Environmental Pollution 308 (2022) 119631

Contents lists available at ScienceDirect

Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol

Fate of river-derived microplastics from the South China Sea: Sources tosurrounding seas, shores, and abysses☆
Kosei Matsushita a, Yusuke Uchiyama a, b, *, Naru Takaura c, Taichi Kosako b
aDepartment of Civil Engineering, Kobe University, Kobe, Japan
Coastal and Estuarine Environmental Department, Port and Airport Research Institute, Yokosuka, Japanc
Tokyo Electric Power Company Holdings, Inc., Tokyo, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
River-derived microplastics
Oceanic transport and dispersal
The south China Sea
Lagrangian particle tracking model
ROMS

Microplastics (MPs) in the ocean have been widely recognized as causing global marine environmental problems.
To gain a quantitative and comprehensive understanding of oceanic MP contamination, detailed numerical
Lagrangian particle tracking experiments were conducted to evaluate the regional oceanic transport anddispersal of MPs in the South China Sea (SCS) derived from three major rivers, Pearl (China), Mekong (Vietnam),and Pasig (the Philippines), which are known to discharge large amounts of plastic waste into the SCS. As
previous field surveys have suggested, MP contamination spreads from the surface to the deeper ocean in thewater column, we thus considered three types of MPs: (1) positively buoyant (light) MPs, (2) positively buoyant(light) MPs with random walk diffusion, and (3) full 3-D tracking of non-buoyant MPs that are passivelytransported by ambient currents. Transport patterns of these MPs from the three rivers clearly showed the intraannual variability associated with seasonally varying circulations driven by the Asian monsoons in the SCS. Many
MPs floating during the prevailing southwest monsoon are transported to the northwest Pacific Ocean and theEast China Sea through the Luzon Strait and the Taiwan Strait to form MP hotspots. Non-buoyant MPs are broadlytransported from the surface layer to depths of approximately 100 m or deeper, where in situ observations are
rare. In addition, the buoyant MPs drifting on the continental shelf originating from southern China tend to bepushed toward the shore and beached by northward wind-induced currents more pronouncedly than the nonbuoyant MPs. Therefore, the river-derived MPs to the SCS were found to serve as sources to adjacent basins
and oceans, to be distributed not only in the upper layer but also in the abyssal ocean (non-buoyant MPs), and to
be transported to the shores (buoyant MPs).

1. Introduction
In recent years, marine plastic pollution has been widely recognized
as one of the most important global environmental issues. Microplastics(MPs), which are plastic fragments degraded into a diameter of 5 mm or
less, flow mainly from land into the ocean and drift for a long time.
Because plastics readily adsorb chemical substances (Mato et al., 2001)
and are accidently ingested by zooplankton (Sun et al., 2017), andmarine organisms (Zhu et al., 2019a), MPs may have a significant impact
on our ecosystems. The annual plastic waste input to the ocean has beenreported to be 4.8 to 12.7 million tons from 192 coastal countries(Jambeck et al., 2015). Among others, many major rivers in Asia are thelargest sources of marine plastic discharge to the ocean, which accountfor approximately 67% of global river discharges of plastic waste (Leb­
reton et al., 2017). In addition, the recent study by Meijer et al. (2021)
evaluated that more than 1000 rivers account for 80% of global annual
emissions, which range between 0.8 million and 2.7 million tons per
year, with small urban rivers among the most polluting. Field samplingof MPs found that East Asian seas have one of the highest contamina­
tions in the world (Isobe et al., 2015), indicating that Asian-origin MPs
have a significant influence on the distribution of marine MPs in thesurrounding seas. To compromise a limited number of laborious andcostly field observations on measuring plastic wastes in a vast expanse ofoceans, numerical modeling studies have been conducted for the Japan
Sea (Iwasaki et al., 2017) and the East China Sea (ECS; Zhang et al.,2020). The transport processes of MPs have thus been gradually clarified

This paper has been recommended for acceptance by Baoshan Xing.
* Corresponding author. Department of Civil Engineering, Kobe University, Kobe, Japan.
E-mail address: uchiyama@harbor.kobe-u.ac.jp (Y. Uchiyama).

☆

https://doi.org/10.1016/j.envpol.2022.119631
Received 4 February 2022; Received in revised form 10 June 2022; Accepted 12 June 2022
Available online 20 June 2022
0269-7491/© 2022 Elsevier Ltd. All rights reserved.

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

in these seas through numerical analyses. However, there are few
modeling studies on MP transport for the South China Sea (SCS, Fig. 1),
although many emission sources exist in the region; the SCS is fringed bythree countries in the top 20 plastic emitting countries to ocean, viz., thePhilippines (1st), China (4th), and Vietnam (8th). In particular, thePhilippines has 19 rivers out of the top 50 plastic emitting rivers (Meijer
et al., 2021). Hence, the quantitative contribution of the rivers as the
major plastic sources in the SCS to plastic pollution in and around the
SCS is an important scientific question to be answered.
The SCS is the world’s largest marginal sea located on the west coastof the Pacific Ocean (Fig. 1). The six coastal SCS countries annuallycontribute 6.1 × 105 tons of the plastic waste, among which thePhilippines, China, and Vietnam emit 3.6 × 105, 7.1 × 104, 2.8 × 104(ton/y), respectively (Meijer et al., 2021). The Pasig River (thePhilippines), the Pearl River (China), and the Mekong River (Vietnam),
all of which have river mouths in the shore of the SCS, discharge 39,000,
110,000, and 23,000 tons of plastic waste into the ocean annually,ranking 8th, 3rd, and 11th, in the world, respectively (Lebreton et al.,2017). The most recent evaluation (Meijer et al., 2021) updated thisranking that the Pasig River may be the world’s largest plastic emitter.
Indeed, MP pollution has been found in the estuaries of these rivers(Emmerik et al., 2020; Lam et al., 2020). In particular, high concen­
trations of MP pollution were found in the coastal areas of southern
China owing to the influence of the Pearl River-derived MPs (Cai et al.,2018; Lam et al., 2020). In contrast, MP pollution in the central SCS is
not as severe as that in MP hotspots such as the coastal SCS (Cai et al.,2018), the ECS (Zhang et al., 2020), and the Japan Sea (Isobe et al.,2015). Therefore, it is necessary to accurately evaluate the transport
processes of MPs derived from major rivers in the SCS and their possible
remote influences on the surrounding seas, while it remains a challenge
to compare MP pollution in the world’s rivers due to the lack of stan­
dardized sampling and analytical protocols (Mai et al., 2019).
Most buoyant MPs are reported to exist within 1 m of the ocean
surface (Reisser et al., 2015). They are also vertically distributed (vanSebille et al., 2020) by vertical eddy mixing, which is enhanced in thesurface boundary layer (Kukulka et al., 2012). As time progresses, thedensity of MPs may be altered owing to fragmentation, refinement, andbiofouling (Kaiser et al., 2017; Fazey and Ryan, 2016). Further

degradation of MPs results in much finer, nanometer-sized pieces ofplastic (Piccardo et al., 2020). Smaller plastic fragments have a larger
surface area relative to their volume, promoting surface adhesion that
increases the MP density, and thus is less likely to have an upwardsurfacing velocity (Chubarenko et al., 2016; Poulain et al., 2019).
Accordingly, once MPs disappear from the ocean surface, they seldom
return to the surface, and thus can be found in the ocean interior below
the surface layer (Kooi et al., 2017). In fact, MPs have occasionally been
observed in the deep ocean at depths ranging from several hundred
meters to several thousand meters (Li et al., 2020). Zhu et al. (2019a)
reported that all deep-sea fish collected from depths of 200–209 m and
453–478 m were contaminated by microplastics on the northern conti­
nental slope of the SCS. Isobe et al. (2019) succeeded in reproducing
surface MP contamination in the Pacific Ocean with their horizontal
two-dimensional (2DH) surface MP transport model if they introduced
an ad hoc sink term crudely activated three years after MP is released to
mimic downward MP transport. Cai et al. (2018) estimated that the top
5 m of the SCS contained around 700 tons of plastics; this is the current
best available estimate of plastic mass present in SCS surface waters.
However, they also pointed out that the MPs found in the surface ocean
are one to four orders of magnitude smaller than the reported emission,
implying both a deficiency of sparse observations and the importance of
non-buoyant plastics. Thus, it is profoundly important to scientificallyunderstand the transport processes of MPs that are not only drifting inthe surface layer, but also being passively transported by ambient cur­rents in the interior ocean where observations of MPs have not yet beenabundant.
In the present study, we aimed to answer the question on the fate ofMPs derived from major rivers in the SCS by using a state-of-the-art
three-dimensional (3D) ocean modeling technique coupled with anoffline Lagrangian particle tracking model. As argued above, anticipatedpossible destinations include shallow coastal areas on the continental
shelf of the SCS, the surface ocean in the middle of the SCS, the sub­surface or abyssal ocean of the SCS, and the surrounding seas such as the
ECS. Because transport and dispersal processes of MPs may differ ac­
cording to their relative density to the seawater; two types of MPs, viz.
positively buoyant (light) and neutrally buoyant (passive) MPs, wereconsidered. Hence, we evaluated the 3D oceanic transport of MPs

Fig. 1. Left: Study area around the South China Sea. Red lines are the borders that separate the East China Sea (ECS), Pacific Ocean, Indian Ocean from the South
China Sea. The black box corresponds to the HYCOM-ROMS model domain. Right: Enlarged view of the left panel with the locations of the three MP release patches
located at the mouths of the Pearl (China), Mekong (Vietnam), and Pasig (Philippines) rivers depicted by red circle marks. Colors show the bathymetry in meter. (Forinterpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)2

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

derived from three major rivers in the SCS, that is, Pearl (China),
Mekong (Vietnam), and Pasig (the Philippines) rivers, as major sourcesof terrestrial plastic waste into the SCS. We investigated the horizontaltransport patterns of MPs in and around the SCS along with possible
driving mechanisms, and quantified the rate of MPs originating from the
SCS that flowed out to the surrounding seas. Based on the modeledoutcomes, we further discuss the possible impacts of high MP concen­
trations on MP pollution in East Asian seas and the differences intransport patterns of MPs of two different weight densities.

the medium-term climatology mode mainly due to ENSO (Table 1a).
More details of the HYCOM-ROMS model can be found in Section S1 of
the Supplemental Material.
We exploited the offline 3-D Lagrangian particle tracking model
originally developed by Carr et al. (2008), forced by the HYCOM-ROMS
model outputs, following the methodology described by Mitarai et al.
(2009), Romero et al. (2013), and Uchiyama et al. (2018b) with several
modifications (See Sections S2 and S3 of Supplemental Material). To
investigate the effects of MP density on the dispersal processes, particle
tracking experiments were conducted for three cases, viz., Case 1:
buoyant MPs forced by the 3D currents, Case 2: buoyant MPs forced by
the surface 2DH currents and placed in the top-most surface layer, andCase 3: non-buoyant MPs that are fully transported passively by the 3D
currents (Table 1b). We presume that MPs are slightly buoyant to park in
the near-surface layer and thus are not to be transported directly bywindage for Cases 1 and 2, while Case 3 accounts for small-sized MPsthat are neutrally buoyant with the rising velocity of wMP = 0. The
particles were released periodically at an interval of 12 h from circular
patches placed at the river mouths of interest (Table 1b–c). The radius of
the circular patch was set to 25 km, in which particles were evenly
placed to complement the under-resolved initial near-field dilution of
MPs around river mouths. More than four million particles were released
over the three years.
To determine the destination of MPs initially released from majorrivers in the SCS, the beaching of MPs is considered in Cases 1 and 3. In
this study, the MPs reaching a grid cell with the representative shallow
water depth of 20 m, which is the pre-defined minimum depth set in the
HYCOM-ROMS model, are automatically deemed “beached.” If theparticle is judged as beached, the tracking is terminated, and the particle
stops at this position. Case 2 is an exception to this protocol because of
the inclusion of the random walk model, in which MPs that drifted onto
the shore grid must be allowed to return to the ocean grid by forced
horizontal MP displacement by the horizontal random walk. However,even for Case 2, the MPs reaching the shore grid are much less mobile as
they spontaneously slow down according to the subgrid velocity inter­
polation towards zero velocity on the land grids. Moreover, we did notinclude the Stokes drift velocity (Uchiyama et al., 2010, 2017a) in thepresent study as argues in Section S2, except for the additional experi­mental run shown in Section S4, as shown in the Supplemental Material.

2. Methods
We conducted particle tracking experiments for both buoyant MPs
and neutrally buoyant (non-buoyant) MPs driven by a pre-computed 3D
Eulerian current velocity. To accurately evaluate the full 3D transport ofMPs with different bulk densities accurately, a detailed threedimensional flow field was evaluated using a high-precision hydrody­
namic model that is capable of precisely reproducing the fully 3D
oceanic flow from the surface to the ocean floor. In addition, for a more
precise determination of MP transport through narrow straits, aroundcomplex lateral topographies near the land, and islands under the in­fluence of eddies; a higher grid resolution model is required. Accord­
ingly, we utilized the HYCOM-ROMS downscaling model (Uchiyama
et al., 2018c, 2019), which is a high-resolution eddy-resolving circula­tion model based on the Regional Oceanic Modeling System (ROMS;
Shchepetkin and McWilliams, 2005, 2009) embedded in a HYCOM
global reanalysis in an offline-nesting configuration (Mason et al., 2010;
Uchiyama et al., 2014, 2017a,b, 2018a,b, 2022; Kamidaira et al., 2017,2018; 2019, 2021; Kurosawa et al., 2020; Tada et al., 2018; Takeda
et al., 2021; Zhang et al., 2019) with tides (Buijsman et al., 2012;
Dauhajre et al., 2017; Masunaga et al., 2018, 2019). The model en­compasses an extensive area as shown in Fig. 1, and a numerical rean­
alysis was performed for a five-year period from 2011 to 2015, avoiding
Table 1
Configurations of (a) HYCOM-ROMS ocean circulation model, (b) Lagrangianparticle tracking experiment for three cases, and (c) mouth conditions of thethree major rivers defined in the particle tracking model.
HYCOM-ROMS model configurations
Computational period
Grid cells
Horizonal grid resolution
Baroclinic time step
Surface wind stress
Surface flux
SST and SSS to restore
Open boundary/initial conditions
TS nudging
Tides
Topography

3. Medium-term oceanic transport of MPs in and around the SCS

1/16/2011–12/31/2015
1024 × 1024 × vertical 40 s-layers
5 km
60 s
GPV-GSMNOAA-COADS (monthly climatology)
HYCOM (20-day averaged)
HYCOM (daily)
HYCOM (10-day averaged)
TPXO 7.0
SIO SRTM30_Pluscase

1. Buoyant(3D)

2. Buoyant (Surface2D +Random walk)3.
Passive

Rising velocity,wMP
Release depth
Number ofpatches
Radius ofpatchesRelease period
Release interval
Release time

2 mm/s

–

–

0.5 m

–

3

3.1. Mean seasonal hydrodynamic variability
As reviewed by Hu et al. (2000) and references therein, overallseasonal circulation in the SCS is cyclonic in winter and anticyclonic insummer with a few stable eddies, mostly driven by monsoon winds.
These surface circulations are formed as intensive western boundary
currents that extend to a depth of around 200 m (summer) to 500 m(winter) (Chu et al., 1999). The seasonal circulation is also affected bywater exchange between the SCS and the adjacent seas. For instance,
observational studies (e.g., Wyrtki, 1961; Song, 2006; Fang et al., 2010)
show that there is volume outflow from the southern SCS into the JavaSea during winter and vice versa in summer, which have been confirmed
by modeling studies (Cai et al., 2005; Fang et al., 2009; Liu et al., 2011;
He et al., 2015; and Daryabor et al., 2016).
The mean seasonal variability of surface hydrodynamics in the SCS
was examined by taking temporal averages of the HYCOM-ROMS model
results for the three-year period from 2012 to 2014 (Figure S3 of the
Supplemental Material). The surface circulation field of the SCS is
composed of basin-scale wind-generated circulations and gyres(Figure S3c, d), which are significantly affected by the seasonallyvarying Asian monsoons developed mainly near the Asian continent(Figure S3a, b). In winter, the mean surface currents showed a pre­dominant cyclonic gyre in the northeastern part of the SCS (viz., east of
the Gulf of Thailand) owing to the northeasterly energetic winter

0.5 m

25 km
1/1/2012–12/31/2014
12 h
2192

River

Country

Longitude

LatitudeReleasedparticles at
each time

Total numberof particles

Mekong
Pearl
Pasig

Vietnam
China
Philippine

107.50◦113.70◦120.75◦10.20◦
22.05◦
14.50◦

214
210
172

1,407,264
1,380,960
1,131,072

3

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

monsoon, with the strengthened southward-drifting western boundary
current off the Vietnam coast (Figure S3d). In contrast, in summer, thenortheastward currents directed towards the Taiwan Strait were derived
from the northward-drifting western boundary current developed off thesouthern China coast, as a part of the broadly distributed northward SCS
warm current provoked by the gentle southerly monsoon (Figure S3c;
Zhu et al., 2019b). As the MP transport in the SCS is anticipated to vary
substantially on the seasonal time scale, we subsequently investigated
the seasonality in the MP transport in the SCS either by the near-surface
currents exerted on buoyant (light) MPs or by the 3D currents respon­sible for the transport of the neutrally buoyant MPs that are fully passive
to the ambient currents, followed by the analysis of the MP fractionstransported out of the SCS.

field observations of Azman et al. (2021), in which higher concentra­
tions of MP pollution along the coastline of the Malay Peninsula were
measured during the northeasterly monsoon season.
The buoyant MPs derived from the Pearl River are dispersed in a strip
over the continental shelf, showing prominent seasonality in dispersal
patterns (Fig. 2b, e). The cross-shelf transport in summer is less obvious
than that from the Mekong River, as those from the Pearl River are more
pronouncedly affected by the northeastward coastal currents(Figure S3c). During the summer monsoon, some MPs were transported
further northeastward, passed through the Taiwan Strait, and eventuallyreached the ECS (Fig. 2b). On the other hand, under the northeasterly
monsoon in winter, most MPs were immediately transported south­
westward, passed the northern coast of Hainan Island, and reached the
Gulf of Tonkin (Fig. 2e).
Such seasonal patterns in MP dispersal are well explained by windgenerated circulations induced by seasonally varying monsoons in thisregion. In the central part of the SCS, strong northeasterly winds blow
from October to March, whereas relatively weaker southwesterly winds
blow from April to September. The oceanic flow field in the SCS is
predominantly composed of monsoon-induced wind-generated circu­
lating gyres (Zhu et al. 2019b). In winter, the mean surface currents forma cyclonic gyre in the northern part of the SCS by the northeasterly
coastal monsoon, which induces an elevated sea surface height near the
shore, leading to a southward current off the Vietnam coast and asoutheastward current off the southern China coast as a part of theintensified western boundary current. In turn, in summer, the westernboundary current is reversed by the southerly to southwesterly monsoon
to generate northeastward currents off the southern China coast as a partof the SCS Warm Current directed to the Taiwan Strait (Zhu et al.
2019b). The transport of MPs in the SCS is obviously affected by thisseasonally varying circulation system.
There are distinctive seasonal differences in the transport distancesof MPs released from the Pasig River. Compared to the summer condi­tion (Fig. 2c), the southwestward transport was enhanced, and the3.2. Seasonality of the MP transport in the SCS
Fig. 2 shows the two-dimensional horizontal distributions of the
Lagrangian PDFs (see Section S3) of the buoyant MP particle displace­ment (i.e., Case 1) derived from the three rivers (i.e., the Pearl, Mekong,and Pasig rivers) in the SCS, averaged for the advection time of 100 days
after each release. To account for the seasonal circulation variability
examined in the preceding subsection, the time-averaged Lagrangian
PDFs are grouped into two typical periods of particle emission: April to
September, representing the summer condition when the gentle south­
westerly monsoon is dominant, while October to March represents thewinter condition when the intensive northeasterly monsoon prevails.
The buoyant MPs (Case 1) derived from the Mekong River in summer
are transported northeastward by the coastal western boundary current
and dispersed over a broad area in the SCS by distributed SCS warm
currents when released during the prevailing southwesterly monsoon insummer (Fig. 2a). On the other hand, the MPs released during the pre­vailing northeasterly monsoon in winter are transported southwestward
and reach the coast of the Malay Peninsula and Gulf of Thailand withmuch less lateral dispersal (Fig. 2d). This result is consistent with theFig. 2. Lagrangian PDFs averaged over the advection time of 100 days (color). Top: For the MPs released from April to September. Bottom: For the MPs released from
October to March. The sources of the MPs are mouths of (a) and (d): the Mekong River, (b) and (e): Pearl River, and (c) and (f): Pasig River, as depicted by red circle
marks. Thin black contours are isobaths on 200 m and 1000 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web
version of this article.)4

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

particles were broadly dispersed in the central part of the SCS during thenortheasterly monsoon in winter (Fig. 2f). Thus, in the SCS, MP trans­
port is characterized by distinctive seasonality influenced by the pre­vailing monsoon over the sea. The MPs originating from the three majorrivers are mostly transported over an extensive area in the SCS, while
they also outflow to the surrounding seas to a certain extent for thethree-year period from 2012 to 2014.

are advected differently by depth-dependent horizontal currents.
In general, regardless of the buoyancy and random walk effects, theMPs released from the three major rivers for all three cases are consis­
tently transported primarily in the circulation direction in the SCS; theoverall differences among the cases are rather minor. However, as ex­pected, the horizontal extent of the non-buoyant MPs (Case 3; Fig. 3g–i)
is wider than that of the buoyant MPs (Case 1; Fig. 3a–c) because of theinfluence of the depth-variant velocity magnitudes and directions in the
spiraling surface Ekman boundary layer. In addition, the non-buoyant
MPs are diluted more considerably in the vicinity of the source areas(i.e., the mouths of the three rivers) than the buoyant MPs. When the
random walk lateral diffusivity is introduced (Case 2; Fig. 3d–f), thehorizontal extent of the MP dispersal is also slightly stretched farther,and the source area dilution occurs more apparently than those for Case
1, while the prevailing transport direction does not change significantly.
Hence, we conclude that both the buoyant and neutrally buoyant MPs
behave quite similarly in the horizontal direction, yet with more effec­
tive lateral diffusivity inherent in the latter. The ad hoc random walkmodel appends further lateral diffusivity, although the overall dispersal
patterns do not change substantially, implying that subgrid-scale mixing

3.3. Effects of buoyancy and random-walk diffusivity on the MP transport
To investigate the buoyancy effects on the MP dispersal on a seasonal
timescale, as well as the effects of under-resolved subgrid-scale eddy
diffusivity on the 2DH Lagrangian transport commonly used in many
previous modeling studies, we compared the horizonal Lagrangian PDFs
for the three cases (Cases 1, 2, and 3) that were averaged for theadvection time of 90 days after each MP release from the three majorrivers (Fig. 3). We note that the differences between Cases 1 and 2 aremainly caused by the random walk diffusivity considered only in Case 2,
while those between Cases 1 and 3 are attributable to the vertical shearinduced dispersion effects in Case 3, in which vertically distributed MPs

Fig. 3. Lagrangian PDFs of the MPs released from April to September averaged over the advection time of 90 days (color). Top row: Case 1 (buoyant MPs), middle
row: Case 2 (buoyant + random walk diffusivity) and bottom row: Case 3 (neutral and fully passive 3D MPs). Left column (a)–(c): MPs released from the MekongRiver, middle column (d)–(f): from the Pearl River, and right column (g)–(i): from the Pasig River, where red circle marks are the release sites located at each of thethree river mouths. Contours are isobaths on 200 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version ofthis article.)5

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

associated with submesoscale eddies may not be crucial for seasonal MPtransport in the SCS.
Fig. 4a–c shows temporal evolutions of the vertical distribution of thedepth-dependent Lagrangian PDFs integrated spatially over the entire
model domain of the neutrally buoyant MPs (Case 3) released from thethree rivers that are transported passively to ambient currents, evalu­
ated for every 10-m depth bin. Although the non-buoyant MPs aremostly suspended in the surface layer within a depth of 10 m from thesurface, some particles are affected by both vertical mixing and advec­
tion and thus are distributed vertically as time progresses. The surface
planetary boundary layer is often called the surface mixed layer, wherevertical turbulent mixing is predominant. Fig. 4d–e shows the mean
mixed layer depths for the warm and cold seasons averaged over thethree-year analysis period. The surface mixed layer develops more
pronouncedly in the cold season to be deeper (approximately 50–100 m)
owing to intensive convection by surface cooling as compared to that in
the warm season (approximately 20–50 m). Even after one year (360days) of advection time after the releases, approximately 50% of theparticles are transported to depths of several tens of meters (roughly inthe surface mixed layer), and approximately 5% of MPs are transported
to depths of 100 m–400 m (below the mixed layer). In particular, among
the three sources, the MPs from the Pasig River dive the deepest; below
the depth of 400 m (Fig. 4c). These deep diving MPs may contribute to
MP pollution in the middle and lower layers of the SCS. Therefore, nonbuoyant MPs that disappear from the surface layer but do not return tothe surface layer again (Kooi et al., 2017) are transported to the deep
ocean, and may contribute to subsurface MP pollution observed down to
depths of several hundred meters over a wide area of the SCS.

3.4. Destinations of the MPs derived from the Pearl River
In the Pearl River Estuary, China, the MP concentration was observed
to be approximately 25 times higher than that in the central SCS (Caiet al., 2018; Lam et al., 2020). Thus, the MPs from the Pearl River are
anticipated to have a significant influence on the distribution of MPs,
especially in the northern SCS. Indeed, the model-predicted horizontal
Lagrangian PDF distributions in Figs. 2b, e and 3b, e, h show that thePearl River-derived MPs were distributed mostly in the along-shelf di­rections over the continental shelf, whereas they were rarely transported
seaward to the central SCS. In addition, when the MPs were released
during the wet/warm season (from April to September) in southern
China, many MPs were transported northeastward along the shore,
passed through the Taiwan Strait, and reached the ECS (Fig. 2b). This
result clearly suggests that the Pearl River can be a source of MPs foundin the ECS. Hence, we conducted further analysis of the fractions of thereleased MPs that reach the surrounding seas (the ECS, the Pacific
Ocean, etc.) by passing several transects (mostly straits) that separate
two seas, as shown by the red lines in the left panel of Fig. 1, to quantify
the MPs that remain in the SCS or those that are transported out of the
SCS to the surrounding seas.
Fig. 5a–f shows time series of the fractions of the Pearl River-derived
MPs existing in the ECS and the Pacific Ocean (labeled as PO) for four
quarterly releasing periods (January–March, April–June, July–Sep­
tember, and October–December) averaged over the three years of themodel results for the buoyant MPs (Case 1), buoyant plus random walk
(Case 2), and neutrally buoyant MPs (Case 3) until the advection time of
180 days after the releases. Evidently, the releases from April to June
resulted in the largest MP discharge out of the SCS. During this period,

Fig. 4. Top: Hovmöller diagrams of the vertical distribution of Lagrangian PDFs of the MPs integrated over the entire study area as a function of advection time until
360 days after the releases of the non-buoyant MPs (viz., Case 3) derived from (a) the Mekong River, (b) the Pearl River, and (c) the Pasig River from April to
September. Blue and red dotted lines show the corresponding 50 and 95 percentiles. Bottom: Surface mixed layer depths for (d) April–September and (e) October toMarch, averaged for the three-year period of 2012–2014. (For interpretation of the references to color in this figure legend, the reader is referred to the Web versionof this article.)6

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

Fig. 5. Fraction of MPs released from the Pearl River as a function of the advection time in day. Line colors correspond to the duration of the MP releases indicated in
the legends. Top six panels: Seasonal variability for (a, d) Case 1 (buoyant MPs), (b, e) Case 2 (buoyant MPs + random walk diffusivity), and (c, f) Case 3 (passiveMPs). (a, b, c) The fraction of MPs that remains in the SCS, and (c, d, e) that was transported to the East China Sea (ECS) and the Pacific Ocean (PO). Bottom six
panels: Inter-annual variability for (g, j) Case 1, (h, k) Case 2, and (i, l) Case 3. The Pearl River-derived MPs released from April to June (g–i) that remain in the SCS
and (j–l) that were transported to the ECS and the PO. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version ofthis article.)

the MPs, to be transported mostly to the ECS and the Pacific Ocean for
half a year of the advection time, in the SCS decreased by approximately
30% (Case 1) or approximately 60% (Cases 2 and 3). In contrast, for theother three quarterly periods, the fractions of the MPs transported out of
the SCS is very low and approximately 80–100% of the MPs released
from the Pearl River remained in the SCS.
Subsequently, we focused on the inter-annual variability of thefraction of the Pearl River-derived MPs released from April to June forthe three cases (Fig. 5g–l). The difference among the cases is not as large
as expected from the similarity in the lateral distributions of the
Lagrangian PDFs shown in Fig. 3, although the inter-annual variability is
pronounced. The fractions of the MPs transported to the ECS and thePacific Ocean for half a year of the advection time after the releases
varied by approximately 5–60% (Case 1; buoyant), 30–80% (Case 2;
7

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

buoyant plus random walk), and 25–80% (Case 3; neutrally buoyant andpassive) of the total MPs released. During the wet/warm season (springand summer) under the influence of relatively weak southerly winds butlarger river discharges, the Pearl River plume is advected eastward andoffshore by the coastal current induced by the upwelling-favorable
winds (Dong et al., 2004: Bai et al., 2015). The MPs from the Pearl
River transported through the Taiwan Strait to the ECS were increased
owing to the Taiwan Strait Warm Current and the Taiwan Warm Cur­rent, both of which are northeastward currents toward the ECS, andintensify in summer (Qi et al., 2017). Because approximately 80% of theannual discharge of the Pearl River occurs during the wet season(April–September; e.g., Yi et al., 2004), the MP emissions also increase
then (Lebreton et al., 2017). Therefore, much of the MPs emitted during
the southwesterly monsoon season in the SCS are anticipated to betransported from the SCS shelf to the ECS through the Taiwan Strait. In
turn, the fraction of MPs remaining in the SCS increased by approxi­
mately 20% in Case 1 compared to Case 2 and Case 3. The cause of thisincrease is discussed in the next section.

advection time of approximately three years, these surface MPs wererandomly eliminated to be consistent with the in situ surface MP con­centrations. Apart from the practical importance of vertical MP trans­
port, their study also suggested that oceanic MP transport could occur
for a long period of time (approximately 3 years). Therefore, it isimportant to evaluate whether the MPs derived from the SCS would
remain in the SCS or be transported to the open ocean and eventually tothe surrounding seas for years. In Section 3, we see that the fraction ofMPs derived from the Pearl River varies from year to year (Fig. 5).
Although the Pearl River-derived MPs tended to exit the SCS to the ECS
in a couple of months, many of the MPs derived from the Mekong andPasig rivers remained in the SCS (Figs. 2 and 3).
Fig. 6 shows the fractions of the MPs released from the three rivers
remaining in the SCS (Fig. 6a–c), reaching the ECS and Pacific Ocean(Fig. 6d–f) and the Indian Ocean (Fig. 6g–i) as a function of the elapsed
advection time for the buoyant (Case 1) and passive (Case 3) MPs. The
MP fractions were compiled into two groups depending on the time ofrelease: the southwesterly monsoon (April to September) and thenortheasterly monsoon (October to March) seasons, consistent with
Fig. 2. The MPs derived from the Mekong River released in the south­
westerly monsoon season were dispersed broadly to the most extensive
area in the SCS under the influence of the northeastward westernboundary current developed in summer (Fig. 2a), while they flowedfrom the SCS to the ECS and Pacific more rapidly than those released in
the northeasterly monsoon season, regardless of the buoyancy of theMPs (Fig. 6a, d). In contrast, less than 10% of the MPs are discharged to
the ECS and the Pacific in the first year for the northeasterly monsoon
season releases, demonstrating the coastal trap of the MPs near the
Malay Peninsula (Fig. 2d), followed by abrupt decreases in the

4. Long-term MP transport
4.1. Long-term transported fractions of MPs
Isobe et al. (2019) numerically studied the surface MP concentra­
tions across the Pacific Ocean for the next 50 years and showed that MPconcentrations may increase in the seas around Japan and in the central
Northern Pacific over the next few decades. In their model, the MPtransport was assumed to be driven purely by the surface currents; thus,
vertical mixing and transport were omitted. However, after the

Fig. 6. Fraction of MPs released from the (left)
Mekong, (middle) Pearl, and (right) Pasig rivers as afunction of the advection time in year. Line colors
correspond to the imposed density of the MPs and thedurations of the MP releases indicated in the legend in(i). Top: the fractions of MPs that remains in the SCS,
middle: that were transported out to the East China
Sea (ECS) and the Pacific Ocean (PO), and bottom:
that were transported out to the Indian Ocean. Red
and blue lines: Case 1 (buoyant MPs), magenta andgreen lines: Case 3 (passive MPs). (For interpretationof the references to color in this figure legend, thereader is referred to the Web version of this article.)8

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

remaining fraction in the second year with much stronger retention of
the SCS for the buoyant MPs. The outflow rate is clearly determined by
the semiannually reversing Asian monsoon. The MPs that remain in the
SCS are retained over a wide area in the central part of the SCS bycounterclockwise circulation in winter (Zhu et al., 2019. Also depicted
in Figure S3a). Then, the outflowing fraction begins to increase again
after approximately one year when the southwesterly monsoon starts toprevail (Fig. 6a, d). Three years after the release in the southwesterly
monsoon season, 40–50% of the MPs were discharged to the ECS and thePacific (Fig. 6d). The MPs released during the prevailing northeasterly
monsoon from the Mekong River were transported predominantly
southwestward (Fig. 2d) with an outflow rate of only 10–20% to the ECS
and Pacific (Fig. 6d). Transport to the Indian Ocean was also observed in
all periods and cases, ranging from 20 to 40% after three years ofadvection (Fig. 6g).
The fraction of MPs derived from the Pearl River depends moresignificantly on the MP buoyancy than those derived from the other two

rivers. The buoyant MPs that keep parking in the surface layer are
obviously attributed to greater retention in the SCS (Fig. 6b) and thus
much less outflow to the ECS and the Pacific (Fig. 6e) than the nonbuoyant MPs. In fact, approximately 60% of the released non-buoyant
MPs (Case 3) were discharged to the ECS and the Pacific after three
years, regardless of the period of release, while approximately 20%(almost 0%) of the buoyant MPs were discharged to the ECS and thePacific when they were released in the southwesterly (northeasterly)
monsoon season.
The MPs derived from the Pasig River behaved almost similarly for
all the scenarios, among those derived from the three rivers (Fig. 6c, f, i).
The Pasig-derived MPs were extensively transported westward to thecentral part of the SCS for an advection time of a few months (Fig. 2c)with more westward extent of the MP dispersal for the northeasterly
monsoon season releases (Fig. 2f). Nevertheless, the timing of the initial
increase in the outflow rate to the ECS and Pacific differs from case tocase (Fig. 6f). For instance, the discharge rate of the buoyant MPs

Fig. 7. Long-term fate of the MPs. Top six panels (a–f): Lagrangian PDFs of the released MPs on the advection time of 3 years for (a–c) Case 2 (buoyant + randomwalk) and (d–f) Case 3 (fully passive MPs). Red circle marks correspond to the released sites, where (a) and (d) from the Mekong River, (b) and (e) from the Pearl
River, and (c) and (f) from the Pasig River. Contours are isobaths on 200 m and 1000 m. Bottom three panels (g–i): The fraction of beached MPs derived from (g) the
Mekong River, (h) the Pearl River, and (i) the Pasig River. The MP releases in 2012 are depicted here for the advection time of up to 3 years. Red, blue, and green lines
are for Case 1, Case 2, and Case 3, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version ofthis article.)9K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

5. Discussion

increases four–eight months for the northeasterly monsoon release(September to March), while from eight months to one year for thesouthwesterly monsoon release (April to September). At the advection
time of three years, a large fraction of the released MPs outflow to the
ECS and the Pacific, approximately 60% for the buoyant MPs, and
40–50% for the non-buoyant MPs. Thus, the MPs from the Pasig River
were transported to the central SCS over tens to hundreds of days, andthen to the ECS and the Pacific by the northeastward current that
occurred with the prevailing southwesterly monsoons. In addition,
10–15% of the MPs were transported to the Indian Ocean at theadvection time of three years (Fig. 6i).

5.1. Performance of the microplastic transport model
Although the HYCOM-ROMS model has been validated thoroughly
as noted in Section S1 of the Supplemental Material, it is worthwhile todemonstrate the validity of the present simplified particle tracking
model. Here we exploited a publicly available data set of satellitetracked surface drifters distributed by the Global Drifter Program(GDP; Elipot et al., 2016), in which more than 30,000 drifters have been
deployed and tracked in the global ocean from 2005 to the present at aquarter day interval. The accuracy of the particle tracking model isconfirmed statistically by comparing the modeled Lagrangian PDFs withthe trajectories of the GDP drifters that have existed in the SCS. Fig. 8
shows examples of such comparisons for the particles released from
Mekong River. We set a transect (107.5◦ E and 9◦ N - 10.5◦ N) near themouth to search for the GDP drifters that passed through the transectfrom 2012 to 2014. Six drifters were detected, while all of them occurredin colder seasons (October to March), not in warmer seasons. To beconsistent with the extracted drifter data, the time-averaged Lagrangian
PDFs are computed for the modeled particles released from October toMarch. Three model results are represented in Fig. 8, whereas Cases 1
and 2 correspond to the drifters that stay at the ocean surface. Threedrifters out of six traveled shorter distances than the others and beachedon the southern coast near the tip of the Indochina Peninsula, where theLagrangian PDFs are very high. The other three drifters were furthertransported westward to enter the inner Gulf of Thailand and approxi­
mately followed the areas with higher Lagrangian PDFs. These results
evidently indicate that the present particle tracking model performed
quite well with good accuracy for Cases 1 and 2. In contrast, modest
discrepancy is found for Case 3 because it represents fully passive MPs
and thus is unsuitable to compare with surface drifters. Interestingly, thepassive MPs in Case 3 were transported more to the south off the Malay
Peninsula than those for the near-surface MPs in Cases 1 and 2. This
result demonstrates again that the buoyancy of the MPs affects theirlateral transport patterns substantially.

4.2. Possible final destinations of the river-derived MPs
To examine the long-term fate of the river-derived MPs in the SCS,
we examine the Lagrangian PDFs at the advection time of three years forthe buoyant and non-buoyant MPs released from the three major rivers(Fig. 7). The results for the non-buoyant, passive MPs (Fig. 7d–f) are
intuitively consistent with the results shown in Fig. 6, which indicatesthat at least approximately 20% of the released MPs remained in the
SCS. Fig. 7d–f shows that the MPs remaining in the SCS mostly stay in
the Gulf of Thailand (the Mekong-derived MPs), Gulf of Tonkin (thePearl-derived MPs), and central SCS (the Pasig-derived MPs). In
contrast, the buoyant MPs were visually missing in the SCS, where theLagrangian PDFs almost vanished (Fig. 7a–c).
The mechanism behind this difference was simple. We found that themissing buoyant MPs were mostly beached. Fig. 7g–i shows the timeseries of the fraction of beached MPs, in which they gradually vary
except for the initial adjustment occurred in the first 2–3 months.
Approximately 40–50% of the buoyant MPs derived from the MekongRiver, approximately 60–90% of the Pearl River-derived MPs, andapproximately 20% of the Pasig River-derived MPs have been beached.
Beaching occurs rather promptly after the release of buoyant MPs,
without prominent temporal evolutions. On the other hand, the nonbuoyant MPs derived from the three rivers were barely beached at lessthan 10% of the released MPs. As a side remark, the random walkdiffusivity places the nearshore MP particles away from the shore (i.e.,
forced to move offshore) that eventually diminishes the beached MPs.
These results indicate that the MPs released from the Pearl River,
Mekong River, and Pasig River tend to be transported to the ECS and thenorthwest Pacific Ocean through the Luzon Strait and Taiwan Strait
during the prevailing southwesterly monsoon. If the MPs remain in the
SCS, many of them are transported to the shore and beached. In reality,
the buoyant MPs that beached may repeatedly drift back and forth be­
tween the shore and the ocean, losing buoyancy under the influence offriction, fragmentation, and biofouling; eventually become non-buoyant
MPs that are likely to be transported over a wide area again. However,such processes have not been modeled in the present study.

5.2. Fate of the river-derived microplastics
In the SCS, pronounced seasonality was observed in the mediumterm (i.e., seasonal timescale) transport of MPs and the outflowing
fractions to the surrounding seas. Therefore, MP pollution in these areas
may vary greatly depending on the season. Field observations withrespect to MPs have normally been conducted for a few months in the
SCS (Cai et al., 2018) and the seas around Japan (Isobe et al., 2015),
whereas year-round monitoring is required to clarify the seasonal dis­tribution of MPs.
The large meso plastic (>5 mm) suspended in the Japan Sea wasFig. 8. Same as Fig. 2d, but six in situ surface drifters’ trajectories (blue curves) and the final positions of them (magenta) are superposed on the Lagrangian PDFs
averaged over the advection time of 120 days (color) for the MPs released from the Mekong River (red circle mark) from October to March. Left: Case 1 (buoyant
MPs), middle: Case 2 (buoyant + random walk diffusivity), and right: Case 3 (neutral and fully passive 3D MPs).
10

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631

reported to be influenced by the prevailing southeastward Stokes drift inthe surface layer and was pushed to the coast of the Japanese islands(Iwasaki et al., 2017). On the other hand, smaller MPs (<5 mm) weretransported broadly over an extensive area of the Japan Sea by thenortheastward current, because they were not affected by the Stokes
drift velocity, which is generally intensified in the surface layer, butrather their vertical distribution was extended to a depth of several
meters by vertical mixing (Iwasaki et al., 2017). The present study
indicated that buoyant MPs from the Pearl River were beached moreconsiderably than the non-buoyant MPs. The buoyant MPs are expected
to drift in the surface layer of the ocean, to repeatedly exchange with theshore, to increase their own density through biofouling and degradation
by ultraviolet rays and friction (Kaiser et al., 2017: Fazey and Ryan,
2016), and eventually lose their buoyancy, thereby increasing theirhorizontal dispersal distance (Isobe et al., 2014).
In addition, a greater fraction of the non-buoyant MPs was trans­
ported to the ECS and Pacific Ocean when the MPs were advected forlong periods of time. If we assume a large meso-plastic, the percentage ofshoreward MP transport may even be higher owing to the influence of
Stokes drift in the surface layer. However, because the wave-driven
littoral currents in and around the surf zone where waves break aremuch more complicated and completely different from the non-wave
current field (Uchiyama et al., 2010, 2017a), a simple linear additionof the Stokes drift velocity to the non-wave Eulerian velocity could cause
large errors in the transport estimation, particularly with numericalmodels. This discrepancy is similarly anticipated in offshore MP trans­
port where the Stokes-Ekman currents develop, as pointed out in Section
S2 of the Supplemental Material. On a global scale, Stokes drift may notcontribute to the large-scale accumulation of microplastics in the sub­tropics rather than Ekman currents and geostrophic currents because of
its non-divergent nature. In turn, Stokes drift may lead to increasedtransport to polar regions where storm-generated waves are larger andoccur more frequently (Onink et al., 2019). We showed that the MPs
suspended in the SCS were transported in the same direction as the
monsoon as the western boundary currents of the basin-scale wind-­generated gyre. Because Stokes drift associated with wind waves mainly
occurs in the same direction as the sea wind (van Sebille et al., 2020), an
ad hoc linear addition of Stokes drift to the HYCOM-ROMS model-gen­erated Eulerian velocity results in enhancement of downwind extent of
the MP transport in the SCS, or leads to very minor effects as seen in
Section S4. Therefore, the effects of waves on MP transport are still
underway and must be carefully studied in the future.
Once MPs are transported shoreward from the offshore, some arebeached permanently, and some are possibly reverted back to theoffshore by back-rushing waves and offshore headed currents such as rip
currents after a certain period of time (Hinata et al., 2017). The resi­
dence time at the shore estimated by observations (Hinata et al., 2017)
led to the development of a model for the behavior of MPs near the shore(Hinata et al., 2020). Hence, more realistic beaching modeling isnecessary in the future to accurately reproduce the nearshore processes
and retention for MP transport along with a wave-averaged 3D circu­
lation model requisite, particularly for surf zone applications.
In previous modeling studies on MP transport, beaching was some­times considered (Zhang et al., 2020), but was omitted at other times(Isobe et al., 2019; van Sebille et al., 2019; Onink et al., 2019). This
study employs a simple beaching model that represents the resolved“stickiness” of the shore by bilinear interpolation of lateral currents that
decrease towards the shore where velocity vanishes, while neglecting
the complicated nearshore physics responsible for actual beachingprocesses. The random walk model used in Case 2 serves as a stochastic
redistributor of the nearshore MPs to drift back offshore, which leads todiminishing the beached MPs by approximately 20–50% relative to Case
1 (Fig. 7g–i). On the other hand, the decrease in the beached MP fraction
is apparent in Case 3, demonstrating that buoyant MPs (Cases 1 and 2)
beaches much more frequently than non-buoyant MPs (Case 3). Becauseof the difference in the weight density of the MPs, the buoyant MPs are

confined in the surface layer, while the non-buoyant MPs can spreadover a wider depth range (Fig. 4a–c). In the surface layer, the intensive
southwestward wind blowing in the southern China coastal areas pro­
motes the onshore Ekman flow to be approximately perpendicular to thecontinent. Thus, the buoyant MPs in the surface layer derived from thePearl River are transported to the coastal area owing to wind-induced
currents that are largely influenced by the northerly to northeasterly
monsoon (Figure S3), attributed to more beached MPs (Fig. 7h). In
contrast, the direction of the subsurface currents at a depth of 5 m israther in the along-shelf direction with a much weaker velocity (notshown). Hence, the non-buoyant MPs distributed widely in the verticaldirection (Fig. 4a–c) are affected by subsurface currents, which do notpromote onshore transport and thus less beaching occurs. This result
suggests that the buoyancy of the MP particles alters their mean vertical
positions, resulting in being driven by different horizontal current ve­
locities and directions in the spiraled Ekman boundary layer.
The seafloor is considered to be a reservoir of MPs because MPs thatlose their buoyancy will reach the seafloor (Woodall et al., 2014). A fieldsurvey conducted on the continental shelf in the coastal area of southern
China confirmed that MP contamination occurs on the seabed over an
extensive area on the shelf (Chen et al., 2020). Our model results for theneutrally buoyant MPs (Case 3) showed that the MPs gradually migrate
vertically down below the surface mixing layer developed approxi­
mately 20–50 m (spring/summer) and approximately 50–100 m (fall/­winter) from the surface (Fig. 4d, e). Consequently, more than 5% of thereleased MPs were transported deeper than 100 m (Fig. 4a–c). If theweight density of MPs increases to be negatively buoyant, that is, to havea downward settling velocity, more MPs are able to dive below themixed layer to possibly reach the abyssal ocean. For instance, Fazey and
Ryan (2016) estimated that after 17–66 days, approximately half of theMPs suspended in coastal areas start to have a settling velocity, while
Kaiser et al. (2017) estimated that this happens after approximately six
weeks.
We examined MP contamination on the seafloor in the SCS based on
the results of the non-buoyant MP model. The MPs derived from thePearl River were broadly transported over the continental shelf(Fig. 4h), and thus have a large chance to contribute to MP pollution on
the continental shelf seabed, consistent with the observations of Chenet al. (2020). On the other hand, the MPs derived from the Mekong andPasig rivers were mostly transported to the central SCS in the advection
time of several tens of days (Fig. 3g, i). If these MPs started to have asettling velocity, they could dive into the deep abyssal oceans at depthsof several thousand meters. However, there are many unansweredquestions about the processes by which MPs acquire settling velocity inthe ocean, and further research is needed to understand MP pollution in
the deep oceans.
6. Conclusions
In the SCS, it was confirmed that the transport pattern of MPs issubstantially influenced by the seasonally varying, basin-scale winddriven circulation system induced by the prevailing monsoon systemdeveloped on the Asian continent. The mean transport direction of theriver-derived MPs, particularly those from the Pearl and Mekong rivers,
was found to be northeastward (southwestward) during the periods ofprevailing southwesterly (northeasterly) monsoon. MPs released fromthe southern coast of China manifest a large proportion of discharge into
the East China Sea (ECS) across the Taiwan Strait, suggesting that theymight be one of MP sources observed in the seas around Japan.
We found that the rate of lateral MP transport has non-trivial inter­annual variability, presumably owing to the long-term variability of the
Asian monsoon and the associated weather system that alter the SCS
circulations as main driving forces. As the advection time progressedfrom several months to several years, few MPs remained in the SCS,
whereas a large fraction of the MPs from the Mekong and Pasig rivers
were transported to the ECS and Pacific Ocean, of which some slightly
11

Environmental Pollution 308 (2022) 119631

K. Matsushita et al.

leaked into the Indian Ocean. The long-term northward transport to the
ECS and the Pacific is evident, particularly during the southwesterly
monsoon, indicating that the summer monsoon plays a significant role inthe dispersal and advective distribution of MPs in the extensive area.
Therefore, the SCS, which has many important emission sources ofplastic waste to the ocean, is anticipated be a major source of MPs in theworld’s oceans.
Although approximately half of the non-buoyant MPs were foundwithin a depth of 10 m from the surface, the remaining MPs weretransported down to 10–100 m from the surface. More than 5% of the
non-buoyant MPs sank down below a depth of 100 m, and some reached
400 m depth, suggesting that the MPs attaining negative buoyancy witha downward settling velocity may be a contributor to abyssal MP
contamination. The buoyant MPs drifting on the continental shelf
originating from southern China tend to be pushed toward the shore by
northward wind-induced currents more pronouncedly than the nonbuoyant MPs, which, in contrast, diffuse to deeper layers owing tovertical mixing and vertical advection. This result suggests that differ­ences in the weight density of MPs affect their beaching processes andthus plastic pollution on beaches.

Carr, S.D., Capet, X., McWillimas, J.C., Pennington, J.T., Chazez, F.P., 2008. The
influence of diel vertical migration on zooplankton transport and recruitment in an
upwelling region: estimates from a coupled behavioral-physical model. Fish.
Oceanogr. 17 (1), 1–15. https://doi.org/10.1111/j.1365-2419.2007.00447.x.
Chen, M., Du, M., Jin, A., Chen, S., Dasgupta, S., Li, J., Xu, H., Ta, K., Peng, K., 2020.
Forty-year pollution history of microplastics in the largest marginal sea of thewestern Pacific. Geochem. Persp. Let 13, 42–47.
Chu, P.C., Edmons, N.L., Fan, C., 1999. Dynamical mechanisms for the south China sea
seasonal circulation and thermohaline variabilities. J. Phys. Oceanogr. 29 (11),2971–2989. https://doi.org/10.1175/1520-0485(1999)029<2971:DMFTSC>2.0.
CO;2.
Chubarenko, I., Bagaev, A., Zobkov, M., Esiukova, E., 2016. On some physical anddynamical properties of microplastic particles in marine environment. Mar. Pollut.
Bull. 108, 105–112. https://doi.org/10.1016/j.marpolbul.2016.04.048.
Daryabor, F., Ooi, S.H., Samah, A.A., Akbari, A., 2016. Dynamics of the water
circulations in the southern south China sea and its seasonal transports. PLoS One 11(7), e0158415. https://doi.org/10.1371/journal.pone.0158415.
Dauhajre, D.P., McWilliams, J.C., Uchiyama, Y., 2017. Submesoscale coherent structureson the continental shelf. J. Phys. Oceanogr. 47, 2,949–2,976. https://doi.org/
10.1175/JPO-D-16-0270.1.
Dong, L., Su, J., Wong, L.A., Cao, Z., Chen, J.C., 2004. Seasonal variation and dynamics
of the Pearl River plume. Continent. Shelf Res. 24, 1761–1777. https://doi.org/
10.1016/j.csr.2004.06.006.
Elipot, S., Lumpkin, R., Perez, R.C., Lilly, J.M., Early, J.J., Sykulski, A.M., 2016. A global
surface drifter data set at hourly resolution. J. Geophys. Res. Oceans 121,
2937–2966. https://doi.org/10.1002/2016JC011716.
Emmerik, T., Klaveren, J., Meijer, L.J.J., Krooshof, J.W., Palmos, D.A.A., Tanchuling, M.
A., 2020. Manila river mouths act as temporary sinks for macroplastic pollution.
Front. Mar. Sci. 7, 545812 https://doi.org/10.3389/fmars.2020.545812.
Fang, G., Wang, Y., Wei, Z., Fang, Y., Qiao, F., Hu, X., 2009. Interocean circulation, heatand freshwater budgets of the South China Sea based on a numerical model. Dynam.
Atmos. Oceans 47, 55–72. https://doi.org/10.1016/j.dynatmoce.2008.09.003.
Fang, G., Susanto, R.D., Wirasantosa, S., Qiao, F., Supangat, A., Fan, B., Wei, Z.,
Sulistiyo, B., Li, S., 2010. Volume, heat and freshwater transports from the South
China Sea to Indonesian seas in the boreal winter of 2007–2008. J. Geophys. Res.
115, C12. https://doi.org/10.1029/2010JC006225.
Fazey, F.M.C., Ryan, P.G., 2016. Biofouling on buoyant marine plastics: an experimental
study into the effect of size on surface longevity. Environ. Pollut. 210, 354–360.
https://doi.org/10.1016/j.envpol.2016.01.026.
He, Z., Feng, M., Wang, D., Slawinski, D., 2015. Contribution of the Karimata Straittransport to the Indonesian Throughflow as seen from a data assimilation model.
Continent. Shelf Res. 92, 16–22. https://doi.org/10.1016/j.csr.2014.10.007.
Hinata, H., Mori, K., Ohno, K., Miyao, Y., Kataoka, T., 2017. An estimation of the average
residence times and onshore-offshore diffusivities of beached microplastics based on
the population decay of tagged meso- and macrolitter. Mar. Pollut. Bull. 122, 17–26.
https://doi.org/10.1016/j.marpolbul.2017.05.012.
Hinata, H., Sagawa, N., Kataoka, T., Takeoka, H., 2020. Numerical modeling of the beachprocess of marine plastics: a probabilistic and diagnostic approach with a particle
tracking method. Mar. Pollut. Bull. 122, 17–26. https://doi.org/10.1016/j.
marpolbul.2020.110910.
Hu, J., Kawamura, H., Hong, H., Qi, Y., 2000. A review on the currents in the south China
sea: seasonal circulation, south China sea warm current and kuroshio intrusion.
J. Oceanogr. 56, 607–624. https://doi.org/10.1023/A:1011117531252.
Isobe, A., Kubo, K., Tamura, Y., Kako, S., Nakashima, E., Fujii, N., 2014. Selective
transport of microplastics and mesoplastics by drifting in coastal waters. Mar. Pollut.
Bull. 89, 324–330. https://doi.org/10.1016/j.marpolbul.2014.09.041.
Isobe, A., Uchida, K., Tokai, T., Iwasaki, S., 2015. East Asian seas, a hot spot of pelagic
microplastics. Mar. Pollut. Bull. 101, 618–623. https://doi.org/10.1016/j.
marpolbul.2015.10.042.
Isobe, A., Iwasaki, S., Uchida, K., Tokai, T., 2019. Abundance of non-conservative
microplastics in the upper ocean from 1957 to 2066. Nat. Commun. 10, 417. https://doi.org/10.1038/s41467-019-08316-9.
Iwasaki, S., Isobe, A., Kako, S., Uchida, K., Tokai, T., 2017. Fate of microplastics andmesoplastics carried by surface currents and wind waves: a numerical model
approach in the sea of Japan. Mar. Pollut. Bull. 121, 85–96. https://doi.org/
10.1016/j.marpolbul.2017.05.057.
Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A.,
Narayan, R., Law, K.L., 2015. Plastic waste inputs from land into the ocean. Science
347, 768–771. https://doi.org/10.1126/science.1260352.
Kaiser, D., Kowalski, N., Waniek, J.J., 2017. Effects of biofouling on the sinking behaviorof microplastics. Environ. Res. Lett. 12, 124003 https://doi.org/10.1088/17489326/aa8e8b.
Kamidaira, Y., Uchiyama, Y., Mitarai, S., 2017. Eddy-induced transport of the kuroshio
warm water around the ryukyu islands in the East China sea. Continent. Shelf Res.
143, 206–218. https://doi.org/10.1016/j.csr.2016.07.004.
Kamidaira, Y., Uchiyama, Y., Kawamura, H., Kobayashi, T., Furuno, A., 2018.
Submesoscale mixing on initial dilution of the radionuclides released from the
Fukushima Dai-ichi nuclear power plant. J. Geophys. Res.: Oceans 123, 2808–2828.
https://doi.org/10.1002/2017JC013359.
Kamidaira, Y., Kawamura, H., Kobayashi, T., Uchiyama, Y., 2019. Development ofregional downscaling capability in STEAMER ocean prediction system based on
multi-nested ROMS model. J. Nucl. Sci. Technol. (Tokyo, Jpn.) 56 (8), 752–763.
https://doi.org/10.1080/00223131.2019.1613269.
Kamidaira, Y., Uchiyama, Y., Kawamura, H., Kobayashi, T., Otosaka, S., 2021.
A modeling study on the oceanic dispersion and sedimentation of radionuclides off

Credit author statement
Kosei Matsushita: Investigation, Software, Validation, Visualiza­
tion, Writing – original draft. Yusuke Uchiyama: Conceptualization,
Methodology, Supervision, Software, Writing- Reviewing and Editing,Funding acquisition. Naru Takaura: Investigation, Software, Valida­tion. Taichi Kosako: Methodology, Software
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
The authors do not have permission to share data.
Acknowledgments
This research was financially supported by the Japan Society for the
Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (Grant# 18H03798) at Kobe University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.envpol.2022.119631.
References
Azman, M.A., Ramli, M.Z., Othman, S.F.C., Shafiee, S.A., 2021. The distribution ofmarine debris along the Pahang coastline, Malaysia during the Southwest andNortheast Monsoons. Mar. Pollut. Bull. 170, 112630 https://doi.org/10.1016/j.
marpolbul.2021.112630.
Bai, Y., Huang, T.H., He, X., Wang, S.L., Hsin, Y.C., Wu, C.R., Zhai, W., Lui, H.K., Chen, C.
T.A., 2015. Intrusion of the Pearl River plume into the main channel of the Taiwan
Strait in summer. J. Sea Res. 95, 1–15. https://doi.org/10.1016/j.
seares.2014.10.003.
Buijsman, M., Uchiyama, Y., McWilliams, J.C., Hill-Lindsay, C.R., 2012. Modelingsemiduirnal internal tides in the Sourthern California Bight. J. Phys. Oceanogr. 42,
62–77. https://doi.org/10.1175/2011JPO4597.1.
Cai, S., Liu, H., Li, W., Long, X., 2005. Application of LICOM to the numerical study ofwater exchange between the South China Sea and its adjacent oceans. Acta Oceanol.
Sin. 24, 10–19.
Cai, M., He, H., Liu, M., Li, S., Tang, G., Wang, W., Huang, P., Wei, G., Lin, Y., Chen, B.,
Hu, J., Cen, Z., 2018. Lost but can’t be neglected: huge quantities of small
microplastics hide in the South China Sea. Sci. Total Environ. 633, 1206–1216.
https://doi.org/10.1016/j.scitotenv.2018.03.197.

12

K. Matsushita et al.

Environmental Pollution 308 (2022) 119631
Sun, X., Li, Q., Zhu, M., Liang, J., Zheng, S., Zhao, Y., 2017. Ingestion of microplastics bynatural zooplankton groups in the northern South China Sea. Mar. Pollut. Bull. 115,
217–224. https://doi.org/10.1016/j.marpolbul.2016.12.004.
Tada, H., Uchiyama, Y., Masunaga, E., 2018. Impacts of two super typhoons on the
Kuroshio and marginal seas on the Pacific coast of Japan. Deep-Sea Res. Part I 132,
80–93. https://doi.org/10.1016/j.dsr.2017.12.007.
Takeda, N., Kashima, M., Odani, S., Uchiyama, Y., Kamidaira, Y., Mitarai, S., 2021.
Identification of coral spawn source areas around Sekisei Lagoon for recovery andpoleward habitat migration by using a particle-tracking model. Sci. Rep. 11, 6963.
https://doi.org/10.1038/s41598-021-86167-5.
Uchiyama, Y., McWilliams, J.C., Shchepetkin, A.F., 2010. Wave-current interaction in an
oceanic circulation model with a vortex force formalism: application to the surfzone. Ocean Model. 34 (1–2), 16–35. https://doi.org/10.1016/j.
ocemod.2010.04.002.
Uchiyama, Y., Idica, E., McWilliams, J.C., Stolzenbach, K.D., 2014. Wastewater effluent
dispersal in Southern California bays. Continent. Shelf Res. 76, 36–52. https://doi.
org/10.1016/j.csr.2014.01.002.
Uchiyama, Y., McWilliams, J.C., Akan, C., 2017a. Three-dimensional transient rip
currents: bathymetric excitation of low-frequency intrinsic variability. J. Geophys.
Res.: Oceans 122, 5826–5849. https://doi.org/10.1002/2017JC013005.
Uchiyama, Y., Suzue, Y., Yamazaki, H., 2017b. Eddy-driven nutrient transport andassociated upper-ocean primary production along the Kuroshio. J. Geophys. Res.:
Oceans 122, 5046–5062. https://doi.org/10.1002/2017JC012847.
Uchiyama, Y., Kanki, R., Takano, A., Yamazaki, H., Miyazawa, Y., 2018a. Mesoscale
reproducibility in regional ocean modeling with a 3-D stratification estimate basedon Aviso-Argo data. Atmos.-Ocean 56 (4), 212–229. https://doi.org/10.1080/
07055900.2017.1399858.
Uchiyama, Y., Odani, S., Kashima, M., Kamidaira, Y., Mitarai, S., 2018b. Influences of thekuroshio on interisland remote connectivity of corals across the nansei archipelago
in the East China sea. J. Geophys. Res. Oceans. 123, 9245–9265. https://doi.org/
10.1029/2018JC014017.
Uchiyama, Y., Sengo, N., Kurosawa, K., Nakayama, A., 2018c. Development of a coupled
HYCOM-ROMS downscaling ocean modeling system and its application to the South
China Sea. In: Proc. 13th International Conference on Hydrodynamics (ICHD2018),
pp. 358–363.
Uchiyama, Y., Takaura, N., Okada, N., Nakayama, A., 2019. Residual circulations andassociated water mass transport in the South China sea analyzed with a coupled
HYCOM-ROMS downscaling ocean model. AIP Conf. Proc. 2157, 020029 https://doi.
org/10.1063/1.5126564.
Uchiyama, Y., Tokunaga, N., Aduma, K., Kamidaira, Y., Tsumune, D., Iwasaki, T.,
Yamada, M., Tateda, Y., Ishimaru, T., Ito, Y., Watanabe, Y.W., Ikehara, K.,
Fukuda, M., Onda, Y., 2022. A storm-induced flood and associated nearshore
dispersal of the river-derived suspended 137Cs. Sci. Total Environ. 816, 151573
https://doi.org/10.1016/j.scitotenv.2021.151573.
van Sebille, E., Delandmeter, P., Schofield, J., Hardesty, B.D., Jones, J., Donnellly, D.,
2019. Basin-scale sources and pathways of microplastic that ends up in the
Galápagos Archipelago. Ocean Sci. 15, 1341–1349. https://doi.org/10.5194/os-151341-2019.
van Sebille, E., Aliani, S., Law, K.L., Maximenko, N., Alsina, J.M., Bagaev, A.,
Bergmann, M., Chapron, B., Chubarenko, I., Cozar, A., Delandmeter, P., Egger, M.,
Fox-Kemper, B., Garaba, S.P., Goddijn-Murphy, L., Hardesty, B.D., Hoffman, M.J.,
Isobe, A., Jongedijk, C.E., Kaandorp, M.L.A., Khatmullina, L., Koelmans, A.A.,
Kukulka, T., Laufkotter, C., Lebreton, L., Lobelle, D., Maes, C., Martinez-Vicente, V.,
Maqueda, M.A.M., Poulain-Zarcos, M., Rodríguez, E., Ryan, P.G., Shanks, A.L.,
Shim, W.J., Suaria, G., Thiel, M., van den Bremer, T.S., Wichmann, D., 2020. The
physical oceanography of the transport of floating marine debris. Environ. Res. Lett.
15, 023003 https://doi.org/10.1088/1748-9326/ab6d7d.
Woodall, L.C., Sanchez-Vidal, A., Canals, M., Paterson, G.L., Coppock, R., Sleight, V.,
Calafat, A., Rogers, A.D., Narayanaswamy, B.E., Thompson, R.C., 2014. The deep sea
is a major sink for microplastic debris. R. Soc. Open Sci. 1, 140317 https://doi.org/
10.1098/rsos.140317.
Wyrtki, K., 1961. Scientific Results of Marine Investigations of the South China Sea andthe Gulf of Thailand 1959–1961, Naga Report 2, Technical Report. Scripps Instituteof Oceanography, p. 195.
Yi, K., Zhang, J., Qian, P.Y., Jian, W., Huang, L., Chen, J., Wu, M.C.S., 2004. Effect ofwind events on phytoplankton blooms in the Pearl River estuary during summer.
Continent. Shelf Res. 24, 1909–1923. https://doi.org/10.1016/j.csr.2004.06.015.
Zhang, X., Uchiyama, Y., Nakayama, A., 2019. On relaxation of the influences of treatedsewage effluent on an adjacent seaweed farm in a tidal strait. Mar. Pollut. Bull. 144,
265–274. https://doi.org/10.1016/j.marpolbul.2019.04.050.
Zhang, Z., Wu, H., Peng, G., Xu, P., Li, D., 2020. Coastal ocean dynamics reduce theexport of microplastics to the open ocean. Sci. Total Environ. 713, 136634 https://doi.org/10.1016/j.scitotenv.2020.136634.
Zhu, L., Wang, H., Chen, B., Sun, X., Qu, K., Xia, B., 2019a. Microplastic ingestion in
deep-sea fish from the South China Sea. Sci. Total Environ. 677, 493–501. https://doi.org/10.1016/j.scitotenv.2019.04.380.
Zhu, Y., Sun, J., Wang, Y., Li, S., Xu, T., Wei, Z., Qu, T., 2019b. Overview of the multilayer circulation in the south China sea. Prog. Oceanogr. 175, 171–182. https://doi.
org/10.1016/j.pocean.2019.04.001.

the coast of Fukushima. J. Environ. Radioact. 238–239. https://doi.org/10.1016/j.
jenvrad.2021.106724, 106724.
Kooi, M., van Nes, E.H., Scheffer, M., Koelmans, A.A., 2017. Ups and downs in the ocean:effects of biofouling on the vertical transport of microplastics. Environ. Sci. Technol.
51, 7963–7971. https://doi.org/10.1021/acs.est.6b04702.
Kukulka, T., Proskurowski, G., Morét-Ferguson, S., Meyer, D.W., Law, K.L., 2012. The
effect of wind mixing on the vertical distribution of buoyant plastic debris. Geophys.
Res. Lett. 39, 7601. https://doi.org/10.1029/2012GL051116.
Kurosawa, K., Uchiyama, Y., Kosako, T., 2020. Development of a numerical marine
weather routing system for coastal and marginal seas using regional oceanic andatmospheric simulations. Ocean. Eng. 195, 106706 https://doi.org/10.1016/j.
oceaneng.2019.106706.
Lam, T.W.L., Fok, L., Lin, L., Xie, Q., Li, H.-X., Xu, X.-R., Ling, C.-Y., 2020. Spatial
variation of floatable plastic debris and microplastics in the Pearl River Estuary,South China. Mar. Pollut. Bull. 158, 111383 https://doi.org/10.1016/j.
marpolbul.2020.111383.
Lebreton, L.C., Van Der Zwet, J., Damsteeg, J.W., Slat, B., Andrady, A., Reisser, J., 2017.
River plastic emissions to the world’s oceans. Nat. Commun. 8, 1–10. https://doi.
org/10.1038/ncomms15611.
Li, D., Liu, K., Li, C., Peng, G., Andrady, A., Wu, T., Zhang, Z., Wang, X., Song, Z.,
Zong, C., Zhang, F., Wei, N., Bai, M., Zhu, L., Xu, J., Wu, H., Wang, L., Chang, S.,
Zhu, W., 2020. Profiling the vertical transport of microplastics in the west Pacific
Ocean and the East Indian Ocean with a novel in situ filtration technique. Environ.
Sci. Technol. 54, 12979–12988. https://doi.org/10.1021/acs.est.0c02374.
Liu, Q., Feng, M., Wang, D., 2011. ENSO-induced interannual variability in thesoutheastern South China Sea. J. Oceanogr. 67, 127–133. https://doi.org/10.1007/
s10872-011-0002-y.
Mai, L., You, S.-N., He, H., Bao, L.-J., Liu, L.-Y., Zeng, E.Y., 2019. Riverine microplastic
pollution in the Pearl River Delta, China: Are modeled estimates accurate? Environ.
Sci. Technol. 53, 11810–11817. https://doi.org/10.1021/acs.est.9b04838.
Mason, E., Molemaker, J., Shchepetkin, A.F., Colas, F., McWilliams, J.C., Sangra, P.,
2010. Procedures for offline grid nesting in regional ocean models. Ocean Model. 35(1), 1–15.
Masunaga, E., Uchiyama, Y., Suzue, Y., Yamazaki, H., 2018. Dynamics of internal tides
over a shallow ridge investigated with a high-resolution downscaling regional ocean
model. Geophys. Res. Lett. 45 (8), 3550–3558. https://doi.org/10.1002/
2017GL076916.
Masunaga, E., Uchiyama, Y., Yamazaki, H., 2019. Strong internal tides generated by theinteraction of the Kuroshio Current and tides over a shallow ridge. J. Phys.
Oceanogr. 49 (11), 2917–2934. https://doi.org/10.1175/JPO-D-18-0238.1.
Mato, Y., Isobe, T., Takada, H., Kanehiro, H., Ohtake, C., Kamimura, T., 2001. Plastic
resin pellets as a transport medium for toxic chemicals in the marine environment.
Environ. Sci. Technol. 35, 318–324. https://doi.org/10.1021/es0010498.
Meijer, L.J.J., van Emmerik, T., van der Ent, R., Schmidt, C., Lebreton, L., 2021. Morethan 1000 rivers account for 80% of global riverine plastic emissions into the ocean.
Sci. Adv. 7, 18. https://doi.org/10.1126/sciadv.aaz5803.
Mitarai, S., Siegel, D.A., Watson, J.R., Dong, C., McWilliams, J.C., 2009. Quantifying
connectivity in the coastal ocean with application to the Southern California Bight.
J. Geophys. Res. 114, C10026 https://doi.org/10.1029/2008JC005166.
Onink, V., Wichmann, D., Delandmeter, P., van Sebille, E., 2019. The role of Ekman
currents, geostrophy and Stokes drift in the accu- mulation of floating microplastic.
J. Geophys. Res.-Oceans 124, 1474–1490. https://doi.org/10.1029/2018JC014547.
Piccardo, M., Renzi, M., Terlizzi, A., 2020. Nanoplastics in the oceans: theory,
experimental evidence and real world. Mar. Pollut. Bull. 157, 111317 https://doi.
org/10.1016/j.marpolbul.2020.111317.
Poulain, M., Mercier, M.J., Brach, L., Martignac, M., Routaboul, C., Perez, E., Desjean, M.
C., ter Halle, A., 2019. Small microplastics as a main contributor to plastic massbalance in the north atlantic subtropical gyre. Environ. Sci. Technol. 53, 1157–1164.
https://doi.org/10.1021/acs.est.8b05458.
Qi, J., Yin, B., Zhang, Q., Yang, D., Xu, Z., 2017. Seasonal variation of the taiwan warm
current water and its underlying mechanism. Chin. J. Oceanol. Limnol. 35,
1045–1060. https://doi.org/10.1007/s00343-017-6018-4.
Reisser, J., Slat, B., Noble, K., du Plessis, K., Epp, M., Proietti, M., de Sonneville, J.,
Becker, T., Pattiaratchi, C., 2015. The vertical distribution of buoyant plastics at sea:
an observational study in the North Atlantic Gyre. Biogeosciences 12, 1249–1256.
https://doi.org/10.5194/bg-12-1249-2015.
Romero, L., Uchiyama, Y., Ohlmann, C., McWilliams, J.C., Siegel, D.A., 2013. Particlepair dispersion in the Southern California coastal zone. J. Phys. Oceanogr. 43,
1862–1879. https://doi.org/10.1175/JPO-D-13-011.1.
Shchepetkin, A.F., McWilliams, J.C., 2005. The regional ocean modeling system (ROMS):
a split-explicit, free-surface, topography-following-coordinate oceanic model. Ocean
Model. 9, 347–404. https://doi.org/10.1016/j.ocemod.2004.08.002.
Shchepetkin, A.F., McWilliams, J.C., 2009. Computational kernel algorithms for finescale, multiprocess, longtime oceanic simulations. In: Temam, R., Tribbia, J. (Eds.),
Handbook of Numerical Analysis Vol. 14: Computational Methods for the Ocean and
the Atmosphere. Elsevier, Amsterdam, pp. 119–181. https://doi.org/10.1016/
S1570-8659(08)01202-0.
Song, Y.T., 2006. Estimation of interbasin transport using ocean bottom pressure: theory
and model for Asian marginal seas. J. Geophys. Res. 111, C11. https://doi.org/
10.1029/2005JC003189.

13