Continental Shelf Research 153 (2018) 16–29

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

Simulation of river plume behaviors in a tropical region: Case study of theUpper Gulf of ThailandT⁎

Xiaojie Yua, Xinyu Guoa, , Akihiko Morimotoa, Anukul BuranaprathepratbabCenter for Marine Environmental Studies (CMES), Ehime University, Japan
Department of Aquatic Science, Faculty of Science, Burapha University, Thailand

A R T I C L E I N F O

A B S T R A C T

Keywords:
River plume
Low latitude
Wind eﬀect
River discharge
Upper Gulf of Thailand
Bulge size

River plumes are a general phenomenon in coastal regions. Most previous studies focus on river plumes in
middle and high latitudes with few studies examining those in low latitude regions. Here, we apply a numericalmodel to the Upper Gulf of Thailand (UGoT) to examine a river plume in low latitudes. Consistent with observational data, the modeled plume has seasonal variation dependent on monsoon conditions. During southwesterly monsoons, the plume extends northeastward to the head of the gulf; during northeasterly monsoons, it
extends southwestward to the mouth of the gulf. To examine the eﬀects of latitude, wind and river discharge on
the river plume, we designed several numerical experiments. Using a middle latitude for the UGoT, the bulge
close to the river mouth becomes smaller, the downstream current ﬂows closer to the coast, and the salinity inthe northern UGoT becomes lower. The reduction in the size of the bulge is consistent with the relationshipbetween the oﬀshore distance of a bulge and the Coriolis parameter. Momentum balance of the coastal current ismaintained by advection, the Coriolis force, pressure gradient and internal stresses in both low and middle
latitudes, with the Coriolis force and pressure gradient enlarged in the middle latitude. The larger pressure
gradient in the middle latitude is induced by more oﬀshore freshwater ﬂowing with the coastal current, which
induces lower salinity. The inﬂuence of wind on the river plume not only has the advection eﬀects of changing
the surface current direction and increasing the surface current speed, but also decreases the current speed due
to enhanced vertical mixing. Changes in river discharge inﬂuence stratiﬁcation in the UGoT but have little eﬀect
on the behavior of the river plume.

1. Introduction
In coastal regions, the discharge of freshwater from rivers is one ofthe principle sources of buoyancy. Through formation of a river plumewith a sharp density gradient between the buoyant freshwater and sea
water, two notable characteristics emerge in the Northern hemisphere:
an anticyclonic bulge in the vicinity of the river mouth and a downstream (in a Kelvin wave sense) coastal current (Chao and Boicourt,1986). The behavior of the plume is inﬂuenced by a variety of factors,
including wind (Fong, 1998; Dzwonkowski et al., 2014), the Coriolis
force (Kasai et al., 2000; Fong and Geyer, 2002), ambient currents(Fong and Geyer, 2002), tides (Guo and Valle-Levinson, 2007; HornerDevine et al., 2009), thermal stratiﬁcation (Wang et al., 2008) and riverdischarge (Yankovsky et al., 2001; Wang et al., 2011; Dzwonkowski
et al., 2014).
Most studies on river plumes are based on observations or model
analysis conducted in middle or high latitude regions, such as the
Chesapeake Bay (Pritchard, 1952, 1954; Guo and Valle-Levinson,⁎2007), Delaware Bay (Münchow and Garvine, 1993; Wong, 1994), theColumbia River (Horner-Devine et al., 2009), and the Yellow River(Wang et al., 2008, 2011). However, fewer studies examining river
plume behaviors have been conducted in regions of low latitude. Low
latitude regions have a small Coriolis parameter, enormous discharges
of freshwater and distinctive wind patterns (Nittrouer and DeMaster,1996). All of these factors make the behavior of river plumes in low
latitudes signiﬁcantly diﬀerent from those in middle and high latitudes.
The main focus on tropical rivers has been on the Amazon River(Lentz and Limeburner, 1995; Lentz, 1995a, 1995b; Nittrouer and
DeMaster, 1996), which discharges freshwater at the equator. Diﬀeringfrom river plumes in middle latitudes, which deﬂect to the right by the
Coriolis force in the Northern Hemisphere, the Amazon River plume
extends leftward to the north Brazilian shelf between the equator and
5°N under the superposition eﬀects of the low latitude location, strong
tides and easterly trade winds (Lentz, 1995a; Nittrouer and DeMaster,1996). There are also other rivers in low latitude regions, not justaround the equator, but between the equator and middle latitudes.

Correspondence to: Center for Marine Environmental Studies, Ehime University, 2–5 Bunkyo-Cho, Matsuyama 790-8577, Japan.
E-mail address: guoxinyu@sci.ehime-u.ac.jp (X. Guo).

https://doi.org/10.1016/j.csr.2017.12.007
Received 27 May 2017; Received in revised form 29 November 2017; Accepted 11 December 2017
Available online 14 December 20170278-4343/ © 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Location and (b) bathymetry (m)of model domain in the Upper Gulf ofThailand (UGoT). Stars indicate the positions of the river mouths of the Maeklong,Thachin, Chaopraya, and Bangpakong, respectively. Solid circles show the stations
where observational data are available for2014 and 2015. Triangles indicate observation stations for tidal harmonic constants,
namely Hua Hin, Sattahip and Bangkok Bar,
respectively. Section AB is indicated by thesolid line. Line CD shows the position of theopen boundary of the model.

ecosystems in the UGoT, it is necessary to enhance our understanding of
the distribution of the river plume in the UGoT and its contribution to
stratiﬁcation. In this study, our objective is to investigate the dynamics
responsible for the river plume and stratiﬁcation in the UGoT, including
the inﬂuences of low latitude, wind and river discharge, through threedimensional numerical modeling.
In Section 2, we brieﬂy introduce in situ data collected in the UGoT
and model conﬁguration. After comparison of the model results withobservational data, the modeled behaviors of the river plume and theinduced stratiﬁcation in the UGoT under monsoons will be described in
Section 3. In Section 4, we examine the inﬂuences of low latitude, windand river discharge on the river plume in the UGoT through a series ofnumerical experiments. The study is summarized in Section 5.

They may not be subject to persistent large river inﬂows and easterly
trade winds, but have seasonal variations in river discharge and wind
patterns such as those in middle latitude regions. Here, we examine thebehavior of a river plume in a tropical region, the Upper Gulf ofThailand (UGoT, Fig. 1), which has seasonal variations in river discharge and is subject to monsoons.
The UGoT, which is a semi-enclosed shallow sea located in a tropical region at about 13°N, has characteristics of estuarine-like systems
due to large freshwater discharges. The sea is surrounded by land on theeastern, northern and western sides and is connected to the main Gulf ofThailand at its southern boundary (Fig. 1a). The maximum depth isapproximately 20 m in the southeastern part of the UGoT. Four rivers,
namely, the Maeklong, Thachin, Chaopraya and Bangpakong from west
to east (Fig. 1b), discharge freshwater into the gulf. The seasonal variation in discharges of these rivers is apparent, with smaller discharges
from December to May and larger discharges from June to November.
The UGoT is also subject to monsoons with dry northeasterly winds
from November to January and wet southwesterly winds from May to
September.
A study on the seasonal variation of water column conditions in the
UGoT (Buranapratheprat et al., 2008) revealed that strong stratiﬁcation
develops in September and October when river discharge is large andheat ﬂux is moderate, while the water column becomes well mixed in
December and January due to surface heat loss, lower discharge fromrivers and strong inputs of wind stress. Using a three-dimensionalmodel, Buranapratheprat et al. (2009) showed that monsoons determine seasonal circulation in the UGoT, which is clockwise during
southwesterly monsoons and counter-clockwise during northeasterly
monsoons. However, they did not pay special attention to the dynamics
of river plumes in the UGoT. Using a numerical model, Saramul andEzer (2014) evaluated dynamics in the UGoT, which are inﬂuenced bysurface heat ﬂux, river runoﬀ, and low latitude. They found that theextension of river plumes diﬀers under diﬀerent monsoon conditions.
The larger Coriolis parameter in middle latitudes in their model likely
pushes the plume farther along the west coast and makes the plumenear the river mouth less axisymmetric. However, the larger Coriolis
parameter cannot lead the plume to turn more to the right, for which
they did not give an explanation. Due to insuﬃcient information on theriver plume in the UGoT, as well as the potential contribution of riverwater-induced stratiﬁcation to the generation of hypoxia (Green et al.,2006), which could have severe consequences on the coastal

2. Observational data and model description
Water temperature and salinity data were collected at 0.1 m intervals through the entire depth at 18 stations (black points, Fig. 1b) oncein August, September, November and December 2014 and in February,
April and June 2015 in the UGoT. Using these observational data, we
determined seasonal variations in the horizontal and vertical distributions of water temperature and salinity, thereby identifying the presence of a river plume.
In order to understand the distribution and controlling factors of theobserved river plume, we developed a numerical model for the UGoT.
The model is based on the Princeton Ocean Model (POM), which is athree-dimensional, primitive equation, sigma-coordinate model(Blumberg and Mellor, 1987; Mellor, 2003). The vertical diﬀusivity
coeﬃcients are calculated by a second momentum turbulent closure
scheme (Mellor and Yamada, 1982), and the horizontal diﬀusivity
coeﬃcients are parameterized by the Smagorinsky formulation(Smagorinsky, 1963). The model domain and bathymetry are shown in
Fig. 1b. Spatial resolution is ~1 km in the horizontal direction and 21
sigma layers in the vertical direction, with higher resolution in thesurface and bottom layers than in the middle layer.
For external forcing, we consider monthly heat ﬂux (Fig. 2a),monthly river discharges (Fig. 2b), and monthly wind stresses (Fig. 2c),
all of which represent climatologic conditions and are from
Buranapratheprat et al. (2008). The net heat ﬂux is the sum of thesensible heat ﬂux, latent heat ﬂux, long wave radiation and short wave
radiation, and has small seasonal variation with a maximum value in17

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Fig. 2. (a) Monthly heat ﬂux (W/m2), (b) monthly river discharges (m3/s), and (c) monthly wind stress (N/m2) in the UGoT.

April (~100 W/m2) and a minimum value in December (~−30 W/m2).
Due to the small area of the UGoT (approximately 100 km × 100 km),we speciﬁed a spatially uniform heat ﬂux in the model.
The model includes the discharges from four rivers, namely the
Maeklong, Thachin, Chaopraya and Bangpakong (Fig. 1b), among
which the Chaopraya is the largest. Seasonal variations in discharges
from the Chaopraya and Bangpakong rivers (Fig. 2b) are the most extreme, with small outputs from December to May (100−200 m3/s) but
large outputs from August to November (500−1000 m3/s). The
Maeklong and Thachin rivers have smaller seasonal variation in discharges, and the former has a large discharge (~300 m3/s) while thelatter has a small discharge (< 50 m3/s) throughout the year.
Wind in the UGoT shows seasonal variations (Fig. 2c), with southerly to southwesterly winds from February to September, northeasterly
winds in November and December, and a transitional period in January
and October. We speciﬁed a spatially uniform wind stress in the modeldue to the small area of the UGoT. It should be noted that the periodwith large river discharges coincides with the three stages of monsoon(September, October and November), which can make the behavior of
the river plume variable.
Along the open boundary of the model in the UGoT, monthly watertemperature and salinity data from Buranapratheprat et al. (2009) areapplied. Four tidal constituents, M2, S2, K1 and O1 with harmonic
constants at two stations (Hua Hin and Sattahip) near the open
boundary from Buranapratheprat et al. (2009) are added to the elevation at the open boundary.
The model calculation was started from the rest state in Januarywith a total integration period of 3 years. The model results were saved
at 1-h intervals. A tide ﬁlter (Hanawa and Mitsudera, 1985) was used toremove tidal components. The analysis reported here is based on model
results in the third year (case 0), which nearly coincide with those in
the second year.

Table 1Comparison of harmonic constants of four tidal constituents between observations
and model results at one station in the UGoT. The phase is with reference to 105°E.
Harmonic constituents

Bangkok Bar
Amplitude (cm) Phase (°)

Observed K1
Modeled K1
Observed O1Modeled O1
Observed M2
Modeled M2
Observed S2Modeled S2

67.0 167.0
66.6 167.7
46.0 117.1
45.3 122.5
55.0 141.0
51.5 146.7
27.0 212.0
27.0 223.6

the UGoT.
Water temperature and salinity in the UGoT show strong seasonalvariations (Figs. 3 and 4). We selected February and September to represent diﬀerent monsoonal seasons, respectively, because both heatﬂux and river discharge have diﬀerent patterns in these two months. In
February, the observed water temperature is about 27.5−27.7 °C overthe entire UGoT (Fig. 3a), and the water column is vertically homogeneous (Fig. 3e) due to surface heat loss and strong winds. In September, the temperature is higher than 30 °C and shows a little spatial
variation, with the northeast of the UGoT around 1.0 °C lower than thesouthwest (Fig. 3c). Meanwhile, stratiﬁcation develops in the watercolumn (Fig. 3g). In the northeast of the UGoT, water temperature at
the surface (29.6 °C) is slightly lower than that at the bottom (30.1 °C).
Temperature inversion is not unusual in coastal regions aﬀected bylarge river discharge in a cooling season (Thadathil et al., 2002). In thelatter part of September, surface temperature in the UGoT begins todecrease due to surface heat loss. At the same time, river discharge
increases substantially. Consequently, the low salinity water at thesurface keeps the low temperature surface water lighter than the hightemperature bottom water.
The modeled water temperature is generally consistent with observational data if we consider that the observations were in 2014 and2015, while the model is based on climatologic conditions. The modeled water temperature in February (Fig. 3b, f) is slightly warmer(~0.1 °C) than the observation, but is also vertically homogeneous. In
September, the modeled temperature (Fig. 3d, h) is slightly lower(~0.3 °C) than the observation, but the pattern of a lower temperaturein the northeast than the southwest is consistent with the observation.
Moreover, a higher temperature at the bottom than at the surface is also
reproduced by the model (Fig. 3h).
Being similar to water temperature, the observed salinity is also
vertically homogeneous in February and has slight spatial variation
with a lower value in the west (32.0) than in the east (32.2) (Fig. 4a, e),which is likely induced by the relatively large river discharge from the

3. Results
3.1. Validation of model results through comparison with observations
Since tidal motion is important for determining the behavior of river
plumes (Guo and Valle-Levinson, 2007), four major tidal constituents(M2, S2, O1 and K1) are considered in our model. To assess model
performance, we compare the modeled tidal harmonic constants of thefour tidal constituents with observations at one station, the Bangkok
Bar station (position shown in Fig. 1b). Since the area of the UGoT issmall, the comparison with observation data at one station (Bangkok
Bar) in the head of the bay enables us to conﬁrm the simulation of tides
over entire bay. Comparisons between model results and observations
indicate good prediction of tidal constituents in the UGoT by the model(Table 1). The tidal range is about 50 cm from the south to the north of18

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Fig. 3. Horizontal (top) and vertical (bottom) distributions of observational data (a, c, e, g) and model results (b, d, f, h) for water temperature (°C) in February and September in the
UGoT. The vertical distribution is shown in section EF in (a) to (d).

Fig. 4. The same as Fig. 3 but for salinity.

3.2. River plume aﬀected by monsoons in the UGoT

Maeklong. In September, salinity decreases greatly and shows apparent
spatial variation with low salinity (< 20) in the northeast and highsalinity (~31) in the southwest (Fig. 4c). Meanwhile, strong stratiﬁcation occurs in the northeast with low salinity water above high salinity water (Fig. 4g). The salinity given by the model generally reﬂects
the pattern of the observation. In February, the modeled salinity issmaller in the northwest than in the southeast (Fig. 4b), and is also
vertically homogeneous (Fig. 4f). In September, the salinity decreases
greatly, and has large spatial variation with lower salinity (< 20) in thenortheast and higher salinity (~29.5) in the southwest (Fig. 4d), and isstrongly stratiﬁed in the northeast (Fig. 4h). The spatial and vertical
patterns are consistent with the observations, although the modeledsalinity values are lower than the observations. This discrepancy ismainly caused by the external forcing used in the model for river discharge and wind, which are climatological conditions instead of actual
observed conditions. However, this discrepancy has little inﬂuence on
the behaviors of the plume noted by this study.

To examine the eﬀect of monsoons on the behavior of the river
plume, we compared model results for salinity and residual currents in
September, October and November, when monsoons are forced by thesouthwesterly wind, weak transitional wind and northeasterly wind,respectively.
In September (Fig. 5a), the prevailing southwesterly wind drives thelow salinity water from the largest river, the Chaopraya, to the eastern
UGoT, and therefore hinders the westward extension of the plume.
Freshwater from the Bangpakong remains near the river mouth, which
results in a large salinity gradient around the river mouth. The plume of
the Maeklong extends upstream to the northeast under the inﬂuence ofthe southwesterly wind. The spatial distribution of salinity shows apattern of gradually increasing from the northeast to the southwest of
the UGoT. The residual current with an average velocity of about
0.08 m/s ﬂows southeastward under the southwesterly wind.
In October (Fig. 5b), when the northeasterly wind is relatively weak
and river discharge is still large, low salinity water from the Chaopraya
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Fig. 5. Horizontal distribution of modeled salinity (color) and residual current (m/s, arrows) in the surface layer (top) and bottom layer (middle), and vertical distribution of salinity(color) and current along section AB (m/s, arrows) (bottom) in September, October and November for case 0. The position of section AB is shown in Fig. 1b. The black lines in (b) show theisohalines around the bulge, and the red line indicates the position of the oﬀshore distance of the bulge. Note that the scales for current speed are diﬀerent for these distributions. (Forinterpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

surface layer, and a similar spatial pattern is found with lower salinity
in the north than in the south. The currents in the bottom layer are allnorthward currents, having the largest velocity in November (0.03 m/s).
The vertical proﬁle of salinity along section AB (shown in Fig. 1b)shows stratiﬁcation at the northern area of the UGoT that is stronger in
October than in September and November (color, Figs. 5g, h, i). This ispartly due to the low salinity water concentrating in the middle regionof the UGoT in October relative to September, and partly due to thelarger river discharge in October compared to November. Residual
current along section AB (arrows, Figs. 5g, h, i) shows a structure of
exchange current, with oﬀshore ﬂow in the surface layer and onshore
ﬂow in the bottom layer. The typical surface current speed is0.03−0.10 m/s and the depth of no motion is at roughly 5 m. Both thesurface current and bottom current are strongest in November, followedby October, and weakest in September.
In order to examine the stratiﬁcation induced by the river discharge
in the whole UGoT, we calculated the potential energy anomaly (PEA)in September and October (Fig. 6). This variable has been used in manystudies as an indicator of vertical stratiﬁcation (Simpson et al., 1977),
and is calculated as follows.

and Bangpakong rivers extends southwestward, forming a jet-like anticyclonic bulge near the river mouth and a downstream current along
the coast with an average current speed of about 0.12 m/s. The salinity
distribution shows a pattern of lower salinity in the north and higher inthe south (~28). The distance of the bulge from the mouth of the
Chaopraya River, as determined from the outmost isohaline that deﬂects toward the river mouth (label of 21.5 on the red line in Fig. 5b), is
about 19.0 km oﬀshore.
In November (Fig. 5c) when the northeasterly wind becomesstronger and the river discharge weakens, the freshwater inﬂow fromthe Charopraya and Bangpakong rivers ﬂows directly to the southwest.
Buoyant water from the Maeklong River stays close to the coast as it
ﬂows southward. During this period, the current is mainly a southwestward current (~0.16 m/s) likely associated with northeasterly
monsoons.
In September and November, the river plume changes its extension
direction with monsoons that are forced by the southwesterly wind andthe northeasterly wind, respectively. Because the wind is relatively
weak in October, a typical river plume can be found in the UGoT.
Therefore, the primary controlling factor on the river plume in the
UGoT is likely the wind.
In the bottom layer (Figs. 5d, e, f), salinity is higher than that in the20

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Fig. 6. Horizontal distribution of potential energy anomaly (J/m3) in September and October for cases 0−5.

PEA =

1
h

0

∫−h (ρ − ρ ) gzdz,

all other conditions the same as in case 0. The modiﬁed condition was
speciﬁed from start of model integration in these experiments. In case 1,
we changed the latitude of the UGoT to about 35°N; in case 2, we set awind stress of 0 over the entire UGoT; in case 3, we kept the same windstress as in case 0 but switched oﬀ its inﬂuence on vertical mixing byusing the vertical eddy viscosity coeﬃcient and vertical eddy diﬀusivity
coeﬃcient from case 2; in cases 4 and 5, we multiplied the river discharges of the four rivers used in case 0 by a factor of 0.5 and 2.0,
respectively.

(1)

where h is the water depth, ρ is the water density, g is gravity acceleration, z is vertical coordinate with origin at sea surface and is positive
0
1
upward, and ρ = h ∫−h ρdz .
In September (Fig. 6a), the largest PEA (> 90 J/m3) appears in theeastern part of the UGoT, while the smallest value appears in the west.
In October (Fig. 6b), the largest PEA value (~65 J/m3) decreases but
the area with a PEA greater than 30 J/m3 increases, covering almost theentire UGoT. The area with a large PEA is strongly stratiﬁed and hypoxia is easily generated in the bottom due to organic material decomposition.

4.1. Eﬀects of latitude
4.1.1. Comparisons of salinity and residual current between middle and low
latitudes
In September (Fig. 7a), since the prevailing southwesterly wind
restrains the spread of the river plume, the eﬀects of a change in latitude on the plume are very limited, with a slight diﬀerence in the vicinity of the Chaopraya and Maeklong river mouths between case 1 andcase 0. Low salinity water discharged from the Chaopraya River ﬂows
more southward in case 1 than in case 0, and that from the Maeklong
River ﬂows more eastward in case 1 instead of northeastward as in case0, which is a rightward deﬂection compared to that of case 0. In October (Fig. 7b) with the situation of a weak wind, the river plumearound the Chaopraya River mouth in case 1 is markedly distinct
compared to case 0. The anticyclonic bulge around the Chaopraya Rivermouth likely retreats to the coast, and is found at a smaller oﬀshore
distance of the bulge in case 1 (~16.0 km, shown by the red isohalinewith a label of 19.5 in Fig. 7b) than in case 0 (~19.0 km). The reason
we used diﬀerent isohaline to deﬁne the budge size in cases 0 and 1 isbecause the average salinity is diﬀerent in two cases. Meanwhile, thedownstream current ﬂows more closely to the coast in case 1 than incase 0, which demonstrates that the coastal current is deﬂecting

4. Sensitivity study
In order to examine the eﬀects of low latitude, wind and river discharge on the river plume in the UGoT, we conducted a series of numerical experiments (Table 2) in which we changed only one condition
from the calculation shown in Section 3 (case 0) and ran the model with
Table 2Model cases and external forcing factors.
Case 0Case 1Case 2

Case 3

Case 4

Case 512°N, tide,
heating,riverdischarge,
wind

35°N, tide,
heating,riverdischarge,
wind

12°N, tide,
heating,riverdischarge,
no wind12°N, tide,
heating,riverdischarge,wind(same Kz
and Kv incase 2),12°N, tide,
heating,
0.5 × riverdischarge,
wind

12°N, tide,
heating, 2
× riverdischarge,
wind

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Fig. 7. The same as Fig. 5 but for case 1.

which is smaller in the northern area in case 1 (Figs. 7d, e, f) than incase 0. The residual current in the bottom layer shows little diﬀerence
between the two cases.
The salinity along section AB shows a lower value in the northern
UGoT (near point B) in case 1 than in case 0 (Figs. 7g, h, i). The
southward component of the oﬀshore current in the upper layer in
October and November is weaker in the north in case 1 than in case 0,
while there is little diﬀerence in the bottom onshore current between
the two cases.
The PEA in case 1 shows a similar pattern to that in case 0, which
has largest value in the east in September (Fig. 6c) and a large value inthe entire UGoT in October (Fig. 6d). However, the PEA maximum ishigher in case 1 than case 0.
Yankovsky and Chapman (1997) classiﬁed river plumes into three
types: surface-advected plumes, bottom-advected plumes and intermediate plumes. Based on the vertical structure of the Chaopraya plumein October, we classify it as a surface-advected plume. The oﬀshore
distance of a surface-advected plume is calculated by
ys = 2(3g′h 0 + vi 2)/[(2g′h 0 + vi 2)1/2f ] (Yankovsky and Chapman, 1997),
where ys is the oﬀshore distance, g′ is the reduced gravity based on theinﬂow density anomaly, g′ = g (Δρ / ρ) ( g is the gravity acceleration, ρ isthe density of ambient water and Δρ is the density diﬀerence between
the river plume and the ambient water at the river mouth), h 0 is theinﬂow depth, vi is the inﬂow velocity, and f is the Coriolis parameter.
The oﬀshore distance of the bulge ( ys ) has an inverse proportion

rightward with increasing latitude. The low salinity water from the
Maeklong ﬂows more southward along the west coast. The salinity inthe northern area of the UGoT is lower in case 1 than in case 0. In
November (Fig. 7c), when the wind blows from the northeast, the inﬂuence of the larger Coriolis force in case 1 than in case 0 is pronounced as low salinity water from the Chaopraya River ﬂows westward more closely to the coast, resulting in a lower salinity in thenorthwest region of the UGoT compared to case 0. A similar result isalso apparent at the Bangpakong River mouth, where the discharged
water extends westward in case 1 instead of southwestward as in case 0.
The low salinity water from the Maeklong also extends more southward
along the west coast.
Saramul and Ezer (2014) reported that the larger Coriolis parameterin middle latitudes pushes the plume in the UGoT farther along the west
coast and makes the plume less axisymmetric near the river mouth, butcannot turn the plume more to the right as compared to the low latitudecase. The former two conclusions are also conﬁrmed in our study.
However, we found that the coastal current in the middle latitude casedeﬂects more rightward along the coast, which will be explained in
Section 4.1.2. Moreover, the bulge size around the Chaopraya Rivermouth in October is smaller in the middle latitude (16.0 km) than in the
low latitude (19.0 km). The salinity in the northern area of the UGoT islower in the middle latitude than in the low latitude, especially in
October and November, which will also be discussed in Section 4.1.2.
The change in the bottom layer is mainly on the salinity value,
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Fig. 8. Vertical averaged values of local acceleration, advection, Coriolis force, pressure gradient, and vertical viscosity (m/s2, color) in the momentum equation for eastward
direction (left) and northward direction (right), and residual
current (m/s, arrows) from the sea surface to the bottom of
the oﬀshore current in October for case 0. Values of momentum have been multiplied by a factor of 106. (For interpretation of the references to color in this ﬁgure legend, thereader is referred to the web version of this article.)

relationship with the Coriolis parameter ( f ), which is 3.2 × 10−5 s−1 at
13°N and 8.3 × 10−5 s−1 at 35°N. The smaller Coriolis parameter in low
latitudes favors a larger oﬀshore distance. This is consistent with theresult of a larger ys in the low latitude (19 km, case 0) than in the
middle latitude (16 km, case 1) in October when the wind is weak.
However, the ratio of ys between the low and middle latitudes is notexactly equal to that of 1/ f , indicating that the density diﬀerence between the river plume and the ambient water at the river mouth (g′)
also aﬀects the oﬀshore distance of the bulge because h 0 and vi are thesame in diﬀerent latitudes.

force, PRE denotes the pressure gradient, and VDIF denotes the internal
stress divergence related to vertical eddy viscosity. The subscripts x andy denote the eastward and northward component, respectively. All
terms are calculated by POM and the tidal components are removed
with a tide killer ﬁlter (Hanawa and Mitsudera, 1985). The vertical
averaged values of these terms from the sea surface to the bottom of theoﬀshore current with low salinity are used to analyze the dynamics ofthe plume in diﬀerent latitudes. The depth of the bottom of the oﬀshore
current with low salinity is determined from the vertical proﬁles of thenorthward component of the current. We mainly analyze the momentum of the bulge and coastal current in October, which are midﬁeld and far-ﬁeld plumes, as described by Horner-Devine et al. (2015).
For the bulge in case 0 (low latitude), we mainly analyze the eastward momentum of the plume from the Chaopraya River. The acceleration term (Fig. 8a) is small, which means the velocity has little
variation over time. The advection term (Fig. 8b) in the bulge is negative. The Coriolis force (Fig. 8c) is negative, which is associated withthe southward current in the bulge. The pressure gradient (Fig. 8d) is
positive. The term for internal stresses (Fig. 8e) is relatively small in thebulge. Therefore, the main balance is among the advection term,

4.1.2. Dynamics of the bulge and coastal current in middle and low latitudes
The momentum equation for the eastward and northward velocityof the plume can be expressed as∂u+ ADVx = CORx + PREx + VDIFx
∂t

(2)

∂v
+ ADVy = CORy + PREy + VDIFy∂t(3)

where ADV denotes the advection terms, COR denotes the Coriolis
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Fig. 9. The same as Fig. 8 but for case 1.

term (Fig. 8g) is negative in the coastal current. The Coriolis force(Fig. 8h) is positive, while the pressure gradient (Fig. 8i) and internal
stresses (Fig. 8j) are both negative. The momentum balance of thecoastal current in case 0 is likely maintained by the advection term,
Coriolis force, pressure gradient and internal stresses. The ratio for theadvection term, Coriolis force, pressure gradient and internal stresses isapproximately 2:2:1:3.
Moving to the middle latitude (case 1), the Coriolis force (Fig. 9h)
and pressure gradient (Fig. 9i) increase greatly, while the advection
term (Fig. 9g) and internal stresses (Fig. 9j) remain almost the same as
in case 0. The larger Coriolis force near the coast demonstrates theimportance of the Coriolis force in middle latitudes, which tends tomake the buoyant water turn more rightward along the coast. The ratioof the advection term, Coriolis force, pressure gradient and internal

Coriolis force and pressure gradient for the bulge around the Chaopraya
River mouth in a low latitude.
Moving to the middle latitude (case 1), the eastward momentum
balance for the bulge around the Chaopraya River mouth is still driven
by advection, the Coriolis force and the pressure gradient (Figs. 9b, c,d). However, both the Coriolis force (Fig. 9c) and pressure gradient(Fig. 9d) are increased compared to case 0. Since the Coriolis parameteris 3.2 × 10−5 s−1 at 13°N and 8.3 × 10−5 s−1 at 35°N, and the current
speed does not change greatly, the Coriolis force increases more than
two-fold in case 1 compared to case 0. The reason for the increase inpressure gradient will be explained later.
For the coastal current, we mainly analyze the northward momentum of the Chaopraya River plume. In case 0, the acceleration term
(Fig. 8f) is smaller than the other terms by one order. The advection

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Fig. 10. Vertical averaged values (m/s2) of barotropic pressure gradient (left), baroclinic pressure gradient (middle), and their sum (right) for the eastward direction (x-direction) andnorthward direction (y-direction) from the sea surface to the bottom of the oﬀshore current in October. The black lines in the left panels indicate the elevation in the surface layer, while
those in middle panels show the vertical averaged density. The upper six panels are for case 0, while the lower six panels are for case 1. The arrows show the vertical averaged residual
current. Values of momentum have been multiplied by a factor of 106. The smaller zonal range for case 1 is caused by its relatively higher latitude location than case 0.

component of the pressure gradient in the bulge (color, Fig. 10g) andthe northward barotropic component of the pressure gradient in the
coastal current (color, Fig. 10j) from the Chaopraya River increase. It is
likely that more freshwater ﬂows out from the bulge, and induces thecloser elevation contour (lines, Fig. 10g) relative to case 0 (lines,
Fig. 10a). The northward baroclinic component of the pressure gradient(color, Fig. 10k) also increases due to more freshwater ﬂowing out fromthe bulge, which enlarges the density gradient (lines, Fig. 10k) in case
1. The greater increase in the barotropic component relative to thebaroclinic component in case 1 results in a larger pressure gradient incase 1 (Figs. 10i, l) than in case 0 (Figs. 10c, f).
According to Fong and Geyer (2002), the shape of the bulge and the
freshwater carried out by the coastal current are dependent on the
Rossby number. For a large Rossby number, the shape of the bulge iscircular, and the transport of the coastal current is smaller; for a small
Rossby number, the shape of the bulge is semi-circular, and the transport of the coastal current is larger. Fong and Geyer (2002) explain thisoccurrence using the Nof's (1988) theory on baroclinic eddies colliding
with a wall, in which a nonlinear, layered, analytic model is used todetermine how an eddy interacts with a wall. When a quasi-geostrophic
anticyclonic eddy interacts with a wall, it will leak ﬂuid toward theright-hand side (looking oﬀshore in the northern hemisphere). If only
the outer edge of the eddy interacts with a wall, the leakage from theeddy is small; if the eddy is closer to the wall, the leakage is larger.
The eddy and the leakage are analogous to the bulge and the
freshwater carried out by the coastal current. In low latitudes with asmall Coriolis parameter ( f ), the Rossby number is large, which leads tostresses is approximately 1:6:4:3 in the middle latitude, indicating thatthe coastal current in the middle latitude becomes more geostrophic
relative to that in the low latitude.
To explain the variation in the pressure gradient in case 0 and case
1, we decomposed the pressure gradient into the barotropic component
induced by the horizontal gradient of water level and the baroclinic
component induced by the horizontal gradient of density. In case 0, theeastward barotropic component of the pressure gradient (color,
Fig. 10a) in the bulge around the Chaopraya River mouth has opposite
signs: positive in the east and negative in the west. This is because theelevation (lines, Fig. 10a) is higher in the central than in the east andwest as freshwater runs out from the river mouth. The baroclinic
component of the pressure gradient (color, Fig. 10b) is opposite to thebarotropic component of the pressure gradient, which is negative in theeast and positive in the west of the bulge around the Chaopraya Rivermouth. The density (lines, Fig. 10b) in the central is smaller than that in
the west and east. The sum of the barotropic and baroclinic components
shows a positive pressure gradient in the bulge of the Chaopraya(Fig. 10c). For the coastal current from the Chaopraya River, thenorthward barotropic component of pressure gradient (color, Fig. 10d)
is negative, which is induced by the higher elevation in the north thanin the south (lines, Fig. 10d). On the other hand, the baroclinic component of the pressure gradient (color, Fig. 10e) is positive due to thelarger density in the south than in the north (lines, Fig. 10e). The larger
barotropic component relative to baroclinic component produces anegative pressure gradient in the coastal current (Fig. 10f).
Moving to the middle latitude (case 1), the eastward barotropic
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Fig. 11. Horizontal distribution of surface salinity (color) and residual current (m/s, arrows) in September, October, and November for case 2 (top) and case 3 (bottom). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 12. Vertical distribution of vertical eddy viscosity Log10 (Kz) (m2/s, color) and current along section AB (m/s, arrows) in September, October and November for case 0 (top) and case
3 (bottom).

the low latitude, as well as the lower salinity in the northern UGoT incase 1 than in case 0.

a large oﬀshore distance of the bulge (eddy away from the wall), andconsequently the freshwater quantity ﬂowing with the coastal current issmaller. In middle latitudes, the situation is opposite; that is, the bulge
is smaller (eddy close to the wall) and the freshwater quantity ﬂowing
with the coastal current is larger. This can explain why the barotropic
and baroclinic pressure gradient is larger in the middle latitude than in

4.2. Eﬀects of wind
The eﬀect of wind on the coastal region is manifested not only in the
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Fig. 13. Horizontal distribution of surface salinity (color) and residual current (m/s, arrows) in September, October and November for case 4 (top) and case 5 (bottom). The black lines in(b) and (e) show the isohalines around the bulge, and the red lines indicate the position of the oﬀshore distance of the bulge. (For interpretation of the references to color in this ﬁgurelegend, the reader is referred to the web version of this article.)

Fig. 14. Vertical distribution of salinity (color) and current (m/s, arrows) along section AB in September, October and November for case 4 (top) and case 5 (bottom). (For interpretationof the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

diﬀerent months are almost identical, with a bulge forming near theriver mouth, and a downstream current ﬂowing along the coast(Figs. 11a, b, c). The only diﬀerence among the months is the salinity
value and current speed, which are determined by monthly river discharges. The coastal current speed from the Chaopraya River is 0.18,
0.15, and 0.11 m/s in September, October, and November, respectively.

changing surface current but also in producing enhanced verticalmixing in the upper layer. To separately examine the two eﬀects ofwind on the river plume, we consider the situation of no wind in case 2
and the situation of wind but without wind-enhanced vertical mixing incase 3 by using the vertical eddy viscosity and diﬀusivity from case 2.
In case 2, when there is no wind, the structures of the river plume in27

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Changing the model domain to the middle latitude results in a different river plume in the UGoT. The bulge near the river mouth extends
farther oﬀshore in the low latitude than in the middle latitude, which
can be qualitatively explained by the inverse relationship between theoﬀshore distance of bulge and the Coriolis parameter in a surface-advected plume (Yankovsky and Chapman, 1997). Geostrophic control on
the coastal current of the plume is weaker in the low latitude than in the
middle latitude, which induces a smaller rightward deﬂected angle ofthe coastal current in the low latitude than in the middle latitude.
Furthermore, salinity in the model domain is higher in the low latitude
than in the middle latitude. This is associated with less freshwater being
carried out by the coastal current from the bulge in the low latitude
than in the middle latitude, which is consistent with the theory of Nof(1988). All these points were either not reported by or are diﬀerent
from the previous study on the river plume in the UGoT (Saramul andEzer, 2014).
Wind has two eﬀects on the surface current that acts as the ambient
current of the river plume: directly changing the surface current withthe addition of momentum to the water and indirectly changing thesurface current with enhancement of vertical mixing. These eﬀects havenot been reported in previous studies. River discharge can change thesize of the bulge and the strength of the coastal current of the river
plume. It also signiﬁcantly inﬂuences stratiﬁcation in the UGoT. A
larger river discharge tends to induce stronger stratiﬁcation.

The PEA shows a similar pattern in September and October (Figs. 6e, f),with a large value around the Chaopraya and Bangpakong rivers, and ahigher value in September due to larger river discharge.
In case 3, the surface currents (Figs. 11d, e, f) are diﬀerent fromthose in case 2, but show a resemblance to those in case 0 in current
direction with a larger current speed than case 0. Comparison of case 3
and case 2 shows a wind-induced surface current in case 3 that ﬂows
northeastward in September and southwestward in October and November under the southwesterly and northeasterly winds, respectively.
The coastal current speed from the Chaopraya River in case 3 is 0.36,
0.22, and 0.40 m/s in September, October, and November, respectively.
Comparison of case 3 with case 0 shows the eﬀect of wind instrengthening the vertical mixing, which weakens the current speed incase 0，especially in the northern area of 13°N. The distribution of PEA
in case 3 shows a similar pattern to that in case 0, which is large in theeast in September and large in the entire UGoT in October (Figs. 6g, h).
Wind produces large vertical eddy viscosity in case 0 (color,
Figs. 12a, b, c) relative to cases 2 and 3 (color, Figs. 12d, e, f) in theupper layer north of 13°N. Large eddy viscosity results in strong internal
friction and therefore weakens the current speed in the surface layer incase 0 (arrows, Fig. 12). Weaker oﬀshore current in the surface layer
induces a weaker onshore current in the bottom layer in case 0 (arrows,
Fig. 12).
4.3. Eﬀects of river discharge

Acknowledgements
We examined the inﬂuence of river discharge on the river plume by
cutting it in half and doubling in cases 4 and 5, respectively. In case 4(case 5), expansions of the river plume (Fig. 13) are similar to those incase 0, but with higher (lower) salinity and weaker (stronger) downstream current than in case 0. The oﬀshore distance of the bulge fromthe Chaopraya River in October increases from 15.7 km in case 4 to23.5 km in case 5 (red lines, Figs. 13b, e). The coastal current from the
Chaopraya River in October also turns more rightward (looking oﬀshore) with increasing river discharge. The response of the plume to themagnitude of the river discharge in the UGoT is similar to that in the
Yellow River (Wang et al., 2008) in both plume range and orientation.
The vertical proﬁles of salinity show the strongest stratiﬁcation incase 5 (color, Figs. 14d, e, f), followed by case 0 (color, Figs. 5g, h, i)and case 4 (color, Figs. 14a, b, c) in the northern part of the UGoT. The
larger river discharge and stronger stratiﬁcation enhance the surface
oﬀshore current and bottom onshore current in case 5 (arrows,
Figs. 14d, e, f).
The patterns of PEA in cases 4 and 5 are similar to those in case 0,
but with a smaller maximum value in case 4 (Figs. 6i, j) and a larger one
in case 5 (Figs. 6k, l). The largest PEA in case 4 is 50 J/m3 in September
and 40 J/m3 in October, while it is 140 J/m3 and 95 J/m3 in case 5.
Therefore, the river discharge can signiﬁcantly change stratiﬁcation in
the UGoT in the rainy season, and inﬂuence the situation of hypoxia
there.

This study was supported by JSPS KAKENHI Grant Number
26302001 and by grants from the government to national university
corporations for the Joint Usage/Research Center, MEXT, Japan and
JSPS Core-to-Core Program.
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Diﬀering from the Amazon River plume that extends leftward to thenorthern hemisphere, in the UGoT, the river plume from four rivers in
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