Origin and pathway of a subsurface maximum of ¹³⁷Cs detected in the winter of 2012 after the Fukushima accident

This study investigates the dispersion behavior of Cs-137 and evaluates its origin (atmospheric deposition or direct ocean release) from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident using a Lagrangian particle tracking model. The ocean circulation fields based on the Modular Ocean Model Version 5 (MOM5) were adopted for the simulation. The MOM5 results represented the formation and migration of subtropical mode water (STMW) comparable with observations and reanalysis data. Particularly, anticyclonic eddies south of the Kuroshio extension promoted surface mixing over 300 m in the cooling season. The particle tracking simulation reproduced well the maximum subsurface activity between 142 and 146(degrees)E, where STMW is deep owing to anticyclonic eddies, compared to the activity found via measurements conducted around 149(degrees)E in the winter of 2012. It also demonstrated that the Cs-137 of the tropical and subtropical regions (10-35(degrees)N, 142-146(degrees)E) in the winter of 2012 almost entirely originated from atmospheric deposition.