A 2011-2018 Fukushima Perspective on North Pacific Mode Water Pathways

This investigation uses the tracer information provided by the 2011 direct ocean release of radio-isotopes, (¹³⁷Cs, ~30-year half-life and ¹³⁴Cs, ~2-year half-life) from the Fukushima Dai-ichi nuclear power plant (FDNPP) together with hydrographic profiles to better understand the origins and pathways of mode waters in the North Pacific Ocean. While using information provided by radionuclide observations taken from across the basin, the main focus is on the eastern basin and results from analyses of two data sets 2015 (GO-SHIP) and 2018 (GEOTRACES) along the 152°W meridian. The study looks at how mode waters formed in the spring of 2011 have spread and mixed, and how they have not. Our radiocesium isotope samples tell a story of a surprisingly confined pathway for these waters and suggest that circulation to the north into the subpolar gyre occurs more quickly than circulation to the south into the subtropical gyre. They indicate that in spite of crossing 6000 km in their journey across the Pacific, the densest 2011 mode waters stayed together spreading by only a few hundred kilometers in the north/south direction, remained subsurface (below ~200 m) for most of the trip, and only saw the atmosphere again as they followed shoaling density surfaces into the boundary of the Alaska Gyre. The more recent data are sparse and do not allow direct measurement of the FDNPP specific ¹³⁴Cs, however they do provide some information on mode water evolution in the eastern North Pacific seven years after the accident.