Journal of Marine Systems 191 (2019) 24–37

Contents lists available at ScienceDirect

Journal of Marine Systems
journal homepage: www.elsevier.com/locate/jmarsys

The age of Yodo River water in the Seto Inland Sea
a,cHaiyan Wang , Xinyu Guo
a
b
c

b,⁎

, Zhe LiuTc,1

National Marine Environmental Forecasting Center, Beijing 100081, China
Center for Marine Environmental Studies, Ehime University, Matsuyama 790-8577, JapanKey Laboratory of Marine Environment and Ecology (Ocean University of China), Ministry of Education, Qingdao 266100, China

A R T I C LE I N FO

A B S T R A C T

Keywords:
Water age
CART
Yodo River
Seto Inland Sea

The age of Yodo River water, the largest river ﬂowing in the Seto Inland Sea (SIS), was calculated using theconstituent-oriented age and residence time theory (CART) in order to understand the long-term materialtransport and environmental issues in this semi-enclosed sea. The Yodo River water was found to have a mean
age of 152 days for the entire SIS. Inside the SIS, the age of Yodo River water shows spatial variation ranging
from 97 days in Osaka Bay, 199 days in the Harima-Nada, and 450 days in other areas. The age of Yodo River
water shows an apparent seasonal variation in the SIS. Tracing water from the Yodo River reveals two pathways
for its movement around Osaka Bay: one brings ~5/8 river water directly into the Kii Channel through the Kitan
Strait; the other brings ~3/8 river water into the Harima-Nada through the Akashi Strait that eventually enters
the Kii Channel through the Naruto Strait. The mean age of Yodo River water is 128 days at the Kitan Strait,
125 days at the Akashi Strait, 203 days at the Naruto Strait, and 271 days at the Bisan Strait. The above estimation considers the return of old Yodo River water into Osaka Bay that does not change the spatial distributionof Yodo River water age inside Osaka Bay but increases the value of water age by ~80%. Without tidal forcing,the mean ages of Yodo River water in Osaka Bay and the Harima-Nada increase by 6% and 51%, respectively,
and nearly all the Yodo River water ﬂows directly into the Kii Channel through the Kitan Strait. Without considering baroclinic forcing, the mean ages of Yodo River water in Osaka Bay and the Harima-Nada increases by
52% and 51%, respectively, while the inﬂuence of local winds and the Yodo River discharge on the age of Yodo
River water are negligible.

1. Introduction
The age of river water at a speciﬁed location in a coastal sea isdeﬁned as the time elapsed since the river water leaves the river mouth,
where its age is prescribed as zero, to its arrival at the location of interest (Bolin and Rodhe, 1973; Takeoka, 1984a). Much land-based
nutrients and pollutants discharge into a coastal sea from a river mouth
and then travel with the river water inside the sea. The age of riverwater, as an important auxiliary variable to describe the transport
timescales, can help us to understand the environmental issues incoastal sea areas.
Many researchers have simulated the age of river water based on theconstituent-oriented age and residence time theory (CART, see www.
climate.be/cart; Deleersnijder et al., 2001). The study areas to which
this theory has been applied covers (1) river estuaries, e.g., York Riverestuary (Shen and Haas, 2004), James River estuary (Shen and Lin,
2006), and Rappahannock River estuary (Gong et al., 2009) in

Chesapeake Bay, Pearl River estuary (Ren et al., 2014), Changjiang
estuary (Wang et al., 2010, 2015), and Columbia River estuary (Karnaand Baptista, 2016), (2) semi-enclosed seas, e.g., Baltic Sea (Meier,2007) and Bohai Sea (Liu et al., 2012; Li et al., 2017), and (3) shelf seas,e.g., New York Bight (Zhang et al., 2010). These studies provide useful
information for the characteristics of freshwater and nutrient transport
in these areas, and the inﬂuences of natural dynamic processes (e.g.,tide, local winds, density, and normal change of river discharge) andhuman activities (e.g., river water regulation event and waterway
constructions) on the river water age.
The Seto Inland Sea (SIS) is a semi-enclosed sea (Fig. 1a) located in
the western part of Japan. The SIS contains several basins, including
Osaka Bay and the Harima-Nada, which are separated by narrow straits,e.g., the Akashi Strait and Bisan Strait. The SIS connects to the Kii andBungo Channels in the Paciﬁc Ocean through the Kitan Strait, Naruto
Strait, and Hayasui Strait. Many rivers discharge into the SIS, and the
Yodo River, directly ﬂowing into Osaka Bay, is the largest of these⁎

Corresponding author at: Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama 790-8577, Japan.
E-mail address: guoxinyu@sci.ehime-u.ac.jp (X. Guo).
1
Present aﬃliation: Department of Earth Sciences, National Natural Science Foundation of China.
https://doi.org/10.1016/j.jmarsys.2018.12.001
Received 19 August 2018; Received in revised form 28 November 2018; Accepted 2 December 2018
Available online 06 December 20180924-7963/ © 2018 Elsevier B.V. All rights reserved.

Journal of Marine Systems 191 (2019) 24–37

H. Wang et al.

Fig. 1. (a) Bathymetry of the Seto Inland Sea (SIS) (unit:
meter). The SIS is separated into several basins, such as the
Harima-Nada and Osaka Bay, which are connected to eachother through narrow straits (e.g., Bisan Strait and Akashi
Strait). The SIS connects the Bungo Channel with Hayasui
Strait, and connects the Kii Channel with the Naruto Strait
and Kitan Strait. The SIS opens to the Paciﬁc Ocean via the
Bungo and Kii Channels. The black star represents thelocation of the river mouth of the Yodo River, and theblack dots represent the locations of the river mouths ofother rivers. The black lines labeled OB1 and OB2 are two
open boundaries.
(b) Line AB denotes the section along which concentration
and age of Yodo River water in Osaka Bay are presented inFig. 4. Line CD in the Harima-Nada and Line EF in Osaka
Bay denote the sections along which concentration, age,and current are presented in Fig. 8 and Fig. 9, respectively.

we present the spatial and seasonal variations of age of Yodo River
water and its pathways in the SIS. In Section 4, we discuss how the
dynamic factors (e.g., tide, local winds, baroclinic forcing, and Yodo
River discharge) and returned Yodo River water aﬀect the water age
using a set of sensitivity experiments. We summarize this study in
Section 5.

rivers. Beneﬁting from human eﬀorts, the present concentrations of
nutrients and phytoplankton in the SIS have signiﬁcantly decreased
compared with those during the 1970s, and the SIS is gradually changing from eutrophic to oligotrophic (Yamamoto, 2003; Nishikawa et al.,2010).
There are only a few studies on the transport timescales such as
residence time in the SIS. The residence time of a water parcel at a
speciﬁed location in the domain is deﬁned as how long the waterparcel, starting from this location, will remain in the domain before
exiting (Zimmerman, 1976; Takeoka, 1984a), and is the complement ofwater age. In theory, under steady conditions, the average residence
time of water parcel in the entire domain can be translated into theaverage age of water parcel in the entire domain, and the average residence time of newly entered water parcel in the domain can betranslated into the average age of water parcel at the outlet of the domain (Takeoka, 1984a). Takeoka (1984b) gives the values of average
residence time of ﬁve water volumes in the entire SIS under the M2 tidein the steady state. The average residence time in the entire SIS demonstrates a decreasing trend in the following order: sea water(460 days), total water (consists of river water and sea water)(440 days), river water (250 days), newly entered river water(230 days), and newly entered sea water (39 days). The hydrodynamicﬁeld used in Takeoka (1984b) is from a laboratory model, which included tidal forcing but neglected earth rotation, wind forcing, andbaroclinic forcing. Since the wind- or buoyancy-driven subtidal currents
in the SIS are usually more than 0.1 ms−1 (Chang et al., 2009), wecannot ignore their inﬂuences on transport time of water.
In this study, embedding a water age calculation module (Liu et al.,2012) into a hydrodynamic numerical model designed to reproduce theseasonal variations (Chang et al., 2009), we present the spatial distribution and temporal variation of water age of the largest river, i.e.,the Yodo River, in the SIS. In Section 2, we describe the conﬁguration of
the hydrodynamic model and the water age calculation. In Section 3,

2. Model description
2.1. Hydrodynamic model
The hydrodynamic model is based on the Princeton Ocean Model(Mellor, 2004), and has been used by Chang et al. (2009) to study theseasonal variations of circulation in the SIS. The model has a horizontal
resolution of about 1 km (1/120 degree in meridional direction and 1/
80 degree in zonal direction), and 21 sigma levels in the vertical direction. The time step is 4 s for the external mode and 120 s for theinternal mode.
In order to reproduce the climatological state with seasonal variations in the hydrodynamic model, monthly atmospheric forcing (e.g.,
local winds, heat ﬂux, precipitation, and evaporation), monthly discharges from 21 major rivers (Fig. 1a), and monthly salinity, watertemperature, normal velocity, and tidal forcing at the two open
boundaries (OB1 and OB2, Fig. 1a) were used. The monthly mean ofeach of these forcing is set at the middle day of each month, and thevalue at a given time step between two monthly mean values is interpolated linearly. Chang et al. (2009) presented a detailed description
and validation of the hydrodynamic model. The hydrodynamic model
results (i.e., sea level, three velocity components, and horizontal andvertical eddy diﬀusivity coeﬃcients) were saved at intervals of 30 min
for 1 year.

25

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H. Wang et al.

2.2. Calculation of Yodo River water age in the SIS

Table 1aVolume-averaged concentration of Yodo River water in the Seto Inland Sea(SIS) and subregions from calculations with boundaries OB1 and OB2.

To calculate the age of Yodo River water in the SIS a(t, x, y, z) basedon CART (Deleersnijder et al., 2001), the equation for the concentrationof Yodo River water C(t, x, y, z) and age concentration of Yodo River
water β(t, x, y, z) need to be solved. C(t, x, y, z) was calculated by Eq. (1):∂C
∂ (uC )∂ (vC )∂ (wC ) ⎤
= −⎡
+
+
⎢
x
y
∂t
∂
∂
∂z ⎥
⎣
⎦
∂
∂C ⎞
∂ ⎛ ∂C ⎞∂ ⎛ ∂C ⎞ ⎤
KH
KV
+
+
+ ⎡ ⎛KH⎢ ∂x ⎝
∂x ⎠
∂y ⎝
∂y ⎠
∂z ⎝
∂z ⎠ ⎥
⎦
⎣
⎜

⎟

(1)

where t is time; x, y, and z are three coordinates in space; u, v, and w arevelocity in the x, y, and z directions; and KH and KV are the horizontal
and vertical eddy diﬀusion coeﬃcients.
β(t, x, y, z) is controlled by Eq. (2), in which C(t, x, y, z) appears on
the right-hand side.

⎜

April

July

October

AnnualSIS
Osaka Bay
Harima-Nada
Kii Channel
Other areas of the SIS
Kitan Strait
Akashi Strait
Naruto Strait
Bisan Strait
Hayasui Strait

0.48
3.04
1.5
0.78
0.0036
1.63
2.13
1.51
0.19
0.00052

0.5
3.12
1.55
0.68
0.0079
1.36
2.63
1.25
0.47
0.00046

0.62
4.17
1.84
0.83
0.0044
1.88
3.45
1.45
0.09
0.00048

0.54
3
1.86
0.69
0.0053
1.64
2.75
1.45
0.39
0.00067

0.54
3.31
1.7
0.76
0.0056
1.63
2.75
1.41
0.31
0.00054

Nada, Kii Channel, and other areas of the SIS are 0.0331, 0.017, 0.0076,
and 0.000056, respectively (Table 1a). Since the concentration of Yodo
River water is low in other areas of the SIS, we focus on results within
Osaka Bay, the Harima-Nada, and Kii Channel (Fig. 2).
In Osaka Bay, the concentration of Yodo River water is higher on theeastern side than on the western side (Fig. 2), and has an inverse correlation with the spatial salinity distribution presented by Tawara(1986). Along the eastern coast of Osaka Bay, there is a concentration
front similar to the salinity front (Tawara, 1986, Fig. 13). The inverse
correlation suggests that the freshwater within Osaka Bay is mainly
from the Yodo River water. In the Harima-Nada, the inverse correlation
between the concentration of Yodo River water and salinity does notexist, indicating that the freshwater there does not originate primarily
from the Yodo River.
The concentration of Yodo River water in Osaka Bay, the HarimaNada, and Kii Channel presents an apparent seasonal variation (Fig. 2).
The high-concentration area oﬀshore of the Yodo River mouth spreads
southward in winter when the low-salinity water also moves southward
along the eastern coast of Osaka Bay. In spring, summer, and autumn,
the high-concentration area moves westward to the northern area of
Osaka Bay. As shown later, the strong northwesterly wind in winter(Fig. 12) is likely responsible for such seasonal variation. According toTable 1a, the seasonal range of concentration (its ratio to annual mean
concentration) is 0.0117 (0.35) for Osaka Bay, 0.0036 (0.21) for the
Harima-Nada, and 0.0015 (0.2) for the Kii Channel.
The annual mean age of Yodo River water in the SIS is 152 days
(Table 1b) and is smallest in the area oﬀshore of the Yodo River mouth
and the spatially increasing order is Osaka Bay < Kii Channel <Harima-Nada < other areas of the SIS (Fig. 3). The annual mean agesof Yodo River water in Osaka Bay, the Kii Channel, Harima-Nada, andother areas of the SIS are 97 days, 165 days, 199 days, and 450 days,
respectively (Table 1b). There is a general inverse relationship between
concentration (Fig. 2) and age of Yodo River water (Fig. 3). However,this relationship does not exist at the Kii Channel, where the concentration of Yodo River water is lower than that in the Harima-Nada
but the age is less (Table 1, Figs. 2–3).
The seasonal variation in the age of Yodo River water in Osaka Bay,the Harima-Nada, and Kii Channel is also apparent (Fig. 3). In the area
oﬀshore of the Yodo River mouth, a front of water age forms with an
obvious seasonal variation. In Osaka Bay, the age is lower on theeastern side than on the western side in winter, and the age in otherseasons are lower on the northern side than on the southern side
(Fig. 3). Apparently, the seasonal variation in the movement of freshwater from the Yodo River mouth (Fig. 2) is the direct cause for suchseasonal variation in water age because the newly discharged riverwater has a low water age.
According to Table 1b, the seasonal range of ages (its ratio to annual
mean age) for Osaka Bay, the Harima-Nada, and Kii Channel are
30 days (0.31), 60 days (0.30), and 30 days (0.18), respectively. The⎟(2)

The water age a(t, x, y, z) was calculated as the ratio of β(t, x, y, z) toC(t, x, y, z).

a = β /C

January

The concentration has been multiplied by a factor of 100. “Other area of the
SIS” refers to all subregions except Osaka Bay and the Harima-Nada.

∂ (wβ ) ⎤∂ (vβ )∂β∂ (uβ )
+
=C−⎡
+
⎢
∂z ⎥
∂y
∂t
⎦
⎣ ∂x
∂β
∂
∂ ⎛ ∂β ⎞∂ ⎛ ∂β ⎞ ⎤KVKH+ ⎡ ⎛KH ⎞ ++⎢ ∂x ⎝ ∂x ⎠yz ⎝ ∂z ⎠ ⎥
y
∂
∂
∂
⎝
⎠
⎣
⎦

Subregions(3)

The sigma coordinate system for Eqs. (1) and (2) converted from thez coordinate are exactly the same as the salinity and water temperatureequations used in the Princeton Ocean Model (Mellor, 2004) and are
not shown here. According to the modules for salinity and water temperature equations in the Princeton Ocean Model, new modules to solve
Eqs. (1) and (2) are developed and have been previously used in simulating the spatial and seasonal variations of age of Yellow River
water in the Bohai Sea (Liu et al., 2012; Li et al., 2017).
Fig. 1a shows the model domain for calculating the age of Yodo
River water in the SIS. Two open boundaries (OB1 and OB2, Fig. 1a) are
set in the model domain for control simulations, through which the
Yodo River water outside the Kii and Bungo channels is not allowed toreturn into the model domain. The exact treatment of the open
boundaries is as follows. If there is an outﬂow at the interior grid pointnext to an open boundary, the concentration C and age concentration βof Yodo River water at an open boundary are set to those at the interior
gird point. If there is an inﬂow at the interior grid point next to an open
boundary, C and β at an open boundary are set to 0. At the river month,
C and β are set to be 1 and 0, respectively. The initial values inside themodel domain are zero for both C and β. The time step is 180 s forsolving Eqs. (1) and (2), a tenth of the time interval (30 min) of hydrodynamic model results (sea level, velocity, and eddy diﬀusivity
coeﬃcient). To solve Eqs. (1) and (2) at a given time step, the hydrodynamic model results are interpolated linearly. Using the annually
repeated hydrodynamic model results, we simulate the water age until
the diﬀerence of concentration and age of Yodo River water between
two years is negligible, and save the results for the last year.
3. Results
3.1. Age of Yodo River water in the SIS
The annual mean concentration of Yodo River water in the SIS is0.0054 (Table 1a), indicating that Yodo River water occupies 0.54% of
the total water volume in the SIS. The concentration of Yodo River
water in all seasons in the area oﬀshore of the Yodo River mouth islarge, and the spatially decreasing order is Osaka Bay > HarimaNada > Kii Channel > other areas of the SIS (Fig. 2). The annual
mean concentrations of Yodo River water in Osaka Bay, the Harima26

Journal of Marine Systems 191 (2019) 24–37

H. Wang et al.

Fig. 2. Vertically averaged concentration of Yodo River water (equal to 1 at the river mouth) in (a) January, (b) April, (c) July, and (d) October. The concentration
inside each panel has been multiplied by a factor of 100. The contour interval is 1.

concentration and age of Yodo River water change little in the vertical(ﬁgure not shown) except for in Osaka Bay, where the seasonal stratiﬁcation is maintained over a timescale of 3 months, which is at thesame order as the mean water age of Yodo River water. In the otherareas, the longer water age allows wind mixing and tidal mixing to
make a vertically homogeneous distribution for the concentration andage of Yodo River water.
We choose section AB that crosses Osaka Bay from the Yodo Rivermouth (see Fig. 1b for its location) to demonstrate the vertical variationof concentration and age of Yodo River water. As expected, the age of
Yodo River water is smaller around the river mouth than in the oﬀshore
area and is smaller in the surface layer than in the bottom layer. The
concentration and age of Yodo River water are homogeneous in winterbut stratiﬁed in other seasons (i.e., spring, summer, and autumn)(Fig. 4). Along the section, the stratiﬁcation is stronger around the river

Table 1b
Volume-averaged age (in days) of Yodo River water in the SIS and subregions
from calculations with boundaries OB1 and OB2.
Subregions
SIS
Osaka Bay
Harima-Nada
Kii Channel
Other areas of the SIS
Kitan Strait
Akashi Strait
Naruto Strait
Bisan Strait
Hayasui Strait

January

April

July

October

Annual

168
112
218
177
524
142
171
210
274
882

173
106
231
179
414
137
141
218
296
939

135
82
188
149
530
117
105
199
319
930

138
91
171
152
417
119
104
191
224
838

152
97
199
165
450
128
125
203
271
891

Fig. 3. Vertically averaged age of Yodo River water (unit: days) in (a) January, (b) April, (c) July, and (d) October. The contour interval is 20 days.
27

Journal of Marine Systems 191 (2019) 24–37

H. Wang et al.

Fig. 4. Vertical distribution of the concentration (left panels) and age (right panels) of Yodo River water along section AB
in January, April, July, and October. The
concentration has been multiplied by afactor of 100. The color bars show the color
ranges for (i) the concentration and (ii) theage. The contour interval for the concentration is 1, and the interval for the age
is 10 days.

+⎧ f YR (x , y0 , z ) = fYR (x , y0 , z ) if fYR (x , y0 , z ) > 0−⎨ fYR(x , y0 , z ) = fYR (x , y0 , z ) if fYR (x , y0 , z ) < 0⎩

mouth than in other areas, corresponding to the salinity stratiﬁcation in
Osaka Bay (Chang et al., 2009).

T

3.2. Budget of Yodo River water in the SIS

fYR (x , y0 , z ) =

1
T

0

Vol

3.2.1. Budget of Yodo River water in Osaka Bay
The annual mean volume of Yodo River water inside Osaka Bay(VYR) is 0.14 × 1010 m3 (Fig. 5a), which occupies 3% of the total water
volume (4.16 × 1010 m3) in Osaka Bay. Every year, 0.8 × 1010 m3 of
Yodo River water is discharged into Osaka Bay from the river mouth(Fig. 5a).
At the Akashi Strait, 1.4 × 1010 m3 of Yodo River water is transported out of Osaka Bay, while 1.1 × 1010 m3 of Yodo River water istransported back (Fig. 5a). Consequently, the yearly net export of Yodo
River water from Osaka Bay to the Harima Nada through the Akashi
Strait is 0.3 × 1010 m3. At the Kitan Strait, 1.1 × 1010 m3 of Yodo River
water is transported out of Osaka Bay, while 0.6 × 1010 m3 is transported back (Fig. 5a), yielding a net export of 0.5 × 1010 m3. Thus, thesum of river water export through the Akashi Strait and the Kitan Strait
is balanced with the input of river water from the Yodo River mouth.
At the Akashi Strait, the Yodo River water ﬂows out of Osaka Bay
through its northern part, and ﬂows back through its southern part(Fig. 6a). At the Kitan Strait, the Yodo River water ﬂows out of Osaka
Bay through its westernmost and east-central parts and ﬂows backthrough its easternmost and west-central parts (Fig. 6b). At both straits,
the area of the Yodo River water ﬂowing out of Osaka Bay is larger thanthat ﬂowing back and the river water transport is larger in the surface
layer than in the bottom layer.

(4)Here, T is the integral time (= one year) and Vol is the volume ofthe study area (i.e., Osaka Bay or the Harima-Nada).
The yearly inﬂow (FYRIN(x0)) and outﬂow (FYROUT(x0)) of Yodo
River water through the Akashi Strait or the Bisan Strait (x = x0), theyearly inﬂow (FYRIN(y0)) and outﬂow (FYROUT(y0)) of Yodo River water
through the Kitan Strait or the Naruto Strait (y = y0) were calculated by
the following equations:∬+IN(x 0) =
f YR
(x 0 , y, z ) dydz
⎧ FYR
⎪
A
−
⎨ F OUT (x 0) =fYR
(x 0 , y, z ) dydz
⎪ YR
A
⎩

∬

(5)

+

⎧ f YR (x 0 , y, z ) = fYR (x 0 , y, z ) if fYR (x 0 , y, z ) > 0−⎨ fYR(x 0 , y, z ) = fYR (x 0 , y, z ) if fYR (x 0 , y, z ) < 0
⎩

(6)

T

fYR (x 0 , y, z ) =∫ u (t, x 0 , y, z ) C (t, x 0 , y, z )dt
0(7)

3.2.2. Budget of Yodo River water in the Harima-Nada
The Naruto Strait is the exit of Yodo River water from the HarimaNada to the Kii Channel. As shown in Fig. 5a, the net export of Yodo
River water from Osaka Bay to the Harima-Nada through the Akashi
Strait is 0.3 × 1010 m3. At the Naruto Strait, 1.5 × 1010 m3 of Yodo∬+IN(y0 ) =
f YR
(x , y0 , z ) dxdz
⎧ FYR
⎪
A
−
⎨ F OUT (y ) =fYR(x , y0 , z ) dxdz
0
⎪ YR
A
⎩

(10)

Here, A is the cross-sectional area across the outlet (Akashi Strait,
Kitan Strait, Bisan Strait, or Naruto Strait); fYR(x0, y, z) is the annual
Yodo River ﬂux at a grid point in the Akashi Strait or the Bisan Strait;fYR(x, y0, z) is the annual Yodo River ﬂux at a grid point in the Kitan
Strait or the Naruto Strait. The overbar in Eqs. (7) and (10) denotes theprocedure of de-tiding.

T∫ ∭ C (t, x, y, z ) dxdydzdt∫ v (t, x, y0 , z ) C (t, x, y0 , z )dt.
0

As the SIS connects to the Paciﬁc Ocean through the Kii and Bungo
Channels, water exchange in these two channels determines the mean
age of Yodo River water in the SIS (Fig. 3). Since the Yodo River water
concentrates in Osaka Bay and the Harima-Nada, we calculate thebudget of freshwater in these two areas.
The annual mean volume of Yodo River water inside Osaka Bay or
the Harima-Nada (VYR) can be calculated by Eq. (4) from the concentration of Yodo River water C that is solution of Eq. (1).

VYR =

(9)

∬

(8)
28

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H. Wang et al.

(caption on next page)
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Journal of Marine Systems 191 (2019) 24–37

H. Wang et al.

Fig. 5. (a) Budgets of Yodo River water in Osaka Bay. VOsaka is the volume of Osaka Bay. See Section 3.2 for the deﬁnition and calculation methods of other variables.
(b) Budgets of Yodo River water in the Harima-Nada. VHarima is the volume of the Harima-Nada.
(c) Pathway of Yodo River water in the SIS.

River water is transported out of the Harima-Nada, and 1.2 × 1010 m3
of Yodo River water is transported inwards (Fig. 5b), giving a net ﬂux of
0.3 × 1010 m3 from the Harima-Nada to the Kii Channel. At the Bisan
Strait, the transports of Yodo River water into and out of the HarimaNada are both 0.03 × 1010 m3, being smaller than those through the
Naruto Strait by one order of magnitude (Fig. 5b). Apparently, it is thenet export of Yodo River water from the Harima-Nada to the Kii
Channel through the Naruto Strait that balances with the input of Yodo
River water from the Akashi Strait (Fig. 5a).
At the Naruto Strait, the Yodo River water ﬂows out of the HarimaNada through the western part and ﬂows back through the eastern part(Fig. 6d). The area of outﬂowing Yodo River water is larger than that of
inﬂowing Yodo River water and river water transport is larger in thesurface layer than in the bottom layer (Fig. 6d). The Yodo Rivertransport through the Bisan Strait (Fig. 6c) is much smaller than thatthrough the Naruto Strait (Fig. 6d).

outlets connecting the SIS with the Bungo and Kii Channels, and the
Akashi Strait and Bisan Strait are the two outlets connecting Osaka Bay,the Harima-Nada, and other areas of the SIS. Among these straits, theYodo River water at the Hayasui Strait has the highest age but lowest
concentration (Table 1). Therefore, we pay more attention to the age of
Yodo River water in the other four straits. Among these, the age of Yodo
River water at the Bisan Strait is higher than that at the Naruto Strait,
which is higher again than those at the Akashi Strait and Kitan Strait(Fig. 3, Table 1b). The annual ages of Yodo River water are 271 days,
203 days, 125 days, and 128 days for the Basin Strait, Naruto Strait,
Akashi Strait, and Kitan Strait, respectively (Table 1b).
According to Table 1b, the ranges of seasonal age variation (its ratio
to annual age) are 95 days (0.35) for the Bisan Strait, 27 days (0.13) for
the Naruto Strait, 67 days (0.54) for the Akashi Strait, and 25 days (0.2)
for the Kitan Strait, indicating that the age of Yodo River water has
strong seasonal variation at all four straits. The age of Yodo River water
is much higher at the Akashi Strait than at the Kitan Strait in winter(Table 1b), when more Yodo River water moves southward due to theintense northwesterly winds (Fig. 12b).

3.3. Age of Yodo River water at the straits along its pathway to the Paciﬁc
Ocean

4. Discussion

By combining the budget of Yodo River water in Osaka Bay and theHarima-Nada, the pathway of Yodo River water in the SIS becomes
clear (Fig. 5c). After discharging into Osaka Bay from the Yodo Rivermouth (~0.8 × 1010 m3), the Yodo River water separates into two
branches. One branch (~0.5 × 1010 m3) ﬂows directly into the Kii
Channel through the Kitan Strait. The other (~0.3 × 1010 m3) ﬂows
into the Harima-Nada through the Akashi Strait, and then ﬂows into theKii Channel through the Naruto Strait. Finally, Yodo River water(~0.8 × 1010 m3) leaves the SIS and ﬂows into the Paciﬁc Ocean
through the Kii Channel.
The Hayasui Strait, Naruto Strait, and Kitan Strait are the three

4.1. Inﬂuences of dynamic processes on the age of Yodo River water
Any change in the dynamic factors (e.g., tide, local winds, baroclinic
forcing, and river discharge) may aﬀect water age. To understand the
major control factor on the age of Yodo River water in the SIS, we use aset of sensitivity simulations to artiﬁcially modify the driving forces andcalculated the water age again.
Four cases (Table 2) are designed to examine the inﬂuences ofdriving forces on the river water age. When running each of these cases,

Fig. 6. Annual mean u (t , x 0 , y, z ) C (t , x 0 , y, z ) at (a) Akashi Strait and (c) Bisan Strait and v (t , x , y0 , z ) C (t , x , y0 , z ) at (b) Kitan Strait and (d) Naruto Strait. See Section
3.2 for the detailed deﬁnition. The contour interval for Fig. 6a, Fig. 6b, and Fig. 6d is 0.1 cm/s, and the interval for Fig. 6c is 0.01 cm/s. Positive values in Fig. 6a andFig. 6c indicate eastward ﬂow, and positive values in Fig. 6b and Fig. 6d indicate northward ﬂow.
30

Journal of Marine Systems 191 (2019) 24–37

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Table 2
Conditions used in sensitivity experiments for Case 1 to Case 4.
Case

Yodo River discharge

Tide

Local winds

Yes
Yes
Yes
Yes
0.5 times

Yes
No
Yes
Yes
Yes

Yes
Yes
No
Yes
Yes

0
1
2
3
4

Table 3b
Relative diﬀerence (%) of volume- and annually averaged concentration of
Yodo River water between the sensitivity experiments and the control case
(Case 0).

Baroclinic forcing
Yes
Yes
Yes
No (T = 15 °C, S = 33)
Yesonly one driving force from the control case (hereafter referred to as
Case 0, whose results were presented in Sections 3.1–3.3) is changed.
The hydrodynamic model and CART module are both calculated again
to obtain a new age of Yodo River water in these cases (Table 2). The
open boundaries for water age are set at OB1 and OB2 in these cases as
in Case 0.

Case 1Case 2

Case 3

Case 4SIS
Osaka Bay
Harima-Nada
Kii Channel
Other areas of the SIS
Kitan Strait
Akashi Strait
Naruto Strait
Bisan Strait

0.54
3.31
1.7
0.76
0.0056
1.63
2.75
1.41
0.31

0.46
3.59
1.04
0.71
0.0102
1.75
2.41
0.82
0.24

0.54
3.29
1.69
0.74
0.0066
1.62
2.87
1.4
0.31

0.87
6.8
2.09
1.4
0.0019
3.41
4.61
2.15
0.18

0.29
1.86
0.91
0.41
0.0026
0.92
1.52
0.75
0.16

Case 2

Case 3

Case 4SIS
Osaka Bay
Harima-Nada
Kii Channel
Other areas of the SIS
Kitan Strait
Akashi Strait
Naruto Strait
Bisan Strait

−15
8
−39
−7
82
7
−12
−42
−23

0
−1
−1
−3
18
−1
4
−1
0

61
105
23
84
−66
109
68
52
−42

−46
−44
−46
−46
−54
−44
−45
−47
−48

4.1.2. Local winds
The local wind in the SIS induces an apparent seasonal variation.
The spring and summer wind is rather weak and northerly winds
commence in autumn. In winter, intense northwesterly wind aﬀects thewhole SIS (Fig. 12a). The exclusion of local winds (Case 2) slightly
aﬀects the concentration and age of Yodo River water in the SIS(Table 3, Fig. 7e, f). Without local winds, the concentration and age of
Yodo River water in Osaka Bay and Harima-Nada change less than 3%of those in Case 0 (Table 3). Correspondingly, the current only changes
slightly in Osaka Bay and the Harima-Nada, e.g., the westward current
becomes larger (Fig. 10c). The net ﬂuxes at the Bisan Strait, Naruto
Strait, Akashi Strait, and Kitan Strait change little (Table 4, Fig. 5),
indicating that the local winds do not change the pathway of Yodo
River water in the SIS.
However, the intense northwesterly wind in winter aﬀects thehorizontal distribution of concentration and age of Yodo River water.
The concentration (age) signiﬁcantly decreases (increases) in theeastern part of Osaka Bay and increases (decreases) in the northern partwithout the northwesterly wind in winter (Fig. 12b, c). Gong et al.
(2009) found a downstream wind generally decreases the water age in
the Rappahannock River, while an upstream wind increases water age.
In Osaka Bay, the northerly wind component is a downstream wind forthe southward moving Yodo River water along the eastern bay, and thewesterly wind component is an upstream wind for the westward
moving of Yodo River water along the northern bay (Fig. 2). Therefore,
the northwesterly wind in winter aﬀects the concentration and age of
Yodo River water in opposite ways in the eastern and northern parts ofthe bay. Speciﬁcally, without the northerly wind component, thesouthward current in the eastern part of Osaka Bay becomes weaker(Fig. 12e), preventing the Yodo River water ﬂowing southward andcorrespondingly reducing (increasing) the concentration (age) of Yodo
River water in the eastern part of the bay (Fig. 12b, c); without thewesterly wind component, the current in the northern part of Osaka Baychanges to ﬂow westward (Fig. 12e), making more Yodo River water

Table 3aVolume- and annually averaged concentration of Yodo River water in the SIS
and subregions from sensitivity experiments (Case 1 to Case 4).
Case 0Case 1(Fig. 11a). Compared to Case 0 (Fig. 6a), the spatial structure of Yodo
River transport at the Akashi Strait changes in Case 1 and the absolutevalue decreases. At the Kitan Strait, every year 0.9 × 1010 m3 of Yodo
River water is transported out of Osaka Bay, while 0.1 × 1010 m3 of
Yodo River water is transported inwards, giving a net ﬂux of
0.8 × 1010 m3 (Table 4). The Yodo River water ﬂows out of Osaka Bay
through the eastern part and the surface layer of the western part of theKitan Strait, and ﬂows into Osaka Bay through the bottom layer of thewestern part (Fig. 11c).
The budget of Yodo River water at the Akashi Strait and Kitan Strait
suggests that the pathway of the Yodo River water changes in the casewithout tide. After discharging into Osaka Bay from the Yodo Rivermouth (~0.8 × 1010 m3), instead of separating into two branches as inCase 0, the Yodo River water (0.8 × 1010 m3) leaves Osaka Bay throughthe Kitan Strait and moves directly into the Kii Channel.

4.1.1. Tide
The exclusion of tide (Case 1, removing tidal oscillation at open
boundaries) reduces the concentration and increases the age of Yodo
River water in the entire SIS (Table 3). However, the eﬀects of tide are
diﬀerent in each sub-region of the SIS. Without tide, the annual concentration of Yodo River water increases by 8% in Osaka Bay but decreases by 39% in the Harima-Nada (Table 3a, Table 3b, Fig. 7c). The
annual mean ages of Yodo River water in Case 1 increase by 6% and
51% in Osaka Bay and the Harima-Nada (Table 3c, Table 3d, Fig. 7d),as compared to those in the control case (Case 0).
We choose sections CD and EF crossing the Harima-Nada and Osaka
Bay (see Fig. 1b for its location) to demonstrate the vertical variation.
Compared with those in Case 0, the concentration and age in the
Harima-Nada become more stratiﬁed without tide (Fig. 8b, e). The
water age destratiﬁcation under tide was also found in James Riverestuary (Shen and Lin, 2006) and Changjiang estuary (Wang et al.,2010). The great reduction (increase) of concentration (age) of Yodo
River water in the Harima-Nada without tide appears from the surface
to the bottom layers (Fig. 8b, e). In Osaka Bay, the eﬀects of tide on thevertical stratiﬁcation and absolute value are much less than those in the
Harima Nada (Fig. 9b, e).
Compared with Case 0 (Fig. 10a), the strong westward current
outside the Akashi Strait in the Harima-Nada disappears, and changes
to ﬂow eastward towards the Akashi Strait without tide (Fig. 10b). The
stronger eastward current in the northern Harima-Nada exists from thesurface to the bottom layers (Fig. 8h), preventing the Yodo River waterﬂowing into the Harima-Nada through the Akashi Strait.
In Case 1, 0.3 × 1010 m3 of Yodo River water is transported out of
Osaka Bay through the Akashi Strait every year, while 0.3 × 1010 m3 istransported inwards. Therefore, there is almost no net export of Yodo
River water from Osaka Bay to the Harima Nada through the Akashi
Strait in Case 1 (Table 4). The Yodo River water ﬂows out of Osaka Bay
mainly in the surface layer and bottom layer of the Akashi Strait, andﬂows into Osaka Bay mainly in the subsurface layer and bottom layer

Subregions

Subregions

The concentration has been multiplied by a factor of 100.
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Fig. 7. Vertically and annually averaged concentration (left panels) and age (right panels) of Yodo River
water in the SIS. The concentration has been multiplied by a factor of 100. (a) (b) Control case (Case 0).
(c) (e) (g) (i) Same as Fig. 7a but for the concentration diﬀerence between the case denoted inside thepanel and Case 0. (d) (f) (h) (j) Same as Fig. 7b but
for the age diﬀerence between the case denoted inside the panel and Case 0. The color bars show thecolor ranges (i) for Fig. 7a and Fig. 7g, (ii) for Fig. 7c,Fig. 7e, and Fig. 7i, (iii) for Fig. 7b, and (iv) for
Fig. 7d, Fig. 7f, Fig. 7h, and Fig. 7j. The contour
interval for the concentration (left panels) is 0.5, andthe interval for the age (right panels) is 10 days.

Table 3c
Volume- and annually averaged age (in days) of Yodo River water in the SIS andsubregions from sensitivity experiments.

Table 3d
Relative diﬀerence (%) of volume- and annually averaged age of Yodo River
water between sensitivity experiments and the control case (Case 0).

Subregions

Case 0Case 1Case 2

Case 3

Case 4

Subregions

Case 1Case 2

Case 3

Case 4SIS
Osaka Bay
Harima-Nada
Kii Channel
Other areas of the SIS
Kitan Strait
Akashi Strait
Naruto Strait
Bisan Strait

152
97
199
165
450
128
125
203
271

185
103
301
158
599
126
159
287
375

152
96
198
166
468
129
117
202
285

207
147
300
247
470
202
212
284
379

160
104
211
178
452
141
136
217
283

SIS
Osaka Bay
Harima-Nada
Kii Channel
Other areas of the SIS
Kitan Strait
Akashi Strait
Naruto Strait
Bisan Strait

22
6
51
−4
33
−2
27
41
38

0
−1
−1
1
4
1
−6
0
5

36
52
51
50
4
58
70
40
40

5
7
6
8
0
10
9
7
4

ﬂow westward and then increasing (decreasing) the concentration (age)in the northern bay (Fig. 12b, c).
Diﬀering from the river water age destratiﬁcation under strong
wind found in the Rappahannock River (Gong et al., 2009), the concentration and age of Yodo River water in the Harima-Nada remains
vertically homogeneous and the vertical stratiﬁcation in Osaka Bay alsobarely changes without the intense northwesterly wind in winter (ﬁgure
not show), since vertical stratiﬁcation is relatively small in winter.

4.1.3. Baroclinic forcing
The exclusion of baroclinic forcing (Case 3, the water temperatureand salinity in the entire SIS is set to be constant) signiﬁcantly increases
the concentration and age of Yodo River water in the entire SIS(Table 3). The annual concentrations of Yodo River water increase by
105% and 23% in Osaka Bay and the Harima-Nada, and the annual ages
increase by 52% and 51%, respectively (Table 3, Fig. 7g, h). The riverwater age with density-induced circulation being less than that without
density-induced circulation was also found in the James River (Shenand Lin, 2006) and Danshuei River (Liu et al., 2008). As expected, the32

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Fig. 8. Vertical distribution of the annual concentration (upper panels), age (middle panels) of Yodo River water, and velocity in the x direction (bottom panels)
along section CD. The concentration has been multiplied by a factor of 100. The contour interval for the concentration is 0.1, the interval for the age is 10 days, andthe interval for the velocity is 0.025 m/s. Positive values in Fig. 8g, Fig. 8h, and Fig. 8i indicate eastward ﬂow.

4.1.4. Yodo River discharge
The variations in the Yodo River discharge signiﬁcantly change its
concentration in the SIS. Halving Yodo River discharge (Case 4) reduces
the concentration of Yodo River water by 40%–50% in the entire andsub-regions of the SIS (Table 3a, Table 3b, Fig. 7i). However, the largechange in Yodo River discharge does not induce a signiﬁcant variationin the age of Yodo River water in the SIS. Halving Yodo River discharge(Case 4) modiﬁes the water age on the order of 0–20 days, which is lessthan 10% of the age of Yodo River water in Case 0 (Table 3c, Table 3d,
Fig. 7j). Correspondingly, the current only changes slightly in Osaka
Bay, e.g., the southward current in the eastern part becomes smaller(Fig. 10e). The reduction of concentration in the SIS is attributed to thelarge reduction of freshwater input.
The insensitivity of water age to the change in river discharge also
occurs in other semi-enclosed seas (e.g., Bohai Sea (Liu et al., 2012)),and is quite diﬀerent from many estuary systems where the age

vertical stratiﬁcation of concentration and age in Osaka Bay and theHarima-Nada disappears, and a great increase of concentration and age
occurs from the surface to the bottom layers without baroclinic forcing(Fig. 8c, f, Fig. 9c, f).
In the absence of baroclinic forcing, the current signiﬁcantly slows
down in the whole of Osaka Bay and the Harima-Nada (Fig. 10d, Fig. 8i,
Fig. 9i). The southward current towards the Kitan Strait in the easternpart of Osaka Bay becomes much smaller (Fig. 10d, Fig. 9i). Thesechanges help to increase the mass and staying time of Yodo River waterin Osaka Bay and the Harima-Nada.
At the Akashi Strait, Kitan Strait, and Naruto Strait, the Yodo River
water transport increases in a similar order and the spatial structure
does not change in Case 3 compared to Case 0 (Fig. 6, Fig. 11b, d, f).
The net ﬂuxes at each Strait barely change (Table 4, Fig. 5), indicatingthat the baroclinic forcing does not change the pathway of Yodo River
water in the SIS.

Fig. 9. Vertical distribution of the annual concentration (upper panels), age (middle panels) of Yodo River water, and velocity in the y direction (bottom panels)
along section EF. The concentration has been multiplied by a factor of 100. The contour interval for the concentration is 0.1, the interval for the age is 10 days, andthe interval for the velocity is 0.025 m/s. Positive values in Fig. 9g, Fig. 9h, and Fig. 9i indicate northward ﬂow.
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Journal of Marine Systems 191 (2019) 24–37

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Fig. 10. Annual mean current (unit: m/s) at 1 m depth in the SIS in (a) Case 0, (b) Case 1, (c) Case 2, (d) Case 3, and (e) Case 4.

discharged Yodo River water from the river mouth. In other words, theenvironmental impact of the reentering and newly entering Yodo River
water is diﬀerent. Therefore, it is important to know the age of newly
entering Yodo River water inside Osaka Bay that corresponds to thewater age without the reentry process through the two outlets (i.e., theAkashi Strait and Kitan Strait).
The hydrodynamic model in this case is same as that used in Case 0.
We only repeat the water age calculation based on CART by setting
open boundaries for concentration and age concentration of Yodo River
water both at the Akashi Strait and Kitan Strait; i.e., if there is an
outﬂow at the outer grid point next to the strait, then the concentration
and age concentration of Yodo River water at the outer grid point were
set to those at the strait; if there is an inﬂow at the outer grid point next
to the strait, then the concentration and age concentration of Yodosigniﬁcantly reduces with increasing river discharge (e.g., York River(Shen and Haas, 2004), James River (Shen and Lin, 2006), Danshuei
River (Liu et al., 2008), Susquehanna River (Hong et al., 2010),
Changjiang (Wang et al., 2010), and Pearl River (Ren et al., 2014)). The
river discharge is probably not an important factor aﬀecting transport
time of freshwater in our study area; however, it signiﬁcantly aﬀects
transport time of freshwater in an estuary.

4.2. Age of Yodo River water in Osaka Bay without the reentry process
As presented in Section 3.2, reentering Yodo River water comprises
a portion of Yodo River water inside Osaka Bay. The materials, such as
nutrients or persistent organic pollutants (POPs), carried by the reentering Yodo River water may diﬀer from those carried by the newly

Table 4
Annual Yodo River water ﬂux (1010 m3/yr) at diﬀerent straits in the sensitivity experiments for Case 1 to Case 3.
Akashi Strait

Kitan Strait

Harima-Nada → Osaka Bay

Osaka Bay → Harima-Nada

Net ﬂux

Kii Channel → Osaka Bay

Osaka Bay → Kii Channel

Net ﬂux

0.3
1.2
1.8

0.3
1.5
2.1

0
0.3
0.3

0.1
0.6
1.5

0.9
1.1
2.0

0.8
0.5
0.5

Case 1Case 2
Case 3

Naruto Strait

Case 1Case 2
Case 3

Bisan Strait

Kii Channel → Harima-Nada

Harima-Nada → Kii Channel

Net ﬂux

Other areas of the SIS → Harima-Nada

Harima-Nada → Other areas of the SIS

Net ﬂux

0.1
1.2
1.9

0.1
1.5
2.2

0
0.3
0.3

0.02
0.03
0.02

0.02
0.03
0.02

0
0
0

The “→” indicates Yodo River water transport from the former to the latter.
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Journal of Marine Systems 191 (2019) 24–37

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Fig. 11. Same as Fig. 6 but for Case 1 (left panels) and Case 3 (right panels).

contribution of reentering Yodo River water to the river water age inOsaka Bay. However, the return ﬂow still exists as in Case 0 and cannot
be simply separated in the hydrodynamic model. Some previous studies
may help us to understand the mechanism of return ﬂow in Osaka Bay.
Sugimatsu and Isobe (2010, 2011) found that the cold, nutrient-richwater with an origin of Kuroshio intermediate water occupies thebottom layers of Osaka Bay in summer and suggested that the estuarine
circulation induced by the river runoﬀ into Osaka Bay brings it into thebay. As the Yodo River water passes the Kitan Strait, the strong verticalmixing there brings the river water down to the bottom layer, becoming
a part of onshore estuarine circulation in the bottom layer, ﬁnally returning to Osaka Bay and consequently increasing the water age inside
the bay.

River water at the outer grid point were set to 0. The age of the Yodo
River water in Osaka Bay decreases by approximately 80% (from approximately 100 days to 20 days) (Table 5, Fig. 3, Fig. 13) in all seasons
without the reentry of river water through both the Akashi Strait andKitan Strait. The signiﬁcant increase in water age caused by reenteredriver water was also found in the Bohai Sea (Liu et al., 2012), indicating
the reentry process through the outlet is important for river water age
in a semi-enclosed sea.
It is interesting that the age of newly entering Yodo River water inOsaka Bay maintains a similar spatial distribution in each of the four
seasons as that of total Yodo River water (Fig. 3, Fig. 13). Therefore, it
is likely that the movement of newly discharged Yodo River water inOsaka Bay, rather than that of returned Yodo River water, controls theage distribution of Yodo River water in Osaka Bay.
If the Yodo River water is not allowed to reenter Osaka Bay throughthe Akashi Strait, i.e., an open boundary is set only at the Akashi Strait
and the same water age calculation is repeated, then its water age inOsaka Bay decreases by 70–80%; if the Yodo River water is not allowed
to reenter Osaka Bay through the Kitan Strait, then its water age inOsaka Bay decreases by 50–60% (Tables 5a and 5b). This result is
consistent with the budget of Yodo River water in Osaka Bay; i.e., theannual transports of freshwater through the Akashi Strait and Kitan
Strait back into Osaka Bay are 1.1 × 1010 m3 and 0.6 × 1010 m3, respectively (Fig. 5a).
Please note that the new water age calculation only removes the

5. Summary
The age of Yodo River water in the SIS was calculated based on
CART (Deleersnijder et al., 2001) using a validated hydrodynamic
model (Chang et al., 2009). While previous studies on water transport
timescales in the SIS mainly focused on the average values in the entire
water volume (e.g., Takeoka, 1984b), this study presents the spatial
distribution and temporal variation of river water transport time. River
water age, from an environmental perspective, provides useful information for quantitative evaluation of the long-term transport ofnutrients, pollutants, and some ecological variables from rivers in the35

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Fig. 12. Eﬀects of local winds in January.
(a) Wind stress (unit: N/m2) in the SIS. (b)Diﬀerence of vertically averaged concentration of Yodo River water between
Case 2 and Case 0. The concentration has
been multiplied by a factor of 100. The
contour interval is 0.5. (c) Diﬀerence ofvertically averaged age of Yodo River water(unit: days) between Case 2 and Case 0. The
contour interval is 10 days. (d) Mean current (unit: m/s) at 1 m depth in the SIS in
Case 0. (e) Same as Fig. 12d but for Case 2.

Table 5aVolume-averaged age (in days) of Yodo River water in the SIS and subregions from calculations with boundaries Akashi Strait and Kitan Strait.
Subregions

Boundaries

Osaka Bay

Akashi + Kitan Strait
OB1 + OB2 + Akashi Strait
OB1 + OB2 + Kitan Strait
Akashi + Kitan Strait
Akashi + Kitan Strait

Kitan Strait
Akashi Strait

January

April

July

October

Annual

23
28
53
31
35

22
33
47
31
26

12
18
35
21
15

11
17
36
20
14

18
25
42
26
19

Table 5b
Relative diﬀerence (%) of volume-averaged age of Yodo River water between calculations with boundaries Akashi Strait, Kitan Strait, and the control case (OB1 andOB2, results are given in Table 1).
Subregions

Boundaries

Osaka Bay

Akashi + Kitan Strait
OB1 + OB2 + Akashi Strait
OB1 + OB2 + Kitan Strait
Akashi + Kitan Strait
Akashi + Kitan Strait

Kitan Strait
Akashi Strait

January

April

July

October

Annual

−79
−75
−53
−78
−80

−79
−69
−56
−77
−82

−85
−78
−57
−82
−86

−88
−81
−60
−83
−87

−81
−74
−57
−80
−85

age from 18 days to 97 days. The tide is an important factor determining the age of Yodo River water in the SIS. Without tidal forcing,the mean ages of Yodo River water in Osaka Bay and the Harima-Nada
increases by 6% and 51%; no net Yodo River water is transported into
the Harima-Nada through the Akashi Strait, and nearly all the Yodo
River water ﬂows directly into the Kii Channel through the Kitan Strait.
Without baroclinic forcing, the mean ages of Yodo River water in Osaka
Bay and Harima-Nada increase by 52% and 51%, but the pathway of
Yodo River water in the SIS does not change.

sea.
The mean age of Yodo River water in the entire SIS is 152 days. The
spatial variation of water age is large in the horizontal direction but
small in the vertical except for in Osaka Bay. The age of Yodo River
water is 97 days in Osaka Bay but 450 days in other areas of the SIS, andboth show strong seasonal variation. After discharging into Osaka Bay,the Yodo River water separates into two branches. One branch(~0.5 × 1010 m3 y−1) ﬂows directly into the Kii Channel through the
Kitan Strait, and the other (~0.3 × 1010 m3 y−1) ﬂows into the HarimaNada through the Akashi Strait, and then ﬂows into the Kii Channel
through the Naruto Strait. This is the ﬁrst understanding of the clear
pathway of Yodo River water in the SIS.
The return of old Yodo River water to Osaka Bay is a primary cause
for the large age of Yodo River water in Osaka Bay, increasing the water

Acknowledgments
This work was completed during H. Wang's visit to Center for
Marine Environmental Studies, Ehime University. She thanks the China
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Journal of Marine Systems 191 (2019) 24–37

H. Wang et al.

Fig. 13. Vertically averaged age of Yodo River water (unit: days) in Osaka Bay from the calculation using the boundary of the Akashi Strait and Kitan Strait in (a)January, (b) April, (c) July, and (d) October. The contour interval is 2 days.

Scholarship Council (CSC) for supporting her stay in Japan. This work is
supported by the Ministry of Education, Culture, Sports, Science andTechnology, Japan (MEXT) to a project on Joint Usage/ResearchCenter-Leading Academia in Marine and Environment Pollution
Research (LaMer) and JSPS KAKENHI (Grant Number JP17H01860).
The module of water age was initially developed by Z. LIU, and thenincluded in Marine Environment Research and Forecasting (MERF)
model as one of its components. For access to MERF and the associatedcode, please contact zliu@ouc.edu.cn.

Ocean Model (Version 2004). Program in Atmospheric and Oceanic Sciences,
Princeton University, Princeton, New Jersey.
Nishikawa, T., Hori, Y., Nagai, S., Miyahara, K., Nakamura, Y., Harada, K., Tanda, M.,
Manabe, T., Tada, K., 2010. Nutrient and phytoplankton dynamics in Harima-Nada,
eastern Seto Inland Sea, Japan during a 35-year period from 1973 to 2007. Estuar.
Coasts 33, 417-417. https://doi.org/10.1007/s12237-009-9198-0.
Ren, Y., Lin, B., Sun, J., Pan, S., 2014. Predicting water age distribution in the Pearl River
Estuary using a three-dimensional model. J. Mar. Syst. 139, 276–287. https://doi.
org/10.1016/j.jmarsys.2014.07.005.
Shen, J., Haas, L., 2004. Calculating age and residence time in the tidal York River using
three-dimensional model experiments. Estuar. Coast. Shelf Sci. 61, 449–461. https://doi.org/10.1016/j.ecss.2004.06.010.
Shen, J., Lin, J., 2006. Modeling study of the inﬂuences of tide and stratiﬁcation on age ofwater in the tidal James River. Estuar. Coast. Shelf Sci. 68, 101–112. https://doi.org/
10.1016/j.ecss.2006.01.014.
Sugimatsu, K., Isobe, A., 2010. Intrusion of Kuroshio-derived bottom cold water into
Osaka Bay and its possible cause. J. Oceanogr. 66, 191–199. https://doi.org/10.
1007/s10872-010-0017-9.
Sugimatsu, K., Isobe, A., 2011. Favorable conditions for cold-water intrusion from theKuroshio intermediate layer into Osaka Bay. J. Oceanogr. 67, 149–158. https://doi.
org/10.1007/s10872-011-0013-8.
Takeoka, H., 1984a. Fundamental concepts of exchange and transport time scales in acoastal sea. Cont. Shelf Res. 3, 311–326. https://doi.org/10.1016/0278-4343(84)90014-1.
Takeoka, H., 1984b. Exchange and transport time scales in the Seto Inland Sea. Cont.
Shelf Res. 3, 327–341. https://doi.org/10.1016/0278-4343(84)90015-3.
Tawara, S., 1986. Studies on the characteristics of oceanographic condition in relation toﬁshing condition in the shallow coastal waters. J. Shimonoseki Univ. Fish. 34, 1–103(In Japanese with English abstract).
Wang, Y., Shen, J., He, Q., 2010. A numerical model study of the transport timescale andchange of estuarine circulation due to waterway constructions in the Changjiang
Estuary, China. J. Mar. Syst. 82, 154–170. https://doi.org/10.1016/j.jmarsys.2010.
04.012.
Wang, Y., Shen, J., He, Q., Zhu, L., Zhang, D., 2015. Seasonal variations of transport timeof freshwater exchanges between Changjiang Estuary and its adjacent regions. Estuar.
Coast. Shelf Sci. 157, 109–119. https://doi.org/10.1016/j.ecss.2015.03.008.
Yamamoto, T., 2003. The Seto Inland Sea - eutrophic or oligotrophic? Mar. Pollut. Bull.
47, 37–42. https://doi.org/10.1016/S0025-326X(02)00416-2.
Zhang, W.G., Wilkin, J.L., Schoﬁeld, O.M.E., 2010. Simulation of water age and residence
time in New York Bight. J. Phys. Oceanogr. 40, 965–982. https://doi.org/10.1175/
2009JPO4249.1.
Zimmerman, J.T.F., 1976. Mixing and ﬂushing of tidal embayments in the Western Dutch
Wadden Sea. Part I: distribution of salinity and calculation of mixing time scales.
Neth. J. Sea Res. 10, 149–191. https://doi.org/10.1016/0077-7579(76)90013-2.

References
Bolin, B., Rodhe, H., 1973. A note on the concepts of age distribution and transit time innatural reservoirs. Tellus 25, 58–62. https://doi.org/10.1111/j.2153-3490.1973.
tb01594.x.
Chang, P.H., Guo, X., Takeoka, H., 2009. A numerical study of the seasonal circulation in
the Seto Inland Sea, Japan. J. Oceanogr. 65, 721–736. https://doi.org/10.1007/
s10872-009-0062-4.
Deleersnijder, E., Campin, J.M., Delhez, E.J.M., 2001. The concept of age in marine
modelling I. Theory and preliminary model results. J. Mar. Syst. 28, 229–267.
https://doi.org/10.1016/S0924-7963(01)00026-4.
Gong, W., Shen, J., Hong, B., 2009. The inﬂuence of wind on the water age in the tidal
Rappahannock River. Mar. Environ. Res. 68, 203–216. https://doi.org/10.1016/j.
marenvres.2009.06.008.
Hong, B., Panday, N., Shen, J., Wang, H.V., Gong, W., Soehl, A., 2010. Modeling waterexchange between Baltimore Harbor and Chesapeake Bay using artiﬁcial tracers:
seasonal variations. Mar. Environ. Res. 70, 102–119. https://doi.org/10.1016/j.
marenvres.2010.03.010.
Karna, T., Baptista, A.M., 2016. Water age in the Columbia River estuary. Estuar. Coast.
Shelf Sci. 183, 249–259. https://doi.org/10.1016/j.ecss.2016.09.001.
Li, Z., Wang, H., Guo, X., Liu, Z., Gao, H., Zhang, G., 2017. Inﬂuence of a water regulation
event on the age of Yellow River water in the Bohai. Ocean. Coast. Sea Res. 16,
727–737. https://doi.org/10.1007/s11802-017-3210-2.
Liu, W.C., Chen, W.B., Kuo, J.T., Wu, C., 2008. Numerical determination of residence timeand age in a partially mixed estuary using three-dimensional hydrodynamic model.
Cont. Shelf Res. 28, 1068–1088. https://doi.org/10.1016/j.csr.2008.02.006.
Liu, Z., Wang, H., Guo, X., Wang, Q., Gao, H., 2012. The age of Yellow River water in the
Bohai Sea. J. Geophys. Res. 117 (C11006). https://doi.org/10.1029/2012JC008263.
Meier, H.E.M., 2007. Modeling the pathways and ages of inﬂowing salt-and freshwater inthe Baltic Sea. Estuar. Coast. Shelf Sci. 74, 610–627. https://doi.org/10.1016/j.ecss.
2007.05.019.
Mellor, G.L., 2004. Users Guide for a Three-dimensional, Primitive Equation Numerical37