The Current System in the Yellow and East China Seas

Home , China Seas, East China Sea, Yellow Sea

Journal of Oceanography, Vol. 58, pp. 77 to 92, 2002

Review

1 2 HIROSHI ICHIKAWA * and ROBERT C. BEARDSLEY

1Faculty of Fisheries, Kagoshima University, Kagoshima 890-0056, Japan 2Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.

(Received 1 June 2001; in revised form 21 September 2001; accepted 21 September 2001)

During the 1990s, our knowledge and understanding of the current system in the Keywords: Yellow and East China Seas have grown significantly due primarily to new technolo- ⋅ East China Sea, gies for measuring surface currents and making high-resolution three-dimensional ⋅ Yellow Sea, ⋅ numerical model calculations. One of the most important new findings in this decade Kuroshio, ⋅ is direct evidence of the northward current west of Kyushu provided by satellite- Tsushima Warm Current, tracked surface drifters. In the East China Sea shelf region, these recent studies indi- ⋅ Changjiang River. cate that in winter the Tsushima Warm Current has a single source, the Kuroshio Branch Current in the west of Kyushu, which transports a mixture of Kuroshio Wa- ter and Changjiang River Diluted Water northward. In summer the surface Tsushima Warm Current has multiple sources, i.e., the Taiwan Warm Current, the Kuroshio Branch Current to the north of Taiwan, and the Kuroshio Branch Current west of Kyushu. The summer surface circulation pattern in the East China Sea shelf region changes year-to-year corresponding to interannual variations in Changjiang River discharge. Questions concerning the Yellow Sea Warm Current, the Chinese Coastal Current in the Yellow Sea, the current field southwest of Kyushu, and the deep circulation in the Okinawa Trough remain to be addressed in the next decade.

1. Introduction main currents, i.e., the Kuroshio, the Tsushima Warm The Yellow and East China Seas are epi-continental Current (TSWC), and the Yellow Sea Warm Current seas bounded by China, Taiwan, the Ryukyu (Nansei) Is- (YSWC) as shown in Fig. 2 (Nitani, 1972). The Kuroshio lands, Kyushu, and the Korean Peninsula (Fig. 1). The enters the East China Sea (ECS) through the strait be- Ryukyu Islands consist of many islands between tween Taiwan and Yonakunijima Island, the easternmost Tanegashima Island south of Kyushu and Yonakunijima island of the Ryukyu Islands, flows northeastward along Island east of Taiwan. The shelf region shallower than the shelf slope, and exits to the Philippine Sea through 200 m occupies more than 70% of the entire Yellow and the Tokara Strait after turning eastward near 30°N. The East China Seas. In the southern and eastern East China TSWC flows into the Japan Sea through the Tsushima Sea lies the deep Okinawa Trough, in which the maxi- Strait, and the YSWC flows into the Yellow Sea from the mum water depth decreases from greater than 2000 m in south of Cheju Island (“Cheju” is represented by “Jeju” its southern section to less than 1000 m in its northeast- when one follows the new “Hangeul” Romanization sys- ern section. At depths greater than 600 m, the Okinawa tem announced by the Korean government in July, 2000). Trough is connected with the Philippine Sea only at the (It should be mentioned that hereafter we call the strait Kerama Gap southwest of Okinawa Island. between Taiwan and Yonakunijima Island as the East Tai- In the 1970s, the surface circulation in the Yellow wan Strait, and that Korean oceanographers call the Ja- and East China Seas was considered to consist of three pan Sea the East Sea and the Tsushima Strait the Korean Strait.) It has been thought that the Kuroshio should play an important role in the driving mechanisms of the TSWC * Corresponding author. E-mail: [email protected] and YSWC. The origin of the TSWC is considered to be u.ac.jp the Kuroshio Branch Current west of Kyushu (KBCWK), Copyright © The Oceanographic Society of Japan. which flows northward along 128.5°E over the western

77 Fig. 2. Schematic of the current system in the Yellow and East China Seas after Nitani (1972), together with the 200-m isobath and location of the repeat transect (section PN) where measurements were made quarterly by the Japan Meteorological Agency. The large numbered dots indicate the locations of tidal gauges: (1) Ishigaki (Ishigakijima Is- land), (2) Keelung (Taiwan), (3) Naze (Amami-Ohshima Island), and (4) Nishinoomote (Tanegashima Island). Fig. 1. Bottom topography in the Yellow and East China Seas. The 50-, 100-, and 200-m isobaths are shown within the green area where the water depth is less than or equal to 500 m. The light blue colored area denotes water depths rather sporadic and obtained in different years. This has between 500 and 1000 m. Water deeper than 1000 m is led to many different interpretations of the circulation in colored dark blue, with the 2000-m isobath included. Num- the Yellow Sea and ECS in the late 1980s. For example, bers indicate the locations of the following: (1) Taiwan there are two schools of thought on the origin of the Strait, (2) Tokara Strait, (3) Tsushima Strait, (4) Kerama TSWC. One school says that the TSWC is a continuation Gap, (5) Taiwan, (6) Yonakunijima Island, (7) Okinawa Is- of the KBCWK that has split from the Kuroshio south- land, (8) the Okinawa Trough, and (9) Changjiang River west of Kyushu (Kondo, 1985; Lie and Cho, 1994), while mouth. the other school says that the source of the TSWC is the Taiwan Warm Current (TWWC), which flows northward through the Taiwan Strait between Taiwan and China (e.g., flank of the northeastern Okinawa Trough. In this paper, Beardsley et al., 1985; Fang et al., 1991; Su, 1998; Isobe, the traditionally used name “Tsushima Warm Current” 1999). represents only the current flowing through the Tsushima During the 1990s, our knowledge and understand- Strait, and the northward current west of Kyushu is called ing of the current system in the Yellow Sea and ECS have the “Kuroshio Branch Current west of Kyushu” as pro- improved significantly due primarily to new technologies posed by Lie et al. (1998a). The origin of the YSWC is for measuring surface currents and conducting high-reso- considered to be the current bifurcated from the KBCWK lution three-dimensional numerical circulation model south of Cheju Island. experiments. The aim of this paper is to summarize re- As fishing activity in the Yellow Sea and ECS has cent developments in the study of the current system in been too strong to allow long-term moored current-meter the Yellow Sea and ECS using historical data analysis measurements, the circulation pattern in the Yellow Sea and numerical model solutions. Relevant review on the and ECS has been examined through analysis of current pattern in the Yellow and East China Seas up to hydrographic and very limited current measurement data. the early 1990s can be found in Su (1998). Vertical sec- However, these data used in many previous studies are tions across the Kuroshio in the central ECS and hori-

78 H. Ichikawa and R. C. Beardsley zontal distributions of water properties near the 50-m moored current-meter data, Fang et al. (1991) estimated depth layer in the Yellow Sea and ECS will be presented the volume transport of TWWC to be 1.0 Sv in winter in Section 2. In Section 3, the main features of surface and 3.1 Sv in summer. From table 5 in Ichikawa and circulation pattern derived by analysis of historical data Beardsley (1993), the mean Kuroshio volume transport will be presented, followed by a description of the cur- through the Tokara Strait from February 1987 to Septem- rent pattern found in recent numerical model studies in ber 1988 is estimated to be 24 Sv. Using moored-array Section 4. Combining the results in Sections 2, 3 and 4, measurements made during 1992Ð1996, Feng et al. (2000) the origin of the TSWC will be discussed in Section 5. estimated the mean total volume transport through the The important but unsolved problems on the current sys- Tokara Strait to be 23.4 Sv, assuming no vertical velocity tem in the Yellow Sea and ECS will be presented in Sec- shear from the sea surface to the top measurement depth tion 6. level (50-m to 280-m) at each mooring site. From sea level difference across the Tokara Strait (Naze minus 2. Seasonal Mean Distributions Derived from Nishinoomote) in 1965Ð1983, Kawabe (1988) indicated Hydrographic Data that the Kuroshio volume transport is maximum in summer and minimum in autumn while its annual cycle 2.1 Seasonal variations of external forcing changes year to year. Kawabe indicated also that the Water properties and the general circulation in the Kuroshio velocity in the Tokara Strait has large Yellow Sea and ECS are strongly influenced by external interannual variations with dominant periods of longer forcing from its surroundings, i.e., the atmosphere, the than five years and around 2.1 years. Takikawa et al. land, and the ocean. The wind over the Yellow Sea and (2001) analyzed the ship-mounted Acoustic Doppler Cur- ECS is monsoonal, northwestward in summer and south- rent Profiler (ADCP) data taken six times a week by a eastward in winter. From surface weather maps collected ferryboat crossing the Tsushima Strait from February 1997 during 1978Ð1987, Na et al. (1992) calculated monthly to February 2001. They found that the total volume trans- mean wind stress over the Yellow Sea and ECS and found port through the Tsushima Strait is a minimum in winter a maximum southward wind stress of about 1 dyne cmÐ2 and maximum in autumn, with an annual mean of about in January over the Yellow Sea and ECS and a maximum 2.7 Sv. The seasonal volume transports through the many northeastward wind stress of about 0.7 dyne cmÐ2 in Sep- straits between the Ryukyu Islands and through the strait tember over the southwestern ECS. From 10-year monthly at the mouth of the Bohai Sea have yet to be estimated. mean values of marine meteorological data in 1961Ð1970 These seasonal and interannual changes of various and marine meteorological data reported by ships in 1975Ð boundary forcing cause large variations in the density 1977, Ishii and Kondo (1987) calculated the monthly mean stratification and horizontal circulation pattern in the net surface heat flux from the atmosphere to the ocean Yellow Sea and ECS as described next. over the Yellow Sea and ECS and found a maximum of 140 W mÐ2 (warming) in June and a minimum of Ð280 2.2 Vertical sections W mÐ2 (cooling) in December, with an annual mean flux Since 1972, the Nagasaki Marine Observatory, the of about Ð58 W mÐ2 (cooling). Japan Meteorological Agency, has been conducting quar- The Changjiang River (Yangtze River) supplies about terly hydrographic observations along the fixed line in 80% of the total discharge of fresh water from rivers the central ECS shown in Fig. 2. Using the hydrographic around the Yellow Sea, ECS and Bohai Sea. Its monthly and current data obtained along this section (hereafter we mean transport has a large seasonal variation from 0.010 call it “section PN” following the originator), studies on Sv (1 Sv = 106 m3sÐ1) in January to 0.048 Sv in July around the seasonal variation of water properties and current in- an annual mean of 0.030 Sv, and large interannual varia- dicate the following. tions in the annual mean from 0.022 to 0.035 Sv during Figures 3(a) and (b) show vertical sections of sea- the 19-year period in 1970 to 1988 (Yanagi, 1994). sonal mean temperature and salinity estimated by The Yellow Sea and ECS has five boundaries with Fujiwara et al. (1987) from hydrographic data obtained surrounding seas, i.e., the Taiwan Strait, East Taiwan in 1972 to 1981. Seasonal variations in water properties Strait, Tokara Strait, Tsushima Strait, and the strait at the are only significant in the top 200-m of the water col- mouth of the Bohai Sea. The Kuroshio transports warm umn. Vertical density stratification is dominant over the saline water into the Yellow Sea and ECS through the shelf region except in winter. In the temperature sections, East Taiwan Strait. From the sea level difference between warm surface water seems to be spreading shoreward Keelung and Ishigaki during 1989Ð1996, Lee et al. (2000) (northwestward) in all seasons while cold bottom water estimated the annual cycle of Kuroshio volume transport seems to be moving seaward (southeastward) over the through the East Taiwan Strait to have a maximum of 24 shelf region in winter and spring. In the salinity sections Sv in summer and a minimum of 20 Sv in autumn. From except in winter, saline water higher than 34.0 psu ap-

Currents in the Yellow and East China Seas 79 Fig. 3. (a) Vertical sections of seasonal mean temperature along section PN in Fig. 2 (Fujiwara et al., 1987). Contour interval is 1°C. (b) Vertical sections of seasonal mean salinity along section PN in Fig. 2 (Fujiwara et al., 1987). Contour interval is 0.1 psu. The station numbers are shown along the top axis, and the horizontal distance between stations 1 and 9 is 463 km (250 nautical miles). pears to be intruding shoreward into the lower layer in the following sections that the main source of summer (deeper than 50-m depth) near the Changjiang River saline water in the lower layer is the Kuroshio Branch mouth while the less saline surface water seems to be Current to the north of Taiwan (KBCNT). spreading seaward towards the Kuroshio region. It should In the “Oceanographic Prompt Report of the Naga- be mentioned that in this paper, the practical salinity unit saki Marine Observatory” published just after each cruise (psu) is adopted even if the older values of salinity were for oceanographic observations, we can see the determined by titration before the introduction of the geostrophic volume transport of the Kuroshio modern salinometer. These differences in these seasonal (northeastward velocity component) referenced to the mean vertical circulations derived respectively from tem- 700-dbar level along section PN together with its sea- perature and salinity sections suggest that we cannot de- sonal mean value. The time series of this relative rive the vertical circulation pattern from only geostrophic volume transport has been used as an index hydrographic data along a fixed section. It will be shown of the long-term variation of the Kuroshio volume trans-

80 H. Ichikawa and R. C. Beardsley port in the ECS since 1972. The long-term mean relative transport during 28 years from 1973 to 2000 is 25.8 Sv, with a mean seasonal maximum of 27.0 Sv in summer and minimum of 23.9 Sv in autumn (Oceanographic Di- vision, Nagasaki Marine Observatory, Japan Meteorologi- cal Agency). From hydrographic and geomagnetic electrokineto- graph (GEK) data collected during 1972Ð1986, Yamashiro et al. (1990) estimated the seasonal mean geostrophic current through section PN using the seasonal mean specific volume anomaly with referenced to the seasonal mean surface current. Vertical sections of seasonal mean geostrophic velocity calculated by them are shown in Fig. 4. In Fig. 4, the Kuroshio axis (the maximum of northeastward velocity component) is located over the upper continental slope where the water depth is 500Ð 1000 m. Hereafter, we call this region where the surface current is larger than 50 cm secÐ1 the “Kuroshio main stream”. The seasonal change in the position of the Kuroshio main stream is not significant. Above the outer shelf where water depth is deeper than 100 m in the left- hand side of the Kuroshio main stream, the surface current flows northeastward with speeds smaller than 40 cm secÐ1 in spring and summer, and smaller than 20 cm secÐ1 in autumn and winter. The seasonal mean volume transport of the Kuroshio including the Kuroshio Fig. 4. Vertical sections of seasonal mean geostrophic current main stream and surrounding weak northeastward current normal to section PN referred to GEK velocity (Yamashiro is estimated to have a maximum of 25.4 Sv in summer et al., 1990). Shaded area indicates southwestward current. and a minimum of 16.2 Sv in autumn, with an annual mean Stations 3, 5, 7, and 9 in this figure correspond respectively of 21.2 Sv. From hydrographic and GEK data collected to stations 2, 3, 4, and 5 in Figs. 3(a) and (b). during 1986Ð1988, Ichikawa and Beardsley (1993) showed the Kuroshio volume transports in each transect to increase with increasing downstream (northeastward Surface southwestward current west of the Ryukyu along-isobath) component of local wind stress. Islands reaches to the bottom layer except in summer. In the bottom layer over the upper slope in Fig. 4, Deep southwestward current exists throughout the year, there is a southwestward current year-round. We call this but it is a little stronger in autumn and winter than other current the “slope counter-current” as proposed by Lie et seasons. These results suggest that vertical sections of al. (1998a). Chen et al. (1992) detected a deep counter water properties and current in the central ECS have large (i.e., southwestward) current over the upper slope near seasonal variations. the section PN in a 1986 hydrographic survey. They at- Here, we should note that the distributions shown in tributed this slope counter-current to be the result of an Figs. 3 and 4 tend to be very smooth, since they are spa- eddy in the left side of the Kuroshio, as shown by satel- tial and temporal averages over large interannual varia- lite imagery. It should be mentioned that the existence of tions and/or short-period fluctuations, especially in fron- the slope counter-current is confirmed not only in the tal regions where the front changes its position signifi- central ECS but also southwest of Kyushu (Lie et al., cantly over time scales of less than one month (e.g., James 1998a; Nakamura et al., 1999) and northeast of Taiwan et al., 1999). By integrating the geostrophic volume trans- (Chuang and Wu, 1991; Hsueh et al., 1993). Lie et al. port referenced with GEK or ship-mounted ADCP veloc- (1998a) suggested that the slope counter-current seems ity in each transect during 1981Ð1992, Ichikawa and to be a quasi-permanent feature in the southwest of Chaen (2000) found that the seasonal mean volume trans- Kyushu, but it is formed intermittently with a period of port of northeastward current through section PN is a 3Ð10 days in the central ECS. The driving mechanisms maximum of 32.1 Sv in summer and a minimum of 20.0 of these counter-currents have not been clarified yet, but Sv in winter, around an annual mean of 27.6 Sv, and these eddy forcing is one clear possibility as suggested by Chen seasonal variations are dominated by seasonal differences et al. (1992). in the volume transport of Kuroshio Surface Water.

Currents in the Yellow and East China Seas 81 2.3 Horizontal distributions of water properties Here it should be mentioned that the artificial In order to avoid the strong influences of large vari- smoothing of spatial distributions (due to averaging ability in atmospheric forcing and river discharge on the interannual variations and/or short-period fluctuations) distributions of sea surface temperature and salinity, must also affect Fig. 5 to some degree. The widths of the Kondo (1985) calculated the seasonal mean temperature temperature fronts shown in Fig. 5 are much larger than and salinity not at the surface but at 50-m depth (or at the those in individual satellite images. bottom in water depths shallower than 50 m) in the Yel- low Sea and ECS, using hydrographic data taken from 2.4 Circulation pattern derived from hydrographic data 1953 to 1970 (Fig. 5). Kondo concluded as follows. From the water property distributions shown in Fig. 1) The distribution of water masses at 50-m depth is 5, Kondo (1985) constructed the following schematics of strongly affected by the warm saline water trans- the summer and winter circulation patterns near 50-m ported by the Kuroshio and the cold, less saline depth in the Yellow Sea and ECS (Fig. 6). Kondo showed water transported by the Chinese Coastal Current. that the same dominant currents—the Kuroshio, the 2) The 50-m distribution does not change much over KBCWK, and the TSWC—as in Fig. 2 exist in both win- the year except in the central ECS shelf region. ter and summer, while the YSWC exists only in winter. It 3) Cold water less than 10°C, called the Yellow Sea should be noticed in Fig. 6 that the KBCWK does not Central Cold Water by Uda (1934), exists from bifurcate from the Kuroshio, but begins from the left of spring to autumn in the middle and bottom layers the Kuroshio. In Fig. 6, the TWWC and the cyclonic eddy of the Yellow Sea. It is the remains of cold water north of Taiwan exist only in summer. Kondo showed also produced in winter by surface cooling and mix- in Fig. 6 the year-round existence of the KBCNT, the ing. southward current transporting Changjiang River Diluted Kondo suggested that the seasonal variations of water Water (CRDW, the mixture of Changjiang River fresh- properties in the central ECS shelf region are caused by water with saline shelf water that leaves the river mouth the seasonal change in the TWWC. mixing region) along the Chinese coast in the ECS, the

(a)

Fig. 5. (a) Horizontal distributions of seasonal mean winter temperature (left) and salinity (right) at 50 m depth (Kondo, 1985). (b) Horizontal distributions of seasonal mean summer temperature (left) and salinity (right) at 50 m depth (Kondo, 1985). Contour intervals are 1°C and 0.2 psu respectively. Contour values at the bottom in areas shallower than 50 m are shown by dashed lines.

82 H. Ichikawa and R. C. Beardsley broad Chinese Coastal Current flowing southward along monsoon (Beardsley et al., 1985). the Chinese coast in the Yellow Sea, the southward cur- Stern and Austin (1995) suggested theoretically that, rent west of the Korean coast, the southward current west while the main branch of the Kuroshio follows the local of the Kyushu coast, and the southwestward current west isobaths when it comes to the steep continental shelf slope of the Ryukyu Islands. at the northeast corner of Taiwan, the inertial of a small It should be noted that Kondo (1985) did not adopt inshore fraction of the oncoming Kuroshio causes it to one unique mechanism but two alternative mechanisms cross the slope, creating an inshore branch. This bifur- to derive each of the currents shown in Fig. 6, i.e., he cated current entering the shelf displaces ambient water showed one current crossing isotherms or isohalines as- of relatively high potential vorticity as a countercurrent, suming that the effect of diffusion or mixing is more domi- which flows seawards across the slope. nant than advection, and another current flowing along Ichikawa et al. (2001) examined correlations of the isotherms or isohalines assuming that the effect of vertically-averaged salinity in the top 30-m layer in au- advection by a geostrophic current is more dominant than tumn at each of eight stations along section PN (Fig. 2) diffusion or mixing. As there is no evidence for adopting with the annual mean discharge of the Changjiang River either of these two alternative mechanisms, we can con- during 1972 to 1988 reported by Yanagi (1994). They clude that while Fig. 5 may represent well the general found a significant negative correlation at two stations, features of the distribution pattern of water masses in the one at 280-m water depth in the left-hand edge of the Yellow Sea and ECS, the seasonal circulation patterns Kuroshio and another at 100-m water depth in the mid- shown in Fig. 6 may be misleading in some regions. shelf region, with lower mean salinities for larger river While Kondo suggested the possibility that the sum- discharge during years of smaller annual mean river dis- mer southward spreading of less saline CRDW in the sur- charge than 0.030 Sv. On the other hand, the correlations face layer may be compensated by northward intrusion are not significant at all stations during years with larger of more saline Kuroshio water in the lower layer, it is river discharge greater than 0.030 Sv. These results sug- generally accepted at present that the CRDW has a bimo- gest that the path of CRDW has large interannual varia- dal pattern of a southward coastal jet and a northeastward tions depending on the Changjiang River discharge. spreading during the summer monsoon, and is transported The seaward countercurrent of KBCNT suggested by to the south along the Chinese coast during the winter Stern and Austin (1995) may explain the negative corre-

(b)

Fig. 5. (continued).

Currents in the Yellow and East China Seas 83 Fig. 6. Schematics of winter (left panel) and summer (right panel) horizontal circulation patterns near 50 m depth, together with the distributions of water masses and oceanic fronts after Kondo (1985). In summer, the Yellow Sea Central Cold Water is denoted YSCCW. The 200-m isobath is shown also. lation between surface salinity in the left-hand side of circulation pattern just in the surface layer while Fig. 6 is the Kuroshio and Changjiang River discharge, i.e., part the seasonal circulation pattern near 50-m depth. of the KBCNT transporting a mixture of Kuroshio Water The KBCWK is not significant in Fig. 7 but is in and CRDW may flow seaward over the shelf as a coun- Fig. 8. The discrepancy between the mean current fields tercurrent and may be entrained into the left-hand edge west of Kyushu shown in Figs. 7 and 8 can be attributed of the Kuroshio main stream. The insignificant correla- to the different features of the Eulerian and Lagrangian tion at all stations during years with larger river discharge current measurements (Uchida et al., 1998). The north- may indicate that the CRDW disperses intermittently in ward surface current with speeds of 30 cm secÐ1 origi- the shelf region during these years. nates from near 30°N, 127.5°E in the shelf region southwest of Kyushu in Fig. 8. This is direct evidence of the 3. Circulation Pattern Derived from Historical Cur- existence of the KBCWK suggested by Kondo (1985). It rent Data should be noted that the KBCWK does not flow over the western flank of the northern Okinawa Trough shown in 3.1 Annual mean current pattern Fig. 2, but over the shelf region with water depths of 100Ð Since currents with periods from 12 hours to several 200 m in Fig. 8. This indicates that it is not the Kuroshio days caused by tide and wind are very large in the Yellow main stream flowing along the 200Ð1000 m isobath but a Sea and ECS, it is necessary to average current data over part of the current on the left-hand side of the Kuroshio a long period to remove these higher frequency compo- main stream that separates to become the KBCWK. nents from the weaker low frequency current system in The southward current west of the Kyushu coast sug- the Yellow Sea and ECS. Qiu and Imasato (1990) con- gested in Fig. 6 can be seen in both Figs. 7 and 8. Hsueh structed a map of the annual mean surface current in the et al. (1996) examined theoretically the driving mecha- ECS by 1/5° × 1/5° areal averaging of current data mea- nism of the KBCWK together with the southward current sured by GEK from 1953 to 1984 (Fig. 7). Lie et al. west of the Kyushu coast, and concluded that the KBCWK (1998a, b) derived another annual mean surface current may, indeed, be a byproduct of the turning of the by 1/3° × 1/3° areal averaging of current data estimated Kuroshio, forced by the shoaling topography of the con- from trajectories of surface drifters from 1989 to 1996 tinental shelf southwest of Kyushu. (Fig. 8). Note that Figs. 7 and 8 represent the annual mean According to Kondo (1985), the KBCNT exists

84 H. Ichikawa and R. C. Beardsley Fig. 7. The annual mean pattern of surface current derived from Fig. 8. The annual mean pattern of surface current derived from GEK data from 1953 to 1984 (Qiu and Imasato, 1990). trajectories of surface drifters from 1989 to 1996 (Lie et al., 1998a, b), together with the 100-, 200-, 500- and 1000- m isobaths. throughout the year but the TWWC is dominant only in spring and summer, and a part of the KBCNT flows north- rent-meter measurements made for periods longer than ward and another flows eastward during both winter and several days, 25-hour current measurements made from summer. The KBCNT and southeastward current can be anchored ships, and surface current measurements ob- seen in Figs. 7 and 8 as an intrusion of the inflow Kuroshio tained with satellite-tracked surface drifters between 1970 and a wide weak seaward current over the continental and 2000, Lin et al. (2001) examined the surface circula- shelf northeast of Taiwan. The TWWC can be identified tion patterns in the warm half-year (MayÐOctober) and in Fig. 8 but not in Fig. 7 due to lack of data used in the cold half-year (NovemberÐApril). Although their data deriving Fig. 7. are rather sporadic, one of their most important conclu- The northwestward current shown in Fig. 8 to the sions is that the surface currents south of Cheju Island, south of Cheju Island does not appear in Fig. 7. As no north of Taiwan, in the central Yellow Sea, and over the surface current has been observed to flow into the Yel- shelf area west of where the Kuroshio turns eastward to- low Sea from south of Cheju Island but a current flowing wards the Tokara Strait are variable and produce eddies. around Cheju Island is dominant, Lie et al. (2000) pro- These variable current regions are the key areas for un- posed to name the mean surface current turning clock- derstanding the water mass distributions shown in Fig. 5 wise around Cheju Island in the northern ESC the Cheju and the driving mechanisms for the circulation pattern Warm Current (CJWC). shown in Fig. 6 as described below. Eastern ECS 3.2 Seasonal mean current patterns From surface current data collected in the eastern As the surface current is largely affected by seasonal ECS during 1900Ð1992 (GEK data in 1953Ð1992, ship and interannual variations in wind forcing and river dis- drift data in 1900Ð1974, and ADCP data in 1985Ð1992), charge, we should keep in mind that Figs. 7 and 8 may Isobe (2000) estimated the monthly mean surface current represent the combined features of currents that are domi- vector at each 1/3° × 1/3° grid point using a Gaussian nant throughout the year with other currents that are strong filter with an e-folding scale of 20 km (Fig. 9). South of only in one season. A current having the same magnitude 32°N in Fig. 9, the current along 128°E is northward with but opposite direction in different seasons may not ap- a speed of 15 cm secÐ1 during July to October, but turns pear in the annual mean. Therefore, using moored cur- northeastward during January to April. These northward

Currents in the Yellow and East China Seas 85 Fig. 9. The seasonal mean patterns of surface current made by Isobe (2000) based on current data collected during 1900Ð1992 (GEK data in 1953Ð1992, ship-drift data in 1900Ð1974, and ADCP data in 1985Ð1992), together with the 100-m, 200-m, and 1000-m isobaths.

and northeastward currents nearly correspond respectively Kuroshio. Kondo (1985) suggested that this bifurcation with the winter and summer patterns of KBCWK shown of the KBCNT occurs during both winter and summer. in Fig. 6. The difference in the average speed of the Katoh et al. (2000) conducted ADCP measurements along KBCWK shown in Fig. 8 from that in Fig. 9 can be at- two lines in the shelf region northeast of Taiwan in July tributed to the differences between their data source. The 1995 when the Changjiang River discharge was large (Zhu southward current west of the Kyushu coast suggested in et al., 2001). They concluded from their diurnally-aver- Fig. 6 is clear also in the January mean surface current aged current pattern at 20-m depth that the main portion field but not in April, July, and October in Fig. 9. of the KBCNT flows northeastward along the 100-m Southwestern ECS isobath and is clearly separated from the TWWC by a Chao (1990) gave a comprehensive description of region of very weak or southward current. Their result is the current system in the southwestern ECS revealed by consistent with Kondo’s suggestion on the existence of various observational results reported in many papers. summer KBCNT. Chao (1990) and Lin et al. (2001) did not divide the Path of CRDW KBCNT from the TWWC, which may cause some confu- Lin et al. (2001) showed that in the warm half-year, sion in understanding the effect of the Kuroshio on the CRDW flows along two paths towards the southeast and circulation in the Yellow Sea and ECS. The conclusion northeast just off the Changjiang River mouth as sug- of Lin et al. (2001) can be rewritten in that the surface gested by Beardsley et al. (1985), with most of its dis- TWWC is dominant in the warm half-year while the sur- charge flowing directly towards Cheju Island after leav- face KBCNT is dominant in the cold half-year, and that ing the river mouth area. They observed two surface drift- during the cold half-year, a part of the surface KBCNT ers released off the Changjiang River mouth to move flows northward along the 60-m isobath, and another northward to Cheju Island with a mean speed of 21.2 flows eastward along the 90-m isobath to merge with the cm sÐ1 after making many circle-like trajectories.

86 H. Ichikawa and R. C. Beardsley South of Cheju Island cific Ocean. Weekly satellite winds, weekly sea surface The summer existence of a northwestward current temperature, and the climatological monthly mean sea south of Cheju Island is suggested by a short current ar- surface salinity were used in a hindcast simulation for row in Fig. 6. This current can be found only in the Octo- the period September 1991 to December 1998. The model ber current map in Fig. 9. Lin et al. (2001) showed that reproduced well the Kuroshio and the TSWC, KBCWK, the current south of Cheju Island is variable and exhibits KBCNT, and other weak currents shown in Fig. 6. The significant eddy motion. The winter existence of a seasonal variations of the TWWC and YSWC were also northeastward current to the north of Cheju Island is sug- reproduced. At the present time, the results of their model gested by a short current arrow west of Cheju Island in seem to explain most well the observational results and Fig. 6. Lin et al. (2001) confirmed the existence of an suggestions given by Kondo (1985). However, their model annual mean CJWC from moored current-meter measure- does not include the Changjiang River discharge and tidal ments, and concluded that there is no branch current flow- forcing, both of which can strongly influence the surface ing westward into the Yellow Sea from the KBCWK. circulation near the Chinese coast and central ECS shelf Yellow Sea region. Lee and Beardsley (1999) examined the M2 tide An overview of circulation studies in the Yellow Sea and residual current generation in the Yellow Sea using up to the mid-1990s is presented by Naimie et al. (2001). the Blumberg and Mellor (1987) numerical coastal ocean Lin et al. (2001) found that in the warm half-year (MayÐ circulation model. They found that stratified tidal rectifi- October), there is a basin-scale large cyclonic circulation cation intensifies the residual currents at the front and at in the Yellow Sea with several small cyclonic or anticy- the top of the bottom boundary layer over the sloping clonic eddies in its central part and a northward current bottom, and the residual currents in the surface layer reach in its northern part. They could not confirm the existence about 40% of the observed mean currents from satellite- of the southward current west of the Korean coast sug- tracked drifters. gested in Fig. 6. Based on the trajectories of two surface Naimie et al. (2001) computed the three-dimensional drifters and one residual current vector, they suggested climatological circulation in the Bohai, Yellow and East that the YSWC flows northward at 124°E, 34°N, near the China Seas using a nonlinear, tide-resolving, baroclinic western flank of the central trough of Yellow Sea but did numerical coastal ocean circulation model (Dartmouth not identify its origin. They also suggested the existence Circulation Model) for the shelf region shallower than of southward coastal currents on both sides of the Yellow 200 m with data inputs of seasonal hydrography, seasonal Sea in the cold half-year (NovemberÐApril) based on the mean wind and river input, and oceanic tides. The sea- trajectories of two surface drifters and two residual cur- ward boundary condition was given by the sea surface rent vectors as suggested in Fig. 6. elevation obtained from simulations on a larger-scale domain. They concluded that the winter and summer cir- 4. Current Patterns Derived from Numerical Model culations in the Bohai and Yellow Seas are partitioned Studies dynamically among tidal rectification, baroclinic pressure Hsueh et al. (1997) calculated the annual mean cir- gradients, wind response, and river input from the culation in the ECS using a high-resolution (1/6° × 1/6°), Changjiang River. As the Kuroshio main stream and three-dimensional (30 levels in 5000-m deep water), lim- KBCNT are not reproduced in their model results, we can ited-area Kuroshio flow-through model based upon the not use their work to evaluate the role of the Kuroshio in Bryan-Cox code. The model reproduced well not only the driving the observed circulation in the Yellow Sea and Kuroshio main stream but also the cyclonic eddy north ECS. of Taiwan, the KBCNT, and the KBCWK. While Hsueh Park and Oh (2001) constructed a three-dimensional et al. (1997) concluded that the Kuroshio flow-through is numerical model (based on POM) to study the dispersion the dominant driving source of the mean circulation pat- of the CRDW with realistic geometry and bottom topog- tern in the Yellow Sea and ECS, they did not look at raphy. Park and Oh’s model features a 1/6° × 1/6° hori- changes in the circulation associated with the large sea- zontal grid with 12 sigma levels in the vertical, and in- sonal variations in the water property distributions sug- cludes M2 tidal forcing and monthly mean wind forcing gested by Kondo (1985). and discharge of the Changjiang River. Their model re- Guo et al. (2001) developed a fully prognostic one- sults indicate that in summer, the CRDW flowing north- way nested model with horizontal resolution of 1/18° × ward into the Yellow Sea is turned clockwise by the tidal 1/18° and 21 sigma-levels in the vertical. This model is currents and disperses directly northeastward due to the based on the Princeton Ocean Model (POM), and is em- northward winds. In winter, the CRDW is confined to the bedded in a 1/6° × 1/6° resolution model that covers the Chinese coast due to the southward winds. When the northwest Pacific Ocean, which is further embedded in a northward winds are weak or the southward winds are 1/2° × 1/2° resolution model that covers the entire Pa- dominant in summer, the CRDW having moved south-

Currents in the Yellow and East China Seas 87 ward from the Changjiang River mouth by the combina- region. However, we emphasize here that the KBCWK is tion of wind and tidal forcing, is entrained into the not a direct result of the bifurcation of the Kuroshio main Kuroshio, and advected by the KBCWK to Cheju Island stream itself, but represents the separation of a part of and the Tsushima Strait. the current on the left-hand side of the main stream of the Kuroshio. The numerical model results of Guo et al. 5. The Origin of the Tsushima Warm Current (2001) suggest that the vertically-integrated annual mean Against the traditional idea on the origin of the circulation pattern in the ECS shelf region may be inap- TSWC shown in Fig. 2, Beardsley et al. (1985) proposed propriate to examine the origin of the TSWC. When we that the origin of the TSWC is the continuation of the regard the KBCNT as a part of the TWWC, we may be TWWC flowing over the mid-shelf (with water depths of able to conclude both the winter circulation pattern and 50Ð100 m) towards the Tsushima Strait. Based on the the vertically-averaged summer circulation pattern to be volume transport per unit width calculated by vertical only weakly influenced by the Kuroshio main stream in integration of 24-hour current records at 138 locations in the central ECS shelf region as shown by Fang et al. the ECS shelf region, Fang et al. (1991) concluded that (1991). the TWWC is the primary contributor to the formation of The ECS shelf water (ECSSW) is formed to the north the TSWC, since the volume transports of the TWWC of Taiwan by mixing KBCNT water with CRDW in the and TSWC are comparable to each other, and the verti- entire water column in winter and only in the upper layer cally-averaged current in the ECS shelf region flows in summer. Katoh et al. (1996) showed using diurnally- mostly towards the northeast. Fang et al. (1991) suggested averaged currents at 20-m depth along many transects in that the main driving force for this ECS through-flow is the ECS shelf region in summers from 1991 to 1994 that the sea level difference between the northeastern South the ECSSW flows northeastward along the 100-m isobath, China Sea and the Tsushima Strait. the low salinity water originating in the TWWC flows Since this second idea was proposed, there has been over the bottom shallower than 90-m depth, and Kuroshio some debate over which idea is more accurate and what water does not generally intrude over the continental shelf is the structure of the actual current system. Lin et al. near 28°N northwest of Okinawa. They concluded that (2001) concluded from their surface current data that in the summer surface TSWC is formed through the conflu- the cold half-year, the KBCWK is the sole source of the ence of the TWWC, the KBCWK, and the flow transport- TSWC. In the warm half-year, the TSWC has multiple ing the ECSSW. It should be noted that, owing to Zhu et sources such as the KBCWK, the TWWC including the al. (2001), among the years of current measurements con- KBCNT, and the currents transporting CRDW and mixed ducted by Katoh et al., the volume transports of the water in the northern ECS. Guo et al. (2001) found in Changjiang River discharge in 1991Ð1993 were larger their numerical model solutions without tidal forcing and than normal while that in 1994 was smaller. From the Changjiang River discharge that the TSWC is supplied observations of hydrography and water movements in the by three sources: the TWWC, KBCNT, and KBCWK. In ECS, Hsueh (2000) concluded that the surface flow along the upper layer (0Ð50 m), the TWWC prevails over the the ECS shelf margin along which the Kuroshio flows is Kuroshio (KBCNT and KBCWK from the East Taiwan marked by a convergence south of 28°N and a divergence Strait) in summer, but the Kuroshio prevails over the north of 28°N. The surface convergent flow in the south TWWC in winter. In the middle layer (50Ð100 m), the is the KBCNT countercurrent suggested by Stern and TWWC has a small proportion and the KBCNT is the main Austin (1995). By this countercurrent, some ECSSW is source of the TSWC. In the lower layer (100Ð150 m), the transported to the left side of the Kuroshio main stream KBCWK is the main source of the TSWC. and finally into the Tsushima Strait by the KBCWK, the From these results, the difference between these two divergence in the north. ideas about the origin of the TSWC can be said to have The above-mentioned results point to the following come from the lack of observational or numerical model interpretation about the origin of the TSWC. In winter, results on the year-round existence of the KBCWK, the the primary source of the TSWC is the KBCWK trans- seasonal and vertical change of horizontal circulation porting ECSSW formed by mixing of KBCNT water with pattern in the central ECS shelf region, and the confusion CRDW. The TWWC water and much of the ECSSW about the northeastward current in the southwest ECS should flow northeastward along isobaths in the wide shelf shelf region whether it contains only TWWC or both region due to the vertically-uniform density field, and may TWWC and KBCNT. It should be mentioned that Isobe flow not into the Tsushima Strait but into the Yellow Sea. (2000) concluded that drifters flowing northward west of In summer, the sources of the TSWC in the surface 50-m Kyushu do not represent the existence of a stable separa- layer are the CRDW, the ECSSW, and the TWWC water. tion branch crossing the steep shelf edge, since almost In years of large Changjiang River discharge, the CRDW all of the drifters were released over the shallow shelf flows directly to the Tsushima Strait, the TWWC water

88 H. Ichikawa and R. C. Beardsley Fig. 10. Schematics of the surface current pattern in summer when the Changjiang River discharge is large (a) or small-to- medium in magnitude (b). The northeastward current along the 100-m isobath is denoted NEC1, the northeastward current along the 60-m isobath is NEC2. The 500-m isobath is shown also.

flows northeastward over bottom depths less than 90 m, rent, Tsushima Warm Current, Kuroshio Branch Current most of the ECSSW flows northeastward along roughly in the west of Kyushu, Kuroshio Branch Current to the the 100-m isobath, and a part of the ECSSW flows south- North of Taiwan, and other currents in the Yellow and eastward to the left side of the Kuroshio main stream to East China Seas. become the KBCWK. In years of small or medium Owing to these developments, our understanding on Changjiang River discharge, the CRDW flows southward the current system in the Yellow and East China Seas, along the Chinese coast and affects largely the salinity of especially on the origin of the Tsushima Warm Current, ECSSW, the current transporting ECSSW is dominant, has grown significantly. We conclude that the difference but the TWWC is variable or weak in the central ESC between the two schools of thought on the origin of the shelf region. At depths greater than 100 m, only the Tsushima Warm Current has come primarily from the lack ECSSW flows northeastward along the Kuroshio to reach of observational and numerical model results on the year- the Tsushima Strait through the KBCWK. A schematic of round existence of the Kuroshio Branch Current in the this summertime surface current pattern is shown in Fig. west of Kyushu and the Kuroshio Branch Current to the 10. North of Taiwan. In winter, the Tsushima Warm Current has a single source, the Kuroshio Branch Current in the 6. Concluding Remarks west of Kyushu, which transports a mixture of Kuroshio This paper attempts to summarize recent develop- Water and Changjiang River Diluted Water northward. ments in the study of the Yellow and East China Seas In summer, the surface Tsushima Warm Current has mul- surface current system based on new observational and tiple sources, i.e., the Taiwan Warm Current, the Kuroshio numerical model results and the analysis of historical data. Branch Current to the north of Taiwan, and the Kuroshio One of the most important observational findings in this Branch Current in the west of Kyushu. The summer sur- decade is the direct confirmation of the Kuroshio Branch face circulation pattern in the East China Sea shelf re- Current in the west of Kyushu using satellite-tracked sur- gion changes year-to-year corresponding to the face drifters made by Lie et al. (1998b). The recent ap- interannual variations of Changjiang River discharge. plication of regional high-resolution three-dimensional In this last decade, the current system schematics numerical circulation models featuring realistic model proposed by Kondo (1985) were useful to help guide ob- topography and boundary forcing has also provided new servational and numerical model studies to explore the insights about the seasonal changes in the Yellow and East current system and attempt to simulate its features through China Seas circulation since these models can reproduce hindcasting. These recent studies indicate that the sum- seasonal variations in the Kuroshio, Taiwan Warm Cur- mer surface circulation pattern in the East China Sea shelf

Currents in the Yellow and East China Seas 89 region has large variability corresponding to the Kyushu, the separation of the Kuroshio Branch Current interannual variation in Changjiang River discharge. This west of Kyushu from the left side of the Kuroshio main result suggests the necessity for composite analysis for stream, the subsurface cyclonic eddy that accompanies different Changjiang River discharge values. In the com- the slope counter current in the northern Okinawa Trough, ing decade, we suggest that observational studies should and the surface anticyclonic eddy associated with the focus on exploring and documenting the subsurface cur- southward current west of the Kyushu coast—should have rents predicted by the numerical model studies, model a close relationship to each other. However, there does studies to focus on improved model physics (especially not appear to be any generally acceptable theory that ex- in the parameterization of turbulent mixing) and more plains these features. Detailed comparisons of the results realistic forcing in order to provide more realistic of numerical model studies with observational results are simulations of the observed (and yet to be observed) cur- urgent tasks to further understanding of the dynamics as- rents, and theoretical studies to elucidate the dynamics sociated with these features and their interconnections, dominating the main features of the established current and to develop new observational efforts. system. We list next some of the more important ques- 4) The deep circulation in the Okinawa Trough tions that need to be addressed to make the next major The properties of deep water in the Okinawa Trough step in understanding circulation in the Yellow and East differ from those to the east of the Ryukyu Islands. To China Sea. help examine which processes set the deep water proper- 1) The Yellow Sea Warm Current ties and drive the deep circulation in the Okinawa Trough, Although the tongue-like distribution of water prop- direct current measurements of the exchange through the erties in winter in the Yellow Sea suggests strongly the Kerama Gap and the deep flow in the southern section of existence of a coherent Yellow Sea Warm Current flow- the Trough should be made with sufficient duration to ing northwestward from south of Cheju Island and some identify both seasonal and annual mean components. The numerical models produced such a flow, the trajectories relationship of the slope counter current southwest of of surface drifters and moored current-meter data do not Kyushu in the northern Okinawa Trough with that north- show such a Yellow Sea Warm Current. In order to better east of Taiwan in the southern Okinawa Trough also needs investigate and identify those processes that produce the to be determined using both observational and theoreti- observed water property distributions in the Yellow Sea, cal approaches. new observational and model studies are required. One Finally, we want to mention that some of the recent hypothesis is that some combination of wind-forced epi- results cited in this review paper were presented in the sodic northward flow with tidal mixing and rectification 11th Pacific Asian Marginal Seas (PAMS)/Japan and East could create the observed tongue-like distribution. China Seas Study (JECSS) Workshop, held on April 11Ð 2) The Chinese Coastal Current 13, 2001, at Cheju Island, Korea. The PAMS/JECSS Chinese coastal water occupies more than one third Workshops, held once every two years, is one of the best of the entire Yellow and East China Seas area, and domi- international meetings to get information about recent nates the distribution of surface water properties in the developments in the study of the oceanography of the Yellow and East China Seas, yet there are few direct meas- Yellow Sea, the East China Sea, and other Asian mar- urements of the currents in the western Yellow Sea that ginal seas. Readers can get copies of the extended ab- can be used to document and understand the primary cur- stracts and proceeding papers at the web site http:// rent patterns. Due to the intense fishing activities near bada.skku.ac.kr/pams2001, or from Prof. Byung Ho Choi the Chinese coast, we encourage a new effort to use sat- ([email protected]), Chair of the Local Organiz- ellite-tracked surface drifters to study the surface currents ing Committee for the 11th PAMS/JECSS Workshop. along the Chinese coast. This effort could use drifters equipped with Global Positioning System (GPS)-track- Acknowledgements ing so that both tidal and subtidal currents can be mea- This review paper is dedicated to Prof. Kenzo Takano sured accurately, and feature drifter releases made along of the University of Tsukuba and the late Prof. Takashi specific cross-isobath transects sufficient times each year Ichiye of Texas A&M University. These two scientists to resolve seasonal variability. Having simultaneous di- developed and promoted the international Japan and East rect measurements of tidal and subtidal currents will al- China Seas Study (JECSS) Workshops in the 1980s dur- low direct comparisons of both components with results ing a period when the strained political relations between from numerical model simulations that include tidal forc- countries around the Yellow and East China Seas pre- ing and stratified tidal rectification. vented much scientific collaboration. Many of the results 3) The Kuroshio Branch Current west of Kyushu described in this paper might not have come about with- The following features—the eastward path of the out their efforts and long-term support for international Kuroshio from the shelf/slope region southwest of research on the oceanography of the Yellow Sea and East

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92 H. Ichikawa and R. C. Beardsley