Numerical Study on the Expansion and Variation of Changjiang Diluted Water in Summer and Autumn

Journal of Marine Science and Engineering

Article Numerical Study on the Expansion and Variation of Changjiang Diluted Water in Summer and Autumn

Wanli Hou 1, Menglin Ba 1, Jie Bai 2 and Jianghua Yu 1,*

1 Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] (W.H.); [email protected] (M.B.) 2 Key Laboratory of Marine Environment and Ecology, College of Environmental Science and Engineering, Ocean University of China, Ministry of Education, Qingdao 266100, China; [email protected] * Correspondence: [email protected]; Tel.: +86-25-58695677; Fax: +86-25-58731090

Abstract: In view of the expansion and directional change mechanisms of Yangtze River water diluted with sea water in the shelf region (also known as Changjiang diluted water [CDW]) during summer and autumn, a three-dimensional hydrodynamic model of the Yangtze River Estuary (YRE) and its adjacent waters was established based on the Finite Volume Community Ocean Model (FVCOM). Compared with the measured data, the model accurately simulates the hydrodynamic characteristics of the YRE. On that basis, the influence of the expansion patterns of the CDW in both summer and autumn was studied. It was found that, in 2019, the CDW expanded to the northeast in the summer and to the southeast in the autumn, and that the route of the CDW is mainly controlled by the wind, not the runoff. Current seasonal winds also change the transportation route of the CDW by affecting its hydrodynamic field. Typhoons are frequent in both summer and autumn, causing abnormalities in both the transportation route and expansion of the CDW. During a typhoon, a large amount of the CDW is transported in a continuous and abnormal manner, accelerating the path turning of the CDW. This paper enhances the existing theoretical research of the CDW and provides a reference with respect to the expansion of diluted water all over the world. Citation: Hou, W.; Ba, M.; Bai, J.; Yu, J. Numerical Study on the Expansion Keywords: Finite Volume Community Ocean Model (FVCOM); Yangtze River Estuary (YRE); and Variation of Changjiang Diluted Changjiang diluted water (CDW); typhoon Water in Summer and Autumn. J. Mar. Sci. Eng. 2021, 9, 317. https:// doi.org/10.3390/jmse9030317 1. Introduction Received: 9 February 2021 Water that is mixed with sea water and the discharge of the Yangtze River (YR) in the Accepted: 6 March 2021 shelf region is called Changjiang diluted water (CDW), and its horizontal expansion range Published: 13 March 2021 varies with the seasons. Based on the records of the Bulletin of China Marine Environmental Status, about 70% of the nutrients discharged into the East China Sea were from the YR Publisher’s Note: MDPI stays neutral during the 2002–2017 period [1]. The CDW usually carries a large amount of nutrients and with regard to jurisdictional claims in organic matter, contributing to higher levels of primary productivity, and has an important published maps and institutional affil- iations. impact on the biogeochemical biology of the estuary. Because many marine biological and chemical activities are very sensitive to changes in nutrients and salinity [2,3], such changes affect the original stability and biodiversity of the marine ecosystem and can have many adverse effects on the development of coastal marine fisheries and the sustainable use of marine ecological resources. Study of the expansion and variation of the CDW is Copyright: © 2021 by the authors. of great scientific significance to further research on marine ecology, and also provides a Licensee MDPI, Basel, Switzerland. reference for other estuaries around the world, such as the Gulf of Mexico and Pamlico This article is an open access article Sound, for example. distributed under the terms and conditions of the Creative Commons The characteristics of the CDW are entirely determined by specific environmental Attribution (CC BY) license (https:// factors including the Kuroshio, baroclinic effects, the wind and the Taiwan warm current. creativecommons.org/licenses/by/ These environmental factors are not only complex but also changeable; their effects are 4.0/). sometimes consistent, while at other times they counteract each other [4,5]. A considerable

J. Mar. Sci. Eng. 2021, 9, 317. https://doi.org/10.3390/jmse9030317 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, 317 2 of 15

body of research has been conducted to understand the expansion of the CDW by means of numerical simulation, theoretical methods and the analysis of measured data. According to the relationship observed between the position of the monthly average diluted water tongue and the runoff of the YR, Le [6] recorded a critical runoff which could influence the flow direction when the runoff exceeds the critical value. Other scholars have proposed that wind is a key factor affecting the path turning of the CDW, that the influence of runoff is limited and only affects its expansion range. The hydrodynamic environment of the East China Sea has been simulated over a long period of time and it was found that the expansion of the CDW to the northeast was mainly affected by the east wind or strong southeast winds, while the expansion area of the CDW near the estuary was mainly controlled by the runoff [7]. This finding was similar to the conclusions of Liu et al. [8], Wang et al. [9] and Zhou et al. [10]. Zhao [5] reported that the direct effect of wind may be circumstantial to the CDW, but the significant change of wind stress vorticity in winter and summer is one of the major reasons for altering the path of the CDW in both the flood and dry seasons. In addition, it is also believed that the Taiwan warm current is a leading factor for the path turning of the CDW [11–13]. Thus, there is no consensus on which factors dominate the transfer or expansion of the CDW. Moreover, while typhoons are one of the factors that affects the expansion of the CDW, there has been little research on the influence of typhoons on the expansion of the CDW. The aim of this study is to show the influence of the monsoon and typhoons on the variation and expansion of the CDW. Considering the dispute on whether the expansion of the CDW is caused by runoff or the wind, the difference between the wind and runoff in summer and autumn is large. In addition, typhoon transit is often during summer and autumn and the impact on the CDW expansion needs to be explored. Therefore, this paper focuses on the variation of the CDW expansion in summer and autumn. Aiming to show the spreading morphology and seasonal variation of the CDW, the expansion variation and turning mechanism of the CDW under these multiple factors were studied by establishing a three-dimensional hydrodynamic model. The research in this paper enhances the theoretical research of the CDW and provides a reference with respect to the expansion of diluted water around the world.

2. Materials and Methods 2.1. Model Description A three-dimensional numerical model, the Finite Volume Community Ocean Model (FVCOM), with unstructured triangular mesh and finite volume is used in this paper [14,15]. The unstructured triangular grid is used in the horizontal direction, which better fits the real coastline, while the sigma-coordinate is used to represent the irregular seabed topography in the vertical direction. The finite volume method, which combines the advantages of the finite element method and the finite difference method, is one of the most commonly used models for numerical solutions [16,17]. The main original equations of the FVCOM are as follows: Continuity equation

∂ζ ∂Du ∂Dv ∂Dw + + + = 0 (1) ∂t ∂x ∂y ∂σ

Momentum equation

∂uD ∂uD ∂uD ∂u ∂t + u ∂x + v ∂y + w ∂σ − f vD 0 (2) ∂ζ gD ∂ R 1 ∂D 1 ∂ ∂u = −gD − [ (D ρdσ ) + ρσ ] + (Km ) + DFx ∂x ρ0 ∂x ∂x D ∂σ ∂z σ

∂vD ∂vD ∂vD ∂v ∂t + u ∂x + v ∂y + w ∂σ − f uD 0 (3) ∂ζ gD ∂ R 1 ∂D 1 ∂ ∂v = −gD − [ (D ρdσ ) + ρσ ] + (Km ) + DFy ∂y ρ0 ∂y ∂y D ∂σ ∂z σ J. Mar. Sci. Eng. 2021, 9, 317 3 of 15

Temperature, salinity and density equations

∂TD ∂TD ∂TD ∂T ∂ ∂T + u + v + w = (K ) + DF + DHˆ (4) ∂t ∂x ∂y ∂σ ∂σ h ∂σ T

∂SD ∂SD ∂SD ∂S ∂ ∂S + u + v + w = (K ) + DF (5) ∂t ∂x ∂y ∂σ ∂σ h ∂σ s ρ = ρ(T, S) (6) where D is the total water depth; ζ is the free surface height; u, v and w are the velocity components of the x axis, the y axis and the z axis respectively in a σ coordinate system; T, S, P, ρ represent temperature, salinity, pressure and density, respectively; g is gravity accel- eration; f is the Coriolis force parameter; Km is the vertical eddy viscosity coefﬁcient, Kh is the thermal vertical eddy viscosity diffusion coefﬁcient; Fu and Fv represent the horizontal momentum diffusion terms; FT, FS are the horizontal temperature and salinity, respectively.

2.2. Model Configuration Considering that there are many factors affecting the CDW (such as the Taiwan warm current) and in order to reduce the experimental error, the grid scope is large and the grid refined area includes the whole YRE. The model grid is shown in Figure1. The calculation range of the model is 2436.7 km long and 1484.1 km wide from north to south, which is from 117◦ E to 137◦ E and 20◦ N to 40◦ N, including Chongming Island, Changxing Island and other major islands. The internal grids of the Yangtze River Estuary (YRE), especially those around the islands, the irregular boundaries and key research areas are more refined compared with the open sea grid. The model contains 12,583 nodes, 23,699 unstructured triangular elements and six sigma layers which are set in the vertical direction. The maximum resolution of the model is 25 km and the minimum resolution is 100 m. Data from ETOPO1, which is a 1 arc-minute global relief model of the Earth’s surface that integrates land topography and ocean bathymetry released by the National Geophysical Data Center [18], are used for the offshore area of the model. The chart of China Navigation’s assurance department is adopted to obtain the depth of the YRE and the shoreline is corrected by using Google Earth, while the large-scale and local water depths are shown in Figure2. In order to reduce the numerical calculation error, the trend of the open boundary should be basically vertical or parallel to the mainstream direction, insofar as possible. The tidal amplitudes of eight main tidal components (M2,S2,K1,O1,N2,K2,P1 and Q1) generated by the tidal fluctuation from the Tidal Model Driver [19] were used for the open boundary conditions. Considering the baroclinic effects caused by temperature and salinity, the initial temperature and salinity field of the model adopts the Hybrid Coordinated Ocean Model (HYCOM) data [20]; the meteorological input files such as heat flux, wind field forcing, evaporation and rainfall are reanalyzed by the Climate Forecast System Reanalysis (CFSR) [21] 6-hourly products provided by the National Center for Environmental Prediction (NCEP), which is widely used in meteorological and hydrological modeling [7,9,22]. The runoff of the YR adopts the monthly average discharge of the Xuliujing section in the Water Resources Monitoring Bulletin of important control sections in the Yangtze River Basin, issued by the Shanghai Water Conservancy Commission (SWCC) [23]. The internal and external mode separation calculation method of the FVCOM is used in the model, in which the external model time step is 1 s and the internal model time step is 10 s. The start-up mode of the model is a cold start and considering that it takes time for the model to be stable, the numerical simulation results were taken from 10 May to 30 November 2019. The calculations continue and the data are calculated by the model output every hour after a stable tidal wave is obtained. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 4 of 15

J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 4 of 15 salinity, the initial temperature and salinity field of the model adopts the Hybrid Coor- dinated Ocean Model (HYCOM) data [20]; the meteorological input files such as heat salinity,flux, wind the initialfield forcing,temperature evaporation and salinity and field rainfall of the are model reanalyzed adopts theby theHybrid Climate Coor- Forecast dinatedSystem OceanReanalysis Model (CFSR) (HYCOM) [21] data 6-hourly [20]; the products meteorological provided input by filesthe Nationalsuch as heat Center for flux,Environmental wind field forcing, Prediction evaporation (NCEP), and which rainfall is arewidely reanalyzed used inby meteorologicalthe Climate Forecast and hydro- Systemlogical Reanalysismodeling (CFSR)[7,9,22]. [21] The 6-hourly runoff ofproducts the YR provided adopts theby monthlythe National average Center discharge for of Environmentalthe Xuliujing sectionPrediction in the(NCEP), Water which Resources is widely Monitoring used in Bulletin meteorological of important and hydro-control sections logicalin the modelingYangtze [7,9,22].River Basin, The runoff issued of theby YRthe adopts Shanghai the monthly Water averageConservancy discharge Commission of the(SWCC) Xuliujing [23]. section The ininternal the Water and Resources external Monitoring mode Bulletinseparation of important calculation control method sections of the inFVCOM the Yangtze is used River in theBasin, model, issued in whichby the theShanghai external Water model Conservancy time step is Commission 1 s and the internal (SWCC)model time[23]. stepThe isinternal 10 s. Theand start-upexternal modemode ofseparation the model calculation is a cold methodstart and of consideringthe FVCOMthat it takes is used time in the for model, the model in which to be the stable external, the model numerical time step simulati is 1 s andon theresults internal were taken model time step is 10 s. The start-up mode of the model is a cold start and considering from 10 May to 30 November 2019. The calculations continue and the data are calculated that it takes time for the model to be stable, the numerical simulation results were taken J. Mar. Sci. Eng. 2021, 9, 317 4 of 15 fromby the 10 modelMay to output30 November every 2019.hour Theafter calculations a stable tidal continue wave and is obtained. the data are calculated by the model output every hour after a stable tidal wave is obtained.

FigureFigure 1. 1. The The computational computationalFigure 1. The domain computational domain and andtriangular domain triangular and mesh triangular meshof the ofmodel. mesh the of model. the model.

Figure 2. Model topography and local topography of the YRE and observation stations. Figure 2. Model topography and local topography of the YRE and observation stations. 3.Figure Results 2. Modeland Analysis topography and local topography of the YRE and observation stations. 3.1. Model Validation3. Results and Analysis 3. Results and Analysis 3.1.1. Validation of3.1. Tidal Model Level Validation 3.1. Model Validation3.1.1. Validation of Tidal Level Five observation stations were selected to verify the model according to the tide Five observation stations were selected to verify the model according to the tide level level3.1.1. and Validation tidal current of Tidal data Levelreleased by the National Marine Data and Information Ser- and tidal current data released by the National Marine Data and Information Service Five observation(NMDIS) stations [24] in order were to verifyselected the accuracy.to verify The the tide model level and according the current to station the layouttide level and tidal currentare shown data in Figure released2. By by comparing the National the observed Marine data Data of the and T1 andInformation T2 stations Ser- with the simulated values of corresponding points, as shown in Figure3, the simulation and observation of the two tide stations are in good agreement. The average errors of the T1 and T2 stations are 6.79 cm and 9.04 cm, respectively. The average error of amplitude of four principal components is 1.42 cm, and the average error of phase is 2.13◦ (Table1). Therefore, the simulation results are in good agreement with the observation results. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 5 of 15

vice (NMDIS) [24] in order to verify the accuracy. The tide level and the current station layout are shown in Figure 2. By comparing the observed data of the T1 and T2 stations with the simulated values of corresponding points, as shown in Figure 3, the simulation and observation of the two tide stations are in good agreement. The average errors of the T1 and T2 stations are 6.79 cm and 9.04 cm, respectively. The average error of amplitude J. Mar. Sci. Eng.of2021 four, 9, 317principal components is 1.42 cm, and the average error of phase is 2.13° (Table 1). 5 of 15 Therefore, the simulation results are in good agreement with the observation results.

◦ ◦ ◦ ◦ FigureFigure 3. Comparison 3. Comparison between between calculated calculated and observed and tideobserved elevation tide (T1: elevation 121.9 E, 31.1(T1: 121.9°N. T2: 122.2E, 31.1°E, 31.4N. T2:N). 122.2° E, 31.4° N). Table 1. Model-observation comparison of the main tidal constituents (O1,K1,M2,S2).

Table 1. Model-observation comparison of the main tidalT1 constituents (O1, K1, M2, S2). T2 Constituents T1Observation Model Error ObservationT2 Model Error Constituents Amplitude (cm) 19.26 18.70 −0.56 15.22 15.13 −0.09 O Observation Model Error Observation Model Error 1 Phase (◦) 28.48 35.39 6.91 0.56 0.69 0.13 Amplitude (cm) 19.26 18.70 −0.56 15.22 15.13 −0.09 O1 Amplitude (cm) 22.23 25.10 2.87 20.34 21.81 1.47 K Phase (°) 1 Phase 28.48 ( ◦) 35.39 67.81 6.91 71.12 0.56 3.31 55.20 0.69 53.87 0.13 −1.33 Amplitude (cm) Amplitude 22.23 (cm) 25.10 126.32 2.87 125.27 20.34−1.05 111.44 21.81 110.99 1.47 −0.45 K1 M Phase (°) 2 Phase 67.81 ( ◦) 71.12 92.13 3.31 93.01 55.20 0.88 84.70 53.87 − 85.221.33 0.52 Amplitude(cm) Amplitude126.32 (cm)125.27 73.12 −1.05 76.13111.44 3.01 66.27110.99 − 68.020.45 1.75 M2 S2 ◦ Phase (°) Phase 92.13 ( ) 93.01 132.81 0.88 136.47 84.70 3.66 118.55 85.22 118.84 0.52 0.29 Amplitude (cm) 73.12 76.13 3.01 66.27 68.02 1.75 S2 Phase (°)3.1.2. Validation 132.81 of the Tidal 136.47 Current 3.66 118.55 118.84 0.29 The calculation results of the tidal current at each station and the verification of the 3.1.2. Validation measuredof the Tidal results Current are shown in Figures4 and5, respectively. The root-mean-square error of spring tide and neap tide velocity at each station is 16.18 cm/s, and the root-mean- The calculationsquare results error ofof directionthe tidal is current less than at 10% each of thestation maximum and the flow verification direction. The of the direction measured resultsof are seawater shown movement in Figures in the4 and open 5, searespectively. area is deflected The byroot-mean-square the influence of the error deflection of spring tide forceand ofneap the Earth’s tide rotation—thevelocity at Corioliseach force—whilestation is islands16.18 cm/s, and the and coast the have the root-mean-squarefunction error of of blockingdirection and is less converging than 10% the offshoreof the maximum current and flow enhancing direction. the influence The of submarine friction. The output results of the model can only be selected in the triangle direction of seawatermesh closestmovement to the in observation the open point,sea area which is isdeflected difficult toby keep the theinfluence same position of the as the deflection force ofactual the observationEarth’s rotation—the point. In addition, Coriolis as force—while the numerical treatmentislands and of large-scale the coast water have the functiondepth of blocking topography and inevitably converging produces the offshore differences current in local and topography, enhancing resulting the inin errors fluence of submarinebetween friction. the calculation The output of tidal results current ofand the themodel measured can only fitting, be it selected is considered in the that the triangle mesh closesthydrodynamic to the observation field of the modelpoint, iswhich well fitted. is difficult to keep the same position

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as the actual observation point. In addition, as the numerical treatment of large-scale as the actual observation point. In addition, as the numerical treatment of large-scale water depth topography inevitably produces differences in local topography, resulting in water depth topography inevitably produces differences in local topography, resulting in J. Mar.errorserrors Sci. Eng. 2021betweenbetween, 9, 317 the the calculation calculation of of tidal tidal current current and and the the measured measured fitting, fitting, it is it considered is considered6 of 15 thatthat thethe hydrodynamichydrodynamic field field of of the the model model is iswell well fitted. fitted.

Figure 4. Tidal current validation of spring tide (a) and neap tide (b). Figure 4. Tidal currentFigure validation 4. Tidal current of spring validation tide of ( springa) and tide neap (a) and tide neap (b tide). (b).

Figure 5. 5.Matching Matching of spring of spring neap tide neap observations tide observations with model values. with (Themodel left figurevalues. shows (The the left flow figure velocity shows fitting andthe the right figure shows the flow direction fitting). flowFigure velocity 5. Matching fitting andof spring the right neap figure tide showsobservations the flow wi directionth model fitting). values. (The left figure shows the flow velocity fitting and3.1.3. the Validation right figure of Salinity shows the flow direction fitting). 3.1.3. Validation of Salinity The salinity calculated with HYCOM data products [20] will be used for salinity 3.1.3.The Validation salinity ofcalculated verificationSalinity with of this HYCOM model, and data the comparison products points [20] arewill shown be used in Figure for6 e,f.salinity Figure 6 shows the comparison of salinity (b, d) calculated by the model with HYCOM (a, c) data verificationThe salinity of this calculatedmodel, and with the comparisonHYCOM data points products are shown [20] inwill Figure be used 6e,f. Figurefor salinity 6 shows the comparisonproducts of salinity [20]. On (b, 2 August, d) calc theulated CDW by calculated the model by the with model HYCOM diffused eastward, (a, c) data which verification of this wasmodel, basically and the the same comparison as the CDW calculated points are by HYCOM shown data in [Figure20]. On 11 6e,f. October, Figure both 6 productsshows the [20]. comparison On 2 August, of salinity the CDW (b, d)calculated calculated by theby themodel model diffused with eastward,HYCOM (a,which c) data was basically the same as the CDW calculated by HYCOM data [20]. On 11 October, both products [20]. On 2 August, the CDW calculated by the model diffused eastward, which the CDW calculated by HYCOM data [20] and the CDW calculated by the model diffused was basically the same as the CDW calculated by HYCOM data [20]. On 11 October, both southward. In addition, by comparing the surface salinity of the two stations (122° E, the CDW calculated by HYCOM data [20] and the CDW calculated by the model diffused 31.9° N; 121.9° E, 29.2° N), the HYCOM data products [20] are highly correlated with the southward. In addition, by comparing the surface salinity of the two stations (122° E, salinity calculated by the model, r = 0.95 (c) and r = 0.88 (f), respectively. 31.9° N; 121.9° E, 29.2° N), the HYCOM data products [20] are highly correlated with the salinity calculated by the model, r = 0.95 (c) and r = 0.88 (f), respectively.

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the CDW calculated by HYCOM data [20] and the CDW calculated by the model diffused J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW southward. In addition, by comparing the surface salinity of the two stations (122◦ E, 31.9◦ 7 of 15 N; 121.9◦ E, 29.2◦ N), the HYCOM data products [20] are highly correlated with the salinity calculated by the model, r = 0.95 (c) and r = 0.88 (f), respectively.

Figure 6. Comparison of model calculation and HYCOM data products ((a,c) are HYCOM data; (b,d) are model calculation Figure 6. Comparisonresults; (e, fof) are model the data calculation comparison of and the sameHYCOM position). data products ((a,c) are HYCOM data; (b,d) are model calculation results; (e,f) are the data comparison of the same position). 3.2. Characteristics of the Wind Field in the YRE 3.2. CharacteristicsThe YRE of is the located Wind in Field the East in Asianthe YRE monsoon region, and the prevailing wind direction exhibits significant seasonal variation. Previous studies on the expansion of the TheCDW YRE in summeris located were in mostly the related East toAsian the sea mo surfacensoon wind region, field [10, 13and,22]. the Differences prevailing in wind direction windexhibits direction significant and speed seasonal may have different variation. effects Previous on the expansion studies of fresh on water,the expansion and of the it is therefore essential to study the characteristics of the wind field in the YRE. Figure7 CDW inshows summer the frequency were mostly chart of windrelated direction to the and se winda surface speed in wind the YRE field (120◦ [10,13,22].E to 124◦ E, Differences in wind29 direction◦ N to 33◦ N) and for eachspeed month, may obtained have fromdifferen the NCEP’st effects CFSR on reanalysis the expansion data. In terms of fresh water, and it isof therefore months, the essential main wind directionsto study at the YRE characteristics from June to August of the in 2019 wind were field southeast, in the YRE. Fig- south and east-northeast, respectively, and the frequency of the main wind direction was ure 7 shows15% in the July frequency 2019. The main chart wind of directions wind direction from September and towind November speed were in similar,the YRE (120° E to 124° E, 29°and mostN to of 33° them N) were for north-northeasteach month, orobtained north due from to the the typhoon NCEP’s transit, CFSR while reanalysis the data. In termsmaximum of months, wind speedthe main was larger wind in August directions and September at the 2019.YRE In from terms ofJune the seasons,to August in 2019 the average wind speed in autumn is stronger than that in summer, with wind from the were southeast,east-southeast, south south and and east-northeasteast-northeast, more commonrespectively, in summer, and while the those frequency from the of the main wind directionnorth-northeast was and15% north in July are dominant 2019. The in autumn. main wind directions from September to No- vember were similar, and most of them were north-northeast or north due to the typhoon transit, while the maximum wind speed was larger in August and September 2019. In terms of the seasons, the average wind speed in autumn is stronger than that in summer, with wind from the east-southeast, south and east-northeast more common in summer, while those from the north-northeast and north are dominant in autumn.

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◦ ◦ ◦ ◦ Figure 7.7. WindWind rose rose of of the the YRE YRE (120 (120°E toE 124to 124°E, 29E, 29°N to N 33 to 33°N) duringN) during June June to November to November 2019. 2019. Length representsrepresents frequency frequency and and color color represents represents speed. speed.

3.3. Effect ofof Wind Wind on on the The Path Path Turning Turning of the of CDW the CDW TheThe CDWCDW is is a a water water mass mass with with a wide a wide horizontal horizontal range range and a and thin thicknessa thin thickness with with characteristics of of low low salt, salt, high oxygen,high oxygen, low transparency low transparency and turbidity. and In orderturbidity. to suitably In order to describe the expansion form of the CDW and referring to the previous deﬁnition of the suitably describe the expansion form of the CDW and referring to the previous definition CDW, the isohaline of 26‰ [3,8] is taken as the core area of the CDW and the expansion ofdirection the CDW, of the the isohaline isohaline of 30 of‰ 26‰[7] is [3,8] used is to taken determine as the the core maximum area of expansion the CDW scope and the ex- pansionin this paper. direction According of the to isohaline Figure8, theof 30‰ core area[7] is of used the CDW to determine mainly extended the maximum to the expan- sionnortheast scope and in northwest this paper. in theAccording summer andto Figure to the southeast8, the core in the area autumn of the of CDW 2019. Themainly ex- tendedCDW expands to the innortheast a northwest and direction northwest with in the the action summer of the and southeast to the wind southeast in June. in With the autumn ofan 2019. increase The in CDW the southeast expands wind in a andnorthwest the runoff direction in the core with area the of action fresh water,of the thesoutheast core wind ◦ inarea June. of the With CDW an reaches increase 124 inE the in Julysoutheast and the wind low saltwater and the tonguerunoff extendsin the core obviously area of fresh water,eastward. the This core is area similar of tothe the CDW direction reaches of the 124° Ekman E in transport, July and that the is, low the CDWsaltwater expands tongue ex- to the right of each wind direction [8]. In August, the wind direction near the YRE changes tends obviously eastward. This is similar to the direction of the Ekman transport, that is, from southeast to east and the Ekman transport to the East is weakened, making the theisohaline CDW 30 expands‰ closer to to the the shore,right of and each the wind trend ofdi therection core [8]. area In of August, the CDW the near wind to the direction nearNorth the Jiangsu YRE coast.changes The from core areasoutheast of the CDWto east expanded and the to Ekman the South transport and contracted to the East is weakened,to the shore making in September. the isohaline This may 30‰ be becausecloser to typhoon the shore, Lingling and the (1913) trend and of typhoon the core area of theTapah CDW (1917) near made to athe large North amount Jiangsu of the CDWcoast.transport The core abnormally area of the to theCDW South. expanded At the to the Southsame time, and thecontracted Ekman transport to the shore under in the September. northeast windThis may had a be westward because component, typhoon Lingling (1913)which madeand typhoon the fresh waterTapah contract (1917) obviouslymade a larg to thee amount West. In of October the CDW and November,transport abnormally the toexpansion the South. of the At core the area same ofthe time, CDW the is similar,Ekman andtransport the core under area of the the CDWnortheast decreases wind had a westward component, which made the fresh water contract obviously to the West. In October and November, the expansion of the core area of the CDW is similar, and the core area of the CDW decreases due to the month by month decrease in runoff. Under the action of a northerly wind, the 30‰ isohaline shrinks to the south, which is consistent

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with Guo et al.’sdue [6] to theconclusion. month by In monthorder to decrease better understand in runoff. Under the expansion the action of of the anortherly CDW wind, the over time, two30 areas‰ isohaline were selected shrinks in to the the nort south,h and which south is of consistent the YRE. with Figure Guo 9 shows et al.’s [that6] conclusion. In with the increaseorder in to runoff, better understandthe salinity theof area expansion A and of area the B CDW gradually over time, decreases. two areas From were selected in July to Augustthe in north 2019, and area south A is ofabove the YRE.26, while Figure area9 shows B is on that the with contrary. the increase In September in runoff, the salinity 2019, the salinityof area of area A and A decreased area B gradually significantly, decreases. while From that Julyof area to AugustB continued in 2019, to inarea A is above crease. This showed26, while that area the Bexpansion is on the direction contrary. of In the September CDW had 2019, changed. the salinity of area A decreased In order significantly,to further study while the that effect of area of wind B continued on the path to increase. turning This of the showed CDW, that this the expansion experiment simulateddirection th ofe thediffusion CDW hadof the changed. CDW with an external wind field and windless conditions based onIn the order FVCOM. to further Figure study 10 cl theearly effect shows of windthe difference on the path in the turning expansion of the CDW, this of the CDW inexperiment summer and simulated autumn, the with diffusion or without of the CDWthe wind with field. an external After adding wind field the and windless wind field, theconditions trend of basedthe CDW on the moves FVCOM. upward, Figure basically 10 clearly in showsthe YRE, the Hangzhou difference in Bay, the expansion of Zhoushan seathe area, CDW while in summerthe CDW and without autumn, the withwind or field without expands the winddownward field. After to 27° adding N the wind in summer. Thefield, difference the trend between of the CDW July movesand August upward, is the basically most obvious. in the YRE, The Hangzhou CDW with Bay, Zhoushan the wind fieldsea changes area, while from thethe CDWNorth without Jiangsu the coastal wind area field to expands the Zhejiang downward coastal to area, 27◦ N in summer. and the core areaThe differenceof the CDW between does not July exceed and August32° N in is autumn. the most The obvious. trend Theof the CDW CDW with the wind without the windfield changesfield is frombasically the North the same, Jiangsu but coastal extends area to to the the ZhejiangNorth Jiangsu coastal coast area, and the core compared witharea the of summer. the CDW Combined does not with exceed the 32 analysis◦ N in autumn. of the wind The field trend in ofthis the paper, CDW without the Section 3.2, itwind was found field is that basically change the of same, the wind but extends field is toconsistent the North with Jiangsu the direction coast compared of with the the core areasummer. of the CDW. Combined In terms with theof th analysise difference of the windof the field residual in this current paper, Sectionfield 3.2, it was → → N found that change of the wind field is consistent with the direction of the core area of the ( = ) in the YRE when there is wind and when there is no wind, →the direction→  i =1V N V Er CDW. In terms of the difference of the residual current field (VEr = ∑i=1 V) in the YRE of the surfacewhen residual there current is wind field and whenis changed there isby no the wind, influence the direction of the ofsurface the surface wind residualin current summer and fieldautumn, is changed as shown by thein Figure influence 11. of The the residual surface windcurrent in summermainly flows and autumn, to the as shown in east or northeastFigure in summer11. The residual and to the current southe mainlyast in flows autumn. to the It eastcan orbe northeastseen that inthe summer sur- and to the face wind cansoutheast change the in direction autumn. of It canthe CD be seenW by that affecting the surface the flow wind field can in change summer the and direction of the autumn. CDW by affecting the flow field in summer and autumn.

Figure 8. MonthlyFigure distribution 8. Monthly distribution of surface salinity of surface in thesalinity YRE. in The the colorYRE. The represents color represents the salinity the (PSU),salinity the (PSU), arrow the represents the monthly averagearrow represents wind ﬁeld, the the monthly color representsaverage wind the field, wind the speed color (m/s) represents and the the black wind solid speed line (m/s) represents and the the isohaline of 26‰, 30‰,black 31‰, solid 32‰, line 33‰ represents and 34‰, the isohaline respectively. of 26‰, 30‰, 31‰, 32‰, 33‰ and 34‰, respectively.

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Figure 9.Figure Salinity 9. changesSalinity in changes the southern in the southernand northern and northernYangtze River Yangtze Estuary. River Estuary. Figure 9. Salinity changes in the southern and northern Yangtze River Estuary.

Figure 10. Distribution of surface salinity in the YRE with and without the wind field in summer Figure 10. Distribution of surface salinity in the YRE with and without the wind field in summer Figure 10. Distributionand autumn. of The surface black salinity solid line in the and YRE the withred solid and withoutline represent the wind the ﬁeldmaximum in summer expansion and autumn. of the The black and autumn. The black solid line and the red solid line represent the maximum expansion of the solid line andCDW the red with solid and line without represent the thewind maximum field, respectively. expansion of the CDW with and without the wind ﬁeld, respectively. CDW with and without the wind field, respectively.

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FigureFigure 11. Surface 11. Surface residual currentresidual fields current with and fields without with the and wind with fieldout in thethe YRE. wind The field red and in the blue YRE. arrows The indicate red theand residualblue current arrows field indicate with and the without residu theal wind current field, respectively.field with and without the wind field, respectively.

3.4. Influence of Typhoons3.4. Influence on The of Typhoons Spreading on the Pattern Spreading of Patternthe CDW of the CDW Many previous studiesMany previous have focused studies have on focusedthe path on turning the path turningand expansion and expansion of the of theCDW CDW in single or multiplein single winds or multiple [8,25,26], winds with [8, 25the,26 CDW], with often the CDW affected often affected by severe by severe weather weather such such as typhoons. Zhao et al. [5] found that the path turning of the CDW often occurs during as typhoons. Zhaothe et flood al. [5] season. found Typhoons that the landing path inturning the East of China the SeaCDW or in often the eastern occurs coastal during areas the flood season. ofTyphoons China usually landing occur in from the JulyEast to China September Sea or [27 in], whichthe eastern represents coastal the flood areas season of China usually occurof the from YR. TyphoonsJuly to September transport a [2 large7], amountwhich represents of the CDW nearthe flood the YRE season to the openof the sea, YR. Typhoons transportaccompanied a large by rainfall. amount Therefore, of the itCDW is necessary near the to studyYRE theto impactthe open of typhoons sea, ac- on the CDW. companied by rainfall.There Therefore, were four typhoonit is necessary events affecting to study the YREthe fromimpact June of to typhoons November 2019,on the along CDW. with typhoon Lekima (1909), typhoon Lingling (1913), typhoon Tapah (1917) and typhoon There were Mitagfour (1918)typhoon as shown events in Figure affecting 12, among the whichYRE from Lekima June (1909) to and November Lingling (1913) 2019, were along with typhoonsuper Lekima typhoons. (1909), During typhoon the passage Lingling of a typhoon, (1913), the strongtyphoon wind Tapah field will (1917) intensify and the typhoon Mitag (1918)vertical as mixing shown of seain Figure water and 12, increase among the which surface Lekima salinity (1909) [28]. Figure and 13Lingling shows the difference between the surface salinity and the bottom salinity before, during and after the (1913) were supertyphoon typhoons. transit. Du Inring order the to better passage explain of thea typhoon, impact of typhoonthe strong hedging wind on thefield expansion will intensify the verticalof the mixing CDW, we of selected sea water two representativeand increase sites the to surface explain thesalinity response [28]. time Figure of the CDW13 shows the differenceto typhoons between (Figure the 14surface). The majoritysalinity of and the CDWthe bottom left the YREsalinity under before, the influence during of a and after the typhoonsoutheast transit. wind In caused order by to typhoon better Lekimaexplain (1909) the impact and approached of typhoon the northwest hedging coastal on area, where the salinity of point A decreased rapidly (Figure 14). After the typhoon passed the expansion of through,the CDW, the CDWwe selected approached two the re Northpresentative Jiangsu coast sites along to with explain the main the wind response direction time of the CDWand to thetyphoons water tongue (Figure changed 14). from The northeast majority to northwestof the CDW in August left the (Figure YRE8). Typhoonsunder the influence of aLingling southeast (1913), wind Tapah caused (1917) andby Mitagtyphoon (1918) Lekima all occurred (1909) in September, and approached generating the wind northwest coastalcircles area, and where vortices the to salinity the east of thepoint YRE. A The decreased CDW was rapidl continuouslyy (Figure transported 14). After to the the typhoon passedZhejiang through, coastal the region CDW under approa the influenceched the of theseNorth typhoons Jiangsu with coast the along same path with and same wind direction, existing for a long period of time under the influence of the wind the main wind directionfields (Figure and 14 the), resulting water intongue the rapid changed downward from expansion northeast of the to CDW northwest in September in August (Figure 8).(Figure Typhoons8). As shownLingling in Figure (1913), 14 ,Tapah under (1917) the influence and Mitag of typhoon (1918) Lekima all occurred (1909), the in September, generatingsalinity of pointwind A circles in the core and area vortices of the CDW to the rapidly east decreased of the YRE. from The 18 to CDW 7 and returned was continuously transportedto the original to statethe Zhejiang after 25 days coastal [27], the region path turning under of thethe CDW influence causing of the these sea water ty- to phoons with the same path and same wind direction, existing for a long period of time under the influence of the wind fields (Figure 14), resulting in the rapid downward expansion of the CDW in September (Figure 8). As shown in Figure 14, under the influence of typhoon Lekima (1909), the salinity of point A in the core area of the CDW rapidly decreased from 18 to 7 and returned to the original state after 25 days [27], the path turning of the CDW causing the sea water to come close in and thereby increasing the salinity. It can be seen that typhoon Lekima (1909) caused the abnormal northward transport of the CDW. After the passage of the typhoon, the monsoon of that month kept the path of the CDW northward, but the abnormally transported CDW cannot exist for

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come close in and thereby increasing the salinity. It can be seen that typhoon Lekima (1909) causedlong, and the abnormal even moved northward southward transport with of the CDW.path of After the theCDW passage under of the typhoon,effect of the sub- the monsoon of that month kept the path of the CDW northward, but the abnormally sequent autumn monsoon (Figure 14). The salinity of point B changed under the influ- transported CDW cannot exist for long, and even moved southward with the path of the CDWence underof continuous the effect of typhoons, the subsequent and autumnthe mini monsoonmum salinity (Figure 14 was). The 8 salinityunder ofthe point influence of Btyphoon changed underTapah the (1917). influence Although of continuous the salinity typhoons, gradually and the rose minimum to 22, salinity it decreased was again 8under under the influenceinfluence of of typhoon abnormal Tapah transportation (1917). Although of the the CDW salinity caused gradually by rosetyphoon to Mitag 22,(1918), it decreased and finally again undermaintained the influence a level of between abnormal 17–19 transportation due to the of thepath CDW turning caused downward byof typhoonthe CDW Mitag in autumn. (1918), and Different finally maintainedfrom Lekima a level (1909), between continuous 17–19 due typhoons to the path cause a large turningamount downward of the CDW of the to CDWbe transported in autumn. abnorm Differentally from to the Lekima South (1909), and continuousthe monsoon of the typhoonscurrent month cause a largealso takes amount the of CDW the CDW path to to be the transported south. However, abnormally continuous to the South typhoons and the monsoon of the current month also takes the CDW path to the south. However, intermittently provide a large amount of the CDW and the CDW is continuously sup- continuous typhoons intermittently provide a large amount of the CDW and the CDW is continuouslyplemented supplementedby the monsoon by theafter monsoon the typhoon after the passes typhoon through passes (Figure through 14). (Figure As 14a ).result, the Asabnormally a result, the transported abnormally transportedflush water flush can waterexist for can a exist long for time. a long time. InIn conclusion, conclusion, typhoons typhoons make make the CDWthe CDW transport transport abnormally. abnormally. In September In September 2019, 2019, followingfollowing the the passage passage of continuousof continuous typhoons, typhoon a larges, a amountlarge amount of flushing of flushing water was water was createdcreated with with continuous continuous abnormal abnormal transportation transpor oftation the CDW of the and CDW the expansion and the directionexpansion direc- oftion the of CDW the alsoCDW changed also changed significantly. significantly.

FigureFigure 12. 12.Typhoon Typhoon track track map map affecting affecting the YRE the inYRE 2019. in The2019. blue The dotted blue arrowdotted and arrow the blueand solidthe blue solid arrowarrow indicate indicate the the direction direction of the of CDW the CDW in summer in summe and autumn,r and autumn, respectively, respective while thely, pink while arrows the pink ar- representrows represent typhoon typhoon Lekima (1909), Lekima typhoon (1909), Lingling typhoon (1913), Lingling typhoon (1913), Tapah typhoon (1917) and Tapah typhoon (1917) Mitag and ty- (1918),phoon respectively. Mitag (1918), respectively.

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Figure 13. Effects of typhoons on the difference between surface salinity and bottom salinity of the Figure 13. EffectsFigure of typhoons 13. Effects on theof typhoons difference on betweenthe difference surface between salinity surface and bottomsalinity and salinity bottom of thesalinity CDW of before,the during and CDWCDW before, before, during during and and after after typhoon typhoon transit. transit. (The color color is is salinity salinity difference, difference, and and the white the white arrow arrow after typhoon transit. (The color is salinity difference, and the white arrow represents the wind ﬁeld). representsrepresents the thewind wind field). field).

Figure 14. Salinity time series of points A and B. (The black arrow represents the different paths of the CDW in summer and autumn).

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4. Conclusions A three-dimensional hydrodynamic model for the YRE and its adjacent waters was established to study the expansion patterns of the CDW in summer and autumn. The main conclusions are as follows. (1) In 2019, the CDW main expansion directions were northeast in summer and south in autumn, as per the monsoon. Seasonal variations of the wind field provide conditions for the CDW to expand in different forms. (2) The influence of wind on the path turning of the CDW is obvious in contrast to the runoff. Wind leads to change of the residual current field, which causes the path turning of the CDW, while the runoff determines how far the core area extends. (3) In the flood season of the YR in 2019, the frequent occurrence of typhoons accelerated change of the spreading form of the CDW. Continuous typhoons make transport a large amount of the CDW continuously and abnormally, which accelerates the path turning of the CDW. This study focused on the impact of multiple factors on the distribution of the CDW, to facilitate understanding of the expansion and variation of the CDW in summer and autumn, and provides an important reference for predicting the expansion direction of the Yangtze River diluted water which is of great significance for further understanding the hydrological environment of the YRE.

Author Contributions: Conceptualization, J.Y. and W.H.; methodology, W.H.; software, W.H.; validation, W.H., M.B. and J.B.; formal analysis, W.H.; investigation, W.H.; resources, J.Y.; data curation, M.B.; writing—original draft preparation, M.B.; writing—review and editing, W.H.; visualization, W.H.; supervision, J.B.; project administration, J.B.; funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript. Funding: The preparation of his work was supported by the National Key R&D Program of China [No.2018YFD0900805]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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