Surface mixing and dispersion of radioactive tracer due to submesoscale eddies off the northeastern Pacific coast of Japan

Kamidaira, Yuki   ; Kawamura, Hideyuki  ; Kobayashi, Takuya ; Uchiyama, Yusuke*

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.