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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 offthe Changjiang Estuary
Jianzhong Ge a, *, Pingxing Ding a, Changsheng Chen b
a The State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, PR Chinab 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
Article history:Accepted 21 October 2014Available online 31 October 2014
Keywords:plume detachmentnon-uniform windEkman transportChangjiang Estuary
* Corresponding author.E-mail addresses: [email protected] (J. Ge
(P. Ding), [email protected] (C. Chen).
http://dx.doi.org/10.1016/j.ecss.2014.10.0120272-7714/© 2014 Elsevier Ltd. All rights reserved.
a b s t r a c t
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-salinityplume over the inner shelf of the East China Sea (ECS). These two mechanisms were re-examined us-ing both observations and a fully three-dimensional (3-D), high-resolution, unstructured-grid, free-surface, primitive-equation, Finite-Volume Community Ocean Model (FVCOM). The observed currentsand salinities showed that the plume was characterized by a two-layer system, in which the upper layeris mainly driven by the river discharge-induced buoyancy flow and the lower layer is predominantlycontrolled by tidal mixing and rectification. The SATM mechanism was based on the model run withoutcalibration against observed currents and salinity around the plume region, so that it should be appliedwith caution to a realistic condition observed on the inner shelf of the ECS. The BI mechanism wasderived under a condition without consideration of tidal mixing. Although BI could still occur along thefrontal zone when tides were included, it was unable to produce a single, large, detached low-salinitylens observed on the inner shelf of the ECS. The process-oriented model experiment results suggestthat for a given river discharge and realistic tidal flow, the spatially non-uniform southwesterly surfacewind during the southeast monsoon-dominant summer could increase frontal spatial variability and thusproduce a significant offshore detachment of low-salinity water on the inner shelf of East China Sea.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The low-salinity plume is a common coastal and oceanic phys-ical phenomenon in river-dominated estuaries such as the AmazonRiver (Lentz, 1995), the Chesapeake Bay (Lentz, 2004; Lentz andLargier, 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) andthe 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 atypical freshwater discharge of ~40000 m3/s during summer (wetseason) and ~10000 m3/s during winter (dry season) (Beardsleyet al., 1985). The Changjiang Estuary (CE) is characterized byshallow shoals, islands and multiple outlet channels in the rivermouth, and submarine canyons in the outer estuary, with dikes andgroins in the river mouth region (Fig. 1). The abundant freshwater
discharge from the CR produces a strong low-salinity plume aroundthe Changjiang Estuary and adjacent inner shelf of the East ChinaSea (ECS), which is a permanent local physical dynamic phenom-enon (Mao et al., 1963; Beardsley et al., 1985; Su and Wang, 1989;Chen et al., 1999). The intensity and structure of this plume varysignificantly with season: weak and generally trapped along thecoast during the dry season but stronger and more unstable duringthe wet season. During the wet season, an isolated low-salinity lensoften occurs as a result of the detachment process along the frontalzone, which directly affects the local and regional ecosystem andsediment 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 physicalmechanisms driving the offshore detachment of low-salinity waterfrom the Changjiang River plume (Chen et al., 2008; Moon et al.,2010; Wu et al., 2011, Xuan et al., 2012). Chen et al., (2008) devel-oped a high-resolution, unstructured-grid, finite-volume, coastalocean model for the ECS (hereafter referred to as ECS-FVCOM) andapplied it to explore the frontal variability of the Changjiang Riverplume. Their results show that baroclinic instability of the plumecould lead to the offshore detachment of the low-salinity water
Fig. 1. Bathymetry and observation stations around the Changjiang Estuary. The red filled circles denote the mooring stations where current, salinity and temperature weremeasured, 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 thisfigure legend, the reader is referred to the web version of this article.)
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e7062
along the frontal zone and form an isolated low-salinity lens on theinner shelf of the ECS. This instability could be enhanced under thesouthwesterly monsoon wind condition, which could produce alarge, isolated low-salinity lens and advect it to the offshore ECSregion. The studies by Chen et al., (2008) were carried out under acondition without inclusion of astronomical tides. The tides aredominant features in the Changjiang Estuary, where the typicalamplitude of tidal currents is about 0.6e1.0 m/s. In the shallowregion (less than 20m), thewater is usually vertically well mixed. Isthe finding reported by Chen et al., (2008) still valid when tidalcurrents and mixing are taken into consideration? To our knowl-edge, this question has not been explored since their work.
Alternative studies were reported by Rong and Li (2012) andMoon et al., (2010) using a coarse-resolution structured-grid oceanmodel with a focus on the contribution of tidal mixing on theoffshore detachment of the low-salinity water from the ChangjiangRiver plume. Although this coarse-resolution model did not resolvethe baroclinic instability process, the simulation results suggestedthat tidally induced vertical mixing via dissipation around theplume was strong enough to overcome the buoyancy effect duringthe spring tide, which could lead to the offshore detachment of thelow-salinity water along the frontal zone. Rong and Li (2012) usedthe same model as Moon et al., (2010), and the configurations inboth their experiments were very similar in horizontal resolution
and tidal forcing. In Table 2 of Rong and Li (2012), the model-produced tidal currents were compared with observations, andthe model overestimated the magnitude of tidal currents by 17%,47% and 60% at MS, SDS, and M2 stations, respectively. Since theSATMmechanismwas based on enhanced tidal current and mixingduring the spring tide, whether or not it could be applied to therealistic condition off the Changjiang Estuary needs a further vali-dation via observed tidal currents and salinity within the plumefrontal zone.
The environmental condition of the Changjiang Estuary hasbeen significantly changed in recent years. This estuary has beenstrongly impacted bymultiple stressors, including the Three GorgesDam in the upstream Changjiang River (Yang et al., 2007, 2011), theDeep Waterway project in the North Passage (Ge et al., 2012) andcoastal land reclamations in Hengsha Shoal and East Nanhui Shoal(Wei et al., 2014). These anthropogenic activities have resulted infast changing estuarine dynamics. Due to the regulation of theThree Gorges Dam and water withdrawal along the ChangjiangRiver, the net freshwater input to the estuary has decreased in thelast decade (Yang et al., 2011). We have collected daily riverdischarge data from January 2000 to June 2014 at Datong Station,which is the nearest hydrology station to the Changjiang RiverEstuary, and statistics of this time series data show that themaximum value during the summer peak period could still reach
Table 1Time 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 werearranged during the neap tidal cycle.
Spring tide Neap tide
09:00Jul 6e11:00Jul 7
17:00Jul 7e19:00Jul 8
09:00Jul 12e11:00Jul 13
17:00Jul 13e19:00Jul 14
JS1 C C
JS2 C
SH1 C C
SH2 C C
SH3 C
SH4 C
SH5 C C
ZJ1 C C
ZJ2 C C
ZJ4 C
ZJ5 C
ZJ6 C
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70 63
66,000 m3/s. However, the river discharge reached 60,000 m3/s fora total of only 45 days over 14 years. Although the river dischargeremained above 55,000 m3/s 35 days in 2010, it was only 3e10 daysin each of the other 6 years. The studies by Chen et al., (2008) andMoon et al., (2010) were based on the historically averagedmaximum river discharge rate of 60,000 m3/s in the summer over a30e60 day simulation period. It is clear that this assumption doesnot apply to the current conditions of the Changjiang Estuary.
It is unclear whether or not previously proposed physicalmechanisms for the offshore detachment of the low-salinity waterfrom the Changjiang River plume are still valid under the currentenvironmental condition. In particular, would baroclinic instabilitytheory still be applicable to the Changjiang River plume afterconsidering the contribution of tidal mixing? How would theconstant versus variable winds contribute to the offshore low-salinity detachment under the condition with tides? To ourknowledge, these questions have not yet been well examined.
In this paper, we attempt to examine the above questions basedon both observations andmodeling. We have developed a regional-estuarine nested high-resolution FVCOM model and used it tosimulate the Changjiang Estuary plume under a realistic conditionand boundary condition. Unlike previous process-oriented mech-anism modeling experiments, the mechanism-oriented numericalexperiments were conducted after a careful validation with thefield measurement data. The results obtained from our studies notonly avoid an unrealistic condition of tidal currents and mixing butalso provide a more comprehensive view of the baroclinic insta-bility process under the realistic tidal-inclusive condition.
The rest of this paper is organized as follows. In Section 2, a briefdescription of the observations and model is given. In Section 3,observed salinity structure and currents are reported, followed bymodel-data comparisons of salinity and its variability for the real-time simulation. In Section 4, the model-guided process-orientedexperiments are carried out to determine the effects of tidal mixing,uniform and non-uniformwinds on the offshore detachment of thelow-salinity water from the Changjiang River plume on the innershelf 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 theinner shelf of the ECS during July 6e15, 2005. The survey areacovered the 10e50-m isobaths region of the Changjiang Estuary,Hangzhou Bay and Zhoushan Archipelago where the river plumewas located (Kong et al., 2007). Twelve moorings (red circles inFig. 1) were deployed, with labels JS1, JS2, SH1, SH2, SH3, SH4 andSH5 in the CE, ZJ1 and ZJ2 in the Hangzhou Bay, ZJ4, ZJ5 and ZJ6around the Zhoushan Archipelago. The measurement durations atindividual mooring stations are listed in Table 1. Currents, salinity,temperature, and turbidity were measured at each mooring. TheSonTek-ADP®-500 KHz (Acoustic Doppler Profiler, SonTek/YSI, Inc.)was used for current measurements, with a cell size of 1.0 m and asensor depth of about 1.0 m below the sea surface. Time intervalwas 120 s. An OBS-3A (Optical Backscatter Sensor, D&A InstrumentCompany) was used to measure the water turbidity (NTU), tem-perature (�C) and salinity (psu). Three additional meteorologicalstations (black triangles in Fig. 1) were also set up at ShengshanIsland (A), Sheshan Island (B) and Dajishan Island (C) to recordhourly wind speed and direction at a 10-m height over the timeperiod of July 1e31, 2005.
A high-resolution, regional-estuarine nested FVCOMmodel wasemployed to simulate the Changjiang River plume and to examinethe driving mechanism of the offshore detachment of the low-salinity water. FVCOM is a prognostic, unstructured-grid, free-sur-face, three-dimensional (3-D), primitive-equation ocean model
(Chen et al., 2003b, 2004, 2006, 2013). FVCOM combines the ad-vantages of the finite-element method for geometric flexibility andof the finite-difference method for high computational efficiency.The finite-volume approach ensures volume andmass conservationin the individual control volume and entire computational domain,which is critical to simulate the river plume on the inner shelf of theECS.
The regional-estuarine nested FVCOM system used in this studyconsisted of two models: ECS-FVCOM and CE-FVCOM. Thecomputational domain of the regional ocean model ECS-FVCOM,developed originally by Chen et al., (2008), covered the entireECS, Yellow and Bohai Seas, and the Japan/East Sea. The computa-tional domain of the high-resolution estuarine model CE-FVCOM,developed originally by Xue et al., (2009), covered the ChangjiangRiver, Hangzhou Bay, Zhoushan Archipelago and the inner shelf ofthe East China Sea (Fig. 2b). The large East China Sea model was runfirst to provide the forcing condition at the nesting boundary withthe small-domain model. In addition to the nesting boundarycondition, the fine-resolution Changjiang Estuary model was alsodriven by river discharges and surfacewind forcing. This model wasdriven by the river discharge at the upstream end of the Changjiangand Qiantangjiang Rivers, surface meteorological forcing, andlateral boundary forcing on the nested boundary provided by ECS-FVCOM. The river discharge rate for the Changjiang River was basedon the daily measurement records, with a mean value of 39,913m3/s and a standard deviation of 2745 m3/s over the period of June 15 -July 30, 2005. For the Qiantangjiang River, a constant summerclimatological river flux of 1000 m3/s was used. ECS-FVCOMincluded eight major astronomical tidal constituents (M2, S2, K2,N2, K1, O1, P1 and Q1) and continental shelf currents such as theTaiwan Warm Current, the Yellow Sea Warm Currents, the Kur-oshio, etc.
Two improvements have been made to ECS-FVCOM in thisstudy. First, we increased the horizontal resolution in both ECS-FVCOM 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 theECS for CE-FVCOM (Figs. 2-c). Second, we included the dike-groynemodule in CE-FVCOM to resolve the realistic bathymetry and con-struction off the Changjiang River mouth (Ge et al., 2012, 2013).
The numerical simulationwas conducted over the period of June15eJuly 30, 2005, for different cases with and without inclusion oftides and winds. For the experiments with winds, we consideredboth constant and variable wind conditions. The non-uniformwindforcing was provided by the high-resolution Weather Research &
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 isshown 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.)
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e7064
Forecast (WRF) model that was validated via measurements atthree 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, JS1and JS2 over the spring tidal cycle had major axes ranging from 0.6to 1.0 m/s and a direction of 120�e140� (Table 2), indicating thatthe inner shelf of the ECS featured a moderate tide. This observedtidal current value was smaller than the simulated value shown inRong and Li (2012), suggesting that the spring-tide mixingmechanism for the offshore detachment of the low-salinity lensproposed by Rong and Li (2012) and Moon et al., (2010) might notbe applicable for the realistic condition of the Changjiang Riverplume.
Table 2Observed tidal ellipse parameters over the spring tide cycle at mooring stations.
Station Major axis (m/s) Minor axis (m/s) Direction (�)
JS1 0.8 0.45 140JS2 0.6 0.37 143SH2 0.98 0.52 96SH4 0.57 0.28 146SH5 1.17 0.13 83ZJ1 1.47 0.17 104ZJ2 1.11 0.12 113ZJ4 0.48 0.24 156ZJ5 0.74 0.38 131ZJ6 0.4 0.31 120
3.2. Vertical distributions of salinity and velocity
Defining six relative depths: surface (0.0H), 0.2H, 0.4H, middlelayer (0.6H), 0.8H and bottom (1.0H), in which H denotes the totalwater depth, we examined the vertical distribution of currents andsalinity relative to the total local water depth. For example, thetemporal variability of vertical profiles of salinity and velocity atstations SH2, SH3 and SH4 over the spring tidal cycle is shown inFig. 3. At the shallow site SH2, the water was vertically well mixed.The salinity at this site varied with tidal excursion scale, with arange of 10 psu over an ebb-flood tidal cycle. At the relativelydeeper sites SH3 and SH4where the plumewas located, the salinityand velocity profiles featured a two-layer structure: the low-salinity water floating in the upper 5e10-m layer, and salty waterin the lower layer from the middle depth to the bottom. Thisobservational evidence clearly suggested that the Changjiang Riverplume, particularly in the frontal zone, was characterized by a two-layer dynamics system described by Chen et al., (2008), and tidalmixing over the spring tidal cycle was not strong enough to breakdown this feature.
3.3. Wind speeds and directions
The wind velocity at the threemeteorological stations located inFig. 1 varied strongly both temporally and spatially during July of2005 (Fig. 4). The wind direction was mainly northward as a resultof the prevailing summertime monsoon. The wind was relativelyweaker, with a speed of ~5e8 m/s during the period of July 4e15and then became much stronger, with a speed reaching 10e12 m/sduring the period of July 16-28. The WRF-simulated wind speedand directionwas compared with these observations. The results of
25
2520 20
2020 15
6 6.5 7 7.5Time (day)
-6
-4
-2
0
Dep
th (m
)
SH2
30
25
7.6 8Time (day)
-16
-8
0
Dep
th (m
)
SH3
302520
6.5 7 7.5Time (day)
-30
-20
-10
0
Dep
th (m
)
SH4
15 20 25 30 35
Salinity (psu)
dept
h
offshore
inner shelf slope
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.
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70 65
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� forthe wind direction. Without data assimilation, the local WRFmodelwas capable of reasonably reproducing the spatial and temporalvariability of the wind field off the CE over the simulation period.
3.4. CE-FVCOM validation
The CE-FVCOM was validated by comparisons with observedsurface tidal elevation, (tidal and subtidal) currents, and salinity.The comparisons for tidal elevationweremade at 32 gauge stationsalong the coasts of CE, Hangzhou Bay and the offshore islands, andresults for the M2 tidal constituent are listed in Table 3. For the M2
tidal constituent, the mean error was less than 10% in amplitudeand less than 10� in phase. Comparisons for tidal constituents S2, K1and O1 were also performed, and the results were in equallyreasonable agreement. The CE-FVCOM was also capable of simu-lating the vertical distribution and temporal variability of observedcurrent and salinity. A model-data comparison for the water
Fig. 4. Variation of wind vectors during July 2005 at three meteorological stations,Shengshan, Sheshan and Dajishan Islands off the Changjiang Estuary.
velocity (speed and direction) in the CE and inner shelf of the ECSwas conducted and described in Ge et al., (2013). At SH1, SH2, andSH5, for example, the maximum velocity during the ebb tidalperiod was >2.0 m/s in the upper surface layer. A pronounced ve-locity shear was revealed between the upper and lower layers. Theshear was mainly dominated by the combined tidal and river-discharge flows from the CR. There was a relatively weaker veloc-ity shear within the lower layer from the mid-depth to the bottom.In this layer, observed and modeled velocities were consistent, andbothwere dominated by the tidal flow. Themodel-data comparisonresults for salinity at measurement stations were illustrated inFig. 11 of Ge et al., (2013), which showed that the CE-FVCOMcorrectly reproduced the salinity variation over tidal cycles. Anexample was shown in Fig. 5 for the comparison of observed andmodeled tidal-cycle averaged vertical salinity profiles. The modelagreed fairly well with observations. At both stations SH1 and SH3,observed salinity showed a strong vertical stratification: the low-salinity water prevailed in the upper layer and the salty water inthe lower layer. This two-layer feature was well captured by themodel. The model also reproduced the well-mixed vertical profilesand horizontal variation of the salinity at the shallow stations SH2and SH5.
4. Model-guided process experiments
Building on the success of the model validation, we applied thishigh-resolution CE-FVCOM to the examination of the physicalmechanism for the offshore detachment of the low-salinity waterfrom the Changjiang River plume under a realistic condition of July2005 in the CE and inner shelf of the ECS. During this period, themodel detected two major surface detachment events: one on July7 and the other on July 26 (Fig. 6). The first detachment eventoccurred around the eastern region of the CE (left column of Fig. 6),and the second took place in the northeastern region (right columnof Fig. 6). These two detachment events can be viewed more clearlyin the vertical section plots along the main axis of the low-salinitydetachment in Fig. 6. The salinity was characterized by a pro-nounced two-layer system, especially offshore of the 20-m isobath.The vertically well-mixed low salinity was mainly constrainedaround the 10-m isobath.
At 16:00 (GMTþ8), July 7, 2005, the water mass bounded by a30-psu contour detached as a continuous bubble shape from thefrontal zone of the plume. The detached water mass graduallydecreased in size over the ebb tide due to tidal mixing and windstirring over the time period from ebb tide maximum to flood tidemaximum. This detachment occurred around the 50-m isobath
Table 3Model 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 Location M2 (model) M2 (obs) M2 (error)
Longitude (�E) Latitude (�N) H (cm) G (�) H (cm) G (�) (Hmod-Hobs)/Hobs G (�)
Wangpan 121.2927 30.5053 165.1 11.4 171.8 12.1 �4 1Liangque 121.6308 30.2732 107.4 346.3 104.5 347.2 3 1Haiwang 121.5003 30.2131 108.5 356.4 111.4 356.0 �3 0Daishan 122.1985 30.2327 94.7 296.8 91.7 298.1 3 1Changtu 122.3011 30.2502 94.2 294.6 97.0 289.4 �3 �5Ganpu 120.9096 30.3584 218.3 36.5 254.1 47.2 �14 11Zhapu 121.0899 30.5905 193.0 12.7 204.2 28.4 �6 16Jinshan 121.3736 30.7288 160.4 359.4 171.1 8.9 �6 10Longshan 121.5829 30.0844 90.8 343.1 91.6 357.1 �1 14Luhuashan 122.5994 30.8163 111.7 292.0 121.3 287.1 �8 �5Daji 122.1656 30.8099 121.2 311.7 125.2 320.7 �3 9Gaoqiao 121.5906 31.3668 106.1 9.9 110.0 13.0 �4 3Shenjiamen 122.3006 29.4581 117.0 247.9 114.6 267.0 2 19Dinghai 122.0993 30.0008 90.2 283.8 93.8 285.6 �4 2Zhenhai 121.7169 29.9882 79.8 329.8 80.0 324.5 0 �5Yuxinnao 121.8633 30.3530 107.3 330.8 107.0 328.0 0 �3Tangnaoshan 121.9711 30.5855 118.8 325.4 117.6 324.5 1 �1Haiyan 120.9524 30.4969 208.8 19.2 212.0 25.9 �2 7Tanhu 121.6131 30.6216 138.1 350.6 144.6 350.1 �4 0Nanhui 121.8475 30.8680 138.5 333.7 145.6 327.0 �5 �7Waikejiao 121.6235 33.0006 168.9 339.1 183.1 335.0 �8 �4Lusi 121.6109 32.1161 181.5 349.1 171.5 352.5 6 3Baozhen 121.5865 31.5173 112.9 16.5 114.4 10.4 �1 �6Sheshan 122.2256 31.3972 122.5 317.5 113.7 311.5 8 �6Wusong 121.5058 31.3989 104.3 18.2 100.1 13.2 4 �5Hengsha 121.8502 31.2740 116.7 343.7 108.6 343.3 8 0Zhongjun 121.9057 31.0948 123.7 330.2 117.2 / 5 /Jiuduan 122.1692 31.0986 127.0 313.9 122.8 312.4 3 �1Luchaogang 121.8267 30.8269 139.8 333.3 144.9 335.3 �4 2Xize 121.8283 29.6114 119.0 268.7 121.0 264.2 �2 �5Shipu 121.9107 29.1940 151.5 245.0 146.1 253.2 4 8Dachen 121.8816 28.4253 148.5 244.7 158.6 247.8 �6 3
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e7066
where the main frontal zone of the plume was located. A narrowneck of low-salinity water linking the detached water and the mainlow-salinity plume was severed due to tidal mixing and the wind-induced water transport. Around 123�E and 31.5�N, the salinitydistribution around the 50-m isobaths featured an unstable frontalzone, where a eastward detachment occurred. The model-
Bottom
0.8H
0.6H
0.4H
0.2H
Surface
Rel
ativ
e D
epth
FVCOM
Observed
SH1
15 20 25 30 35Salinity(psu)
Bottom
0.8H
0.6H
0.4H
0.2H
Surface
Rel
ativ
e D
epth
SH1
15 20Sal
Fig. 5. Model-data comparisons for tidal-cycle-averaged salinity profiles at SH1, SH2 and SHdotted curves are simulated; blue ones are observed). (For interpretation of the references
produced detached salinity distribution in this region agreed wellwith previous salinity measurement results during summer (Zhuet al., 2003).
At 21:00 (GMTþ8), July 26, 2005, the other bulge-shapeddetachment event occurred off the 20-m isobath around thenortheastern region (123�E, 32�N) off the CE (Fig. 6: right panel),
SH2 SH3
25 30 35inity(psu)
SH2
15 20 25 30 35Salinity(psu)
SH5
3 during the spring tidal cycle, and at SH1, SH2 and SH5 during the neap tidal cycle (redto color in this figure legend, the reader is referred to the web version of this article.)
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 thesection 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 linesindicate the 20-m and 50-m isobaths respectively.
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70 67
where a strong unstable salinity gradient was found near the 20-misobath. The salinity of the detached water was relatively uniform,with relatively high-salinity water forming a northward intrusionfrom the tip of the 50-m isobath. The direction of the intrusionwasidentical with the flood tidal direction (~120�e140�). The bubblesand bulge contours all showed a northwestern flattening along theflood tidal direction as a result of tidal mixing.
To identify and quantify the physical driving mechanism for theoffshore detachment of the low-salinity water from the ChangjiangRiver plume detected in our real-time simulation, we re-ran themodel for the cases with a) only river discharge (Case A), b) riverdischarge plus tides (Case B), c) river discharge, tides and constantwind (Case C), and d) river discharge, tides and variablewinds (CaseD). Case D is the simulation case we have shown in Fig. 6. Theconstant wind in Case C is a July 2015monthlymean value of hourlyWRF-simulated wind velocity.
Case A is an experiment repeated from Chen et al., (2008), butwith reduced river discharges. The results clearly showed thatunder this forcing condition, the plume was characterized by thebulge shape along the 20-m isobath (Fig. 7, upper-left panel), whichwas very similar to the pattern detected in Chen et al.’s (2008)simulation. In contrast to Chen et al. 's (2008) experiments, thehorizontal diffusion coefficient used in our experiment was200 m2/s, about 10 times larger than the value used in their work,and the river discharge rate used in our experiment was 40,000m3/s, 20,000 m3/s smaller than that used in their work. Applying thebaroclinic instability criterion to our case, we have.
Eh ¼ Ahf
ðg0QeÞ2=3z0:11< Ehcz0:34� 0:57
where Eh is horizontal Ekman number, Ehc is the critical Ekmannumber for baroclinic instability, and Qe, Ah, g0 and f are river
discharge per unit length, horizontal eddy viscosity, reducedgravity acceleration and Coriolis parameter, respectively (Chenet al., 2008). This value of Eh still satisfied the baroclinic insta-bility criterion given above. This indicates that even for the casewith reduced river discharge rate and larger horizontal diffusioncoefficient, the plume produced in the river-discharge-only casewas in a baroclinically unstable condition, even though no eddieswere generated in this case. As horizontal diffusion coefficient di-minishes, we saw eddies form along the frontal zone as a result ofbaroclinic instability. Our focus here is on examining the physicalmechanism driving the offshore low-salinity detachment discov-ered in Case D, where the horizontal diffusion coefficient wasspecified by Smagorinsky's turbulence closure scheme. For thisreason, we did not alter the horizontal diffusion coefficient tomatch that used in Chen et al., (2008). Based on the lateral mixingcoefficient used in our case, the baroclinic instability seemed not tobe a key physical mechanism in producing the large isolatedoffshore low-salinity detachment found in Case D.
When tidal forcingwas added in Case B, the bulge shape along the20-m isobaths was significantly smoothed as a result of enhancedtidally induced vertical mixing, even though the plume still satisfiedthe baroclinic instability criterion (Fig. 7, lower left panel). Tidalmixingwasmainly causedby the tidal current shear near the bottom,and the tidally induced mixed layer above the bottom agreed withthe analytical solution derived by Chen and Beardsley (1995).Balanced by the buoyancy input and turbulent dissipation, the tidallyinduced mixing depth (hm) could be determined by.
hm ¼�16gdDTU3
N2p
�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
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 caseswith 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 variableand spatially non-uniform winds.
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e7068
time period DT, the typical value of which is suggested to be3.7� 10�3 by Simpson and Hunter (1974); DT is suggested to be ~14days in the strong stratification case (Lee and Beardsley, 1999). U isthe typical tidally averaged and vertically averaged current (with atypical value of ~0.5 m/s along the plume); N is the BrunteV€ais€al€afrequency (with a typical value of 0.02 s�1 for the strong stratifi-cation case in the plume frontal zone). The calculated mixing depthwas ~22 m, which is identical to the isobath of the northern plumeregion shown in the lower left panel of Fig. 7. This suggests that thevertically well-mixed salinity patterns in the region shallower than~22 m was caused by tidal mixing. The baroclinic instability waslikely to occur in the deeper region of ~50 m, where the frontalstructure was characterized by a two-layer dynamical system. Itshould be noted that the tidal mixing depth determined by Moonet al., (2010) using the same formula was 36 m, about 14 mhigher than the value we found in our experiments. Since the initialcondition of stratifications was different and the model forcing wasnot the same, it was not surprising to see such a difference betweentwo models.
Adding a constant monthly-averaged wind to Case B, weexamined the impact of the wind on the plume variability for CaseC. The southwesterly wind produced an offshore Ekman transport,which advected the plume offshore. The offshore frontal movingspeed ufront satisfied the Ekman transport theory given as.
ufront ¼tw
rfhc
where tw, f, and hc are wind shear stress, Coriolis parameter andthickness of plume front, respectively (Fong and Geyer, 2001). The
interaction of the uniform wind and non-uniform plume velocitytended to enhance the plume spatial variability, which wasconsistent with the finding reported by Chen et al., (2008). Underthe conditions given in our experiment, however, no offshore low-salinity detachment occurred in this case.
The situation significantly changed when a variable wind wasused in Case D. Given the same river discharge rate, tidal forcingand lateral mixing coefficient, the change of the speed and direc-tion of the wind significantly enhanced the temporal and spatialvariability of the plume. As a result, the low-salinity water in thenorthern plume area was detached offshore from the frontal zone.The detachment under this condition was relatively strong. A largebody of the low-salinity water was detached as an isolated lens,similar to what has often been observed in that region. It is clearthat the variable wind played a key role in enhancing the instabilityand spatial-temporal variability of the plume. Since the formationof the isolated low-salinity lens occurred under the wind forcingcondition, the flow within the lens was not an eddy. The mecha-nism driving the offshore-detachment was very similar to the casedetected by Chen (2000) on the South Atlantic Bight shelf wherethe isolated low-salinity lens was often observed under theupwelling-favorable wind condition. He found that the detachmentprocess happened in two stages. First, the spatially non-uniformresponse of current to the upwelling-favorable wind enhances awavelike frontal shape at the outer edge of the frontal zone. Then,the isolated low-salinity lenses formed at the crest, when water onthe shoreward side of the crest was displaced by relatively high-salinity water advected from the upstream trough south of thecrest and diffused upward from the deep region. In our case, we
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 ofJuly 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. (Forinterpretation 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 69
found that the upwelling-favorable wind-induced Ekman transportadvected the plume offshore, which caused the plume to becomemore baroclinically unstable. Variation of the wind enhanced thenon-uniform response of current to the wind, and detachmentoccurred at the bulge-shaped (crest) region of the plume, whichproceeded in two steps as described by Chen (2000).
Unlike the constant wind case, the non-uniform wind causedthe spatial-varying offshore movement speed of the plume, whichdirectly enhanced the spatial variability of the plume as that foundin the uniform wind condition. This can be clearly seen in Fig. 8,which shows that the offshore Ekman transport varied significantlyalong the frontal zone of the plume when the variable wind wasused. During the first detachment period on July 7, 2005, theoffshore Ekman transport was relatively larger around 30.5�N thanin the surrounding area. During the second detachment period, themaximum offshore Ekman transport shifted to the northern regionat around 31.5�N. In this event, the surface windwasmuch strongerthan in the previous event. Non-uniform offshore Ekman transportsobserved in both events played a key role in enhancing the along-frontal variability of the plume, and thus led to the offshoredetachment of the low-salinity water from the plume.
5. Summary
The temporal and spatial variability of the Changjiang Riverplume was examined using both field measurements and aregional-estuarine nested high-resolution model. The observationsshowed that due to anthropogenic activities, the Changjiang Riverdischarge rate in summer has been significantly reduced. Bothsalinity and velocity measurements showed that the plume wascharacterized by a two-layer structure: the low-salinity waterfloating in the upper 5e10-m layer, and salty water occupying thelower layer from the middle depth to the bottom.
The high-resolution ECS-FVCOM and CE-FVCOM nested modelsystem was capable of simulating the vertical distribution andtemporal variability of the observed current and salinity of theChangjiang River plume. The real-time simulation over theobserved period revealed two significant offshore detachments ofthe low-salinity water from the plume. Process-oriented
experiments suggest that the non-uniform distribution and vari-ability of the wind played a key role in driving these two offshoredetachment events. The spatially non-uniform wind field causedthe spatially varying offshore movement speed of the plume. Thelarge spatial variability of the plume caused by the non-uniformoffshore Ekman transport increased the plume instability.Although tidal mixing tended to stabilize the frontal structure ofthe plume, the isolated low-salinity lenses could be formed at thebulge-shaped area of the plume whenwater on the shoreward sideof the crest was displaced by relatively high-salinity water advectedfrom the upstream trough south of the crest and diffused upwardfrom the deep region.
Our finding was consistent with previous theories suggested byChen (2000) and Chen et al., (2008). The key difference is that ourcase was done with a larger horizontal diffusion coefficient andtidal mixing. Under this condition, we found that the spatially non-uniform wind could be more critical than the baroclinic instabilityin causing the offshore low-salinity detachment from the Chang-jiang River plume. In our experiments, we did not find the offshoredetachment during the spring tidal period, which was suggested byMoon et al., (2010) and Rong and Li (2012). One reason is that thetidal currents have significantly changed since dikes and groyneswere constructed off the Changjiang Estuary. Our measurementsshowed that the magnitude of the M2 tidal currents at the JS2 sitewas 0.6 m/s during the spring period of July 2005. This spring tidalcurrent magnitude changed significantly with stratification. Itdropped to 0.46 m/s in October 2005 and 0.53 m/s in May 2006 asstratification became weak. This site was very close to the M2 andM4 stations discussed in Rong and Li (2012), and the computedmagnitude of regular tidal currents in their model was 0.68e0.8 m/s, which is the same as or larger than the observed spring tidalcurrent magnitudes. As a result of such environmental change,whether or not the SATM will still be applicable to the ChangjiangEstuary plume needs further validation via comparisons.
Acknowledgments
Jianzhong Ge and Pingxing Ding are supported by the Fund fromNatural Sciences Foundation of China (No. 41021064; No.
J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e7070
41306080), Public Service Programme of State Ocean Administra-tion (No. 201205017-2), China Science and Technology SupportProgramme (No. 2013BAB12B03-Z1) and the SKELC fund (No.SKLEC-2011RCDW03). The authors would like to express theirappreciation to two anonymous reviewers, who provided veryhelpful suggestions to improve the manuscript.
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