs 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 137 Cs 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 137 Cs was transported downward below the mixed layer by eddies with the major contribution being from ASCs induced by submesoscale eddies.