Low-Salinity Plume Detachment Under Non-Uniform Summer Wind Off the Changjiang Estuary

Estuarine, Coastal and Shelf Science 156 (2015) 61e70

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Estuarine, Coastal and Shelf Science

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Low-salinity plume detachment under non-uniform summer wind off the 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 China b School for Marine Science and Technology, University of Massachusetts-Dartmouth, New Bedford, MA 02744, United States article info abstract

Article history: In the past, two physical mechanisms, baroclinic instability (BI) and strong asymmetric tidal mixing Accepted 21 October 2014 (SATM) during the spring tidal period, were proposed for the offshore detachment of the low-salinity Available online 31 October 2014 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, free- Keywords: surface, primitive-equation, Finite-Volume Community Ocean Model (FVCOM). The observed currents plume detachment and salinities showed that the plume was characterized by a two-layer system, in which the upper layer non-uniform wind is mainly driven by the river discharge-induced buoyancy flow and the lower layer is predominantly Ekman transport fi Changjiang Estuary controlled by tidal mixing and recti cation. 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.

1. Introduction discharge from the CR produces a strong low-salinity plume around the Changjiang Estuary and adjacent inner shelf of the East China The low-salinity plume is a common coastal and oceanic phys- Sea (ECS), which is a permanent local physical dynamic phenom- ical phenomenon in river-dominated estuaries such as the Amazon enon (Mao et al., 1963; Beardsley et al., 1985; Su and Wang, 1989; River (Lentz, 1995), the Chesapeake Bay (Lentz, 2004; Lentz and Chen et al., 1999). The intensity and structure of this plume vary Largier, 2006), the Connecticut River mouth (O'Donnell, 1990), signiﬁcantly with season: weak and generally trapped along the the southeast U.S. continental shelf (Kourafalou et al., 1996a; 1996b, coast during the dry season but stronger and more unstable during Chen et al., 1999, 2000), the Columbia River (Hickey et al., 1998) and the wet season. During the wet season, an isolated low-salinity lens the Changjiang River (CR) (Beardsley et al., 1985; Chen et al., 2008; often occurs as a result of the detachment process along the frontal Xue et al., 2009). The CR, one of largest rivers in the world, has a zone, which directly affects the local and regional ecosystem and typical freshwater discharge of ~40000 m3/s during summer (wet sediment transport on the inner shelf of the ECS (Tian et al., 1993a, season) and ~10000 m3/s during winter (dry season) (Beardsley 1993b; Chen et al., 1999, 2003a; Gao et al., 2008, 2009). et al., 1985). The Changjiang Estuary (CE) is characterized by Several studies have been conducted to examine the physical shallow shoals, islands and multiple outlet channels in the river mechanisms driving the offshore detachment of low-salinity water mouth, and submarine canyons in the outer estuary, with dikes and from the Changjiang River plume (Chen et al., 2008; Moon et al., groins in the river mouth region (Fig. 1). The abundant freshwater 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 * Corresponding author. E-mail addresses: [email protected] (J. Ge), [email protected] plume. Their results show that baroclinic instability of the plume (P. Ding), [email protected] (C. Chen). could lead to the offshore detachment of the low-salinity water http://dx.doi.org/10.1016/j.ecss.2014.10.012 0272-7714/© 2014 Elsevier Ltd. All rights reserved. 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 and tidal forcing. In Table 2 of Rong and Li (2012), the model- inner shelf of the ECS. This instability could be enhanced under the produced tidal currents were compared with observations, and southwesterly monsoon wind condition, which could produce a the model overestimated the magnitude of tidal currents by 17%, large, isolated low-salinity lens and advect it to the offshore ECS 47% and 60% at MS, SDS, and M2 stations, respectively. Since the region. The studies by Chen et al., (2008) were carried out under a SATM mechanism was based on enhanced tidal current and mixing condition without inclusion of astronomical tides. The tides are during the spring tide, whether or not it could be applied to the dominant features in the Changjiang Estuary, where the typical realistic condition off the Changjiang Estuary needs a further vali- amplitude of tidal currents is about 0.6e1.0 m/s. In the shallow dation via observed tidal currents and salinity within the plume region (less than 20 m), the water is usually vertically well mixed. Is frontal zone. the finding reported by Chen et al., (2008) still valid when tidal The environmental condition of the Changjiang Estuary has currents and mixing are taken into consideration? To our knowl- been significantly changed in recent years. This estuary has been edge, this question has not been explored since their work. strongly impacted by multiple stressors, including the Three Gorges Alternative studies were reported by Rong and Li (2012) and Dam in the upstream Changjiang River (Yang et al., 2007, 2011), the Moon et al., (2010) using a coarse-resolution structured-grid ocean Deep Waterway project in the North Passage (Ge et al., 2012) and model with a focus on the contribution of tidal mixing on the coastal land reclamations in Hengsha Shoal and East Nanhui Shoal offshore detachment of the low-salinity water from the Changjiang (Wei et al., 2014). These anthropogenic activities have resulted in River plume. Although this coarse-resolution model did not resolve fast changing estuarine dynamics. Due to the regulation of the the baroclinic instability process, the simulation results suggested Three Gorges Dam and water withdrawal along the Changjiang that tidally induced vertical mixing via dissipation around the River, the net freshwater input to the estuary has decreased in the plume was strong enough to overcome the buoyancy effect during last decade (Yang et al., 2011). We have collected daily river the spring tide, which could lead to the offshore detachment of the discharge data from January 2000 to June 2014 at Datong Station, low-salinity water along the frontal zone. Rong and Li (2012) used which is the nearest hydrology station to the Changjiang River the same model as Moon et al., (2010), and the configurations in Estuary, and statistics of this time series data show that the both their experiments were very similar in horizontal resolution maximum value during the summer peak period could still reach 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 for Table 1 a total of only 45 days over 14 years. Although the river discharge 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 remained above 55,000 m3/s 35 days in 2010, it was only 3e10 days arranged during the neap tidal cycle. 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 Spring tide Neap tide maximum river discharge rate of 60,000 m3/s in the summer over a 09:00 17:00 09:00 17:00 30e60 day simulation period. It is clear that this assumption does Jul 6e11:00 Jul 7e19:00 Jul 12e11:00 Jul 13e19:00 not apply to the current conditions of the Changjiang Estuary. Jul 7 Jul 8 Jul 13 Jul 14 It is unclear whether or not previously proposed physical JS1 CC C mechanisms for the offshore detachment of the low-salinity water JS2 SH1 CC from the Changjiang River plume are still valid under the current SH2 CC environmental condition. In particular, would baroclinic instability SH3 C theory still be applicable to the Changjiang River plume after SH4 C considering the contribution of tidal mixing? How would the SH5 CC CC constant versus variable winds contribute to the offshore low- ZJ1 ZJ2 CC salinity detachment under the condition with tides? To our ZJ4 C knowledge, these questions have not yet been well examined. ZJ5 C In this paper, we attempt to examine the above questions based ZJ6 C on both observations and modeling. We have developed a regional- estuarine 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 mech- (Chen et al., 2003b, 2004, 2006, 2013). FVCOM combines the ad- anism modeling experiments, the mechanism-oriented numerical vantages of the finite-element method for geometric flexibility and experiments were conducted after a careful validation with the of the finite-difference method for high computational efficiency. field measurement data. The results obtained from our studies not The finite-volume approach ensures volume and mass conservation only avoid an unrealistic condition of tidal currents and mixing but in the individual control volume and entire computational domain, also provide a more comprehensive view of the baroclinic insta- which is critical to simulate the river plume on the inner shelf of the bility process under the realistic tidal-inclusive condition. ECS. The rest of this paper is organized as follows. In Section 2, a brief The regional-estuarine nested FVCOM system used in this study description of the observations and model is given. In Section 3, consisted of two models: ECS-FVCOM and CE-FVCOM. The observed salinity structure and currents are reported, followed by computational domain of the regional ocean model ECS-FVCOM, model-data comparisons of salinity and its variability for the real- developed originally by Chen et al., (2008), covered the entire time simulation. In Section 4, the model-guided process-oriented ECS, Yellow and Bohai Seas, and the Japan/East Sea. The computa- experiments are carried out to determine the effects of tidal mixing, tional domain of the high-resolution estuarine model CE-FVCOM, uniform and non-uniform winds on the offshore detachment of the developed originally by Xue et al., (2009), covered the Changjiang low-salinity water from the Changjiang River plume on the inner River, Hangzhou Bay, Zhoushan Archipelago and the inner shelf of shelf of the ECS. The conclusions are summarized in Section 5. 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 2. Field measurements and design of model experiments the small-domain model. In addition to the nesting boundary condition, the fine-resolution Changjiang Estuary model was also An interdisciplinary cruise was conducted in the CE and the driven by river discharges and surface wind forcing. This model was inner shelf of the ECS during July 6e15, 2005. The survey area driven by the river discharge at the upstream end of the Changjiang covered the 10e50-m isobaths region of the Changjiang Estuary, and Qiantangjiang Rivers, surface meteorological forcing, and Hangzhou Bay and Zhoushan Archipelago where the river plume lateral boundary forcing on the nested boundary provided by ECS- was located (Kong et al., 2007). Twelve moorings (red circles in FVCOM. The river discharge rate for the Changjiang River was based Fig. 1) were deployed, with labels JS1, JS2, SH1, SH2, SH3, SH4 and on the daily measurement records, with a mean value of 39,913 m3/ SH5 in the CE, ZJ1 and ZJ2 in the Hangzhou Bay, ZJ4, ZJ5 and ZJ6 s and a standard deviation of 2745 m3/s over the period of June 15 - around the Zhoushan Archipelago. The measurement durations at July 30, 2005. For the Qiantangjiang River, a constant summer individual mooring stations are listed in Table 1. Currents, salinity, climatological river flux of 1000 m3/s was used. ECS-FVCOM temperature, and turbidity were measured at each mooring. The included eight major astronomical tidal constituents (M2, S2, K2, ® SonTek-ADP -500 KHz (Acoustic Doppler Profiler, SonTek/YSI, Inc.) N2, K1, O1, P1 and Q1) and continental shelf currents such as the was used for current measurements, with a cell size of 1.0 m and a Taiwan Warm Current, the Yellow Sea Warm Currents, the Kur- sensor depth of about 1.0 m below the sea surface. Time interval oshio, etc. was 120 s. An OBS-3A (Optical Backscatter Sensor, D&A Instrument Two improvements have been made to ECS-FVCOM in this Company) was used to measure the water turbidity (NTU), tem- study. First, we increased the horizontal resolution in both ECS- perature (C) and salinity (psu). Three additional meteorological FVCOM and CE-FVCOM off the Changjiang Estuary (Ge et al., stations (black triangles in Fig. 1) were also set up at Shengshan 2013), with a grid size as fine as 250 m in the inner shelf of the Island (A), Sheshan Island (B) and Dajishan Island (C) to record ECS for CE-FVCOM (Figs. 2-c). Second, we included the dike-groyne hourly wind speed and direction at a 10-m height over the time module in CE-FVCOM to resolve the realistic bathymetry and con- period of July 1e31, 2005. struction off the Changjiang River mouth (Ge et al., 2012, 2013). A high-resolution, regional-estuarine nested FVCOM model was The numerical simulation was conducted over the period of June employed to simulate the Changjiang River plume and to examine 15eJuly 30, 2005, for different cases with and without inclusion of the driving mechanism of the offshore detachment of the low- tides and winds. For the experiments with winds, we considered salinity water. FVCOM is a prognostic, unstructured-grid, free-sur- both constant and variable wind conditions. The non-uniform wind face, three-dimensional (3-D), primitive-equation ocean model 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 3.2. Vertical distributions of salinity and velocity three meteorological stations (Ge et al., 2013). Defining six relative depths: surface (0.0H), 0.2H, 0.4H, middle 3. Observed salinity structure and model-data comparisons 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 3.1. Tidal elevations and currents salinity relative to the total local water depth. For example, the temporal variability of vertical profiles of salinity and velocity at Observed tidal current ellipses at observation stations SH4, JS1 stations SH2, SH3 and SH4 over the spring tidal cycle is shown in and JS2 over the spring tidal cycle had major axes ranging from 0.6 Fig. 3. At the shallow site SH2, the water was vertically well mixed. to 1.0 m/s and a direction of 120e140 (Table 2), indicating that The salinity at this site varied with tidal excursion scale, with a fl the inner shelf of the ECS featured a moderate tide. This observed range of 10 psu over an ebb- ood tidal cycle. At the relatively tidal current value was smaller than the simulated value shown in deeper sites SH3 and SH4 where the plume was located, the salinity fi Rong and Li (2012), suggesting that the spring-tide mixing and velocity pro les featured a two-layer structure: the low- fl e mechanism for the offshore detachment of the low-salinity lens salinity water oating in the upper 5 10-m layer, and salty water proposed by Rong and Li (2012) and Moon et al., (2010) might not in the lower layer from the middle depth to the bottom. This be applicable for the realistic condition of the Changjiang River observational evidence clearly suggested that the Changjiang River plume. plume, particularly in the frontal zone, was characterized by a two- layer dynamics system described by Chen et al., (2008), and tidal mixing over the spring tidal cycle was not strong enough to break Table 2 down this feature. Observed tidal ellipse parameters over the spring tide cycle at mooring stations.

Station Major axis (m/s) Minor axis (m/s) Direction () 3.3. Wind speeds and directions JS1 0.8 0.45 140 JS2 0.6 0.37 143 The wind velocity at the three meteorological stations located in SH2 0.98 0.52 96 Fig. 1 varied strongly both temporally and spatially during July of SH4 0.57 0.28 146 2005 (Fig. 4). The wind direction was mainly northward as a result SH5 1.17 0.13 83 ZJ1 1.47 0.17 104 of the prevailing summertime monsoon. The wind was relatively ZJ2 1.11 0.12 113 weaker, with a speed of ~5e8 m/s during the period of July 4e15 ZJ4 0.48 0.24 156 and then became much stronger, with a speed reaching 10e12 m/s ZJ5 0.74 0.38 131 during the period of July 16-28. The WRF-simulated wind speed ZJ6 0.4 0.31 120 and direction was compared with these observations. The results of J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70 65

SH2 SH3 SH4 0 0 0 20 25 2530 30 -2 20 15 20 -10 -8

-4 25 -20

20 Depth (m) Depth (m) -16 Depth (m)

25 -6 -30

6 6.5 7 7.5 7.6 8 6.5 7 7.5 Time (day) Time (day) Salinity (psu) Time (day) inner shelf slope 15 20 25 30 35 depth 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. velocity (speed and direction) in the CE and inner shelf of the ECS (2013). The RMS error was 2.1 m/s for the wind speed and 23 for was conducted and described in Ge et al., (2013). At SH1, SH2, and the wind direction. Without data assimilation, the local WRF model SH5, for example, the maximum velocity during the ebb tidal was capable of reasonably reproducing the spatial and temporal period was >2.0 m/s in the upper surface layer. A pronounced ve- variability of the wind field off the CE over the simulation period. locity shear was revealed between the upper and lower layers. The shear was mainly dominated by the combined tidal and river- 3.4. CE-FVCOM validation discharge flows from the CR. There was a relatively weaker velocity shear within the lower layer from the mid-depth to the bottom. The CE-FVCOM was validated by comparisons with observed In this layer, observed and modeled velocities were consistent, and surface tidal elevation, (tidal and subtidal) currents, and salinity. both were dominated by the tidal flow. The model-data comparison The comparisons for tidal elevation were made at 32 gauge stations results for salinity at measurement stations were illustrated in along the coasts of CE, Hangzhou Bay and the offshore islands, and Fig. 11 of Ge et al., (2013), which showed that the CE-FVCOM results for the M2 tidal constituent are listed in Table 3. For the M2 correctly reproduced the salinity variation over tidal cycles. An tidal constituent, the mean error was less than 10% in amplitude example was shown in Fig. 5 for the comparison of observed and and less than 10 in phase. Comparisons for tidal constituents S2, K1 modeled tidal-cycle averaged vertical salinity profiles. The model and O1 were also performed, and the results were in equally agreed fairly well with observations. At both stations SH1 and SH3, reasonable agreement. The CE-FVCOM was also capable of simu- observed salinity showed a strong vertical stratification: the low- lating the vertical distribution and temporal variability of observed salinity water prevailed in the upper layer and the salty water in current and salinity. A model-data comparison for the water 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

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 ﬂ Fig. 4. Variation of wind vectors during July 2005 at three meteorological stations, stirring over the time period from ebb tide maximum to ood tide Shengshan, Sheshan and Dajishan Islands off the Changjiang Estuary. 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 ( )isM2 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 41 Liangque 121.6308 30.2732 107.4 346.3 104.5 347.2 3 1 Haiwang 121.5003 30.2131 108.5 356.4 111.4 356.0 30 Daishan 122.1985 30.2327 94.7 296.8 91.7 298.1 3 1 Changtu 122.3011 30.2502 94.2 294.6 97.0 289.4 3 5 Ganpu 120.9096 30.3584 218.3 36.5 254.1 47.2 14 11 Zhapu 121.0899 30.5905 193.0 12.7 204.2 28.4 616 Jinshan 121.3736 30.7288 160.4 359.4 171.1 8.9 610 Longshan 121.5829 30.0844 90.8 343.1 91.6 357.1 114 Luhuashan 122.5994 30.8163 111.7 292.0 121.3 287.1 8 5 Daji 122.1656 30.8099 121.2 311.7 125.2 320.7 39 Gaoqiao 121.5906 31.3668 106.1 9.9 110.0 13.0 43 Shenjiamen 122.3006 29.4581 117.0 247.9 114.6 267.0 2 19 Dinghai 122.0993 30.0008 90.2 283.8 93.8 285.6 42 Zhenhai 121.7169 29.9882 79.8 329.8 80.0 324.5 0 5 Yuxinnao 121.8633 30.3530 107.3 330.8 107.0 328.0 0 3 Tangnaoshan 121.9711 30.5855 118.8 325.4 117.6 324.5 1 1 Haiyan 120.9524 30.4969 208.8 19.2 212.0 25.9 27 Tanhu 121.6131 30.6216 138.1 350.6 144.6 350.1 40 Nanhui 121.8475 30.8680 138.5 333.7 145.6 327.0 5 7 Waikejiao 121.6235 33.0006 168.9 339.1 183.1 335.0 8 4 Lusi 121.6109 32.1161 181.5 349.1 171.5 352.5 6 3 Baozhen 121.5865 31.5173 112.9 16.5 114.4 10.4 1 6 Sheshan 122.2256 31.3972 122.5 317.5 113.7 311.5 8 6 Wusong 121.5058 31.3989 104.3 18.2 100.1 13.2 4 5 Hengsha 121.8502 31.2740 116.7 343.7 108.6 343.3 8 0 Zhongjun 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 1 Luchaogang 121.8267 30.8269 139.8 333.3 144.9 335.3 42 Xize 121.8283 29.6114 119.0 268.7 121.0 264.2 2 5 Shipu 121.9107 29.1940 151.5 245.0 146.1 253.2 4 8 Dachen 121.8816 28.4253 148.5 244.7 158.6 247.8 63 where the main frontal zone of the plume was located. A narrow produced detached salinity distribution in this region agreed well neck of low-salinity water linking the detached water and the main with previous salinity measurement results during summer (Zhu low-salinity plume was severed due to tidal mixing and the wind- et al., 2003). induced water transport. Around 123E and 31.5N, the salinity At 21:00 (GMTþ8), July 26, 2005, the other bulge-shaped distribution around the 50-m isobaths featured an unstable frontal detachment event occurred off the 20-m isobath around the zone, where a eastward detachment occurred. The model- northeastern region (123E, 32N) off the CE (Fig. 6: right panel),

Surface SH1 SH2 SH3 0.2H

0.4H

0.6H FVCOM Relative Depth 0.8H Observed Bottom

Surface SH1 SH2 SH5 0.2H

0.4H

0.6H Relative Depth 0.8H

Bottom 15 20 25 30 35 15 20 25 30 35 15 20 25 30 35 Salinity(psu) Salinity(psu) Salinity(psu)

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 discharge per unit length, horizontal eddy viscosity, reduced isobath. The salinity of the detached water was relatively uniform, gravity acceleration and Coriolis parameter, respectively (Chen with relatively high-salinity water forming a northward intrusion et al., 2008). This value of Eh still satisfied the baroclinic insta- from the tip of the 50-m isobath. The direction of the intrusion was bility criterion given above. This indicates that even for the case identical with the flood tidal direction (~120e140). The bubbles with reduced river discharge rate and larger horizontal diffusion and bulge contours all showed a northwestern flattening along the coefficient, the plume produced in the river-discharge-only case flood tidal direction as a result of tidal mixing. was in a baroclinically unstable condition, even though no eddies To identify and quantify the physical driving mechanism for the were generated in this case. As horizontal diffusion coefficient di- offshore detachment of the low-salinity water from the Changjiang minishes, we saw eddies form along the frontal zone as a result of River plume detected in our real-time simulation, we re-ran the baroclinic instability. Our focus here is on examining the physical model for the cases with a) only river discharge (Case A), b) river mechanism driving the offshore low-salinity detachment discov- discharge plus tides (Case B), c) river discharge, tides and constant ered in Case D, where the horizontal diffusion coefficient was wind (Case C), and d) river discharge, tides and variable winds (Case specified by Smagorinsky's turbulence closure scheme. For this D). Case D is the simulation case we have shown in Fig. 6. The reason, we did not alter the horizontal diffusion coefficient to constant wind in Case C is a July 2015 monthly mean value of hourly match that used in Chen et al., (2008). Based on the lateral mixing WRF-simulated wind velocity. coefficient used in our case, the baroclinic instability seemed not to Case A is an experiment repeated from Chen et al., (2008), but be a key physical mechanism in producing the large isolated with reduced river discharges. The results clearly showed that offshore low-salinity detachment found in Case D. under this forcing condition, the plume was characterized by the When tidal forcing was added in Case B, the bulge shape along the bulge shape along the 20-m isobath (Fig. 7, upper-left panel), which 20-m isobaths was significantly smoothed as a result of enhanced was very similar to the pattern detected in Chen et al.’s (2008) tidally induced vertical mixing, even though the plume still satisfied simulation. In contrast to Chen et al. 's (2008) experiments, the the baroclinic instability criterion (Fig. 7, lower left panel). Tidal horizontal diffusion coefficient used in our experiment was mixing was mainly caused by the tidal current shear near the bottom, 200 m2/s, about 10 times larger than the value used in their work, and the tidally induced mixed layer above the bottom agreed with and the river discharge rate used in our experiment was 40,000 m3/ the analytical solution derived by Chen and Beardsley (1995). s, 20,000 m3/s smaller than that used in their work. Applying the Balanced by the buoyancy input and turbulent dissipation, the tidally baroclinic instability criterion to our case, we have. induced mixing depth (hm) could be determined by.

= 3 1 3 ¼ Ahf z : < z : : 16gdDTU Eh 0 11 Ehc 0 34 0 57 h ¼ 0 2=3 m 2 ðg QeÞ N p where Eh is horizontal Ekman number, Ehc is the critical Ekman where g is the bottom friction coefﬁcient, usually taken as 0.0025; 0 number for baroclinic instability, and Qe, Ah, g and f are river 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 interaction of the uniform wind and non-uniform plume velocity 3.7 10 3 by Simpson and Hunter (1974); DT is suggested to be ~14 tended to enhance the plume spatial variability, which was days in the strong stratification case (Lee and Beardsley, 1999). U is consistent with the finding reported by Chen et al., (2008). Under the typical tidally averaged and vertically averaged current (with a the conditions given in our experiment, however, no offshore low- typical value of ~0.5 m/s along the plume); N is the BrunteVais€ al€ a€ salinity detachment occurred in this case. frequency (with a typical value of 0.02 s 1 for the strong stratifi- The situation significantly changed when a variable wind was cation case in the plume frontal zone). The calculated mixing depth used in Case D. Given the same river discharge rate, tidal forcing was ~22 m, which is identical to the isobath of the northern plume and lateral mixing coefficient, the change of the speed and direc- region shown in the lower left panel of Fig. 7. This suggests that the tion of the wind significantly enhanced the temporal and spatial vertically well-mixed salinity patterns in the region shallower than variability of the plume. As a result, the low-salinity water in the ~22 m was caused by tidal mixing. The baroclinic instability was northern plume area was detached offshore from the frontal zone. likely to occur in the deeper region of ~50 m, where the frontal The detachment under this condition was relatively strong. A large structure was characterized by a two-layer dynamical system. It body of the low-salinity water was detached as an isolated lens, should be noted that the tidal mixing depth determined by Moon similar to what has often been observed in that region. It is clear et al., (2010) using the same formula was 36 m, about 14 m that the variable wind played a key role in enhancing the instability higher than the value we found in our experiments. Since the initial and spatial-temporal variability of the plume. Since the formation condition of stratifications was different and the model forcing was of the isolated low-salinity lens occurred under the wind forcing not the same, it was not surprising to see such a difference between condition, the flow within the lens was not an eddy. The mecha- two models. nism driving the offshore-detachment was very similar to the case Adding a constant monthly-averaged wind to Case B, we detected by Chen (2000) on the South Atlantic Bight shelf where examined the impact of the wind on the plume variability for Case the isolated low-salinity lens was often observed under the C. The southwesterly wind produced an offshore Ekman transport, upwelling-favorable wind condition. He found that the detachment which advected the plume offshore. The offshore frontal moving process happened in two stages. First, the spatially non-uniform speed ufront satisfied the Ekman transport theory given as. response of current to the upwelling-favorable wind enhances a wavelike frontal shape at the outer edge of the frontal zone. Then, ¼ tw ufront the isolated low-salinity lenses formed at the crest, when water on rfhc the shoreward side of the crest was displaced by relatively high- where tw, f, and hc are wind shear stress, Coriolis parameter and salinity water advected from the upstream trough south of the thickness of plume front, respectively (Fong and Geyer, 2001). 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 experiments suggest that the non-uniform distribution and vari- advected the plume offshore, which caused the plume to become ability of the wind played a key role in driving these two offshore more baroclinically unstable. Variation of the wind enhanced the detachment events. The spatially non-uniform wind field caused non-uniform response of current to the wind, and detachment the spatially varying offshore movement speed of the plume. The occurred at the bulge-shaped (crest) region of the plume, which large spatial variability of the plume caused by the non-uniform proceeded in two steps as described by Chen (2000). offshore Ekman transport increased the plume instability. Unlike the constant wind case, the non-uniform wind caused Although tidal mixing tended to stabilize the frontal structure of the spatial-varying offshore movement speed of the plume, which the plume, the isolated low-salinity lenses could be formed at the directly enhanced the spatial variability of the plume as that found bulge-shaped area of the plume when water on the shoreward side in the uniform wind condition. This can be clearly seen in Fig. 8, of the crest was displaced by relatively high-salinity water advected which shows that the offshore Ekman transport varied significantly from the upstream trough south of the crest and diffused upward along the frontal zone of the plume when the variable wind was from the deep region. used. During the first detachment period on July 7, 2005, the Our finding was consistent with previous theories suggested by offshore Ekman transport was relatively larger around 30.5N than Chen (2000) and Chen et al., (2008). The key difference is that our in the surrounding area. During the second detachment period, the case was done with a larger horizontal diffusion coefficient and maximum offshore Ekman transport shifted to the northern region tidal mixing. Under this condition, we found that the spatially non- at around 31.5N. In this event, the surface wind was much stronger uniform wind could be more critical than the baroclinic instability than in the previous event. Non-uniform offshore Ekman transports in causing the offshore low-salinity detachment from the Chang- observed in both events played a key role in enhancing the along- jiang River plume. In our experiments, we did not find the offshore frontal variability of the plume, and thus led to the offshore detachment during the spring tidal period, which was suggested by detachment of the low-salinity water from the plume. Moon et al., (2010) and Rong and Li (2012). One reason is that the tidal currents have significantly changed since dikes and groynes 5. Summary were constructed off the Changjiang Estuary. Our measurements showed that the magnitude of the M2 tidal currents at the JS2 site The temporal and spatial variability of the Changjiang River was 0.6 m/s during the spring period of July 2005. This spring tidal plume was examined using both field measurements and a current magnitude changed significantly with stratification. It regional-estuarine nested high-resolution model. The observations dropped to 0.46 m/s in October 2005 and 0.53 m/s in May 2006 as showed that due to anthropogenic activities, the Changjiang River stratification became weak. This site was very close to the M2 and discharge rate in summer has been significantly reduced. Both M4 stations discussed in Rong and Li (2012), and the computed salinity and velocity measurements showed that the plume was magnitude of regular tidal currents in their model was 0.68e0.8 m/ characterized by a two-layer structure: the low-salinity water s, which is the same as or larger than the observed spring tidal floating in the upper 5e10-m layer, and salty water occupying the current magnitudes. As a result of such environmental change, lower layer from the middle depth to the bottom. whether or not the SATM will still be applicable to the Changjiang The high-resolution ECS-FVCOM and CE-FVCOM nested model Estuary plume needs further validation via comparisons. system was capable of simulating the vertical distribution and temporal variability of the observed current and salinity of the Acknowledgments Changjiang River plume. The real-time simulation over the observed period revealed two significant offshore detachments of Jianzhong Ge and Pingxing Ding are supported by the Fund from the low-salinity water from the plume. Process-oriented Natural Sciences Foundation of China (No. 41021064; No. 70 J. Ge et al. / Estuarine, Coastal and Shelf Science 156 (2015) 61e70

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