To interpret the observed radionuclide elevations off the coast of Japan, the short-term numerical model simulations are conducted using the passive tracers over a time scale of several months. The tracer simulations demonstrated some key features of the observed Cs distribution in the 2011 cruise. First, the absence of 137Cs south of the Kuroshio Current was attributed to the transport barrier that the Kuroshio was preventing the southward progression of 137Cs. Second the largest concentrations of 137Cs were shown to be associated with the semi-permanent near-shore eddy that was entraining 137Cs-rich coastal waters from the FNPP vicinity and retaining them through stirring around the eddy perimeter. Finally, the intermediate 137Cs concentrations at the westernmost meridional column of stations were explained by the fact that these stations contained the more recent coolant water that had continued to leak from the reactor after the initial discharge. The pronounced non-uniform spatial structure of the measured 137Cs fields is consistent with our modeling results, which suggest that the June distribution of 137Cs was still streaky and patchy and was dominated by advection processes rather than by diffusion.
The numerical simulations determine that >95% of Cs remaining in the water within ~600km of Fukushima in mid-June 2011 was due to the direct oceanic discharge, and the estimated strength of the ocean source is 16.2± 1.6PBq based on minimizing the model-data mismatch. Total releases from Fukushima are not well constrained, with estimates from atmospheric fallout and direct ocean discharge spanning 4 to 90 peta Becquerels (PBq), but are most likely in the 15–30 PBq range. (Read more here.)
For the prediction of the Fukushima-derived radionuclides crossing the North Pacific Ocean, the drifter trajectories from the Global Drifter Program (GDP) dataset are employed to construct probability maps by tracking the drifters released near the Fukushima area over a time scale of 5 years. Using a two-iteration method to connect the source area to different geographical locations throughout the Pacific Ocean, the probability maps are greatly improved to interpret the radionuclide observations. The drifter-based maps indicate that the Cs-rich waters reach the International Date Line between 30°N and 40°N approximately 1 year after the accident. After 1.5 years, the leading edge of the tracer distribution extends diagonally from southwest to northeast. The drifter-based predictions about the progression of the contaminated waters across the basin with time were shown to compare favorably with the available 2012 and 2013 measurements of near-surface Fukushima-related 134Cs. This front moves eastward across the basin, and by 2 years its northeastern part follows the Alaska Current into the Alaskan gyre. (Read more here.)
