Continental Shelf Research 143 (2017) 206–218

Contents lists available at ScienceDirect

Continental Shelf Research
journal homepage: www.elsevier.com/locate/csr

Research papers

Eddy-induced transport of the Kuroshio warm water around the Ryukyu
Islands in the East China Sea

MARK

⁎

Yuki Kamidairaa, , Yusuke Uchiyamab, Satoshi MitaraicaNuclear Science and Engineering Center, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan
Department of Civil Engineering, Kobe University, Kobe, Hyogo, JapancMarine Biology Unit, Okinawa Institute of Science and Technology, Onna, Okinawa, Japanb

A R T I C L E I N F O

A BS T RAC T

Keywords:
Submesoscale eddy
Kuroshio
Topography
East China Sea
ROMS

In this study, an oceanic downscaling model in a double-nested conﬁguration was used to investigate the role
played by the Kuroshio warm current in preserving and maintaining biological diversity in the coral coasts
around the Ryukyu Islands (Japan). A comparison of the modeled data demonstrated that the innermost
submesoscale eddy-resolving model successfully reproduced the synoptic and mesoscale oceanic structures even
without data assimilation. The Kuroshio ﬂows on the shelf break of the East China Sea approximately 150–
200 km from the islands; therefore, eddy-induced transient processes are essential to the lateral transport ofmaterial within the strip between the Kuroshio and the islands. The model indicated an evident predominanceof submesoscale anticyclonic eddies over cyclonic eddies near the surface of this strip. An energy conversion
analysis relevant to the eddy-generation mechanisms revealed that a combination of both the shear instability
due to the Kuroshio and the topography and baroclinic instability around the Kuroshio front jointly provoke
these near-surface anticyclonic eddies, as well as the subsurface cyclonic eddies that are shed around the shelf
break. Both surface and subsurface eddies ﬁt within the submesoscale, and they are energized more as the grid
resolution of the model is increased. An eddy heat ﬂux (EHF) analysis was performed with decomposition into
the divergent (dEHF) and rotational (rEHF) components. The rEHF vectors appeared along the temperature
variance contours by following the Kuroshio, whereas the dEHF properly measured the transverse transportnormal to the Kuroshio's path. The diagnostic EHF analysis demonstrated that an asymmetric dEHF occurs
within the surface mixed layer, which promotes eastward transport toward the islands. Conversely, below themixed layer, a negative dEHF tongue is formed that promotes the subsurface westward warm water transport.

1. Introduction
Coral reefs are home to the most diverse range of marine life in theworld. They are of great importance to marine ecosystems, hostingfavorable habitat to a wide variety of ﬂora and fauna. An estimated 25%of all marine life is supported by coral reefs, even though they cover <0.1% of the world's oceans and represent one of the most fragile andendangered marine ecosystems in the world (e.g., Spalding et al.,2001). Coral reefs also represent a vital resource for humankind interms of tourism and ﬁshing. Cesar et al. (2003) reported that coral
reefs provide approximately US$29.8 billion in net beneﬁt streams perannum in goods and services to world economies, including tourism(US$9.6 billion), ﬁsheries (US$5.7 billion), and coastal protection (US$9.0 billion). Similarly, coral reefs have great economic value in Japan,
generating as much as US$1.6 billion per annum domestically. In
particular, the Ryukyu Islands, located in the subtropical region of⁎Japan that fringes the East China Sea (ECS; Fig. 1), have ecologicallyabundant coral reefs situated at their northernmost end at the border
between the Paciﬁc and Indian oceans. These corals lie within a region
that supports the highest diversity of indigenous species in the world.
Water temperature is widely known as a factor that has considerable eﬀect on coral growth. The ideal range of ambient temperature forreef corals is narrow; most corals cannot survive in temperatures much
below 16–18 °C even for a few weeks. High temperatures also have aserious eﬀect on coral growth and can lead to “coral bleaching,” aprocess that results in devastating mass mortality of the coral during
which they expel their symbiotic algae. Therefore, the habitat of coral isgenerally restricted to a latitudinal band between 30°N and 30°S
because decreasing temperature follows increasing latitude.
The sea around the Ryukyu Islands in the ECS, located between
25°N and 30°N, provides an environment for coral growth even though
it lies at the northernmost extreme of the habitable region. Major warm

Corresponding author.
E-mail addresses: kamidaira.yuki@jaea.go.jp (Y. Kamidaira), uchiyama@harbor.kobe-u.ac.jp (Y. Uchiyama), satoshi@oist.ac.jp (S. Mitarai).

http://dx.doi.org/10.1016/j.csr.2016.07.004
Received 8 December 2015; Received in revised form 16 May 2016; Accepted 10 July 2016
Available online 15 July 20160278-4343/ © 2016 Elsevier Ltd. All rights reserved.

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 1. Double-nested ROMS model domains and bathymetry (color: m). Left: the ROMS-L1 and L2 domains embedded in the JCOPE2 domain. Right: a zoomed-in region of the
ROMS-L2 domain. Black thick line indicates the JMA PN Line transect.

currents of the North Paciﬁc subtropical gyre, enters the ECS from theeast coast of Taiwan. It turns northeastward and drifts along thecontinental shelf slope fringing the ECS around the Ryukyu Islands(Qiu, 2001). The Kuroshio not only plays an essential role in themeridional transport of large amounts of warm and salty tropical water
northward (e.g., Ichikawa and Beardsley, 1993; Ichikawa and Chaen,
2000; Imawaki et al., 2001; Johns et al., 2001; Andres et al., 2008;
Yang et al., 2011) but also inﬂuences the regional climatic system of the
ECS (e.g., Xu et al., 2011; Sasaki et al., 2012). Temperature measurements recorded continuously by more than 100 thermometers inconjunction with satellite SST (sea surface temperature) measurements
have revealed that areas of high SST are formed oﬀ the middle of thewest coast of Okinawa Island because of the Kuroshio warm water(Nadaoka et al., 2001).
Several numerical studies have been undertaken to investigate thephysical processes and eﬀects of the Kuroshio in the ECS. Guo et al.
(2003) were successfully demonstrated that the path and verticalstructure of the Kuroshio in the ECS are reproduced more realisticallyas the horizontal resolution of a model increases on the basis of a triply
nested ocean modeling using the Princeton Ocean Model (Blumberg
and Mellor, 1987). Based on a study using the Meteorological ResearchInstitute Community Ocean Model (Usui et al., 2006), Usui et al.
(2008) reported that frontal waves are generated as a result of thecollisions between anticyclonic mesoscale eddies with diameters atorders of 100 km. These eddies are considered to have nontrivial
inﬂuence on mass and heat transport between the Kuroshio and the
Ryukyu Islands. Their study suggested that eddy-induced lateralmixing must be substantial to promote the warm water intrusion

Table 1
Computational configurations for the ROMS-L1 and ROMS-L2 models.

currents, such as the Kuroshio, allow the development of reefs up toand beyond the ordinary habitable latitudinal limit. These ocean
currents play important roles in transporting coral larvae and warm
water to such areas, thus maintaining favorable environments for reef
corals.
The Kuroshio, which is one of the world's major western boundary

Fig. 2. Time series of the volume-averaged surface (z > −400 m) kinetic energy from the ROMS-L1 (red), ROMS-L2 (blue), and JCOPE2 (black) models. The abscissa indicates theelapsed time in days since December 27, 2010, UTC. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

207

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 3. Plan view plots of: (a) time-averaged surface velocity magnitude and (b) SSH variance. Top: AVISO data, middle: JCOPE2, and bottom: ROMS-L2 on L1.

toward the Ryukyu Islands, because the main body of the Kuroshio is
persistently located approximately 150–200 km to the west of theislands, restricting its direct impact.

Recently, the eﬀects of submesoscale eddies (at typical horizontal
scales of several to tens of km or less) on the mean oceanic structure,
stratiﬁcation, and frontal processes have been studied actively to208

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 4. Seasonally averaged temperature (left) and salinity (right) for spring from JMA observations (upper panels) and ROMS-L2 (lower panels) in the vertical section along the PN
Line.

situated on the eastern side of the ECS continental shelf break where
the Kuroshio persistently ﬂows northeastward. The islands obstruct the
westward-propagating mesoscale eddies that detach from the Kuroshio
recirculation (Nakamura et al., 2009). Therefore, this obstruction may
result in the emergence of unique turbulence such as island wakes andassociated eddy shedding, as has been investigated in the Southern
California Bight (e.g., Dong and McWilliams, 2007). These geographical conﬁgurations are presumed to set preferable conditions for thedevelopment of submesoscale-eddy mixing through baroclinic andbarotropic instability due to the Kuroshio fronts and topographic shear
within the study area.
In the present study, a submesoscale-eddy-resolving numericalexperiment was conducted for the area around the Ryukyu Islands.
The study was based on a double-nested ocean downscaling conﬁguration using the ROMS, embedded in the assimilative Japan Coastal
Ocean Predictability Experiments (JCOPE2) oceanic reanalysis(Miyazawa et al., 2009) with atmospheric forcing from the assimilative
GPV-GSM (e.g., Roads, 2004) and MSM (e.g., Isoguchi et al., 2010)
reanalysis products. The innermost ROMS model domain (the principal focus of this analysis) had 1-km horizontal grid spacing, which was
suitably ﬁne for full representation of submesoscale activities (Capetet al., 2008a, 2008a, 2008b). Particular attention was given to themodel's reproducibility, statistical description of intrinsic submesoscale eddies, possible mechanisms for eddy inducement, and inﬂuenceof the eddies on the lateral mixing that promotes transport of theKuroshio water toward the islands. The remainder of this paper isorganized as follows. A description of the modeling framework used forthe hindcast experiment for the years 2010–2013 is given in Section 2.
Section 3 illustrates an extensive comparison between the model
results and ﬁeld observation and satellite altimetry data in order tovalidate the model's capability of reproducing the Kuroshio and 3-D
oceanic structure. Section 4 considers the impact of downscaling,which is followed by analyses of both the energy conversion andinstability relevant to eddy kinetic energy in Section 5 and of the heatﬂux in Section 6. Conclusions are given in Section 7.

Table 2
Seasonally averaged volume flux in Sverdrup across the PN Line from the in situ
observations, JCOPE2, ROMS-L1, and ROMS-L2 models.

observation
JCOPE2
ROMS-L1
ROMS-L2

SpringSummerFall

Winter

26.4
30.0
27.8
27.9

26.9
32.9
29.5
29.2

25.1
31.4
27.4
27.3

26.0
31.0
29.2
28.0

enhance our understanding of the dynamic processes of the upper
oceans (e.g., Boccaletti et al., 2007; Badin et al., 2011; Callies et al.,2015; Kunze et al., 2015). Capet et al. (2008a, 2008b, 2008c)
conducted a high-resolution numerical experiment of the idealized
California Current System using the Regional Oceanic Modeling
System (ROMS; Shchepetkin and McWilliams, 2005, 2008). They
demonstrated that submesoscale eddies occur through frontogenesis,
which sharpens the surface density fronts, forming in the regions ofhigh strain on the ﬂanks of mesoscale eddies, down to horizontal scalesof a few kilometers or less in association with strong vertical ageostrophic secondary circulations in the surface boundary layer. A multiple nesting technique (e.g., Marchesiello et al., 2003; Penven et al.,2006; Mason et al., 2010) has enabled submesoscale eddy-resolving
ocean modeling to investigate submesoscale stirring and mixing in theupper oceans and associated material dispersal. For example, Romero
et al. (2013) conducted a Lagrangian particle tracking in the Santa
Barbara Channel, CA, USA, using quadruple-nested high-resolution
ROMS modeling with a 75-m horizontal grid size. Uchiyama et al.
(2014) performed a Eulerian passive tracer tracking for sewage outfalls
in the Santa Monica and San Pedro bays in the Southern California
Bight using a similar quadruple-nested downscaling ocean modeling.
Both studies exhibited anisotropic along- and cross-shelf dispersal ofmaterial concentrations and particles on the continental shelves andnearshore areas, markedly dominated by submesoscale-eddy mixing.
In addition to those studies focusing on the eastern boundary currents,
several other studies have investigated the western boundary currents,
such as the Kuroshio and its extension region oﬀ Japan (e.g., Sasaki
et al., 2014) and the Gulf Stream oﬀ the U. S. east coast (e.g., Gula
et al., 2014). However, the inﬂuence of submesoscale eddies on upperocean dynamics and the resultant dispersal and transport of materials,
including the Kuroshio-derived warm water, nutrients, and corallarvae, has not yet been investigated adequately around the Ryukyu
Islands in the ECS.
Another important aspect of the Ryukyu Islands is their upheaved
shallow topography on the relatively deep Ryukyu Trough, which is

2. Model conﬁguration
Fig. 1 shows the numerical domains of the oceanic downscaling
model in a double-nested conﬁguration embedded in the JCOPE2(Miyazawa et al., 2009) domain. The JCOPE2 is a numerical reanalysis
product for the northwestern Paciﬁc Ocean assimilated with a vast
amount of satellite and in situ data, including ARGO ﬂoats using 3DVAR. The JCOPE2 product is provided as daily averaged sea surface
height (SSH), temperature, salinity, and meridional and zonal hor209

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 5. Left panels—surface eddy kinetic energy (EKE), Ke, from: (a) JCOPE2, (b) ROMS-L1, and (c) ROMS-L2. Right panels—instantaneous spatial distributions of surface vorticity
normalized by planetary vorticity, ζ/f (dimensionless) on January 7, 2012 from: (d) JCOPE2, (e) ROMS-L1, and (f) ROMS-L2. The black line in (f) indicates transect AA′ for the crosssectional plots.

monthly freshwater discharge of the Yangtze River into the ECS, whichis reported to range approximately between 838–907 km3/yr (e.g., Daiet al., 2009), was taken into account. The innermost ROMS-L2 domain
was 832×608 km with 1-km horizontal resolution and 32 vertical slayers, which covered the entire chain of the Ryukyu Islands, from theAmami Islands of Kagoshima Prefecture in the north to the Yaeyama
Islands of Okinawa Prefecture in the south. Table 1 lists the numericalconﬁguration of the ROMS models.

izontal current velocities. We relied on a one-way oﬄine nesting
approach (Mason et al., 2010) to reduce the horizontal grid size fromapproximately 10 (JCOPE2) to 3 km (ROMS-L1), and ultimately, down
to 1 km (ROMS-L2). The parent ROMS domain (ROMS-L1) had ahorizontal size of 2304×2304 km with uniformly square 3-km grid
spacing and vertically stretched 32 s-layers, designed to encompass awide area to consider all possible impacts of the Kuroshio ﬂowing infrom the Taiwan Strait and the Luzon Strait. The climatological210

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

could interfere with the spontaneous development and decay of
intrinsic eddies. We note that the present model does not include tidal
forcing since it is considered to have minor eﬀects on mean and eddyﬁeld in such an open ocean conﬁguration. For instance, Romero et al.
(2013) pointed out that dispersal and mixing in Santa Barbara
Channel, CA, USA, are dominated much prominently by submesoscale
stirring, not by tides. The L2 model computational period wasapproximately 33 months, from December 27, 2010 to September
14, 2013, UTC. The statistical analyses conducted in the present study
exploit the model results for the same period between March 27, 2011
and September 14, 2013, unless otherwise noted.
3. Model validation
In this section, we compare the model results with satellite, in situ
observations, and the assimilative JCOPE2 reanalysis. Fig. 2 shows thetime series of the volume-averaged surface kinetic energy (KE) for thethree model results (i.e., JCOPE2, ROMS-L1, and ROMS-L2). The
volume average is taken over the entire ROMS-L2 domain from thesurface to a depth of 400 m, encompassing the region in which the
Kuroshio main body is most inﬂuential. The temporal variations of theupper-ocean KE in the three models are similar. Given the fact that
JCOPE2 is assimilated with multiple satellite altimetry data, SST,
ARGO, and in situ mooring data, the two ROMS models provide
realistic estimates of the near-surface eddy activities. The ROMS-L2
generally yields slightly larger KE than the other cases because it is a
submesoscale eddy-resolving model that results in more energetic KE,
while retaining adequate seasonal variability. This result is achieved if
the L1 model is run with weak TS nudging toward the low-frequency
JCOPE2 3-D density ﬁeld, to dissipate KE appropriately for realistic
replication of the Kuroshio's behavior. Otherwise, the KE in the L1model increases signiﬁcantly with unrealistically large meandering of
the Kuroshio path (not shown). Conversely, the L2 model with theassimilated L1 boundary forcing behaves favorably, as shown in Fig. 2,
without any controls such as TS nudging.
Extensive model-data comparisons are performed using satellite
altimetry data and JMA observations to demonstrate the reproducibility of the double-nested ROMS model. For validating the mean
structure and temporal variance of the surface currents, including theKuroshio, we exploited the gridded composite of multiple satellite
altimetry data provided by AVISO (e.g., Traon et al., 1998). The
delayed-time AVISO-SSH data set is available daily with horizontal
spacing of 1/4°. The magnitude of the time-averaged geostrophic
current velocity, estimated from the AVISO-SSH, exhibits comparable
magnitude with the corresponding patterns of the JCOPE2 and ROMSL2 on the L1 (Fig. 3). However, the Kuroshio intrusion into the South
China Sea from Luzon Strait in the ROMS-L1 model occurs more
apparently than that in AVISO and JCOPE2 where the westward
meander is weakened with generating a leaped eddy or a ring. The
looping in the Luzon Strait could be realistic since it has been reported
both observationally and computationally (e.g., Centurioni et al., 2004
and Miyazawa et al., 2004). Nevertheless, Luzon Strait is locatedsuﬃciently far from the study area, and thus we conclude the plots of
the ROMS velocity magnitude also show reasonable agreement with the
Kuroshio path of the other two data sets. The SSH variance is viewed asa proxy that measures the intensity of the temporal variability insynoptic and mesoscale signals mostly due to eddies and the Kuroshio
meanders. The ROMS-derived SSH variance reproduces several important features with equivalent magnitudes to the AVISO data. For
instance, the variance is smaller on the persistent Kuroshio path on thewestern side of the Ryukyu Islands, compared with the other side,
where the westward-traveling Rossby waves and mesoscale eddies
collide with the topographic ridge around the islands. Another energetic area commonly arises north of 29°N, oﬀ the southwest coast of
Kyushu Island.
The modeled stratiﬁcation is subsequently compared with in situ

Fig. 6. Probability density functions of the normalized relative vorticity at 2 m depth
along transect AA′ (see Fig. 5f) from: (a) JCOPE2, (b) ROMS-L1, and (c) ROMS-L2
models, as a function of distance from Okinawa Island (km). The black lines are the mean
Kuroshio axes.

The outermost boundary and initial conditions of ROMS-L1 wereobtained from the spatiotemporally interpolated ﬁelds of the daily
averaged JCOPE2 data. The model topography was obtained from the
SRTM 30 Plus product (SRTM: Shuttle Radar Topography Mission;
Rodriguez et al., 2005; Becker et al., 2009), which covers the global
ocean at 30 geographic arc seconds, or roughly 1 km. We utilized the
QuikSCAT-ECMWF blended wind (e.g., Bentamy et al., 2006) for
2005–2007 and the JMA GPV-GSM product (JMA: Japan
Meteorological Agency, GPV-GSM: grid point value of the Global
Spectral Model) with horizontal resolution of 0.2°×0.25° for 2008–2013 for surface momentum forcing, depending on the availability ofthese data sets. Surface heat, freshwater and radiation ﬂuxes weretaken from the COADS (Comprehensive Ocean–Atmosphere Data Set;
Woodruﬀ et al., 1987) monthly climatology. The 20-day averaged
JCOPE2 data were applied to the SST and sea surface salinity (SSS)
restoration with a time scale of 90 days to correct long-term biases
caused by the imposed climatological surface ﬂuxes. The monthly
climatology of the major river discharges in Dai et al. (2009) wasapplied for the Yangtze River. A four-dimensional TS nudging (a.k.a.
robust diagnostic; e.g., Marchesiello et al., 2003) with a weak nudging
time scale of 1/20 per day was applied to the 10-day averaged JCOPE2
temperature and salinity ﬁelds for consistency of the Kuroshio path
reproduced by the ROMS-L1 with that of JCOPE2. The L1 model wasused for more than eight years from January 1, 2005 until September
14, 2013, UTC.
The innermost L2 model was initialized and forced along theboundary perimeters by the spatiotemporally interpolated daily averaged L1 output. The hourly output of the JMA GPV-MSM (Mesoscale
Model) reanalysis, which encompasses the entire L2 domain withhorizontal resolution of 0.05°×0.0625°, was used for the L2 model
instead of the GPV-GSM. Similar to the L1 model, SST and SSS
restoration for surface ﬂux correction was included. The other numerical conditions were the same as for the L1 model. Hence, the L2 model
was run freely without any assimilation such as the TS nudging that211

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 7. Left panels: (a) barotropic conversion rate, KmKe, (b) baroclinic conversion rate, PeKe, and (c) EKE, Ke, integrated vertically over the mixed layer from the ROMS-L2 model
results. Right panels: same as the left panels, but integrated vertically from the surface down to 1200 m depth. The gray contours represent surface velocity magnitude > 0.5 m/s withintervals of 0.25 m/s.

Line. Comparisons of the seasonally averaged temperature and salinity
clearly illustrate that the present model is capable of reproducing theobserved stratiﬁcation, not just in spring (Fig. 4) but in all seasons (notshown). A tilted thermocline and halocline are formed toward the ECS
shelf region with subsurface salinity maxima in the trough region.
Table 2 summarizes the modeled and observed volume ﬂux (transport)in Sverdrup along the PN Line. The observed volume ﬂuxes are
estimated geostrophically from the slope of the isobaric surface, basedon the seasonal climatology of the temperature and salinity (Fig. 4) byassuming the transport vanishes at 1000 m depth. The volume ﬂuxes
obtained by the models principally contain the ageostrophic component, which results in slightly larger transport than those observed.
However, the modeled volume ﬂuxes adequately capture the observed
seasonal variability, such as the increase in summer and the decrease in
fall. Interestingly, the ROMS-L2, with the ﬁnest grid resolution without
TS nudging, provides a better estimate of the transport (compared withthe observations) than that evaluated using the coarser-resolution
models (viz., ROMS-L1 and JCOPE2), both of which employ data
assimilation to some extent. This is likely attributable to the occurrenceof an appropriate spontaneous ﬂux adjustment in the ROMS-L2
through submesoscale lateral mixing and associated dissipation at

Fig. 8. Vertically integrated KmKe (red thin line), PeKe (red thick line) and Ke (blue line)
over the mixed layer from ROMS-L2 along the transect shown by the black line in Fig. 5f.
(For interpretation of the references to color in this ﬁgure legend, the reader is referred to
the web version of this article.)observations from the vertical section along the PN Line transect (e.g.,
Miyazawa et al., 2009), indicated by the thick black lines in Fig. 1. The
PN Line measurements comprise 16 CTD (conductivity, temperatureand depth) casts that have been obtained seasonally since 1972 by JMA
research vessels. As this transect favorably transverses the Kuroshio
path in the ECS, we can estimate the volume transport across the PN
212

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 9. Cross-sectional plots of: (a) barotropic conversion rate, KmKe, (b) baroclinic
conversion rate, PeKe, and (c) EKE, Ke, from the ROMS-L2 model. The correspondingtransect is shown by the black line in Fig. 5f. The white lines are the mean mixed-layer
depth estimated from the KPP model in ROMS. The black contours represent the mean
streamwise velocity normal to the transect.

Fig. 10. (a) Cross-sectional plot of normalized relative vorticity ζ/f on January 7, 2012,
along transect AA′ (shown by the black line in Fig. 5f). The white line is the mixed-layer
depth estimated from the KPP model. (b) Normalized relative vorticity ζ/f in thehorizontal plane at z=−400 m on January 7, 2012.

the resolved scales of the mean KE around the Kuroshio path. In
summary, the presented double-nested ROMS model is shown satisfactorily capable of reproducing the mesoscale behavior of the Kuroshio
and the mean 3-D oceanic structure.

ageostrophy is more evident. Vorticity is generally distributed asstreaks and ﬁlaments around the Kuroshio axis where the change ofsign occurs. However, enclosed circular eddies are dominant away fromthe axis, in particular, in the two ROMS model results. The two
distinctive high EKE (Ke) regions in Fig. 5a–c are consistent withthese vorticity distributions. As the resolution becomes ﬁner, the extent
and magnitudes of the resolved vortices become prominently diversiﬁed and enhanced, coinciding with the high EKE region on the easternside of the Kuroshio (Fig. 5a–c). The higher-resolution model renders
smaller submesoscale eddies that typically have diameters of several
kilometers.
We notice that negative vorticity, viz., counter-clockwise-rotating
cyclonic eddies, develops more vigorously and widely on the east side of
the Kuroshio than on the other side, where positive vorticity dominates. The innermost model with the highest resolution (ROMS-L2)
captures the negative vorticity that is retained signiﬁcantly on theeastern side of the Kuroshio, while the centrifugally stable positive
vorticity is attenuated rather quickly there. The ROMS-L2 model has
the smallest eddies and the largest negative vorticity near the islands.
This negative bias near the islands, in the direction transverse to the
Kuroshio path, is presumably caused by the increase of the resolved
eddies with the increased model resolution. To conﬁrm this negative
bias quantitatively, the probability density function (PDF) of thenormalized relative vorticity (ζ/f) at the surface was determined as afunction of the westward transverse distance from Okinawa Island
along transect AA′, as shown in Fig. 5f. This transect is deﬁned normal
to the mean Kuroshio axis, averaged over the computational period,
which is inclined at 35° relative to the geographical coordinate. Fig. 6
indicates that the ﬁner-resolution models yield stronger vortices withgentler PDF slopes along the ordinates. Although the PDFs are

4. Downscaling eﬀects
The unassimilated L2 model is capable of fully resolving submesoscale eddies, whereas the L1 and JCOPE2 are submesoscale-permitting (Capet et al., 2008a, 2008b, 2008c). Therefore, eddy activity
should be enhanced by the grid reﬁnement of the downscaling via
the increase and strengthening of the resolved eddies. To examine thedownscaling eﬀects, surface eddy kinetic energy (EKE), Ke, can be
estimated as follows:

Ke =

1⎛ 2
⎜u′ + v′ 2 ),2⎝

(1)

where (u, v) is the horizontal velocity and the overbar represents anensemble-averaging operator. The overbars denote an ensemble-averaging operator. The variables assigned with the prime are the ﬂuctuating eddy components obtained by removing the seasonal variations
with a low-pass Butterworth ﬁlter in the frequency domain (the ﬁrst
and last 10% of the analysis period cannot be used because of the
Butterworth ﬁlter's properties). Fig. 5a–c demonstrates that the surface
EKE increases markedly as the model grid spacing decreases from 10 to
3 and to 1 km. The higher EKE mostly emerges in two distinct regions:
one is on the Kuroshio axis and the other is on its eastern side, close to
Okinawa (Main) Island.
Fig. 5d–f illustrates the daily averaged, surface relative vorticity
normalized by the background rotation f (the Coriolis parameter), viz.,representing the emergence of mesoscale and submesoscale eddies ineach model. The variable ζ/f is also known as the vortical Rossby
number, the absolute value of which is greater than unity when
213

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Fig. 11. Eddy heat ﬂux (EHF) vectors vertically integrated (left) over the mixed layer and (right) from the surface to depth of 1200 m, superposed on the across-Kuroshio component of
the labeled EHF (in color). (upper) total EHF, (middle) rotational component, rEHF, and (lower) divergent component, dEHF. The gray contours are surface velocity magnitude >0.5 m/s with intervals of 0.25 m/s.

tions of the anticyclonic negative vorticity on the eastern side of theKuroshio axis. However, on the other side, anticyclones and cyclones
compete with the activated positive vorticity near the Kuroshio axis.
This transverse asymmetry is a unique structure that characterizes theeddy ﬁeld of the study area, which is perhaps related to both the
topographic ridge near the island chain and the continental shelf breakalong which the Kuroshio persistently drifts (Fig. 1), as well as frontal

distributed nearly symmetrically with respect to the Kuroshio axis, they
peak at ζ/f < 0 on the eastern side of the Kuroshio axis, even adjacent toOkinawa Island. This negative bias on the east is most evident at thehighest resolution. On the west of the Kuroshio, large positive vorticity
appears immediately next to the Kuroshio, while the PDF peaks
converge to zero away from the axis to the west. In summary, theRyukyu Islands are considered to enhance both intensity and ﬂuctua-

214

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

⎛
∂u
∂u
∂u
∂v
∂v
∂v ⎞
KmKe = −⎜u′u′+ u′v′+ u′w′+ v′u′+ v′v′+ v′w′ ⎟ ,
∂x
∂y
∂z
∂x
∂y
∂z ⎠
⎝

PK
e e =−

g
ρ′w′,
ρ0

(2)

(3)

where (x, y, z) are the horizontal and vertical coordinates, w is thevertical velocity, ρ is the density of sea water, ρ0=1027.5 kg m−3 is theBoussinesq reference density, and g is gravitational acceleration. The
vertically integrated KmKe, PeKe, and Ke (EKE) over the mixed layerfrom the ROMS-L2 model are plotted in Fig. 7a–c. The averaged
mixed-layer depth estimated by the KPP model (Large et al., 1994)
used in the ROMS is approximately 50 m in the L2 domain. The mixedlayer integrated PeKe is positive almost everywhere with two distinctly
high regions around the Kuroshio axis and the neighboring ﬂank on theeastern side to the islands (Fig. 7b). This PeKe distribution illustrates
the importance of baroclinic instability in the vorticity generation
within these two regions. In contrast, an axisymmetric pair of large
positive and negative areas of KmKe can be observed in the narrow
strips on both sides of the Kuroshio, representing the lateral shear
instability induced by the Kuroshio (Fig. 7a). In general, the regionswith positive PeKe and positive KmKe coincide with the areas of high Ke(Fig. 7c).
The area of highly positive PeKe is distributed widely between the
Kuroshio path and the Ryukyu Islands, whereas the highly positive
KmKe appears mostly near the Kuroshio and on the western side of theislands near the topography. The vertically integrations of KmKe, PeKe,and Ke over the mixed layer along the transect are plotted in Fig. 8.
Consistent with Fig. 7a–c, PeKe is positive and larger than KmKe almost
everywhere along the transect, indicating that baroclinic instability is
the dominant mechanism for eddy generation near the surface,especially, on the eastern side of the Kuroshio path where high valuesof Ke appear. Therefore, it is manifest that the negative vorticity on theeastern side of the Kuroshio (Fig. 5f) is provoked by a combination of
the lateral shear aﬀected by the Kuroshio, topographic eddy sheddingnear the islands, and baroclinic instability due to the Kuroshio front.
The negative KmKe on the western side of the Kuroshio suggests that
positive vorticity is suppressed by the lateral shear through an inverse
energy cascade while baroclinically destabilized by the competing
positive PeKe.
The EKE (Ke) budget is examined further for the subsurface water,where the Kuroshio is inﬂuential, by vertical integration of KmKe, PeKe,and Ke from the surface down to a depth of 1200 m with the L2 result(Fig. 7d–f). The L2 model detects large positive barotropic andbaroclinic conversion rates near the Kuroshio that coincide with theregion of high Ke. In addition, an increase of the subsurface PeKe andresultant intensiﬁcation of Ke are evident on the eastern side of theislands, due to a branch of the Kuroshio known as the Ryukyu (Under)
Current (e.g., Kawabe, 2001; Andres et al., 2008). This large PeKe could
induce further subsurface westward lateral mixing and intrusion of the
Ryukyu Current. However, this is beyond the scope of the present study
and it will be examined elsewhere. The subsurface structure on thewestern side of the islands is illustrated in Fig. 9 with respect to thevertical cross-section along transect AA′ (see Fig. 5f). The Kuroshio
main body is inclined on the shelf slope with a mean streamwise
velocity of > 0.2 m/s, even at 600 m depth. High Ke is distributed
widely near the surface to the east, coinciding with the positive KmKenear the Kuroshio and positive PeKe extending between the Kuroshio
and Okinawa Island. Conversely, high Ke is mostly conﬁned along theshelf break from the surface to 400 m depth to the west, where both
KmKe and PeKe increase in magnitude with the sign change.
Below the mixed layer, the Kuroshio is squeezed strongly against
the shelf slope on the eastern side, which provokes large velocity shear
and thus large positive KmKe, which is associated with the shear
instability due to topographic eddy shedding. Around the inclined
Kuroshio core, competing large positive PeKe and large negative KmKe

Fig. 12. Cross-sectional plots of: (a) mean streamwise velocity normal to the transect(contours) and mean temperature (color), (b) temperature variance, and (c) acrossKuroshio component of the divergent eddy heat ﬂux, dEHF (eastward positive toward theislands) from the ROMS-L2 results, along the transect shown by the black line in Fig. 5f.
White line shows the mean mixed-layer depth estimated from the KPP model.

Fig. 13. Vertically integrated dEHF (eastward positive toward the islands) over themixed layer from ROMS-L2 along transect AA′ (as shown in Fig. 5f). The black line
indicates the mean position of the Kuroshio axis.

processes associated with the Kuroshio warm water.

5. Energy conversion analysis
Energy conversion rates in the eddy kinetic energy (Ke) conservation equation are often used to quantify the relative importance ofinstability and eddy-mean interaction mechanisms (e.g., Marchesielloet al., 2003; Dong et al., 2006; Klein et al., 2008). If the conversion ofmean kinetic energy to eddy kinetic energy KmKe (viz., barotropic
conversion rate) is positive, it implicates the occurrence of shear
instability in the extraction of Ke to energize eddies. If the conversion
of eddy potential energy to eddy kinetic energy PeKe (viz., baroclinic
conversion rate) is positive, baroclinic instability is expected. We focus
on these two primary quantities, as expressed in the following
equations, in the investigation of the stimulation mechanisms of Ke:
215

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

anticyclonic submesoscale eddies developed around the Ryukyu
Islands (Section 4).
Fig. 11d–f shows the vertically integrated EHF vectors from thesurface to 1200 m depth. In general, the vectors are similar to those
integrated over the mixed layer, although several substantial diﬀerences can be observed. The total EHF and rEHF occurs mainly in the
direction of the Kuroshio path, whereas the major transport bifurcatesaround Ishigaki Island, which is located near the lower-left corner ofthe domain, forming the Ryukyu Current EHF branch that passes on
the eastern side of Okinawa Island. As this subsurface branch drifts
close to several islands, including Okinawa Island, the inﬂuence of theKuroshio on the Ryukyu Islands is partially brought by this undercurrent. Other diﬀerences include the attenuated positive acrossKuroshio transport (dEHF) between the Kuroshio and Okinawa
Island and the southeastward subsurface dEHF on the eastern side of
the islands due to the Ryukyu Current. These ﬁndings illustrate that thenear-surface dEHF brings the Kuroshio warm water to the islands,
whereas the subsurface dEHF aﬀects them in a diﬀerent way.
Fig. 12 shows cross-sectional plots of mean temperature, temperature variance, and dEHF (eastward positive to Okinawa Island) along
the transect. The mean thermocline and mixed-layer depths become
shallower toward the ECS shelf from Okinawa Island. However, theKuroshio induces additional eﬀects such that the mean thermocline isinclined to shallow both toward the ECS shelf and toward Okinawa
Island, with a near-surface bulge of warm water around the Kuroshio
axis.
The maximum lateral temperature gradient is formed adjacent to
the Kuroshio core that is inclined on the shelf slope. The overall
stratiﬁcation is increased by this inclined thermocline, established fromthe thermal wind relation with the cross-sectional velocity structure
due to the Kuroshio. Although the across-Kuroshio dEHF is positiveand conﬁned mostly in the mixed layer in the east, a tongue of negativedEHF is formed in the west, which penetrates to a depth of 400 m
along the slope (Fig. 12c). The temperature variance (Fig. 12b) is large
where the dEHF and Ke (see Fig. 8c) are consistently large.
Interestingly, the temperature variance is increased around the mean
mixed-layer depth on the ECS shelf, perhaps provoked by temporal
ﬂuctuations of the thermocline.
The mixed-layer integrated dEHF along the transect (Fig. 13)
indicates that energetic lateral eddy heat transport is induced within
and around the surface mixed layer, leading to the zonal transport of
the Kuroshio warm water. The positive eddy ﬂux develops morestrongly on the eastern side of Kuroshio than does the negative ﬂux
on the other side in the mixed layer. Nevertheless, the largest
temperature variance emerges between the Kuroshio and the slope
where the tongue of negative dEHF exists. The subsurface topographic
eddy shedding on the slope (Fig. 10) promotes this tongue of negativedEHF, which results in subsurface westward heat transport via thewarm water brought up from the bottom of the Kuroshio to the ECS
shelf. As a consequence of all these processes, lateral eddy heattransport occurs asymmetrically relative to the Kuroshio path.

are formed simultaneously below the mixed layer down to a depth of
600 m. Fig. 10 shows a snapshot of the daily averaged, normalized
relative vorticity (ζ/f) ﬁeld in the vertical section along the transect andin the horizontal section at z=−400 m from the L2 model. In Fig. 10a,negative vorticity (anticyclonic submesoscale eddies) appears dominantly near the surface on the eastern side of the Kuroshio toward
Okinawa Island, while positive cyclonic vorticity appears around theKuroshio core from the surface down to depths beyond 500 m along theshelf slope. The diameter of this cyclone is approximately 50 km, whichstill ﬁts within a typical submesoscale range. In Fig. 10b, cyclonic eddy
shedding occurs quasi-periodically from the shelf slope topography.
Therefore, a combination of topographic shear and baroclinic instability promotes the near-surface anticyclonic eddies and subsurface
cyclonic eddies, both of which are submesoscale.
6. Heat ﬂux analysis
The submesoscale anticyclonic eddies induced by the Kuroshio are
anticipated to promote eastward material transport to the west coast of
Okinawa Island through lateral eddy mixing. To quantify this eﬀect, we
assessed the lateral turbulent mixing of a tracer (i.e., heat) in the upper
ocean. The time-averaged, vertically integrated heat (potential temperature) transport equation is represented as (e.g., Marchesiello et al.,2003):
η

⎛

η

⎞

⎛

′ ′

′ ′⎞

η

∫ ⎜⎝ ∂∂uTx + ∂∂vTy ⎟⎠dz + ∫ ⎜⎝ ∂u∂xT + ∂v∂yT ⎟⎠dz + ∫ [Q(T ) + D(T )]dz=0,
−h

−h

−h

(4)
where T is potential temperature, Q is the sea surface heat ﬂux, D is theparameterized vertical and horizontal subgrid-scale mixing of heat, h isdepth, and η is surface elevation. We focus on the advective transport
by eddying ﬂow, which is a divergence of lateral eddy heat ﬂuxes(EHFs) F:

F = (Fx, Fy ) = (ρ0 Cpu′T ′, ρ0 Cpv′T ′)
−1

(5)

−1

where Cp=4000 J kg °C , which is the heat capacity of seawater at aconstant pressure. To quantify the eddy heat transport to the islands, adivergent component of the EHF is evaluated. The EHF can be
decomposed into divergent and rotational components using
Helmholtz's theorem (e.g., Aoki et al., 2013) such thatF = k × ∇ψ + ∇φ ≡ rEHF + dEHF,(6)

where k is a vertical unit vector, and ψ and φ are scalar quantities
similar to a streamfunction and a velocity potential, respectively. We
introduce the notation where rEHF and dEHF are the rotational anddivergent components of the EHF. This decomposition is conducted bynumerically solving the Poisson Eq. (6) with Neumann boundary
conditions.
The mixed-layer integrated EHF, rEHF, and dEHF vectors, superimposed on their transverse component relative to the mean Kuroshio
path from the L2 result, are plotted in Fig. 11a–c. The total EHF(Fig. 11a) is properly decomposed into the rEHF (Fig. 11b) and dEHF
(Fig. 11c). The rEHF vectors mainly follow the prevailing direction of
the northeastward-drifting Kuroshio path with recurring southwestward eddy heat transport near the islands. The eddy heat transport in
the opposite direction to the Kuroshio near the islands is obviously due
to a mesoscale secondary circulation often known as the Kuroshio
Counter Current, as reported in previous studies (e.g., Qiu andImasato, 1990). However, the mixed-layer integrated dEHF properly
measures the contribution normal to the Kuroshio axis, which manifests the lateral eddy heat transport toward the islands. Fig. 11c also
demonstrates that the near-surface heat transport to the islands occurs
more strongly on the eastern side of the Kuroshio, although a weaker
northwestward heat transport occurs on the other side. This nearsurface heat transport toward the islands is obviously induced by

7. Conclusions
Eddy-induced lateral mixing due to the Kuroshio around the
Ryukyu Islands in the ECS was investigated using a double-nested
ROMS model that downscales the assimilative JCOPE2 oceanic reanalysis to the innermost submesoscale eddy-resolving model with 1-km
grid spacing. An extensive model-data comparison was performed
against ﬁeld observations and satellite altimetry data to demonstrate
the model's capability of reproducing the Kuroshio and 3-D oceanic
structure. The model-data comparison demonstrated that the elaborated innermost high-resolution ROMS-L2 model successfully reproduced mesoscale structures spontaneously without any data assimilation.
The L2 models simulated signiﬁcant negative vorticity bias, com216

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

prising anticyclonic mesoscale and submesoscale eddies, on thewestern side of the islands. The PDF of the normalized relative vorticity
along the transect normal to the mean Kuroshio path supported thisasymmetric appearance of negative vorticity. Positive vorticity was
conﬁned mostly to the vicinity of the Kuroshio, while the peak vorticity
PDF converged to zero (viz., almost no positive and negative bias)
toward the ECS shelf. These results reinforce the speculation thateddies are generated because of interactions between the Kuroshio
warm water and the unique local topography, including the ridge of theislands to the east and the ECS continental shelf break to the west,
along which the Kuroshio persistently ﬂows.
The energy conversion analysis focusing on the barotropic andbaroclinic conversion rates suggested that the near-surface anticyclonicnegative vorticity on the eastern side of the Kuroshio and the subsurface cyclonic positive vorticity on the western side are generated via the
combination of shear instability and baroclinic instability, both ofwhich are evidently inﬂuenced by the Kuroshio. Conversely, thenegative barotropic conversion rate, which appeared near the
Kuroshio axis, suggested that cyclonic positive vorticity is suppressedby the Kuroshio's lateral shear near the surface. The resultant surface
EKE is thus also asymmetric with respect to the Kuroshio, with greater
EKE distributed widely on the eastern side of the path. However, thesubsurface water below the mixed layer reﬂected a pronouncedly
diﬀerent energy balance. The magnitude of the subsurface barotropic
conversion rate is large on the shelf break, where a positive conversion
rate appears near the slope, whereas a negative rate appears to the east,
where it competes with a large positive baroclinic conversion rate.
The heat ﬂux analysis solidly explained that these eddies promote
lateral material transport from the Kuroshio. Helmholtz decomposition
was introduced to the EHFs to evaluate the rotational and divergent
components of the EHF, rEHF, and dEHF. The decomposed rEHF
detected the contribution from the EHFs that mainly follow the Kuroshio
and the anticyclonic recurring secondary circulation referred to as theKuroshio Counter Current. Conversely, the dEHF measured the contribution normal to the Kuroshio axis, which represents the transverse
eddy-induced transport to the islands. The surface lateral eddy heattransport occurs asymmetrically relative to the Kuroshio axis, withgreater transverse eastward transport than toward the ECS shelf. Thisoccurs because of the more energetic anticyclonic submesoscale eddies
on the eastern side of the Kuroshio. Consistent with the subsurface
energy conversion rates, the depth-integrated EHFs were visibly diﬀerent from those near the surface. Although the depth-integrated EHF andrEHF occur mainly in the direction of the Kuroshio path, the acrossKuroshio transport (viz., dEHF) showed that they can be enhanced
signiﬁcantly near the surface, which promotes warm water transport inboth transverse directions relative to the Kuroshio path. However, a“negative dEHF tongue” was found to form uniquely on the shelf slope
and thus the subsurface warm water is brought upward along the slope
toward the ECS shelf. This negative dEHF tongue was attributed tosubsurface eddies generated by a combination of the baroclinic andshear instability, according to the energy conversion analysis. Thesesubsurface eddies are evidently shed on the ECS shelf slope down to adepth of 600 m as energetic cyclonic submesoscale eddies.
The present study clariﬁed that the Kuroshio warm water undoubtedly inﬂuences the biologically diverse ecosystems with abundantcorals that have formed around the Ryukyu Islands through mechanical intrusion. Based on the modeling results, it was established that theKuroshio-derived waters approach the islands in at least three ways: 1)
by transverse eddy-induced lateral mixing near the surface, 2) via aclockwise recurring ﬂow known as the Kuroshio Counter Current, and3) via a subsurface pathway associated with the Ryukyu Current. This
study focused primarily on the ﬁrst mechanism that is accompanied bysubsurface submesoscale eddy transport toward the ECS shelf, induced
by topographic eddy shedding on the slope. Further analysis will be
required to elucidate the detailed mechanisms leading to the other two
processes.

Acknowledgements
We are grateful to James C. McWilliams, Alexander F. Shchepetkin,
and M. Jeroen Molemaker of UCLA and Mayumasa Miyazawa of
JAMSTEC for their help and comments on the numerical modeling.
We are also grateful to Shohei Nakada of OIST for his help on theorganizing JMA's research vessels data. This study was supported by
JSPS Grant-in-Aid for Scientiﬁc Research C and B (KAKENHI grant
numbers: 24560622 and 15H04049).
References
Andres, M., Park, J., Wimbush, M., Zhu, X., Chang, K., Ichikawa, H., 2008. Study of theKuroshio/Ryukyu current system based on satellite-altimeter and in situ
measurements. J. Oceanogr. 64, 937–950.
Aoki, K., Minobe, S., Tanimoto, Y., Sasai, Y., 2013. Southward eddy heat transport
occurring along southern ﬂanks of the Kuroshio extension and the gulf stream in a 1/
108 global ocean general circulation model. J. Phys. Oceanogr. 43, 1899–1910.
Badin, G., Tandon, A., Mahadevan, A., 2011. Lateral mixing in the pycnocline bybaroclinic mixed layer eddies. J. Phys. Oceanogr. 41, 2080–2101.
Becker, J.J., Sandwell, D.T., Smith, W.H.F., Braud, J., Binder, B., Depner, J., Fabre, D.,
Factor, J., Ingalls, S., Kim, S.-H., Ladner, R., Marks, K., Nelson, S., Pharaoh, A.,
Trimmer, R., Von Rosenberg, J., Wallace, G., Weatherall, P., 2009. Global
bathymetry and elevation data at 30″ resolution: SRTM30_PLUS. Mar. Geod. 32 (4),355–371.
Bentamy, A., Ayina, H.-L., Queﬀeulou, P., Croize-Fillon, D., 2006. Improved near realtime surface wind resolution over the Mediterranean Sea. Ocean Sci. Discuss. 3 (3),435–470.
Blumberg, A.F., Mellor, G.L., 1987. A description of a three-dimensional coastal ocean
circulation model. In: Heaps, N. (Ed.), Three-Dimensional Coastal Ocean Models.
Coastal and estuarine Sciences. Ameri. Geophys. Union, Washington, D.C., USA,
1–16.
Boccaletti, G., Ferrari, R., Fox-Kemper, B., 2007. Mixed layer instabilities andrestratiﬁcation. J. Phys. Oceanogr. 37, 2228–2250.
Callies, J., Ferrari, R., Klymak, J.M., Gula, J., 2015. Seasonality in submesoscale
turbulence. Nat. Commun. 6, Article number: 6862.
Capet, X., McWilliams, J.C., Molemaker, J.M., Shchepetkin, A.F., 2008a. Mesoscale tosubmesoscale transition in the California current system. Part I: ﬂow structure, eddy
ﬂux, and observational tests. J. Phys. Oceanogr. 38, 29–43.
Capet, X., McWilliams, J.C., Molemaker, J.M., Shchepetkin, A.F., 2008a. Mesoscale tosubmesoscale transition in the California current system. Part II: frontal processes.
J. Phys. Oceanogr. 38, 44–64.
Capet, X., McWilliams, J.C., Molemaker, J.M., Shchepetkin, A.F., 2008b. Mesoscale tosubmesoscale transition in the California current system. Part III: energy balanceand ﬂux. J. Phys. Oceanogr. 38, 2256–2269.
Cesar, H.J.S., Burke, L., Pet-Soede, L., 2003. The Economics of Worldwide Coral Reef
Degradation. Cesar Environmental Economics Consulting, Arnhem, and WWFNetherlands, Zeist, The Netherlands. 23pp.
Centurioni, L.R., Niiler, P.P., Lee, D.K., 2004. Observations of inﬂow of Philippine sea
surface water into the South China Sea through the Luzon Strait. J. Phys. Oceanogr.
34, 113–121.
Dai, A., Qian, T., Trenberth, K.E., Milliman, J.D., 2009. Changes in continental
freshwater discharge from 1948–2004. J. Clim. 22, 2773–2791.
Dong, C., McWilliams, J.C., Shchepetkin, A.F., 2006. Island wakes in deep water. J. Phys.
Oceanogr. 37, 962–981.
Dong, C., McWilliams, J.C., 2007. A numerical study of island wakes in the Southern
California bight. Cont. Shelf Res. 27, 1233–1248.
Gula, J., Molemaker, M.J., Mcwilliams, J.C., 2014. Submesoscale cold ﬁlaments in the
Gulf Stream. J. Phys. Oceanogr. 44, 2617–2643.
Guo, X., Hukuda, H., Miyazawa, Y., Yamagata, T., 2003. A triply nested ocean model for
simulating the Kuroshio—roles of horizontal resolution on JEBAR. J. Phys.
Oceanogr. 33, 146–169.
Ichikawa, H., Beardsley, R.C., 1993. Temporal and spatial variability of volume transportof the Kuroshio in the East China Sea. Deep-Sea Res. I 40, 583–605.
Ichikawa, H., Chaen, M., 2000. Seasonal variation of heat and freshwater transports by
the Kuroshio in the East China Sea. J. Mar. Syst. 24, 119–129.
Imawaki, S., Uchida, H., Ichikawa, H., Fukazawa, M., Umatani, S., the ASUKA Group,
2001. Satellite altimeter monitoring the Kuroshio transport south of Japan. Geophys.
Res. Lett. 28 (1), 17–20.
Isoguchi, O., Shimada, M., Kawamura, H., 2010. Characteristics of ocean surface winds
in the lee of an isolated island observed by synthetic aperture radar. Mon. Weather
Rev. 139, 1744–1761.
Johns, W.E., Lee, T.N., Zhang, D., Zantopp, R., 2001. The Kuroshio East of Taiwan:
Moored transport observations from the WOCE PCM-1 array. J. Phys. Oceanogr. 31,
1031–1053.
Kawabe, M., 2001. Interannual variations of sea level at Nansei Islands and volumetransport of the Kuroshio due to wind changes. J. Phys. Oceanogr. 57, 189–205.
Klein, P., Hua, B.-L., Lapeyre, G., Capet, X., Le Gentil, S., Sasaki, H., 2008. Upper ocean
turbulence from high-resolution 3D simulations. J. Phys. Oceanogr. 38, 1748–1763.
Kunze, E., Klymak, J.M., Lien, R.-C., Ferrari, R., Lee, C.M., Sundermeyer, M.A.,
Goodman, L., 2015. Submesoscale water-mass spectra in the Sargasso Sea. J. Phys.
Oceanogr. 45, 1325–1338.

217

Continental Shelf Research 143 (2017) 206–218

Y. Kamidaira et al.

Simulations of nearshore particle-pair dispersion in Southern California. J. Phys.
Oceanogr. 443, 1862–1879.
Sasaki, H., Klein, P., Qiu, B., Sasai, Y., 2014. Impact of oceanic-scale interactions on theseasonal modulation of ocean dynamics by the atmosphere. Nat. Commun. 5, Article
number: 5636.
Sasaki, Y., Minobe, S., Inatsu, M., 2012. Inﬂuence of the Kuroshio in the East China Sea
on the early summer (Baiu) rain. J. Clim. 25, 6627–6645.
Shchepetkin, A.F., McWilliams, J.C., 2005. The regional ocean modeling system (ROMS):
a split-explicit, free-surface, topography-following-coordinate oceanic model. Ocean
Model. 9, 347–404.
Shchepetkin, A.F., McWilliams, J.C., 2008. Computational kernel algorithms for ﬁnescale, multiprocess, longtime oceanic simulations. In: Temam, R., Tribbia, J. (Eds.),
Handbook of Numerical Analysis. Elsevier, Amsterdam, 119–181.
Spalding, M.D., Ravilious, C., Green, E.P., 2001. World Atlas of Coral Reefs. The UNEPWorld Conservation Monitoring Centre, University Of California Press, Berkeley,
USA, 432.
Traon, P.Y.L., Nadal, F., Ducet, N., 1998. An improved mapping method of multisatellite
altimeter data. J. Atmos. Ocean. Technol. 15, 522–534.
Uchiyama, Y., Idica, E.Y., McWilliams, J.C., Stolzenbach, K.D., 2014. Wastewater eﬄuent
dispersal in Southern California Bays. Cont. Shelf Res. 76, 36–52.
Usui, N., Ishizaki, S., Fujii, Y., Tsujino, H., Yasuda, T., Kamachi, M., 2006. MeteorologicalResearch Institute Multivariate Ocean Variational Estimation (MOVE) system: someearly results. Adv. Space Res. 37, 806–822.
Usui, N., Tsujino, H., Fujii, Y., Kamachi, M., 2008. Generation of a trigger meander forthe 2004 Kuroshio large meander. J. Geophys. Res. 113, C01012.
Woodruﬀ, S.D., Slutz, R.J., Jenne, R.L., Steurer, P.M., 1987. A comprehensive oceanatmosphere data set. Bull. Am. Meteor. Soc. 68, 1239–1250.
Xu, H., Xu, M., Xie, S., Wang, Y., 2011. Deep atmospheric response to the spring
Kuroshio current over the East China Sea. J. Clim. 24, 4959–4972.
Yang, D., Yin, B., Liu, Z., Feng, X., 2011. Numerical study of the ocean circulation on the
East China Sea shelf and a Kuroshio bottom branch northeast of Taiwan in summer.
J. Geophys. Res. 116, C05015.

Large, W.G., McWilliams, J.C., Doney, S.C., 1994. Oceanic vertical mixing: a review andmodel with a nonlocal boundary layer parameterization. Rev. Geophys. 32, 363–403.
Marchesiello, P., McWilliams, J.C., Shchepetkin, A., 2003. Equilibrium structure anddynamics of the California current system. J. Phys. Oceanogr. 33, 753–783.
Mason, E., Molemaker, J., Shchepetkin, A.F., Colas, F., McWilliams, J.C., Sangrà, P.,
2010. Procedures for oﬄine grid nesting in regional ocean models. Ocean Model. 35,
1–15.
Miyazawa, Y., Guo, X., Yamagata, T., 2004. Roles of mesoscale eddies in the Kuroshio
paths. J. Phys. Oceanogr. 34, 2203–2222.
Miyazawa, Y., Zhang, R., Guo, X., Tamura, H., Ambe, D., Lee, J., Okuno, A., Yoshinari,
H., Setou, T., Komatsu, K., 2009. Water mass variability in the Western North Paciﬁc
detected in 15-year eddy resolving ocean reanalysis. J. Oceanogr. 65, 737–756.
Nadaoka, K., Nihei, Y., Wakaki, K., Kumano, R., Kakuma, S., Moromizato, S., Omija, T.,
Iwao, K., Shimike, K., Taniguchi, H., Nakano, Y., Ikema, T., 2001. Regional variationof water temperature around Okinawa coasts and its relationship to oﬀshore thermal
environments and coral bleaching. Coral Reefs 20, 373–384.
Nakamura, H., Nonaka, M., Sasaki, H., 2009. Seasonality of the Kuroshio pathdestabilization phenomenon in the Okinawa Trough: a numerical study of its
mechanism. J. Phys. Oceanogr. 40, 530–550.
Penven, P., Debreu, L., Marchesiello, P., McWilliams, J.C., 2006. Evaluation andapplication of the ROMS 1-way embedding procedure to the California current
upwelling system. Ocean Model. 12, 157–187.
Qiu, B., Imasato, N., 1990. A numerical study on the formation of the Kuroshio countercurrent and the Kuroshio branch current in the East China Sea. Cont. Shelf Res. 10,
165–184.
Qiu, B., 2001. Kuroshio and Oyashio Currents. In: Encyclopedia of Ocean Sciences.
Academic Press, pp. 1413–1425.
Roads, J., 2004. Experimental weekly to seasonal U.S. forecasts with the regional spectral
model. Bull. Am. Meteor. Soc. 85, 1887–1902.
Rodriguez, E., Morris, C.S., Belz, J.E., Chapin, E.C., Martin, J.M., Daﬀer, W., Hensley, S.,
2005. An Assessment of the SRTM Topographic Products. Jet Propulsion
Laboratory, Pasadena, California, 143.
Romero, L., Uchiyama, Y., Ohlman, J.C., McWilliams, J.C., Siegel, D.A., 2013.

218