1991년부터 2017년까지 표층 뜰개 자료를 이용하여 계산한 동해의 평균 표층 해류와 해류 변동성

Estimation of Mean Surface Current and Current Variability in the East Sea using Surface Drifter Data from 1991 to 2017

Abstract

동해의 평균 표층 순환과 표층 해류의 변동성을 이해하기 위하여 1991년부터 2017년까지 동해를 지나간 표층 뜰개들의 궤적을 분석하였다. 표층 뜰개 자료를 분석하여 동해 표층 해류들을 그 주경로 별로 분류하고, 이들 해류의 변동을 조사하였다. 동한난류는 한국 동해안을 따라 북쪽으로 흐르며 $36{\sim}38^{\circ}N$에서 이안한 후 동해 중앙($131{\sim}137^{\circ}E$)에서 동쪽으로 흐른다. 이때 해류 경로의 평균 위도는 $36{\sim}40^{\circ}N$의 범위를 가지며, 남북으로 큰 진폭을 갖고 사행한다. 표층 뜰개 경로의 평균 위도가 $37.5^{\circ}N$ 이남(이북)일 때 사행진폭이 상대적으로 크며(작으며) 진폭은 약 100 (50) km이다. 동해 중앙에서 표층 뜰개들은 $37.5{\sim}38.5^{\circ}N$를 따라 동쪽으로 흐르는 경로를 가장 빈번하게 지나간다. 동해 북부 블라디보스토크 연안에 투하된 표층 뜰개들은 연안을 따라 남서쪽으로 이동하다가 일본분지 서쪽에서 반시계방향 순환을 따라 남동쪽으로 이동한 후 $39{\sim}40^{\circ}N$에서 동쪽으로 사행하여 이동한다. 다음으로 동해를 $0.25^{\circ}$ 간격으로 격자를 나누어 각 격자를 통과하는 표층 뜰개들의 이동 속도 벡터 자료로 동해 평균 표층 해류 벡터장과 속력장을 구하였다. 그리고 $0.5^{\circ}$ 격자 간격으로 해류장의 분산타원을 계산하였다. 울릉분지 서쪽에서는 동한난류의 경로가 매년 변화하고, 야마토분지에서는 해류의 사행과 소용돌이가 많아 해류의 변동성(분산)이 크다. 표층 뜰개의 주 이동 경로, 평균 해류 벡터장, 분산을 모두 반영하여 표층 뜰개 자료에 근거한 동해 표층 해류 모식도를 제시하였다. 이 연구는 그동안 인공위성 고도계 자료를 이용하여 구한 표층 지형류와 해양수치모델로 모의한 해류를 중심으로 연구해 왔던 동해 표층 순환을 라그랑지 관측 자료를 통해 정리했다는 데 의의가 있다.

To understand the mean surface circulation and surface currents in the East Sea, trajectories of surface drifters passed through the East Sea from 1991 to 2017 were analyzed. By analyzing the surface drifter trajectory data, the main paths of surface ocean currents were grouped and the variation in each main current path was investigated. The East Korea Warm Current (EKWC) heading northward separates from the coast at $36{\sim}38^{\circ}N$ and flows to the northeast until $131^{\circ}E$. In the middle (from $131^{\circ}E$ to $137^{\circ}E$) of the East Sea, the average latitude of the currents flowing eastward ranges from 36 to $40^{\circ}N$ and the currents meander with large amplitude. When the average latitude of the surface drifter paths was in the north (south) of $37.5^{\circ}N$, the meandering amplitude was about 50 (100) km. The most frequent route of surface drifters in the middle of the East Sea was the path along $37.5-38.5^{\circ}N$. The surface drifters, which were deployed off the coast of Vladivostok in the north of the East Sea, moved to the southwest along the coast and were separated from the coast to flow southeastward along the cyclonic circulation around the Japan Basin. And, then, the drifters moved to the east along $39-40^{\circ}N$. The mean surface current vector and mean speed were calculated in each lattice with $0.25^{\circ}$ grid spacing using the velocity data of surface drifters which passed through each lattice. The current variance ellipses were calculated with $0.5^{\circ}$ grid spacing. Because the path of the EKWC changes every year in the western part of the Ulleung Basin and the current paths in the Yamato Basin keep changing with many eddies, the current variance ellipses are relatively large in these region. We present a schematic map of the East Sea surface current based on the surface drifter data. The significance of this study is that the surface ocean circulation of the East Sea, which has been mainly studied by numerical model simulations and the sea surface height data obtained from satellite altimeters, was analyzed based on in-situ Lagrangian observational current data.

Fig. 1. Bathymetry of the East Sea (Modified from Choi et al., 2012). Contour lines denote 1000, 2000 and 3000 m isobaths. KS stands for Korea Strait, TS Tsugaru Strait, SS Soya Strait, TTS Tatar Strait, UB Ulleung Basin, JB Japan Basin, YB Yamato Basin, YR Yamato Rise, EKB East Korea Bay, KP Korea Plateau, UD Ulleungdo, DD Dokdo, OI Oki Islands, and NP Noto Peninsula.

Fig. 2. Trajectories of surface drifters from the Korea Strait along the East Korea Warm Current (EKWC). Surface drifters were grouped by the mean latitude (L) of their trajectories between 131°E to 137°E. Here, n is the number of surface drifters in each group.

Fig. 3. The number of surface drifters which traveled eastward along each latitude bin in the middle (131~137°E) of the East Sea. Bin size is 1°. For example, the number in 38°N bin represents the number of surface drifters whose mean latitude were between 37.5°N and 38.5°N.

Fig. 4. Trajectories of surface drifters along (a) the Japan Nearshore Branch (JNB) along the coast (the Tsushima Warm Current) and (b) the JNB separated from the coast. Here, n is the number of surface drifters in each group.

Fig. 5. Trajectories of surface drifters released or started off Vladivostok in the northwester East Sea. Surface drifters were grouped by the mean latitude (L) of their trajectories within 133-137.5°E and 35°N-42°N. Here, n is the number of surface drifters in each group.

Fig. 6. Composites of absolute dynamic topography (m) (contours at 0.05 m intervals) and drifter trajectories (a) from October 1999 to February 2000 and (b) from January 2007 to June 2007. Red (black) lines represent trajectories of surface drifters released from the Korea Strait (off Vladivostok, Russia). Vectors are surface geostrophic currents estimated from the satellite altimeter data.

Fig. 9. Trajectories of the surface drifters along the EKWC. Surface drifters were grouped by the spatial mean northward speed ($_{\upsilon}^{-}$) within 129.3-130°E and 35.5-37°N. (a) $_{\upsilon}^{-}$ < 30 cm/s, n = 8 (b) 30 cm/s ≤ $_{\upsilon}^{-}$< 50 cm/s, n = 31 (c) 50 cm/s ≤ $_{\upsilon}^{-}$ < 70 cm/s, n = 30 (d) $_{\upsilon}^{-}$ ≥ 70 cm/s, n = 15. Here, n the number of surface drifters in each group.

Fig. 10. Schematic surface current map of the East Sea estimated from surface drifter trajectories. Red (Blue) lines represent warm current (cold current). (a) Vectors are mean surface current vectors (cm/s) and (b) Isobaths representing bottom topography of the East Sea are overlaid with schematic surface current map. Contour lines denote 1000, 2000 and 3000 m isobaths, respectively.

Fig. 7. (a) Mean surface current vectors on 0.25° × 0.25° grid and (b) mean surface current speed (cm/s).

Fig. 8. (a) Variance ellipses and (b) the number of surface current vectors used for variance estimation with major axis and minor axis on 0.5° × 0.5° grid.