Lagrangian approach is applied to study near-surface large-scale transport in the Kuroshio Extension area using a simulation with synthetic particles advected by AVISO altimetric velocity field. A material line technique is applied to find the origin of water masses in cold-core cyclonic rings pinched off from the jet in summer 2011. Tracking and Lagrangian maps provide the evidence of cross-jet transport. Fukushima derived caesium isotopes are used as Lagrangian tracers to study transport and mixing in the area a few months after the March of 2011 tsunami that caused a heavy damage of the Fukushima nuclear power plant (FNPP). Tracking maps are computed to trace the origin of water parcels with measured levels of Cs-134 and Cs-137 concentrations collected in two R/V cruises in June and July 2011 in the large area of the Northwest Pacific. It is shown that Lagrangian simulation is useful to finding the surface areas that are potentially dangerous due to the risk of radioactive contamination. The results of simulation are supported by tracks of the surface drifters which were deployed in the area.
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