Estuarine, Coastal and Shelf Science 156 (2015) 61e70
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Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier.com/locate/ecss
Low-salinity plume detachment under non-uniform summer wind off
the Changjiang Estuary
Jianzhong Ge a, *, Pingxing Ding a, Changsheng Chen b
a
b
The State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, PR China
School for Marine Science and Technology, University of Massachusetts-Dartmouth, New Bedford, MA 02744, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Accepted 21 October 2014
Available online 31 October 2014
In the past, two physical mechanisms, baroclinic instability (BI) and strong asymmetric tidal mixing
(SATM) during the spring tidal period, were proposed for the offshore detachment of the low-salinity
plume over the inner shelf of the East China Sea (ECS). These two mechanisms were re-examined using both observations and a fully three-dimensional (3-D), high-resolution, unstructured-grid, freesurface, primitive-equation, Finite-Volume Community Ocean Model (FVCOM). The observed currents
and salinities showed that the plume was characterized by a two-layer system, in which the upper layer
is mainly driven by the river discharge-induced buoyancy flow and the lower layer is predominantly
controlled by tidal mixing and rectification. The SATM mechanism was based on the model run without
calibration against observed currents and salinity around the plume region, so that it should be applied
with caution to a realistic condition observed on the inner shelf of the ECS. The BI mechanism was
derived under a condition without consideration of tidal mixing. Although BI could still occur along the
frontal zone when tides were included, it was unable to produce a single, large, detached low-salinity
lens observed on the inner shelf of the ECS. The process-oriented model experiment results suggest
that for a given river discharge and realistic tidal flow, the spatially non-uniform southwesterly surface
wind during the southeast monsoon-dominant summer could increase frontal spatial variability and thus
produce a significant offshore detachment of low-salinity water on the inner shelf of East China Sea.
© 2014 Elsevier Ltd. All rights reserved.
Keywords:
plume detachment
non-uniform wind
Ekman transport
Changjiang Estuary
1. Introduction
The low-salinity plume is a common coastal and oceanic physical phenomenon in river-dominated estuaries such as the Amazon
River (Lentz, 1995), the Chesapeake Bay (Lentz, 2004; Lentz and
Largier, 2006), the Connecticut River mouth (O'Donnell, 1990),
the southeast U.S. continental shelf (Kourafalou et al., 1996a; 1996b,
Chen et al., 1999, 2000), the Columbia River (Hickey et al., 1998) and
the Changjiang River (CR) (Beardsley et al., 1985; Chen et al., 2008;
Xue et al., 2009). The CR, one of largest rivers in the world, has a
typical freshwater discharge of ~40000 m3/s during summer (wet
season) and ~10000 m3/s during winter (dry season) (Beardsley
et al., 1985). The Changjiang Estuary (CE) is characterized by
shallow shoals, islands and multiple outlet channels in the river
mouth, and submarine canyons in the outer estuary, with dikes and
groins in the river mouth region (Fig. 1). The abundant freshwater
* Corresponding author.
E-mail addresses: [email protected] (J. Ge), [email protected]
(P. Ding), [email protected] (C. Chen).
http://dx.doi.org/10.1016/j.ecss.2014.10.012
0272-7714/© 2014 Elsevier Ltd. All rights reserved.
discharge from the CR produces a strong low-salinity plume around
the Changjiang Estuary and adjacent inner shelf of the East China
Sea (ECS), which is a permanent local physical dynamic phenomenon (Mao et al., 1963; Beardsley et al., 1985; Su and Wang, 1989;
Chen et al., 1999). The intensity and structure of this plume vary
significantly with season: weak and generally trapped along the
coast during the dry season but stronger and more unstable during
the wet season. During the wet season, an isolated low-salinity lens
often occurs as a result of the detachment process along the frontal
zone, which directly affects the local and regional ecosystem and
sediment transport on the inner shelf of the ECS (Tian et al., 1993a,
1993b; Chen et al., 1999, 2003a; Gao et al., 2008, 2009).
Several studies have been conducted to examine the physical
mechanisms driving the offshore detachment of low-salinity water
from the Changjiang River plume (Chen et al., 2008; Moon et al.,
2010; Wu et al., 2011, Xuan et al., 2012). Chen et al., (2008) developed a high-resolution, unstructured-grid, finite-volume, coastal
ocean model for the ECS (hereafter referred to as ECS-FVCOM) and
applied it to explore the frontal variability of the Changjiang River
plume. Their results show that baroclinic instability of the plume
could lead to the offshore detachment of the low-salinity water
62
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
Fig. 1. Bathymetry and observation stations around the Changjiang Estuary. The red filled circles denote the mooring stations where current, salinity and temperature were
measured, and black triangles show the wind observation sites A (Shengshan Island), B (Sheshan Island) and C (Dajishan Island). (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
along the frontal zone and form an isolated low-salinity lens on the
inner shelf of the ECS. This instability could be enhanced under the
southwesterly monsoon wind condition, which could produce a
large, isolated low-salinity lens and advect it to the offshore ECS
region. The studies by Chen et al., (2008) were carried out under a
condition without inclusion of astronomical tides. The tides are
dominant features in the Changjiang Estuary, where the typical
amplitude of tidal currents is about 0.6e1.0 m/s. In the shallow
region (less than 20 m), the water is usually vertically well mixed. Is
the finding reported by Chen et al., (2008) still valid when tidal
currents and mixing are taken into consideration? To our knowledge, this question has not been explored since their work.
Alternative studies were reported by Rong and Li (2012) and
Moon et al., (2010) using a coarse-resolution structured-grid ocean
model with a focus on the contribution of tidal mixing on the
offshore detachment of the low-salinity water from the Changjiang
River plume. Although this coarse-resolution model did not resolve
the baroclinic instability process, the simulation results suggested
that tidally induced vertical mixing via dissipation around the
plume was strong enough to overcome the buoyancy effect during
the spring tide, which could lead to the offshore detachment of the
low-salinity water along the frontal zone. Rong and Li (2012) used
the same model as Moon et al., (2010), and the configurations in
both their experiments were very similar in horizontal resolution
and tidal forcing. In Table 2 of Rong and Li (2012), the modelproduced tidal currents were compared with observations, and
the model overestimated the magnitude of tidal currents by 17%,
47% and 60% at MS, SDS, and M2 stations, respectively. Since the
SATM mechanism was based on enhanced tidal current and mixing
during the spring tide, whether or not it could be applied to the
realistic condition off the Changjiang Estuary needs a further validation via observed tidal currents and salinity within the plume
frontal zone.
The environmental condition of the Changjiang Estuary has
been significantly changed in recent years. This estuary has been
strongly impacted by multiple stressors, including the Three Gorges
Dam in the upstream Changjiang River (Yang et al., 2007, 2011), the
Deep Waterway project in the North Passage (Ge et al., 2012) and
coastal land reclamations in Hengsha Shoal and East Nanhui Shoal
(Wei et al., 2014). These anthropogenic activities have resulted in
fast changing estuarine dynamics. Due to the regulation of the
Three Gorges Dam and water withdrawal along the Changjiang
River, the net freshwater input to the estuary has decreased in the
last decade (Yang et al., 2011). We have collected daily river
discharge data from January 2000 to June 2014 at Datong Station,
which is the nearest hydrology station to the Changjiang River
Estuary, and statistics of this time series data show that the
maximum value during the summer peak period could still reach
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
66,000 m3/s. However, the river discharge reached 60,000 m3/s for
a total of only 45 days over 14 years. Although the river discharge
remained above 55,000 m3/s 35 days in 2010, it was only 3e10 days
in each of the other 6 years. The studies by Chen et al., (2008) and
Moon et al., (2010) were based on the historically averaged
maximum river discharge rate of 60,000 m3/s in the summer over a
30e60 day simulation period. It is clear that this assumption does
not apply to the current conditions of the Changjiang Estuary.
It is unclear whether or not previously proposed physical
mechanisms for the offshore detachment of the low-salinity water
from the Changjiang River plume are still valid under the current
environmental condition. In particular, would baroclinic instability
theory still be applicable to the Changjiang River plume after
considering the contribution of tidal mixing? How would the
constant versus variable winds contribute to the offshore lowsalinity detachment under the condition with tides? To our
knowledge, these questions have not yet been well examined.
In this paper, we attempt to examine the above questions based
on both observations and modeling. We have developed a regionalestuarine nested high-resolution FVCOM model and used it to
simulate the Changjiang Estuary plume under a realistic condition
and boundary condition. Unlike previous process-oriented mechanism modeling experiments, the mechanism-oriented numerical
experiments were conducted after a careful validation with the
field measurement data. The results obtained from our studies not
only avoid an unrealistic condition of tidal currents and mixing but
also provide a more comprehensive view of the baroclinic instability process under the realistic tidal-inclusive condition.
The rest of this paper is organized as follows. In Section 2, a brief
description of the observations and model is given. In Section 3,
observed salinity structure and currents are reported, followed by
model-data comparisons of salinity and its variability for the realtime simulation. In Section 4, the model-guided process-oriented
experiments are carried out to determine the effects of tidal mixing,
uniform and non-uniform winds on the offshore detachment of the
low-salinity water from the Changjiang River plume on the inner
shelf of the ECS. The conclusions are summarized in Section 5.
2. Field measurements and design of model experiments
An interdisciplinary cruise was conducted in the CE and the
inner shelf of the ECS during July 6e15, 2005. The survey area
covered the 10e50-m isobaths region of the Changjiang Estuary,
Hangzhou Bay and Zhoushan Archipelago where the river plume
was located (Kong et al., 2007). Twelve moorings (red circles in
Fig. 1) were deployed, with labels JS1, JS2, SH1, SH2, SH3, SH4 and
SH5 in the CE, ZJ1 and ZJ2 in the Hangzhou Bay, ZJ4, ZJ5 and ZJ6
around the Zhoushan Archipelago. The measurement durations at
individual mooring stations are listed in Table 1. Currents, salinity,
temperature, and turbidity were measured at each mooring. The
SonTek-ADP®-500 KHz (Acoustic Doppler Profiler, SonTek/YSI, Inc.)
was used for current measurements, with a cell size of 1.0 m and a
sensor depth of about 1.0 m below the sea surface. Time interval
was 120 s. An OBS-3A (Optical Backscatter Sensor, D&A Instrument
Company) was used to measure the water turbidity (NTU), temperature ( C) and salinity (psu). Three additional meteorological
stations (black triangles in Fig. 1) were also set up at Shengshan
Island (A), Sheshan Island (B) and Dajishan Island (C) to record
hourly wind speed and direction at a 10-m height over the time
period of July 1e31, 2005.
A high-resolution, regional-estuarine nested FVCOM model was
employed to simulate the Changjiang River plume and to examine
the driving mechanism of the offshore detachment of the lowsalinity water. FVCOM is a prognostic, unstructured-grid, free-surface, three-dimensional (3-D), primitive-equation ocean model
63
Table 1
Time coverage of mooring stations during the spring tidal cycle and neap tidal cycle.
Six survey vessels took shifts during the spring tide cycle, and three vessels were
arranged during the neap tidal cycle.
Spring tide
09:00
Jul 6e11:00
Jul 7
JS1
JS2
SH1
SH2
SH3
SH4
SH5
ZJ1
ZJ2
ZJ4
ZJ5
ZJ6
Neap tide
17:00
Jul 7e19:00
Jul 8
09:00
Jul 12e11:00
Jul 13
C
C
C
C
17:00
Jul 13e19:00
Jul 14
C
C
C
C
C
C
C
C
C
C
C
C
C
C
(Chen et al., 2003b, 2004, 2006, 2013). FVCOM combines the advantages of the finite-element method for geometric flexibility and
of the finite-difference method for high computational efficiency.
The finite-volume approach ensures volume and mass conservation
in the individual control volume and entire computational domain,
which is critical to simulate the river plume on the inner shelf of the
ECS.
The regional-estuarine nested FVCOM system used in this study
consisted of two models: ECS-FVCOM and CE-FVCOM. The
computational domain of the regional ocean model ECS-FVCOM,
developed originally by Chen et al., (2008), covered the entire
ECS, Yellow and Bohai Seas, and the Japan/East Sea. The computational domain of the high-resolution estuarine model CE-FVCOM,
developed originally by Xue et al., (2009), covered the Changjiang
River, Hangzhou Bay, Zhoushan Archipelago and the inner shelf of
the East China Sea (Fig. 2b). The large East China Sea model was run
first to provide the forcing condition at the nesting boundary with
the small-domain model. In addition to the nesting boundary
condition, the fine-resolution Changjiang Estuary model was also
driven by river discharges and surface wind forcing. This model was
driven by the river discharge at the upstream end of the Changjiang
and Qiantangjiang Rivers, surface meteorological forcing, and
lateral boundary forcing on the nested boundary provided by ECSFVCOM. The river discharge rate for the Changjiang River was based
on the daily measurement records, with a mean value of 39,913 m3/
s and a standard deviation of 2745 m3/s over the period of June 15 July 30, 2005. For the Qiantangjiang River, a constant summer
climatological river flux of 1000 m3/s was used. ECS-FVCOM
included eight major astronomical tidal constituents (M2, S2, K2,
N2, K1, O1, P1 and Q1) and continental shelf currents such as the
Taiwan Warm Current, the Yellow Sea Warm Currents, the Kuroshio, etc.
Two improvements have been made to ECS-FVCOM in this
study. First, we increased the horizontal resolution in both ECSFVCOM and CE-FVCOM off the Changjiang Estuary (Ge et al.,
2013), with a grid size as fine as 250 m in the inner shelf of the
ECS for CE-FVCOM (Figs. 2-c). Second, we included the dike-groyne
module in CE-FVCOM to resolve the realistic bathymetry and construction off the Changjiang River mouth (Ge et al., 2012, 2013).
The numerical simulation was conducted over the period of June
15eJuly 30, 2005, for different cases with and without inclusion of
tides and winds. For the experiments with winds, we considered
both constant and variable wind conditions. The non-uniform wind
forcing was provided by the high-resolution Weather Research &
64
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
Fig. 2. Unstructured model grid nested in the East China Sea model (panel a). The blue grids in panel b indicate the nesting boundary. The enlarged view of the river mouth grids is
shown in lower panel c. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Forecast (WRF) model that was validated via measurements at
three meteorological stations (Ge et al., 2013).
3. Observed salinity structure and model-data comparisons
3.1. Tidal elevations and currents
Observed tidal current ellipses at observation stations SH4, JS1
and JS2 over the spring tidal cycle had major axes ranging from 0.6
to 1.0 m/s and a direction of 120 e140 (Table 2), indicating that
the inner shelf of the ECS featured a moderate tide. This observed
tidal current value was smaller than the simulated value shown in
Rong and Li (2012), suggesting that the spring-tide mixing
mechanism for the offshore detachment of the low-salinity lens
proposed by Rong and Li (2012) and Moon et al., (2010) might not
be applicable for the realistic condition of the Changjiang River
plume.
Table 2
Observed tidal ellipse parameters over the spring tide cycle at mooring stations.
Station
Major axis (m/s)
Minor axis (m/s)
Direction ( )
JS1
JS2
SH2
SH4
SH5
ZJ1
ZJ2
ZJ4
ZJ5
ZJ6
0.8
0.6
0.98
0.57
1.17
1.47
1.11
0.48
0.74
0.4
0.45
0.37
0.52
0.28
0.13
0.17
0.12
0.24
0.38
0.31
140
143
96
146
83
104
113
156
131
120
3.2. Vertical distributions of salinity and velocity
Defining six relative depths: surface (0.0H), 0.2H, 0.4H, middle
layer (0.6H), 0.8H and bottom (1.0H), in which H denotes the total
water depth, we examined the vertical distribution of currents and
salinity relative to the total local water depth. For example, the
temporal variability of vertical profiles of salinity and velocity at
stations SH2, SH3 and SH4 over the spring tidal cycle is shown in
Fig. 3. At the shallow site SH2, the water was vertically well mixed.
The salinity at this site varied with tidal excursion scale, with a
range of 10 psu over an ebb-flood tidal cycle. At the relatively
deeper sites SH3 and SH4 where the plume was located, the salinity
and velocity profiles featured a two-layer structure: the lowsalinity water floating in the upper 5e10-m layer, and salty water
in the lower layer from the middle depth to the bottom. This
observational evidence clearly suggested that the Changjiang River
plume, particularly in the frontal zone, was characterized by a twolayer dynamics system described by Chen et al., (2008), and tidal
mixing over the spring tidal cycle was not strong enough to break
down this feature.
3.3. Wind speeds and directions
The wind velocity at the three meteorological stations located in
Fig. 1 varied strongly both temporally and spatially during July of
2005 (Fig. 4). The wind direction was mainly northward as a result
of the prevailing summertime monsoon. The wind was relatively
weaker, with a speed of ~5e8 m/s during the period of July 4e15
and then became much stronger, with a speed reaching 10e12 m/s
during the period of July 16-28. The WRF-simulated wind speed
and direction was compared with these observations. The results of
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
SH2
SH3
0
Depth (m)
20
25
-8
Depth (m)
15
25
SH4
0
30
20
-6
25
-4
20
20
Depth (m)
0
-2
65
-16
20
25
30
-10
-20
-30
6
6.5
7
Time (day)
7.5
7.6
8
Time (day)
6.5
Salinity (psu)
7
7.5
Time (day)
depth
inner she
lf slope
15
20
25
30
35
offshore
Fig. 3. Variation of salinity processes at SH2, SH3 and SH4 from shallow to deep region off the Changjiang Estuary during the spring tidal cycle.
a detailed comparison were described and discussed in Ge et al.
(2013). The RMS error was 2.1 m/s for the wind speed and 23 for
the wind direction. Without data assimilation, the local WRF model
was capable of reasonably reproducing the spatial and temporal
variability of the wind field off the CE over the simulation period.
3.4. CE-FVCOM validation
The CE-FVCOM was validated by comparisons with observed
surface tidal elevation, (tidal and subtidal) currents, and salinity.
The comparisons for tidal elevation were made at 32 gauge stations
along the coasts of CE, Hangzhou Bay and the offshore islands, and
results for the M2 tidal constituent are listed in Table 3. For the M2
tidal constituent, the mean error was less than 10% in amplitude
and less than 10 in phase. Comparisons for tidal constituents S2, K1
and O1 were also performed, and the results were in equally
reasonable agreement. The CE-FVCOM was also capable of simulating the vertical distribution and temporal variability of observed
current and salinity. A model-data comparison for the water
velocity (speed and direction) in the CE and inner shelf of the ECS
was conducted and described in Ge et al., (2013). At SH1, SH2, and
SH5, for example, the maximum velocity during the ebb tidal
period was >2.0 m/s in the upper surface layer. A pronounced velocity shear was revealed between the upper and lower layers. The
shear was mainly dominated by the combined tidal and riverdischarge flows from the CR. There was a relatively weaker velocity shear within the lower layer from the mid-depth to the bottom.
In this layer, observed and modeled velocities were consistent, and
both were dominated by the tidal flow. The model-data comparison
results for salinity at measurement stations were illustrated in
Fig. 11 of Ge et al., (2013), which showed that the CE-FVCOM
correctly reproduced the salinity variation over tidal cycles. An
example was shown in Fig. 5 for the comparison of observed and
modeled tidal-cycle averaged vertical salinity profiles. The model
agreed fairly well with observations. At both stations SH1 and SH3,
observed salinity showed a strong vertical stratification: the lowsalinity water prevailed in the upper layer and the salty water in
the lower layer. This two-layer feature was well captured by the
model. The model also reproduced the well-mixed vertical profiles
and horizontal variation of the salinity at the shallow stations SH2
and SH5.
4. Model-guided process experiments
Fig. 4. Variation of wind vectors during July 2005 at three meteorological stations,
Shengshan, Sheshan and Dajishan Islands off the Changjiang Estuary.
Building on the success of the model validation, we applied this
high-resolution CE-FVCOM to the examination of the physical
mechanism for the offshore detachment of the low-salinity water
from the Changjiang River plume under a realistic condition of July
2005 in the CE and inner shelf of the ECS. During this period, the
model detected two major surface detachment events: one on July
7 and the other on July 26 (Fig. 6). The first detachment event
occurred around the eastern region of the CE (left column of Fig. 6),
and the second took place in the northeastern region (right column
of Fig. 6). These two detachment events can be viewed more clearly
in the vertical section plots along the main axis of the low-salinity
detachment in Fig. 6. The salinity was characterized by a pronounced two-layer system, especially offshore of the 20-m isobath.
The vertically well-mixed low salinity was mainly constrained
around the 10-m isobath.
At 16:00 (GMTþ8), July 7, 2005, the water mass bounded by a
30-psu contour detached as a continuous bubble shape from the
frontal zone of the plume. The detached water mass gradually
decreased in size over the ebb tide due to tidal mixing and wind
stirring over the time period from ebb tide maximum to flood tide
maximum. This detachment occurred around the 50-m isobath
66
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
Table 3
Model validation of amplitude and phase of the M2 tidal constituent at 32 gauge stations in the Changjiang Estuary, Hangzhou Bay and adjacent coastal regions (H (cm) is M2
tidal amplitude and G ( ) is M2 tidal phase).
Station
Wangpan
Liangque
Haiwang
Daishan
Changtu
Ganpu
Zhapu
Jinshan
Longshan
Luhuashan
Daji
Gaoqiao
Shenjiamen
Dinghai
Zhenhai
Yuxinnao
Tangnaoshan
Haiyan
Tanhu
Nanhui
Waikejiao
Lusi
Baozhen
Sheshan
Wusong
Hengsha
Zhongjun
Jiuduan
Luchaogang
Xize
Shipu
Dachen
M2 (model)
Location
M2 (obs)
M2 (error)
Longitude ( E)
Latitude ( N)
H (cm)
G ( )
H (cm)
G ( )
(Hmod-Hobs)/Hobs
G ( )
121.2927
121.6308
121.5003
122.1985
122.3011
120.9096
121.0899
121.3736
121.5829
122.5994
122.1656
121.5906
122.3006
122.0993
121.7169
121.8633
121.9711
120.9524
121.6131
121.8475
121.6235
121.6109
121.5865
122.2256
121.5058
121.8502
121.9057
122.1692
121.8267
121.8283
121.9107
121.8816
30.5053
30.2732
30.2131
30.2327
30.2502
30.3584
30.5905
30.7288
30.0844
30.8163
30.8099
31.3668
29.4581
30.0008
29.9882
30.3530
30.5855
30.4969
30.6216
30.8680
33.0006
32.1161
31.5173
31.3972
31.3989
31.2740
31.0948
31.0986
30.8269
29.6114
29.1940
28.4253
165.1
107.4
108.5
94.7
94.2
218.3
193.0
160.4
90.8
111.7
121.2
106.1
117.0
90.2
79.8
107.3
118.8
208.8
138.1
138.5
168.9
181.5
112.9
122.5
104.3
116.7
123.7
127.0
139.8
119.0
151.5
148.5
11.4
346.3
356.4
296.8
294.6
36.5
12.7
359.4
343.1
292.0
311.7
9.9
247.9
283.8
329.8
330.8
325.4
19.2
350.6
333.7
339.1
349.1
16.5
317.5
18.2
343.7
330.2
313.9
333.3
268.7
245.0
244.7
171.8
104.5
111.4
91.7
97.0
254.1
204.2
171.1
91.6
121.3
125.2
110.0
114.6
93.8
80.0
107.0
117.6
212.0
144.6
145.6
183.1
171.5
114.4
113.7
100.1
108.6
117.2
122.8
144.9
121.0
146.1
158.6
12.1
347.2
356.0
298.1
289.4
47.2
28.4
8.9
357.1
287.1
320.7
13.0
267.0
285.6
324.5
328.0
324.5
25.9
350.1
327.0
335.0
352.5
10.4
311.5
13.2
343.3
/
312.4
335.3
264.2
253.2
247.8
4
3
3
3
3
14
6
6
1
8
3
4
2
4
0
0
1
2
4
5
8
6
1
8
4
8
5
3
4
2
4
6
1
1
0
1
5
11
16
10
14
5
9
3
19
2
5
3
1
7
0
7
4
3
6
6
5
0
/
1
2
5
8
3
where the main frontal zone of the plume was located. A narrow
neck of low-salinity water linking the detached water and the main
low-salinity plume was severed due to tidal mixing and the windinduced water transport. Around 123 E and 31.5 N, the salinity
distribution around the 50-m isobaths featured an unstable frontal
zone, where a eastward detachment occurred. The model-
Surface
produced detached salinity distribution in this region agreed well
with previous salinity measurement results during summer (Zhu
et al., 2003).
At 21:00 (GMTþ8), July 26, 2005, the other bulge-shaped
detachment event occurred off the 20-m isobath around the
northeastern region (123 E, 32 N) off the CE (Fig. 6: right panel),
SH1
SH2
SH3
SH1
SH2
SH5
Relative Depth
0.2H
0.4H
0.6H
FVCOM
0.8H
Observed
Bottom
Surface
Relative Depth
0.2H
0.4H
0.6H
0.8H
Bottom
15
20
25
30
Salinity(psu)
35
15
20
25
30
Salinity(psu)
35
15
20
25
30
Salinity(psu)
35
Fig. 5. Model-data comparisons for tidal-cycle-averaged salinity profiles at SH1, SH2 and SH3 during the spring tidal cycle, and at SH1, SH2 and SH5 during the neap tidal cycle (red
dotted curves are simulated; blue ones are observed). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
67
Fig. 6. Distributions of surface salinity at two significant low-salinity water detachments on July 7 (left column) and July 26 (right column), 2005. The dashed black line shows the
section along the main axis during the detachment. The vertical distributions of the salinity along the sections are plotted in the lower row. The white-dashed and white-solid lines
indicate the 20-m and 50-m isobaths respectively.
where a strong unstable salinity gradient was found near the 20-m
isobath. The salinity of the detached water was relatively uniform,
with relatively high-salinity water forming a northward intrusion
from the tip of the 50-m isobath. The direction of the intrusion was
identical with the flood tidal direction (~120 e140 ). The bubbles
and bulge contours all showed a northwestern flattening along the
flood tidal direction as a result of tidal mixing.
To identify and quantify the physical driving mechanism for the
offshore detachment of the low-salinity water from the Changjiang
River plume detected in our real-time simulation, we re-ran the
model for the cases with a) only river discharge (Case A), b) river
discharge plus tides (Case B), c) river discharge, tides and constant
wind (Case C), and d) river discharge, tides and variable winds (Case
D). Case D is the simulation case we have shown in Fig. 6. The
constant wind in Case C is a July 2015 monthly mean value of hourly
WRF-simulated wind velocity.
Case A is an experiment repeated from Chen et al., (2008), but
with reduced river discharges. The results clearly showed that
under this forcing condition, the plume was characterized by the
bulge shape along the 20-m isobath (Fig. 7, upper-left panel), which
was very similar to the pattern detected in Chen et al.’s (2008)
simulation. In contrast to Chen et al. 's (2008) experiments, the
horizontal diffusion coefficient used in our experiment was
200 m2/s, about 10 times larger than the value used in their work,
and the river discharge rate used in our experiment was 40,000 m3/
s, 20,000 m3/s smaller than that used in their work. Applying the
baroclinic instability criterion to our case, we have.
Eh ¼
Ah f
ðg 0 Qe Þ
2=3
z0:11 < Ehc z0:34 0:57
where Eh is horizontal Ekman number, Ehc is the critical Ekman
number for baroclinic instability, and Qe, Ah, g0 and f are river
discharge per unit length, horizontal eddy viscosity, reduced
gravity acceleration and Coriolis parameter, respectively (Chen
et al., 2008). This value of Eh still satisfied the baroclinic instability criterion given above. This indicates that even for the case
with reduced river discharge rate and larger horizontal diffusion
coefficient, the plume produced in the river-discharge-only case
was in a baroclinically unstable condition, even though no eddies
were generated in this case. As horizontal diffusion coefficient diminishes, we saw eddies form along the frontal zone as a result of
baroclinic instability. Our focus here is on examining the physical
mechanism driving the offshore low-salinity detachment discovered in Case D, where the horizontal diffusion coefficient was
specified by Smagorinsky's turbulence closure scheme. For this
reason, we did not alter the horizontal diffusion coefficient to
match that used in Chen et al., (2008). Based on the lateral mixing
coefficient used in our case, the baroclinic instability seemed not to
be a key physical mechanism in producing the large isolated
offshore low-salinity detachment found in Case D.
When tidal forcing was added in Case B, the bulge shape along the
20-m isobaths was significantly smoothed as a result of enhanced
tidally induced vertical mixing, even though the plume still satisfied
the baroclinic instability criterion (Fig. 7, lower left panel). Tidal
mixing was mainly caused by the tidal current shear near the bottom,
and the tidally induced mixed layer above the bottom agreed with
the analytical solution derived by Chen and Beardsley (1995).
Balanced by the buoyancy input and turbulent dissipation, the tidally
induced mixing depth (hm) could be determined by.
hm ¼
16gdDTU 3
N2 p
1=3
where g is the bottom friction coefficient, usually taken as 0.0025;
d is the efficiency of tidal kinetic energy dissipation over the given
68
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
Fig. 7. Distributions of the surface salinity distributions during the first detachment process (2005-07-07 T16:00) around the eastern region of the Changjiang Estuary for the cases
with a) only river discharge; b) river discharge plus tides; c) river discharge plus tides and a constant wind; and d) the real-time simulation with river discharge, tides, and variable
and spatially non-uniform winds.
time period DT, the typical value of which is suggested to be
3.7 103 by Simpson and Hunter (1974); DT is suggested to be ~14
days in the strong stratification case (Lee and Beardsley, 1999). U is
the typical tidally averaged and vertically averaged current (with a
typical value of ~0.5 m/s along the plume); N is the BrunteV€
ais€
al€
a
frequency (with a typical value of 0.02 s1 for the strong stratification case in the plume frontal zone). The calculated mixing depth
was ~22 m, which is identical to the isobath of the northern plume
region shown in the lower left panel of Fig. 7. This suggests that the
vertically well-mixed salinity patterns in the region shallower than
~22 m was caused by tidal mixing. The baroclinic instability was
likely to occur in the deeper region of ~50 m, where the frontal
structure was characterized by a two-layer dynamical system. It
should be noted that the tidal mixing depth determined by Moon
et al., (2010) using the same formula was 36 m, about 14 m
higher than the value we found in our experiments. Since the initial
condition of stratifications was different and the model forcing was
not the same, it was not surprising to see such a difference between
two models.
Adding a constant monthly-averaged wind to Case B, we
examined the impact of the wind on the plume variability for Case
C. The southwesterly wind produced an offshore Ekman transport,
which advected the plume offshore. The offshore frontal moving
speed ufront satisfied the Ekman transport theory given as.
ufront ¼
tw
rfhc
where tw, f, and hc are wind shear stress, Coriolis parameter and
thickness of plume front, respectively (Fong and Geyer, 2001). The
interaction of the uniform wind and non-uniform plume velocity
tended to enhance the plume spatial variability, which was
consistent with the finding reported by Chen et al., (2008). Under
the conditions given in our experiment, however, no offshore lowsalinity detachment occurred in this case.
The situation significantly changed when a variable wind was
used in Case D. Given the same river discharge rate, tidal forcing
and lateral mixing coefficient, the change of the speed and direction of the wind significantly enhanced the temporal and spatial
variability of the plume. As a result, the low-salinity water in the
northern plume area was detached offshore from the frontal zone.
The detachment under this condition was relatively strong. A large
body of the low-salinity water was detached as an isolated lens,
similar to what has often been observed in that region. It is clear
that the variable wind played a key role in enhancing the instability
and spatial-temporal variability of the plume. Since the formation
of the isolated low-salinity lens occurred under the wind forcing
condition, the flow within the lens was not an eddy. The mechanism driving the offshore-detachment was very similar to the case
detected by Chen (2000) on the South Atlantic Bight shelf where
the isolated low-salinity lens was often observed under the
upwelling-favorable wind condition. He found that the detachment
process happened in two stages. First, the spatially non-uniform
response of current to the upwelling-favorable wind enhances a
wavelike frontal shape at the outer edge of the frontal zone. Then,
the isolated low-salinity lenses formed at the crest, when water on
the shoreward side of the crest was displaced by relatively highsalinity water advected from the upstream trough south of the
crest and diffused upward from the deep region. In our case, we
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
69
Fig. 8. Distribution of upwelling-favorable, wind-induced Ekman volume transport (VEk ¼ t/f) along the 20-m-isobath (green line in left panel) during the first detachment period of
July 7, 2005 (red lines in right panel) and second detachment period of July 26, 2005 (blue line in right panel). t is the surface wind stress and f is the local Coriolis parameter. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
found that the upwelling-favorable wind-induced Ekman transport
advected the plume offshore, which caused the plume to become
more baroclinically unstable. Variation of the wind enhanced the
non-uniform response of current to the wind, and detachment
occurred at the bulge-shaped (crest) region of the plume, which
proceeded in two steps as described by Chen (2000).
Unlike the constant wind case, the non-uniform wind caused
the spatial-varying offshore movement speed of the plume, which
directly enhanced the spatial variability of the plume as that found
in the uniform wind condition. This can be clearly seen in Fig. 8,
which shows that the offshore Ekman transport varied significantly
along the frontal zone of the plume when the variable wind was
used. During the first detachment period on July 7, 2005, the
offshore Ekman transport was relatively larger around 30.5 N than
in the surrounding area. During the second detachment period, the
maximum offshore Ekman transport shifted to the northern region
at around 31.5 N. In this event, the surface wind was much stronger
than in the previous event. Non-uniform offshore Ekman transports
observed in both events played a key role in enhancing the alongfrontal variability of the plume, and thus led to the offshore
detachment of the low-salinity water from the plume.
5. Summary
The temporal and spatial variability of the Changjiang River
plume was examined using both field measurements and a
regional-estuarine nested high-resolution model. The observations
showed that due to anthropogenic activities, the Changjiang River
discharge rate in summer has been significantly reduced. Both
salinity and velocity measurements showed that the plume was
characterized by a two-layer structure: the low-salinity water
floating in the upper 5e10-m layer, and salty water occupying the
lower layer from the middle depth to the bottom.
The high-resolution ECS-FVCOM and CE-FVCOM nested model
system was capable of simulating the vertical distribution and
temporal variability of the observed current and salinity of the
Changjiang River plume. The real-time simulation over the
observed period revealed two significant offshore detachments of
the low-salinity water from the plume. Process-oriented
experiments suggest that the non-uniform distribution and variability of the wind played a key role in driving these two offshore
detachment events. The spatially non-uniform wind field caused
the spatially varying offshore movement speed of the plume. The
large spatial variability of the plume caused by the non-uniform
offshore Ekman transport increased the plume instability.
Although tidal mixing tended to stabilize the frontal structure of
the plume, the isolated low-salinity lenses could be formed at the
bulge-shaped area of the plume when water on the shoreward side
of the crest was displaced by relatively high-salinity water advected
from the upstream trough south of the crest and diffused upward
from the deep region.
Our finding was consistent with previous theories suggested by
Chen (2000) and Chen et al., (2008). The key difference is that our
case was done with a larger horizontal diffusion coefficient and
tidal mixing. Under this condition, we found that the spatially nonuniform wind could be more critical than the baroclinic instability
in causing the offshore low-salinity detachment from the Changjiang River plume. In our experiments, we did not find the offshore
detachment during the spring tidal period, which was suggested by
Moon et al., (2010) and Rong and Li (2012). One reason is that the
tidal currents have significantly changed since dikes and groynes
were constructed off the Changjiang Estuary. Our measurements
showed that the magnitude of the M2 tidal currents at the JS2 site
was 0.6 m/s during the spring period of July 2005. This spring tidal
current magnitude changed significantly with stratification. It
dropped to 0.46 m/s in October 2005 and 0.53 m/s in May 2006 as
stratification became weak. This site was very close to the M2 and
M4 stations discussed in Rong and Li (2012), and the computed
magnitude of regular tidal currents in their model was 0.68e0.8 m/
s, which is the same as or larger than the observed spring tidal
current magnitudes. As a result of such environmental change,
whether or not the SATM will still be applicable to the Changjiang
Estuary plume needs further validation via comparisons.
Acknowledgments
Jianzhong Ge and Pingxing Ding are supported by the Fund from
Natural Sciences Foundation of China (No. 41021064; No.
70
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70
41306080), Public Service Programme of State Ocean Administration (No. 201205017-2), China Science and Technology Support
Programme (No. 2013BAB12B03-Z1) and the SKELC fund (No.
SKLEC-2011RCDW03). The authors would like to express their
appreciation to two anonymous reviewers, who provided very
helpful suggestions to improve the manuscript.
References
Beardsley, R., Limeburner, R., Yu, H., Cannon, G.A., 1985. Discharge of the Changjiang
(Yangtze River) into the East China Sea. Cont. Shelf Res. 4, 57e76.
Chen, C., 2000. A modeling study of episodic cross-frontal water transports over the
inner shelf of the South Atlantic Bight. J. Phys. Oceanogr. 30, 1722e1742.
Chen, C., Beardsley, R.C., 1995. A numerical study of stratified tidal rectification over
finite-amplitude banks. Part I: symmetric banks. J. Phys. Oceanogr. 25,
2090e2110.
Chen, C., Zheng, L., Blanton, J., 1999. Physical processes controlling the formation,
evolution, and perturbation of the low-salinity front in the inner shelf off the
Southeastern U.S: a modeling study. J. Geophys. Res. 104, 1259e1288.
Chen, C., Zhu, J., Beardsley, R.C., Franks, P.S., 2003a. Physical-biological sources for
the Dense Algal Bloom over the Western Shelf of the East China Sea. Geophys.
Res. Lett. 30 (10), 1515, 22e1:4.
Chen, C., Liu, H., Beardsley, R.C., 2003b. An unstructured, finite-volume, threedimensional, primitive equation ocean model: application to coastal ocean and
estuaries. J. Atmos. Oceanic Technol. 20, 159e186.
Chen, C., Cowles, G., Beardsley, R.C., 2004. An Unstructured Grid, Finite-volume
Coastal Ocean Model: FVCOM User Manual, p. 183. SMAST/UMASSD Technical
Report-04e0601.
Chen, C., Beardsley, R.C., Cowles, G., 2006. An unstructured grid, finite-volume
coastal ocean model (FVCOM) system. Special Issue entitled “Advance in
Computational Oceanography”. Oceanography 19 (1), 78e89.
Chen, C., Xue, P., Ding, P., Beardsley, R.C., Xu, Q., Mao, X., Gao, G., Qi, J., Li, C., Lin, H.,
Cowles, G., Shi, M., 2008. Physical mechanisms for the offshore detachment of
the Changjiang diluted water in the East China Sea. J. Geophys. Res. 113, C02002.
http://dx.doi.org/10.1029/2006JC003994.
Chen, C., Beardsley, R.C., Luettich Jr., R.A., Westerink, J.J., Wang, H., Perrie, W., Xu, Q.,
Donahue, A.S., Qi, J., Lin, H., Zhao, L., Kerr, P.C., Meng, Y., Toulany, B., 2013.
Extratropical storm inundation testbed: intermodel comparisons in Scituate,
Massachusetts. J. of Geophys. Res. Oceans 118, 1e20. http://dx.doi.org/10.1002/
jgrc.20397.
Fong, D.A., Geyer, W.R., 2001. Response of a river plume during an upwelling
favorable wind event. J. Geophys. Res. 106 (C1), 1067e1084. http://dx.doi.org/
10.1029/2000JC900134.
Gao, L., Li, D., Ding, P., 15 October 2008. Variation of nutrients in response to the
highly dynamic suspended particulate matter in the Changjiang (Yangtze River)
plume. Cont. Shelf Res. ISSN: 0278-4343 28 (17), 2393e2403. http://dx.doi.org/
10.1016/j.csr.2008.05.004.
Gao, L., Li, D., Ding, P., January 2009. Quasi-simultaneous observation of currents,
salinity and nutrients in the Changjiang (Yangtze River) plume on the tidal
timescale. J. Mar. Syst. ISSN: 0924-7963 75 (1e2), 265e279. http://dx.doi.org/
10.1016/j.jmarsys.2008.10.006.
Ge, J., Chen, C., Qi, J., Ding, P., Beardsley, R.C., 2012. A dikeegroyne algorithm in a
terrain-following coordinate ocean model (FVCOM): development, validation
and application. Ocean. Model. 47 (C), 26e40. http://dx.doi.org/10.1016/
j.ocemod.2012.01.006.
Ge, J., Ding, P., Chen, C., Hu, S., Fu, G., Wu, L., 2013. An integrated East China SeaChangjiang Estuary model system with aim at resolving multi-scale regionalshelf-estuarine dynamics. Ocean. Dyn. 63 (8), 881e900. http://dx.doi.org/
10.1007/s10236-013-0631-3.
Hickey, B., Pietrafesa, L., Jay, D., Boicourt, W., 1998. The Columbia River Plume study:
subtidal variability in the velocity and salinity fields. J. Geophys. Res. 103 (C5),
10339e10368.
Kong, Y., Ding, P., He, S., October 2007. Analysis of basic characteristics of residual
current in the Changjiang River Estuary and adjacent sea areas. Adv. Mar. Sci. 25
(4) (in Chinese).
Kourafalou, V., Oey, L.Y., Wang, J., Lee, T., 1996a. The fate of river discharge on the
continental shelf 1. Modeling the river plume and the inner shelf coastal current. J. Geophys. Res. 101 (C2), 3415e3434.
Kourafalou, V., Lee, T., Oey, L.Y., Wang, J., 1996b. The fate of river discharge on the
continental shelf 2. Transport of coastal low e salinity waters under realistic
wind and tidal forcing. J. Geophys. Res. 101 (C2), 3435e3455.
Lee, S.H., Beardsley, R.C., 1999. Influence of stratification on residual tidal currents in
the Yellow Sea. J. Geophys. Res. Oceans 104 (C7), 15679e15701.
Lentz, S., 1995. Seasonal variations in the horizontal structure of the Amazon Plume
inferred from historical hydrographic data. J. Geophys. Res. 100 (C2),
2391e2400.
Lentz, S., 2004. The response of Buoyant coastal plumes to upwelling-favorable
winds. J. Phys. Oceanogr. 34 (11), 2458e2469.
Lentz, S., Largier, John, 2006. The influence of wind forcing on the Chesapeake Bay
Buoyant Coastal current. J. Phys. Oceanogr. 36 (7), 1305e1316.
Mao, H., Kan, Tze-chun, Lan, Shu-Fang, 1963. A preliminary study of the Yangtze
diluted water and its mixing processes. Oceanol. Limnol. Sinica 5 (3), 183e206.
Moon, J.-H., Hirose, N., Yoon, J.-H., Pang, I.-C., 2010. Offshore detachment process of
the low-salinity water around changjiang Bank in the East China sea. J. Phys.
Oceanogr. 40 (5), 1035e1053. http://dx.doi.org/10.1175/2010JPO4167.1.
O'Donnell, J., 1990. The formation and fate of a river plume: a numerical model.
J. Phys. Oceanogr. 20 (4), 551e569.
Rong, Z., Li, M., 2012. Estuarine, coastal and shelf science. Estuar. Coast. Shelf Sci. 97
(C), 149e160. http://dx.doi.org/10.1016/j.ecss.2011.11.035.
Simpson, J.H., Hunter, J.R., 1974. Fronts in the Irish Sea. Nature 250, 404e406.
http://dx.doi.org/10.1038/250404a0.
Su, Jilan, Wang, Kangshan, January 1989. Changjiang river plume and suspended
sediment transport in Hangzhou Bay. Cont. Shelf Res. ISSN: 0278-4343 9 (1),
93e111. http://dx.doi.org/10.1016/0278-4343(89)90085-X.
Tian, R.C., Hu, F.X., Saliot, A., 1993a. Biogeochemical processes controlling nutrients
at the turbidity maximum and the plume water fronts in the changjiang estuary. Biogeochemistry 19 (2), 83e102 (1992e1993).
Tian, R.C., Hu, F.X., Martin, J.M., 1993b. Summer nutrient fronts in the changjiang
(Yangtze river) estuary. Estuar. Coast. Shelf Sci. ISSN: 0272-7714 37 (1), 27e41.
http://dx.doi.org/10.1006/ecss.1993.1039. July 1993.
Wei, W., Tang, Z., Dai, Z., Lin, Y., Ge, Z., Gao, J., 2014. Variations in tidal flats of the
Changjiang (Yangtze) estuary during 1950se2010s: future crisis and policy
implication.
Ocean
Coast.
Manag.
1e8.
http://dx.doi.org/10.1016/
j.ocecoaman.2014.05.018.
Wu, H., Zhu, J., Shen, J., Wang, H., 2011. Tidal modulation on the Changjiang River
plume in summer. J. Geophys. Res. 116 (C8), C08017. http://dx.doi.org/10.1029/
2011JC007209.
Xuan, J.-L., Huang, D., Zhou, F., Zhu, X.-H., Fan, X., 2012. The role of wind on the
detachment of low salinity water in the Changjiang Estuary in summer.
J. Geophys. Res. 117 (C10), C10004. http://dx.doi.org/10.1029/2012JC008121.
Xue, P., Chen, C., Ding, P., Beardsley, R.C., Lin, H., Ge, J., Kong, Y., 2009. Saltwater
intrusion into the Changjiang River: a model-guided mechanism study.
J. Geophys. Res. 114, C02006. http://dx.doi.org/10.1029/2008JC004831.
Yang, S.L., Zhang, J., Xu, X.J., 2007. Influence of the three Gorges Dam on downstream delivery of sediment and its environmental implications, Yangtze River.
Geophys. Res. Lett. 34 (10), L10401. http://dx.doi.org/10.1029/2007GL029472.
Yang, S.L., Milliman, J.D., Li, P., Xu, K., 2011. 50,000 dams later: erosion of the Yangtze
River and its delta. Glob. Planet. Change 75 (1e2), 14e20. http://dx.doi.org/
10.1016/j.gloplacha.2010.09.006.
Zhu, J., Ding, P., Hu, D., May 2003. observation of the diluted water and plume front
off the Changjiang River Estuary during august 2000. Oceanol. Limnol. Sinica 34
(3), 249e255.