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Kamidaira, Yuki; Kawamura, Hideyuki
no journal, ,
Recent studies suggested that turbulent mixing by submesoscale eddies is prominent for oceanic dispersion of materials in continental shelves and nearshore areas. A multiple nesting methodology enables us to reproduce submesoscale eddies in order to investigate their effect on surface mixing and associated material transport. In this study, downscaling simulations in a coastal area off Fukushima with a horizontal resolution of 1 km were conducted by using the Regional Ocean Modeling System (ROMS). Initial and boundary conditions for oceanic field such as temperature and salinity were given by reanalysis data calculated by an ocean data assimilation system MOVE developed at Japan Meteorological Agency (JMA). A comparison with the satellite altimetry data demonstrated a good agreement with measurement. Analysis of surface normalized relative vorticity suggested that meso- and submeso-scale eddies are enhanced as a horizontal resolution is finer. In addition, we simulated oceanic dispersion of Fukushima tracer to investigate the effect of submesoscale eddies on transport of Fukushima-derived radioactive materials. It was demonstrated that movement of Fukushima tracer in the coastal area depends on horizontal resolution, which suggests an importance of submesoscale eddies to evaluate oceanic pollution associated with the Fukushima disaster.
Otosaka, Shigeyoshi; Sato, Yuhi; Suzuki, Takashi; Kuwabara, Jun
no journal, ,
Transport processes of biohilic radionuclides, derived from the accident of Fukushima Daiichi Nuclear Power Plant, are discussed from distribution of 
I in seabed sediment off Fukushima. Concentrations of I in the seabed sediment in 2011 ranged between 0.02 and 0.45 mBq/kg, and were generally higher in the coastal region. In January 2013, about two years after the accident, concentrations of I in the sediment surface slightly increased in the shelf-edge region (bottom depth: 200
400 m), and such a trend was not observed for radiocesium. The preferential accumulation of I in the shelf-edge sediment was explained by lateral transport of sedimentary I from the coastal to shelf-edge regions. Considering the geochemical characteristics of iodine, it can be considered that particulate organic matter near the seabed efficiently carried I to the offshore.
Kamidaira, Yuki; Kawamura, Hideyuki; Kobayashi, Takuya; Uchiyama, Yusuke*
no journal, ,
A multiple nesting technique enables us to examine submesoscale eddies and their effects on surface mixing and associated material transport (e.g., Kamidaira et al., 2016). In this study, downscaling simulations for the coastal margin off Fukushima are conducted by using ROMS with a horizontal resolution of 1 km. The oceanic initial and boundary conditions are given by an oceanic data assimilation system MOVE developed at Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA). Energy conversion analysis exhibits that surface eddy potential energy to eddy kinetic energy (EKE) conversion through baroclinic instability is substantial in vorticity generation in this area. A further simulation is performed on oceanic dispersion of a radioactive tracer released from the Fukushima Dai-ichi Nuclear Power Plant to investigate eddy-induced mixing using an offline, oceanic tracer dispersion model SEA-GEARN developed at Japan Atomic Energy Agency (JAEA) (Kobayashi et al., 2007). A spatial moment analysis with respect to the center of gravity of the concentration reveals that tracer dispersion occurs with two distinct regimes leading to anisotropic transport in the zonal and meridional directions. The first regime lasting for about 5-8 days after the tracer release are mainly caused by coastally-trapped alongshore jet and submesoscale eddy-mixing. The subsequent regime occurs after the tracer is sufficiently dispersed offshore where mesoscale eddies play more dominant roles. Moreover, time series of zonal and meridional tracer variances fluctuate with enstrophy, area averaged density anomaly and EKE. These results clearly illustrate that fluctuating submesoscale and mesoscale eddy field induced by baroclinic insatiability evidently affects the tracer dispersion off Fukushima.