Proceedings of the Twenty-fourth (2014) International Ocean and Polar Engineering Conference
Busan, Korea, June 15-20, 2014
Copyright © 2014 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1 880653 91-3 (Set); ISSN 1098-6189 (Set)www.isope.org

Effects of the Submesoscale Anticyclonic Eddies Induced by Kuroshio in East China Sea
Yuki Kamidaira 1, Yusuke Uchiyama 1, Satoshi Mitarai 2and Taichiro Sakagami 21

Department of Civil Engineering, Kobe University
Kobe, Hyogo, Japan2Marine Biophysics Unit, Okinawa Institute of Science and Technology
Kunigami, Okinawa, Japan

submesoscale eddies and their stirring, and 2) shallow island
topography, in order to better understand dynamics and mixing
between the Kuroshio and the islands. We thus conduct numericalexperiments based on a detailed doubly nested ocean downscaling
model using ROMS (Regional Ocean Modeling System; Shchepetkinand McWilliams, 2005) at horizontal resolution down to 1km aroundthe Ryukyu Islands, forced by realistic assimilative meteorological andoceanic products to approach these scientific questions.

ABSTRACT
For maintaining biological diversity in the coral coasts around Ryukyu
Islands, Japan, a role played by the adjacent Kuroshio warm current isanticipated to be necessary for larval and nutrient transport. In order tounderstand dynamics and mixing between Kuroshio and the islands, wedevelop a detailed ocean downscaling model around Ryukyu Islands ina doubly nested configuration using ROMS at horizontal resolutions
down to 1km, forced by the assimilative JCOPE2 and JMAGSM/MSM. The model successfully reproduces anticyclonic eddies
that are significantly retained on the western side of the islands topromote lateral mixing in the area.

NUMERICAL MODEL
Figure 1 shows the oceanic downscaling model in doubly nested
configuration embedded in the JCOPE2 (Japan Coastal Ocean
Predictability Experiments; Miyazawa et al., 2009) domain. We rely on
an one-way offline nesting approach to successively reduce thehorizontal grid size from about 10 km (JCOPE2)  3 km (ROMS-L1)
 1 km (ROMS-L2). The outer domain is ROMS-L1 with a horizontal
resolution of 3km and 32 vertical -layers, designed to encompass afairly wide area to consider possible impacts of the Kuroshio flowing infrom the Taiwan Strait and the Luzon Strait, and climatologicalfreshwater discharge of the Yangtze River. The inner ROMS-L2

KEY WORDS: ROMS; downscaling; submesoscale; eddy mixing;
Kuroshio; Ryukyu Island; East China Sea

INTRODUCTION
Okinawa is home to ecologically significant coral reefs situated at thenorthernmost end of the border between the Pacific and the IndianOceans. These corals lie within a biodiversity hotspot that supports thehighest diversity of endemic species, plants and animals in the world.
For preserving biodiversity and marine ecosystem in the coral coasts
around Ryukyu Islands, a role played by the adjacent Kuroshio warm
current is considered to be substantial for larval and nutrient transport.
Guo et al. (2003) showed that the path and vertical structure of theKuroshio in the East China Sea, including the Ryukyu Islands, become
more realistic as the model’s horizontal resolution increases on thebasis of a triply nested ocean modeling using POM. Another numerical
study using a high-resolution ocean model in this area indicates that thesouthwestward counter-Kuroshio Current and mesoscale eddies, at O(100) km, have non-trivial influence on volume and heat transport
between Kuroshio and the Islands (Nadaoka et al., 2006).
Recently, to enhance the clarity of the dynamical processes in theupper oceans, studies on effects of submesoscale dynamics, at O (10)km or less, on the mean structure, eddies, frontal processes,stratification, etc. have been conducted quite actively (e.g., Uchiyama
et al., 2012). However, effects of the submesoscale phenomenon on theoceanic structure have not been fully investigated yet around theRyukyu Island. Another important point here is that the Kuroshio in thearea is considered to be largely affected by the shallow island
topography on its east side that may result in unique turbulence.
Therefore, we focus in the present study on two effects of 1)

Fig. 1. Bathymetry and domains of doubly nested models: ROMSL1 (outer box) and ROMS-L2 (inner box). The red line shows theJMA ’s transect called PN-line.

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Table 1. The configlations for ROMS-L1 and ROMS-L2.

Table 2. The specifications of “camphor”.

L1

L2Machine

Cray XE6MPI Tile

128(32core×4node)128(32core×4node)

Number of Nodes

940

Time step

240 sec

60 secTheoretical Peak
Performance

300.8 teraflops

1/1/2005

12/27/2010~10/21/2012~10/21/2012

Horizontal grids

768× 768 (×32 level)

832×608 (×32 level)

Horizontal resolution

3.0 km

1.0 kmSpecifications
Total Memory Capacity 59 TB

Computational period

Surface wind stress(1/1/2005~12/31/2007)

QuikSCATECMWF(daily )Surface wind stress

JMA GPVMSM(hourly,
12/27/2010~

Network Topology

3-D torus interconnect

Bisection Bandwidth

1.7TB/s

Processor (Core)

2 (2x16 = 32)

Theoretical Peak
Performance

320 gigaflops

MemoryDDR3-1600 64GB
102.4GB/s

Interconnect

Gemini 9.3GB/s or4.6GB/s

Processor

AMD Opteron 6000 series
processors

Architecture

x86-64Clock2.5 GHz

Number of Cores

16

Theoretical Peak
Performance

160 gigaflops

Node Specifications

JMA GPV-GSM(daily)(1/1/2008~)
Surface flux
SSTSSSCOADS （monthly）

COADS (monthly)

JCOPE2(20-dayaveraged )JCOPE2(20-dayaveraged )Dai and Trenberth2009(monthly)n/a

Processor Specifications
Major riverdischarges(Yangtze River)

Boundary ・Initialcondition

JCOPE2(daily)

L1（daily）

T-Snudging(1/20day−1)

JCOPE2(10-dayaveraged )n/aTopography

SRTM30

SRTM30

reproduced by the ROM-L1 with that of JCOPE2. The modelcomputational period is about 22 months, from December 27, 2010 toOctober 21, 2012.

COMPUTATINAL RESOURCES
We make use of a linux-based supercomputer Cray XE6 named“camphor” at Kyoto University to run our model. Table 2 shows thespecifications of “camphor”. We typically use 128 AMD Opteron coreswith parallelization based on MPI. For instance, the 16 months
consecutive model run on the finest resolution took us about 2 days
with 128 MPI processes.

domain with a horizontal resolution of 1km and 32 vertical -layers
covers the entire chain of the Ryukyu Islands, from the Amami Islandsof Kagoshima Prefecture to the north, all the way down to the Yaeyama
Islands of Okinawa Prefecture.
Table 1 shows the configlation for the numerical model. The outermost boundary and initial conditions are provided from the interpolated
fields of the assimilative JCOPE2 daily-averaged data. The model
topography is taken from SRTM 30 (Shuttle Radar TopographyMission), which covers the global ocean at 30 geographic arc seconds(about 1 km). We utilize the QuikSCAT-ECMWF blended wind andJMA GPV-MSM/GSM (Japan Meteorological Agency Meso-ScaleModel/Global Spectral Model) data for the surface momentum forcing.
Surface heat and freshwater fluxes are given by the COADS (TheComprehensive Ocean-Atmosphere Data Set) monthly climatology. A
20 day-averaged JCOPE2 data is applied to the sea surface temperatureand salinity for the surface flux restoration. The monthly climatology ofmajor river discharges in Dai et al. (2009) is considered for Yangtze
River in ROMS-L1. A four-dimensional TS-nudging (nudging strength= 1/20 day−1) towards the 10 day-averaged JCOPE2 temperature andsalinity fields is performed for consistency of the Kuroshio path

RESULT
An extensive model-data comparison is performed against the fieldobservation and satellite altimetry data to demonstrate the capability of
reproducing the Kuroshio and 3D oceanic structure quite well. For
example, seasonally averaged temperature and salinity in the verticalsection along the transect are compared with the JMA’s observationalhydrography (Fig.1; red line). Both of two model results exhibit an
overall agreement with the observed temperature and salinity data not
only in spring and summer (Fig. 2), but in all the four seasons (notshown here). Furthermore, the modeled mean and eddy kinetic energy
at surface agree well with the altimeter-derived kinetic energy (notshown).
Figure 3 shows daily-averaged, surface relative vorticity normalized
by the background rotation f, representing the significance of1005

Fig.2. Vertical sections of seasonally averaged temperature (upper) and salinity (lower) from
ROMS-L2 and in-situ data along the PN-line (Spring and Summer).

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Fig.3. Surface relative vorticity normalized by the planetary vorticity, ζ / f (no dimension) on December 23, 2011, from the 4 models. The black lineindicetes the transect for the vertical cross-sectional plots in Fig. 3 (b).

Fig.4. Cross sections (indicated by the black line in Figure 3) for normalized relative vorticity on December 23, 2011, from the 4 models.

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eddy shedding associated with the ridge topographic affected by themeridional mean momentum transport due to Kuroshio.

SUMMARY AND CONCLUSION
An elaborated high-resolution numerical oceanic downscaling model
is developed to investigate submesoscale dynamics and mixing around
Ryukyu Islands. An extensive model-data comparison is performed
against the field observation and satellite altimetry data to demonstratea quite close agreement. ROMS-L2 successfully reproduces negative
vorticity significantly retained on the western side of the islands. No
islands experiment suggests that the negative bias on the eastern side ofKuroshio is caused by topographic eddy shedding. It is expected thatnutrients, larvae and other marine substances are transported from
Kuroshio to the islands through these anticyclonic submesoscale eddies
and associated lateral mixing in the upper ocean.

REFERENCES
Fig. 5. Bathymetry and domains of “No Islands” experiment.

Dong, C. and McWilliams, J.C. (2007). “A numerical study of island
wakes in the Southern California Bight”, Cont. Shelf Res., Vol. 27,
pp. 1233-1248.
Dai, A., Qian, T., Trenberth, K.E. and Milliman, J.D. (2009). “Changesin continental freshwater discharge from 1948-2004”, J. Climate,Vol. 22, pp. 2773-2791.
Guo, X., Hukuda, H., Miyazawa, Y. and Yamagata, T. (2003). “ATriply Nested Ocean Model for Simulating the Kuroshio --- Rolesof Horizontal Resolution on JEBAR”, J. Phys. Oceanogr., Vol. 33，pp. 146-169.
Marchesiello, P., McWilliams, J.C. and Shchepetkin, A.F. (2003).
“Equilibrium Structure and Dynamics of the California Current
System”, J. Phys. Oceanogr., Vol. 33, pp. 753–783.
Nadaoka, K., Suzuki, Y., Nishimoto, T., Tamura, H., Miyazawa, Y. andYasuda, N. (2006). “Numerical Analysis of Current and Larval
Transport around the Ryukyu Islands to Clarify Coastal
Ecosystem Network among Distant Reefs”，Proc. JSCE Coastal
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Miyazawa, Y., Zhang, R., Guo, X., Tamura, H., Ambe, D., Lee, J.-S.,Okuno, A., Yoshinari, H., Setou, T., and Komatsu, K. (2009).
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Uchiyama, Y., Ishii, S., and Miyazawa, Y. (2012). “OceanicDownscaling Effects on the Kuroshio Extension Jet using a
JCOPE2-ROMS System”, J. JSCE, Ser. B2 (CoastalEngineering), Vol.68，No. 2, pp. I_436-I_440. (in Japanese withEnglish abstract)

submesoscale eddies with its negative values. As resolution gets finer,size and magnitude of the resolved vortices are pronouncedlyenhanced. We notice that negative vorticity in blue develops morestrongly and widely on the east side of the Kuroshio than those on theother side, where positive vorticity in red dominates. ROMS-L2 hassmallest eddies and largest negative vorticity near the island while thecentrifugally-stable positive vorticity on the west side of the Kuroshio
is attenuated rather quickly. This negative bias in the zonal direction
near the islands is likely to be related to the model’s grid resolutions.
These findings suggest that submesoscale eddies on the east tend to be
more anti-cyclones enhanced by the island topography. In order toevaluate this negative bias of relative vorticity with the island
topography, we conduct an alternative numerical experiment directly
examining the island topography effects around the Ryukyu islands.
For the case with “no islands” (see its topography in Fig. 5), the islands
and shallow topography h(m) < 1000 are sunk to h(m) = 1000 m while
the other configurations are held unchanged as in the original ROMSL2 model.
The submesoscale negative vorticity on the east side of the Kuroshio
in the no island case is much weaker than that in ROMS-L2 andnegative bias is almost disappeared (see the lower right of Fig.3).
Figure 4 shows vertical slices of the daily-averaged vertical normalized
relative vorticity near the Okinawa main island. ROMS-L2 has largest
negative vorticity not only at surface but also at depth than the othercases. These largest subdepth negative vorticity, however, is easily
attenuated in the “no island” case. Therefore, emergence of the uniquenegative bias of surface relative vorticity between Kuroshio and theRyukyu Islands is presumably caused by the lateral shear and resultant

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