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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 dur- ing 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.
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