The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY (한국해양학회지:바다)

The Korean Society of Oceanography (한국해양학회)
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Migration of the Dokdo Cold Eddy in the East Sea

동해 독도 냉수성 소용돌이의 이동 특성

  • KIM, JAEMIN (Department of Oceanography, Chonnam National University) ;
  • CHOI, BYOUNG-JU (Department of Oceanography, Chonnam National University) ;
  • LEE, SANG-HO (Department of Architecture Ocean Construction Convergence Engineering, Kunsan National University) ;
  • BYUN, DO-SEONG (Ocean Research Division, Korea Hydrographic and Oceanographic Agency) ;
  • KANG, BOONSOON (Ocean Research Division, Korea Hydrographic and Oceanographic Agency)
  • 김재민 (전남대학교 해양학과) ;
  • 최병주 (전남대학교 해양학과) ;
  • 이상호 (군산대학교 건축해양건설융합 공학부) ;
  • 변도성 (국립해양조사원 해양과학조사연구실) ;
  • 강분순 (국립해양조사원 해양과학조사연구실)
  • Received : 2019.01.23
  • Accepted : 2019.05.27
  • Published : 2019.05.31
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Abstract

The cold eddies around the Ulleung Basin in the East Sea were identified from satellite altimeter sea level data using the Winding-Angle method from 1993 to 2015. Among the cold eddies, the Dokdo Cold Eddies (DCEs), which were formed at the first meandering trough of the East Korea Warm Current (EKWC) and were pinched off to the southwest from the eastward flow, were classified and their migration patterns were analyzed. The vertical structures of water temperature, salinity, and flow velocity near the DCE center were also examined using numerical simulation and observation data provided by the Hybrid Coordinate Ocean Model and the National Institute of Fisheries Science, respectively. A total of 112 DCEs were generated for 23 years. Of these, 39 DCEs migrated westward and arrived off the east coast of Korea. The average travel distance was 250.9 km, the average lifespan was 93 days, and the average travel speed was 3.5 cm/s. The other 73 DCEs had moved to the east or had hovered around the generated location until they disappeared. At 50-100 m depth under the DCE, water temperature and salinity (T < $5^{\circ}C$, S < 34.1) were lower than those of ambient water and isotherms made a dome shape. Current faster than 10 cm/s circulates counterclockwise from the surface to 300 m depth at 38 km away from the center of DCE. After the EKWC separates from the coast, it flows eastward and starts to meander near Ulleungdo. The first trough of the meander in the east of Ulleungdo is pushed deep into the southwest and forms a cold eddy (DCE), which is shed from the meander in the south of Ulleungdo. While a DCE moves westward, it circumvents the Ulleung Warm Eddy (UWE) clockwise and follows U shape path toward the east coast of Korea. When the DCE arrives near the coast, the EKWC separates from the coast at the south of DCE and circumvents the DCE. As the DCE near the coast weakens and extinguishes about 30 days later after the arrival, the EKWC flows northward along the coast recovering its original path. The DCE steadily transports heat and salt from the north to the south, which helps to form a cold water region in the southwest of the Ulleung Basin and brings positive vorticity to change the separation latitude and path of the EKWC. Some of the DCEs moving to the west were merged into a coastal cold eddy to form a wide cold water region in the west of Ulleung Basin and to create a elongated anticlockwise circulation, which separated the UWE in the north from the EKWC in the south.

인공위성이 관측한 해수면 높이 자료를 활용하여 울릉분지 일대에서 발생하는 냉수성 소용돌이들을 1993년부터 2015년까지 Winding-Angle 방법을 이용하여 탐지하고 분류하였다. 냉수성 소용돌이들 중에서 동한난류 사행의 첫 번째 골에서 형성되어 동쪽으로 흐르는 해류의 주경로로부터 남서쪽으로 떨어져 나온 독도 냉수성 소용돌이(Dokdo Cold Eddy, DCE)를 구분하였고, 그 이동 특성을 분석하였다. 또한 국립수산과학원(National Institute of Fisheries Science)이 관측한 수온과 염분 자료와 Hybrid Coordinate Ocean Model의 수치모의 결과를 이용하여 DCE 중심 근처에서 수온과 유속의 수직구조를 살펴보았다. DCE는 23년 동안 총 112개 발생하였고, 이 중 39개의 DCE가 서쪽으로 이동하여 한국 동해안 근처 연안에 도달하였으며, 평균 이동 거리는 250.9 km, 평균 수명은 93일, 평균 이동 속도는 3.5 cm/s였다. 나머지 73개의 DCE는 동쪽으로 이동하거나 생성된 위치 주변을 맴돌다가 소멸하였다. DCE 아래 50~100 m에서 수온(T)과 염분(S)이 주변보다 낮아(T < $5^{\circ}C$, S < 34.1) 등온선들과 등염선들이 돔(dome, 반구형으로 된 지붕 모양) 구조를 보였다. 또한 DCE의 중심에서 평균 38 km 떨어진 곳에서 10 cm/s 이상의 해류가 표층부터 수심 300 m까지 반시계방향으로 원을 그리며 흐른다. 동한난류가 이안하여 동쪽으로 흐르다가 울릉도 북쪽에서 울릉도를 끼고 시계방향으로 흘러서 사행을 시작하고, 울릉도 동쪽에 위치한 사행의 첫 번째 골이 남서쪽으로 깊이 파고들면, 해류사행의 마루와 마루가 연결되고 골 부분이 독립적으로 떨어져 나와 반시계방향 순환을 형성하면서 DCE가 생성된다. DCE가 서쪽으로 이동할 때 울릉 난수성 소용돌이(Ulleung Warm Eddy, UWE)의 가장자리를 따라 우회하여 시계방향으로 U 모양을 그리며 한국 동해안 쪽으로 이동한다. DCE가 연안 부근에 도달하면, 동한난류는 냉수성 소용돌이 보다 더 남쪽에서 이안하고, 냉수성 소용돌이의 가장자리를 따라 우회하여 북쪽으로 흐른다. 연안에서 독도 냉수성 소용돌이가 약화되고 약 30일 후에 소멸하면, 동한난류가 다시 한국 동해안을 따라 북쪽으로 흘러서 본래의 경로를 회복한다. DCE는 열과 염을 북쪽에서 남쪽으로 꾸준히 수송하고 울릉분지 남서쪽에 냉수해역 형성에 도움을 주며, 양의 상대와도를 가지고 와서 동한난류의 경로를 변경시키는 역할을 한다. 서쪽으로 이동하는 DCE 중에서 일부는 연안 냉수성 소용돌이와 병합되어 울릉분지 서쪽에 넓고 긴 냉수해역을 만들고 반시계 방향의 순환을 형성한다. 이와 같이 병합된 소용돌이는 북쪽에 UWE를 남쪽에 동한난류로부터 분리시킨다.

Keywords

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Fig. 1. ADT (color and contour) and surface geostrophic current (vector) on (a) November 25, 2000 and (b) April 1, 2008. Blue line represents trajectory of a drifter from October 14, 2000 to January 18, 2001 and from April 4, 2008 to June 5, 2008. Red (black) dot indicates starting (end) location of a surface drifter.

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Fig. 2. Trajectories of (a) all cold eddies, (b) DCE (Dokdo Cold Eddy), (c) CCE (Coastal Cold Eddy), and (d) OCE (Other Cold Eddy) in the Ulleung Basin. Red dot represents formation location and black dot represents decay location. Blue line, which connects red and black dots, represents trajectory of a cold eddy. Trajectories (blue lines) of moving cold eddies overlapped each other.

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Fig. 4. Trajectories of DCEs (a) which moved toward the east coast of Korea, DCE-W (N = 39) and (b) which moved eastward or hovered around their formation locations and decayed, DCE-E (N = 73). N represents the number of eddies. Red dot represents formation location and black dot represents decay location. Blue line, which connects red and black dots, represents trajectory of a DCE. Trajectories (blue lines) of moving cold eddies overlapped each other.

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Fig. 8. Composites of ADT (color and contour) and surface geostrophic current (vector) on the days of DCE generation. Red (black) dot represents generation (decay) location. DCEs were grouped by their mean latitude (ML) during lifespan: (a) ML < 36.0°N, (b) 36.0° < ML < 36.5°N, (c) 36.5° < ML < 37.0°N, (d) ML > 37.0°N.

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Fig. 9. ADT (color and contour) and surface geostrophic current (vector) every 30 days relative to the day of DCE generation. Black dot represents DCE location. The number and mean latitude of DCEs used in this composite analysis are 16 and 36.0°‒ -36.5°N, respectively.

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Fig. 10. ADT with surface geostrophic currents on (a) October 14, (c) November 4, and (d) December 2, 1995. Horizontal distribution of 100 m depth temperature in (b) October 1995 and (e) December 1995. Contour intervals of sea surface height and temperature are 0.05 m and 2°C, respectively.

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Fig. 11. ADT with surface geostrophic currents on (a) June 24, (c) July 15, and (d) August 12, 2000. Horizontal distribution of 100 m depth temperature in (b) June 2000 and (e) August 2000. Contour intervals of sea surface height and temperature are 0.05 m and 2°C, respectively.

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Fig. 12. ADT with surface geostrophic currents on (a) August 12, (c) September 9, and (d) October 7, 2007. Horizontal distribution of 100 m depth temperature in (b) August 2007 and (e) October 2007. Contour intervals of sea surface height and temperature are 0.05 m and 2°C, respectively.

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Fig. 3. (a) The number of DCE (Dokdo Cold Eddy), CCE (Coastal Cold Eddy) and OCE (Other Cold Eddy) detected in each year from 1993 to 2015, (b) the total number of eddy formation in each month from 1993 to 2015.

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Fig. 5. (a) Absolute dynamic topography with surface geostrophic currents on October 14, 2013, (b) horizontal distribution of 100 m depth temperature, (c) zonal section of temperature along 37°N, (d) meridional section of temperature along 131.2°E, (e) zonal section of salinity along 36°N, and (f) meridional section of salinity along 130.0°E in October 2013. Contour intervals of sea surface height, temperature and salinity are 0.05 m, 2°C, and 0.1, respectively.

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Fig. 6. (a) Absolute dynamic topography with surface geostrophic currents on December 23, 2013, (b) horizontal distribution of 100 m depth temperature, (c) zonal section of temperature along 36.5°N, (d) meridional section of temperature along 130°E, (e) zonal section of salinity along 36.5°N, and (f) meridional section of salinity along 130°E in december 2013. Contour intervals of sea surface height, temperature and salinity are 0.05 m, 2°C, and 0.1, respectively.

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Fig. 7. (a) Absolute dynamic height with surface geostrophic height from satellite observation, (b) sea surface height (SSH), horizontal distributions of (c) 100 m and (d) 200 m depths temperature, (e) zonal section of SSH, temperature and meridional velocity(v) along 36.3°N, and (f) meridional section of SSH, temperature and zonal velocity (u) along 131.7°E from HYCOM simulation on October 21, 2013.

Table 1. Lifespan, displacement distance and propagation speed of DCE (Dokdo Cold Eddy) calculated from satellite altimeter and tide gauge sea level data for 23 years (1993-2015). DCE-W (DCE-E) stands for DCE propagating westward (eastward).

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