The Age of Yodo River Water in the Seto Inland

Journal of Marine Systems 191 (2019) 24–37

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Journal of Marine Systems

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The age of Yodo River water in the Seto Inland Sea T ⁎ Haiyan Wanga,c, Xinyu Guob, , Zhe Liuc,1 a National Marine Environmental Forecasting Center, Beijing 100081, China b Center for Marine Environmental Studies, Ehime University, Matsuyama 790-8577, Japan c Key Laboratory of Marine Environment and Ecology (Ocean University of China), Ministry of Education, Qingdao 266100, China

ARTICLE INFO ABSTRACT

Keywords: The age of Yodo River water, the largest river flowing in the Seto Inland Sea (SIS), was calculated using the Water age constituent-oriented age and residence time theory (CART) in order to understand the long-term material CART transport and environmental issues in this semi-enclosed sea. The Yodo River water was found to have a mean Yodo River age of 152 days for the entire SIS. Inside the SIS, the age of Yodo River water shows spatial variation ranging Seto Inland Sea 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 esti- mation considers the return of old Yodo River water into Osaka Bay that does not change the spatial distribution of 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 con- sidering 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 Chesapeake Bay, Pearl River estuary (Ren et al., 2014), Changjiang estuary (Wang et al., 2010, 2015), and Columbia River estuary (Karna The age of river water at a specified location in a coastal sea is and Baptista, 2016), (2) semi-enclosed seas, e.g., Baltic Sea (Meier, defined as the time elapsed since the river water leaves the river mouth, 2007) and Bohai Sea (Liu et al., 2012; Li et al., 2017), and (3) shelf seas, where its age is prescribed as zero, to its arrival at the location of in- e.g., New York Bight (Zhang et al., 2010). These studies provide useful terest (Bolin and Rodhe, 1973; Takeoka, 1984a). Much land-based information for the characteristics of freshwater and nutrient transport nutrients and pollutants discharge into a coastal sea from a river mouth in these areas, and the influences of natural dynamic processes (e.g., and then travel with the river water inside the sea. The age of river tide, local winds, density, and normal change of river discharge) and water, as an important auxiliary variable to describe the transport human activities (e.g., river water regulation event and waterway timescales, can help us to understand the environmental issues in constructions) on the river water age. coastal sea areas. The Seto Inland Sea (SIS) is a semi-enclosed sea (Fig. 1a) located in Many researchers have simulated the age of river water based on the the western part of Japan. The SIS contains several basins, including constituent-oriented age and residence time theory (CART, see www. Osaka Bay and the Harima-Nada, which are separated by narrow straits, climate.be/cart; Deleersnijder et al., 2001). The study areas to which e.g., the Akashi Strait and Bisan Strait. The SIS connects to the Kii and this theory has been applied covers (1) river estuaries, e.g., York River Bungo Channels in the Pacific Ocean through the Kitan Strait, Naruto estuary (Shen and Haas, 2004), James River estuary (Shen and Lin, Strait, and Hayasui Strait. Many rivers discharge into the SIS, and the 2006), and Rappahannock River estuary (Gong et al., 2009)in 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: [email protected] (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 2018 0924-7963/ © 2018 Elsevier B.V. All rights reserved. H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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 each other 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 the location of the river mouth of the Yodo River, and the black dots represent the locations of the river mouths of other 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 in Fig. 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.

rivers. Benefiting from human efforts, the present concentrations of we present the spatial and seasonal variations of age of Yodo River nutrients and phytoplankton in the SIS have significantly decreased water and its pathways in the SIS. In Section 4, we discuss how the compared with those during the 1970s, and the SIS is gradually chan- dynamic factors (e.g., tide, local winds, baroclinic forcing, and Yodo ging from eutrophic to oligotrophic (Yamamoto, 2003; Nishikawa et al., River discharge) and returned Yodo River water affect the water age 2010). using a set of sensitivity experiments. We summarize this study in There are only a few studies on the transport timescales such as Section 5. 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 water parcel, starting from this location, will remain in the domain before 2. Model description exiting (Zimmerman, 1976; Takeoka, 1984a), and is the complement of water age. In theory, under steady conditions, the average residence 2.1. Hydrodynamic model time of water parcel in the entire domain can be translated into the average age of water parcel in the entire domain, and the average re- The hydrodynamic model is based on the Princeton Ocean Model sidence time of newly entered water parcel in the domain can be (Mellor, 2004), and has been used by Chang et al. (2009) to study the translated into the average age of water parcel at the outlet of the do- seasonal variations of circulation in the SIS. The model has a horizontal main (Takeoka, 1984a). Takeoka (1984b) gives the values of average resolution of about 1 km (1/120 degree in meridional direction and 1/ residence time of five water volumes in the entire SIS under the M2 tide 80 degree in zonal direction), and 21 sigma levels in the vertical di- in the steady state. The average residence time in the entire SIS de- rection. The time step is 4 s for the external mode and 120 s for the monstrates a decreasing trend in the following order: sea water internal mode. (460 days), total water (consists of river water and sea water) In order to reproduce the climatological state with seasonal varia- (440 days), river water (250 days), newly entered river water tions in the hydrodynamic model, monthly atmospheric forcing (e.g., (230 days), and newly entered sea water (39 days). The hydrodynamic local winds, heat flux, precipitation, and evaporation), monthly dis- field used in Takeoka (1984b) is from a laboratory model, which in- charges from 21 major rivers (Fig. 1a), and monthly salinity, water cluded tidal forcing but neglected earth rotation, wind forcing, and temperature, normal velocity, and tidal forcing at the two open baroclinic forcing. Since the wind- or buoyancy-driven subtidal currents boundaries (OB1 and OB2, Fig. 1a) were used. The monthly mean of − in the SIS are usually more than 0.1 ms 1 (Chang et al., 2009), we each of these forcing is set at the middle day of each month, and the cannot ignore their influences on transport time of water. value at a given time step between two monthly mean values is inter- In this study, embedding a water age calculation module (Liu et al., polated linearly. Chang et al. (2009) presented a detailed description 2012) into a hydrodynamic numerical model designed to reproduce the and validation of the hydrodynamic model. The hydrodynamic model seasonal variations (Chang et al., 2009), we present the spatial dis- results (i.e., sea level, three velocity components, and horizontal and tribution and temporal variation of water age of the largest river, i.e., vertical eddy diffusivity coefficients) were saved at intervals of 30 min the Yodo River, in the SIS. In Section 2, we describe the configuration of for 1 year. the hydrodynamic model and the water age calculation. In Section 3,

25 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

2.2. Calculation of Yodo River water age in the SIS Table 1a Volume-averaged concentration of Yodo River water in the Seto Inland Sea To calculate the age of Yodo River water in the SIS a(t,x,y,z) based (SIS) and subregions from calculations with boundaries OB1 and OB2. on CART (Deleersnijder et al., 2001), the equation for the concentration Subregions January April July October Annual of 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): SIS 0.48 0.5 0.62 0.54 0.54 Osaka Bay 3.04 3.12 4.17 3 3.31 ∂C ∂()uC ∂ ()vC ∂ (wC ) Harima-Nada 1.5 1.55 1.84 1.86 1.7 =−⎡ + + ⎤ ⎢ ⎥ Kii Channel 0.78 0.68 0.83 0.69 0.76 ∂t ⎣ ∂x ∂y ∂z ⎦ Other areas of the SIS 0.0036 0.0079 0.0044 0.0053 0.0056 Kitan Strait 1.63 1.36 1.88 1.64 1.63 ⎡ ∂ ∂C ∂ ⎛ ∂C ⎞ ∂ ∂C ⎤ + ⎛K ⎞ + ⎜⎟K + ⎛K ⎞ ⎢ HHV⎥ Akashi Strait 2.13 2.63 3.45 2.75 2.75 ⎣∂x ⎝ ∂xy⎠ ∂ ⎝ ∂yz⎠ ∂ ⎝ ∂z ⎠⎦ (1) Naruto Strait 1.51 1.25 1.45 1.45 1.41 Bisan Strait 0.19 0.47 0.09 0.39 0.31 where t is time; x, y, and z are three coordinates in space; u, v, and w are Hayasui Strait 0.00052 0.00046 0.00048 0.00067 0.00054 velocity in the x, y, and z directions; and KH and KV are the horizontal and vertical eddy diffusion coefficients. The concentration has been multiplied by a factor of 100. “Other area of the β(t,x,y,z) is controlled by Eq. (2), in which C(t,x,y,z) appears on SIS” refers to all subregions except Osaka Bay and the Harima-Nada. the right-hand side. Nada, Kii Channel, and other areas of the SIS are 0.0331, 0.017, 0.0076, ∂β ∂()uβ ∂ ()vβ ∂ (wβ ) =−C ⎡ + + ⎤ and 0.000056, respectively (Table 1a). Since the concentration of Yodo ∂t ⎣⎢ ∂x ∂y ∂z ⎦⎥ 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). ⎡ ∂ ⎛ ∂β ⎞ ∂ ⎛ ∂β ⎞ ∂ ⎛ ∂β ⎞⎤ + KHHV+ ⎜⎟K + K In Osaka Bay, the concentration of Yodo River water is higher on the ⎢∂x ⎝ ∂xy⎠ ∂ ∂yz∂ ⎝ ∂z ⎠⎥ ⎣ ⎝ ⎠ ⎦ (2) eastern side than on the western side (Fig. 2), and has an inverse cor- The water age a(t,x,y,z) was calculated as the ratio of β(t,x,y,z)to relation with the spatial salinity distribution presented by Tawara C(t,x,y,z). (1986). Along the eastern coast of Osaka Bay, there is a concentration front similar to the salinity front (Tawara, 1986, Fig. 13). The inverse aβ= /C (3) correlation suggests that the freshwater within Osaka Bay is mainly The sigma coordinate system for Eqs. (1) and (2) converted from the from the Yodo River water. In the Harima-Nada, the inverse correlation z coordinate are exactly the same as the salinity and water temperature between the concentration of Yodo River water and salinity does not equations used in the Princeton Ocean Model (Mellor, 2004) and are exist, indicating that the freshwater there does not originate primarily not shown here. According to the modules for salinity and water tem- from the Yodo River. perature equations in the Princeton Ocean Model, new modules to solve The concentration of Yodo River water in Osaka Bay, the Harima- Eqs. (1) and (2) are developed and have been previously used in si- Nada, and Kii Channel presents an apparent seasonal variation (Fig. 2). mulating the spatial and seasonal variations of age of Yellow River The high-concentration area offshore of the Yodo River mouth spreads water in the Bohai Sea (Liu et al., 2012; Li et al., 2017). southward in winter when the low-salinity water also moves southward Fig. 1a shows the model domain for calculating the age of Yodo along the eastern coast of Osaka Bay. In spring, summer, and autumn, River water in the SIS. Two open boundaries (OB1 and OB2, Fig. 1a) are the high-concentration area moves westward to the northern area of set in the model domain for control simulations, through which the Osaka Bay. As shown later, the strong northwesterly wind in winter Yodo River water outside the Kii and Bungo channels is not allowed to (Fig. 12) is likely responsible for such seasonal variation. According to return into the model domain. The exact treatment of the open Table 1a, the seasonal range of concentration (its ratio to annual mean boundaries is as follows. If there is an outflow at the interior grid point concentration) is 0.0117 (0.35) for Osaka Bay, 0.0036 (0.21) for the next to an open boundary, the concentration C and age concentration β Harima-Nada, and 0.0015 (0.2) for the Kii Channel. of Yodo River water at an open boundary are set to those at the interior The annual mean age of Yodo River water in the SIS is 152 days gird point. If there is an inflow at the interior grid point next to an open (Table 1b) and is smallest in the area offshore of the Yodo River mouth boundary, C and β at an open boundary are set to 0. At the river month, and the spatially increasing order is Osaka Bay < Kii Channel < C and β are set to be 1 and 0, respectively. The initial values inside the Harima-Nada < other areas of the SIS (Fig. 3). The annual mean ages model domain are zero for both C and β. The time step is 180 s for of Yodo River water in Osaka Bay, the Kii Channel, Harima-Nada, and solving Eqs. (1) and (2), a tenth of the time interval (30 min) of hy- other areas of the SIS are 97 days, 165 days, 199 days, and 450 days, drodynamic model results (sea level, velocity, and eddy diffusivity respectively (Table 1b). There is a general inverse relationship between coefficient). To solve Eqs. (1) and (2) at a given time step, the hydro- concentration (Fig. 2) and age of Yodo River water (Fig. 3). However, dynamic model results are interpolated linearly. Using the annually this relationship does not exist at the Kii Channel, where the con- repeated hydrodynamic model results, we simulate the water age until centration of Yodo River water is lower than that in the Harima-Nada the difference of concentration and age of Yodo River water between but the age is less (Table 1, Figs. 2–3). two years is negligible, and save the results for the last year. 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 ff 3. Results o shore of the Yodo River mouth, a front of water age forms with an obvious seasonal variation. In Osaka Bay, the age is lower on the 3.1. Age of Yodo River water in the SIS eastern side than on the western side in winter, and the age in other seasons are lower on the northern side than on the southern side The annual mean concentration of Yodo River water in the SIS is (Fig. 3). Apparently, the seasonal variation in the movement of fresh- 0.0054 (Table 1a), indicating that Yodo River water occupies 0.54% of water from the Yodo River mouth (Fig. 2) is the direct cause for such the total water volume in the SIS. The concentration of Yodo River seasonal variation in water age because the newly discharged river water in all seasons in the area offshore of the Yodo River mouth is water has a low water age. large, and the spatially decreasing order is Osaka Bay > Harima- According to Table 1b, the seasonal range of ages (its ratio to annual Nada > Kii Channel > other areas of the SIS (Fig. 2). The annual mean age) for Osaka Bay, the Harima-Nada, and Kii Channel are mean concentrations of Yodo River water in Osaka Bay, the Harima- 30 days (0.31), 60 days (0.30), and 30 days (0.18), respectively. The

26 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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.

Table 1b concentration and age of Yodo River water change little in the vertical Volume-averaged age (in days) of Yodo River water in the SIS and subregions (figure not shown) except for in Osaka Bay, where the seasonal strati- from calculations with boundaries OB1 and OB2. fication is maintained over a timescale of 3 months, which is at the Subregions January April July October Annual same order as the mean water age of Yodo River water. In the other areas, the longer water age allows wind mixing and tidal mixing to SIS 168 173 135 138 152 make a vertically homogeneous distribution for the concentration and Osaka Bay 112 106 82 91 97 age of Yodo River water. Harima-Nada 218 231 188 171 199 Kii Channel 177 179 149 152 165 We choose section AB that crosses Osaka Bay from the Yodo River Other areas of the SIS 524 414 530 417 450 mouth (see Fig. 1b for its location) to demonstrate the vertical variation Kitan Strait 142 137 117 119 128 of concentration and age of Yodo River water. As expected, the age of Akashi Strait 171 141 105 104 125 Yodo River water is smaller around the river mouth than in the offshore Naruto Strait 210 218 199 191 203 area and is smaller in the surface layer than in the bottom layer. The Bisan Strait 274 296 319 224 271 Hayasui Strait 882 939 930 838 891 concentration and age of Yodo River water are homogeneous in winter but stratified in other seasons (i.e., spring, summer, and autumn) (Fig. 4). Along the section, the stratification is stronger around the river

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 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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 a factor of 100. The color bars show the color ranges for (i) the concentration and (ii) the age. The contour interval for the concentration is 1, and the interval for the age is 10 days.

mouth than in other areas, corresponding to the salinity stratiﬁcation in + ⎧ fYR (, xyz00 , )=> fYR (, xyz , )if f YR (, xyz 0 , ) 0 Osaka Bay (Chang et al., 2009). ⎨ f− (, xyz , )=< f (, xyz , )if f (, xyz , ) 0 ⎩ YR 00YR YR 0 (9)

3.2. Budget of Yodo River water in the SIS T fYR (,xy0 ,) z= ∫ vtxy (,,00 ,) zCtxy (,, ,) zdt . 0 (10) As the SIS connects to the Pacific Ocean through the Kii and Bungo Channels, water exchange in these two channels determines the mean Here, A is the cross-sectional area across the outlet (Akashi Strait, age of Yodo River water in the SIS (Fig. 3). Since the Yodo River water Kitan Strait, Bisan Strait, or Naruto Strait); fYR(x0,y,z) is the annual concentrates in Osaka Bay and the Harima-Nada, we calculate the Yodo River flux at a grid point in the Akashi Strait or the Bisan Strait; budget of freshwater in these two areas. fYR(x,y0,z) is the annual Yodo River flux at a grid point in the Kitan The annual mean volume of Yodo River water inside Osaka Bay or Strait or the Naruto Strait. The overbar in Eqs. (7) and (10) denotes the the Harima-Nada (VYR) can be calculated by Eq. (4) from the con- procedure of de-tiding. centration of Yodo River water C that is solution of Eq. (1).

T 3.2.1. Budget of Yodo River water in Osaka Bay 1 The annual mean volume of Yodo River water inside Osaka Bay VYR = ∫∭Ctxyzdxdydzdt(, , , ) T 10 3 0 Vol (4) (VYR) is 0.14 × 10 m (Fig. 5a), which occupies 3% of the total water volume (4.16 × 1010 m3) in Osaka Bay. Every year, 0.8 × 1010 m3 of Here, T is the integral time (= one year) and Vol is the volume of Yodo River water is discharged into Osaka Bay from the river mouth the study area (i.e., Osaka Bay or the Harima-Nada). (Fig. 5a). fl IN fl OUT The yearly in ow (FYR (x0)) and out ow (FYR (x0)) of Yodo At the Akashi Strait, 1.4 × 1010 m3 of Yodo River water is trans- River water through the Akashi Strait or the Bisan Strait (x = x0), the ported out of Osaka Bay, while 1.1 × 1010 m3 of Yodo River water is fl IN fl OUT yearly in ow (FYR (y0)) and out ow (FYR (y0)) of Yodo River water transported back (Fig. 5a). Consequently, the yearly net export of Yodo through the Kitan Strait or the Naruto Strait (y = y0) were calculated by River water from Osaka Bay to the Harima Nada through the Akashi the following equations: 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 trans- IN + 10 3 ⎧ FYR () x00= ∬ fYR (,,) x y z dydz ported back (Fig. 5a), yielding a net export of 0.5 × 10 m . Thus, the ⎪ A sum of river water export through the Akashi Strait and the Kitan Strait ⎨FOUT () x= f− (,,) x y z dydz ⎪ YR 00∬ YR is balanced with the input of river water from the Yodo River mouth. ⎩ A (5) 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 ⎧ fYR (,,) x00 yz=> fYR (,,)if(,,) x yz f YR x 0 yz 0 fl ⎨ − Bay through its westernmost and east-central parts and ows back fYR (,,) x00 yz=< fYR (,,)if(,,) x yz f YR x 0 yz 0 (6) ⎩ through 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 than T that flowing back and the river water transport is larger in the surface f (xyz ,,)= utxyzCtxyzd (, ,,) (, ,,)t YR 0 ∫ 00 layer than in the bottom layer. 0 (7)

3.2.2. Budget of Yodo River water in the Harima-Nada ⎧ FIN () y= f+ (,,) x y z dxdz YR 00∬ YR The Naruto Strait is the exit of Yodo River water from the Harima- ⎪ A Nada to the Kii Channel. As shown in Fig. 5a, the net export of Yodo ⎨FOUT () y= f− (,,) x y z dxdz ⎪ YR 00∬ YR River water from Osaka Bay to the Harima-Nada through the Akashi A (8) ⎩ Strait is 0.3 × 1010 m3. At the Naruto Strait, 1.5 × 1010 m3 of Yodo

28 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

(caption on next page)

29 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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 outlets connecting the SIS with the Bungo and Kii Channels, and the of Yodo River water is transported inwards (Fig. 5b), giving a net flux of Akashi Strait and Bisan Strait are the two outlets connecting Osaka Bay, 0.3 × 1010 m3 from the Harima-Nada to the Kii Channel. At the Bisan the Harima-Nada, and other areas of the SIS. Among these straits, the Strait, the transports of Yodo River water into and out of the Harima- Yodo River water at the Hayasui Strait has the highest age but lowest Nada are both 0.03 × 1010 m3, being smaller than those through the concentration (Table 1). Therefore, we pay more attention to the age of Naruto Strait by one order of magnitude (Fig. 5b). Apparently, it is the Yodo River water in the other four straits. Among these, the age of Yodo net export of Yodo River water from the Harima-Nada to the Kii River water at the Bisan Strait is higher than that at the Naruto Strait, Channel through the Naruto Strait that balances with the input of Yodo which is higher again than those at the Akashi Strait and Kitan Strait River water from the Akashi Strait (Fig. 5a). (Fig. 3, Table 1b). The annual ages of Yodo River water are 271 days, At the Naruto Strait, the Yodo River water flows out of the Harima- 203 days, 125 days, and 128 days for the Basin Strait, Naruto Strait, Nada through the western part and flows back through the eastern part Akashi Strait, and Kitan Strait, respectively (Table 1b). (Fig. 6d). The area of outflowing Yodo River water is larger than that of According to Table 1b, the ranges of seasonal age variation (its ratio inflowing Yodo River water and river water transport is larger in the to annual age) are 95 days (0.35) for the Bisan Strait, 27 days (0.13) for surface layer than in the bottom layer (Fig. 6d). The Yodo River the Naruto Strait, 67 days (0.54) for the Akashi Strait, and 25 days (0.2) transport through the Bisan Strait (Fig. 6c) is much smaller than that for the Kitan Strait, indicating that the age of Yodo River water has through the Naruto Strait (Fig. 6d). 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 3.3. Age of Yodo River water at the straits along its pathway to the Pacific (Table 1b), when more Yodo River water moves southward due to the Ocean intense northwesterly winds (Fig. 12b).

By combining the budget of Yodo River water in Osaka Bay and the 4. Discussion Harima-Nada, the pathway of Yodo River water in the SIS becomes clear (Fig. 5c). After discharging into Osaka Bay from the Yodo River 4.1. Influences of dynamic processes on the age of Yodo River water mouth (~0.8 × 1010 m3), the Yodo River water separates into two branches. One branch (~0.5 × 1010 m3) flows directly into the Kii Any change in the dynamic factors (e.g., tide, local winds, baroclinic Channel through the Kitan Strait. The other (~0.3 × 1010 m3) flows forcing, and river discharge) may affect water age. To understand the into the Harima-Nada through the Akashi Strait, and then flows into the major control factor on the age of Yodo River water in the SIS, we use a Kii Channel through the Naruto Strait. Finally, Yodo River water set of sensitivity simulations to artificially modify the driving forces and (~0.8 × 1010 m3) leaves the SIS and flows into the Pacific Ocean calculated the water age again. through the Kii Channel. Four cases (Table 2) are designed to examine the influences of The Hayasui Strait, Naruto Strait, and Kitan Strait are the three driving forces on the river water age. When running each of these cases,

Fig. 6. Annual mean utx(,00 , yzCtx , ) (, , yz , )at (a) Akashi Strait and (c) Bisan Strait and vtxy(, ,00 , zCtxy ) (, , , 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 and Fig. 6c indicate eastward flow, and positive values in Fig. 6b and Fig. 6d indicate northward flow.

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Table 2 Table 3b Conditions used in sensitivity experiments for Case 1 to Case 4. Relative diﬀerence (%) of volume- and annually averaged concentration of Yodo River water between the sensitivity experiments and the control case Case Yodo River discharge Tide Local winds Baroclinic forcing (Case 0).

0 Yes Yes Yes Yes Subregions Case 1 Case 2 Case 3 Case 4 1 Yes No Yes Yes 2 Yes Yes No Yes SIS −15 0 61 −46 3 Yes Yes Yes No (T = 15 °C, S = 33) Osaka Bay 8 −1 105 −44 4 0.5 times Yes Yes Yes Harima-Nada −39 −123−46 Kii Channel −7 −384−46 Other areas of the SIS 82 18 −66 −54 only one driving force from the control case (hereafter referred to as Kitan Strait 7 −1 109 −44 − − Case 0, whose results were presented in Sections 3.1–3.3) is changed. Akashi Strait 12 4 68 45 Naruto Strait −42 −152−47 The hydrodynamic model and CART module are both calculated again Bisan Strait −23 0 −42 −48 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. (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 absolute 4.1.1. Tide value decreases. At the Kitan Strait, every year 0.9 × 1010 m3 of Yodo The exclusion of tide (Case 1, removing tidal oscillation at open River water is transported out of Osaka Bay, while 0.1 × 1010 m3 of boundaries) reduces the concentration and increases the age of Yodo Yodo River water is transported inwards, giving a net flux of River water in the entire SIS (Table 3). However, the effects of tide are 0.8 × 1010 m3 (Table 4). The Yodo River water flows out of Osaka Bay different in each sub-region of the SIS. Without tide, the annual con- through the eastern part and the surface layer of the western part of the centration of Yodo River water increases by 8% in Osaka Bay but de- Kitan Strait, and flows into Osaka Bay through the bottom layer of the creases by 39% in the Harima-Nada (Table 3a, Table 3b, Fig. 7c). The western part (Fig. 11c). annual mean ages of Yodo River water in Case 1 increase by 6% and The budget of Yodo River water at the Akashi Strait and Kitan Strait 51% in Osaka Bay and the Harima-Nada (Table 3c, Table 3d, Fig. 7d), suggests that the pathway of the Yodo River water changes in the case as compared to those in the control case (Case 0). without tide. After discharging into Osaka Bay from the Yodo River We choose sections CD and EF crossing the Harima-Nada and Osaka mouth (~0.8 × 1010 m3), instead of separating into two branches as in Bay (see Fig. 1b for its location) to demonstrate the vertical variation. Case 0, the Yodo River water (0.8 × 1010 m3) leaves Osaka Bay through Compared with those in Case 0, the concentration and age in the the Kitan Strait and moves directly into the Kii Channel. Harima-Nada become more stratified without tide (Fig. 8b, e). The water age destratification under tide was also found in James River estuary (Shen and Lin, 2006) and Changjiang estuary (Wang et al., 4.1.2. Local winds 2010). The great reduction (increase) of concentration (age) of Yodo The local wind in the SIS induces an apparent seasonal variation. River water in the Harima-Nada without tide appears from the surface The spring and summer wind is rather weak and northerly winds to the bottom layers (Fig. 8b, e). In Osaka Bay, the effects of tide on the commence in autumn. In winter, intense northwesterly wind affects the vertical stratification and absolute value are much less than those in the whole SIS (Fig. 12a). The exclusion of local winds (Case 2) slightly Harima Nada (Fig. 9b, e). affects the concentration and age of Yodo River water in the SIS Compared with Case 0 (Fig. 10a), the strong westward current (Table 3, Fig. 7e, f). Without local winds, the concentration and age of outside the Akashi Strait in the Harima-Nada disappears, and changes Yodo River water in Osaka Bay and Harima-Nada change less than 3% to flow eastward towards the Akashi Strait without tide (Fig. 10b). The of those in Case 0 (Table 3). Correspondingly, the current only changes stronger eastward current in the northern Harima-Nada exists from the slightly in Osaka Bay and the Harima-Nada, e.g., the westward current surface to the bottom layers (Fig. 8h), preventing the Yodo River water becomes larger (Fig. 10c). The net fluxes at the Bisan Strait, Naruto flowing into the Harima-Nada through the Akashi Strait. Strait, Akashi Strait, and Kitan Strait change little (Table 4, Fig. 5), In Case 1, 0.3 × 1010 m3 of Yodo River water is transported out of indicating that the local winds do not change the pathway of Yodo Osaka Bay through the Akashi Strait every year, while 0.3 × 1010 m3 is River water in the SIS. transported inwards. Therefore, there is almost no net export of Yodo However, the intense northwesterly wind in winter affects the River water from Osaka Bay to the Harima Nada through the Akashi horizontal distribution of concentration and age of Yodo River water. Strait in Case 1 (Table 4). The Yodo River water flows out of Osaka Bay The concentration (age) significantly decreases (increases) in the mainly in the surface layer and bottom layer of the Akashi Strait, and eastern part of Osaka Bay and increases (decreases) in the northern part flows into Osaka Bay mainly in the subsurface layer and bottom layer without the northwesterly wind in winter (Fig. 12b, c). Gong et al. (2009) found a downstream wind generally decreases the water age in Table 3a the Rappahannock River, while an upstream wind increases water age. Volume- and annually averaged concentration of Yodo River water in the SIS In Osaka Bay, the northerly wind component is a downstream wind for and subregions from sensitivity experiments (Case 1 to Case 4). the southward moving Yodo River water along the eastern bay, and the Subregions Case 0 Case 1 Case 2 Case 3 Case 4 westerly wind component is an upstream wind for the westward moving of Yodo River water along the northern bay (Fig. 2). Therefore, SIS 0.54 0.46 0.54 0.87 0.29 the northwesterly wind in winter affects the concentration and age of Osaka Bay 3.31 3.59 3.29 6.8 1.86 Yodo River water in opposite ways in the eastern and northern parts of Harima-Nada 1.7 1.04 1.69 2.09 0.91 fi Kii Channel 0.76 0.71 0.74 1.4 0.41 the bay. Speci cally, without the northerly wind component, the Other areas of the SIS 0.0056 0.0102 0.0066 0.0019 0.0026 southward current in the eastern part of Osaka Bay becomes weaker Kitan Strait 1.63 1.75 1.62 3.41 0.92 (Fig. 12e), preventing the Yodo River water flowing southward and Akashi Strait 2.75 2.41 2.87 4.61 1.52 correspondingly reducing (increasing) the concentration (age) of Yodo Naruto Strait 1.41 0.82 1.4 2.15 0.75 Bisan Strait 0.31 0.24 0.31 0.18 0.16 River water in the eastern part of the bay (Fig. 12b, c); without the westerly wind component, the current in the northern part of Osaka Bay The concentration has been multiplied by a factor of 100. changes to flow westward (Fig. 12e), making more Yodo River water

31 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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 the panel 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 the color 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, and the interval for the age (right panels) is 10 days.

Table 3c Table 3d Volume- and annually averaged age (in days) of Yodo River water in the SIS and Relative diﬀerence (%) of volume- and annually averaged age of Yodo River subregions from sensitivity experiments. water between sensitivity experiments and the control case (Case 0).

Subregions Case 0 Case 1 Case 2 Case 3 Case 4 Subregions Case 1 Case 2 Case 3 Case 4

SIS 152 185 152 207 160 SIS 22 0 36 5 Osaka Bay 97 103 96 147 104 Osaka Bay 6 −1527 Harima-Nada 199 301 198 300 211 Harima-Nada 51 −1516 Kii Channel 165 158 166 247 178 Kii Channel −4 1 50 8 Other areas of the SIS 450 599 468 470 452 Other areas of the SIS 33 4 4 0 Kitan Strait 128 126 129 202 141 Kitan Strait −2 1 58 10 Akashi Strait 125 159 117 212 136 Akashi Strait 27 −6709 Naruto Strait 203 287 202 284 217 Naruto Strait 41 0 40 7 Bisan Strait 271 375 285 379 283 Bisan Strait 38 5 40 4

flow westward and then increasing (decreasing) the concentration (age) 4.1.3. Baroclinic forcing in the northern bay (Fig. 12b, c). The exclusion of baroclinic forcing (Case 3, the water temperature Differing from the river water age destratification under strong and salinity in the entire SIS is set to be constant) significantly increases wind found in the Rappahannock River (Gong et al., 2009), the con- the concentration and age of Yodo River water in the entire SIS centration and age of Yodo River water in the Harima-Nada remains (Table 3). The annual concentrations of Yodo River water increase by vertically homogeneous and the vertical stratification in Osaka Bay also 105% and 23% in Osaka Bay and the Harima-Nada, and the annual ages barely changes without the intense northwesterly wind in winter (figure increase by 52% and 51%, respectively (Table 3, Fig. 7g, h). The river not show), since vertical stratification is relatively small in winter. water age with density-induced circulation being less than that without density-induced circulation was also found in the James River (Shen and Lin, 2006) and Danshuei River (Liu et al., 2008). As expected, the

32 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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, and the interval for the velocity is 0.025 m/s. Positive values in Fig. 8g, Fig. 8h, and Fig. 8i indicate eastward flow. vertical stratification of concentration and age in Osaka Bay and the 4.1.4. Yodo River discharge Harima-Nada disappears, and a great increase of concentration and age The variations in the Yodo River discharge significantly change its occurs from the surface to the bottom layers without baroclinic forcing concentration in the SIS. Halving Yodo River discharge (Case 4) reduces (Fig. 8c, f, Fig. 9c, f). the concentration of Yodo River water by 40%–50% in the entire and In the absence of baroclinic forcing, the current significantly slows sub-regions of the SIS (Table 3a, Table 3b, Fig. 7i). However, the large down in the whole of Osaka Bay and the Harima-Nada (Fig. 10d, Fig. 8i, change in Yodo River discharge does not induce a significant variation Fig. 9i). The southward current towards the Kitan Strait in the eastern in the age of Yodo River water in the SIS. Halving Yodo River discharge part of Osaka Bay becomes much smaller (Fig. 10d, Fig. 9i). These (Case 4) modifies the water age on the order of 0–20 days, which is less changes help to increase the mass and staying time of Yodo River water than 10% of the age of Yodo River water in Case 0 (Table 3c, Table 3d, in Osaka Bay and the Harima-Nada. Fig. 7j). Correspondingly, the current only changes slightly in Osaka At the Akashi Strait, Kitan Strait, and Naruto Strait, the Yodo River Bay, e.g., the southward current in the eastern part becomes smaller water transport increases in a similar order and the spatial structure (Fig. 10e). The reduction of concentration in the SIS is attributed to the does not change in Case 3 compared to Case 0 (Fig. 6, Fig. 11b, d, f). large reduction of freshwater input. The net fluxes at each Strait barely change (Table 4, Fig. 5), indicating The insensitivity of water age to the change in river discharge also that the baroclinic forcing does not change the pathway of Yodo River occurs in other semi-enclosed seas (e.g., Bohai Sea (Liu et al., 2012)), water in the SIS. and is quite different from many estuary systems where the age

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, and the interval for the velocity is 0.025 m/s. Positive values in Fig. 9g, Fig. 9h, and Fig. 9i indicate northward ﬂow.

33 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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. significantly reduces with increasing river discharge (e.g., York River discharged Yodo River water from the river mouth. In other words, the (Shen and Haas, 2004), James River (Shen and Lin, 2006), Danshuei environmental impact of the reentering and newly entering Yodo River River (Liu et al., 2008), Susquehanna River (Hong et al., 2010), water is different. Therefore, it is important to know the age of newly Changjiang (Wang et al., 2010), and Pearl River (Ren et al., 2014)). The entering Yodo River water inside Osaka Bay that corresponds to the river discharge is probably not an important factor affecting transport water age without the reentry process through the two outlets (i.e., the time of freshwater in our study area; however, it significantly affects Akashi Strait and Kitan Strait). transport time of freshwater in an estuary. 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 4.2. Age of Yodo River water in Osaka Bay without the reentry process 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 As presented in Section 3.2, reentering Yodo River water comprises and age concentration of Yodo River water at the outer grid point were a portion of Yodo River water inside Osaka Bay. The materials, such as set to those at the strait; if there is an inflow at the outer grid point next nutrients or persistent organic pollutants (POPs), carried by the re- to the strait, then the concentration and age concentration of Yodo entering 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

Case 1 0.3 0.3 0 0.1 0.9 0.8 Case 2 1.2 1.5 0.3 0.6 1.1 0.5 Case 3 1.8 2.1 0.3 1.5 2.0 0.5

Naruto Strait 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

Case 1 0.1 0.1 0 0.02 0.02 0 Case 2 1.2 1.5 0.3 0.03 0.03 0 Case 3 1.9 2.2 0.3 0.02 0.02 0 The “→” indicates Yodo River water transport from the former to the latter.

34 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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

River water at the outer grid point were set to 0. The age of the Yodo contribution of reentering Yodo River water to the river water age in River water in Osaka Bay decreases by approximately 80% (from ap- Osaka Bay. However, the return flow still exists as in Case 0 and cannot proximately 100 days to 20 days) (Table 5, Fig. 3, Fig. 13) in all seasons be simply separated in the hydrodynamic model. Some previous studies without the reentry of river water through both the Akashi Strait and may help us to understand the mechanism of return flow in Osaka Bay. Kitan Strait. The significant increase in water age caused by reentered Sugimatsu and Isobe (2010, 2011) found that the cold, nutrient-rich river water was also found in the Bohai Sea (Liu et al., 2012), indicating water with an origin of Kuroshio intermediate water occupies the the reentry process through the outlet is important for river water age bottom layers of Osaka Bay in summer and suggested that the estuarine in a semi-enclosed sea. circulation induced by the river runoff into Osaka Bay brings it into the It is interesting that the age of newly entering Yodo River water in bay. As the Yodo River water passes the Kitan Strait, the strong vertical Osaka Bay maintains a similar spatial distribution in each of the four mixing there brings the river water down to the bottom layer, becoming seasons as that of total Yodo River water (Fig. 3, Fig. 13). Therefore, it a part of onshore estuarine circulation in the bottom layer, finally re- is likely that the movement of newly discharged Yodo River water in turning to Osaka Bay and consequently increasing the water age inside Osaka Bay, rather than that of returned Yodo River water, controls the the bay. age distribution of Yodo River water in Osaka Bay. If the Yodo River water is not allowed to reenter Osaka Bay through the Akashi Strait, i.e., an open boundary is set only at the Akashi Strait 5. Summary and the same water age calculation is repeated, then its water age in Osaka Bay decreases by 70–80%; if the Yodo River water is not allowed The age of Yodo River water in the SIS was calculated based on to reenter Osaka Bay through the Kitan Strait, then its water age in CART (Deleersnijder et al., 2001) using a validated hydrodynamic Osaka Bay decreases by 50–60% (Tables 5a and 5b). This result is model (Chang et al., 2009). While previous studies on water transport consistent with the budget of Yodo River water in Osaka Bay; i.e., the timescales in the SIS mainly focused on the average values in the entire annual transports of freshwater through the Akashi Strait and Kitan water volume (e.g., Takeoka, 1984b), this study presents the spatial Strait back into Osaka Bay are 1.1 × 1010 m3 and 0.6 × 1010 m3, re- distribution and temporal variation of river water transport time. River spectively (Fig. 5a). water age, from an environmental perspective, provides useful in- Please note that the new water age calculation only removes the formation for quantitative evaluation of the long-term transport of nutrients, pollutants, and some ecological variables from rivers in the

35 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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

Subregions Boundaries January April July October Annual

Osaka Bay Akashi + Kitan Strait 23 22 12 11 18 OB1 + OB2 + Akashi Strait 28 33 18 17 25 OB1 + OB2 + Kitan Strait 53 47 35 36 42 Kitan Strait Akashi + Kitan Strait 31 31 21 20 26 Akashi Strait Akashi + Kitan Strait 35 26 15 14 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 and OB2, results are given in Table 1).

Subregions Boundaries January April July October Annual

Osaka Bay Akashi + Kitan Strait −79 −79 −85 −88 −81 OB1 + OB2 + Akashi Strait −75 −69 −78 −81 −74 OB1 + OB2 + Kitan Strait −53 −56 −57 −60 −57 Kitan Strait Akashi + Kitan Strait −78 −77 −82 −83 −80 Akashi Strait Akashi + Kitan Strait −80 −82 −86 −87 −85 sea. age from 18 days to 97 days. The tide is an important factor de- The mean age of Yodo River water in the entire SIS is 152 days. The termining the age of Yodo River water in the SIS. Without tidal forcing, spatial variation of water age is large in the horizontal direction but the mean ages of Yodo River water in Osaka Bay and the Harima-Nada small in the vertical except for in Osaka Bay. The age of Yodo River increases by 6% and 51%; no net Yodo River water is transported into water is 97 days in Osaka Bay but 450 days in other areas of the SIS, and the Harima-Nada through the Akashi Strait, and nearly all the Yodo both show strong seasonal variation. After discharging into Osaka Bay, River water flows directly into the Kii Channel through the Kitan Strait. the Yodo River water separates into two branches. One branch Without baroclinic forcing, the mean ages of Yodo River water in Osaka − (~0.5 × 1010 m3 y 1) flows directly into the Kii Channel through the Bay and Harima-Nada increase by 52% and 51%, but the pathway of − Kitan Strait, and the other (~0.3 × 1010 m3 y 1) flows into the Harima- Yodo River water in the SIS does not change. Nada through the Akashi Strait, and then flows into the Kii Channel through the Naruto Strait. This is the first understanding of the clear Acknowledgments pathway of Yodo River water in the SIS. The return of old Yodo River water to Osaka Bay is a primary cause This work was completed during H. Wang's visit to Center for for the large age of Yodo River water in Osaka Bay, increasing the water Marine Environmental Studies, Ehime University. She thanks the China

36 H. Wang et al. Journal of Marine Systems 191 (2019) 24–37

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.

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