Numerical simulation on the long-term variation of radioactive cesium concentration in the North Pacific due to the Fukushima disaster

Numerical simulations on oceanic ¹³⁴Cs and ¹³⁷Cs dispersions were intensively conducted in order to assess an effect of the radioactive cesium on the North Pacific environment with a focus on the long-term variation of the radioactive cesium concentration after the Fukushima disaster that occurred in March 2011. The amounts of ¹³⁴Cs and ¹³⁷Cs released into the ocean were estimated using oceanic monitoring data, whereas the atmospheric deposition was calculated through atmospheric dispersion simulations. The highly accurate ocean current reanalyzed through a three-dimensional variational data assimilation enabled us to clarify the time series of the ¹³⁴Cs and ¹³⁷Cs concentrations in the North Pacific. It was suggested that the main radioactive cesium cloud due to the direct oceanic release reached the central part of the North Pacific, crossing 170°W one year after the Fukushima disaster. The radioactive cesium was efficiently diluted by meso-scale eddies in the Kuroshio Extension region and its concentration in the surface, intermediate, and deep layers had already been reduced to the pre-Fukushima background value in the wide area within the North Pacific 2.5 years after the Fukushima disaster. - Highlights: • We assessed an effect of the Fukushima disaster on the North Pacific. • The main radioactive cloud reached the central North Pacific one year later. • The ¹³⁷Cs had been reduced to the pre-Fukushima value 2.5 years later