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The Marine Environment of Dokdo

HOME > Marine Science Reports > The Marine Environment of Dokdo
Distribution of water mass in the sea areas surrounding Dokdo: The role of topography
Water mass distribution in the upper layers of the East Sea is related to sea water exchange with the outer seas through four nearby straits (Korea Strait, Tsugaru Strait, Soya Strait, and Tatar Strait). Usually, a cold water mass resides in the northern half of the surface layer of the East Sea, while a warm water mass exists in the southern half. The warm water originates from the waters of Kuroshio that flow from the East China Sea, and in the summer months is also affected by low salinity water that comes from areas such as the Yangtze River. While the sea areas surrounding Ulleungdo and Dokdo are usually under the influence of such warm water masses, the incessant movement of sea water continues to bring into this area sea water of different temperatures and salinity. Water mass in the deeper waters of the East Sea, however, is governed by independent oceanic movements that occur within the East Sea. Deep sea oceanic movements are almost entirely influenced by topographical features, and therefore the distribution of water mass in the deep sea surrounding Dokdo is controlled mostly by deep sea water movements in the Ulleung Basin and the Korea Gap (the deep sea channel between Ulleungdo and Dokdo that was named ‘Korea Gap’ by the Korean Committee on Marine Geographical Names through a public announcement in the government gazette by the Ministry of Maritime Affairs and Fisheries on December 7, 2005).
Surface seawater characteristics in the sea areas surrounding Dokdo: The role of polar fronts and eddies
Figure 1. A map of cold and warm current of the surface layer of the East Sea
Figure 1. A map of cold and warm current of the surface layer of the East Sea
After travelling through the Korea Strait, the Tsushima Warm Current splits into two or three branches as it flows into the northern and eastern parts of the East Sea.
The East Korea Warm Current travels up Korea’s east coast and turns to the east at a latitude of 37.5° N towards the Tsugaru Strait. As this current flows through the East Sea, it meets a cold water mass that originates from the North Korean Cold Current, causing the formation of a front called the ‘polar front’.
Although the location and shape of this polar front can vary widely, unless the East Korea Warm Current is not generated or is very weak, the polar front will form to the north of Ulleungdo. The Dokdo area, therefore, will generally be under the influence of warm water. (Figure 1)
Figure 2. Observational analysis on the Ulleung warm eddy and the Dok Cold Eddy (Mitchell, 2005)
Figure 2. Observational analysis on the Ulleung warm eddy and the Dok Cold Eddy (Mitchell, 2005)
Sea water may sometimes swirl either clockwise or counterclockwise to form eddies. Along with currents, eddies - which can sometimes reach hundreds of kilometers in diameter - have a major influence on sea weather. As the East Korea Warm Current curves east and away from Korea’s east coast, a portion of the current is drawn to the southwest and re-circulated into the main current, generating a warm eddy (where the water temperature is higher within the eddy than outside) close to Ulleungdo that almost always exists. A smaller cold eddy (where the water temperature is lower within the eddy) has also been observed. The warm eddy is called the ‘Ulleung Warm Eddy’, and it maintains its clockwise flow as it extends its reach hundreds of meters deep into the sea. The counter-clockwise swirl of the cold eddy is usually generated southeast of the warm eddy in the southern seas surrounding Dokdo and travels westward within the Ulleung Basin. D.A. Mitchell, an oceanographer from the US, named this the ‘Dok Cold Eddy’ after Dokdo. (Figure 2)
Figure 3. Sea surface temperature distribution as seen through infra-red satellite images (April 29, 2004)
Figure 3. Sea surface temperature distribution as seen through infra-red satellite images (April 29, 2004)
The water mass in the upper layer of the sea areas surrounding Dokdo are governed by the location and the degree by which the aforementioned polar front meanders and the formation along with the movement of the Ulleung Warm Eddy and the Dok Cold Eddy. Temperature distribution in the East Sea (Figure 3) as observe by satellites shows that, aside from the summer months, there exist many eddies and quite a lot of meandering on the part of the polar front.
In addition to seawater movements, there are also other factors that could affect the temperature and salinity of sea water in the Dokdo area, such as the volume transport of the Tsushima Warm Current and the Chinese coastal waters that come in through the Korea Strait, precipitation, wind, solar radiation and other external atmospheric conditions.
Deep seawater characteristics in the sea areas surrounding Dokdo: The mini conveyor belt
Thermohaline circulation’ is a massive circulatory system that is caused by differences in density in seawater. Surface water flows to the polar seas near Greenland and the Antarctic and sinks as the wind and the lower temperature makes the water heavier. This water turns into bottom water and becomes the source of all oceans’ deep seawater. It continues to move and sometimes flows upward into the upper levels, where it travels along the upper layer circulation system until it once again reaches the polar seas. This conveyor belt system is a very slow system that usually takes more than 1,000 years to complete one cycle; and any change to the pace of this system will cause drastic changes in the world’s climate. A similar thermohaline circulation exists in the East Sea. The cycle for the East Sea’s circulation is in the hundreds of years, which is much faster than that in the bigger oceans. For this very reason, the East Sea can be utilized as a laboratory through which we can understand and predict the process of change that occurs in the large oceans. In the East Sea, deep seawater is generated in the northern seas closer to Vladivostok. The heavy deep seawater that is generated here during the winter is lower in salinity, and most of this deep seawater travels south along the east coast of Korea and is supplied into the Ulleung Basin, while a portion travels from the Japan Basin through the Korea Gap into the Ulleung Basin. This seawater remains hundreds of meters deeper than the eddies and the polar front, and is called ‘East Sea Intermediate Water’. Changes to ‘East Sea Intermediate Water’ critically affect the characteristics of seawater in the sea areas surrounding Dokdo.

In the oceans, the water temperature generally drops below 2°C when the water is deeper than 1,000 meters. In the Ulleung Basin, however, water that is colder than 2°C can be found at depths of only 100 to 200 meters, while of course this colder water is found at greater depths when warm eddies are present. Also, extremely cold water that is just 0.2°C is found at depths of only 1,000 meters in the Ulleung Basin. Sea water in the Ulleung Basin is therefore mostly very cold water that is below 2°C, making the area a great resource for deep seawater projects.
Mixing seawater: The Dokdo Effect
In general, seawater in areas surrounding islands are mixed together quite forcefully in a turbulent flow that is maintained as a result of friction between geographical features and tidal currents (repetitive water flows accompanying tides) and ocean currents. As this mixing of warmer upper layer seawater and cooler lower layer seawater occurs more forcefully in areas closer to islands, the temperature of the upper layer seawater is lower in waters closer to the island. This is called the ‘island mixing effect’. Observations discovered that this island mixing effect also occurs in the seas surrounding Dokdo. According to data compiled by the Korea Institute of Ocean Science & Technology (KIOST), the island mixing effect is sometime clearly present and sometimes not so clear in the sea areas around Dokdo (Figure 4). In the autumn months, the temperature of the surface seawater close to Dokdo was found clearly to be about 1°C colder than surface water in sea areas that were far away. This phenomenon was therefore named “The Dokdo Effect”. Meanwhile, the vertical mixing of seawater did not occur very significantly in spring. Scientists looked at satellite images of seawater temperature to analyze the reasons for the differences between seasons, and found that the mixing effect was not that great when ocean currents in the Dokdo area were very weak, due to either the absence of eddies or the absence of currents. It could therefore be interpreted that the Dokdo Effect is influenced more by ocean currents related to water mass movements than those related to tidal currents. This would mean that the vertical mixing of seawater in the Dokdo area happens actively when turbulent flows occur as a result of friction between island topography and seawater exceeding a certain flow velocity. Thus, rather than being caused by tidal currents, this island effect induced vertical mixing near Dokdo is more likely controlled by water flows caused by broader oceanographic characteristics such as eddies and the meandering of the polar front in the Dokdo area.
Figure 4. East-west vertical temperature distribution near Dokdo (November 2006) The upper layer seawater closer to Dokdo is lower in temperature than in the farther seas due to vertical mixing caused by friction between ocean currents and Dokdo.
Figure 4. East-west vertical temperature distribution near Dokdo (November 2006) The upper layer seawater closer to Dokdo is lower in temperature than in the farther seas due to vertical mixing caused by friction between ocean currents and Dokdo.
Deep sea currents in the sea areas surrounding Dokdo: The Dokdo Abyssal Current
Figure 5. Deep sea currents In the deep layers between Ulleungdo and Dokdo, seawater mostly flows into the Ulleung Basin from the Japan Basin and flows out near Dokdo by way of a strong current (The Dokdo Abyssal Current).
Figure 5. Deep sea currents In the deep layers between Ulleungdo and Dokdo, seawater mostly flows into the Ulleung Basin from the Japan Basin and flows out near Dokdo by way of a strong current (The Dokdo Abyssal Current).
Seawater flows that travel in the deep layers are generally weaker than those in the surface layers, but deep currents can sometimes be quite strong. Up to the mid-1990s, Korea did not have any data on deep water currents in the East Sea. Since 1996, however, KIOST, Seoul National University and Pukyong National University have been observing deep currents below 350 meters’ depth at 5 stations across the Korea Gap. According to the data that has since been collected, the most notable characteristic of the deep flows travelling along the Korea Gap is that there is a two-way flow that travels along an east-west axis. Throughout most of the Korea Gap, water flows from the Japan Basin towards the Ulleung Basin. Meanwhile, near the eastern area close to Dokdo, water that entered into the Ulleung Basin flows out to the Japan Basin in a strong northbound current. It was observed that the deep flows near Dokdo were very consistent, and the highest observed flow velocity was about 33.9cm/s -a speed much faster that the few centimeters per second that is usually observed in other deep sea currents. A research team from Seoul National University named this consistently-observed deep sea current the ‘Dokdo Abyssal Current’ and reported on its existence in 2008 to international academia. (Figure 5)