Radiocaesium derived from the TEPCO Fukushima accident in the North Pacific Ocean: Surface transport processes until 2017

We report temporal variations of Cs-137 activity concentrations in surface waters of six regions of the western and central North Pacific Ocean during 2011-2017 using a combination of 1264 previously published data and 42 new data. In the western and central North Pacific Ocean at latitudes of 30-42 degrees N and longitudes of 140 degrees E to 160 degrees W, eastward transport of radiocaesium was clearly apparent. Cs-137 activity concentrations in surface water decreased rapidly to similar to 2-3 Bq m(-3) in 2015/2016, still a bit higher than Cs-137 activity concentrations before the FNPP1 accident (1.5-2 Bq m(-3)). Cs-134/Cs-137 activity ratios decay-corrected to 11 March 2011 were similar to 0.5-0.8. To the south of 30 degrees N and between 130 degrees E and 160 degrees W in the western and central Pacific Ocean, Cs-137 activity concentrations were around 1-7 Bq m(-3) in 2011/2012 but then stabilized at a few Bq m(-3) up to 2017.Cs-134 activity concentrations were detected at levels of 0.1-0.9 Bq m(-3), and Cs-134/Cs-137 activity ratios decay-corrected to 11 March 2011 were similar to 0.3-0.5. Temporal variations of model-simulated Cs-137 activity concentrations in surface water in the region of interest showed good agreement with observations, except in the southwestern North Pacific Ocean.