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Journal of Oceanography Vol. 49, pp. 443 to 458. 1993 The Northward Intruding Eddy along the East Coast of Korea YUTAKA ISODA1 and SEI-ICHI SAITOH2 * 1Department of Civil and Ocean Engineering, Ehime University, Matsuyama 790, Japan Institute, Japan Weather Association, Kaiji Center Bldg., 5, 4-chome, Kojimachi, Chiyoda-ku, Tokyo 102, Japan 2Research (Received 3 September 1992; in revised form 22 January 1993; accepted 25 January 1993) The current structures and their seasonal variations in the East Korean Warm Current (EKWC) region, which plays a significant role in the northward transport of warm and saline waters, were described by combining the sea surface temperature (SST) data of consecutive satellite inferred (IR) images and hydrographic data. The SST patterns in winter–spring clearly showed that the small meander of thermal front originating from the Tsushima/Korea Strait formed close to the Korean coast and grew an isolated warm eddy with horizontal dimension of order 100 km. Such warm eddy began to intrude slowly northward from spring to summer. At that time, interactions with neighboring synoptic warm eddy [Ks] around the Ulleung Basin were found to have strongly influence the movement of the intruding eddy and its structural change. In autumn, after the northward movement stopped at the north of eddy [Ks], the relative stable northward current along the Korean coast were formed. The evidence from observational results does not support a persistent branching of the EKWC from the Tsushima/Korea Strait, but a seasonal episodic supply of warm and saline waters due to the northward intruding eddy process described above. 1. Introduction It was proposed by Suda and Hidaka (1932), Uda (1934) and Kawabe (1982a) that the Tsushima Current formed three branches just after it enters the Japan Sea through the Tsushima/ Korea Strait as schematically shown in Fig. 1. The 1st (nearshore) branch is the extension flow along the Japanese coast, which enters the Japan Sea through the eastern channel of the strait. The 2nd (offshore) branch and the 3rd branch (the East Korean Warm Current; referred to as the EKWC) enters the Japan Sea through the western channel of the strait and they are separated from each other northeast of the strait. The EKWC may be particularly significant, since it carries heat and water mass northward and creates the warm surface layer throughout the basin south of the oceanic thermal front, i.e. the polar front in the Japan Sea. The dynamical process has been reported regarding the formation of this EKWC using the numerical experiments, e.g. Yoon (1982a, b), Kawabe (1982b) and Sekine (1986). These experiments have explained the EKWC as the steady western boundary current generated by the external forcing of outflow at the Tsugaru Strait. Namely, the EKWC extends northward until it reaches the latitudes of the Tsugaru Strait located at 41°N and becomes an eastward zonal current to flow out through the outlet. However, the observations as discussed below show some serious discrepancies of model results. *Now at Faculty of Fisheries, Hokkaido University, 3-1-1, Minato-cho, Hakodate 041, Japan. 444 Y. Isoda and S. Saitoh Fig. 1. Bathymetric chart in the Japan Sea. Three branches of the Tsushima Current near the Tsushima/ Korea Strait are schematically shown. Broken thick outline is the area covered by a consecutive satellite IR images in 1987 utilized in this study. Figure 2(a) shows the monthly isotherms of 100 m depth from the climatological mean from 1906 to 1987 for February and August (Japan Maritime Safety Agency, 1992). It is important to note that there may be two or three fronts with the synoptic meanders in the EKWC region. These meanders are taken to be accompanied by warm and cold eddies. Recently, statistical studies were made on the existing frequency distributions in terms of the synoptic warm eddies with horizontal dimension of order a few hundred km as shown in Fig. 2(b) (Isoda and Nishihara, 1992). This map implies that the movements of synoptic warm eddies are not turbulence, which transfers heat northward steadily (Toba et al., 1984), but rather stable at the following five areas, i.e. [Kn], [Ks], [Y], [O] and [N] in Fig. 2(b). From Figs. 2(a) and (b), it is found that the actual separation of the EKWC occurs south of 38°N at the depth of 100 m and the local northern limit of the polar front corresponds with the location of eddy [Y] on the Yamato Rise at 40°N. Furthermore, a cold water region may be always situated to the northwest of the Oki spur (see Fig. 2(a)), which is corresponding well to the low frequency area less than 10–20% in Fig. 2(b). It is likely that warm eddies do not enter into this area. In fact, the Shimane Cold Water might be frequently formed there, e.g. Tanioka (1962). The existence of such eddies causes a large meandering paths in the EKWC region. Besides, the recent observations have shown that the EKWC is highly energetic and characterized by The Northward Intruding Eddy along the East Coast of Korea 445 Fig. 2. (a) Monthly isotherms of 100 m depth from the climatological mean from 1906 to 1987 for February and August (Japan Maritime Safety Agency, 1992). (b) The existing frequency of synoptic warm eddies during the statistical period from 1980 to 1990 (Isoda and Nishihara, 1992). [Y], [O], [N], [Kn] and [Ks] indicate the sea areas with a local maximum value and suggest the stable existence areas of synoptic warm eddies. mesoscale features. In particular, satellite IR images off the Korean coast have shown some warm filaments extending seaward or mesoscale warm eddies with horizontal dimension of order less than 100 km, e.g. Tameishi (1987) and Kim and Legeckis (1986). We infer that these observations provide evidence for synoptic or mesoscale warm eddies as an important role for the current structures in the EKWC region. 446 Y. Isoda and S. Saitoh In the present study, relatively cloud-free satellite IR images over the EKWC region were obtained in spring and autumn from 1985 to 1989. In particular, based on a consecutive images in 1987, we investigated the shapes and trajectories of the mesoscale warm eddies. The results derived from these images were compared with the shallower and deeper hydrographic structures in 1987. We clarified the relation between the synoptic processes of the ocean and mesoscale ones in the EKWC region and their seasonal variability. 2. Data Plate 1 shows the satellite IR images in the western part of the Japan Sea from 1985 to 1989, which were processed by the Japan Weather Association. They are typical SST patterns of enhanced images in spring (a: March–April) and autumn (b: October–December), in which darker (brighter) tones enhanced warmer (colder) waters. To investigate the temporal and spatial evolution of the mesoscale features in detail, the consecutive satellite IR images over the EKWC region were collected in 1987. Study area of these images is shown by the broken thick outline in Fig. 1. Figures 3 and 6 show the selected images as the SST contour maps revealing the evolution of mesoscale warm eddies in winter– spring (from 8 January to 17 April) and the northward flow close to the Korean coast in autumn (4 October and 16 December) 1987, respectively. Routine hydrographic survey were conducted in 1987, by the Fisheries Research and Development Agency, Korea (1988) and the Shimane Prefectural Fisheries Experimental Stations (Fisheries Agency Japan, 1990) in Japan. It has been considered that the 100 m isotherms well represent the streamlines of the Tsushima Current. However, the trouble was that the quasisynoptic patterns of 100 m isotherms denoted the very complicated streamlines as for the presence of the EKWC at each observation. There are examples of both absence and presence of the northward branch from the Tsushima/Korea Strait (Kim and Legeckis, 1984) and the existence of an isolated warm eddy or a simple one-mean-dering current off the Korean coast (Tanioka, 1968). The fact is that such drastic changes at the middle layer, i.e. 100 m depth, occurs due to the couple with mesoscale feature with the shallower depths and synoptic scale feature with the deeper ones, as will be described in the present study. Therefore, we depicted the temperature distributions of near-surface layer (75 m depth) which indicate the spatial structure of mesoscale eddies and those of 200 m depth which clearly indicate synoptic warm eddies [Kn] and [Ks] (Fig. 4). Furthermore, the structural changes of the Tsushima Current can be estimated on the results from the distributions of high salinity water which flows into the Japan Sea through the Tsushima/Korea Strait in spring every year. Figure 7 shows the temporal changes of the salinity distributions of 100 m depth in 1987. To indicate the vertical features of synoptic warm eddies, i.e. [Ks] and [Kn], and the offshore branch, the vertical sections of water temperature and salinity observed along 103 and 105 lines off the Korean coast and the seaward line off Hamada in Japan are shown in Fig. 5. Their locations are shown as the thick solid lines in Fig. 4. 3. Typical SST Patterns of the Enhanced Images in Spring and Autumn It is apparent that the spring SST patterns of surface water system in the EKWC region differ from the autumn SST patterns. The SST in spring is reflected by the spatial structure and location of some thermal fronts, while the SST gradients in autumn are relatively small and it is impossible to detect such thermal fronts. Therefore, it is noticed that a strong contrast of enhanced images in autumn does not necessarily coincide with a sharp SST front. Satellite IR images taken in spring (Plate 1(a)) indicated that the narrow-belt of warmest The Northward Intruding Eddy along the East Coast of Korea 447 waters, about 50 km width, was formed close to the Korean coast, accompanied by some warm filaments. Such filaments suggested the complex eddy-like structures or the existence of a few turning flows from the Korean coast. So, it is difficult that the northern end of the EKWC is clearly detected from the satellite SST patterns. Thus, the EKWC in spring does not appear to be a stationary current but flows in a very turbulent way. On the other hand, the satellite SST images in autumn (Plate 1(b)) showed the relative stable flow pattern every year. The northward EKWC was clearly seen as the warm belt close to the Korean coast, which was somewhat broader 50 km to 100 km width. It turned at 38°–39°N sharply eastward, where the existence of a warm eddy can be inferred from the anticyclonic rotation pattern of warm filament. Thus, in autumn, the sharp SST fronts accompanied by eddies could not be seen except for the northern end of the EKWC. SST features commonly observed in both seasons could be also detected. First, since we refer to the warm belt as the trajectory of northward current, axis of the EKWC seems to approach near the Korean coast in the upper oceans. Second, not only the southward counter-current flowing along the eastern side of northward current close to the Korean coast, but also the eastward zonalcurrent which was directly connected to warm water on the Yamato Rise was not formed. Isoda et al. (1992) studied the spatial structures around the warm eddy [Y] and their seasonal variations. They suggested a possibility that warm water in eddy [Y] was not transported from an extension of the EKWC, but composed of water flowing northeastward from the Oki spur. Presumably, the EKWC cross-exchange as “leakage” or “diffusivity” process might play an important role in heat and water mass transport to the interior region of the Tsushima Current. Thus, the SST distributions of the upper oceans in the EKWC region have a seasonal variability. The mesoscale features may dominate in spring, but not in autumn. Namely, the mesoscale eddies will evolve rapidly in spring, and their drastic structural changes will occur during spring to autumn. In the following analysis, we focus the discussion on their temporal and spatial variations using the satellite and hydrographic data in 1987. 4. Northward Intruding Eddy along the Korean Coast 4.1 Formation process in winter to spring The northward intruding eddy was born and evolved from January to April 1987. The process of this formation can be seen in the satellite SST contour maps (Fig. 3). The SST from the IR image on 8 January was higher than the SST on 5 March by about 4°C in most parts of the EKWC region, which presumably indicates a uniform sea surface cooling due to wintertime convection. On 8 January the sharp thermal fronts A, B, C and D–E appeared roughly perpendicular to the Korean coast. Each front from A to C seems to be formed at the northern end of mesoscale anticyclonic eddy. Concerning the thermal front B and C situated around 37°–38°N, we find that on the following days these fronts near the Korean coast decreased in the horizontal temperature gradient and amount of warm water in the southern mesoscale eddy engulfed into the northern mesoscale eddy as the warm filament. Then, on 5 March the thermal front A and B disappeared and the front C moved northward until around 39°N. Another thermal front D–E originated near the southeastern part of the Korean Peninsula and extended eastward on 8 January. This front corresponds to the offshore branch of the Tsushima Current flowing along the shelf edge off the Japanese coast. On 28 January the thermal front at side D started to form as the small meander moved northward along the Korean coast. On 27 March this meander grew as an isolated mesoscale warm eddy in size about 100 km in diameter. Plate 1(a). The enhanced satellite IR images in spring from 1985 to 1989. 448 Y. Isoda and S. Saitoh Plate 1(b). The enhanced satellite IR images in autumn from 1985 to 1989. The Northward Intruding Eddy along the East Coast of Korea 449 450 Y. Isoda and S. Saitoh On 17 April the thermal front at side D had separated from the offshore branch of the Tsushima Current and can no longer be connected to the thermal front at side E. Thus, a consecutive satellite IR images showed the northward migration of the southern warm water wedge associated with the southern separation point. Although mesoscale eddies with the thermal front A and B took the form of a fully developed warm filaments, they diffused rapidly during a few months. On the other hand, similar fully developed mesoscale eddies with the thermal front C and D were born from January to March and grew in size about 100 km in diameter. After this, we could clearly see a tendency for these individual eddies to intrude northward. 4.2 Northward movement from spring to autumn After formation, the northward intrusion process of eddy can be seen in the near-surface temperature maps of 75 m depth (the upper panels in Fig. 4). It is worth noticing that these patterns entirely differ from those of 200 m depth (the lower panels in Fig. 4), particularly at the southern part of the EKWC region. On February–March two synoptic warm eddies [Kn1] around the Korean Plateau and [Ks] around the Ulleung Basin were clearly seen at the deeper hydrographic distributions of 200 m depth, whereas two warm eddies [a] and [b] along the Korean coast were seen at the shallower ones. Eddy [a] with an SST front C corresponded to the eddy [Kn1]. It moved slowly northward at least by June. Eddy [b] with an SST front D also moved northward at the western side of eddy [Ks] from February to August. According to the hydrographic section on February 1987 in Fig. 5, the vertical features of both eddies [Kn1] (or [a]) and [Ks] are characterized by about 5°C isotherm contour. The structure of eddy [Ks] was evident to a depth of more than 300 m. This eddy [Ks] contained an intruding eddy [b] at the western upper layer of itself, which could be characterized by the saline waters more than 34.3 psu and about 10°C isotherm contour. This kind of superimposed eddies-structure was also seen along 105 and 103 lines on June and along 103 line on October in Fig. 5, and has been frequently observed as seen in Fig. 8 of Gong et al. (1985), Fig. 8 of Kim, K. et al. (1991) and Fig. 4 of Kim, C. H. et al. (1991). The vertical distributions of saline water in an intruding eddy [b] were almost the same as those conferring the shelf waters off Hamada throughout the year in 1987 (see Fig. 5). This implies that the intruding eddy [b] is certainly separated from the offshore branch with the shelf waters. On June, eddy [Ks] formed a warm-water protrusion [Kn2] to the north and changed shape to an ellipse when eddy [b] began to intrude along the northwest edge of eddy [Ks] (Fig. 4). At that time, vertical section (along 105 line on June in Fig. 5) showed the superimposed eddiesstructure in an eddy [Kn2], accompanied by eddy [b]. Namely, the intrusion of eddy [b] has a strong influence on the surface structure of eddy [Ks] shallower than 150–200 m, but not the core structure. On August, the northward movement of eddy [b] (or [Kn2]) stopped at the north of eddy [Ks] and, then, completely separated from the Korean coast (Fig. 4). After this, the superimposededdies structure in an eddy [Kn2] had disappeared and became to an isolated eddy, characterized by about 5°C isotherm contour (along 105 line on October in Fig. 5). Although the northward movement of eddy [a] (or [Kn1]) after August was not clear from Fig. 4, we inferred that this eddy had to dissipate in the northern EKWC region during July to October. From Fig. 6, the satellite SST contour maps on October and December capture a broader warm belt close to the Korean coast, which are represented by the northward intruding SST contour of 19°C and 13°C, respectively. Comparing with Fig. 4 and Fig. 6, it is found that there is no warm eddy at the north of eddy [b]. Eddy [b] with the thermal front F was formed as the Fig. 3. The consecutive SST contour maps over the EKWC region from 8 January to 17 April 1987 showing the northward movement of mesoscale eddies. Arrows A, B, C and D–E indicate the thermal fronts in SST. Dashed arrow indicates the warm filament and its flow direction. Cross grid denotes the cloud data, and the ship mark shows the period of the hydrographic survey. The Northward Intruding Eddy along the East Coast of Korea 451 Fig. 4. Near-surface (75 m depth) temperature distributions showing the warm eddies [a], [b] and [c], and temperature distributions at the depth of 200 m showing the synoptic warm eddies [Kn] (Kn1 , Kn2, Kn3) and [Ks] in 1987. The solid lines on February, June and October indicate survey observation lines for the vertical distributions of temperature and sanity in Fig. 5. Warm eddy [b] is the northward intruding eddy along the Korean coast. 452 Y. Isoda and S. Saitoh Fig. 5. Vertical temperature cross-sections of the offshore branch of the Tsushima Current off Hamada in Japan and the synoptic warm eddies [Kn] and [Ks] along 103 and 105 lines on February, June and October in 1987. Shadow areas show the saline waters more than 34.3 psu. The Northward Intruding Eddy along the East Coast of Korea 453 454 Y. Isoda and S. Saitoh northern end of the warm belt. However, the dissipation mechanism of eddy [a] could not be clarified in the present study because it is difficult to observe off the north Korea and detect such phenomena due to its rapid dissipation. On December, a significant offshore current flowing along the shelf edge off Japanese coast was formed again as the thermal front (Fig. 4). This current could be also seen at the SST maps in winter–spring (Fig. 3). When we compared the two cross-sections of eddy [a] (or [Kn1]) and eddy [b] (or [Kn2]) in Fig. 5, one finds that they are very similar structure. The saline waters could be found in the core of both northern eddies [Kn1 ] and [Kn2 ], beyond the southern eddy [Ks]. These results suggest that the formation of the northern synoptic eddy [Kn] might be due to the supplying of warm and saline waters accompanied by the northward intruding eddy in the upper oceans. 4.3 Structural changes during the northward intruding As mentioned in the previous sections, salinity maps of 100 m depth in 1987 (Fig. 7) reveal a similar story, confirming the structural changes of intruding eddy [b]. Eddy [b] with a core of saline waters moved slowly northward from February–March to June. On June, two developed saline filaments appeared. The shape of these filaments shows that saline waters in eddy [b] leaked outside of the northeast of eddy [Ks]. At that time, eddy [Ks] was elliptical in shape, and accompanied by eddy [b] at the north rims itself. Such saline waters spread in all EKWC regions, horizontally at the middle layer from 50 m to 150 m depth. From October to December, they were detected in a core of eddy [b] and two patches along the rims of eddy [Ks]. Thus, the northward intruding eddy in spring generally have sharp temperature gradients and well-defined patterns of salinity in the upper oceans, while those in summer to autumn gradually obscured by warm and saline waters engulfment into the southern synoptic eddy [Ks]. 5. Discussion Satellite IR images in 1987 with concurrent hydrographic data provide that the warm and saline waters in the EKWC region are episodically intruded northward by the mesoscale eddylike plumes of the shelf waters near the Tsushima/Korea Strait. Seasonal variability of such flow patterns is schematically represented in Fig. 8. In winter to spring, the small meander of thermal front originating from the Tsushima/Korea Strait forms close to the Korean coast and grows an isolated mesoscale warm eddy. In summer to autumn, this mesoscale warm eddy intrudes slowly northward along the Korean coast around 36°–37°N and gradually leaks outside of the synoptic eddy [Ks] around 37°–38°N. At that time, the density structure of intruding eddy is found at the depth shallower than 150–200 m. Below this depth, the structures are superimposed on the synoptic eddy [Ks], suggesting the intruding eddy may be driven by the deep of eddy [Ks]. After the movement of the intruding eddy, the relative stable flow along the Korean coast and the warm eddy [Kn] as the northern end of the EKWC are formed, but may be temporary only in autumn. As the results of wintertime convection and diffusive process of saline waters, some mesoscale-size water patches will be formed during winter, as seen in the satellite IR images in Plate 1(a) and Fig. 3. The present observation suggests that the intruding eddy process occurs regularly with one-year cycle from winter to autumn. Therefore, the seasonal difference of current structure in the EKWC region can be explained by a series of intruding eddy process described above, with the addition of structural changes during winter. The existence of the southern synoptic eddy [Ks] is so important as the dynamical process of the intruding eddy. From Fig. 2(b), eddy [Ks] is always in existence and its location is almost The Northward Intruding Eddy along the East Coast of Korea 455 Fig. 6. The two SST contour maps over the EKWC region on 4 October and 16 December 1987 showing the warm belt close to the Korean coast. Arrow F indicates the thermal front in SST. Cross grid denotes the cloud data. Fig. 7. Salinity distributions of 100 m depth showing the structural changes of the Tsushima Current in 1987. Shadow and black areas show the saline waters more than 34.3 psu and 34.6 psu, respectively. 456 Y. Isoda and S. Saitoh Fig. 8. Schematic illustration of the intruding eddy process in the EKWC region, showing that the northward movement of intruding eddy through the interaction with the southern synoptic eddy [Ks] and the formation of the northern synoptic warm eddy [Kn], which was due to the supplying of warm and saline waters accompanied by the northward intruding eddy. Shadow water mass shows the warm and saline shelf waters which are intruded from the exit of the Tsushima/Korea Strait. constant around the Ulleung Basin. Kim, K. et al. (1991) suggested that the stationary eddy [Ks] was generated by the topographic control on the movement of the abyssal waters, but such phenomena as the interaction with neighboring currents have not been well investigated. To confirm the hypothesis that northward intruding eddy indicate the interaction with eddy [Ks], it would be necessary in future to make an additional quantitative study of the velocity structure of eddy [Ks] and to make concurrent measurements of northward current structure along the shelf off the Korean coast. Finally, we consider that the intruding eddy process mentioned above may occur due to the following reasons. Outflows from the strait can be treated from the law of potential vorticity conservation, e.g. Ichiye (1991). Namely, the flow just after leaving the Tsushima/Korea Strait may conserve the potential vorticity near the strait. Furthermore, it can be assumed that near the Tsushima/Korea Strait the relative vorticity, i.e. O(10–6 s–1) from (the horizontal shear/the width of the strait) = 40 cm s–1/200 km referred to Isoda and Yamaoka (1991), is very small compared to the local Coriolis parameter, i.e. O(10–4 s–1). Therefore, the amount of potential vorticity largely depends on the distributions of local water depth. Most of the outflow waters from the The Northward Intruding Eddy along the East Coast of Korea 457 Tsushima/Korea Strait, where the mean depth is less than 150 m, are occupied by warm waters more than 10°C throughout the year, e.g. Ogawa (1983) and Isoda and Yamaoka (1991). On the other hand, the warm waters of the synoptic eddy [Ks], characterized by about 5°C isotherm contour, were found at the depths deeper than 250–300 m. Then, it is inferred that the water masses through the shallower strait cannot directly connect to those in the synoptic eddy [Ks] because of a large difference in the potential vorticity between deep water columns and shallow ones. The intruding eddy process, in which the potential vorticity can be conserved in a local water mass, will be most available features for the supply mechanism of warm waters into the EKWC region. 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