Inomata et al. (2018) reported that radioactive cesium (¹³⁷Cs) released into the North Pacific Ocean due to the Fukushima Daiichi Nuclear Power Plant accident has been advected to the East China Sea (ECS) and the Japan Sea (JS). The density σ_Θ at which ¹³⁷Cs activity was detected in the ECS and JS was approximately 25.2 kg m^-3, indicating that the ¹³⁷Cs subducted into the North Pacific subtropical mode water (STMW) has reached the East China Sea and the Japan Sea. In this study, the advection and diffusion of ¹³⁷Cs were simulated using the Regional Ocean Modelling System (ROMS). Since the formation area of the STMW covers a wide area around the North Pacific Kuroshio Current and the Kuroshio Extension, we divided the formation area into seven rectangles and set tracers (dye-01 (0.33±0.03 PBq): 28–35°N, 130–140°E; dye-02 (0.36±0.02 PBq): 28–35°N, 140–150°E; dye-03 (0.04±0.01 PBq): 35–42°N, 140–150°E; dye-04 (0.23±0.05 PBq): 28–42°N, 150–160°E; dye-05 (0.2±0.03 PBq): 28–35°N, 135–140°E; dye-06 (0.18±0.02 PBq): 28–35°N, 140–145°E; dye-07 (0.17±0.01 PBq): 28–35°N, 145–150°E) uniformly at 10 Bq m^-3 within the density range of σ_Θ from 25.0 to 25.4 kg m-3 in each rectangle at 0:00 on April 1.After simulating the diffusion of the tracers in the North Pacific Ocean, we examined the time series of the total inventories in the ECS and the JS from the monthly averaged tracer concentrations. The number of simulation ensembles for the tracers is four, as the start years of tracer simulations correspond to four consecutive years. From the inventories in the ECS, JS, and the North Pacific, we calculated the monthly flux balance between the Pacific Ocean and the East China Sea, and between the East China Sea and the Sea of Japan. However, since the matrices for the calculation of the balance were not full-rank, we calculated them using the least squares method by adding conditions to minimize the flux balance values. For ease of explanation, the start years of the tracer simulations are referenced as April 2011. The inventory of these tracers showed the following results: (1) About 80% of dye-04 was transported east of 160°E in April 2012. Furthermore, the dye-04 arrived in the ECS in 2013 and the JS in 2014, with these arrivals being later than those of the other dyes.; (2) Approximately half of dye-01 and dye-02 were transported east of 160°E in 2013.; (3) In the ECS, the inventories of dye-01 and dye-02 peaked around 2012 and 2012 to 2013, respectively. At their peaks, approximately 1.5% of the initial total amounts of the dyes were in the ECS.; (4) In the JS, the inventories of dye-01 and dye-02 began increasing around the summer of 2012 and then gradually rose (by approximately 0.2% and 0.3% of the initial total amounts, respectively). The increase peaked around 2012 and 2013 to 2014, respectively.; (5) Dye-05, dye-06, and dye-07 reached the ECS in this order, with arrivals gradually delayed from September 2011 to February 2012, and gradually increased in the JS from around the summer of 2012 (approximately 0.2 to 0.3% of the initial total amounts). The flux balance of dye-05, dye-06, and dye-07 indicated that the inflow of each tracer from the Pacific Ocean to the southern part of the ECS peaked around September 2011, October 2011, and February 2012, respectively, while the outflow through the Tokara Strait peaked around April 2012. The transport of the tracers into the JS remained at a relatively constant level after their arrival.

Cs,North pacific,Subtropical Mode water,SE,East