Fukushima Cs pathway in the western North Pacific

Radionuclide samples collected as part of the spring 2013 CLIVAR P02 at 30N revealed that the Fukushima-derived ¹³⁴Cs had reached a depth of 600 m west of dateline with distinct zonal extent. Physical processes responsible for the deep ¹³⁴Cs penetration in the western Pacific appear to be related to specific water mass subduction pathways, such as Subtropical Mode Water (NPSTMW) and Intermediate Water (NPIW). The Fukushima ocean discharge was deposited in the Oyashio-Kuroshio mixed region and the CLIVAR P02 observations suggest a rapid deep penetration of Cs at 30N. This short subduction time scale is key to understanding the Cs pathway spreading into the mid-depth western North Pacific Ocean. Here, a detailed analysis of water mass property characteristics is presented. The study is based on a number of hydrographic dataset collected from the coastal survey near Japan between 2011 to 2015. Mixed layer depth is calculated from Argo float profiles in western North Pacific to examine the interannual changes in the distinct water mass production rates.