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

The Korean Society of Oceanography (한국해양학회)
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First Observational Finding of Submesoscale Intrathermocline Eddy in the East Sea using Underwater Glider

수중글라이더를 활용한 동해 아중규모 중층성 소용돌이 발견

  • PARK, JONGJIN (Research Institute for Dok-do and Ulleung-do Island/Kyungpook Institute of Oceanography/School of Earth System Sciences, Kyungpook National University)
  • 박종진 (경북대학교 자연과학대학 지구시스템과학부/경북해양과학연구소/울릉도.독도연구소)
  • Received : 2019.02.20
  • Accepted : 2019.04.09
  • Published : 2019.05.31
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Abstract

Zonal hydrographic section measurements at $39.7^{\circ}N$ were conducted between $129.0^{\circ}E$ and $131.3^{\circ}E$ from August 7 to 25 in 2017 using an underwater glider. The glider traveled about 440 km for about 18 days along the 106 line of the regular shipboard measurements in the National Institute of Fishery Science (NIFS) and obtained twice a hydrographic section with high horizontal resolution. Even under the strong East Korea Warm Current with maximum speed of 0.8 m/s across the section, the glider successfully maintained the designated path within an RMS distance of 400 m. By comparing with the NIFS shipboard hydrographic section, it is confirmed that high spatial resolution measurements obtained from a glider were necessary to properly observe front and eddy variability in the East Sea where a typical spatial scale is smaller than the open oceans. From the glider section measurements, a new lens-shaped eddy was found in the thermocline. The lens-shaped anticyclonic eddy had 10~13 km in horizonal width and about 200 m in height like a typical submesoscale eddy resided within the thermocline, which was firstly named as Korea intrathermocline eddy (Keddy). The Keddy has the distinguishing characteristics of a typical intrathermocline eddy, such as a central core with anomalously weak stratification, a convex shaped lens bounded by the stratification anomaly, an interior maximum of velocity at 170 m, no surface appearance of the geopotential field, a small or comparable horizontal width relative to the first baroclinic Rossby radius of deformation, and the Rossby nubmer of 0.7.

2017년 8월 7일부터 25일까지 수중글라이더를 활용하여 $37.9^{\circ}N$ 위도 라인를 따라 동경 $129.0^{\circ}E{\sim}131.3^{\circ}E$ 사이를 왕복하는 단면 관측을 수행하였다. 해당 경로는 국립수산과학원 정기선박 관측라인 중 106 라인을 따른 것으로 이 경로를 따라 약 18일간 운용하였으며 위치 유지 모드로 동작했던 시간을 제외하고 총 440 km를 비행하였고, 그동안 고해상도 수온 및 염분의 공간 단면을 관측하였다. 본 관측 해역은 약 0.8 m/s의 강한 유속을 갖는 동한난류가 북상하고 있는 상황이었음에도 불구하고, 해당 수중글라이더는 지정된 경로에서부터 RMS 거리 400 m 이내를 벗어나지 않고 정확하게 106 라인을 따라 비행하였다. 본 관측에서 얻어진 고해상도 물성 단면 구조를 국립수산과학원 정선 관측 자료와 비교함으로써 해양환경에 지대한 영향을 끼치는 전선역이나 소용돌이와 같은 현상을 관측하기 위해서 고해상도 관측이 얼마나 중요한지 확인할 수 있었다. 이러한 수중글라이더 관측을 통해 이제까지 발견하지 못했던 새로운 소용돌이를 발견할 수 있었다. 이 소용돌이는 수평폭이 10~13 km, 수직폭은 200 m 가량되는 렌즈 형태를 가지고 있으며 시계방향으로 회전하는 아중규모 중층성 소용돌이(submesoscale intrathermocline eddy)다. 수온약층 내 혹은 직하부에 존재하면서 아중규모의 렌즈형태의 구조를 갖는 이러한 중층성 소용돌이는 동해에서 처음 발견되었기 때문에 Korea intrathermocline eddy(Keddy)로 명명하였다. 이 Keddy는 다음과 같은 전형적인 중층성 소용돌이(intrathermocline eddy)의 특징을 가지고 있다. Keddy는 수온약층 하부인 수심 약 170 m, 즉 중층에서 유속최대값을 갖는 특징이 있고, 따라서 해표면에는 해당 지오포텐셜 구조가 드러나지 않는 2차 순압성 구조를 가지고 있다. 또한 중앙부의 성층화가 주변보다 약하고, 수평크기가 1차 순압성 로스비 변형반경과 유사하며, 로스비 수가 0.7로 1에 근접한다.

Keywords

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Fig. 1. Slocum Glider (kg557) taken right before flying along the NIFS 106 line on August 7, 2017.

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Fig. 3. Longitude (blue) and Latitude (black) GPS position of the underwater glider in time. Blue lines denote longitudinal location of the NIFS 106 stations.

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Fig. 4. Zonal sections of potential temperature (a), salinity (b), and meridional absolute geostrophic velocity (c) along the offshore-bound glider trajectory. The contour intervals are 1.0℃ in (a) and 0.1 g/kg in (b). The black straight lines in (a) and (b) show the stations of the NIFS 106 line. Unit in (c) is cm/s.

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Fig. 5. Same as Fig. 4 but along the onshore-bound glider trajectory.

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Fig. 6. Zonal sections of potential temperature (a) and salinity (b) along the NIFS 106 line obtained from the shipboard CTD measurements from Aug. 13 to Aug. 14 which are interpolated with the cubic spline method in 2 km x 1 m bin. The contour intervals are 1℃ in (a) and 0.1 g/kg in (b). The black straight lines denote the stations of the NIFS 106 line.

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Fig. 8. Depth-averaged velocities estimated by glider trajectories. Blue lines present meridional components and red ones are zonal components. Dotted lines denote original data and solid lines are the data after removing linear trends between 150 km and 180 km in zonal distance.

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Fig. 9. Vertical profiles of potential temperature (a), salinity (b), and potential density (c) inside (red) and outside of Keddy (black).

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Fig. 10. First baroclinic Rossby deformation radii estimated from the glider hydrographic measurements along the offshore-bound trajectory.

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Fig. 2. (a) Offshore-bound trajectory of the underwater glider. Open circles denote the stations of the NIFS 106 line. The black arrow presents the location where Keddy was detected by the glider (b) Onshore-bound glider trajectory. (c) A map of Sea surface height anomaly (unit: meter) composited with multi-satellite measurements from Aug. 9 to Aug. 15, 2017 (http://marine.coperinicus.eu). Black dots denote the glider GPS positions.

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Fig. 7. (a) Zonal sections of potential temperature in Fig. 4 (a) zoomed in between 150 km and 180 km in zonal distance. White lines show meridional absolute geostrophic velocity. (b) Absolute geopotential heights estimated from the glider measurements. Black contours denote the Gaussian geopotential model fitted into the glider observation. (c) Azimuthal cyclo-geostrophic velocity (unit: cm/s). Black straight lines present the locations where the maximum velocities appear.

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