Spreading of Fukushima-derived radiocesium in the western north Pacific ocean by the end of 2014

Abstract

We reviewed studies concerning the spreading of radiocesium derived from the accident of Fukushima Daiichi Nuclear Power Plant in March 2011 into the North Pacific Ocean by the end of 2014, including our latest observational data. In the north of Kuroshio Front, which is derived from eastward-flowing Kuroshio and its extension currents in the south of Japan, namely in the mixing and subarctic areas, radiocesium atmospheric-deposited and directly discharged had been transported eastward from off Japan to Gulf of Alaska along the North Pacific Current in the mid-latitude region in the surface layer above about 200 m depth. Just south of the front, the northern area of the subtropical area, atmospheric-deposited radiocesium had been transported southward in the subsurface layer between 200 and 400 m depth, approximately, due to the southward subduction of the subtropical mode water in the winter season. These observational results indicate that the Fukushima-derived radiocesium had spread into the whole western North Pacific Ocean by the end of 2014.