Transport simulation of the radionuclide from the shelf to open ocean around Fukushima

We simulate radionuclide distribution in the ocean due to direct emission from the Fukushima Daiichi nuclear power plant (FNPP) for the period from 21 March to 6 May 2011. Dispersion of Cesium-137 is modeled by a transport equation with advection and diffusion by three dimensional ocean current. The ocean current data are provided from a data-assimilative, tide-resolving ocean general circulation model with horizontal resolution of 1/36°. Results show that the radionuclide expands from the shelf region into open ocean in April by the ocean currents and farther transported eastward along the Kuroshio Extension front in May. Sensitivity experiments demonstrate that the mesoscale geostrophic currents basically governed the transport processes in the open ocean after April. In the shelf region, on the other hand, the wind-driven currents facilitate the north–south extension of the radionuclide distribution through the repeated generations of the shelf waves. Influences of tide and river discharges cannot be neglected.