Hindcast and Forecast of 137Cs Activities in the North Pacific Ocean Waters from 1945 to 2020 by Eddy-resolving ROMS

Abstract:

We conducted a hindcast and forecast of ¹³⁷Cs activities in the North Pacific waters from 1945 to 2020, before and after the Fukushima Dai-ichi Nuclear Power Plant (F1NPP) accident. We used the Regional Ocean Model System (ROMS) with high resolution (1/12º-1/4º in horizontal, 45 levels in vertical), of which domain was the North Pacific Ocean. The model was driven by the exactly repeating “Normal Year” forcing Coordinated Ocean Reference Experiment (CORE) forcing dataset (Large and Yeager, 2008) using bulk formulae and the model-predicted sea surface temperature and the 50 years averaged SODA data as boundary conditions. The reconstructed global fallout due to atmospheric nuclear weapons’ tests and Chernobyl accident was employed for atmospheric flux of ¹³⁷Cs from 1945 to 2011. After the accident, the atmospheric deposition and direct release of ¹³⁷Cs from F1NPP were also employed for input condition. Five ensemble calculations of ¹³⁷Cs activities in seawater were conducted under different initial conditions, but had identical forcing. The net input of 16 PBq of ¹³⁷Cs from F1NPP, which was employed in this study, corresponded to 26% of the total amount (61 PBq) of ¹³⁷Cs that was estimated in the North Pacific before the F1NPP accident in 2011. Before the accident in 2011, the ¹³⁷Cs on surface ranged from 0.75 to 1.7 Bq m^-3. The direct comparison between simulated and observed ¹³⁴Cs activities in the surface layer represented that the root-mean-square error and correlation coefficient were 5.6 Bq m^–3 and 0.86, respectively, suggesting the model result were consistent with the observations. The main body of high ¹³⁷Cs activity water from F1NPP was transported to south of the Subarctic Front around 42°N via the Oyashio Coastal Current, the Oyashio intrusion, and the Kuroshio bifurcation and then to the western North Pacific. This model simulation suggested that the ¹³⁷Cs activities in surface waters at P26 (P04) would increase to 4.1 Bq m^–3 (4.3 Bq m^–3 ) in 2015 (2016) and then decrease to 1.3 Bq m^–3 (1.8 Bq m^–3 ) in last 2020.