Long-range transport of radiocaesium derived from global fallout and the Fukushima accident in the Pacific Ocean since 1953 through 2017. Part 1. Source term and surface transport

Long range transport of radiocaesium derived from local fallout occurred early 1950s, global fallout which occurred mainly late 1950s and early 1960s and the Fukushima accident occurred in 2011 were investigated and presented for ocean surface in the Pacific Ocean. HAM database and its update were used in this study to present whole history of radiocaesium transport in surface layer in the interested region. Since both the main local/global fallout regions and injection of radiocaesium by Fukushima accident occurred in the western North Pacific and constrain of surface current systems which governed surface transport processes were subtropical gyre and subarctic gyre, radiocaesium transport in surface water in the mid latitude was characterized as rapid eastward transport along Kuroshio and Kuroshio extension. Behaviors were similar and repeated for local/global fallout and Fukushima derived radiocaesium. A part of radiocaesium transported/deposited/injected in the mid latitude subducted into ocean interior and the radiocaesium activity concentrations were kept higher rather than those in surface water. (author)