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Variation of Semidiurnal Internal Tides Along the Southeastern Coast of Korea Induced by Typhoons

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Variation of Semidiurnal Internal Tides Along the Southeastern Coast of Korea Induced by Typhoons Journal of Marine Science and Engineering Article Variation of Semidiurnal Internal Tides along the Southeastern Coast of Korea Induced by Typhoons Jeong-Yeob Chae 1 , Chanhyung Jeon 2 , Pyeongjoong Kim 3, Naoki Hirose 4 , Ahyoung Ku 1 and Jae-Hun Park 1,* 1 Department of Ocean Sciences, Inha University, Incheon 22212, Korea; [email protected] (J.-Y.C.); [email protected] (A.K.) 2 Department of Oceanography, Pusan National University, Busan 46241, Korea; [email protected] 3 Underwater Survey Technology 21 Corp., Incheon 21999, Korea; [email protected] 4 Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan; [email protected] * Correspondence: [email protected]; Tel.: +82-32-860-7703 Abstract: Typhoon-induced strong winds can dramatically change the oceanic environment, occa- sionally resulting in sudden stratification changes. In July 2015, two consecutive typhoons, Chanhom and Nangka, passed over the Yellow and East/Japan Seas within a week. Remarkable temperature variations were observed near the southeastern coast of Korea, caused by typhoon-induced up- welling and downwelling events, which altered the energy of semidiurnal internal tides. During the typhoon-induced downwelling event, the energy of semidiurnal internal tides near the southeastern coast of Korea varied independently from the barotropic tidal forcing. Data-assimilated numerical simulation results reveal that the pycnocline, which is typically tilted toward the coast, enables the semidiurnal internal tidal energy to propagate toward shallow regions after being generated off the coast. Meanwhile, the downwelling event deepens the pycnocline near the coast and reflects and Citation: Chae, J.-Y.; Jeon, C.; Kim, P.; Hirose, N.; Ku, A.; Park, J.-H. concentrates the semidiurnal internal tide energy near the bottom off the coast. A simple mechanism Variation of Semidiurnal Internal using the ratio between the wave characteristic slope and the bottom slope is proposed to explain the Tides along the Southeastern Coast of observed variations of semidiurnal internal tide energy near the coast. This paper demonstrates a Korea Induced by Typhoons. J. Mar. case study showing that typhoon passage can modify the energetics of internal tides, which has the Sci. Eng. 2021, 9, 328. https:// potential to cause unusual short-term coastal environmental changes. doi.org/10.3390/jmse9030328 Keywords: semidiurnal internal tides; typhoon; southeast coast of Korea; ADCP observation; data- Academic Editor: Wei-Bo Chen assimilated numerical model Received: 8 February 2021 Accepted: 13 March 2021 Published: 16 March 2021 1. Introduction The southeastern coast of Korea is well known for its frequent upwelling events that Publisher’s Note: MDPI stays neutral occasionally last longer than a week during summer months. The water temperature near with regard to jurisdictional claims in the coast is sensitive to wind variations; however, the upwelling process is associated with published maps and institutional affil- upwelling-favored winds, in addition to the baroclinic structures and bathymetry near the iations. coast [1,2]. Typhoon passages can also be the dominant factor inducing water property changes in coastal regions [3]. Strong cyclonic winds blow around a typhoon and typically create an outward Ekman transport from its center. Considering the southeastern coast of Korea, typhoons passing over the Yellow Sea can create an eastward Ekman transport Copyright: © 2021 by the authors. and typhoons passing over the East/Japan Sea can create a westward Ekman transport, Licensee MDPI, Basel, Switzerland. inducing coastal upwelling and downwelling processes, respectively [4,5]. This article is an open access article Unexpected temperature variations along coastal areas adversely affect human activi- distributed under the terms and ties by causing mass mortalities at fish farms under influence of cold-shock stress [6], and conditions of the Creative Commons efficiency decreases in power plants changing condenser cooling water temperature [7]. Attribution (CC BY) license (https:// The temperature variations over days can be related to the vertical movement of water creativecommons.org/licenses/by/ 4.0/). along coastal areas, caused by upwelling and downwelling processes. In a relatively shorter J. Mar. Sci. Eng. 2021, 9, 328. https://doi.org/10.3390/jmse9030328 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 2 of 12 shorter time scale, internal waves can cause vertical variations. In particular, internal tides can induce temperature fluctuations with tidal frequencies, because they make water masses move vertically based on tidal periods. Stratification changes induced by the upwelling and downwelling processes are crit- ical for internal tides as they modify the depth of the pycnocline. Park and Watts [8] showed that the generation and propagation of semidiurnal internal tides at the south- western corner of the East/Japan Sea changed based on the background current and strat- ification conditions. Lee et al. [9] explained that the energy generated near the Western J. Mar. Sci. Eng. 2021, 9, 328 2 of 12 Korea Strait propagates near the southeastern coast by using the Regional Oceanic Mod- eling System. They showed that background stratification changes could modify the in- ternal wave field. timeIn July scale, 2015, internal There waves were can two cause successiv verticale typhoons, variations. Chanhom In particular, and internal Nangka, tides that can passedinduce over temperature the Yellow fluctuations and East/Japan with tidalSeas frequencies,(Figure 1a,b). because The two they typhoons make water induced masses upwellingmove vertically and downwelling based on tidal events periods. in series along the eastern coast of Korea [3]. The events madeStratification an abrupt changes 6–8 induced°C decrease by the and upwelling a consecutive and downwelling restorationprocesses within approxi- are critical matelyfor internal 5 days, tideswhich as was they recorded modify thenear depth the southeastern of the pycnocline. coast of Park Korea and using Watts an [8] ocean showed buoythat measuring the generation temperatures and propagation at three dept of semidiurnalhs (Figure 1c). internal A bottom-moored tides at the southwestern Acoustic Dopplercorner Current of the East/Japan Profiler (ADCP) Sea changed was deployed based on at the a site background close to the current ocean and buoy stratification (Figure 1c)conditions. for approximately Lee et al. half [9] a explained month in that July the 2015, energy which generated overlapped near with the Western the period Korea of the Strait twopropagates typhoons. nearThe observed the southeastern current coastprofile by ti usingmeseries the showed Regional energetic Oceanic tidal Modeling frequency System. fluctuationsThey showed with that abrupt background appearances stratification and disappearances, changes could which modify should the be internal a modulation wave field. of internalIn July tides 2015, affected There wereby the two stratifi successivecation typhoons, changes Chanhomthat occurred and Nangka, because that of passedthe upwellingover the and Yellow downwelling and East/Japan events. Seas (Figure1a,b). The two typhoons induced upwelling and downwelling events in series along the eastern coast of Korea [3]. The events made an In this study,◦ we first show the variations in the observation data and compare them withabrupt outputs 6–8 ofC the decrease high-resolution and a consecutive numerica restorationl ocean model, within called approximately DR_M, based 5 days, on whichthe data-assimilatedwas recorded nearResearch the southeastern Institute for coast Applied of Korea Mechanics using anocean ocean model buoy (RIAMOM) measuring temper-[10]. atures at three depths (Figure1c). A bottom-moored Acoustic Doppler Current Profiler The whole domain and study area of DR_M is exhibited in Figure 1b,c. Subsequently, (ADCP) was deployed at a site close to the ocean buoy (Figure1c) for approximately half a typhoon-induced changes in the stratification and the associated variation in semidiurnal month in July 2015, which overlapped with the period of the two typhoons. The observed internal tides are elucidated using the DR_M outputs. Lastly, dynamical explanations for current profile timeseries showed energetic tidal frequency fluctuations with abrupt ap- the variation in semidiurnal internal tides are presented. pearances and disappearances, which should be a modulation of internal tides affected by the stratification changes that occurred because of the upwelling and downwelling events. Figure 1. (a) Track of the two typhoons, Chanhom and Nangka, around the Korean Peninsula in July 2015. The colors of the Figurecircle 1. denote(a) Track the of scale the oftwo the typhoons, typhoon Chanhom based on the and Saffir–Simpson Nangka, around hurricane the Korean wind Peninsula scale, as shownin July in2015. the topThe rightcolors corner; of thebathymetry circle denote of the (b) thescale entire of the domain typhoon of thebased high-resolution on the Saffir–Simpson numerical hurricane ocean model wind (DR_M) scale, andas shown (c) the in southeast the top right coast of corner;Korea. bathymetry The red square of (b) inthe (b entire) indicates domain the studyof the areahigh-resolution exhibited in numerical (c). The magenta, ocean model blue, and(DR_M) red markers and (c) the in ( csoutheast) denote the sites of the Acoustic Doppler Current Profiler (ADCP) mooring, ocean buoy,
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