A Fukushima Tracer Perspective On Four Years Of North Pacific Mode Water Evolution

We present the results of a multi-platform investigation that utilizes tracer information provided by the 2011 release of radioisotopes from the Fukushima Dai-ichi Nuclear Power Plant to better understand the pathways, mixing and transport of mode waters formed in the North Pacific Ocean. The focus is on transition region mode waters and radiocesium (Cs-137 and Cs-134) observations obtained from the May-June 2015 GO-SHIP occupation of the 152 degrees W line in the Northeast Pacific. Samples include profiles from the surface to 1000 m and surface/sub-surface pairs that provide an average 1 degrees of latitude spacing along 152 degrees W. We find a clear Fukushima (134Cs) signal from the surface to 400 m. The core signal (Cs-134-10 Bq m(-3), 137Cs-12 Bq m(-3)) at 41 degrees-43 degrees N lies at 30-220 m where mode waters formed through deep winter mixing in 2011 outcropped in the western North Pacific. The strongest 2015 152 degrees W Fukushima-source radiocesium signal is associated with Dense-Central Mode Waters consistent with the densest variety of these mode waters being formed off the coast of Japan 4 years earlier. The radionuclide signal transited the basin along subsurface mode water isopycnals mainly on the northern side of the subtropical gyre before outcropping at and to the east of the 152 degrees W line. In 2015, the densest 152 degrees W waters with Cs-134 lie at-435 m in the bottom range of Dense-Central Mode Water at 40 degrees N. There is a weak, but detectable, signal in the boundary current off both Kodiak and Sitka. The deepest detectable Cs-137 (weapon's testing background) are found at and to the north of 45 degrees N at 900-1000 m. With the exception of a single subsurface sample near Hawaii, as of spring 2015, the southernmost Cs-134 was found above 200 mat 30 degrees N. A total date-corrected Cs-134 inventory of 11-16 PBq is estimated. Qualitative comparison to model output suggests good consistency in terms of general location, latitudinal breadth, and predicted depth of penetration, allowing discussion of the bigger picture. However, the model's 2015 152 degrees W radiocesium signal is quantitatively weaker and the core is offset in latitude, potentially due to the lack of consideration of atmospheric deposition.