th14 Annual Meeting of Asia Oceania Geosciences Society (AOGS 2017) @ Singapore, Aug. 6-11, 2017
OS02-A023

Submesoscale Eddy-Induced Mixing and Dispersion of Radioactive Tracer
Off the Northeastern Pacific Coast of JapanYuki KAMIDAIRA

1,2#+

1

12, Hideyuki KAWAMURA , Takuya KOBAYASHI , Yusuke UCHIYAMA , Akiko FURUNO
12Japan Atomic Energy Agency, Japan, Kobe University, Japan#+Corresponding author: manakenta@gmail.com Presenter

1

Recent studies have suggested that turbulent mixing by submesoscale eddies has an inevitable importance inoceanic dispersion of materials in continental shelves and nearshore areas. In this study, a downscaling oceanic
dispersion modeling system is developed for the coastal margin off Fukushima, Japan, to investigate eddy-induced
mixing of dissolved radionuclide. The system consists of a double nested Regional Oceanic Modeling System(ROMS) and the offline oceanic radionuclides dispersion model, SEA-GEARN-FDM. The oceanic initial and boundary
conditions are given by an oceanic data assimilation system MOVE-WNP developed at Meteorological ResearchInstitute/Japan Meteorological Agency. Surface wind stress is provided by the GPV-MSM atmospheric reanalysis,
while surface heat and freshwater fluxes are from a monthly climatological dataset. The developed submesoscale
eddy-resolving system is applied to the Fukushima accident occurred in March 2011. The direct release ofradionuclides from the Fukushima Dai-ichi Nuclear Power Plant and the atmospheric deposition are taken intoaccount. An extensive comparison between the model results and observation data demonstrates the model’s
capability of reproducing Cs-137 distribution.
A diagnostic vertical Cs-137 flux analysis demonstrates that ageostrophic secondary circulations induced bysubmesoscale eddies promote seasonally-varying subduction of the Cs-137 in spring. The fluctuating eddy
components of vertical Cs-137 flux are extracted by removing the seasonal variations with a low-pass Butterworth
filter in the frequency domain. The decomposition demonstrates that eddy components predominantly contribute tothe vertical transport.
In order to investigate the source of Cs-137 subducted deeply into the ocean interior, the numerical experiment
without atmospheric deposition is conducted. It is suggested that the Cs-137 originated from the direct release
dominates in the deeper layer, whereas the airborne Cs-137 remains in the subsurface layer rather than subducting
the deeper layer.

th14 Annual Meeting of Asia Oceania Geosciences Society (AOGS 2017) @ Singapore, Aug. 6-11, 2017
OS18-A003

A High-Resolution Coastal Forecasting System with a 3DVAR Assimilation
Optimal for a Semi-Enclosed Estuary
1#+1

12,3Kenta KUROSAWA , Yusuke UCHIYAMA , Takemasa MIYOSHI
23
Kobe University, Japan, RIKEN Advanced Institute for Computational Science, Japan, University of Maryland, United States#+Corresponding author: vppbjb@gmail.com Presenter

Ocean data assimilation, which combines observations and numerical models to obtain the statistically best estimateof the ocean state, has widely been used for various purposes such as ocean monitoring, forecast, and reanalysis.
Because of the progress of numerical ocean models and enhancement of ocean observation data such as Argo
floats and satellite-derived sea surface data in the past decade, many researchers have developed realistic data
assimilation systems for various ocean areas. However, in semi-enclosed estuaries including the Seto Inland Sea(SIS), Japan, data assimilation has rarely been adapted because satellite data and Argo floats, which are essential tosuccessful oceanic predictions, are unavailable, whereas forecast is highly demanded such as for fisheries, vessel
navigation, marine construction, offshore platform management, marine monitoring, etc.
In the present study, we develop a data assimilation system toward rigorous coastal predictions around Japan. The
system consists of a three-dimensional variational (3DVAR) assimilation scheme compatible with Regional OceanicModeling Systems (ROMS) and a fine-resolution coastal modeling configuration based on ROMS encompassing afairly extensive area around the SIS. The SIS, the largest estuary in Japan, has been equipped with a number ofautonomous in-situ monitoring stations for vertical profiles of temperature and salinity, tens of tidal gages, as well as
continuous surface current measurement using HF radars.
We first investigate a theoretical framework of the 3DVAR algorithm optimal for the high resolution configuration.
Subsequently, the developed 3DVAR is coupled with the SIS ROMS model to compare the model outcomes against
observed data. The 3DVAR ROMS model for the SIS is shown to perform much better than the accompanying
forward model without assimilation with good model skill scores, and to reproduce quite complex flows in the SIS
because of its complicated topography with more than 3,000 islands in there. Furthermore, we will share technicaldifficulties encountered during the experiment.

th

14 Annual Meeting of Asia Oceania Geosciences Society (AOGS 2017) @ Singapore, Aug. 6-11, 2017
OS02-A002

Numerical Simulations of the Interaction Between Internal Waves
and the Kuroshio Current Over the Izu-Ogasawara Ridge
1#+1234

5

6Eiji MASUNAGA , Guillaume AUGER , Matt RAYSON , Oliver FRINGER , Yusuke UCHIYAMA , Hidekatsu YAMAZAKI
234Ibaraki University, Japan, IBM Research Watson, United States, University of Western Australia, Australia, Stanford University,
56United States, Kobe University, Japan, Tokyo University of Marine Science and Technology, Japan#+Corresponding author: eiji.masunaga.office@vc.ibaraki.ac.jp Presenter

This study reports physical oceanic processes over the Izu-Ogasawara Ridge, off the Japan mainland. It is wellknown that this area is strongly influenced by both tides and the Kuroshio current. In order to investigate the physicalprocesses in the study area, we used three numerical modeling approaches: (1) the SUNTANS model, (2) the ROMS
model, and (3) a one-way coupled ROMS-SUNTANS model. The SUNTANS model was forced with only tidal
elevations and climatological stratification to reproduce the internal tides. Numerical results from SUNTANS showedstrongly enhanced internal wave energy around islands located on the Izu-Ogasawara Ridge. For the ROMS model,
we used triple nested domains embedded in the assimilative JCOPE2 oceanic reanalysis data and the domains wereforced by spatially and temporally-variable climatological conditions and tides. The ROMS model reproduced physicalprocesses associated with internal waves and the Kuroshio off the Japan coast. Tides significantly contributed toenhanced internal wave energy over the ridge except in the regions with strong Kuroshio currents. In addition to thetwo separated approaches, we attempted to develop the ROMS-SUNTANS coupling model, namely the highresolution SUNTANS model nested into the ROMS domain. The coupled model showed small scale physicalprocesses associate with tides, winds and eddies due to the Kuroshio, processes that are absent from thestandalone ROMS and SUNTANS simulations.

th14 Annual Meeting of Asia Oceania Geosciences Society (AOGS 2017) @ Singapore, Aug. 6-11, 2017
OS02-A016

Impact of the Kuroshio on Dispersal of Coral Spawn and Larvae
Around Ryukyu Islands in the East China Sea1#+

1

121,34Sachika ODANI , Yusuke UCHIYAMA , Motohiko KASHIMA , Yuki KAMIDAIRA , Satoshi MITARAI
234Kobe University, Japan, Kobe Gakuin University, Japan, Japan Atomic Energy Agency, Japan, Okinawa Institute of Science andTechnology Graduate University, Japan#+Corresponding author: sa34chika@gmail.com Presenter

Ryukyu Islands in the East China Sea are in a subtropical climate, hosting desirable environment for abundant coralecosystem. Kamidaira et al. (2016) suggested that the Kuroshio warm water maintains warmer water temperaturefavorable to corals around Okinawa Main Island due mainly to intermittent transverse eddy heat transport. It isanticipated that the Kuroshio and associated eddy mixing also promote the transport and dispersal of coral spawn
and larvae across the islands, whereas the area has suffered from coral bleaching in the recent decades. For optimalpreservation and protection of the coral habitats around Ryukyu Island, we conduct a double nested high-resolution
synoptic ocean modeling using ROMS with grid spacing down to 1 km coupled with an offline Lagrangian particle
tracking model. The modeled dispersal of coral spawn and larvae released from about 20 major islands and lagoons
are examined to quantify connectivity using Lagrangian probability density functions (PDFs) of the Lagrangianparticles (e.g., Mitarai et al., 2009) among Ryukyu Islands.
We introduce 145 patches with a diameter of 1km around Sekisei Lagoon between Yaeyama and Ishigaki Islands
close to Taiwan to compare with an in-situ surface drifter measurement. Lagragian particles are released from thepatches in spring for 2012-2015 and are tracked for the advection time of 3 weeks suitable to representing active
spawning and lifespan of coral spawn and larvae. The PDF analyses suggest that the particles mostly remain near
the released areas, while about 10% of the particles are entrained by the Kuroshio to travel long distance, leading tointer-island connectivity. Some of these particles are further transported northeastward, and then partially trapped bythe Kuroshio Counter Current formed between the Kuroshio and Main Island to approach Okinawa Main Island. We
reveal that eastward current in the south the lagoon occurred shortly after the releases suppresses the entrainment
and the resultant long-distance transport.

th14 Annual Meeting of Asia Oceania Geosciences Society (AOGS 2017) @ Singapore, Aug. 6-11, 2017
OS02-A030

Intrinsic Low-Frequency Variability in Transient Three-Dimensional Rip Currents
1#+1

2

2

Yusuke UCHIYAMA , James C. MCWILLIAMS , Cigdem AKAN
2
Kobe University, Japan, University of California, Los Angeles, United States#+Corresponding author: uchiyama@harbor.kobe-u.ac.jp Presenter

The ROMS-WEC model (Uchiyama et al., 2010) based on an Eulerian wave-averaged vortex-force asymptotic theoryof McWilliams et al. (2004) is applied to 3D transient rip currents in the surf zone on a surveyed topography undertime-independent, alongshore-uniform offshore incident waves. Substantial depth-dependency is manifested in the3D rip currents due to the vertical recirculation formed in the surf zone. The 3D rip currents are found to consist ofpairs of counter-rotating longitudinal vortex tubes that promote surface convergence along the rip center. The vortex
force plays the most dominant role in vorticity budget, other than the breaker acceleration torque that is the primaryvorticity source. Overall, the vortex force torque preconditions vorticity reduction from the rip eddies, while itconversely recharges vorticity to the rip eddies in the shoreward of the bars (viz., neck region). These rip currents are
intrinsically unstable and contribute about 70% to kinetic energy (KE) as eddy kinetic energy (EKE). The estimateddominant fluctuation period is about 18 minutes, which fits the very low frequency (VLF) band, consistent with a fieldmeasurement (Reniers et al., 2007). The EKE contained in the VLF band explains about 70% of the total EKE.

th14 Annual Meeting of Asia Oceania Geosciences Society (AOGS 2017) @ Singapore, Aug. 6-11, 2017
OS02-A032

Inter-Annual and Seasonal Variability of Upper-Ocean Primary Production
Along the Kuroshio Off Japan#+Xu ZHANG , Yusuke UCHIYAMA
Kobe University, Japan#+Corresponding author: zhangxu51876@gmail.com Presenter

The Kuroshio is accompanied by intense eddy activities influencing on biogeochemical processes around its path. In
general, localized upwelling due to mesoscale cyclonic eddies brings nutrient-rich subsurface water to the nutrientdepleted upper ocean to enhance near-surface primary productivity. However, a recent climatological modeling study
has revealed that topographic eddy shedding as well as baroclinic instability around the Kuroshio front concurrently
promote diapycnal eddy-driven downward nutrient transport in Enshunada Sea, Japan (Uchiyama et al., 2017). This
downward transport competes with upwelling-induced nutrient supply, resulting in suppressing the near-surface
productivity substantially as compared with that in the Kuroshio Extension region. In the present study, a synoptic,
retrospective downscaling ocean modeling is carried out for the marginal seas along the Kuroshio off Japan based on
ROMS (Shchepetkin and McWilliams, 2005; 2008) coupled with an NPZD biogeochemical model (Fasham etal., 1990; Gruber et al., 2006). The model domain encompasses both the Kuroshio and its Extension regions in asubmesoscale eddy-permitting configuration with horizontal grid spacing of 3 km. The simulation is conducted for 9years from 2008 to 2016 to take into account variety of the Kuroshio path. We focus on inter-annual and seasonalvariability of eddy-induced vertical nutrient fluxes that play an essential role in maintaining the upper-ocean primary
productivity. For instance, mixed layer deepening and surface cooling provoke vigorous eddy activity in colder
seasons that enhances diapycnal nutrient transport. We further discuss influences of the transient Kuroshio path,
which occasionally takes meandered paths rather than relatively straight courses, on eddy generation and associatedbiogeochemical responses.