5:15 PM - 7:15 PM
[MAG34-P06] Transport of Cs-137 subducted into the North Pacific subtropical mode water at the end of March to the Sea of Japan
Keywords:Cs-137, Subtropical Mode water, Sea of Japan, East China Sea
Inomata et al. (2018) reported that radioactive cesium (137Cs) into the North Pacific Ocean due to the Fukushima Daiichi Nuclear Power Plant accident was advected to the East China Sea (ECS) and Japan Sea (JS). The density σΘ at which 137Cs activities were detected in the ECS and JS was approximately 25.2 kg m-3, indicating that 137Cs subducted into the North Pacific subtropical mode water (STMW) after the accident reached the East China Sea and Japan Sea. In this study, the advection and diffusion of 137Cs were numerically calculated using the North Pacific Region Model (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 at 0:00 on April 1 into multiple rectangles and set tracers (dye01, 02 ,...) uniformly at 10 Bq m-3 in the depth of σΘ from 25.0 to 25.4 kg m-3 in the rectangles. After simulating the diffusions for tracers in the North Pacific Ocean, we examined the time series of the total amount of the tracers in the ECS and the JS. The number of simulation ensembles for the tracers is four, as the start years of tracer simulations correspond to four consecutive one-year periods.
First, we set the tracers corresponding to the rectangles: dye-01 (0.33±0.03 PBq) in 28-35°N and 130-140°E, dye-02 (0.36±0.02 PBq) in 28-35°N and 140-150°E, dye-03 (0.04±0.01 PBq) in 35-42°N and 140- 150°E, dye-04 (0.23±0.05 PBq) in 28-42°N and 150- 160°E and simulated the advection-diffusion of each tracer. The inventory of these tracers showed the following results:
(1) The 80% of dye-04 were transported east of 160°E in one year after the initial simulation.
(2) Approximately half of dye-01 and 02 were east of 160°E two years after the start.
(3) In the ECS, the inventories of dye-01 and 02 had peaks around 1 year and 2 to 3 years after the start, respectively.
(4) At the time of the peak, approximately 1.5% of the total amount, initially set, for dye-01 and 02 is present in the ECS.
(5) In the JS, the inventories of dyes-01 and 02 began to increase around 1.5 years after the start and gradually increased (to about 0.2 and 0.3% of the initially set total amount in 5 years).
Secondly, the area for the tracer dye-02 was divided into eastern and western segments, and tracer dye-06 and dye-07, which were set in the area, were simulated. The tracer dye-06 and 07 exhibited comparable time series of the inventories in the ECS and the JS.
First, we set the tracers corresponding to the rectangles: dye-01 (0.33±0.03 PBq) in 28-35°N and 130-140°E, dye-02 (0.36±0.02 PBq) in 28-35°N and 140-150°E, dye-03 (0.04±0.01 PBq) in 35-42°N and 140- 150°E, dye-04 (0.23±0.05 PBq) in 28-42°N and 150- 160°E and simulated the advection-diffusion of each tracer. The inventory of these tracers showed the following results:
(1) The 80% of dye-04 were transported east of 160°E in one year after the initial simulation.
(2) Approximately half of dye-01 and 02 were east of 160°E two years after the start.
(3) In the ECS, the inventories of dye-01 and 02 had peaks around 1 year and 2 to 3 years after the start, respectively.
(4) At the time of the peak, approximately 1.5% of the total amount, initially set, for dye-01 and 02 is present in the ECS.
(5) In the JS, the inventories of dyes-01 and 02 began to increase around 1.5 years after the start and gradually increased (to about 0.2 and 0.3% of the initially set total amount in 5 years).
Secondly, the area for the tracer dye-02 was divided into eastern and western segments, and tracer dye-06 and dye-07, which were set in the area, were simulated. The tracer dye-06 and 07 exhibited comparable time series of the inventories in the ECS and the JS.