  We present the results of in-situ measurements of $^{134}$Cs and $^{137}$Cs
released from the Fukushima Nuclear Power Plant (FNPP) collected at surface and
different depths in the western North Pacific in June and July 2012. It was
found that 15 month after the incident concentrations of radiocesium in the
Japan and Okhotsk seas were at background or slightly increased level, while
they had increased values in the subarctic front area east of Japan. The
highest concentrations of $^{134}$Cs and $^{137}$Cs up to 13.5 ${\pm}$ 0.9 and
22.7 ${\pm}$ 1.5 Bq m$^{-3}$ have been found to exceed ten times the background
levels before the accident. Maximal content of radiocesium was observed within
subsurface and intermediate water layers inside the cores of anticyclonic
eddies (100 - 500 m). Even slightly increased content of radiocesium was found
at some eddies at depth of 1000 m. It is expected that convergence and
subduction of surface water inside eddies are main mechanisms of downward
transport of radionuclides. In situ observations are compared with the results
of simulated advection of these radioisotopes by the AVISO altimetric velocity
field. Different Lagrangian diagnostics are used to reconstruct the history and
origin of synthetic tracers imitating measured seawater samples collected in
each of those eddies. The results of observations are consistent with the
simulated results. It is shown that the tracers, simulating water samples with
increased radioactivity to be measured in the cruise, really visited the areas
with presumably high level of contamination. Fast water advection between
anticyclonic eddies and convergence of surface water inside eddies make them
responsible for spreading, accumulation and downward transport of cesium rich
water to the intermediate depth in the frontal zone.
