The Influence of Wind Direction During Storms on Sea Temperature in The

Journal of Marine Science and Engineering

Article The Inﬂuence of Wind Direction during Storms on Sea Temperature in the Coastal Water of Muping, China

Xiangyang Zheng 1,2,3, Yana Ding 4, Yandong Xu 3,5 , Tao Zou 1,2, Chunlei Wang 6 and Qianguo Xing 1,2,*

1 CAS Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research (YIC), Chinese Academy of Sciences (CAS), Yantai 264003, China; [email protected] (X.Z.); [email protected] (T.Z.) 2 Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China 3 Shandong Key Laboratory of Marine Ecological Restoration, Yantai 264006, China; [email protected] 4 Yantai Engineering & Technology College, Yantai 264006, China; [email protected] 5 Shandong Marine Resource and Environment Research Institute, Yantai 264006, China 6 College of Artiﬁcial Intelligence, North China University of Science and Technology, Tangshan 063210, China; [email protected] * Correspondence: [email protected]

Abstract: Sea temperature structures are important for water stratiﬁcation and marine ecosystems. In the coastal water of Muping, China, stationary measurements of sea temperature captured temporal temperature changes during two summer storm events. The north component of the wind during the two storms was opposite. The temperature responded differently to wind directions in the two storm events. A well-validated numerical ocean model was used to investigate the mechanism of sea temperature variation of the coast of Muping. The model revealed that the southerly and

easterly wind was upwelling-favorable in the study area. They generated the shoreward transport of bottom cold water, which induced bottom water cooling, enhanced stratiﬁcation, and weakened

Citation: Zheng, X.; Ding, Y.; Xu, Y.; vertical mixing. On the other hand, the northerly and westerly wind was downwelling-favorable Zou, T.; Wang, C.; Xing, Q. The and enhanced turbulent mixing. The alongshore upwelling-favorable wind caused more cross-shore Inﬂuence of Wind Direction during transport than cross-shore upwelling-favorable wind, which resulted in stronger bottom cooling. Storms on Sea Temperature in the Similarly, alongshore downwelling-favorable wind generated lower temperature than cross-shore Coastal Water of Muping, China. J. wind. A surface cold-water band was formed in the second storm. Although it was formed during Mar. Sci. Eng. 2021, 9, 710. https:// upwelling-favorable wind, the temperature balance analysis indicated that vertical mixing and doi.org/10.3390/jmse9070710 westward horizontal advection were the two dominant processes compared to upwelling.

Academic Editor: Kyung-Ae Park Keywords: storms; wind direction; temperature structure; upwelling-favorable wind; stratification; ocean models Received: 2 June 2021 Accepted: 23 June 2021 Published: 27 June 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Muping is a town in the north coast of the Shandong Peninsula, China (Figure1). In published maps and institutional affil- recent years, bottom hypoxia (Dissolved Oxygen < 2 mg/L) has appeared in the summer iations. in the coastal water of Muping [1]. Low dissolved oxygen (DO) concentration in this area strongly impacts the sea cucumber farming. Similar to other well-known hypoxia zones such as the Chesapeake Bay and the Yangtze River estuary [2,3], water columns in the coastal water of Muping are also well stratified in summer when hypoxia occurs. Vertical turbulent mixing in stratified water is very weak, and bottom DO may decrease Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. significantly due to the slow replenishing rate from surface water. This article is an open access article The coastal water of Muping is part of the north Yellow Sea. In summer (June–August), distributed under the terms and the north of China is mainly influenced by weak southerly wind and strong positive net conditions of the Creative Commons heat flux [4]. Both contribute to the formation of water stratification. Many parts of the Attribution (CC BY) license (https:// Yellow Sea are strongly stratified in summer [5–7]. There are two bottom cold-water masses, ◦ creativecommons.org/licenses/by/ and the bottom temperature at the two centers of cold-water masses is below 7 C in the ◦ ◦ 4.0/). summer [7,8]. One cold water mass center is approximately located at (35.3 N, 123.6 E)

J. Mar. Sci. Eng. 2021, 9, 710. https://doi.org/10.3390/jmse9070710 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, 710 2 of 19

and the other is located at (38.3◦ N, 122.2◦ E). The locations of cold-water center in the Yellow Sea are not fixed but vary with the meteorological forcing [9,10]. Muping is about 90 km southwest of the second cold-water center, and the coastal water of Muping may be influenced by the bottom cold-water mass. With the increase of air temperature from March to July, temperature stratification in the Yellow Sea is enhanced. The temperature stratification in the Yellow Sea can be very stable under normal weather conditions in the summer. In July and August, strong wind during storm events may be the only driving force that temporarily relieves or breaks down water stratification. When typhoon storms sweep the Yellow Sea, a strong wind could mix the whole water columns and erode the temperature stratification [11,12]. Vertical mixing of the water column is enhanced by strong winds, and sea surface temperature (SST) could be sharply reduced during storms [13,14]. Storms usually last from 1 day to several days, but their impact on the J. Mar. Sci. Eng. 2021, 9, 710 temperature structure may affect annual variations. Lee et al. (2016) found a negative2 of 20 peak of the summer SST anomalies in the Yellow Sea, which was caused by the typhoon Muifa [11].

FigureFigure 1. 1. (A(A) The) The location location of of the the study study area area is is in in the the red red frame. frame. (B (B) )The The study study area area is is indicated indicated in in the the blueblue rectangular rectangular frame. frame. The The black isolines areare isobathsisobaths (m)(m) in in the the study study area area and and adjacent adjacent waters; waters; the thered red triangular triangular is theis the location location of aof stationary a stationary measurement measurement (S1). (S1). Wind increases velocity shear and thus enhances turbulent mixing, which alters verti- The coastal water of Muping is part of the north Yellow Sea. In summer (June–Au- cal temperature structures. Also, wind-induced current redistributes water temperature gust), the north of China is mainly influenced by weak southerly wind and strong positive and changes the vertical temperature structure. In freshwater influenced coastal waters, net heat flux [4]. Both contribute to the formation of water stratification. Many parts of the wind direction regulates the stratification and turbulent mixing [15,16]. This process is Yellow Sea are strongly stratified in summer [5–7]. There are two bottom cold-water called wind straining. The down-estuary wind increases stratification, while up-estuary masses, and the bottom temperature at the two centers of cold-water masses is below 7 °C has the opposite effect. According to Ekman’s theory, the surface current is to the right of in the summer [7,8]. One cold water mass center is approximately located at (35.3° N, wind direction in the Northern Hemisphere [17]. In a narrow bay, the Ekman transport due 123.6° E) and the other is located at (38.3° N, 122.2° E). The locations of cold-water center to along-channel wind induces upwelling at one side of the channel and downwelling at an- in the Yellow Sea are not fixed but vary with the meteorological forcing [9,10]. Muping is other side, which raises or lowers the isolines of temperature [18]. Lentz et al. (2003) found about 90 km southwest of the second cold-water center, and the coastal water of Muping that the breakdown of the seasonal thermocline in fall was primarily due to westward may be influenced by the bottom cold-water mass. With the increase of air temperature wind, while eastward wind stress of similar magnitude did not reduce the stratification [19]. fromDuring March storms, to July, both temperature wind mixing stratification and horizontal in the advection Yellow Sea are is magnifiedenhanced. The considerably. temper- atureThus, stratification temperature in structure the Yellow would Sea can be significantlybe very stable altered under innormal comparison weather with conditions its state inunder the summer. normal weatherIn July and conditions. August,The strong surface wind cooling during due storm to vertical events mixingmay be underthe only the drivingtyphoon force Megi that in temporarily October 2010 relieves was upor br toeaks 4.2 ◦ downC[20]. water Before strati thefication. landfall When of the typhoon tropical stormscyclone, sweep it was the found Yellow that Sea, wind a directionstrong wind plays could different mix roles the whole in the evolutionwater columns of stratifica- and erodetions. the The temperature downwelling-favorable stratification wind [11,12]. is more Vertical effective mixing in breakingof the water down column stratification is en- hancedthan upwelling-favorable by strong winds, and wind sea surface [21]. temperature (SST) could be sharply reduced during stormsSummer [13,14]. storms Storms are important usually last in breakingfrom 1 day the to water several stratification, days, but their relieving impact the on bottom the temperaturewater hypoxia, structure and mitigating may affect the annual economic variat lossesions. Lee due et to al. the (2016) low found oxygen a negative [22]. However, peak ofthe the frequency summer ofSST windstorms anomalies inin thethe coastalYellow water Sea, which of Muping was caused is relatively by the low typhoon in the summer. Muifa [11]. Wind increases velocity shear and thus enhances turbulent mixing, which alters vertical temperature structures. Also, wind-induced current redistributes water temperature and changes the vertical temperature structure. In freshwater influenced coastal waters, wind direction regulates the stratification and turbulent mixing [15,16]. This process is called wind straining. The down-estuary wind increases stratification, while up-estuary has the opposite effect. According to Ekman’s theory, the surface current is to the right of wind direction in the Northern Hemisphere [17]. In a narrow bay, the Ekman transport due to along-channel wind induces upwelling at one side of the channel and downwelling at another side, which raises or lowers the isolines of temperature [18]. Lentz et al. (2003) found that the breakdown of the seasonal thermocline in fall was primarily due to westward wind, while eastward wind stress of similar magnitude did not reduce the stratification [19]. During storms, both wind mixing and horizontal advection are magnified considerably. Thus, temperature structure would be significantly altered in comparison with

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On average, there are about 3 days in July and August with strong wind larger than Beaufort force 9 at Muping, which is much lower than that in other months (about 5–6 days). The average number of typhoons influencing the north Yellow Sea is one [23]. As wind direction could affect the water stratification, as shown by previous studies [18,19,21], it is necessary to know which wind directions during summer storm events are in favor of the breakdown of water stratification in a specific study area. The aims of this study are to investigate how sea temperature and stratification respond to different wind direction during storms and to determine the mechanism of the different responses in the coastal water of Muping. The paper consists of four sections. In Section2, the study area as well as the data and numerical model used in the study are introduced. In Section3, the influence of wind direction during storms on sea temperature in the coastal water of Muping is examined using data and numerical model results. The discussions and summary of this study are provided in Section4.

2. Materials and Methods 2.1. Study Area Muping is located in the north coast of Shandong Peninsula. The coastal water of Muping is part of the north Yellow Sea, as shown in Figure1A. The study area considered herein is limited in the blue frame in Figure1B, which is about 40 km long (west–east) and 30 km wide (south–north). In most parts of the study area, bottom hypoxia occurs in the summer. The depth of the water in the study area ranges from 5 m to 25 m. The isobath 10 m is only 3 km away from the coastline, while the isobath 20 m is located farther in the west of 121.9◦ E. A trough exists around 121.9◦ E, and the isobath 20 m is perpendicular to the coastline in the trough. The depth of the cross-shore axis of the trough reaches up to 23 m. The time zone of the study area is UTC+8. All time-related data or ﬁgures are recorded in local time.

2.2. Two Storm Events The 2 storm events occurred in July of 2015 and 2016, as shown in Figure2. They are hereafter called Storm I and Storm II, respectively. Both storms were listed in annual China’s Marine Disaster Bulletin published by the State Ocean Administration of China as the impacted coastal areas suffered significant losses. Storm I was a typhoon named Chan-hom, which was the ninth typhoon in 2015. It originated in the northwest Pacific Ocean. Its intensity grew as it moved toward the northwest. On 12 July, the typhoon center moved northeastward and entered the Yellow Sea. As it approached Muping, the wind speed in the coastal water of Muping increased up to 15.1 m/s at 17:00 on 12 July 2015. The intensity of the typhoon was weaker as it moved northward, and the influence of the typhoon on the coastal water of Muping disappeared at 12:00 on 13 July. The wind direction during Storm I in the coastal water of Muping varied from northeast to northwest (see Figure2C). The wind direction was almost northerly when the wind speed was maximum. Storm II was induced by an extratropical cyclone system. The low-pressure center was about 1000 km in the southwest of Muping. The dominating wind directions during Storm II were southeast and south. From 00:00 on 20 July 2016 to 05:00 on 22 July 2016, the wind speed in the coastal water of Muping was more than 8 m/s during this storm. The maximum wind speed during Storm II was 11.1 m/s at 20:00 on 20 July 2016.

2.3. Stationary Measurements In early May 2015, an SBE 21 Thermosalinograph CTD (conductivity, temperature, and depth) sensor was placed on the seabed of site S1 (Figure1B). The CTD was retrieved in late August of 2015. Timeseries of the bottom sea temperature were measured by the CTD during this period. The same CTD sensor was deployed on the seabed at site S1 again from early July to late August 2016, and the bottom water temperature was measured. At the same time in 2016, a buoy was installed about 200 m in the east of the CTD, measuring the J.J. Mar. Mar. Sci. Sci. Eng. Eng. 20212021,, 99,, 710 710 4 4of of 20 19

speedsurface in temperature the coastal water during of thisMuping period. was The more time than interval 8 m/s of during all stationary this storm. measurements The maxi- mumof sea wind temperature speed during was 10 Storm min. II was 11.1 m/s at 20:00 on 20 July 2016.

FigureFigure 2. 2. AirAir pressure pressure and and wind wind speed speed (10 (10 m) m)of the of thetwo two storm storm events. events. (A) Air (A )pressure Air pressure (hPa, (hPa,blue isolines)blue isolines) and wind and windfield (red ﬁeld vect (redors) vectors) over the over Yellow the Yellow Sea and Sea the and Bohai the BohaiSea at Sea 11:00 at 11:0012 July 12 2015 July (Storm2015 (Storm I). The I). green The line green indicates line indicates the track the of trackTyphoon of Typhoon Chan-hom; Chan-hom; 4 red diamonds 4 red diamonds are the locations are the oflocations typhoon of centers typhoon at centers00:00, 08:00, at 00:00, 15:00 08:00, on 12 15:00 July on 2015, 12 Julyand 2015,02:00 andon 13 02:00 July on 2015 13 Julyfrom 2015 south from to north.south ( toB): north. Air pressure (B) Air (blue pressure isolines) (blue and isolines) wind andfield wind (red vectors) ﬁeld (red over vectors) Yellow over Sea Yellow and Bohai Sea andSea atBohai 17:00 Sea on at 20 17:00 July on 2016 20 July(Storm 2016 II (Storm). (C) Timeseries II). (C) Timeseries of wind of vectors wind vectors at site atS1 site during S1 during Storm Storm Ⅰ. (D) I. Timeseries(D) Timeseries of wind of wind vectors vectors at S1 at S1during during Storm Storm II. IIr.Wind Wind speed speed (10 (10 m) m) and and air air pressure pressure data data are are ECMWFECMWF interim interim data. data.

2.3.2.4. Stationary Model Configuration Measurements InIn early this paper, May 2015, a 3-dimension an SBE 21 coastal Thermosali oceannograph model, Finite CTD Volume (conductivity, Coastal temperature, Ocean Model and(FVCOM), depth) wassensor used was to placed study the on influencethe seabed of of storms site S1 on (Figure the temperature 1B). The CTD structure. was retrieved FVCOM inwas late developed August of by 2015. Chen Timeseries et al. (2003) of th [24e ].bottom It has thesea attributestemperature of simplewere measured discrete coding,by the CTDis computationally during this period. efficient, The same and isCTD geometrically sensor was flexible.deployed It on is verythe seabed suitable at forsite hydrody-S1 again fromnamic early modelling July to late of estuarine August 2016, and coastal and the regions bottom with water a complextemperature irregular was measured. coastline and At thetopographical same time in geometry. 2016, a buoy For detailedwas installed descriptions about 200 of m FVCOM, in the east please of the refer CTD, to themeasuring manual the(https://wiki.fvcom.pml.ac.uk/ surface temperature during this; accessed period. on Th 26e June time 2021). interval FVCOM of all hasstationary been widely measure- used mentsin ocean of sea modelling temperature studies was [25 10–27 min.]. As shown in Figure3, the model domain covers the whole Yellow Sea and the Bohai 2.4.Sea. Model The boundaryConfiguration between the Yellow Sea and East China sea was the open boundary of theIn modelthis paper, domain. a 3-dimens The meshion resolutioncoastal ocean near model, the open Finite boundary Volume was Coastal set to Ocean 15 km Modeland was (FVCOM), increased was to used about to 1study km in the the influence study area. of storms The on bathymetry the temperature data used structure. in the FVCOMmodel was was extracted developed from by nauticalChen et charts.al. (2003) On [24] the. openIt has boundary, the attributes the hourlyof simple water discrete level coding,forecasted is computationally by TPXO 7.0 [28] efficient, was added. and The is geometrically initial temperature flexible. and It salinity is very conditions, suitable for as hydrodynamicwell as the monthly modelling temperature of estuarine and salinityand coastal data regions on the with open a boundary, complex irregular were provided coast- by the Hybrid Coordinate Ocean Model (HYCOM, http://www.hycom.org; accessed on line and topographical geometry. For detailed descriptions of FVCOM, please refer to the 26 June 2021) [29]. The meteorological data on the sea surface were the ECMWF interim manual (https://wiki.fvcom.pml.ac.uk/; accessed on 26 June 2021). FVCOM has been data [30] with a spatial resolution of 0.25◦ and a time interval of 3 h. The meteorological widely used in ocean modelling studies [25–27]. data included the wind speed, air temperature at 2 m, air pressure, relative humidity, As shown in Figure 3, the model domain covers the whole Yellow Sea and the Bohai Sea. The boundary between the Yellow Sea and East China sea was the open boundary of

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the model domain. The mesh resolution near the open boundary was set to 15 km and was increased to about 1 km in the study area. The bathymetry data used in the model was extracted from nautical charts. On the open boundary, the hourly water level forecasted by TPXO 7.0 [28] was added. The initial temperature and salinity conditions, as well as the monthly temperature and salinity data on the open boundary, were provided by the Hybrid Coordinate Ocean Model (HYCOM, http://www.hycom.org; accessed on 26 June 2021) [29]. The meteorological data on the sea surface were the ECMWF interim data [30] with a spatial resolution of 0.25° and a time interval of 3 h. The meteorological data included the wind speed, air temperature at 2 m, air pressure, relative humidity, J. Mar. Sci. Eng. 2021, 9, 710 shortwave heat flux, and downward longwave heat flux. The wind stress and the net 5heat of 19 flux were calculated based on the meteorological data. The model started on 1 January 2013. After a 2-year spin-up period, the model results ofshortwave 2015 and heat2016 flux, were and stored downward for further longwave analysis. heat In flux.this paper, The wind the stressmodel and results the netduring heat theflux 2 werestorms calculated were used. based on the meteorological data.

FigureFigure 3. 3. (A(A) )The The whole whole model model mesh mesh of of FVCOM. FVCOM. The The re redd line line indicates indicates the the open open boundary. boundary. ( (BB)) The The zoom-inzoom-in model model mesh mesh surrounding surrounding the the study study area. area. The The red red horizontal horizontal line line indicates indicates a transection a transection C1 usedC1 used for analysis for analysis in Section in Section 3.3. The3.3. 3 The black 3 black triangles triangles D1, D2, D1, and D2, D3 and are D3 used are usedfor temperature for temperature bal- ancebalance analysis analysis in Section in Section 3.4. 3.4.

3. ResultsThe model started on 1 January 2013. After a 2-year spin-up period, the model results 3.1.of 2015Model and Validation 2016 were stored for further analysis. In this paper, the model results during the 2 storms were used. Figure 4 shows the comparison between the recorded and the modelled temperature data3. Results at location S1 during Storm I (Figure 4A) and Storm II (Figure 4B). Since no surface temperature3.1. Model Validation was observed during Storm I, only the bottom temperature was compared. The modelFigure simulated4 shows the the comparison surface and betweenbottom temp the recordederature before and the the modelled two storms temperature relatively well,data as at the location RMSD S1 before during the Storm storm I (Figurewas less4A) than and 1 Storm°C. During II (Figure Storm4B). I, the Since observational no surface datatemperature shows that was bottom observed temperature during Storm experience I, onlyd thea sharp bottom increase temperature from 18.5 was °C compared. to about 22The °C model within simulated 7 h. The thebottom surface temperature and bottom af temperatureter the storm before was thealmost two stormsthe same relatively in the modelwell, as and the the RMSD observation, before the but storm the time was lessat which than 1the◦C. bottom During temperature Storm I, the increased observational sig- nificantlydata shows occurred that bottom in the temperaturemodel 8 h earlier experienced than in the a sharp observation. increase During from 18.5 Storm◦C toII,about both the22 ◦surfaceC within and 7 h.bottom The bottom temperature temperature decreased after wh theen storm the storm was almost started the to sameimpact in the modelstudy area,and theas indicated observation, in the but observational the time at which data the. The bottom decreasing temperature rate of increased surface temperature significantly wasoccurred greater in thethan model the bottom 8 h earlier temperature. than in the Co observation.nsequently, During the temperature Storm II, both difference the surface be- tweenand bottom the surface temperature and the decreasedbottom decreased when the from storm 4 °C started to lessto than impact 1 °C theafter study the midday area, as onindicated 20 July. in After the observationalthat, the surface data. temperature The decreasing was raterelatively of surface stable, temperature and the bottom was greater tem- peraturethan the decreased bottom temperature. slightly. At Consequently, 22:00 on 21 July, the the temperature surface temperature difference between increased the sharply surface byand 2.5 the °C bottomwithin 3 decreased h, while the from bottom 4 ◦C totemper less thanature 1 remained◦C after the stable. midday This onmade 20 July.the strati- After ficationthat, the recover surface to temperature a similar level was before relatively Storm stable, II. For and the the second bottom storm, temperature the model decreased repro- ducedslightly. the At decrease 22:00 on in 21the July, surface the surfaceand bottom temperature temperature increased very well. sharply However, by 2.5 the◦C model within 3 h, while the bottom temperature remained stable. This made the stratification recover to a similar level before Storm II. For the second storm, the model reproduced the decrease in the surface and bottom temperature very well. However, the model overestimated the vertical mixing at the surface, and the bottom temperature was completely mixed at 00:00 on 21 July while this was not the case in the observation. Overall, the model well simulated the temporal variation of the seawater temperature at the site S1. Furthermore, it captured different evolution patterns of the water temperature during the two storms with different wind directions. Thus, the model results could be used to investigate the influencing mechanism of the wind directions during storms on seawater temperature structure. J. Mar. Sci. Eng. 2021, 9, 710 6 of 20 J. Mar. Sci. Eng. 2021, 9, 710 6 of 20

overestimated the vertical mixing at the surface, and the bottom temperature was com- overestimated the vertical mixing at the surface, and the bottom temperature was completely mixed at 00:00 on 21 July while this was not the case in the observation. pletelyOverall, mixed the at 00:00 model on well 21 July simulated while this the was temp notoral the variation case in the of observation.the seawater tempera- Overall, the model well simulated the temporal variation of the seawater temperature at the site S1. Furthermore, it captured different evolution patterns of the water tem- ture at the site S1. Furthermore, it captured different evolution patterns of the water temperature during the two storms with different wind directions. Thus, the model results peraturecould be duringused to the investigate two storms the with influencing different mechanism wind directions. of the Thus,wind thedirections model duringresults J. Mar. Sci. Eng. 2021, 9, 710 could be used to investigate the influencing mechanism of the wind directions during6 of 19 storms on seawater temperature structure. storms on seawater temperature structure.

Figure 4. Comparisons of temperature between model results and observational data during Storm FigureIFigure (A) and 4. 4. ComparisonsStormComparisons II (B). of of temperature temperature between model resultsresults andand observationalobservational datadataduring during Storm Storm I I( A(A)) and and Storm Storm II II ( B(B).). 3.2. Responses of Temperature Structure during the Two Storm Events 3.2.3.2. Responses Responses of of Temperature Temperature Structure during the Two Storm Events 3.2.1. Temperature Structure Variation during the First Storm 3.2.1.3.2.1. Temperature Temperature Structure Structure Variation during the First Storm Figure 5 shows the temporal change of temperature over the whole water column at Figure5 shows the temporal change of temperature over the whole water column at S1 duringFigure the 5 shows first storm. the temporal Before the change storm of (wind temperature speed 8 m/s). The surface temperature decreased from 22 °C◦ to 20 °C◦ while areaarea (wind (wind speed speed > 8 8 m/s). m/s). The The surface surface temper temperatureature decreased decreased from from 22 22 °CC to to 20 20 °CC while while the bottom temperature increased by 2 °C◦ within 7 h. At 14:00 on 12 July, the surface tem- thethe bottom bottom temperature temperature increased increased by by 2 °C 2 withinC within 7 h. 7 At h. 14:00 At 14:00 on 12 on July, 12 July,the surface the surface temperature and the bottom temperature were almost the same (about 20 °C),◦ which means peraturetemperature and andthe thebottom bottom temperature temperature were were almost almost the the same same (about (about 20 20 °C),C), which which means means that the water column became well mixed during the storm. After 11:00 on 13 July, the thatthat the the water water column column became became well well mixed mixed during during the the storm. storm. After After 11:00 11:00 on on 13 13 July, July, the the windwind speed decreased and was inferior to 8 m/s.m/s. Although Although the the temperature temperature in in the whole windwater speedcolumn decreased increased and within was inferior the 2 days to 8 m/s.following Although the thestorm, temperature the water in column the whole re- waterwater column increasedincreased within within the the 2 days2 days following following the storm,the storm, the waterthe water column column remained remained well mixed. mainedwell mixed. well mixed.

Figure 5. Time-depth diagram of the modelled temperature at location S1 during Storm I. The blue and red lines indicate the start and end time of Storm I, respectively (wind speed > 8 m/s).

Figure6 shows the distribution of the surface and bottom temperature before and after the Storm I. Before the storm (Figure6A,B), the surface temperature over the study area was relatively uniform, ranging from 22 ◦C to 24 ◦C. The bottom temperature was more variable over space, whereby the bottom temperature nearshore was up to 8 ◦C higher than offshore at 37.8◦ N. The storm that occurred at 14:00 on 13 July 2015 caused the surface temperature to decrease by 1–5 ◦C from nearshore to offshore compare to before the J. Mar. Sci. Eng. 2021, 9, 710 7 of 20

Figure 5. Time-depth diagram of the modelled temperature at location S1 during Storm I. The blue and red lines indicate the start and end time of Storm I, respectively (wind speed > 8 m/s).

Figure 6 shows the distribution of the surface and bottom temperature before and after the Storm Ⅰ. Before the storm (Figure 6A,B), the surface temperature over the study area was relatively uniform, ranging from 22 °C to 24 °C. The bottom temperature was J. Mar. Sci. Eng. 2021, 9, 710 more variable over space, whereby the bottom temperature nearshore was up to 78 of°C 19 higher than offshore at 37.8° N. The storm that occurred at 14:00 on 13 July 2015caused the surface temperature to decrease by 1–5 °C from nearshore to offshore compare to be- forestorm, the whereasstorm, whereas the bottom the bottom temperature temperat increasedure increased by 1–3 ◦byC. 1–3 The °C. surface The surface and the and bottom the bottomtemperature temperature were almost were almost the same the insame the southin the ofsouth isobath of isobath 19 ◦C, 19 indicating °C, indicating that water that watercolumns columns were completelywere completely mixed mi inxed most in ofmost the of study the study area. area.

FigureFigure 6. 6. SpatialSpatial distribution distribution of of the the surface surface (A (A,C,C) )and and bottom bottom (B (B,D,D) )temperature temperature before before (12:00 (12:00 on on 11 11 JulyJuly 2015 2015 (A (A,B,B)))) and and during during (14:00 (14:00 on on 13 13 July July 2015 2015 ( (CC,D,D)))) Storm Storm I. I.

3.2.2.3.2.2. Temperature Temperature Structure Structure Variation Variation during during the the Second Storm FigureFigure 7 showsshows the the temporal temporal change of th thee temperature temperature over the whole water column atat location location S1 during thethe secondsecond storm. storm. Before Before the the storm, storm, the the water water column column was was in the in statethe ◦ stateof stratification. of stratification. The The surface surface temperature temperature was was about about 24 24C, °C, and and the the bottom bottom temperature tempera- ◦ turewas was below below 20 C.20 Under °C. Under the influence the influence of strong of strong storm storm wind, wind, both the both surface the surface and bottom and bottomtemperatures temperatures decreased, decreased, which which did not did occur not occur during during Storm Storm I, therefore I, therefore representing represent- a ingnotable a notable difference difference between between the the two two storm storm events. events. From From 10:00 10:00 to to 18:0018:00 on 20 July, July, the the decreasingdecreasing rate rate of of surface surface temperature temperature was was accelerated, accelerated, and and the the depth depth of of the the upper upper mixed mixed layerlayer was was increased. increased. At At 18:00 18:00 on on 20 20 July, July, the the water water column column was was completely completely mixed mixed with with a a temperature less than 19 ◦C, and the bottom temperature continued to decrease to 18 ◦C. temperature less than 19 °C, and the bottom temperature continued to decrease to 18 °C. Even if the wind speed was lower than 8 m/s after 02:00 on 22 July, the bottom temperature remained below 18 ◦C for more than 24 h. However, the temperature in the middle and upper layers increased soon after the storm and water column became stratified once again. Figure8 shows the distribution of the surface and bottom temperature before and after the second storm. Before the storm (Figure8A,B), the surface temperature and the bottom temperature were within the range 22–24.5 ◦C and 12–23 ◦C, respectively. During the storm at 00:00 on 22 July, the surface temperature generally decreased by 1–5 ◦C compared with that before the storm. The surface temperature nearshore experienced a stronger decrease than in the offshore. The lowest surface temperature was less than 18 ◦C in the southeast J. Mar. Sci. Eng. 2021, 9, 710 8 of 19

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corner of the study area, which was at the southern tip of the trough. A cold surface water band could be seen extending from the cold-water center toward the west. Figure8D shows Eventhat all if the isolines wind ofspeed the bottomwas lower temperature than 8 m/s moved after 02:00 toward on 22 the July, coastline the bottom in comparison temperature with remainedbefore the below storm. 18 For °C example, for more thethan isolines 24 h. However, 16–18 ◦C movedthe temperature 15–20 km in and the were middle very and close upperto the coastlinelayers increased during soon the storm. after the The stor bottomm and temperature water column isolines became exhibited stratified an once abrupt again.turn to the coastline in the trough area, which ﬁtted well the isobath of the trough.

Figure 7. 7. Time-depthTime-depth diagram diagram of of modelled modelled temperature temperature at atlocation location S1 S1 during during Storm Storm II. II.The The blue blue and and red line line indicate indicate the the start start and and end end ti timeme of of Storm Storm I (wind I (wind speed speed > 8 > m/s). 8 m/s).

3.3. NumericalFigure 8 shows Experiments the distribution of the surface and bottom temperature before and after3.3.1. the Experiments second storm. Settings Before the storm (Figure 8A,B), the surface temperature and the bottomIn temperature summary, during were thewithin second the range storm 22–24 occurring.5 °C inand July 12–23 2016, °C, the respectively. temperature During showed thea different storm at variability 00:00 on 22 pattern July, the than surface that observedtemperature during generally the first decreased storm. Theseby 1–5 differences°C com- paredlie in twowith aspects: that before (1) During the storm. Storm The I, the surface surface temperature temperature nearshore decreased experienced and the bottom a strongertemperature decrease increased, than in while the offshore. during Storm The lowest II, both surface the surface temperature and the bottomwas less water than were18 °Ccooled, in the and southeast the nearshore corner of surface the study water area, experienced which was a sharperat the southern decrease tip than of the during trough. Storm AI; andcold (2)surface Storm water I was band more could effective be seen in breakingextending down from the stratification cold-water than center Storm toward II. the west.The Figure maximum 8D shows wind that speedall isolines during of the Storm bottom I in temperature the coastal water moved of toward Muping the was coast- more linethan in 15.0 comparison m/s, which with issignificantly before the storm. larger For than example, that recorded the isolines in Storm 16–18 II (11.1 °C moved m/s). Hence,15– 20it iskm unclear and were whether very theclose wind to the direction coastline was during responsible the storm. for The the bottom temperature temperature structure isolinesdifference exhibited during an the abrupt two storms.turn to the To coastline address in this the point, trough a area, numerical which experimentfitted well the was isobathdesigned of basedthe trough. on the validated model during Storm II (Model_ref). The wind directions (during the storm period) of the other models in the numerical experiment were modified based on the wind data in the reference model. The wind speed magnitude remained the same in all the models in the experiment. The model settings in the experiment are shown in Table1.

3.3.2. Model_ref vs. Model1 During Storms I and II, the north component of the wind was the dominating component, and the signs of the north component were opposite (Figure2). It is estimated that the different signs of the north component of wind led to the different patterns of temperature change. Therefore, Model1 was designed to test the response of the temperature to the north component of the wind. Figure9A shows the timeseries of surface and bottom temperature at S1 simulated by Model_ref and Model1 in the numerical experiment. Unlike in Model_ref, the bottom temperature in Model1 increased and the surface temperature decreased during the storm, which is consistent with the bottom temperature changes ob-

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served during Storm I. The surface water cooling was noticeably less significant in Model1, only about 3 ◦C higher than that in Model_ref. After 22 July, the water column at site S1 in Model1 was almost mixed. Figure9B,C show the surface and bottom temperature all over the study area at 00:00 on 22 July in Model1. In comparison with the temperature recorded on 19 July before the storm (Figure8A,B), the surface temperature decreased, while the bottom temperature increased all over the study area in Model1, showing similar patterns to the temperature change in Storm I (Figure6). This indicates that the temperature variation was sensitive to the north component of the wind and the northward wind component may have influenced the decrease of bottom temperature during Storm II. Stratification was reduced over the study area in Model1 compared with Model_ref. The water columns nearshore became well mixed, but water in the offshore was still more stratified than that J. Mar. Sci. Eng. 2021, 9, 710 9 of 20 during Storm I (Figure6)). This was obviously due to the stronger wind speed recorded during Storm I.

FigureFigure 8.8. Spatial distribution of of surface surface (A (A,C,C) )and and bottom bottom (B (B,D,D) temperature) temperature before before (12:00 (12:00 on on 19 19July July 20162016 ((AA,,BB)))) andand during (00:00 on on 21 21 July July 2016 2016 ( (CC,D,D)))) Storm Storm II. II.

Table3.3. Numerical 1. Model settingsExperiments of the numerical experiment. 3.3.1. ExperimentsModel Run Settings Descriptions In summary,Model_ref during the second storm The well-validated occurring in model Julyduring 2016, Stormthe temperature II showed a Model1different variability pattern South componentthan that ofobserved the realistic during wind the→ northfirst storm. component These differencesModel2 lie in two aspects: (1) During Storm realistic I, the wind surface→ southerly temperature wind decreased and the bottomModel3 temperature increased, while during realistic Storm wind II, both→ northerly the surface wind and the bottom water wereModel4 cooled, and the nearshore surface realisticwater experienced wind → easterly a sharper wind decrease than → during StormModel5 I; and (2) Storm I was more effective realistic in wind breakingwesterly down wind stratification than Storm II. The maximum wind speed during Storm Ⅰ in the coastal water of Muping was more than 15.0 m/s, which is significantly larger than that recorded in Storm II (11.1 m/s). Hence, it is unclear whether the wind direction was responsible for the temperature structure difference during the two storms. To address this point, a numerical experiment was designed based on the validated model during Storm II (Model_ref). The wind directions (during the storm period) of the other models in the numerical experiment were modified based on the wind data in the reference model. The wind speed magnitude remained the same in all the models in the experiment. The model settings in the experiment are shown in Table 1.

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Table 1. Model settings of the numerical experiment.

Model Run Descriptions Model_ref The well-validated model during Storm II Model1 South component of the realistic wind  north component Model2 realistic wind  southerly wind Model3 realistic wind  northerly wind Model4 realistic wind  easterly wind Model5 realistic wind  westerly wind

3.3.2. Model_ref vs. Model1 During Storms I and II, the north component of the wind was the dominating component, and the signs of the north component were opposite (Figure 2). It is estimated that the different signs of the north component of wind led to the different patterns of temperature change. Therefore, Model1 was designed to test the response of the temperature to the north component of the wind. Figure 9A shows the timeseries of surface and bottom temperature at S1 simulated by Model_ref and Model1 in the numerical experiment. Un- like in Model_ref, the bottom temperature in Model1 increased and the surface temperature decreased during the storm, which is consistent with the bottom temperature changes observed during Storm I. The surface water cooling was noticeably less significant in Model1, only about 3 °C higher than that in Model_ref. After 22 July, the water column at site S1 in Model1 was almost mixed. Figure 9B,C show the surface and bottom temperature all over the study area at 00:00 on 22 July in Model1. In comparison with the temperature recorded on 19 July before the storm (Figure 8A,B), the surface temperature decreased, while the bottom temperature increased all over the study area in Model1, showing similar patterns to the temperature change in Storm I (Figure 6). This indicates that the temperature variation was sensitive to the north component of the wind and the northward wind component may have influenced the decrease of bottom temperature during Storm II. Stratification was reduced over the study area in Model1 compared with Model_ref. The water columns nearshore became well mixed, but water in the offshore J. Mar. Sci. Eng. 2021, 9, 710 was still more stratified than that during Storm I (Figure 6)). This was obviously due10 of to 19 the stronger wind speed recorded during Storm I.

Figure 9. (A) Timeseries of temperature simulated by Model_ref and Model1. (B) Spatial distribution of surface temperature simulated by Model1 at 00:00 on 22 July 2016. (C) Spatial distribution of bottom temperature simulated by Model1 at 00:00 on 22 July 2016. 3.3.3. The Response of the Temperature to the Alongshore Wind and Cross-Shore Wind To gain a better understanding of the response of temperature to the wind directions, the roles of the alongshore and cross-shore winds were investigated. In Model2–5, the wind magnitude remained the same as in Model_ref, and the wind directions were set to south, north, east, and west, respectively. The orientation of the coastline roughly extended west to east. Therefore, the southerly and northerly wind was cross-shore, and the easterly and westerly wind was alongshore. As shown in Figure 10, the easterly wind and southerly wind had a similar effect on the surface and bottom temperature variation, whereby both the surface and bottom temperature decreased during strong wind at site S1. The bottom temperature under the easterly wind (Model4) at S1 was slightly lower than under the southerly wind (Model2) before 12:00 21 July. After that time, their temperature difference was greater than 1 ◦C. The surface temperature was about 2 ◦C lower in Model4. On the other hand, the northerly wind and the westerly wind completely mixed the water column at S1. The surface temperature under northerly wind was about 1 ◦C higher than under westerly wind. Figure 11 shows the snapshots of the surface and bottom temperature at 00:00 on 22 July simulated by Model2–5. In Model2 and Model4, the bottom temperature decreased all over the study area at 00:00 on 22 July compared with that observed at 19:00 on 19 July (Figure8), while the bottom temperature increased in Model3 and Model5. However, the isolines during the easterly windstorm were located closer to the coastline than during the northerly wind storm. This indicates that the easterly wind brought more cold water from the deeper sea to the study area. The surface temperature in Model4 was also colder than in Model2 and similar to the cold-water band along 37.5◦ N in the Model_ref (Figure8C ), and there was also a cold-water band at the same location in Model4. However, this cold-water band did not appear in Model2, which implies that the cold-water band was related to the easterly wind component in Model_ref. Under the inﬂuence of the northerly wind and westerly wind, the surface temperature and the bottom temperature were almost the same. The surface temperature was about 1 ◦C higher than the bottom temperature only at 37.7◦ N near the northmost boundary of the study area. Although water columns were J. Mar. Sci. Eng. 2021, 9, 710 11 of 20

Figure 9. (A) Timeseries of temperature simulated by Model_ref and Model1. (B) Spatial distribution of surface temperature simulated by Model1 at 00:00 on 22 July 2016. (C) Spatial distribution of bottom temperature simulated by Model1 at 00:00 on 22 July 2016.

3.3.3. The Response of the Temperature to the Alongshore Wind and Cross-Shore Wind To gain a better understanding of the response of temperature to the wind directions, the roles of the alongshore and cross-shore winds were investigated. In Model2–5, the wind magnitude remained the same as in Model_ref, and the wind directions were set to south, north, east, and west, respectively. The orientation of the coastline roughly extended west to east. Therefore, the southerly and northerly wind was cross-shore, and the easterly and westerly wind was alongshore. As shown in Figure 10, the easterly wind and southerly wind had a similar effect on the surface and bottom temperature variation, whereby both the surface and bottom temperature decreased during strong wind at site S1. The bottom temperature under the east- J. Mar. Sci. Eng. 2021, 9, 710 erly wind (Model4) at S1 was slightly lower than under the southerly wind (Model2)11 ofbe- 19 fore 12:00 21 July. After that time, their temperature difference was greater than 1 °C. The surface temperature was about 2 °C lower in Model4. On the other hand, the northerly windwell mixedand the in westerly the two models,wind completely the water mixed in Model3 the water was generally column warmerat S1. The than surface in Model5 tem- peratureat 00:00 onunder 22 July. northerly wind was about 1 °C higher than under westerly wind.

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Figure 10. Figure 10. TimeseriesTimeseries of of surface surface and and bottom bottom te temperaturemperature simulated simulated by by Model2–5. Model2–5.

Figure 11 shows the snapshots of the surface and bottom temperature at 00:00 on 22 July simulated by Model2–5. In Model2 and Model4, the bottom temperature decreased all over the study area at 00:00 on 22 July compared with that observed at 19:00 on 19 July (Figure 8), while the bottom temperature increased in Model3 and Model5. However, the isolines during the easterly windstorm were located closer to the coastline than during the northerly wind storm. This indicates that the easterly wind brought more cold water from the deeper sea to the study area. The surface temperature in Model4 was also colder than in Model2 and similar to the cold-water band along 37.5° N in the Model_ref (Figure 8C), and there was also a cold-water band at the same location in Model4. However, this cold- water band did not appear in Model2, which implies that the cold-water band was related to the easterly wind component in Model_ref. Under the influence of the northerly wind and westerly wind, the surface temperature and the bottom temperature were almost the same. The surface temperature was about 1 °C higher than the bottom temperature only at 37.7° N near the northmost boundary of the study area. Although water columns were well mixed in the two models, the water in Model3 was generally warmer than in Model5 at 00:00 on 22 July.

FigureFigure 11. 11. DistributionDistribution of of surface surface ( (AA,,CC,E,E,G,G) )and and bottom (B,D,,F,,HH)) temperature simulated by Model2–5. Model2–5.

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3.3.4. Current Velocity Induced by Alongshore and Cross-Shore Wind The notable difference between the two storms was the intrusion of cold water from offshore during Storm II, which implies that cross-shore transport may have been significantly enhanced during the second storm. The current velocity and the cross-shore discharge were also investigated. The hourly current velocity in Model2–5 was averaged over the period from 00:00 on 20 July to 01:00 on 21 July 2016. The time-averaged current velocity excluded tidal component and kept the wind-induced current. The surface and bottom time-averaged currents in Model2–5 are shown in Figure 12. The surface current in the four models is generally consistent with the surface Ekman velocity theory. The surface current directions were approximately 45◦ to the right of the wind directions in most parts of the study area. Near the coastline, the surface current directions were almost parallel to the coastline. The cross-shore components of surface current pointed toward the north in Model2 and Model4 and toward the south in Model3 and Model5. The southerly wind and the easterly wind produced a shoreward bottom current, while in the other two Models, the bottom current was overall directed toward the northeast. The orientation of the coastline in the study area was almost west–east. The easterly wind was upwelling- favorable, and the westerly wind was downwelling-favorable according to the generally accepted viewpoint on the wind-upwelling system. From the surface and bottom current velocities in Figure 12, one can infer that the southerly wind was upwelling-favorable and the northerly wind was downwelling-favorable. A strong bottom temperature gradient existed before the storm, whereby the bottom temperature decreased rapidly from nearshore to offshore (Figure6B). During the upwelling-favorable wind, the bottom current transported colder water toward nearshore. The bottom cooling due to the intruding cold-water had a greater impact than the warming rate due to vertical mixing, which resulted in the continuous decrease of the temperature at S1, as shown in Figure 10. The decrease in the bottom temperature helped stabilize the water column and reduced vertical turbulent mixing. On the other hand, under the influence of downwelling-favorable wind, the bottom current transported warmer water toward the offshore and the surface current transported colder water to the nearshore. Both the surface and the bottom transport had a positive effect on vertical mixing. Therefore, it was easier to reduce stratification during the downwelling-favorable wind. As shown in Figure2, the wind direction in Storm I ranged from the northeast to northwest. The north component of the wind was always southward and significantly larger than the east component. Therefore, the wind during Storm I was downwelling-favorable, while the wind during Storm II was upwelling-favorable. The different patterns of temperature structure during Storm I and II could be well explained by the above analysis. As cross-shore transport play a critical role in temperature structure variation, cross- shore discharge over 20 July through a transect C1 (see Figure3B) was investigated. The transect is on 37.55◦ N and extends from 121.7◦ E to 122.0◦ E. The discharge per unit area (m3) is defined as R t2 vdt, where t is 00:00 on 20 July, t2 is 00:00 on 21 July, and v is the t1 1 cross-shore component (north component) of the current velocity on a certain layer. The depth-variable discharge per unit area through the transect C1 in Model2–5 is presented in Figure 13. During the upwelling-favorable wind in Model2 and Model4, water flowed northward in the upper layers and southward in the lower layers. The wind-induced Ekman surface current was right of the wind direction. The subsurface current then rotated clockwise and was smaller than the surface current. Thus, the cross-shore component of the current velocity decreased rapidly with depth during the southerly wind. During the easterly wind, the current velocity also decreased with depth, but its direction was from northwest to north with depth. The cross-shore component may not have decreased as rapidly as during northerly wind. Therefore, the thickness of the outflow layer was smaller in Model2 (southerly wind) than in Model4 (easterly wind) (Figure 13A,C). The easterly wind transported more upper-layer water toward the north than northerly wind (<2 × 105 m3), which resulted in a stronger discharge (2∼4 × 105 m3) in the lower layers. This explains why the bottom temperature was lower during the easterly wind than during J. Mar. Sci. Eng. 2021, 9, 710 13 of 20

Models, the bottom current was overall directed toward the northeast. The orientation of the coastline in the study area was almost west–east. The easterly wind was upwelling- favorable, and the westerly wind was downwelling-favorable according to the generally accepted viewpoint on the wind-upwelling system. From the surface and bottom current velocities in Figure 12, one can infer that the southerly wind was upwelling-favorable and the northerly wind was downwelling-favorable. A strong bottom temperature gradient existed before the storm, whereby the bottom temperature decreased rapidly from nearshore to offshore (Figure 6B). During the upwelling-favorable wind, the bottom current transported colder water toward nearshore. The bottom cooling due to the intruding cold- J. Mar. Sci. Eng. 2021, 9, 710 13 of 19 water had a greater impact than the warming rate due to vertical mixing, which resulted in the continuous decrease of the temperature at S1, as shown in Figure 10. The decrease in the bottom temperature helped stabilize the water column and reduced vertical turbu- lentthe southerlymixing. On wind the other (Figure hand, 11). under Both the in northerlyfluence of wind downwelling-favorable and westerly wind producedwind, the bottomupper-inflow current and transported lower-outflow warmer discharge water towa patterns,rd the butoffshore the upper-layer and the surface discharge current in transportedModel5 (westerly colder wind) waterwas to the larger nearshore. than in Both Model3 the (northerlysurface and wind). the bottom The offshore transport surface had awater positive was effect colder on than vertical the mi nearshorexing. Therefore, surface water.it was easier Therefore, to reduce more stratification surface cold during water theflowed downwelling-favorable southward and caused wind. the surface temperature to lower, as shown in Figure 11.

Figure 12. Surface and bottom time-averaged current velocities in Model2–5. Surface and bottom Figure 12. Surface and bottom time-averaged current velocities in Model2–5. Surface and bottom currentcurrent velocities velocities simulated simulated by by the the Model2 Model2 are are displayed displayed in in ( (AA,,BB)) respectively. respectively. Surface Surface and and bottom bottom current velocities simulated by the Model3 are displayed in (C,D) respectively. Surface and bottom current velocities simulated by the Model4 are displayed in (E,F) respectively. Surface and bottom current velocities simulated by the Model5 are displayed in (G,H) respectively.

3.4. The Formation Mechanism of a Surface Cold-Water Band Figure8 shows that a cold-water band formed during Storm II at around 37.5 ◦ N and extended to about 20 km from west to east. Site S1 was inside the west part of the cold-water band. The substantial surface water cooling and reduced stratiﬁcation at S1 are attributed to the surface cold-water band. As the minimum surface temperature during Storm II was about 19 ◦C, the isolines 19 ◦C of surface temperature at different times are presented in Figure 14. The surface temperature of 19 ◦C did not appear until at 15:00 on 20 July. At that time, surface water at temperatures lower than 19 ◦C was limited in a small J. Mar. Sci. Eng. 2021, 9, 710 14 of 19

nearshore area around 122.9◦ E (enclosed by the blue isoline), where the depth was smaller than 8 m. At 18:00 on 20 July, the cold-water zone (lower than 19 ◦C) was expanded toward the west. A westward cold-water tongue extended from 121.75◦ N to 122.05◦ N at 00:00 on 21 July. After that, the cold-water tongue receded and moved a short distance northward. In the numerical experiment, the easterly wind generated the westward surface current over the cold-water band, while the southerly wind-generated northward surface current (Figure 12). Accordingly, in Model4 with the easterly wind, there was also a cold-water band (Figure 11) as in Model_ref (Figure8), but no cold-water band appeared in Model2 J. Mar. Sci. Eng. 2021, 9, 710 15 of 20 with the southerly wind. This implies that the westward component of wind during Storm II played an important role in the formation of the cold-water band.

Figure 13. Discharge (m3) per unit area across the section C1 over 20 July 2016 in Model2 (A), Figure 13. Discharge (m3) per unit area across the section C1 over 20 July 2016 in Model2 (A), Model3 (BModel3), Model4 (B), ( Model4C), and Model5 (C), and ( Model5D). The (signD). The of inflow sign of (toward inﬂow (towardthe coast) the was coast) negative was negative (dashed (dashed lines), −5 andlines), outflow and outﬂow was positive was positive (solid lines). (solid For lines). better For display, better display,the isoline the values isoline are values 10ିହ are times10 oftimes discharge.of discharge.

3.4. TheA temperatureFormation Mechanism balance analysisof a Surface was Cold-Water performed Band using the FVCOM model output to explain the formation of the cold-water band quantitatively. The temperature equation is Figure 8 shows that a cold-water band formed during Storm II at around 37.5° N and written as extended to about 20 km from west to east. Site S1 was inside the west part of the cold- water band. The∂T substantial ∂(uT surface) water∂(vT cooling) and∂( reducedwT) stratification∂ ∂T at S1 are at- = − + − + − + K (1) tributed to the∂ tsurface cold-water∂x band.∂ yAs the minimum∂z surface∂z temperatureh ∂z during Storm II was about 19 °C, the isolines 19 °C of surface temperature at different times are presentedwhere T is in the Figure water 14. temperature; The surface temperaturet is the time; ofu ,19v ,°C and didw notare appear the east, until the at north, 15:00 andon 20the July. vertical At that component time, surface of currentwater at velocity,temperatures respectively; lower than and 19 K°Ch wasis the limited turbulent in a small eddy nearshoreviscosity. Thearea left-hand-sidearound 122.9° termE (enclosed in Equation by th (1)e blue represents isoline), the where local the temporal depth was temperature smaller thanchange. 8 m. At Local 18:00 temperature on 20 July, the change cold-water is caused zone by (lower the four than terms 19 °C) on was the expanded right-hand toward side. theThese west. are A the westward east component cold-water of tongue the horizontal extended advection from 121.75° (XADV), N to the122.05° north N componentat 00:00 on 21of July. horizontal After that, advection the cold-water (YADV), tongue the vertical receded advection and moved (VADV), a short and distance the vertical northward. mixing In the numerical experiment, the easterly wind generated the westward surface current over the cold-water band, while the southerly wind-generated northward surface current (Figure 12). Accordingly, in Model4 with the easterly wind, there was also a cold-water band (Figure 11) as in Model_ref (Figure 8), but no cold-water band appeared in Model2 with the southerly wind. This implies that the westward component of wind during Storm II played an important role in the formation of the cold-water band.

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(VMIX). Along with the cold-water band, three sites D1, D2, and D3 were speciﬁed for the surface temperature balance analysis (see Figure3B). The water depth at the sites was 6.5 m, 14.1 m, and 18.0 m at D1, D2, and D3, respectively. Site D1 was located where the cold water was originally formed. D3 was at the same location as S1, where the instrument was deployed. D2 was between D1 and D3 on the cold-water band. During the whole day of 20 July, the surface temperature decreased at all the three sites (Figure 15D). At D1, the vertical mixing dominated the surface cooling. At D2, surface cooling was mainly caused by vertical mixing in the ﬁrst 2 h. Then, XADV became the primary process causing the reduction of surface water temperature throughout the remainder of 20 July. The surface cooling was so strong that the surface water was even colder than its adjacent lower layer temperature as VMIX was positive. At D3, YADV was the dominant process of the surface cooling before 15:00 on 20 July. After that, the surface cooling was mainly driven by XADV. Figure 15 shows that the cooling mechanism at D1 was different from D2 and D3. The depth at D1 was only 6.5 m. The strong wind and tidal current could mix the surface and lower-layer cold water. The surface cold water at D1 was subsequently transported toward the west. The cold surface water from D1 arrived at D2 earlier than D3, which explains why the minimum XADV at D2 was earlier than at D3. VADV was very small compared J. Mar. Sci. Eng. 2021, 9, 710 16 of 20 with the vertical mixing and the horizontal advection at all the three sites, although the wind in Storm II was upwelling-favorable.

◦ Figure 14.14. IsolinesIsolines 1919 °CC of surface temperature at different timestimes duringduring StormStorm II.II.

A temperature balance analysis was performed using the FVCOM model output to explain the formation of the cold-water band quantitatively. The temperature equation is written as ߲ܶ ߲ሺݑܶሻ ߲ሺݒܶሻ ߲ሺݓܶሻ ߲ ߲ܶ ൌሺെ ሻ൅ሺെ ሻ൅ሺെ ሻ൅ ൬ܭ ൰ (1) ߲ݐ ߲ݔ ߲ݕ ߲ݖ ߲ݖ ௛ ߲ݖ where ܶ is the water temperature; ݐ is the time; ݑ, ݒ, and ݓ are the east, the north, and the vertical component of current velocity, respectively; and ܭ௛ is the turbulent eddy vis- cosity. The left-hand-side term in Equation (1) represents the local temporal temperature change. Local temperature change is caused by the four terms on the right-hand side. These are the east component of the horizontal advection (XADV), the north component of horizontal advection (YADV), the vertical advection (VADV), and the vertical mixing (VMIX). Along with the cold-water band, three sites D1, D2, and D3 were specified for the surface temperature balance analysis (see Figure 3B). The water depth at the sites was 6.5 m, 14.1 m, and 18.0 m at D1, D2, and D3, respectively. Site D1 was located where the cold water was originally formed. D3 was at the same location as S1, where the instrument was deployed. D2 was between D1 and D3 on the cold-water band. During the whole day of 20 July, the surface temperature decreased at all the three sites (Figure 15D). At D1, the vertical mixing dominated the surface cooling. At D2, surface cooling was mainly caused by vertical mixing in the first 2 h. Then, XADV became the primary process causing the reduction of surface water temperature throughout the remainder of 20 July. The surface cooling was so strong that the surface water was even colder than its adjacent lower layer temperature as VMIX was positive. At D3, YADV was the dominant process of the surface cooling before 15:00 on 20 July. After that, the surface cooling was mainly driven by XADV. Figure 15shows that the cooling mechanism at D1 was different from D2 and D3. The depth at D1 was only 6.5 m. The strong wind and tidal current could mix the surface and lower-layer cold water. The surface cold water at D1 was subsequently transported toward the west. The cold surface water from D1 arrived at D2 earlier than D3, which explains why the minimum XADV at D2 was earlier than at D3. VADV was very small compared with the vertical mixing and the horizontal advection at all the three sites, although the wind in Storm Ⅱ was upwelling-favorable.

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FigureFigure 15. 15. SurfaceSurface temperature temperature balance balance terms terms during during 20 July 20 July 2016 2016 at D1 at ( D1A), (D2A), ( D2B), and (B), D3 and (C D3). ( (AC–). C(A) share–C) share the same the same legend legend as (A as). ((AD).) (SurfaceD) Surface temperature temperature at D1, at D1,D2, D2,and and D3 during D3 during 20 July 20 July 2016. 2016.

4.4. Discussions Discussions and and Summaries Summaries ThisThis study study investigated investigated the the effect effect of of two two summer summer storm storm events events with with opposite opposite wind wind directionsdirections on on water water temperature temperature structures structures in in the the coastal coastal water water of of Muping, Muping, China. China. A A well- well- validatedvalidated numerical numerical ocean ocean model model FVCOM FVCOM was was used used for for this this study. study. MostMost of of the the time time in in the the summer, summer, a a strong strong te temperaturemperature gradient gradient exists exists in in the the offshore offshore bottombottom water water of of Muping. Muping. The The patterns patterns of of the the water water temperature temperature variation variation during during the the two two stormsstorms were were different. different. Wind Wind during during the the St Stormorm I Iwas was downwelling-favorable, downwelling-favorable, which which means means thatthat the the bottom bottom current current was was toward toward offshore. offshore. Wind Wind during during the the Storm Storm II II was was upwelling- upwelling- favorable,favorable, and and the the induced induced bottom bottom current current was was shoreward. shoreward. During During Storm Storm I, I, the the surface surface temperaturetemperature decreased decreased and and the the bottom bottom temperature temperature increased. increased. The The temperature temperature of of the the waterwater columns columns over over the the study study area area became became almost almost mixed. mixed. However, However, during during the the second second windstorm,windstorm, both both the the surface surface and and bottom bottom temperature temperature in in the the study study area decreased. TheThe downwelling-favorable downwelling-favorable wind wind was was more more effective effective at at eroding eroding stratification stratification than than thethe upwelling-favorable upwelling-favorable wind. wind. During During the the downwelling-favorable downwelling-favorable wind, wind, the the shoreward shoreward surfacesurface current current transported transported colder colder water water from from offshore, offshore, and and the the nearshore nearshore warmer warmer bottom bottom waterwater was was transported transported to to the the direction direction of of offshore. offshore. Therefore, Therefore, the the horizontal horizontal advection advection of of thethe heat heat during during the the downwelling-favorable downwelling-favorable wind wind weakened weakened the the stratification. stratification. On On the the other other hand,hand, during during the the upwelling-favorable upwelling-favorable wind, wind, th thee effect effect of of the the bottom bottom intruding intruding cold cold water water was greater than the vertical wind mixing, causing continuous bottom water cooling in the was greater than the vertical wind mixing, causing continuous bottom water cooling in study area. The bottom cold water made the water column more stable and suppressed the study area. The bottom cold water made the water column more stable and suppressed mixing. These results are consistent with the findings reported when hurricanes attacked mixing. These results are consistent with the findings reported when hurricanes attacked the east coast of the United States [21,31]. the east coast of the United States [21,31]. In the study area, the coastline extended from west to east. It is generally regarded In the study area, the coastline extended from west to east. It is generally regarded that upwelling-favorable or downwelling-favorable wind is parallel to the coastline [32]. that upwelling-favorable or downwelling-favorable wind is parallel to the coastline [32]. Few previous studies have recognized that the cross-shore wind is upwelling-favorable Few previous studies have recognized that the cross-shore wind is upwelling-favorable or downwelling-favorable. The numerical experiment in this paper indicates that the or downwelling-favorable. The numerical experiment in this paper indicates that the cross-shore wind could also be upwelling-favorable (southerly) or downwelling-favorable

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(northerly) as it could also produce cross-shore transport with opposite directions in the upper and lower layers. This may be valid in shallow coastal waters and under the influence of strong wind. Strong cross-shore wind transported significantly more surface water than moderate wind along the cross-shore direction. Then, the lower-layer water had to be transported to the opposite direction due to the mass balance. However, under the easterly wind, both the upper-layer and lower-layer transport were larger than the southerly wind. More bottom deeper cold water intruded nearshore, making the surface and bottom water colder than during the southerly wind. During a storm, the northerly wind is also as downwelling-favorable as the westerly wind. Due to stronger surface transport, surface water was colder during the westerly wind than during the northerly wind. Apart from in the study area investigated herein, a cross-shore wind of moderate magnitude can also be upwelling-favorable, such as in small-scale bays (2 × 3 km) [33] and narrow bays (5 × 40 km) [34]. Therefore, whether cross-shore wind can be upwelling- favorable or downwelling-favorable depends on wind speed magnitude, the coastal water size, and the coastline shape. Although wind during Storm II was upwelling-favorable, the effect of upwelling to cool surface water was minor. As the wind is very strong during a storm, vertical mixing is the primary process for cooling surface water other than upwelling. Apart from wind mixing, large wind can also generate a strong alongshore surface current which redistributes surface temperature. A distinct feature was the appearance of a cold- water band during Storm II. Longshore cold-water bands are common in upwelling- influenced coastal waters [35–37]. The positions of cold-water bands depend on the wind magnitude [38] and topography [37,39]. Unlike classic upwelling-related cold-water bands, the cold-water band observed herein was formed due to the vertical mixing and westward surface advection. Although the southerly wind was also upwelling-favorable, such surface cold-water band did not appear under the southerly wind. Strong summer stratification in the study area slows down the vertical diffusion of DO from upper layers to lower layers and may sometimes lead to hypoxia. Storms are important to relieve DO deficiency. Storms are rare and short in summer, but strong winds may significantly alter the stratification process in shallow coastal waters as shown in this paper. The wind direction during storms was found to play an important role in sea temperature variation. Under normal weather conditions, the dominant wind direction was south or southwest in the study area. The wind speed was weak but lasted for a longer time than the storms. Future research will be conducted to explore the influence of wind direction under normal weather conditions on temperature structures and stratification and their effects on annual variation of sea temperature.

Author Contributions: Conceptualization, X.Z.; methodology, X.Z.; modelling, Y.D.; validation, T.Z.; formal analysis, X.Z.; investigation, Y.X.; writing—original draft preparation, X.Z.; writing—review and editing, Y.D. and C.W.; visualization, Y.D. and X.Z.; supervision, Q.X.; project administration, X.Z.; funding acquisition, Q.X. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Shandong Key Laboratory of Marine Ecological Restoration (Grant No.: 201913), Key Deployment Project of Center for Ocean Mega-Research of Science, Chinese academy of sciences with No. COMS2019J05, the National Natural Science Foundation of China under Grant 41801264, and the Key Research and Development Project of Yantai (2017ZH095). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Request to the corresponding author of this article. Acknowledgments: The authors appreciate the reviewers for their comments and suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest. J. Mar. Sci. Eng. 2021, 9, 710 18 of 19

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