AGU Fall Meeting, Dec. 3-7, 2012

Coastal dispersal of radionuclides released from the Fukushima
Nuclear Power Plant
Uchiyama, Y.
Department of Civil Engineering, Kobe University, JapanIshii, T.
Department of Civil and Earth Resources Engineering, Kyoto University, Japan
Tsumune, D.
Environmental Science Research Laboratory, CRIEPI, Japan
Miyazawa, Y.
Research Institute for Global Change, JAMSTEC, Japan

ABSTRACTThe 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. In the present study, a retrospective,
double-nested high-resolution numerical experiment is conducted to evaluate oceanic/coastal
dispersion of the released cesium-137 (137Cs). The model successfully reproduces the overall
ocean state as well as the monitored cesium-137 concentrations. Alongshore distribution of
the concentrations is found to be highly inhomogeneous with diluted patterns distributed
widely in the south of FNPP, while medium concentration appears in the north. The
probability density function of the concentration in the coastal area demonstrates that hotspots
may exist along the Sanriku coast, a rias coastline located north of FNPP. Whereas theprevious works reported that the FNPP-derived cesium-137 would be transported offshore
rather promptly, the present model indicates that cesium-137 substantially sticks to the shore
based on a flux budget analysis near the source location. Time-integrated cumulative
cesium-137 fluxes at the northern and southern (alongshore) boundaries of the control volume
indicate apparent outgoing alongshore transport. In contrast, a net cross-shore flux at theeastern boundary (50 - 100 km offshore) almost vanishes or even becomes negative, viz., anet incoming flux. Therefore the leaked cesium-137 remains in the coastal area with mostly
being transported alongshore back and forth. However, we found a meridional asymmetry of
the cesium-137 fluxes with more southward transport, mainly attributed to the southward
eddy transport.

AGU Fall Meeting, Dec. 3-7, 2012

Submesoscale Frontal Dynamics in the Kuroshio Extension JetIshii, S.
Department of Civil Engineering, Kobe University, Japan
Uchiyama, Y.
Department of Civil Engineering, Kobe University, Japan
Miyazawa, Y.
Research Institute for Global Change, JAMSTEC, JapanABSTRACT
The Kuroshio current flowing up off the eastern side of Japan is well known to be controlled
by mesoscale dynamics, but effects of submesoscale dynamics, O (10) km or less, on themean structure, eddies, frontal processes, stratifications, etc. have not been fully understood.
To this end, and towards a rigorous submesoscale-resolving Kuroshio model, we conducted
detailed numerical downscaling experiments with a double nested ROMS at horizontal
resolutions of 3 km and 1 km, forced by the assimilative JCOPE2 at 1/12 degree as theboundary condition. According to an energy analysis along the principal frontal directions,energy conversion systems observed in the Kuroshio extension jet can be approximately
categorized into two groups by their locations. The first one is right on the Kuroshio axis
where the meandering and scale transitions occur mainly through baroclinic frontal instability,
while submesoscale eddies are suppressed by the horizontal shear near the Kuroshio axis. The
second one is at 1 - 2 degrees apart from the axis with emergence of coherent submesoscaleeddies; by comparing to the case in the eastern boundary current system (Capet et al., 2008),
the energy conversion rates on the both sides of the Pacific look quite similar to each other,suggesting that the effect of shear instability weakens as going away from the axis. As a result,if sufficiently distant from the Kuroshio axis, the dynamics becomes much closer to that of
the eastern boundary current, leading to the forward energy cascade down to thesubmesoscale mainly through frontogenesis and associated baroclinic instability. In ameridional transect along 143 deg. E across the Kuroshio front, these submesoscale eddies areassociated with strong ageostrophic secondary circulations and vertical transport of potential
vorticity in the upper ocean.

AGU Fall Meeting, Dec. 3-7, 2012

Impact of the Kurishio paths on oceanic and estuarine
circulations in and around Seto Inland Sea, Japan
Kuriyama, T.
Department of Civil Engineering, Kobe University, Japan
Uchiyama, Y.
Department of Civil Engineering, Kobe University, Japan
Miyazawa, Y.
Research Institute for Global Change, JAMSTEC, JapanABSTRACT
The estuarine circulation and associated material dispersal in Seto Inland Sea (SIS), Japan,have been reported largely affected by the intrusion of the Kuroshio water through its twonarrow channels, the Bungo and Kii Channels (Figures 1 and 2). In addition, freshwater
inputs and the complicated coastline, topography and over 3,000 islands could modify theinfluxes through nonlinear processes, and substantially enhance intrinsic variability in theestuary. In the present study, a double nested oceanic modeling framework based on ROMS
downscaled from the JCOPE2 dataset is developed to examine the interaction between theestuarine and the Kuroshio water at a horizontal grid resolution of 600 m. The model result iscompared with the observation of tidal and subtidal surface elevations, stratification, andvelocity structure around the Kuroshio path to show a reasonable agreement. A mass flux
budget analysis is then conducted by estimating the fluxes at the two channels, the freshwater
discharges from the major rivers in SIS, and the precipitation/evaporation; the former has acomparable contribution to the sum of the latters. The net flux at the channels are found to beclosely related to the patterns of the Kuroshio path, measured by a distance from the eachchannel, suggesting that the Kuroshio intrusion plays an important role in forming a seasonalvariability of SIS.

Figure1: The geographical location of the Seto Inland Sea model (ROMS-L2).

Figure2: Isobaths (white, yellow and blue lines denoting 10, 30, 100 m deep, respectively) on
a monthly-averaged surface velocity magnitude in April, 2009 (color) around Seto Inland Sea.