High-Resolution Observations of Upwelling and Front in Daya Bay, South China Sea

Journal of Marine Science and Engineering

Article High-Resolution Observations of Upwelling and Front in Daya Bay, South China Sea

Huabin Mao 1,2,3, Yongfeng Qi 1,4,*, Chunhua Qiu 5, Zhenhua Luan 6, Xia Wang 1,4, Xianrong Cen 4, Linghui Yu 1,4, Shumin Lian 1 and Xiaodong Shang 1,3,4

1 Key Laboratory of Science and Technology on Operational Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; [email protected] (H.M.); [email protected] (X.W.); [email protected] (L.Y.); [email protected] (S.L.); [email protected] (X.S.) 2 Ocean College, Zhejiang University, Zhoushan 316021, China 3 Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China 4 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; [email protected] 5 School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China; [email protected] 6 Sate Key Laboratory of Nuclear Power Safety Monitoring Technology and Equipment, China Nuclear Power Engineering Co., Ltd., Shenzhen 518124, China; [email protected] * Correspondence: [email protected]

Abstract: Field observations of coastal regions are important for studying physical and biological features. Observations of high-resolution coastal phenomena were obtained by using a tow-yo instrument and a turbulence profiler at Daya Bay in the South China Sea in October 2015. Details of coastal phenomena, including warm water from a nuclear plant discharge, as well as an upwelling, and front, were obtained. The upwelling, with a width of 2 km, resulted in saltier and more turbid water near the bottom, with low chlorophyll-a and dissolved oxygen contents being transported Citation: Mao, H.; Qi, Y.; Qiu, C.; upward to the surface layer and changing the local water environment. The front, with the lateral Luan, Z.; Wang, X.; Cen, X.; Yu, L.; Lian, S.; Shang, X. High-Resolution salinity variations as large as 0.7 psu across 1 km, was active at the water intersection of the South Observations of Upwelling and Front China Sea and Daya Bay. Such events commonly form during weak stratification periods in autumn. in Daya Bay, South China Sea. J. Mar. Continuous measurements from VMP-250 profiler over circa 22 h revealed active fronts and an −8 −5 2 Sci. Eng. 2021, 9, 657. https:// averaged dissipation rate of 8 × 10 W/kg and diffusivity of 5.8 × 10 m /s (i.e., one order of doi.org/10.3390/jmse9060657 magnitude larger than in the open ocean) in the thermocline. The front was accompanied by strong mixing, indicating that it had formed at the intersection of different water masses and played an Academic Editor: important role in energy dissipation in Daya Bay, further affecting the distribution of ecological Charitha Pattiaratchi elements.

Received: 13 May 2021 Keywords: Daya Bay; high-resolution observation; upwelling; front; turbulent mixing Accepted: 11 June 2021 Published: 13 June 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- The South China Sea (SCS) is the largest marginal sea connected to the Western Pacific iations. and is dominated by the East Asian monsoon. In winter, northeasterly winds overlie the SCS, whereas in summer, southwesterly winds prevail across the region [1,2]. In the Northern SCS, surface currents (e.g., the Kuroshio Current, South China Sea Warm Current, and cyclonic currents) display complicated dynamics. All of these large-scale currents are generally associated with wind, wind stress curl, and the processes from the Pacific [3–7]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Mesoscale to small-scale structures prevail in shelf seas of the Northern SCS, including This article is an open access article eddies, fronts, and upwellings/downwellings [8]. These features play very important roles distributed under the terms and in regulating oceanic heat and energy exchange, are vital to the transport of biochemicals, conditions of the Creative Commons and influence interannual/decadal climate variations [9]. Daya Bay (DYB) is located in the Attribution (CC BY) license (https:// subtropical ocean and is one of the largest and most important gulfs along the southern creativecommons.org/licenses/by/ coast of China. Physical and biochemical processes in DYB have been the focus of many 4.0/). previous studies. Thermal pollution has been identified in the vicinity of a nuclear power

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station in DYB [10]. High dissolved oxygen (DO) and nutrient concentrations were reported to be trapped within surface water [11,12]. The investigators surmised that the high surface nutrient concentrations resulted from stratification and the intrusion of upwelling water. The water refresh time was found to be approximately 26 days and wind driven. Fronts and associated eddies have been widely reported by oceanographers in Southern California [13], Africa [14], and Alaska [15]. Coastal fronts have mainly been examined at the interface of coastal water and the open ocean. According to their different formation mechanisms, coastal fronts can include tidal mixed, river plume, continental-shelf, upwelling, cape, and island fronts. Both eddies and fronts have mainly been observed via satellite or in situ ship-based observations. Understanding the small-scale physical and biological processes in coastal and es- tuarine areas is a pressing issue in oceanography. In recent studies, small-scale physical processes in these areas have been observed [11,16,17]. Anthropogenic activities are considered the most important factor in the degradation of the DYB marine environment in recent years [10,18]. Therefore, studies of the hydrological and ecological processes in DYB are important for understanding the impacts of human activities on the ecology of DYB. Fixed-site and remotely sensed observations are commonly used in these studies [10,19–22]. For example, Tang et al. [10] obtained remote-sensing data from the Advanced Very High- Resolution Radiometer (AVHRR) satellite to study the thermal plume discharged by the nuclear power station into DYB and analyze its seasonal characteristics. Sun et al. [19] and Yang et al. [22] studied the long-term changes in phytoplankton response patterns in DYB, using fixed-site observations. However, satellite remote-sensing and fixed-site station observations have a disadvantage in that these observational methods cannot be used to obtain high-resolution internal profile characteristics. Satellite remote-sensing observation can only observe sea surface information. In situ conductivity–temperature–depth profiler (CTD) observation of fixed-site station often lacks continuity of spatial and temporal results. Daya Bay covers an area of 600 km2, with a width of approximately 15 km and a north to south length of approximately 30 km. It has a complex topography, contains many islands, and therefore, has a rich dynamic structure. Low-resolution station observations cannot be used to explain the structure of the fine-scale, dynamic thermal, and ecological processes in the bay. A useful method for observing oceanic processes is towed observation. With this method, high-resolution data can be obtained rapidly and continuously. In this study, we investigated the rich structural characteristics of DYB, which included the thermal plume discharged from the nuclear power station, as well as an upwelling, and front, using high-resolution Yoing Ocean Data Acquisition (YODA) profiler data and a microstructure turbulence profiler. We then describe the fine-scale and turbulence features of these structures.

2. Materials and Methods 2.1. Instruments The main data used in the study were collected by a YODA profiler and a VMP-250 turbulence profiler. The YODA profiler (manufactured by JFE Advantech Co. Ltd., Japan; Figure1a) is a tow-yo instrument that can be used to profile the water column with high resolution from small boats, without occupying much space [11]. The instrument is equipped with a deployment winch and sensors that measure conductivity, temperature, pressure, chlorophyll, turbidity, and DO. The brush at the top of the instrument provides a stabilizing effect on the free-fall sinking speed, which is approximately constant at 0.2 m/s. The sampling frequency is 10 Hz. All data are stored internally and downloaded to a PC through a wet connector and interface. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 3 of 18

The sampling frequency is 10 Hz. All data are stored internally and downloaded to a PC through a wet connector and interface. The VMP-250 (manufactured by Rockland Scientific inc., Canada; Figure 1b) is a coastal-zone profiler for the measurement of microscale turbulence. The profiler is designed for operation off small vessels with limited deck space (e.g., zodiacs) or where electrical power facilities are limited or missing (e.g., ice camps). The instrument is equipped with two shear probes, one thermistor, and a nose-mounted conductivity– temperature sensor. The VMP-250 has a constant free-fall sinking speed of approximately J. Mar. Sci. Eng. 2021, 9, 657 3 of 18 0.7 m/s. The sampling frequency is 1024 Hz. All data are stored internally and downloaded to a PC through a wet connector and interface.

(a) (b)

Figure 1. (a) The Yoing Ocean Data Acquisition (YODA) profiler. (b) The VMP-250 turbulence profiler. Figure 1. (a) The Yoing Ocean Data Acquisition (YODA) profiler. (b) The VMP-250 turbulence profiler The. VMP-250 (manufactured by Rockland Scientific inc., Canada; Figure1b) is a coastal-zone profiler for the measurement of microscale turbulence. The profiler is designed for operation off small vessels with limited deck space (e.g., zodiacs) or where electrical 2.2. Observations power facilities are limited or missing (e.g., ice camps). The instrument is equipped with twoDYB shear is probes, a large one semi thermistor,-enclosed and bay a nose-mounted (ca. 24 km conductivity–temperaturein length and 13–25 km sensor. in width, coveringThe VMP-250 an area has of a ca. constant 600 km free-fall2). The sinking bay has speed an irregular of approximately coastline 0.7 and m/s. contains The sampling more than 50 frequencyislands. It is is 1024 loc Hz.ated All along data are the stored south internally coast of and China downloaded (114°29.7′ to a– PC114 through°49.7′ E, a wet22°31.2′– 22°connector50.0′ N), andas shown interface. in Figure 2. Measurements were taken between 20 October and 22 October2.2. Observations 2015. On 20 October 20 2015, we obtained conductivity, temperature, pressure, chlorophyll, turbidity, and DO data, using YODA along the western boundary of DYB DYB is a large semi-enclosed bay (ca. 24 km in length and 13–25 km in width, covering (S1)an and area a of cross ca. 600-section km2). of The the bay mouth has an of irregular the bay coastline (S2) (Figure and contains 2). The more S1 observation than 50 islands. transect ranIt from is located the north along to the the south south, coast with of China the survey (114◦29.7 starting0–114◦49.7 at 012:20E, 22 ◦local31.20–22 time◦50.0 and0 N), ending as at 14:50,shown whereas in Figure S22 . ran Measurements from the werewest taken to the between east, with 20 October the survey and 22 October starting 2015. at 16:00 On and ending20 October at 18:30. 20 2015, During we obtained the cruise, conductivity, YODA temperature, obtained 53 pressure, and 56 chlorophyll, profiles at turbidity, S1 and S2, respectively.and DO data, The using average YODA space along theinterval western between boundary successive of DYB (S1) YODA and a measurements cross-section of was approtheximately mouth of the230 bay m, (S2)enabling (Figure fine2). The-scale S1 observations observation transect of physical ran from phenomena. the north to the south, with the survey starting at 12:20 local time and ending at 14:50, whereas S2 ran from the west to the east, with the survey starting at 16:00 and ending at 18:30. During the cruise, YODA obtained 53 and 56 profiles at S1 and S2, respectively. The average space interval between successive YODA measurements was approximately 230 m, enabling fine-scale observations of physical phenomena. The VMP-250 profiler was used to measure turbulence at a fixed station at the mouth of DYB (yellow triangle in Figure2b). We obtained 74 turbulence profiles from 10:00 on 21 October to 7:30 on 22 October 2015. The time intervals between VMP-250 measurements ranged from 10 to 30 min; therefore, any rapid changes in physical phenomena could be observed.

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Figure 2. (a) Location of Daya Bay (DYB) along the southern coast of China. The arrow indicates the sea wind direction. (b) Bathymetric map of DYB, in which S1 and S2 indicate the two YODA observation transects (red lines). The yellow triangle and square indicate the locations of VMP-250 profiler observations and the blue arrows indicate vessel route directions. T1 Figure(black 2. circle) (a) isLocation the observation of Daya station Bay of tidal(DYB) elevation. along the southern coast of China. The arrow indicates the sea wind direction. (b) Bathymetric map of DYB, in which S1 and S2 indicate the two YODA observation transects (red lines).There The are no yellow major riverstriangle discharged and square into DYB, indicate but three the locations small streams of VMP discharged-250 into Dapeng Cove. From September to October 2015, the average rainfall in DYB is less profiler observations and the blue arrows indicate vessel route directions. T1 (black circle) is the than 30 mm. The sea surface wind was northeasterly and was less than 10 m/s during the observation station of tidalobservation elevation. period, indicating that our observation was less affected by surface runoff and sea surface wind field. Therefore, the effects of rainfall, runoff and sea surface wind are The VMP-250 profilerignored was in the used latter analysis.to measure The tidal turbulence current in DYB at a is fixed characterized station by at diurnal the mouth tide and semidiurnal tide and the semidiurnal tide is more prominent ([18]). of DYB (yellow triangle in Figure 2b). We obtained 74 turbulence profiles from 10:00 on 21 October to 7:30 2.3. on Calculation 22 October of Turbulence 2015. The time intervals between VMP-250 measurements ranged fromThe shear 10 probes to 30 in min; the VMP-250 therefore, profiler any measured rapid high-frequency changes in velocity physical fluctu- phenomena could be ations,observed. which were used to calculate the local turbulent dissipation rate. The dissipation rate was estimated by using the following isotropic formula: There are no major rivers discharged into DYB, but three small streams discharged 15 ∂u 15 Z kmax into Dapeng Cove. From September to Octoberε = v 2015,= the vaverageψ(k) drainfallk in DYB is less(1) than 30 mm. The sea surface wind was northeasterly2 ∂z and2 was0 less than 10 m/s during the observation period, indicatingwhere ν is the that kinematic our observation molecular viscosity, was the less overline affected indicates by asurface spatial average, runoffu and sea surface windis field. either Therefore, one of the two the horizontal effects componentsof rainfall of, runoff velocity, andz is thesea vertical surface coordinate, wind ψ(k) is the spectrum of vertical shear, and k is the vertical wavenumber [23]. The upper are ignored in the latterlimit analysis. of the integration, The tidalkmax current, is variable. in DYB The iskmax characterizedvalue was calculated by diurnal by using tide the and semidiurnal tide methodand the recommended semidiurnal by Shangtide is et more al. [24], prominent which states that(Song it should et al., be 2016). the smallest of the following values: the lowest frequency that shows corruption of the shear signal by vibrations, a wavenumber of 150 cpm due to the spatial resolution of the shear probe, the 2.3. Calculation of Turbulencecutoff frequency of the antialiasing filter, an estimate of the wavenumber that resolves 90% The shear probesof the in shear the variance VMP according-250 profiler to the Nasmyth measured spectrum, high using-frequency the method of velocity Lueck [25], or the location of the spectral minimum determined with a low-order fit to the spectrum in fluctuations, which werelog–log used space. Diapycnal to calculate diffusivity the [23 local] was turbulentcalculated based dissipation on the dissipation rate. rate The (ε ) dissipation rate was estimatedand stratification by using as follows: the following isotropic formula: ε κ = Γ (2) ρ N2 15  u  15 kmax ε  v   v ψkdk (1) 2  z  2 0 where is the kinematic molecular viscosity, the overline indicates a spatial average, u is either one of the two horizontal components of velocity, z is the vertical coordinate, () is the spectrum of vertical shear, and k is the vertical wavenumber [23]. The upper limit of the integration, kmax, is variable. The kmax value was calculated by using the method recommended by Shang et al. [24], which states that it should be the smallest of the following values: the lowest frequency that shows corruption of the shear signal by vibrations, a wavenumber of 150 cpm due to the spatial resolution of the shear probe, the cutoff frequency of the antialiasing filter, an estimate of the wavenumber that resolves 90% of the shear variance according to the Nasmyth spectrum, using the method of Lueck [25], or the location of the spectral minimum determined with a low-order fit to the

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spectrum in log–log space. Diapycnal diffusivity [23] was calculated based on the dissipation rate (ε) and stratification as follows: J. Mar. Sci. Eng. 2021, 9, 657  5 of 18   Γ (2)  N 2 where Γ is the mixing efficiency, which was set to 0.2. The stratification was calculated as where Γ is the mixing efﬁciency, which was set to 0.2. The stratiﬁcation was calculated as 2 g  follows: N2 = − g ∂ρ , where g is the acceleration due to gravity. follows: N ρ ∂z , where g is the acceleration due to gravity.

2.4.2.4. A WorkflowWorkflow Graph FigureFigure3 3 provides provides a a workflow workflow graph graph toto recap on the logic logic path path of of the the study. study. The The workflowworkflow mainly consists of of three three parts, parts, including including the the used used instruments, instruments, the the data data types, types, andand thethe observed costal phenomena.

FigureFigure 3.3. The workflow workﬂow graph to to recap recap on on the the logic logic path path of of the the study. study.

3.3. ResultsResults 3.1. High-Resolution Observations by YODA 3.1. High-Resolution Observations by YODA In the section, features of coastal phenomenon, including the nuclear power station In the section, features of coastal phenomenon, including the nuclear power station discharged warm water, upwelling and front, are revealed by high spatial resolution discharged warm water, upwelling and front, are revealed by high spatial resolution observationsobservations by YODA. 3.1.1.3.1.1. Water Properties OurOur efforts efforts to to obtain obtain high-resolution high-resolution data data for for DYB DYB by using by using the YODA the YODA proﬁler profiler proved toproved be successful. to be successful. Figures Figures4 and5 show4 and 5 the show observed the observed data, which data, which indicated indicated that the that water the environmentwater environment of DYB of was DYB complex, was complex, with obvious with obvioushorizontal horizontal spatial and spatial vertical and differences vertical indiffe temperaturerences in temperature and salinity and characteristics. salinity characteristics. The temperature The temperature varied from varied 26.2 from to 27.4 26.2◦ C, andto 27.4 salinity °C, and varied salinity from varied 32.5 to from 33.2 32.5 psu. to The 33.2 water psu. The along water transect along S1 transect was warm S1 was and warm fresh to theand north, fresh coldto the and north, salty cold in the and middle, salty in and the warm middle, and and fresh warm farther and to fresh the south. farthe Ther to waterthe wassouth. warm The andwater salty was to warm the west, and andsalty cold to the and west, fresh and to the cold east. and The fresh temperature–salinity to the east. The (T–S)temperature sections–salinity (Figures (T4– S)and sections5) and (Figures T–S diagrams 4 and 5 (Figure) and T–6S) diagrams indicate that(Figure there 6) indicate were at leastthat there ﬁve distinctwere at least water five masses distinct along water transect masses S1along and transect three distinct S1 and three water distinct masses water along transectmasses along S2. The transect different S2. The water different masses water might masses have might been have formed been byformed different by different dynamic characteristics.dynamic characteristics.

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Figure 4. 4. ((aa)) Temperature, Temperature, (b (()bb salinity,)) salinity,salinity, and andand (c) (density(cc)) densitydensity along alongalong transect transecttransect S1 as S1 S1measured asas measuredmeasured by the by byYODA thethe YODA YODA proﬁler.profiler.

Figure 5. (a) Temperature, (b) salinity, and (c) density along transect S2 as measured by the YODA profiler. FigureFigure 5. (a) Temperature, ((bb)) salinity,salinity, andand ((cc)) densitydensity alongalong transecttransect S2S2 asas measuredmeasured byby thethe YODAYODA proﬁler.profiler.

The different water masses along transect S1 could be separated into the following ﬁve distinct areas. Area 1 had a latitude greater than 22.56◦ N, with warmer and fresher water found at the surface. Water in this area was affected by thermal discharges from the nuclear power station. Area 2 was found between the latitudes 22.525◦ N and 22.535◦ N, with saltier water present throughout the whole depth of the water column and no obvious changes in temperature with depth. Area 3 was found between the latitudes 22.505◦N and 22.525◦ N, with warmer water present near the surface and no obvious changes in salinity with depth. Area 4 had a latitude lower than 22.505◦ N, with cooler water present throughout the whole depth of the water column. Area 5 was found between the latitudes 22.535◦ N and 22.56◦ N and represented a transition region.

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FigureFigure 6. 6.Temperature–salinity Temperature–salinity (T–S) (T–S) diagrams diagrams for for transects transects ( a()a) S1 S1 and and ( b(b)) S2. S2. Dashed Dashed lines lines indicate indicate potential densities. potential densities.

TheThe differentdifferent waterwater massesmasses alongalong transecttransect S2S1 could could be be separated separated into into the the following following threefive distinct distinct areas. areas. Area Area 1 1 had had a a latitude longitude greater greater than than 22.56° 114.7 N,◦ withE, with warmer cold and and fresher fresher waterwater in found the upper at the layer.surface. Area Water 2 was in foundthis area between was affected the longitudes by thermal 114.655 discharges◦ E and from 114.7 the◦ E,nuclear with saltier power water station. present Area near 2 was the found bottom between and no the obvious latitudes changes 22.525° in temperature N and 22.535° with N, depth.with saltier Area 3 water had a longitude present throughout less than 114.655 the whole◦ E, with depth warmer of the and water fresher column water present and no nearobvious the surface. changes Generally, in temperature water with on a depth.shelf is Area cooler 3 and was fresher found thanbetween in the the adjoining latitudes ocean,22.505°N leading and to 22.525° the formation N, with of warmer a shelf water break frontpresent [26 near]. Therefore, the surface it was and reasonable no obvious to assumechanges that in thesalinity cold with water depth. at longitudes Area 4 greaterhad a latitude than 114.7 lower◦ E originated than 22.505° from N, the with bay. cooler The areawater between present 114.655 throughout◦ E and the 114.7 whole◦ E was depth considered of the a water transition column. region. Area 5 was found between the latitudes 22.535° N and 22.56° N and represented a transition region. 3.1.2.The Influence different of the water Nuclear masses Power along Station transect S2 could be separated into the following threeAlthough distinct areas. the observation Area 1 had area a longitude was small, greater the water than characteristics 114.7° E, with of cold DYB and exhibited fresher obviouswater in spatial the upper variability layer. Area over 2 a distancewas found of between approximately the longitudes 12.5 km. 114.655° E and 114.7° E, withA significant saltier water feature present of near water the temperature bottom and wasno obvious that it waschanges clearly in temperature affected by with the thermaldepth. Area discharge 3 had a from longitude the nuclear less than power 114.655° station E, with in warmer the north, and and fresher the water temperature present wasnear significantly the surface. higherGenerally, in the water north on than a shelf in other is cooler places and (Figure fresher4a). than The in surface the adjoining water ◦ temperatureocean, leading was to asthe highformation as 27.4 of aC shelf in the break northernmost front [26]. Therefore, observation it was position, reasonable which to ◦ wasassume approximately that the cold 2 kmwater from at thelongitudes nuclear greater power station.than 114.7° This E was originated 1 C higher from than the bay. the waterThe are temperaturea between 114.655° in the south. E and The 114.7° warm E was water considered from the thermala transition discharge region. was mainly concentrated in the upper 10 m and extended approximately 5 km to the south of the ◦ release3.1.2. Influence orifice, which of the correspondedNuclear Power to Station 22.56 N (Figure4a). This range at which water dischargeAlthough from the the observation nuclear power area station was small, influenced the water water characteristics temperature of was DYB consistent exhibited withobvious the satellitespatial variability remote-sensing over a study distance of [10 of]. approximately It has been reported 12.5 km. that during the winter months, the thermal plume formed by water discharge from the nuclear power station A significant feature of water temperature was that it was clearly affected by the is localized to an area within a few kilometers of the power plant. During the summer thermal discharge from the nuclear power station in the north, and the temperature was months, there is a larger thermal plume that extends 8–10 km south along the coast [10]. significantly higher in the north than in other places (Figure 4a). The surface water The strong seasonal difference in the thermal plume is related to vertical mixing in the temperature was as high as 27.4 °C in the northernmost observation position, which was water column in winter and stratification in summer. In addition to the warm water being approximately 2 km from the nuclear power station. This was 1 °C higher than the water discharged from the nuclear power station, a cloud of warm water with a radius of 2 km at temperature in the south. The warm water from the thermal discharge was mainly the surface between 22.5◦ N and 22.52◦ N was also observed. This area of water was 0.3 ◦C concentrated in the upper 10 m and extended approximately 5 km to the south of the warmer than the surrounding water. The temperature difference between the surface and release orifice, which corresponded to 22.56° N (Figure 4a). This range at which water bottom was less than 1 ◦C, indicating weak stratification of the water column during the discharge from the nuclear power station influenced water temperature was consistent observation period. with the satellite remote-sensing study of [10]. It has been reported that during the winter The salinity and chlorophyll-a content were also affected by the power plant. In themonths, area influenced the thermal by plume discharge formed water, by thewater salinity discharge (Figure from4b) the and nuclear chlorophyll-a power station content is localized to an area within a few kilometers of the power plant. During the summer months, there is a larger thermal plume that extends 8–10 km south along the coast [10].

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The strong seasonal difference in the thermal plume is related to vertical mixing in the water column in winter and stratification in summer. In addition to the warm water being discharged from the nuclear power station, a cloud of warm water with a radius of 2 km at the surface between 22.5° N and 22.52° N was also observed. This area of water was 0.3 °C warmer than the surrounding water. The temperature difference between the surface J. Mar. Sci. Eng. 2021, 9, 657 and bottom was less than 1°C, indicating weak stratification of the water column during8 of 18 the observation period. The salinity and chlorophyll-a content were also affected by the power plant. In the area influenced by discharge water, the salinity (Figure 4b) and chlorophyll-a content (Figure7c) were lower than in the south. There was no signiﬁcant difference in DO (Figure(Figure 7c)7 d).were lower than in the south. There was no significant difference in DO (Figure 7d).

Figure 7. Three-dimensional view of (a) temperature, (b) salinity, (c) chlorophyll-a, (d) dissolved oxygen, and (e) turbidity distributions. Figure 7. Three-dimensional view of (a) temperature, (b) salinity, (c) chlorophyll-a, (d) dissolved oxygen, and (e) turbidity distributions. 3.1.3. The Front Water in DYB is cooler and fresher than in the adjoining SCS, resulting in a strong salinity front that is the most notable hydrographic feature in this region (Figure4). There

was a lateral density gradient identiﬁed along both transects S1 and S2. For transect S1, YODA measurements revealed lateral salinity variations as large as 0.7 psu across 1 km between 22.53◦ N and 22.54◦ N. Transect S2 also had a similar variation in salinity between 114.65◦ E and 114.66◦ E. The width of the front was approximately 1 km, and the water depth, h, was 20 m. The p Rossby deformation radius R = g0h/ f was approximately 6 km. The width of the front J. Mar. Sci. Eng. 2021, 9, 657 9 of 18

was much less than the Rossby deformation radius, indicating that it was not maintained 0 0 by geostrophic balance. Here, g is the reduced gravity, which is given by g = g(r2 − r1)/r, where r1 and r2 are the densities of the upper and lower layers, respectively, r is the mean density, g = 9.81 m/s2, and f is the Coriolis parameter. In the front area along transect S1, chlorophyll-a increased from 3 to 8 g/L, and the turbidity decreased from 4 to 1 FTU. We assumed that the strong front in S1 was formed at the intersection of different water masses and was strengthened by an upwelling system, in which cooler and saltier water at the bottom was dragged to the upper layer (Section 3.1.4). The presence of the front also prevented water from the adjoining SCS from entering the inner area of DYB.

3.1.4. The Upwelling A cold salty water mass dominated the area between 22.52◦ N and 22.54◦ N. The turbidity was relatively high from the bottom to the sea surface, whereas the DO and chlorophyll-a concentrations were relatively low. The distributions of the parameters indicated a pumping process. The width of the upwelling was approximately 2 km. The most direct evidence of upwelling was the turbidity signal. Figure7e shows that the turbidity was much higher at depth than in the upper layer. It was induced by friction between water layers, making the water in the bottom boundary layer turbid. In the area between 22.52◦ N and 22.54◦ N, the whole water column was more turbid than the surrounding water. This could only be explained by the turbidity of the entire water body being derived from the turbid water body at the bottom. The occurrence of an upwelling is closely related to wind conditions. In the Northern Hemisphere, when the wind blows parallel to the coast for a long time, under the action of a geostrophic deflection force, the wind drift formed by the wind causes the surface water to leave the coast. This in turn causes the shallower water near the shore to rise and form an upwelling. In the North SCS, the most notable upwellings are the Eastern Guangdong upwelling and the Eastern Hainan upwelling, which occur in summer months [27–29]. In some case, the wind in particular direction acting on the vorticity field can generate vertical motions. For example, Chang et al. [30] reported that the seaward increasement of cyclonic vorticity near the Kuroshio’s western edge favors a stronger Ekman transport away from the jet, producing upwelling at the shelfbreak under a northeasterly wind. The passage of high-energy atmospheric systems also may stir up the upwelling [31]. In this study, however, the upwelling mechanism described above cannot explain the upwelling observed here in October. The upwelling areas caused by large-scale wind fields are more extensive than the upwelling area observed in this study, which had a width of approximately 2 km. The sea surface wind field far from the high-energy atmospheric system during the observation period is also not a cause of the vertical motions of water. When the surface water is blocked by the coast or an island, sea water will gather and diverge in the horizontal direction, and an upwelling may occur in the vertical direction. In the study region, the terrain is flat in the area where the upwelling was observed (Figure1), which indicates that the upwelling was induced by other dynamic mechanisms. We assumed that the upwelling was formed by the interaction between tidal currents and the topography. Upwelling can bring nutrients from the bottom layer to the surface layer, make plankton grow in large quantities, and provide food for fish. Therefore, upwelling areas are often important fishing grounds, such as the Peru fishing grounds, which benefit from the cold Peru Current (upwelling compensation current). In our study area, the upwelling may also have an important impact on the local environment. As shown in Figures7 and8, the upwelling changed the distribution of chlorophyll-a and DO in the local area and carried turbid material from the bottom up to the surface. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 10 of 18

We assumed that the upwelling was formed by the interaction between tidal currents and the topography. Upwelling can bring nutrients from the bottom layer to the surface layer, make plankton grow in large quantities, and provide food for fish. Therefore, upwelling areas are often important fishing grounds, such as the Peru fishing grounds, which benefit from the cold Peru Current (upwelling compensation current). In our study J. Mar. Sci. Eng. 2021, 9, 657 area, the upwelling may also have an important impact on the local environment.10 of 18As shown in Figures 7 and 8, the upwelling changed the distribution of chlorophyll-a and DO in the local area and carried turbid material from the bottom up to the surface.

FigureFigure 8. 8.( a(a)) Temperature, Temperature, salinity, salinity, (b ()b chlorophyll-a,) chlorophyll-a, dissolved dissolved oxygen, oxygen and, and turbidity turbidity at aat depth a depth of 3of m3 alongm along transect transect S1. TheS1. The ﬁlled filled backgrounds backgrounds indicate indicate the upwelling the upwelling (gray), (gray), front (orange), front (orange), and warm and warm discharge water from the nuclear reactor (pink). discharge water from the nuclear reactor (pink).

3.2.3.2. High-ResolutionHigh-Resolution ObservationsObservations byby thethe VMP-250VMP-250 ProfilerProfiler InIn the the section, section, feature feature of coastal of coastal phenomenon phenomenon of front andof front its accompanied and its accompanied turbulence areturbulence revealed are by revealed high temporal by high resolution temporal observations resolution observations by YODA. by YODA. 3.2.1. Water Properties 3.2.1. Water Properties We used the VMP-250 profiler to observe turbulence for 22 h at a fixed location and We used the VMP-250 profiler to observe turbulence for 22 h at a fixed location and obtained 74 profiles. The time intervals varied between 10 and 30 min (Figure 9c). A nose- obtained 74 profiles. The time intervals varied between 10 and 30 min (Figure9c). A mounted conductivity–temperature sensor was installed on the VMP-250 profiler, nose-mounted conductivity–temperature sensor was installed on the VMP-250 profiler, enabling the analysis of temperature and salinity characteristics observed at continuous enabling the analysis of temperature and salinity characteristics observed at continuous monitoring stations. monitoring stations. The measurements indicated that both the temperature and salinity changed rapidly The measurements indicated that both the temperature and salinity changed rapidly with time. Using density data, we calculated the depth of the surface mixed layer and the with time. Using density data, we calculated the depth of the surface mixed layer and the thickness of the bottom mixed layer (Figure 9c). The depth of the mixed layer was defined thickness of the bottom mixed layer (Figure9c). The depth of the mixed layer was defined 3 asas thethe point point at at which which the the density density varied varied by by less less than than 0.05 0.05 kg/m kg/m3.. TheThe depthdepth of of the the upper upper mixedmixed layerlayer waswas 88 m,m, whereaswhereas thethe thicknessthickness ofof thethe bottom bottom mixed mixed layerlayer extended extended toto 8 8 m m (the(the waterwater depth depth at at the the observation observation station station was was ca. ca. 20 20 m). m). Sometimes, Sometimes, aa bottombottom mixed mixed layerlayer could could not not be be discerned. discerned. Activities Activities in in the the upper upper water water extended extended to to the the sea sea bottom bottom at at timestimes (e.g., (e.g., at at 20:00 20:00 on on 21 21 October October 2015). 2015). The The bottom bottom mixed mixed layer layer was was mainly mainly affected affected by by tides,tides, with with the the mixed mixed layer layer depth depth increasing increasing at high at tide high (Figure tide9 c,f). (Figure The largest9c,f). The buoyancy largest fluxesbuoyancy were fluxes in the were stratified in the region stratified just above region the just bottom above mixed the bottom layer (Figuremixed 9layerd). (Figure 9d). The T–S sections and T–S diagrams (Figure 10) indicate that the water masses at variousThe stations T–S sections comprised and T distinct–S diagrams water (Figure masses. 10 For) indicate example, that between the water 21:00 masses on 21 at Octobervarious 2015 stations and comprised 3:00 on 22 October distinct 2015,water the masses. water For temperature example, was between low and 21:00 salinity on 21 was high, especially in the bottom mixed layer. This was a period of high tide, indicating that SCS water was dominant during this period. From 6:00 to 8:00 on 22 October 2015, the upper layer of water exhibited a low temperature and low salinity. This was the ebb tide period, indicating that DYB water was the dominant water mass. Compared with the Figure6, the characteristics of the seawater was confirmed. The observation station was located at the confluence of the SCS and DYB, which changed constantly under the action of tides. With the interactions at the confluence, an abundance of thermohaline structures was formed. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 11 of 18

October 2015 and 3:00 on 22 October 2015, the water temperature was low and salinity was high, especially in the bottom mixed layer. This was a period of high tide, indicating that SCS water was dominant during this period. From 6:00 to 8:00 on 22 October 2015, the upper layer of water exhibited a low temperature and low salinity. This was the ebb tide period, indicating that DYB water was the dominant water mass. Compared with the J. Mar. Sci. Eng. 2021, 9, 657 Figure 6, the characteristics of the seawater was confirmed. The observation station was11 of 18 located at the confluence of the SCS and DYB, which changed constantly under the action of tides. With the interactions at the confluence, an abundance of thermohaline structures was formed.

Figure 9. Time courses of (a) temperature, (b) salinity, (c) dissipation rate, (d) buoyancy frequency, (e) diffusivity, (f) water level during the observation periods, and (g) water level of October 2015. Isopycnal contours (solid lines) in (a–e) represent density. The front is marked with an inverted triangle in (a). In (c–e), the red line indicates the surface mixed layer, whereas the yellow line indicates the bottom mixed layer. Vertical dotted lines in (f) indicate the time points of VMP-250 observations. The water-level data in (f,g) were derived at the tidal elevation station T1. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 12 of 18

Figure 9. Time courses of (a) temperature, (b) salinity, (c) dissipation rate, (d) buoyancy frequency, (e) diffusivity, (f) water level during the observation periods, and (g) water level of October 2015. Isopycnal contours (solid lines) in (a–e) represent density. The front is marked with an inverted triangle in (a). In (c–e), the red line indicates the surface mixed layer, whereas the yellow line J. Mar. Sci. Eng. 2021, 9, 657 indicates the bottom mixed layer. Vertical dotted lines in (f) indicate the time points of VMP12 of-250 18 observations. The water-level data in (f,g) were derived at the tidal elevation station T1.

FigureFigure 10.10. T–ST–S diagramdiagram ofof VMP-250VMP-250 measurements.measurements. DashedDashed lineslines indicateindicate potentialpotential densities.densities.

3.2.2.3.2.2. The Front InIn addition to YODA YODA measurements, measurements, lateral lateral density density gradients gradients were were also also identified identified in inthe the study study area area from from the the fixed fixed station station observations observations (Figure (Figure 99).). These These gradients gradients were were controlledcontrolled by by variations variations in both in both salinity salinity and temperature. and temperature. Stratification Stratification persisted persiste through-d outthroughout the upper the water upper column, water column, with considerable with considerable variability variability along along isopycnal. isopycnal. VMP-250 VMP- ◦ measurements250 measurements indicated indicated lateral lateral temperature temperature variations variations as large as as large 0.4 Cas and 0.4 lateral°C and salinity lateral variationssalinity variations as large as 0.3large psu as over0.3 psu 30-min overperiods 30-min atperiods 11:30 andat 11:30 20:30 and on 20:30 21 October on 21 October 2015. It is2015. noted It is that noted the conventionalthat the conventional definition definition of density of front density (dρ/d frontx) is (d relatedρ/dx) is to related the variation to the ofvariation horizontal of horizontal distance of distance density. of Here, density. we assume Here, we that assume the density that the front density does not front change does innot a change certain in period a certain of time period when of time it moves when in it moves the flow in direction,the flow direction, dρ/dt = dUρd/dρ/dt = xU.dρ/d Thus,x. theThus, larger the thelarger variation the variation in dρ/d int, d theρ/d strongert, the stronger the density the density front. Therefront. wereThere two were strong two 3 frontsstrong atfronts these at two these moments, two moments, with density with density variations variations of approximately of approximately 0.2 kg/m 0.2over kg/m a3 30-minover a 30 period.-min period. At 20:30 At on 20:30 21 October on 21 October 2015, the 201 front5, the extended front extended to the bottom. to the The bottom. width The of thewidth front of wasthe front approximately was approximately 1 km (Section 1 km 3.1.3 (Section), and it3. took1.3), and approximately it took approximately 30 min for the 30 frontmin for to crossthe front the observationto cross the observation position. Therefore, position. we Therefore, inferred we that inferred the speed that of the the speed front wasof the approximately front was approximately 0.5 m/s, which 0.5 ism/s, similar which to is the similar tidal currentto the tidal velocity current [18 ].velocity [18]. 3.2.3. Turbulence 3.2.3. Turbulence Examples of dissipation spectra at specified depth segments from the VMP-250 mea- Examples of dissipation spectra at specified depth segments from the VMP-250 surement at 12:15 on 21 October 2015 are shown in Figure 11. There were two shear probes measurement at 12:15 on 21 October 2015 are shown in Figure 11. There were two shear installed at the head of profiler, and data from both probes are shown. The spectra were ap- probes installed at the head of profiler, and data from both probes are shown. The spectra proximately consistent with the Nasmyth universal spectrum, indicating the credibility of were approximately consistent with the Nasmyth universal spectrum, indicating the the measurement. The triangles indicate the upper limit of integration. The raw microscale velocitycredibility shear of the was measurement. weak between The 4 and triangles 8 m; therefore, indicate the estimatedupper limit dissipation of integration. rate wasThe smallraw microsc (ε~10−7aleW/kg). velocity The shear microscale was weak velocity between shear strengthened4 and 8 m; therefore, between 8the and estimated 12 m, as −7 diddissipation the dissipation rate was (ε~10 small−6 W/kg). (ε~10 W/kg). The microscale velocity shear strengthened between 8 and 12 m, as did the dissipation (ε~10−6 W/kg).

J. Mar. Sci. Eng. 2021, 9, 657 13 of 18 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 13 of 18

FigureFigure 11. (a 11.) Examples(a) Examples of raw of microscale raw microscale velocity velocity shear shear in specified in speciﬁed depth depth segments segments during during data data recordingrecording at 12:15 at 12:15 on 21 on October 21 October 2015 2015and ( andb,c) (theb,c )corresponding the corresponding dissipation dissipation spectra. spectra. The smooth The smooth curves overlapping the dissipation spectra represent the Nasmyth universal spectrum, and triangles curves overlapping the dissipation spectra represent the Nasmyth universal spectrum, and triangles indicate the upper limit of integration. indicate the upper limit of integration.

It hasIt been has been reported reported that thatdissipation dissipation and andmixing mixing on the on shelf the shelf are mainly are mainly controlled controlled by nearby near-bottom-bottom friction friction and andshear shear from from low- low-frequencyfrequency internal internal waves waves and solibores and solibores [26]. [26]. However,However, according according to our to observatio our observations,ns, the front the front that thatformed formed at the at intersection the intersection of the of the SCS SCSand DYB and DYBalso alsoplays plays an important an important role rolein local in local energy energy dissipation. dissipation. Figure Figure 9c shows9c shows the the dissipationdissipation rates rates and andassociated associated diapycnal diapycnal diffusivities diffusivities for each for each profile. profile. The Thedissipation dissipation ratesrates of the of thermocli the thermoclinene and bottom and bottom boundary boundary layer layer were were of the of same the same order order of magnitude. of magnitude. The thicknessThe thickness of the of bottom the bottom mixing mixing layer layer has the has characteristics the characteristics of periodic of periodic variation, variation, and and its increaseits increase is consistent is consistent with with the period the period of high of high tide. tide. The Thedissipation dissipation rate rateat the at bottom the bottom also alsoincrease increase during during the high the hightide tideperiods periods (Figure (Figure 9e). 9Takene). Taken together together with with Figure Figure 9c,e,f,9c,e,f, it it was wasconcluded concluded that thatthe turbulence the turbulence in the in bottom the bottom mixed mixed layer layer was driven was driven by tidal by tidalcurrents. currents. In theIn upper the upper mixing mixing layer, layer, ocean ocean mixing mixing is mainly is mainly controlled controlled by buoyancy by buoyancy flux fluxand andwind wind momentummomentum flux flux[32]. [In32 ].this In study, this study, we focused we focused on the on dissipation the dissipation in the in thermocline. the thermocline. The dissipationThe dissipation rate rate of the of thethermocline thermocline was was mainly mainly between between 1 1× ×1010−9 −and9 and 1 ×1 10×−610 W/kg,−6 W/kg, withwith an average an average value value of 8 × of10 8−8 ×W/kg.10− 8TheW/kg. maximum The maximum value was value mainly was record mainlyed at recorded 11:30, at 16:00,11:30, and 16:00,20:30 on and 21 20:30 October on 21 2015, October as well 2015, as as 3:00 well on as 22 3:00 October on 22 2015, October at which 2015, at point which the point densitythe densityprofile changed profile changed substantially. substantially. Peak dissipation Peak dissipation at 5:00 at on 5:00 22 on October 22 October 2015 2015was was inducedinduced in a patch in a patch near nearthe bottom. the bottom. Average Average diffusivity diffusivity profiles profiles were were calculated calculated from from the the averageaverage dissipation dissipation rate rateand andstratification. stratification. Because Because diffusivity diffusivity was wasnot well not welldefined defined in the in the wellwell-mixed-mixed boundary boundary layers, layers, average average diffusivity profiles profiles are are only only shown shown for data for excluding data excludingthe boundary the boundary layer layer (Figure (Figure 12a). 1 The2a). averageThe average diffusivity diffusivity associated associated with with mid-column, mid- − column,stratified stratified turbulence turbulence was was 5.8 5.8× 10 × 105 −5m m2/s2/s for the whole whole profile, profile, which which is is one one order order of of magnitudemagnitude greater greater than than the the level level in in an an open open-ocean-ocean thermocline thermocline [33,34] [33,34. ].

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FigureFigure 12.12. TimeTime coursescourses ofof thethe ((aa)) averageaverage dissipationdissipation raterate andand diffusivity,diffusivity, andand ((bb)) averageaverage densitydensity andand variationvariation inin ddρ/dt.ρ/dt. ( cc)) Relationship Relationship between between the the dissipation dissipation rate rate and variation in dρ/dt. dρ/dt.

ToTo analyzeanalyze thethe relationshiprelationship betweenbetween frontfront andand turbulentturbulent mixingmixing inin thethe thermocline,thermocline, thethe densitydensity ofof waterwater waswas averaged,averaged, excludingexcluding thethe boundaryboundary layer.layer. ChangesChanges inin averageaverage densitydensity withwith timetime (d(dρρ/d/dt) were calculated. Active frontsfronts existedexisted onon 2121 NovemberNovember butbut therethere werewere fewerfewer frontsfronts onon 2222 NovemberNovember (Figure(Figure 12 12b).b). TheThe frontsfronts detecteddetected betweenbetween 15:0015:00 andand 18:0018:00 onon 2121 OctoberOctober 20152015 werewere affectedaffected byby thethe variationvariation inin densitydensity nearnear thethe surfacesurface mixedmixed layer.layer. ItIt cancan bebe seenseen fromfrom FigureFigure 10 10 that that the the dissipation dissipation rate rate and and variation variation in in d dρρ/d/dtt werewere positivelypositively correlated,correlated, i.e.,i.e., aa largerlarger ddρρ/d/dtt tendedtended toto produceproduce aa largerlarger dissipationdissipation rate.rate. ThisThis indicatesindicates thatthat aa densitydensity frontfront inin thethe studystudy areaarea isis importantimportant forfor locatinglocating thethe sourcessources ofof energyenergy dissipation.dissipation. TheThe turbulentturbulent mixingmixing processprocess inin thethe oceanocean determinesdetermines thethe redistributionredistribution ofof seawaterseawater characteristics.characteristics. This This plays plays a key a key role role in the in redistribution the redistribution of nutrients, of nutrients, DO, and DO, other and ecolog- other icalecological elements elements and substances and substances in the ocean, in the whichocean, then which affects then marine affects productivitymarine producti [35–vity38]. In[35 this–38] study,. In this we study, did not we make did observations not make observations of ecological of elements ecological when elements VMP-250 when measure- VMP- ments250 measurements were recorded. were However, recorded. it was However, apparent it thatwas theapparent front regulated that the front the distribution regulated the of ecologicaldistribution elements of ecological (Figure 7 elements), while turbulent (Figure 7 mixing), while also turbulent played an mixing important also play role.ed an important role. 4. Discussion Comparison with Observations in July 2019 4. Discussion Daya Bay is located in the tropical monsoon region. In this area, the northeast monsoon Comparison with Observations in July 2019 is strong in winter, whereas the weaker southwest monsoon is dominant in summer [39]. DuringDaya winter, Bay the is located water in in DYB theis tropical vertically monsoon mixedunder region. the In influence this area, of the northeast northeast monsoonmonsoon [40 is ]. strong Water in stratification winter, whereas in DYB the varies weaker seasonally. southwest The monsoon development is dominant of physical in phenomenasummer [39] in. Dur theing ocean winter, is closely the water related in DYB to stratification, is vertically andmixed seasonal under variationthe influence in the of formationthe northeast of a front monsoon and its effect[40]. on Water energy stratification dissipation in in the DYB observation varies area seasonally. was obvious. The developmentTo confirm of the physical seasonal phenomena variation inin the the front ocean in is DYB, closely we analyzedrelated to observations stratification, of and the areaseasonal (114.68 variati◦ E,on 22.58 in◦ theN) formation from July 2019of a front (Figure and1). its At effect the observation on energy dissipation site, 138 profiles in the wereobservation obtained area by was YODA obvious. within 48 h, with a time interval of 20 min. The temperature differenceTo confirm between the the seasonal surface variation and bottom in the was front more in than DYB, 6 ◦ C,we and analyzed the salinity observations difference of wasthe area more (114.68° than 2 E, psu 22.58° (Figure N) from 13), whichJuly 2019 were (Figure much 1). larger At the than observation the values site, measured 138 profiles in Octoberwere obtained (Figure by 10 ).YODA The fresh within water 48 h, input with during a time the interval summer of rainy20 min. season The temperature diminished thedifference surface between salinity, the and surface water withand bottom a lower was temperature more than and 6 °C, high and salinitythe salinity intruded difference into thewas bay more along than the 2 psu bottom (Figure from 1 the3), which SCS. Unlike were much the observations larger than madethe values in October, measured there in wasOctober a stable (Figure bottom 10). mixed The fresh layer water in July input that wasduring more the than summer 10 mthick rainy (Figure season 13 diminishedd). Above thethe bottomsurface salinity, mixing layerand water was a with thermocline a lower temperature reflecting strong and high stratification. salinity intruded In the Julyinto 2019the bay observations, along the therebottom was from no obviousthe SCS. density Unlike front,the obs butervations a high-frequency made in October, internal wavethere atwas the a stable thermocline bottom wasmixed observed. layer in July As that shown was in more Figure than 13 10c, m the thick density (Figure (in 1 the3d). regionAbove ofthe 1022.5 bottom kg/m mixing2) oscillated layer was within a thermocline a thickness reflecting of 4 m strong in the stratification. thermocline. In According the July 2019 to

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J. Mar. Sci. Eng. 2021, 9, 657 observations, there was no obvious density front, but a high-frequency internal wave15 of 18at the thermocline was observed. As shown in Figure 13c, the density (in the region of 1022.5 kg/m2) oscillated within a thickness of 4 m in the thermocline. According to observations observationsfrom October from 2015, October vertical 2015, fluctuation vertical of ﬂuctuation the isotherm of the exceeded isotherm 10 exceeded m, and the 10 m,density and thechanged density substantially changed substantially at different attimes different and at times certain and depths. at certain depths.

FigureFigure 13.13. TimeTime coursescourses ofof ((aa)) temperature,temperature, ((bb)) salinity,salinity, ( c(c)) density, density, and and ( d(d)) buoyancy buoyancy frequency frequency as as 3 measuredmeasured byby YODA in in July July 2019 2019.. The The black black line line in in(c) (indicatesc) indicates a density a density of 1022.5 of 1022.5 kg/m kg/m3. Red dotted. Red dottedlines in lines (d) indicate in (d) indicate the surface the surface and bottom and bottom mixed mixedlayers. layers.

InIn autumn, autumn, the the upwelling upwelling along along the the coast coast of of Guangdong Guangdong disappeared disappeared due due to to the the transitiontransition of of the the monsoon. monsoon. Low-temperature Low-temperature and and high-salinity high-salinity water water could could not intrudenot intrude into theinto bay the along bay along the bottom the bottom from the from SCS, the leading SCS, leading to the temperature to the temperature and salinity and ofsalinity the bottom of the waterbottom being water warmer being andwarmer lower, and respectively, lower, respectively, than in summer than in (Figures summer9 and (Figures 13 ). The 9 and larger 13). temperatureThe larger temperature and salinity differencesand salinity between differences the surface between and the bottom surface in summer and bottom led to in strongersummer stratificationled to stronger in stratification July 2019 compared in July 2019 to October compared 2015. to As October shown 2015. in Figure As shown 14, the in buoyancyFigure 14,frequency the buoyancy in summer frequency in the in summer thermocline in the was thermocline more than was one more order than of magnitude one order higher. The strong stratification prevented the upper dynamic waves from crossing the of magnitude higher. The strong stratification prevented the upper dynamic waves from thermocline, and the front was absent in summer. Thus, it was concluded that weak crossing the thermocline, and the front was absent in summer. Thus, it was concluded that stratification in autumn is important for the formation of a front. In the final analysis, the weak stratification in autumn is important for the formation of a front. In the final analysis, transition of the monsoon was the primary factor causing changes in stratification. the transition of the monsoon was the primary factor causing changes in stratification.

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FigureFigure 14. 14.AveragedAveraged profiles proﬁles of buoyancy of buoyancy frequency frequency measured measured in October in October 2015 2015 (summer) (summer) and and July July 20192019 (autumn). (autumn).

5. Conclusions5. Conclusions WeWe used used a tow a tow-yo-yo CTD CTD instrument, instrument, the the YODA YODA profiler, profiler, to to record record high high-resolution-resolution physicalphysical and and biological biological data data in DYB in DYB from from a small a small vessel vessel in October in October 2015. 2015. Observations Observations in thein DYB the DYBrevealed revealed complex complex coastal coastal phe phenomena,nomena, which which included included warm warm-water-water discharge discharge fromfrom a nuclear a nuclear plant, plant, as as well well as as an an upwelling, upwelling, and and strong strong density density front. front. Those Those coastal coastal phenomenaphenomena changed changed the the distribution distribution of ofecological ecological elements. elements. The The upwelling upwelling resulted resulted in in saltiersaltier and and more more turbid turbid water water near near the the bottom, bottom, with with low low chlorophyll chlorophyll-a-a and and DO DO contents, contents, beingbeing transported transported upward upward to the to the surface surface layer layer and and changing changing the the local local water water environment. environment. TheThe front front had had a width a width of of1 km 1 km and and formed formed at the at the intersection intersection of ofthe the SCS SCS and and DYB. DYB. It was It was strengthenedstrengthened by bythe the upwelling upwelling sys system.tem. TurbulenceTurbulence over over a 22 a- 22-hh period period at atthe the mouth mouth of ofDYB DYB was was also also observed observed by byusing using a a microstructuremicrostructure turbulence turbulence profiler. profiler. The The water water column column was was divided divided into into a surface a surface mixed mixed layer,layer, thermocline, thermocline, and and bottom bottom mixed mixed layer. layer. The The turbulence turbulence was was focused focused within within the the therthermocline.mocline. The The dissipation dissipation rate rate and and diapycnal diapycnal diffusivity diffusivity in inthe the thermocline thermocline displayed displayed clearclear variability. variability. The The average average (in (intime time and and space) space) dissipation dissipation rate rate and and diapycnal diapycnal diffusivity diffusivity × −8 × −5 2 in inthe the thermocline thermocline were were 8 8× 1010−8 W/kgW/kg and and 5.8 5.8 × 10−510 m2/s,m respectively,/s, respectively, whic whichh were were one one orderorder of magnitude of magnitude larger larger than than the the respective respective levels levels in an in anopen open-ocean-ocean thermocline. thermocline. Active Active density fronts were observed in the thermocline and were associated with strong turbulent density fronts were observed in the thermocline and were associated with strong mixing. The active front was induced by weak stratification during the observation period. turbulent mixing. The active front was induced by weak stratification during the The results indicate that, rather than internal waves, it was the front that had formed at the observation period. The results indicate that, rather than internal waves, it was the front intersection of different water bodies that played the primary role in energy dissipation at that had formed at the intersection of different water bodies that played the primary role the mouth of DYB. in energy dissipation at the mouth of DYB. Author Contributions: Conceptualization, Y.Q.; methodology, Y.Q.; validation, X.W.; formal analysis, Author Contributions: Conceptualization, Y.Q.; methodology, Y.Q.; validation, X.W.; formal C.Q.; investigation, L.Y., X.W., and X.S. and X.C.; data curation, Y.Q.; writing—original draft prepara- analysis, C.Q.; investigation, L.Y., X.W., and X.S. and X.C.; data curation, Y.Q.; writing—original tion, H.M.; writing—review and editing, H.M.; supervision, S.L. and Z.L.; project administration, draft preparation, H.M.; writing—review and editing, H.M.; supervision, S.L. and Z.L.; project H.M.; funding acquisition, S.L. All authors have read and agreed to the published version of the administration, H.M.; funding acquisition, S.L. All authors have read and agreed to the published manuscript. version of the manuscript. Funding: This study is supported by the National Key R&D Plan of China under contract Nos Funding: This study is supported by the National Key R&D Plan of China under contract Nos 2017YFC0305904, 2017YFC0305804 and 2016YFC1401404; the National Natural Science Foundation 2017YFC0305904, 2017YFC0305804 and 2016YFC1401404; the National Natural Science Foundation of China under contract Nos 42006020, 41630970, and 41806037; the Guangdong Science and Tech- of China under contract Nos 42006020, 41630970, and 41806037; the Guangdong Science and nology Project under contract Nos 2019A1515111044 and 2018A0303130047; the Dedicated Fund Technology Project under contract Nos 2019A1515111044 and 2018A0303130047; the Dedicated for Promoting High-quality Economic Development in Guangdong Province (Marine Economic Fund for Promoting High-quality Economic Development in Guangdong Province (Marine EconomicDevelopment Development Project) under Project) contract under No. contract GDOE[2019]A03; No. GDOE[2019]A03; the CAS Key the Technology CAS Key Talent Technology Program Talent(202012292205). Program (202012292205).

J. Mar. Sci. Eng. 2021, 9, 657 17 of 18

Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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