Abstract submitted to JpGU 2013
A-CG06: Continental-Oceanic Mutual Interaction: Global-scale Material Circulation throughRiver Runoff

Initial oceanic dispersion mechanism of radionuclides
released from Fukushima
Yusuke Uchiyama, Takafumi Yamanish, Daisuke Tsumune and Yasumasa Miyazawa

The 2011 earthquake off the Pacific coast of Tohoku, Japan, and the subsequent tsunami
caused a severe nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FNPP), leading
to radionuclides leaking into the coastal ocean.

A retrospective, double-nested high-resolution

numerical model at 1 km horizontal resolution based on the JCOPE2-ROMS downscaling
framework (Uchiyama et al., 2012) is utilized to evaluate oceanic/coastal dispersion of thereleased cesium-137 (137Cs) from FNPP.

A rational leakage submodel proposed by Tsumune

et al. (2011) based on the iodine-cesium ration is employed for the realistic cesium-137 release.
Among several oceanic radionuclide dispersal models including the present one, there still is adiscrepancy in the prevailing direction of initial dispersion (for the first several weeks) of theleaked materials, which remains an open question.
The present model successfully reproduces the overall oceanic structure as well as thedispersal of cesium-137, according to an extensive model-data comparison exploiting thesatellite altimetry, the in situ cesium-137 concentration monitored by TEPCO and MEXT, andthe aerial snapshot taken by NSSA.

Alongshore distribution of the concentrations is found tobe highly inhomogeneous with diluted patterns distributed widely in the south of FNPP, while
medium concentration appears in the north.

The initial dispersion predominantly occurs in thenorthward direction for the first 30 days, followed by the southward cesium-137 transport down
to the Kuroshio extension to be drifted far to the North Pacific.

The model also demonstrates

that an isolated, anti-cyclonic mesoscale eddy persists off Kashima Coast (south of Fukushima)
shed around Cape Inubo where the Kuroshio separation takes place.

This standinganti-cyclone enhances the poleward coastal jet about less than 50 km from the shore, especially
3-4 weeks after the accident.

A cross-spectral analysis exhibits a prominent correlationbetween northward wind and along-shelf oceanic currents in a fairly broard period band of 72 –240 hours peaked at 168 hours.

The highest coherence (coh2 > 0.92) is most evident in thenearshore area within about 10 km from the shore, while it exponentially decreases offshore.
This result strongly suggests the presence of wind-driven shelf waves as a potential agent of thealongshore cesium transport as argued by Miyazawa et al. (2012).

In contrast, a positive

buoyancy input through four major rivers discharging into Sendai Bay (about 80 km north ofFNPP) is found to be substantial to the southward cesium transport.

This buoyancy input isattributed to a prevailing equatorward flow contracted in a quite narrow coastal strip within
about 10 km from the shore.
In summary, the initial dilution and dispersal of the leaked radionuclide from FNPP areprimarily constrained in the nearshore area around Fukushima Coast with reduction of theequatorward transport.

The narrow coastal strip of the predominant alongshelf transport ischaracterized by the “three-layered system” in the cross-shelf direction consisting of 1) theinnermost southward transport by the positive buoyancy (about 0-10 km off the shore), 2) theprevailing poleward coastal jet associated with a standing anti-cyclonic mesoscale eddy formed
off Kashima Coast that most likely influenced by wind-driven shelf waves, and (3) the overall
equatorward transport in the offshore farther than about 50 km.