Dispersion behavior of Fukushima-derived 137Cs over the North Pacific with emphasis on its sensitivity to vertical velocity and diffusion

This study simulated the dispersion of ¹³⁷Cs in the North Pacific using a Lagrangian particle model, incorporating basin-wide atmospheric deposition and direct release from the Fukushima accident. Three experiments examined the impact of vertical diffusion and velocity on dispersion behavior. EXP01 and EXP02 assumed zero vertical velocity with different vertical diffusion coefficients (1 × 10^-4 and 2 × 10^-4 m²/s, respectively), while EXP03 used a 3-day average vertical velocity and the same diffusion coefficient as EXP01. EXP01 significantly overestimated ¹³⁷Cs concentrations offshore North America. In contrast, EXP03 successfully reproduced the timing and magnitude of cesium peak at P26, the eastern Pacific monitoring station. Although EXP02 and EXP03 showed marginal differences in vertical structures of ¹³⁷Cs in the northeastern Pacific, they differed distinctly in the northwestern Pacific. While reproducing realistic vertical profiles was challenging, EXP03 reasonably represented the vertical penetration of ¹³⁷Cs, particularly its deep intrusion in the Kuroshio Extension region. EXP02 reproduced a shallow and nearly homogeneous pattern of ¹³⁷Cs penetration over the northwestern Pacific. Along 149°E, EXP03 was more consistent with the observational study by Kumamoto et al. (2014) than EXP02 in describing the ¹³⁷Cs penetration depth of 2.0 Bq/m³. This study highlights the importance of adequate vertical velocity for realistic modeling of radioactive dispersion across different scales in the North Pacific. These findings will contribute to an effective response to emergency situations, such as the incidents involving the release of radioactive material.