1.Introduction

The East Sea is a peculiar ocean system characterizing by the warm and cold region divided by the polar front. It has called a miniature of the ocean. The southwest region of the East Sea shows very complicated oceanographic conditions because this region is the passage of the East Korean Warm Current. Also, there exists some of cold and warm eddies. Interactions of these oceanic conditions show varieties of phenomena.

Eddies are important to biological productivity, because of their association with upwelling of subsurface water. One of the mechanisms responsible for nutrient input into the euphotic zone is uplifting of the nutricline by cyclonic eddies, known as “eddy pumping,” which results in enhanced primary production inside a cyclonic eddy. High primary productivity in the East Sea during late spring and early fall appears to be sustained by the interaction between eddies and wind, as well as other factors such as coastal upwelling and terrestrial nutrient input during the rainy season (Chang et al., 2016). Ulleung Warm Eddy (UWE) exists in the Ulleung Basin by the effect of bottom topography (Kim et al., 1991; An et al., 1994; Lie et al., 1995; etc.). Its form and position are related with the intensity of East Korean Warm Current (EKWC) (Cho et al., 1990; Isoda and Saitoh, 1993). Ulleung Warm Eddy also significantly affects the phytoplankton biomass seasonally (Kim et al., 2007).

Meanwhile, there exists the East Sea Intermediate Water (ESIW) under the lower portion of the Ulleung Warm Eddy (Kim et al., 1991; Kim et al., 2006). This water formed in the northern basin of the East Sea are spreading southward below the permanent thermocline in the basin (Kim et al., 1991). The ESIW lies approximately at 265 m depth with average thickness of 175 m, but the depth of the ESIW shows vertical variation influenced by the Ulleung Warm Eddy (Kim et al., 2006). The ESIW is characterized roughly 1~5°C in temperature ranges with salinity minimum (< 34.1) (Kim et al., 1991; Choi and Cho, 1997; Shin et al, 1998; Kim and Kim, 1999; Kim et al., 2006).

Therefore, we conjecture the interaction between the ESIW and UWE in the mid-depth or under the depth of permanent thermocline. Of course, the upper layer of UWE varies seasonally in temperature as well as salinity due to the atmospheric forcings (wind, heat flux, inflow of low saline water, etc.) and EKWC. But this is out of our scope in this study. Then, it would be a meaningful work to know an aspect of warm eddy, especially the interaction of UWE and ESIW in the mid-depth. The purpose of this study is to clarify the temperature variation of warm eddy in relation to the ESIW in the mid-depth.

2.Data and Approaches

Expendable bathythermography (XBT) observations were conducted in the southwestern region of the East Sea in the odd month during July 2013 to July 2015 by the vessel of Korea Coast Guard (KCG), Ministry of Public Safety and Security. The number of observation was 12 times and we omitted in May 2014 due to the accident of the passenger’s vessel, Sewolho, in the West Sea of Korea. At that time, most of vessels of KCG dispatched to the accident area.

The number of stations was 8 from the port of KCG in Donghae to Dok Island with equidistant interval. The distance is about 260 km from the KCG port in Donghae to Dok Island and the distance between the XBT stations is about 30 km. We used to omit some station due to bad weather or operational trouble of XBT system during the observation. But the number of observation occupied usually 8 to 11 times in each station during the whole period of observations.

In order to see the variation of temperature recorded by XBT, we calculated the coefficient of variation (CV), CV = SD (T) / mean (T), here SD (T) is the standard deviation of temperature and mean (T) is average of temperature in the observation data.

The observation line always went across the area of warm eddy, called Ulleung Warm Eddy. We focused on the variation of warm eddy with temperature distribution using the data both XBT (Table 1) and serial oceanographic observation from National Institute of Fisheries Science (NIFS). The data from serial oceanographic observation of NIFS were used to see the Ulleung Warm Eddy in horizontal and vertical distributions of temperature from June 2013 to August 2015 (Fig. 1).

3.Results

Fig. 2 shows all temperature profiles that were observed during July 2013 to July 2015. The surface temperature varied from 9°C to 26°C. The seasonal thermocline located at the depth of 20~30 m and the permanent thermocline was the depth of 200~300 m. It was relatively stable in temperature below the depth of 400 m.

Fig. 3 denotes the vertical distribution of mean temperature and the temperature variation by coefficient of variation (CV) according to the observation line by XBT. The mean temperature varied from 18°C at the surface to 2°C at 300 m in depth. The thermocline is shallow in the coastal region (stations 1 to 3) but deep in the region of stations 5 to 6 that was shown the shape of warm eddy. The coefficient of variation (CV) of temperature is 0.8 at 250 m in depth of station 4 and it was the largest value in the vertical distribution in temperature. And the large values of CV (0.6~0.8) biased upward to the coast and the surface layer.

Fig. 4 shows the horizontal distribution at 250 m depth of mean temperature and the temperature variation by CV using the data of serial oceanographic observation by National Institute of Fisheries Science. The core of warm eddy showed 5°C in temperature in the southwestern region of Ulleung island. The shape of warm eddy was covered around Ulleung island based on 3°C in isothermal line. The large value of coefficient of variation is 0.8 in the area between the east coast of Korea and Ulleung island. This region with large CV value seemed to be elongated along the north-south direction.

To see the vertical shape of warm eddy according to the seasonal variation, the vertical distributions of temperature from the data by XBT (odd months) and serial oceanographic observation (even months) were shown in Fig. 5. The surface mixing layer formed from December to March. The seasonal thermocline formed in April and lasted to November. The whole shape of warm eddy were shown in February and August. On February and August, the line 104 of serial oceanographic observation goes across Dok island so that the number of stations are extended comparing with other months as in Fig. 1. The isothermal line of 11°C in February in 2014 and 2015 were extended to the 200 m depth. These isothermal line of 11°C shrink to the 150 m depth in 2014 and to the 100 m depth in 2015. The shape of warm eddy persisted during the year except the forming of seasonal thermocline when the temperature increases.

Fig. 6 shows the horizontal distributions of temperature in the 250 m depth to see the core of warm eddy. The core temperature of warm eddy varied from 5°C (December 2013) to 10°C (August 2015). These warm eddy core located usually around Ulleung island or southwest region of Ulleung island except December 2013. At this time, the warm eddy located northeast region of Ulleung island exceptionally.

In order to examine the movement of warm eddy, the center of the warm eddy at 250 m depth was drawn in Fig. 7. We digitized the point of the center of the warm eddy. Some were not shown whole shape of the warm eddy. In this case, we digitized near the center in regard to the whole circle or ellipse. The warm eddy moved from 36°N to 38°N and from 130°E to 131°E. The movement scale of north-south direction was about 170 km and east-west about 120 km.

Fig. 8 shows T-S diagram based on the serial oceanographic observation during the observation period. The arrow indicates 3°C in temperature and 33.9 in salinity to see the law saline water. The low saline water showed the range of temperature between 3°C to 5°C.

4.Discussion

The form and position of warm lenses were related with the intensity of the Tsushima Warm Current and the formation of warm lenses were related with the bottom topography (Cho et al., 1990). Kim et al.(1991) suggested that anticyclonic circulation in the Ulleung Basin is controlled strongly by the shoaling bottom. An et al.(1994) referred that the warm eddies are elliptical in shape and the mean size is about 130 km in diameter, and suggested that the development and the movement of warm eddies are controlled by the Ulleung Basin.

In this study, the centers digitized in the center part of the warm eddy tended to move along the east coast of Korea between 130°E and 131°E as was shown in Fig 7. Also, the movement scale is larger in the north-south direction (about 170 km) than the east-west direction (about 120 km). These show that the movement of warm eddy is larger in the direction of the north-south than that of the east-west indicating that the warm eddy is trapped in the Ulleung Basin.

Isoda and Saitoh (1993) insisted that 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. And in summer to autumn, this mesoscale warm eddy intrudes slowly northward along the Korean coast. Gordon et al.(2002) referred that the winter mixed layer off the coast of Korea closely matches the intrathermocline eddies (ITE) core characteristics. Also, Shin et al.(2005) said that a homogeneous layer (lens) caused by atmospheric cooling in the core of warm eddy formed from the surface layer to a maximum depth of 250 m throughout the winter. And Shin et al.(1995) explained that interior of upper layer of warm eddy occupied homogeneous water with 10°C in temperature and 34.2 psu in salinity. This homogeneous water shrinks with time after winter passes.

As same as above studies, we showed that the surface mixed layer in winter lasted to summer in the vertical distribution of temperature as in Fig. 5. This means that Ulleung Warm Eddy formed through the surface mixing layer in winter, which water supplied from the East Korean Warm Current. Most of surface mixing layer was 10°C in winter in the Ulleung Warm Eddy. But in summer, the homogeneous water layer shrinks remarkably.

The cold waters, both East Sea Intermediate Water and East Sea Proper Water, formed in the northern basin of the East Sea are spreading southward below the permanent thermocline in the basin (Kim et al., 1991). The East Sea Intermediate Water (ESIW) characterized by the salinity minimum layer shows the range of potential temperature between 1 to 5°C and salinity lower than 34.06 psu. The ESIW lies approximately at 265 m depth with average thickness of 175 m. This thickness of the ESIW continues to be relatively uniform regardless of spatio-temporal space. However, the depth of the ESIW shows vertical variation influenced by the Ulleung Warm Eddy (Kim et al., 2006). Moreover, the intrusion of the East Sea Intermediate Water and lateral mixing between the warm eddy gradually reduced warm eddy’s diameter and maximum depth (Shin et al., 2005).

As in Fig. 8, the law saline water denoted that the range of temperature was between 3°C and 5°C. This range of temperature also indicated the East Sea Intermediate Water with the lowest salinity. Based on the above studies, the variation of temperature at the depth of 250 m influenced by the interaction between the Ulleung Warm Eddy and the East Sea Intermediate Water. In conclusion, the largest variation of temperature in the depth of 250 m indicates that the interaction between the lower portion of warm eddy and the East Sea Intermediate Water is very active. Of course, there would be the salinity variation. It will be studied about the variation of salinity in realtion to the East Sea Intermediate Water and Ulleung Warm Eddy in the near future. In this case, lateral mixing would be an important factor in the T-S relation. This also must be studied in the next theme.