The Tōhoku earthquake and tsunami of March 11, 2011, resulted in unprecedented radioactivity releases from the Fukushima Dai-ichi nuclear power plants to the Northwest Pacific Ocean. Results are presented here from an international study of radionuclide contaminants in surface and subsurface waters, as well as in zooplankton and fish, off Japan in June 2011. A major finding is detection of Fukushima-derived 134 Cs and 137 Cs throughout waters 30–600 km offshore, with the highest activities associated with near-shore eddies and the Kuroshio Current acting as a southern boundary for transport. Fukushima-derived Cs isotopes were also detected in zooplankton and mesopelagic fish, and unique to this study we also find 110m Ag in zooplankton. Vertical profiles are used to calculate a total inventory of ∼2 PBq 137 Cs in an ocean area of 150,000 km 2 . Our results can only be understood in the context of our drifter data and an oceanographic model that shows rapid advection of contaminants further out in the Pacific. Importantly, our data are consistent with higher estimates of the magnitude of Fukushima fallout and direct releases [Stohl et al. (2011) Atmos Chem Phys Discuss 11:28319–28394; Bailly du Bois et al. (2011) J Environ Radioact , 10.1016/j.jenvrad.2011.11.015]. We address risks to public health and marine biota by showing that though Cs isotopes are elevated 10–1,000× over prior levels in waters off Japan, radiation risks due to these radionuclides are below those generally considered harmful to marine animals and human consumers, and even below those from naturally occurring radionuclides.

The March of 2011 earthquake and tsunami that caused a loss of power at the Fukushima nuclear power plants (FNPP) resulted in emission of radioactive isotopes into the atmosphere and the ocean. In June of 2011, an international survey of various radionuclide isotopes, including ¹³⁷Cs, was conducted in surface and subsurface waters off Japan. This paper presents the results of numerical simulations aimed at interpreting these observations, investigating the spread of Fukushima-derived radionuclides off the coast of Japan and into the greater Pacific Ocean, studying the dominant mechanisms governing this process, as well as estimating the total amount of radionuclides in discharged coolant waters and atmospheric airborne radionuclide fallout. The numerical simulations are based on two different ocean circulation models, one inferred from AVISO altimetry and NCEP/NCAR reanalysis wind stress, and the second generated numerically by the NCOM model. Our simulations determine that >95% of ¹³⁷Cs remaining in the water within ~600 km of Fukushima, Japan in mid-June 2011 was due to the direct oceanic discharge. The estimated strength of the oceanic source is 16.2 ± 1.6 PBq, based on minimizing the model-data mismatch. We cannot make an accurate estimate for the atmospheric source strength since most of the fallout cesium would have moved out of the survey area by mid-June. The model explained several features of the observed ¹³⁷Cs distribution. First, the absence of ¹³⁷Cs at the southernmost stations is attributed to the Kuroshio Current acting as a transport barrier against the southward progression of ¹³⁷Cs. Second, the largest ¹³⁷Cs concentrations were associated with a semi-permanent eddy that entrained ¹³⁷Cs-rich waters collecting and stirring them around the eddy perimeter. Finally, the intermediate ¹³⁷Cs concentrations at the westernmost stations were attributed to younger, and therefore less Cs-rich, coolant waters that continued to leak from the reactor in June of that year

The Great East Japan Earthquake and tsunami that caused a loss of power at the Fukushima nuclear power plants (FNPP) resulted in emission of radioactive isotopes into the atmosphere and the ocean. In June of 2011, an international survey measuring a variety of radionuclide isotopes, including ¹³⁷Cs, was conducted in surface and subsurface waters off Japan. This paper presents the results of numerical simulations specifically aimed at interpreting these observations and investigating the spread of Fukushima-derived radionuclides off the coast of Japan and into the greater Pacific Ocean. Together, the simulations and observations allow us to study the dominant mechanisms governing this process, and to estimate the total amount of radionuclides in discharged coolant waters and atmospheric airborne radionuclide fallout. The numerical simulations are based on two different ocean circulation models, one inferred from AVISO altimetry and NCEP/NCAR reanalysis wind stress, and the second generated numerically by the NCOM model. Our simulations determine that > 95% of ¹³⁷Cs remaining in the water within ~600 km of Fukushima, Japan in mid-June 2011 was due to the direct oceanic discharge. The estimated strength of the oceanic source is 16.2 ± 1.6 PBq, based on minimizing the model-data mismatch. We cannot make an accurate estimate for the atmospheric source strength since most of the fallout cesium had left the survey area by mid-June. The model explained several key features of the observed ¹³⁷Cs distribution. First, the absence of ¹³⁷Cs at the southernmost stations is attributed to the Kuroshio Current acting as a transport barrier against the southward progression of ¹³⁷Cs. Second, the largest ¹³⁷Cs concentrations were associated with a semi-permanent eddy that entrained ¹³⁷Cs-rich waters, collecting and stirring them around the eddy perimeter. Finally, the intermediate ¹³⁷Cs concentrations at the westernmost stations are attributed to younger, and therefore less Cs-rich, coolant waters that continued to leak from the reactor in June of that year

Strongly nonlinear surface eddies are identified and analyzed in a general circulation model. Agulhas rings and Gulf Stream cold-core eddies are examples of eddies that cannot be properly characterized using linear geostrophic dynamics. These eddies are compact, highly circular, persistent in time, and travel long distances while maintaining their characteristics. The nonlinear eddies can be identified by a large Rossby number and high circularity. The majority of the anomalous eddies are anticyclones. Calculation of the balance of forces on these eddies demonstrates that the centrifugal force associated with strong curvature is significant, and the force balance shifts from geostrophy toward a gradient wind balance. Using geostrophy instead of the gradient wind balance produces large errors in estimates of rotational velocity of these eddies. The gradient wind velocity can be calculated from geostrophic velocity and eddy radius. Comparison between the results demonstrates that even when only sea surface height and associated geostrophic velocities are available, strongly nonlinear eddies can be identified and properly characterized. This analysis is then applied to altimetric maps of sea surface height. Nonlinear eddies are present in the altimetric maps, but are less common and not as strongly nonlinear. This analysis demonstrates that by properly accounting for the dynamics of the eddy field, a more complete statistical description including nonlinear terms can be obtained from readily available observations.

[1] Interannual variability of the circulation in the northeast Pacific Ocean is explored through a joint analysis of expendable bathythermograph (XBT) and expendable conductivity-temperature-depth (XCTD) data, satellite altimetry, and output from a model that was constrained by ocean data. XBT temperature profiles with high spatial resolution are available in the eastern North Pacific along two repeated transects. These ship tracks, along with the coast of North America, define a closed ''box'' which is used to study the time-mean circulation and its variability on interannual timescales. Geostrophic velocities from XBT data are compared with geostrophic velocities from model output as well as the full model velocity fields. Correlations in variability on interannual timescales between transport in the subpolar gyre and in the subtropical gyre are present in both model output and data. The nature of the variability, and its relation to the changes of the strength of the North Pacific Current (NPC), which supplies the water for both gyres, are explored. Interannual variability in gyre transport is found to be related to both the bifurcation of the NPC, resulting in an anticorrelation in transport between the two gyres, and to variations in NPC strength, resulting in simultaneous changes in the two gyres. The dominant signal is found to be a long-term increase in the NPC, which results in a strengthening of the subtropical gyre. Possible connections with local-scale wind stress changes and with the El Niño/Southern Oscillation phenomenon are also explored.