Fukushima-derived radiocesium in the western North Pacific Ocean the decade after the accident

In March 2011, radiocesium (¹³⁴Cs and ¹³⁷Cs) was released from the damaged Fukushima Dai-ichi Nuclear Power Plant (FNPP1) into the North Pacific Ocean. In this study, vertical profiles of radiocesium activity concentration at 19 stations in the western North Pacific Ocean and Bering Sea were measured between 2020 and 2023. Using these data, the transport pathways and inventory of the FNPP1-derived ¹³⁴Cs in the North Pacific Ocean over the decade following the FNPP1 accident were synthesized. In the subarctic area, ¹³⁴Cs completed a single circulation in the surface layer along with the cyclonic gyre current and began recirculating within the decade. In the western subtropical area, the FNPP1-derived ¹³⁴Cs was transported into the subsurface layer due to the subduction of the subtropical mode water (STMW). The concentration of ¹³⁴Cs decreased by approximately 50% from 2013 to the early 2020s because it was exported from the western subtropical area. However, the subsurface maximum remained in the early 2020s. This indicated that the STMW-derived ¹³⁴Cs circulated several times along with the anti-cyclonic circulation of STMW during the decade. The total inventories of ¹³⁴Cs in the western subtropical area, estimated in this study, suggested that previous studies underestimated the total release of ¹³⁴Cs into the North Pacific Ocean in 2011.