Journal of Oceanography, Vol. 65, pp. 721 to 736, 2009

A Numerical Study of the Seasonal Circulation in the
Seto Inland Sea, JapanP IL-HUN CHANG1*, X INYU GUO 2 and HIDETAKA TAKEOKA2
1

National Institute of Meteorological Research,
Kisangcheng-gil, Dongjak-gu, Seoul 156-720, Korea
2Center for Marine Environmental Studies, Ehime University,
Bunkyo-cho, Matsuyama 790-8577, Japan(Received 3 February 2009; in revised form 17 March 2009; accepted 12 June 2009)

The seasonal variation of water circulation in the Seto Inland Sea is investigated using a high resolution, three-dimensional numerical ocean model. The model results
are assessed by comparison with long-term mean surface current and hydrographic
data. The simulated model results are consistent with observations, showing a distinct summer and winter circulation patterns. In summer the sea water is highly stratified in basin regions, while it is well mixed near the straits due to strong tidal mixing
there. During this period, a cold dome is formed in several basins, setting up stable
cyclonic eddies. The cyclonic circulation associated with the cold dome develops fromMay and disappears in autumn when the surface cooling starts. The experiment without freshwater input shows that a basin-scale estuarine circulation coexists with cyclonic eddy in summer. The former becomes dominant in autumn circulation after
the cold dome disappears. In winter the water is vertically well mixed, and the winterwinds play a significant role in the circulation. The northwesterly winds induce upwind (downwind) currents over the deep (shallow) water, forming a “double-gyrepattern” in the Suo-Nada, two cyclonic eddies in Hiuchi-Nada, and anticyclonic circulation in Harima-Nada in vertically averaged current fields.

Keywords:⋅ Numerical simulation,⋅ general circulation,⋅ seasonal variation,⋅ cold dome,⋅ cyclonic eddy,⋅ wind-induced
circulation,⋅ Seto Inland Sea.

The character of the water in the SIS is strongly seasonal. The great freshwater runoff from rivers (see Fig. 2
for the location of major rivers) and rainfall produce lowsalinity water during summer. For example, a water mass
with salinity of about 31 psu distributes widely in HarimaNada in summer. In combination with the freshwater input, the heat flux from spring to summer gives rise tosummer stratification in the basins where tidal mixing isrelatively weak. It also should be mentioned that a domeshaped bottom cold water mass (hereinafter, cold dome)
is formed in several Nadas; for instance, the cold dome
with anoxic water and/or cyclonic flow has been studiedin Hiuchi-Nada (e.g., Ochi and Takeoka, 1986; Guo etal., 2004; Kasai et al., 2007); residual flow related to cold
dome was investigated in Iyo-Nada (Guo et al., 2006). In
addition, hydrographic observation along the SIS during
summer has revealed a cold dome in Iyo- and HarimaNada (Kobayashi et al., 2006), similar to Fig. 6(b) (shown
later).
The abovementioned circulation features related tothe cold dome have also been studied in other seas wherestratified water is surrounded by vertically homogeneous

1. Introduction
The Seto Inland Sea (SIS), located in western Japan,
is a semi-enclosed, elongated coastal region (Fig. 1). It
has a unique geographical feature, consisting of several
basins (called “Nada” in Japanese), such as Suo-Nada,
Iyo-Nada, Hiuch-Nada and Harima-Nada, which are connected with each other through narrow straits. The SIS
opens to the Pacific Ocean via the Bungo and Kii Channels, through which a tidal wave with similar phase propagates into the Inland Sea to converge near the Bisan Strait.
Due to these geographical features, tidal mixing is strong
near the straits and relatively weak in Nada (refer to figure 4 of Yanagi and Okada, 1993). Such regional difference in tidal mixing in the SIS determines the spatial distribution of density stratification in summer (e.g.,
Takeoka, 1985).

* Corresponding author. E-mail: phchang@korea.kr
Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

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Fig. 1. Model domain and bathymetry. Thick solid line indicates southern boundary of present prognostic model. Results of adiagnostic model that covers the whole domain are imposed along the open boundaries, i.e., dotted lines at the entrance toBungo and Kii Channels. Water depths are in meters.

Fig. 2. Locations of samples and sections. Small squares indicate samples of hydrographic data for model validation (see Figs. 6
and 7), and solid dots are meteorological stations for calculation of short-wave radiation. The SST observation station isrepresented by a solid star in northern Harima-Nada. Circles with letters along the coast are the location of river inflow (seeTable 1 for annual mean discharge). Sections across eddies in summer and winter circulations are denoted by solid and dottedlines, respectively. See Figs. 9(b) and 13 for eddies calculated in summer and winter, respectively.

water. For example, through extensive field observations,
Hill (1994) revealed the presence of a cold dome and its
relationship with cyclonic eddy in the Irish Sea in summer. The observed cyclonic circulation was subsequently
studied with a two-layer (Hill, 1996) and three-dimensional models (e.g., Xing and Davis, 2001; Horsburgh andHill, 2003).
On account of surface cooling the water is homogeneous through the whole water column over the SIS in
winter. During this period, a northwesterly wind prevails
over the sea surface (refer to Fig. 3(d)), which is expected
to play an important role in the water circulation. For
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example, Fujiwara and Higo (1986) suggested that the
winter wind enhances eastward volume transport in the
SIS. Despite its importance, its role in the winter circulation in the SIS has not been well understood hitherto,
partly due to the difficulties measuring currents and partly
due to a comparative lack of modeling effort.
As mentioned above, it is anticipated that seasonally varying surface momentum, heat and salt fluxes, andriver discharge possibly cause a unique seasonal circulation in the SIS, competing with regional difference in tidal
mixing. However, little effort has been expended so far
in understanding the seasonal variation of general circu-

Fig. 3. Monthly mean wind stress fields in (a) April, (b) July, (c) October and (d) January.

lation in the SIS, in particular from the viewpoint of theentire SIS. Since the tidal effect is important in the SIS,
many numerical simulations have been performed to investigate the tides and tide-induced residual flow (e.g.,
Oonishi, 1977), tidal exchange near the narrow strait(Awaji et al., 1980; Imasato et al., 1980), and the effectof tidal stirring on density stratification (Takeoka, 1985;
Kobayashi et al., 2006). In addition, density-induced circulation in the SIS has been examined using a diagnostic
numerical model (Murakami et al., 1978, 1985; Yanagi
and Igawa, 1992; Guo et al., 2004). As Takeoka (2002)
pointed out, most numerical studies of the SIS circulation have focused on the individual processes, such astides, tide-induced residual flow, density-driven currents,
and water exchange. Furthermore, most of these studies
were limited to a basin scale, or else they dealt with theentire SIS as one-dimensional channel system. Thus, there
is still no comprehensive numerical study of seasonal circulation in the SIS.
In the present study a high-resolution, three-dimensional numerical model is applied to the SIS to examine
the seasonal variation of water circulation. Besides thetidal forcing, the model is forced by monthly surface
fluxes, river run-off and oceanic lateral fluxes via the
Bungo and Kii Channels. Here we mainly examine theseasonal variability of general circulation and the forcings
determining seasonality.
2. Model Description
The numerical ocean model adopted in this study isbased on the Princeton Ocean Model (POM), describedin detail by Blumberg and Mellor (1987). The POM is athree-dimensional, primitive-equation, sigma-coordinate

model. It incorporates thermodynamics and the second
momentum turbulent closure scheme of Mellor and
Yamada (1982) to calculate vertical mixing coefficients.
In addition, horizontal mixing coefficients areparameterized using a Smagorinsky (1963) formulation.
The POM is applied to the SIS for prognostic
modeling studies. Figure 1 shows the model domain andbottom topography. The model grid has a resolution of
about 1 km in the horizontal (1/120 degree in meridianal
direction and 1/80 degree in zonal direction), and 21 sigma
levels in the vertical. Bottom topography is obtained fromthe Japan Oceanographic Data Center (JODC) dataset with
1 km resolution. The bathymetry data in some regions,however, are somewhat incorrect, so they have been modified by depth data read from a nautical map. The minimum depth is set to 5.0 m.
The monthly surface fluxes used to drive the model
include surface momentum, heat and water fluxes. The
wind forcing is based on a recent 3-year (2003–2005)
results of wind velocity from Grid Point Value of MesoScale Model (GPV-MSM) provided by Japan Meteorological Agency (JMA). The monthly wind stress is computed from the wind velocity of GPV-MSM with 6 hourand 10 km resolutions at 10 meters height, adopting thedrag coefficient of Large and Pond (1981). Figure 3 shows
seasonal variation of wind stress in the SIS. In general,
the spring wind is weak and its direction is region-dependent. In July, southeasterly and southerly winds are
dominant but the stress is still relatively small. Thereafter, northerly winds commence in October. In January,
intense northwesterly wind stress covers the whole SIS.
The net surface heat flux, Q, excluding the shortwave radiation, is calculated by:

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Table 1. Annual mean river discharges of 21 major rivers in
the Seto Inland Sea, based on the 8-year (1993–2000) datasetof Ministry of Land, Infrastructure and Transport (MLIT).
The location of river mouths is shown in Fig. 2.

Fig. 4. Seasonal variations of long-wave radiation, sensible
and latent heat fluxes at the station in Harima-Nada, spatially averaged short-wave radiation over whole model domain and the sum of four components of heat flux, i.e., net
heat flux.

Q = QO + γ T (T − Tclim. ),(1)

where QO is the surface heat flux. The monthly mean QO
of is computed by adopting bulk formulae described by
Hirose et al. (1999), utilizing daily sea surface temperature (SST) observed at a coastal station in Harima-Nadaby the Fisheries Experiment Station Okayama Prefecture
and the atmospheric data from the meteorological stationin Okayama during 23 years (1974–1996). The locationsof the coastal station for SST and the meteorological station are given in Fig. 2. The calculated QO is applied uniformly throughout the model domain and is modified to
Q by the second term in (1), in which T is the calculated
SST, and Tclim. is the monthly mean SST based on the
Marine Information Research Center (MIRC) dataset covering 30 years, and γT is chosen as –40.0 W m–2degree–1.
The short-wave radiation Qs is calculated by the formulaof Kim and Kimura (1995), using daily fractional cloud
cover observed at 19 meteorological stations (see the location in Fig. 2) around the SIS during 1974–1996. Figure 4 shows the seasonal variation of spatially averaged
QO (i.e., long-wave radiation, sensible and latent fluxes)
and short-wave radiation Qs.
Surface salinity flux, W s is calculated as follows:
Ws = ( P − E ) S + γ S ( S − Sclim. ).

(2 )For the term γS we adopted the weak value of –10.0/365
m day–1. P and E are precipitation and evaporation given
by Tawara (1986). S is calculated surface salinity, Sclim.
and is climatological monthly surface salinity from MIRC

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P.-H. Chang et al.

No.

River

a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u

Kino
Yamato
Yodo
Kako
Ibo
Yoshii
Asahi
Takahashi
Ashida
Oota
Oze
Saba
Yamakuni
Oita
Oono
BanshoHiji
Sigenobu
Doki
YoshinoNaka

Discharge (m3 s – 1 )
48.63
26.27
242.42
43.56
27.33
63.25
55.43
64.61
11.50
76.74
12.84
17.67
21.03
27.25
62.28
14.26
37.84
8.93
1.97
37.70
61.28

dataset.
The present prognostic model employs the results ofa robust diagnostic model, which was previously used for
Hiuchi-Nada (Guo et al., 2004), as boundary conditions
at the open boundaries (i.e., Bungo and Kii Channels).
The grid size and bathymetry of diagnostic model are thesame as in the present model, while the south boundaryof the diagnostic model extends to 32°N (see Fig. 1), including the Kuroshio region. The initial temperature andsalinity fields used in diagnostic model are produced bymerging the MIRC dataset in the SIS region and the model
results of Guo et al. (2003) in the Kuroshio region. Twelve
cases of diagnostic calculations, corresponding to 12
months, were performed by adopting monthly density andwind fields mentioned above. The de-tided velocity, temperature, salinity, and surface elevation obtained from thediagnostic model are then specified along the open boundary of the prognostic model (see thick dotted line in Fig.
1). Along the open boundary, tidal forcing due to four
major tidal constituents M2, S2, O1 and K 1, provided by
Matsumoto et al. (2000), are also linearly superimposedon the sub-tidal fields from the diagnostic model.
In the prognostic model the normal velocity along
the open boundary, U n , is computed according to theFlather (1976) radiation condition:

Fig. 5. Surface residual current distributions. (a) Schematic map of annual mean residual current based on the observations(Yanagi and Higuchi, 1979) and (b) simulated current distribution averaged from May to August. See text for explanation of
the reason why we use current field averaged from May to August.

()

Un = U Diag. + UT + g / H η − ηT − η Diag. ,(3)

where U Diag. and ηDiag. represent de-tided normal velocity and surface elevation from the diagnostic model, respectively. UT and ηT denote the tidal flow and the tidal
elevation computed using the harmonic constants of four
major tidal components, provided by Matsumoto et al.
(2000). η denotes predicted sea elevation in the model,
and H water depth along the open boundary.
The present model includes major 21 rivers, of which
the locations are shown in Fig. 2. The monthly river discharges averaged over 8 years (1993–2000), reported by
Ministry of Land, Infrastructure and Transport (MLIT),
were used in the calculation. The annual variation of allriver discharges shows that the discharge reaches its maximum (2.08 × 10 3 m3s –1) in July and minimum (0.49 × 103
m3s–1) in December. In general, large rivers, in terms ofvolume discharge, are located along the northern coastsof SIS, as can be understood from the annual mean discharges of each river (Table 1).
The initial temperature and salinity fields in January
are from the MIRC dataset, and then the model was run
for two years. All the monthly forcings on the surface
and lateral boundaries were interpolated linearly in timeat each time step. Here, results in the second year areused to describe the circulation in the SIS.

3.

Comparison between Model Simulations and Observations

3.1 Validation of surface current fields
To assess the model’s performance, in this section
we compare surface currents fields and hydrography between observations and simulated results. As briefly mentioned in Section 1, the observed current data covering
the entire SIS are not sufficient to describe the seasonal
water circulation. Yanagi and Higuchi (1979) collected
observed surface current data and depicted annual-mean
residual flow for the entire SIS. However, a large numberof those collected data were observed in the period from
May to August. Thus, although part of their data was obtained in other seasons, their annual mean flow basically
represents the current distribution in late spring and summer.
Here we first compare the schematic map of surface
current (Fig. 5(a)) given by Yanagi and Higuchi (1979),
and the simulated result averaged from May to August(Fig. 5(b)). In the observation and model results, large
cyclonic and anticyclonic eddies are developed near theentrance and inner part of Suo-Nada, respectively. These
two eddies were also suggested from the movement ofdrift bottles observed during the spring season (Kamizono
et al., 1991) and a diagnostic model forced by monthlyhydrographic data in warm seasons (Balotro et al., 2002).
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725

Associated with the well-developed large cycloniceddy in Suo-Nada, the southward flow hugging Kunisaki
Peninsula reaches Hayasui Strait in both the observed andmodel results. In our model, the southward flow has amagnitude of 0.1–0.2 m s–1 during the warming season
and is strengthened in autumn (shown later in Fig. 9(c)).
Based on the long-term mean current fields observed during June through October, Tawara (1986) also pointed outthat a strong southward current of 0.2–0.3 m s –1 was seenalong the Kunisaki Peninsula. He also mentioned that thesouthward current become weak in winter, as shown in
our model (refer to Fig. 9(d)). The model seems to illustrate the southward current flowing to the western Bungo
Channel via the Hayasui Strait. In the eastern part of
Bungo Channel, however, the current direction in themodel turns out to be northward, while that in the observed direction is southward.
Observation shows an apparent southwestward current in the northern Iyo-Nada. A strong southwestward
current (over 0.15 m s–1) also appears in the model, and acyclonic eddy coexists with this outward current, as recent field observations have reported (Guo et al., 2006).
The observed result indicates that the southward current
from the Bisan Strait is directed to the center of HiuchiNada, where it bifurcates into the eastern and westernsides. Analogous patterns are shown in the model, but
the velocity seems to be weak.
The model also well reproduces the observed circulation features in the Harima-Nada, showing two eddies.
In Osaka Bay, the southward current along the eastern
coast and anticyclonic flow near the Akashi Strait arefound in both the observed and model results. Fujiwara
et al. (1989) described the residual current system inOsaka Bay, and the present model results coincide wellwith their conclusions. Moreover, a cyclonic eddy isclearly seen in Kii Channel in both the observed and model
results. In the model, the outflow with a magnitude of
0.1–0.25 m s–1 from Osaka Bay to Kii Channel is directedto the western part of the channel (also, refer to Fig. 9(b)).
Similar surface current fields have been shown in previous observations. On the basis of ADCP observation insummer 1996, Fujiwara et al. (1997) demonstrated thatthe outflow over 0.1 m s–1 from Osaka Bay turns to thewestern Kii Channel due to the cyclonic eddy existingaround 34°N. Tawara (1986) also showed that a southward surface current of about 0.2 m s–1 existed along thewestern Kii Channel.
Simulated currents in some regions, such as thenorthern coast of Harima-Nada and Bungo Channel, however, differ from the observed patterns. One possible cause
might arise from the different observation periods. For
instance, the coastal current in northern Harima-Nada
exhibits strong seasonality in our model; it is westward
in summer and eastward in winter (shown later in Fig. 9).

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Fig. 6. Vertical distributions of calculated temperature and salinity, (a) and (c), along a section across the whole Seto
Inland Sea in July (see Fig. 2 for location of stations). Also
shown are those for the observation, (b) and (d). The dotsin (b) and (d) denote data depths.

This means that there is a limitation to comparing all theobserved current patterns with the model results. Nevertheless, the overall features of modeled surface currents
are qualitatively quite consistent with the observations.
3.2 Summer and winter hydrography
In addition to the surface current fields, here we compare the vertical distributions of temperature and salinity
along the SIS, as between observation and model (Figs. 6
and 7; sample locations adopted are depicted in Fig. 2).
Observed results are based on the 30-year MIRC dataset.
Figure 6 shows the temperature and salinity sections of
the model and observation in July. In general, both modeland observed results exhibit quite well the mixed wateraround the straits and the well stratified water in the
Nadas. In temperature sections (Figs. 6(a) and (b)), sur-the Bungo Channel. Toward Bungo and Kii Channels, lesssaline water flows outward in the surface layer.
Figure 7 shows vertical distributions of temperatureand salinity in winter. The model and observed results
show the well-mixed water through the water column inthe entire SIS. In general, warm water distributes in Bungo
and Kii Channels, and water temperature becomes lower
toward the inside SIS (Figs. 7(a) and (b)). The lowest temperature is detected in Bisan Strait in both model andobserved value, although the model result is about 2.0°C
higher than the observation (see samples near 30). Simulated temperature in Harima-Nada and Osaka Bay is similar to the observed value, showing about 11.0°C and 12°C,
respectively (see samples near 34 and 40).
Similar to the summer, the modeled salinity in winter is higher about 0.25–0.5 psu than the observed value,
probably due to the underestimation of river discharges
into the model domain. However, the overall feature issimilar to the observed pattern; high salinity water over
34 psu is found in Bungo Channel, and relatively low
salinity water appears in the eastern part of SIS (Figs.
7(c) and (d)). In addition, cold, less saline water exists inthe opening of Osaka Bay (areas of 40–45), forming afront structure against the relatively warm, saline waterin Kii Channel.

Fig. 7. As Fig. 6, but for January.

face warm pools and bottom cold water are formed in the
Nadas. Similar to the observations, a cold dome is wellestablished in Iyo-Nada, Harima-Nada and Kii Channel
in the model (Fig. 6(a)). However, the calculated temperature at the Bisan Strait (i.e., areas of 27–30 in Fig. 2)
is higher than that the observed value, and the bottom
temperature in Harima-Nada is lower than that observedby about 2.5°C (area 34). Consequently, a strong bottom
front in Harima-Nada is shown in our model.
Since the present model does not consider all riverdischarges in the SIS, the spatially-averaged salinity inthe entire SIS is slightly higher in the model than the observed value (Figs. 6(c) and (d)). Nevertheless, the simulated results quite well represents the observed salinity
pattern. The less saline water covers the entire SIS during summer, except for Bungo Channel. In particular,
strong salinity stratification is detected near Osaka Bay,the location of the largest river in terms of freshwater
volume discharge (see areas 40–45). Moreover, the vertical salinity variation is greater in the Kii Channel than in

4. Model Results: Circulation Inferred from the ModelIn this section we examine water circulation in the
SIS inferred from the model results. Since our major concern is to investigate the seasonal variation of circulation, we subtract tide-induced residual current from tidally
averaged mean current. To do this, we begin with tideinduced residual current in the next subsection. In addition, we describe other experiments without wind stress(i.e., the without-wind case) in Subsection 4.4 to elucidate its role in the winter circulation.
4.1 Tide-induced residual current
The tide-induced residual current is obtained by running the 3-D ocean model (i.e., POM) driven only by thetidal forcing. The bathymetry and harmonic constants
mentioned in Section 2 are given, and are also the same
conditions as those used by Guo (2005). The homogeneous density field is applied in the calculation. The simulated tides and tidal flow are almost the same as theirresults (Guo, 2005), which present model validation anddetailed results on tides and tidal flows. Figure 8 displays
computed tide-induced residual currents. The simulatedtide-induced residual current exhibits similar features tothose shown in previous numerical studies; for example,
anticyclonic eddies in the vicinity of Akashi Strait(Oonishi, 1979; Fujiwara et al., 1994) and a cyclonic residual current near the western Hiuchi Nada (Guo et al.,2004). On the whole, the tide-induced residual current is

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727

Fig. 8. Tide-induced surface residual current at 10 m depth. Tones indicate magnitude of velocity in m s–1. Currents weaker than
0.005 m s –1 are omitted to avoid overcrowding.

strong near the straits (>0.06 m s–1), while it is relatively
weak in the Nadas and Kii Channel.
4.2 Seasonal variation of current fields
In this section we investigate the seasonal variationof surface and bottom current fields based on the model
results. In order to present seasonally varying flow fields(i.e., density- and wind-driven flows), the calculated
Eulerian tide-induced residual currents described in theabove subsection are removed from tidally averaged current fields. Hereinafter, we describe the modeled monthly
current fields in April, July, October, and January, defined as representative months in spring, summer, autumn,
and winter, respectively (Figs. 9 and 10). In April, thenorthward warm current from the open sea is clearly seenin the surface layer of Bungo Channel, and the current
extends to the western Iyo-Nada forming a large anticyclonic circulation at the north to Hayasui Strait (Fig. 9(a)).
Furthermore, the distinct cyclonic circulation is shownin Kii-Channel. In general, the circulation inside SIS in
spring is weak, compared to the circulation in summer.
Current distribution in July is similar to Fig. 5(b),showing several cyclonic eddies in the Nadas (Fig. 9(b)).
In our model those eddies are formed from May when thethermal stratification begins, to become fully developedin July. In addition to these eddies, two basin scale outflow systems toward Bungo and Kii Channels are shownin the surface layer, as suggested by Murakami et al.
(1985): an outward flow from Suo- and Aki-Nada via
Bungo Channel in the western SIS, and a coastal current
at the southern Harima-Nada and the flow connectingfrom Osaka Bay to western Kii Channel in the eastern
SIS. Since wind stress is not strong (Fig. 3(b)), it is natural to anticipate that the surface current in July could bedensity-driven. The cyclonic eddies related to density
structure will be described in the next subsection.
Eddies that were dominant in Suo-, Iyo- and Harima728

P.-H. Chang et al.

Nada during summer are no longer discernable in October (Fig. 9(c)). It is, however, interesting that the strong
outward flows in summer are still dominant in the surface layer. The southward flow hugging Kunisaki Peninsula reaches 0.17 m s–1, which is its maximum throughout the year. The current from Aki-Nada now reveals anorthwestward excursion to the entrance of Suo-Nada,
where it merges with the southward flow offshore
Kunisaki Peninsula. In Contrast to the summer case, ananticyclonic circulation now forms in Kii Channel, andthe strong outward flow of about 0.1–0.2 m s–1 from Osaka
Bay merges with it and flows out via the eastern part ofthe channel.
In January, when the water is destratified and thenorthwesterly wind stress is strong, the current becomes
weak, except for regions near the open sea (Fig. 9(d)).
The southward flow and a strong anticyclonic circulation
are dominant on the surface layer in Bungo Channel andthe western Iyo-Nada, respectively. In addition, a southward coastal current appears along the eastern side of
Bungo and Kii Channels. Although the flow velocity is
weak, it is likely that parts of the anticyclonic flow in thewestern Iyo-Nada penetrate into the Suo-Nada. In HarimaNada, an eddy-like flow with a magnitude of about 0.03–
0.05 m s–1 is found in the surface layer.
Figure 10 shows the current distribution near thebottom layer (i.e., 20th layer in the model) in the corresponding month of Fig. 9. Compared to the surface currents, the flow in the bottom layer is relatively weak andunorganized. Moreover, it is generally directed to the inner sea during summer and autumn when the less saline
water covers the entire inland sea. In April there is no
distinct circulation pattern, except for Bungo Channel.
The outward flow of 0.03–0.04 m s–1 is clearly seen in
Bungo Channel, forming an inverse estuarine circulation
along with the surface current (Fig. 9(a)).
In summer, the bottom current becomes intensive

Fig. 9. Monthly mean surface residual current distributions at
10 m depth in (a) April, (b) July, (c) October, and (d) January, after subtracting tide-induced residual current shownin Fig. 8.

especially in the western part of SIS; an inflow of about
0.05 m s–1 exists in Bungo Channel, and a non-organized
but inward flow pattern is shown in Aki-Nada (Fig. 10(b)).
The inward flow pattern extends to the Bisan Strait. Although the bottom current is weak in the eastern SIS, it isalso directed to the inner sea. It appears that the inflows
from the western and eastern SIS converge at Bisan Strait.
In addition, the bottom current is weak in the regions
where the cyclonic circulations exist in the surface layer;in particular, divergence is found in Suo- and Iyo-Nada.
Associated with the disappearance of cyclonic eddies, the divergence in bottom layer can no longer be discerned (Fig. 10(c)). Simultaneously, the flow toward theinside SIS becomes intense. It is likely that a two-layer
opposite flow (i.e., estuarine circulation) becomes strong

Fig. 10. As Fig. 9, but for near-bottom current fields.

in autumn, especially in Suo- and Iyo-Nada. Moreover,
the alongshore current in the Harima-Nada to the Bisan
Strait becomes stronger, and a typical up-estuarine flow
is found in the eastern Osaka Bay.
The near-bottom flow to the inside SIS is no longer
dominant in winter (Fig. 10(d)). On the whole, the bottom current opposite the northwesterly wind develops in
the SIS. In the Hiuchi- and Harima-Nada, the upwind
currents are stronger in the deeper region than the shallow water. Moreover, an eddy-like flow pattern is displayed in the northern Harima-Nada. These winter circulation patterns are examined more detail in Subsection
4.4.
4.3 Cyclonic circulation: summer feature
Summer current fields described in the previous section demonstrate that the cyclonic eddies are dominant inseveral Nadas where tidal mixing is relatively weak. AlSeasonal Circulation in the Seto Inland Sea

729

Fig. 11. Temperature difference between surface and bottom layers in July. Vectors indicate vertically averaged residual current
after subtracting tide-induced residual current.

Fig. 12. Monthly mean cross-sectional distributions of (a)–(c) temperature (°C); (d)–(f) salinity (psu); (g)–(i) density, σ t;(j)–(l) normal velocity (m s–1) for the Suo- (left panels), Iyo- (middle panels), and Harima-Nada (right panels) in July. See
Fig. 2 for location of the sections.

though the current fields in Fig. 9(b), in which tidal residual flow is subtracted, also include a wind-driven component and nonlinear interaction of the residual flow, it
is mostly driven by the density gradient because of weaksummer winds. Figure 11 indicates the temperature dif730

P.-H. Chang et al.

ference between the surface and bottom layers as well as
vertically averaged current fields after removing tide residual flow. Since outward flow in the surface layer iscanceled by inward flow in the bottom layer, the cycloniceddies become salient. Due to strong tidal mixing, tem-Fig. 13. Monthly mean distributions of (a) vertically averaged current field and (b) surface elevation in January. Grids deeper
than the average water depth in each Nada (surrounded by three boxes) are shaded. Spatially averaged wind direction is given
in upper-left corner of each panel.

perature difference is negligible near the straits. By contrast, thermal stratification is strong in the Nadas, where
the cyclonic eddies are clearly overlapped by the large
temperature difference (see the light colored region; ∆t >4.0°C). On the other hand, the salinity difference does
not indicate a distinct relationship with the cyclonic circulation (not shown). Since the temperature structure ismostly responsible for the density structure in those
Nadas, that in turn induces the cyclonic circulation.
Figure 12 presents vertical distributions of temperature, salinity, density and normal velocity at sections in
the Suo-, Iyo- and Harima-Nada in July where cycloniceddies are established (see marked solid line in Fig. 2 forsections). An increase of solar heating and river run-off
induce high temperature and low salinity water in thesurface layer during summer. Although its scale is different, the warm pool is also discernible in the surface layer,
especially in Iyo-Nada. The most remarkable structure isthe cold dome of about 16.0–18.0°C beneath thethermocline in the middle of each Nada (Figs. 12(a)–(c)).
Although a similar structure is shown in the salinity distribution of Harima-Nada, it is considered that the temperature (i.e., cold dome) is most responsible for the symmetric density structure. Associated with these cold domes
are the cyclonic flows that are in geostrophic balance withthe density fields through the thermal wind relation (Figs.
12(j)–(l)). The cyclonic flow reveals a robust horizontal

structure. In addition, high velocities of about 0.1 m s–1
exist in the subsurface layer; this feature is particularly
distinct in Iyo-Nada, because of the interference of anticyclonic flow in the surface layer due to the warm pool
(Davis and Xing, 2006).
Based on ship-mounted ADCP and CTD observations, Guo et al. (2006) revealed an eddy structure in thevelocity field in Iyo-Nada during late July, 2005. Notethat their observation section is located near the marked
line in Fig. 2. The cyclonic subtidal current has a magnitude of 0.1–0.2 m s–1; the outflow reaches around 0.2
m s–1, while the inflow is less than 0.1 m s–1 in the subsurface layer. Although the model could not directly compare with the above field observation, it is considered thatthe model results are comparable to observations madein the Iyo-Nada during summer.
As inferred from horizontal current fields in Figs. 9
and 10, an estuarine circulation exists in the SIS during
summer and autumn. We can confirm the flow structurein the velocity section of northern Iyo-Nada (>33.7°N)
and southern Suo-Nada (i.e., offshore Kunisaki Peninsula,<33.7°N). It is expected that the sharp gradient of low
salinity water induces those circulations there, implyingthat the increased freshwater input during summer has an
influence on the circulation in the SIS. The roles of freshwater discharge in the summer and autumn circulation
are discussed in Section 5.
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731

4.4 Wind-driven circulation: winter feature
As shown in the vertical distributions of temperature and salinity (Fig. 7), the water in winter is verticallywell mixed, except for Osaka Bay. On the other hand, thestrong northwesterly wind stress is predominant in thewhole inland sea. To examine wind effects on the circulation, we begin with the vertically averaged current fields
in January (Fig. 13(a)). The computed current field reveals that it is downwind in the shallow coastal region,
while a upwind current concentrates at deep parts of theFig. 14. Monthly mean cross-sectional distributions of normal
velocity (m s –1) for the Suo- (upper panel), Hiuchi- (middle
panel), and Harima-Nada (lower panel). Note that positive(negative) values indicate downwind (upwind) current. See
marked dotted lines in Fig. 2 for position of sections. Contour interval is 0.01 m s –1.

Nadas (see shaded region), forming basin-scale eddies: adouble-gyre in the Suo-Nada, and two cyclonic eddies in
Hiuchi-Nada and an anticyclonic eddy in Harima-Nada.
Besides those eddies, a southward coastal current is foundin the eastern part of Bungo and Kii Channels, while astrong anticyclonic current is developed in western IyoNada, supplying warm and salt water into Suo-Nada. Figure 13(b) shows the surface elevation during the same
periods. The distribution of sea level shows that the sea
level is high at the eastern part in each Nada and low at
the straits, as examined in Fujiwara and Higo (1986). In
general, sea level slope confronts the northwesterly winds
throughout the whole SIS, suggesting that the geostrophic
component only could not explain the flow patterns described above.
Here we describe the vertical distributions of above
eddies, adopting sections marked in Fig. 2 (see thick dotted lines). Note that negative (positive) values represent
upwind (downwind) in normal velocity component. Fromthe modeled results, it can be seen that the wind drives
downwind circulation throughout the water column overthe shallow water (Fig. 14(a)). The maximum velocity,
over 0.05 m s–1, exists at the surface layer, and the velocity decreases with increasing bottom friction. On the other
hand, the upwind current is predominant in the deep region, and the current core is found at the mid-layer in
Suo-Nada and the deeper layer in Hiuchi- and HarimaNada. When considering the distribution of surface elevation, it is expected that the pressure gradient forcing
due to the sea level slope induces the upwind current in
the deep region (deeper than average water depth), as
described by Csanady (1982).
To elucidate the effect of wind forcing, we performed
an additional experiment without wind forcing. Figure 15
shows the vertically averaged current fields in the without-wind case. There is no distinct circulation pattern in
the results of the without-wind case, and the circulationin shallow water, such as eddies in Suo-, Hiuchi-, andHriama-Nada, is no longer discerned. By contrast, an

Fig. 15. Vertically averaged current distribution in January, obtained from without-wind experiment.
732

P.-H. Chang et al.

Fig. 16. Monthly variations of the temperature at sections across cyclonic eddies in the Suo- (left panels), Iyo- (middle panels),and Harima-Nada (right panels) from May to September. Contour interval is 1.0°C.

outward flow pattern along the deep region in Suo-Nada
appears due to the gravitational effect of dense water in ashallow region. The without-wind experiment evidently
indicates that the eddies shown in shallow Nada are mostly
wind-driven. In addition, the winter wind reinforces the
anticyclonic circulation in the western Iyo-Nada.
5. Conclusions and Discussion
A high-resolution, three dimensional numerical ocean
model based on the POM has been applied to the SIS tosimulate the seasonal variation of general circulation.
Annually varying forcings, such as surface fluxes, riverdischarge and lateral flux are imposed on the model sea,
as well as tidal forcing. Since this is the first time that thethree dimensional model with realistic conditions has been
applied to the entire SIS, the general circulation features
obtained by the model are presented and compared withobservation. The model reproduces well the observationsof long-term mean surface current and hydrographic features, and the simulated water circulation reveals remarkable seasonal variability. The seasonal variation of current field is summarized below.
In early spring there are no remarkable currents in
the Nadas, but inverse estuarine circulation and a cycloniceddy are established in Bungo and Kii Channels, respectively. As stratification becomes strong during summer,
cyclonic eddies associated with a cold dome appear inseveral Nadas, such as Suo-, Iyo-, and Harima-Nada.
Moreover, an outward flow toward Bungo and Kii Channels exists in the surface layer, while there is an inward
flow near the bottom layer. The cyclonic eddies disappear in autumn, while the two-layer opposite circulation
system remains. In winter, the surface current exhibits arelatively weak circulation. During this period the antiSeasonal Circulation in the Seto Inland Sea

733

Fig. 17. Simulated surface current distributions for the case ofwithout freshwater input, in (a) July and (b) October. Also
given are differences between cases with and without freshwater input in (c) July and (d) October.

cyclonic circulation in the western Iyo-Nada is developed,
and the current below the surface layer generally shows
an oppositely directed flow to the intense northwesterly
wind stress, especially over the deep water of Suo-,
Hiuchi-, and Harima-Nada.
One of the most important implications of our model
results relates to the presence of a cold dome, which issubstantial in the summer circulation. The sharp pressure
gradient due to the cold dome is the main forcing thatgenerates cyclonic eddies in the Nadas. To examine thetemporal variation of the cold dome, here we illustrate
the vertical temperature distributions from May to September at each Nada (Fig. 16). The model results indicate that the cold dome generally forms in May and weakens from August. With the extinction of the cold dome,

734

P.-H. Chang et al.

cyclonic eddies also disappear in our model. The rate of
temperature change in the cold dome is roughly 2.0–2.6°C month–1 during the warming season (May–August),
which is similar to that in the surface layer (2.5–3.2°C month–1). When considering strong summer stratification, this implies that the horizontal advection/mixing
might play an important role in supplying heat from thesurrounding well-mixed region to the cold dome (e.g.,
Takeoka, 1985, 2002).
In addition to the thermal structure, freshwater discharge during summer has also a great effect on the density distribution in the SIS (Takeoka, 1985). To elucidate
the effect of freshwater input, a simple experiment without freshwater input was carried out after the model had
achieved a stable state. The noticeable differences in flow
fields with and without freshwater input are mostly revealed in the summer and autumn periods. Figure 17
shows surface current without freshwater input in Julyand October (Figs. 17(a) and (b)), and the difference between cases with and without freshwater input in the samemonths (Figs. 17(c) and (d)). The current field without
freshwater influx in July is similar to the major summer
circulation in Fig. 9(b), showing cyclonic eddies in the
Nadas. However, a relatively large difference is found in
the outward flow (Fig. 17(c)): outward surface currents
from Iyo Nada (Osaka Bay) to Bungo Channel (Kii Channel).
The effect of freshwater input is more apparent in
October. It can be seen that there is no remarkable flow
pattern in the SIS, except in both channels (Fig. 17(b)).
As in summer, the outward velocity anomaly is pronounced in the difference plot (Fig. 17(d)), showing asimilar pattern to the surface current field in autumn (seeFig. 9(c)). This is also the same for the inward flow near
the bottom layer (not shown). It is therefore clear that the
freshwater influx underlies the maintenance of the strong
estuarine circulation in the transition periods, which isrepresented in the present model study.
The validated model could be useful in understanding a basic winter circulation feature in the SIS, in whichobservations of flow velocity are deficient during thewinter season. The computed winter circulation demonstrates that the intense winter wind has a significant effect on the circulation in the SIS. Since the water is well
mixed throughout the water column during winter, theintense wind stress easily acts on the deeper water. The
downwind current is thus dominant over the shallow water of each Nada, implying that acceleration due to windstress is stronger than the pressure gradient. By contrast,in the region where the depth is deeper than average, thepressure gradient induces an upwind current. As a consequence, the vertically integrated flow pattern represents
an eddy structure in several Nadas: a “double-gyre pattern” in the Suo-Nada, two anticyclonic eddies in Hiuchi-Nada, and anticyclonic circulation in Harima-Nada.
The present study shows that the cold dome is an
important water structure that determines the summer circulation in several Nadas. Based on the field observations in 2004 and 2005, Guo et al. (2006) reported a significant inter-annual variability of the cold dome in the
Iyo-Nada and suggested the possibility of a competition
between the cyclonic eddy and estuarine circulation in
Iyo-Nada, which is located on the pathway of a basinscale estuarine circulation in the SIS. For a full understanding of such field observations, it is necessary to
clarify the factors controlling the formation of cold domesin the SIS. The fact that the cold dome and the surface
layer above have a similarly increasing rate of temperature indicates the importance of horizontal advection/
mixing process between the stratified water in the Nadas
and ambient homogeneous water in the straits. Our next
study will examine the different role of horizontal andvertical processes in heat and salt transports in the SIS, afundamental issue related to the formation of the colddome.
Acknowledgements
The authors express their sincere thanks to two
anonymous reviewers for their useful comments. P.-H.
Chang is grateful for the financial support from the Japan Society for the Promotion of Science (JSPS),
Postdoctoral Fellowship for Foreign Researchers. The
study was also supported by a Grant-in-Aid for the Global COE Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), andthe Japan Society for the Promotion of Science (JSPS).
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