Fukushima-derived radiocesium in the northwestern Pacific Ocean in February 2012

• Fukushima-derived radiocesium was found in the northwestern Pacific in February 2012.
• The highest activity concentration in the transition area was due to the direct discharge.
• Direct discharged radiocesium was conveyed southwardly across the Kuroshio Current.
• More than 80% of Fukushima-derived radiocesium was derived from the direct discharge.

We measured radiocesium (134Cs and 137Cs) in seawaters collected at stations in the northwestern Pacific Ocean in February 2012. Activity concentration of Fukushima-derived radiocesium was highest in the transition area between the subarctic and subtropical regions, which was due to the direct discharge. The direct discharged radiocesium was transported southwardly across the Kuroshio Extension along isopycnal mixing. More than 80% of the Fukushima-derived radiocesium at stations both in the transition area and subtropical region was derived from the direct discharge.