278 Responses of coastal waters in the to Typhoon Bolaven

Responses of coastal waters in the Yellow Sea to Typhoon Bolaven

Chang S. Kim†, Hak-Soo Lim†, Jin Yong Jeong†, Jae-Seol Shim†, Il-Ju Moon‡∞, You Jung Oh‡, Hak Yoel You₸

†Coastal Disaster Research Center, ‡College of Ocean Science, Jeju National ₸Oceanographic Division, Korea Institute of Ocean Science & Technology, University, 102 Jeju-Univ. St., Jeju, Korea, Hydrographic and Oceanographic www.cerf-jcr.org 787 Haeanro, Ansan 426-744, Korea [email protected], [email protected] Administration, 351 Haeyang-ro,

[email protected], [email protected] ∞ Corresponding author Yeongdo-gu, , Korea [email protected], [email protected] [email protected]

ABSTRACT

Kim, C.S., Lim, H.S., Jeong, J.Y., Shim, J.S., Moon, I.J., Oh, Y.J., You, H.Y., 2014. Response of Coastal Waters in Yellow Sea to Typhoon Bolaven. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 278-283, ISSN 0749-0208. www.JCRonline.org In August 2012, Typhoon Bolaven (1215) passed through the East Sea (ECS) and the Yellow Sea (YS), leading to severe coastal damages in Korea. This study investigated the responses of the coastal waters along the meridional direction of the YS and the ECS to Typhoon Bolaven. This included the causes of record-breaking high waves in the ECS, the possible danger of coincident peak surges and high tides, resonant coupling between the typhoon, tides and topography, and the impact of Bolaven-induced sea surface cooling on the intensity of (1214). Analyses were conducted using observations from an ocean platform, buoys, and tidal stations, as well as a numerical model during the passage of Bolaven. Results revealed that the western coast of the Korean Peninsula fortunately avoided severe damages due to weak tidal action during the passage of Bolaven, although pure surge components were significantly high. However, we found that there was the possibility of resonant coupling between surges, tides and topography in the YS, which would contribute to further enhancement of the storm surge. Based on the wave simulations, it was revealed that a straight track and fast translation of Bolaven maximized the production of record-breaking high waves in the ECS.

ADDITIONAL INDEX WORDS: Typhoon Bolaven, sea surface cooling, coastal water responses, storm surge, tide, resonance, waves

INTRODUCTION the oceanic conditions ahead of Tembin, resulting in the weakened On average, about three tropical cyclones (hereafter referred to intensity of Tembin. This assumption was also verified in this as typhoons) per year influence the Korean peninsula (KP) and study. about one typhoon per year makes landfall on the KP. These typhoons sometimes lead to extensive damages from strong winds, DATA AND METHODS high waves, storm surges, and flooding, which has long been one To investigate the responses of waters along the coast of the KP of the most serious natural hazards to people living in Korea. to Bolaven, we collected all available data observed during the In 2012, four typhoons made landfall on the KP, which was the passage of the typhoon. The observations came from the Ieodo largest number recorded since 1960. Among them, Typhoon Ocean Research Station (IORS), two Korea Ocean Gate Arrays Bolaven (1215), which hit the Jeju Island (JI) and passed through (KOGA), four Korean Meteorological Administration (KMA) the Yellow Sea (YS), caused serious property damages, buoys, and 14 tidal stations (see Figure 1 and Table 1). In particularly for aquaculture and breakwaters along the southwest particular, IORS, the KOGA buoys, and the Marado buoy were coast of the KP due to high waves. At 00 UTC 28 August, the very close to Bolaven’s track (Figure 1). We also used satellite maximum wind speed (MWS) of Bolaven was 33 m/s (10 min images (Kim et al., 2013; Ryu et al., 2012) and numerical model average value) around the JI, but ocean waves that were higher results (Lim et al., 2013; Moon et al., 2003b) for the analysis. The than expected destroyed many breakwaters along the coast. This surge heights were calculated by subtracting the predicted tidal study investigated the possible causes for the unusually high elevation from the observed sea level at the 14 tidal stations. The waves based on wave simulations and measurements. In addition, predicted tidal elevation was estimated using a harmonic analysis. we examined the potential danger of storm surges along the coast Wave simulations were conducted using the WAVEWATCH III of the YS due to coincidence of peak surges and high tides as well (WW3) model. The WW3 is an ocean surface wave model as resonant coupling between the typhoon, tides, and topography. developed at NOAA/NCEP in the spirit of the WAM model. The Two days after the passage of Bolaven, Typhoon Tembin WW3 has been used in many research programs to study surface (1214) also made landfall on the KP, but the intensity of the wave dynamics, and as the operational wave model of NCEP for typhoon was not so strong. It was assumed that Bolaven changed global and regional wave forecasts (Tolman 2002; Tolman et al., 2002). ______DOI: 10.2112/SI70-047.1 received 29 November 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2014

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Responses of Coastal Waters in the Yellow Sea to Typhoon Bolaven 279

2.5°C cooling in the inner core is sufficient to shut down the entire energy production of a storm (Emanuel et al., 2004), the large SST drop seemed to have a significant weakening effect on Typhoon Tembin when it passed over the area on 28-29 August 2012 (Figure 3).

Potential Danger of Coincident Peak Surges and High Tides The west coast of Korea is one of the strongest tidal areas in the world. The tidal range is about 4 m in the south and increases to about 10 m in the north. During the high spring tides, the tidal elevation is significantly increased. If the astronomically enhanced tide levels coincide with the passage of a typhoon, it might be very threatening to the low land of coastal regions in Korea (Moon et al., 2003a). Figure 4 shows the temporal variations of minimum central pressures, wind speeds, and surge heights during the passages of Typhoons Bolaven and Tembin. Here, the surge heights were calculated by subtracting the predicted tidal level from observed Figure 1. Observation locations and tracks of Typhoons Bolaven sea level. For the 14 tidal stations, we also calculated peak surge and Tembin. Names of tidal station are listed in Table 1. height (A) and time (tA), predicted tide level (B) at tA, nearest high tide level (C) and time (tC), nearest spring high tide level (D) and time (tD), observed sea level (A+B) at tA, extreme sea level scenario 1 (A+C) at tC, extreme sea level scenario 2 (A+D) at tD, and time differences (|tC-tA|, |tD-tA|) during the passage of Typhoon Bolaven (see Table 1). The peak surge heights ranged from 50.2 cm (Busan) to 167.3 cm (Goheung). The surge heights above 140 cm were among the highest records in this region. However, it was fortunate that the peak surges occurred near low tide (04h 45m < |tC-tA| < 06h 57m) at most stations on the western coast along Bolaven’s track (1–10, including JI). Furthermore, Bolaven’s arrival avoided the spring tide (|tD-tA| > 67h). We estimated extreme sea levels at the tidal stations assuming the worst case scenarios that Bolaven landfall at high tide (scenario 1, A+C) and at spring high tide (scenario 2, A+D). For example, the observed sea level of 458 cm at YB reached 985 cm in scenario 1 and 1094 cm in scenario 2. At the other stations, potential extreme sea levels were estimated based on these Figure 2. Change in (SST) after the scenarios (Table 1). These results suggested that the coincidence passage of Bolaven: (a) before arrival of Bolaven (26 August), (b) of peak surge and high tides present tremendous danger along the after the passage (28 August). Tembin passed through the cooled coast of the KP and proper preparedness is necessary for the worst areas before landfall on the KP. case scenarios.

RESULTS

Bolaven-induced Sea Surface Cooling and its Impacts In summer, the surface waters in the YS are moderately warm, approximately 27°C–28°C, while the bottom waters are relatively cool at approximately 10°C (Kim et al., 2004) due to the year- round presence of Yellow Sea Bottom Cold Water (YSBCW). Considering that the average water depth in the YS is about 60 m, this area is distinguished by a large vertical temperature gradient of up to 18°C. This unique environment, rarely found in other regions, produces a strong sea surface cooling when typhoons pass over the area (Moon and Kwon, 2012). Figure 2 shows the sea surface cooling after the passage of Typhoon Bolaven. Warm surface waters of 27°C–28°C were suppressed to 16°C–20°C, particularly in the southern and western Figure 3. Change in intensity of typhoon Tembin (1214) in terms coastal waters of the KP. Typhoon Tembin passed over the cooled of minimum central pressure and maximum wind speed. areas two days after the passage of Bolaven. Considering that

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280 Kim et al.

Figure 4. Time series of minimum central pressure, instant wind speed, 10 min average wind speed (a, c, e, g) and predicted tidal elevation, observed sea level, surge height (b, d, f, h) with the time difference (DT) between high tide and surge peak occurrence at various coastal stations in Korea: (a, b) Incheon, (c, d) Daesan, (e, f) Yeonggwang, and (g, h) Goheung during the passages of Typhoons Bolaven and Tembin.

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Responses of Coastal Waters in the Yellow Sea to Typhoon Bolaven 281

Figure 5. Temporal variation of wind speed, significant wave height, and maximum wave height observed at oceanographic buoys and platforms during the passing of Typhoon Bolaven.

Resonant Coupling Between Typhoon, Tides, and 2013), the tidal wave propagates from south to north along the Topography western coast of Korea and there is a six hour difference in high tide between Jeju Harbour in the south and Incheon Harbour in In this region, another critical situation can occur due to mid-western Korea. The distance between the two tidal stations is resonant coupling of typhoon and tide when the storm translation approximately 438 km, yielding a phase speed of about 70 km/h speed (STS) is comparable to that of the tide. According to the tidal chart (Fang et al., 2004) and observed tidal data (Lee et al.,

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282 Kim et al.

Table1. Peak surge height (A) and time (tA), predicted tide level (B) at tA, nearest high tide level (C) and time (tC), nearest spring high tide level (D) and time (tD), observed sea level (A+B) at tA, extreme sea level scenario 1 (A+C) at tC, extreme sea level scenario 2 (A+D) at tD, and time differences (|tC-tA|, |tD-tA|) at 14 tidal stations during the passage of Typhoon Bolaven. The unit of sea level is [cm]. Here, |tC-tA | = 0 hour and |tC-tA | = 6 hours represent that the peak surges occur at high and low tides, respectively.

Station A B C D A+B A+C A+D |tC-tA| |tD-tA| 1 Yeongjong Bridge (YB) 159.6 298.4 825.4 934.6 458 984.9 1094.1 04h 50m 79h 06m 2 Incheon 151.5 289.5 807.3 911.7 441 958.8 1063.2 04h 49m 79h 01m 3 Ansan 142.3 270.7 763.1 862.9 413 905.4 1005.2 04h 45m 78h 55m 4 Daesan 110.8 211.2 712.2 797.5 322 823 908.3 05h 16m 79h 26m 5 Boreong 143.2 178.9 676.1 757 322 819.2 900.1 06h 46m 80h 54m 6 Gunsan(out) 124.5 164.5 645.4 721.4 289 769.9 845.9 05h 58m 80h 07m 7 Yeonggwang 119.9 151.1 599.2 670.8 271 719.2 790.7 05h 43m 79h 55m 8 Daeheuksando 84.4 145.7 324.4 362.2 230 408.7 446.6 06h 57m 92h 21m 9 Chujado 98.4 139.6 297.6 317.2 238 396 415.6 06h 24m 67h 37m 10 Seogwipo 111.8 145.2 262 295.3 257 373.8 407.1 06h 06m 92h 24m 11 Wando 136.6 258.4 285.3 384.9 395 421.9 521.5 01h 41m 88h 27m 12 Goheung 167.3 262.7 268.6 369.2 430 436 536.5 00h 47m 87h 37m 13 Gwangyang 101.3 258.7 287.8 385 360 389.1 486.3 01h 35m 85h 14m 14 Busan 50.2 111.8 113.8 145.9 162 164 196.1 00h 40m 86h 07m

for the co-tidal line. If the STS is similar to the phase speed, the enhancement of the storm surge always exists in this region due to storm surge can ride on the high tide. the coupling. Based on the best track data of the typhoon, the STS of Bolaven was about 40 km/h when it passed over the YS, which is much Occurrence of Record-Breaking High Waves slower than the propagation of the co-tidal line in this region. One of the most interesting responses of coastal waters during However, the STS of some historical storms such as Kompasu in the passage of Typhoon Bolaven was the observation of high 2010 reached up to 70 km/h, suggesting that in the worst case maximum wave heights (MWH). The MWHs observed at KOGA- scenario, the storm surge peaks could be amplified by a resonant S01, KOGA-S04, IORS, and Mardo buoys were 20.7m, 17.2m, coupling of tides and typhoon along the west coast. On the other hand, the YS is a semi-enclosed marginal sea of the northwestern Pacific Ocean, surrounded by the KP, the Chinese coast, and the . If we assume that the YS is an open channel closed at one end with a length of 820 km and depth of 60 m, the natural period of the channel, Tn, can be given by: 4L T (1) n n gh where n is the number of nodes (n = 1, 3, . . . ), L is the length of the channel, g is the gravity acceleration, and h is the water depth. For n = 1 and n = 3, the natural periods, T1 and T3, are 37.8 hours and 12.6 hours, respectively. The period T3, with three nodes, is known to reinforce tides by resonance with the semi-tidal period, resulting in strong tides in this area (Choi, 1980). If the natural periods (T1 and T3) are similar to the predominant period (Tsurge) of the surge generated from typhoons passing over this region, the storm surge can be significantly enhanced by the resonant coupling (Moon et al., 2003a). The Tsurge can be expressed by:

Lsurge Tsurge (2) Vstrom where Lsurge is the length scale of the typhoon and Vstorm is the STS. In the case of Typhoon Bolaven, if we consider that Lsurge was 600 km (based on mean distributions of wind and air pressure) and Figure 6. (a) Swath of SWH during the passage of Typhoon Vstorm was 40 km/h, the predominant period of the surge was about Bolaven. Line is the storm track. (b) Dominant MWL (×10, red 15 hours, resulting in no significant resonant coupling. However, solid line), MWS (red dotted line), and STS (black solid line) depending on the combination of STS and storm size, a potential along the typhoon track.

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17.3 m, and 13.7 m respectively (Figure 5). The extraordinarily funded by the National Institute of Meteorological Research. high values broke observational records in these regions although Partial support was provided by the projects entitled, Bolaven was not the strongest typhoon based on the historical “Construction of Ocean Research Station and their Application best-track archives of the Regional Specialized Meteorological Studies”, “Development of Coastal Erosion Control Technology”, Center (RSMC). and “KOOS II”, funded by the Ministry of Oceans and Fisheries, It is known that as the STS increases and becomes comparable Korea, and this support was greatly appreciated. to the group speed of dominant waves, waves to the right of the typhoon track are exposed to prolonged forcing from wind; that is, they become “trapped” within the typhoon (resonance effect or LITERATURE CITED dynamic fetch) and grow continuously (Moon et al., 2003b). The Choi, B.H., 1980. A tidal model of the Yellow Sea and the . straight translation of the typhoon can maximize the increase of Korean Ocean Research and Development Institute (KORDI) Report the dynamic fetch. 80-02, 72p. Typhoon Bolaven was moving quickly along a straight track Emanuel, K.A., DesAutels, C., Holloey, C. and Korty, R., 2004. with a high STS of about 9–18 m/s in the ECS and YS (Figure 6), Environmental control of tropical cyclone intensity. Journal of the which may have provided favorable conditions for wave growth. Atmosphric Sciences, 61, 843-857. To support this hypothesis, we simulated the significant wave Fang, G.H., Wang, Y.G., Wei, Z., Choi, B.H., Wang, X. and Wang, J., 2004. Empirical Cotidal Charts of the Bohai, Yellow, and East China height (SWH) for Typhoon Bolaven using the WW3 model Ses from 10 years of TOPEX/Poseidon altimetry. Journal of (Tolman, 2002) and calculated the maximum SWH at all grid Geophysical Research,109, c11006. points (swath in Figure 6) as well as the dominant mean wave Kim, C.S., Lim, H.S., Yoon, J.J., Oh, J.H. and Fangli, Q., 2004. Numerical length (MWL) along the typhoon track (red solid line in Figure 6). simulation of hydrodynamics and water properties in the Yellow Sea. This indicated that asymmetric distribution of the SWH was Journal of Korean Society of Oceanography,39(10), 72-95. evident due to the resonance effect, resulting in high waves to the Kim, C.S., Park, Y.J., Park, K.S., Shim, J.S. and Lim, H.S., 2013. right of the typhoon track. Application of GOCI Satellite Data to Ocean Modeling. Journal of Coastal Research, SI65, pp. 1409-1414. Lee, S.H., Kang, C.Y., Choi, B.J. and Kim, C.S., 2013. Surface Current CONCLUSIONS Response to Wind and Plumes in a Bay-shape Estuary of the eastern In 2012, five typhoons affected the Korean coastal waters, and Yellow Sea. Ocean Science Journal, 48, 117-139. among them, three typhoons made landfall on the KP from late Lim, H.S. Chun, I.S. Kim, C.S. Park, K.S. Shim, J.S. and Yoon, J.J., 2013. August to early September. This investigation focused on the High-resolution operational coastal modeling system for the prediction response of coastal waters to the passage of Typhoon Bolaven of hydrodynamics in Korea using a wave-current coupled model. Journal of Coastal Research, SI65, pp. 314-319. (1215) which was the strongest to hit the KP in 2012, and passed Moon, I.-J. and Kwon, S.J., 2012. Impact of upper-ocean thermal structure through the ECS and YS, leading to severe damages in Korea. We on the intensity of Korean peninsular landfall typhoons. Progress in analyzed observations from an ocean platform, buoys, and tidal Oceanography, 105, 61-66. stations as well as numerical modeling results during the passage Moon, I.-J., Oh, I.S., Murty, T. and Yoon, Y.H., 2003a. A study on the of Bolaven. cause of the unusual coalstal flooding generated by typhoon Winnie Analysis revealed that the western coast of the KP avoided along the western coast of Korea.Natural Hazards, 29(3), 485-500. severe storm surge damages due to weak tidal action during the Moon, I.-J., Ginis, I., Hara, T., Tolman, H.,Wright, C.W. and Walsh, E.J., passage of Bolaven, although pure surge components were 2003b. Numerical simulation of sea-surface directional wave spectra under hurricane wind forcing. Journal of Physical Oceanography, 33, significantly high. In fact, sea level at the coastal stations along 1680-1706. the track of Bolaven reached peak surges ranging from 50 to 150 Ryu, J.H., Han, H.J., Cho, S., Park, Y.J. and Ahn, Y.H., 2012. Overview of cm over the tidal elevation. However, the peak surges fortunately Geostationary Ocean Color Imager (GOCI) and GOCI Data Processing occurred with a time difference of approximately 4–5 hours from System (GDPS). Ocean Science Journal, 47(3), 223-233. high tide. Tolman, H.L., 2002. Validation of WAVEWATCH III version 1.15 for a We examined the possible dangers due to coincident peak global domain. NOAA/NWS/NCEP/OMB Tech. Note 213, 33 pp. surges and high tides as well as resonant coupling between the [Available online at http://polar.ncep.noaa.gov/waves/references.html] typhoon, tides, and topography. Results suggested that if the STS Tolman, H.L., Balasubramaniyan, B., Burroughs, L.D., Chalikov, D., Chao, Y.Y., Chen, H.S. and Gerald, V.M., 2002. Development and is similar to the co-tidal amplitude propagation (70 km/h) or the implementation of wind-generated ocean surface wave models at NCEP. predominant period of the surge is similar to the natural period of Weather and Forecasting, 17, 311–333. the YS (12.6 or 37.8 hours), the coasts along the YS would be very vulnerable to coastal inundation during high and spring tides, requiring a systematic warning system for the worst situation. Wave observations and modeling results showed that the straight and fast translation of Typhoon Bolaven caused unusually high waves in this region. The SST measurements also indicated that when Typhoon Bolaven passed through the YS, the surface water was vertically mixed, lowering the SST from 27°C–28°C to 16°C–20°C. Due to the cooler SST, Typhoon Tembin, which followed closely after Bolaven, lost energy and ended up a moderate typhoon.

ACKNOWLEDGEMENTS This research was supported by the project entitled, “Functional improvement of the Korea Ocean Satellite Center”, funded by the Korea Institute of Ocean Science & Technology as well as the project entitled, “Advanced Research on Applied Meteorology”,

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