ADVANCES IN THE STUDIES ON THE YELLOW
SEA DYNAMICAL PROCESS
GUO Binghuo and HU Xiaomin
First Institute of Oceanography, SOA, Qingdao 266061, China
I.INTRODUCTIONThe Yellow Sea is a semi-closed and shallow shelf sea adjacent to the East China Sea on the south and is linked up with the Bohai Sea on the north. The bottom topography of the Yellow Sea is characterized by the Yellow Sea trough with a water depth of 70 to 100m stretching from southeast to north and the isobaths basically parallel to the coastline on both sides of the Yellow Sea trough with quite a slope on the east side and an expansive shallow water area on the west side. Taking about 124°E meridian as a demarcation line, China and Korea made their respective investigations in the Yellow Sea on each side in the past, however all these past investigations were not able to cover the whole Yellow Sea with a quasi-synchronous marine environmental investigation program, which resulted in difficulties in understanding the main dynamical problems with the Yellow Sea. Therefore, the scientists from both China and Korea had made joint efforts to carry out the project “China-Korea Joint Study on Ocean Circulation Dynamics in the Yellow Sea” from 1996 to 1998. After that the program “Comprehensive Investigation and Study on the Marine Environment in the China's Seas”(1997-2002) and the program“Chinese Sea Circulation Study (1999-2004)” have been conducted. The above-mentioned investigations and studies have enabled people to further understand the Yellow Sea marine environments and variabilities, and in this paper, a general review on the main advances in the Yellow Sea dynamical studies is presented.
1. Yellow Sea Warm CurrentThe Yellow Sea Warm Current is a main component of the Yellow Sea circulation, and brings the external warm saline water into the Yellow Sea to affect the Yellow Sea environment. The Yellow Sea Warm Current used to be thought of as a branch separated from the Tsushima Warm Current in the sea area southeast of Cheju Island and to have features being stronger in winter and weaker in summer. However, the traditional thoughts on the Yellow Sea Warm Current have been queried since 1980s, and some important problems such as the origin and pathway of the Yellow Sea Warm Current have been intriguing the physical oceanographers. The main advances in the investigations and studies in this field in recent years are as follows.
(1) Indicative thermohaline characteristics of the Yellow Sea Warm CurrentThe warm saline water tongue intruding into the Yellow Sea has long been considered as a main indicative feature of the Yellow Sea Warm Current, but it is shown from many satellite remote sensing images and observed temperature distributions that the warm water tongue in winter and spring stretches northwestward in the area southwest of Cheju Island, and then turns to the north pointing to the east end of Shandong Peninsula, which indicates that the Yellow Sea Warm Current flows northward along the isobaths of 50 to 60 m rather than along the Yellow Sea trough (Guo et al., 2000; Tang et al., 2000; Zang et al., 2001). It is shown from the comparison between the Yellow Sea temperature distribution in October of 1996 and that of February of 1997 (Fig.1) that the temperature in the warm water tongue area in February of 1997 was about 1.5℃ higher than that in October of 1996, and the temperature on two sides of the warm water tongue in winter (February of 1997) was obviously lower than that in autumn, especially that on the west side it was lower by over than 5℃ than that in autumn, which indicates that there was obvious thermal advection in the warm water tongue in winter, so the Yellow Sea warm water tongue can certainly be taken as a indicator of the Yellow Sea Warm Current (Guo et al. , 2000). It is shown from the analysis of saline water tongues intruding into the Yellow Sea in different months that the saline water tongue exists in the bottom layer almost all the year round, and its northward stretching extent has an annual variation cycle, namely, it stretches northward to only 34°N in January; it stretches northward quickly to 35°40¢N in February (Fig.2) with a pattern similar to that of the warm water tongue; it stretches northward slowly in spring and summer, the 33.0 isohahine goes as far north as 37°N, but its tongue looks less sharp; the saline water tongue shrinks southward in autumn, the tongue in the bottom layer shrinks southward quickly to the Yellow Sea trough northwest of Cheju Island in December because the convectional mixing in December has reached the bottom so as to result in a vertically uniform temperature profile, and the saline water tongue keeps on shrinking southward until January (Guo et al., 2000). It is suggested that the reason why the warm water tongue in January was not synchronized with the saline water tongue in January be that the temperature field in the Yellow Sea in autumn has a distribution pattern different from that of the salinity field, i.e., the isotherms have an east-west trend, and the isohalines have a north-south trend, so after the Yellow Sea Warm Current brings the warm saline water into the Yellow Sea, the warm water tongue appears at once, but the saline water tongue does not appear until the salinity value in the warm current water has exceeded that on both sides of the warm current. In summer and autumn, the Yellow Sea Cold Water Mass occupies the Yellow Sea trough bottom layer so as to make the warm water tongue disappear, but the saline water tongue still exists because the weak water exchange in the Yellow Sea trough area enables the saline water tongue in the bottom layer to be maintained. Therefore, it is suggested that the warm water tongue in the Yellow Sea be more indicative of the Yellow Sea Warm Current than the saline water tongue.
Fig. 2. Positions of isohaline 33.0 at the bottom layer in January and February, dash dot line was in February 1997, dash line in February 1959, dash cross line in January 1986 and solid line in January 1991 (after Guo et al., 2000).
(2) Seasonal characteristics of the Yellow Sea Warm Current
Based on the results by analyzing the hydrographic data observed in the Yellow Sea and the East China Sea from the period October of 1997 to January of 1999, it is suggested that in winter the Subei coastal current intrude into the northern East China Sea along the Yangtse River shoal due to the driving of strong northerly wind, and the Yellow Sea Warm Current intrude into the southern Yellow Sea as a compensation current. In spring the Subei coastal current intruding into the East China Sea weakens, and the Yellow Sea Warm Current flowing northward weakens consequently; in summer the Yellow Sea Cold Water Mass occupies the whole Yellow Sea basin, and the Subei coastal current has no longer intruded into the East China Sea and joins the Yellow Sea circulation, so the Yellow Sea Warm Current has disppeared (Guo et al., 2000; Tang et al., 2000). It is shown from the observed data in October of 1996 that there is an oceanic front in the area nearby the line connecting the Yangtse River month and the Cheju Island in autumn with the temperature front being dominant in the eastern part of the front and the salinity front being dominant in the western part (Fig.3), which indicates that there is no significant water exchange between the Yellow Sea and the East China Sea in autumn, so there is no Yellow Sea Warm Current entering the Yellow Sea (Zou et al., 1999). The above-mentioned facts indicate that the Yellow Sea Warm Current is a seasonal compensation current, which occurs mainly in winter, quickly weakens in spring, and disappears in summer and autumn. These are contrary to the traditional thoughts on the Yellow Sea Warm Current.
Fig. 4. Trajectories of satellite-tracked drifters with daily mean currents at 3 days interval and distributions of 34.0 isohaline (after Zang et al., 2001) .
2. Cheju Warm Current and Origins of Yellow Sea Warm Current
Guo et al. (1992) calculated the transport passing through the Cheju Strait in studying the origin of the Tsushima Warm Current. They concluded that the current passing through the Cheju Strait flows eastward all the year round, and is named the Cheju Strait Current. Lie et al. (1999) named the current flowing round the Cheju Island the Cheju Warm Current. Zang et al. (2001) made a further analysis of the water mass characteristics west of Cheju Island and pointed out that the sea area west of Cheju Island was demarcated by the 34.0 isohaline in winter with the saline water (salinity is 34.0 to 34.5) east of the isohaline being the Cheju Warm Current and the mixed water (salinity is 33.0 to 34.0) west of the isohaline being the origin of the Yellow Sea Warm Current (Fig.4).
3. Observed Current Evidence of the Yellow Sea Warm CurrentThe current observations for the three consecutive days were made at the fixed point of 34°N, 123°30¢E where the Yellow Sea warm water tongue reached in April of 1996, and the observed data indicate that the surface and bottom residual current directions at the point were roughly north by west, the residual current direction at the 20 m depth layer was northeast, and its residual current speed was about 2 to 3 cm/s. In addition, Argos drifters were deployed in the Yellow Sea warm water tongue in April of 1996 and February of 1997. Among them, the Argos drifter deployed at 34°N, 122°E in February of 1997 moved northwestward at a speed of 2.4 cm/s for a long period of time and then turned to the south west. Therefore, it is shown from the direct current observations that the Yellow Sea Warm Current is quite weak with a speed of less than 5cm/s (Zang et al., 2001). And it is an intermittent northwestward current during the winter monsoon when the strong northerly winds prevail (Lie et al., 1999).
II.YELLOW SEA INTERMEDIATE COLD WATER IN SPRING
The intermediate cold water in the sea area southeast of the Shandong Peninsula was first found in April of 1996. The water mass distributions in the upper layer of Yellow Sea in spring are shown in Fig.5, where the intermediate cold water area is the region enclosed by a dashed line. Fig.6 illustrates the profiles observed at 6 CTD stations, which shows that the temperature in the intermediate layer is lower by 0.5 to 2.5℃ than that in the both upper and lower layers. Zou et al. (2000) pointed out that the intermediate cold water occurred in the shelf-front area (Fig.5), whose formation mechanism could be stated as follows. In early spring, the Yellow Sea Warm Current gets weaker first in the upper layer, while the temperature variations get slower in the bottom layer; then the upper layer shelf-front moves eastward along with weakening of the northward flowing warm current, and the mixed water in the western Yellow Sea extends southeastward; after that the mixed water in early spring is formed in a shallow water area of a water depth of about 30m with a significant rise in water temperature in the upper layer and at the same time the temperature of the lower layer (at depths of more than 15m) keeps its wintertime one, which is lower by about 2℃ than that of the neighboring Yellow Sea Warm Current water, and its salinity and density are all lower than those of the warm current water, thus, when the mixed water extends southeastward and overlies the Yellow Sea Warm Current water, the lower layer mixed water becomes the intermediate cold water with a water temperature of being lower than that in the upper and lower layers.
Fig. 5. Upper layer water mass distributions in the Yellow Sea in spring and the intermediate cold water area enclosed by a dashed line (after Zou et al., 2000).
III.WATER EXCHANGE BETWEEN THE YELLOW SEA AND EAST CHINA SEA
Based on the CTD data observed in the Yellow Sea and East China Sea in 4 seasons during 1997 to 1999, Guo et al. (2000,2002) analysed the water exchange process between the Yellow Sea and East China Sea and proposed the main patterns of the water exchange. In winter, the Subei coastal water extends southeastward to intrude into the northern East China Sea, and the Yellow Sea Warm Current intrudes into the southern Yellow Sea as a compensation current to form a pattern of the S-shaped isothermal lines, which indicate the features of water exchange (Fig. 7a). In spring, the Subei coastal water begins to retreat, the Taiwan Warm Current water extends northward to affect the Yangtze River estuary, which results in the water pattern without a tongue of low temperature and low salinity in the surface layer (0-10m). However, the low temperature and low salinity water tongue at depths of more than 30 m is still remained. Moreover, the northern East China Sea cold water (cold eddy) and the associated cyclonic circulation occur in the area southwest of Cheju Island. The Yellow Sea Warm Current water intruding into the southern Yellow Sea has been cut off to form an isolated warm water mass. Therefore, a pattern of the distribution composed of a cold water mass in the northern East China Sea and a warm water mass in the southern Yellow Sea has been formed (Fig.7b).