Journal of Oceanography, Vol. 58, pp. 77 to 92, 2002
Review
The Current System in the Yellow and East China Seas
HIROSHI ICHIKAWA1* and ROBERT C. B EARDSLEY2
1
2
Faculty of Fisheries, Kagoshima University, Kagoshima 890-0056, Japan
Department 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
Yellow and East China Seas have grown significantly due primarily to new technologies for measuring surface currents and making high-resolution three-dimensional
numerical model calculations. One of the most important new findings in this decade
is direct evidence of the northward current west of Kyushu provided by satellitetracked surface drifters. In the East China Sea shelf region, these recent studies indicate 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 Water 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.
Keywords:
⋅ East China Sea,
⋅ Yellow Sea,
⋅ Kuroshio,
⋅ Tsushima Warm
Current,
⋅ Changjiang River.
main currents, i.e., the Kuroshio, the Tsushima Warm
Current (TSWC), and the Yellow Sea Warm Current
(YSWC) as shown in Fig. 2 (Nitani, 1972). The Kuroshio
enters the East China Sea (ECS) through the strait between Taiwan and Yonakunijima Island, the easternmost
island of the Ryukyu Islands, flows northeastward along
the shelf slope, and exits to the Philippine Sea through
the Tokara Strait after turning eastward near 30°N. The
TSWC flows into the Japan Sea through the Tsushima
Strait, and the YSWC flows into the Yellow Sea from the
south of Cheju Island (“Cheju” is represented by “Jeju”
when one follows the new “Hangeul” Romanization system announced by the Korean government in July, 2000).
(It should be mentioned that hereafter we call the strait
between Taiwan and Yonakunijima Island as the East Taiwan Strait, and that Korean oceanographers call the Japan 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
and YSWC. The origin of the TSWC is considered to be
the Kuroshio Branch Current west of Kyushu (KBCWK),
which flows northward along 128.5°E over the western
1. Introduction
The Yellow and East China Seas are epi-continental
seas bounded by China, Taiwan, the Ryukyu (Nansei) Islands, Kyushu, and the Korean Peninsula (Fig. 1). The
Ryukyu Islands consist of many islands between
Tanegashima Island south of Kyushu and Yonakunijima
Island east of Taiwan. The shelf region shallower than
200 m occupies more than 70% of the entire Yellow and
East China Seas. In the southern and eastern East China
Sea lies the deep Okinawa Trough, in which the maximum water depth decreases from greater than 2000 m in
its southern section to less than 1000 m in its northeastern section. At depths greater than 600 m, the Okinawa
Trough is connected with the Philippine Sea only at the
Kerama Gap southwest of Okinawa Island.
In the 1970s, the surface circulation in the Yellow
and East China Seas was considered to consist of three
* Corresponding author. E-mail: [email protected]
Copyright © The Oceanographic Society of Japan.
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 Island), (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
between 500 and 1000 m. Water deeper than 1000 m is
colored dark blue, with the 2000-m isobath included. Numbers indicate the locations of the following: (1) Taiwan
Strait, (2) Tokara Strait, (3) Tsushima Strait, (4) Kerama
Gap, (5) Taiwan, (6) Yonakunijima Island, (7) Okinawa Island, (8) the Okinawa Trough, and (9) Changjiang River
mouth.
flank of the northeastern Okinawa Trough. In this paper,
the traditionally used name “Tsushima Warm Current”
represents only the current flowing through the Tsushima
Strait, and the northward current west of Kyushu is called
the “Kuroshio Branch Current west of Kyushu” as proposed by Lie et al. (1998a). The origin of the YSWC is
considered to be the current bifurcated from the KBCWK
south of Cheju Island.
As fishing activity in the Yellow Sea and ECS has
been too strong to allow long-term moored current-meter
measurements, the circulation pattern in the Yellow Sea
and ECS has been examined through analysis of
hydrographic and very limited current measurement data.
However, these data used in many previous studies are
78
H. Ichikawa and R. C. Beardsley
rather sporadic and obtained in different years. This has
led to many different interpretations of the circulation in
the Yellow Sea and ECS in the late 1980s. For example,
there are two schools of thought on the origin of the
TSWC. One school says that the TSWC is a continuation
of the KBCWK that has split from the Kuroshio southwest of Kyushu (Kondo, 1985; Lie and Cho, 1994), while
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.,
Beardsley et al., 1985; Fang et al., 1991; Su, 1998; Isobe,
1999).
During the 1990s, our knowledge and understanding of the current system in the Yellow Sea and ECS have
improved significantly due primarily to new technologies
for measuring surface currents and conducting high-resolution three-dimensional numerical circulation model
experiments. The aim of this paper is to summarize recent developments in the study of the current system in
the Yellow Sea and ECS using historical data analysis
and numerical model solutions. Relevant review on the
current pattern in the Yellow and East China Seas up to
the early 1990s can be found in Su (1998). Vertical sections across the Kuroshio in the central ECS and hori-
zontal distributions of water properties near the 50-m
depth layer in the Yellow Sea and ECS will be presented
in Section 2. In Section 3, the main features of surface
circulation pattern derived by analysis of historical data
will be presented, followed by a description of the current pattern found in recent numerical model studies in
Section 4. Combining the results in Sections 2, 3 and 4,
the origin of the TSWC will be discussed in Section 5.
The important but unsolved problems on the current system in the Yellow Sea and ECS will be presented in Section 6.
2.
Seasonal Mean Distributions Derived from
Hydrographic Data
2.1 Seasonal variations of external forcing
Water properties and the general circulation in the
Yellow Sea and ECS are strongly influenced by external
forcing from its surroundings, i.e., the atmosphere, the
land, and the ocean. The wind over the Yellow Sea and
ECS is monsoonal, northwestward in summer and southeastward in winter. From surface weather maps collected
during 1978–1987, Na et al. (1992) calculated monthly
mean wind stress over the Yellow Sea and ECS and found
a maximum southward wind stress of about 1 dyne cm–2
in January over the Yellow Sea and ECS and a maximum
northeastward wind stress of about 0.7 dyne cm–2 in September over the southwestern ECS. From 10-year monthly
mean values of marine meteorological data in 1961–1970
and marine meteorological data reported by ships in 1975–
1977, Ishii and Kondo (1987) calculated the monthly mean
net surface heat flux from the atmosphere to the ocean
over the Yellow Sea and ECS and found a maximum of
140 W m–2 (warming) in June and a minimum of –280
W m –2 (cooling) in December, with an annual mean flux
of about –58 W m–2 (cooling).
The Changjiang River (Yangtze River) supplies about
80% of the total discharge of fresh water from rivers
around the Yellow Sea, ECS and Bohai Sea. Its monthly
mean transport has a large seasonal variation from 0.010
Sv (1 Sv = 106 m3s–1) in January to 0.048 Sv in July around
an annual mean of 0.030 Sv, and large interannual variations in the annual mean from 0.022 to 0.035 Sv during
the 19-year period in 1970 to 1988 (Yanagi, 1994).
The Yellow Sea and ECS has five boundaries with
surrounding seas, i.e., the Taiwan Strait, East Taiwan
Strait, Tokara Strait, Tsushima Strait, and the strait at the
mouth of the Bohai Sea. The Kuroshio transports warm
saline water into the Yellow Sea and ECS through the
East Taiwan Strait. From the sea level difference between
Keelung and Ishigaki during 1989–1996, Lee et al. (2000)
estimated the annual cycle of Kuroshio volume transport
through the East Taiwan Strait to have a maximum of 24
Sv in summer and a minimum of 20 Sv in autumn. From
moored current-meter data, Fang et al. (1991) estimated
the volume transport of TWWC to be 1.0 Sv in winter
and 3.1 Sv in summer. From table 5 in Ichikawa and
Beardsley (1993), the mean Kuroshio volume transport
through the Tokara Strait from February 1987 to September 1988 is estimated to be 24 Sv. Using moored-array
measurements made during 1992–1996, Feng et al. (2000)
estimated the mean total volume transport through the
Tokara Strait to be 23.4 Sv, assuming no vertical velocity
shear from the sea surface to the top measurement depth
level (50-m to 280-m) at each mooring site. From sea level
difference across the Tokara Strait (Naze minus
Nishinoomote) in 1965–1983, Kawabe (1988) indicated
that the Kuroshio volume transport is maximum in summer and minimum in autumn while its annual cycle
changes year to year. Kawabe indicated also that the
Kuroshio velocity in the Tokara Strait has large
interannual variations with dominant periods of longer
than five years and around 2.1 years. Takikawa et al.
(2001) analyzed the ship-mounted Acoustic Doppler Current Profiler (ADCP) data taken six times a week by a
ferryboat crossing the Tsushima Strait from February 1997
to February 2001. They found that the total volume transport through the Tsushima Strait is a minimum in winter
and maximum in autumn, with an annual mean of about
2.7 Sv. The seasonal volume transports through the many
straits between the Ryukyu Islands and through the strait
at the mouth of the Bohai Sea have yet to be estimated.
These seasonal and interannual changes of various
boundary forcing cause large variations in the density
stratification and horizontal circulation pattern in the
Yellow Sea and ECS as described next.
2.2 Vertical sections
Since 1972, the Nagasaki Marine Observatory, the
Japan Meteorological Agency, has been conducting quarterly hydrographic observations along the fixed line in
the central ECS shown in Fig. 2. Using the hydrographic
and current data obtained along this section (hereafter we
call it “section PN” following the originator), studies on
the seasonal variation of water properties and current indicate the following.
Figures 3(a) and (b) show vertical sections of seasonal mean temperature and salinity estimated by
Fujiwara et al. (1987) from hydrographic data obtained
in 1972 to 1981. Seasonal variations in water properties
are only significant in the top 200-m of the water column. Vertical density stratification is dominant over the
shelf region except in winter. In the temperature sections,
warm surface water seems to be spreading shoreward
(northwestward) in all seasons while cold bottom water
seems to be moving seaward (southeastward) over the
shelf region in winter and spring. In the salinity sections
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
(deeper than 50-m depth) near the Changjiang River
mouth while the less saline surface water seems to be
spreading seaward towards the Kuroshio region. It should
be mentioned that in this paper, the practical salinity unit
(psu) is adopted even if the older values of salinity were
determined by titration before the introduction of the
modern salinometer. These differences in these seasonal
mean vertical circulations derived respectively from temperature and salinity sections suggest that we cannot derive the vertical circulation pattern from only
hydrographic data along a fixed section. It will be shown
80
H. Ichikawa and R. C. Beardsley
in the following sections that the main source of summer
saline water in the lower layer is the Kuroshio Branch
Current to the north of Taiwan (KBCNT).
In the “Oceanographic Prompt Report of the Nagasaki Marine Observatory” published just after each cruise
for oceanographic observations, we can see the
geostrophic volume transport of the Kuroshio
(northeastward velocity component) referenced to the
700-dbar level along section PN together with its seasonal mean value. The time series of this relative
geostrophic volume transport has been used as an index
of the long-term variation of the Kuroshio volume trans-
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 Division, Nagasaki Marine Observatory, Japan Meteorological Agency).
From hydrographic and geomagnetic electrokinetograph (GEK) data collected during 1972–1986, 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 500–
1000 m. Hereafter, we call this region where the surface
current is larger than 50 cm sec–1 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 lefthand side of the Kuroshio main stream, the surface current flows northeastward with speeds smaller than 40
cm sec –1 in spring and summer, and smaller than 20
cm sec–1 in autumn and winter. The seasonal mean volume transport of the Kuroshio including the Kuroshio
main stream and surrounding weak northeastward current
is estimated to have a maximum of 25.4 Sv in summer
and a minimum of 16.2 Sv in autumn, with an annual mean
of 21.2 Sv. From hydrographic and GEK data collected
during 1986–1988, Ichikawa and Beardsley (1993)
showed the Kuroshio volume transports in each transect
to increase with increasing downstream (northeastward
along-isobath) component of local wind stress.
In the bottom layer over the upper slope in Fig. 4,
there is a southwestward current year-round. We call this
current the “slope counter-current” as proposed by Lie et
al. (1998a). Chen et al. (1992) detected a deep counter
(i.e., southwestward) current over the upper slope near
the section PN in a 1986 hydrographic survey. They attributed this slope counter-current to be the result of an
eddy in the left side of the Kuroshio, as shown by satellite imagery. It should be mentioned that the existence of
the slope counter-current is confirmed not only in the
central ECS but also southwest of Kyushu (Lie et al.,
1998a; Nakamura et al., 1999) and northeast of Taiwan
(Chuang and Wu, 1991; Hsueh et al., 1993). Lie et al.
(1998a) suggested that the slope counter-current seems
to be a quasi-permanent feature in the southwest of
Kyushu, but it is formed intermittently with a period of
3–10 days in the central ECS. The driving mechanisms
of these counter-currents have not been clarified yet, but
eddy forcing is one clear possibility as suggested by Chen
et al. (1992).
Fig. 4. Vertical sections of seasonal mean geostrophic current
normal to section PN referred to GEK velocity (Yamashiro
et al., 1990). Shaded area indicates southwestward current.
Stations 3, 5, 7, and 9 in this figure correspond respectively
to stations 2, 3, 4, and 5 in Figs. 3(a) and (b).
Surface southwestward current west of the Ryukyu
Islands reaches to the bottom layer except in summer.
Deep southwestward current exists throughout the year,
but it is a little stronger in autumn and winter than other
seasons. These results suggest that vertical sections of
water properties and current in the central ECS have large
seasonal variations.
Here, we should note that the distributions shown in
Figs. 3 and 4 tend to be very smooth, since they are spatial and temporal averages over large interannual variations and/or short-period fluctuations, especially in frontal regions where the front changes its position significantly over time scales of less than one month (e.g., James
et al., 1999). By integrating the geostrophic volume transport referenced with GEK or ship-mounted ADCP velocity in each transect during 1981–1992, Ichikawa and
Chaen (2000) found that the seasonal mean volume transport of northeastward current through section PN is a
maximum of 32.1 Sv in summer and a minimum of 20.0
Sv in winter, around an annual mean of 27.6 Sv, and these
seasonal variations are dominated by seasonal differences
in the volume transport of Kuroshio Surface Water.
Currents in the Yellow and East China Seas
81
2.3 Horizontal distributions of water properties
In order to avoid the strong influences of large variability in atmospheric forcing and river discharge on the
distributions of sea surface temperature and salinity,
Kondo (1985) calculated the seasonal mean temperature
and salinity not at the surface but at 50-m depth (or at the
bottom in water depths shallower than 50 m) in the Yellow Sea and ECS, using hydrographic data taken from
1953 to 1970 (Fig. 5). Kondo concluded as follows.
1) The distribution of water masses at 50-m depth is
strongly affected by the warm saline water transported by the Kuroshio and the cold, less saline
water transported by the Chinese Coastal Current.
2) The 50-m distribution does not change much over
the year except in the central ECS shelf region.
3) Cold water less than 10°C, called the Yellow Sea
Central Cold Water by Uda (1934), exists from
spring to autumn in the middle and bottom layers
of the Yellow Sea. It is the remains of cold water
produced in winter by surface cooling and mixing.
Kondo suggested that the seasonal variations of water
properties in the central ECS shelf region are caused by
the seasonal change in the TWWC.
Here it should be mentioned that the artificial
smoothing of spatial distributions (due to averaging
interannual variations and/or short-period fluctuations)
must also affect Fig. 5 to some degree. The widths of the
temperature fronts shown in Fig. 5 are much larger than
those in individual satellite images.
2.4 Circulation pattern derived from hydrographic data
From the water property distributions shown in Fig.
5, Kondo (1985) constructed the following schematics of
the summer and winter circulation patterns near 50-m
depth in the Yellow Sea and ECS (Fig. 6). Kondo showed
that the same dominant currents—the Kuroshio, the
KBCWK, and the TSWC—as in Fig. 2 exist in both winter and summer, while the YSWC exists only in winter. It
should be noticed in Fig. 6 that the KBCWK does not
bifurcate from the Kuroshio, but begins from the left of
the Kuroshio. In Fig. 6, the TWWC and the cyclonic eddy
north of Taiwan exist only in summer. Kondo showed also
in Fig. 6 the year-round existence of the KBCNT, the
southward current transporting Changjiang River Diluted
Water (CRDW, the mixture of Changjiang River freshwater with saline shelf water that leaves the river mouth
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
monsoon (Beardsley et al., 1985).
Stern and Austin (1995) suggested theoretically that,
while the main branch of the Kuroshio follows the local
isobaths when it comes to the steep continental shelf slope
at the northeast corner of Taiwan, the inertial of a small
inshore fraction of the oncoming Kuroshio causes it to
cross the slope, creating an inshore branch. This bifurcated current entering the shelf displaces ambient water
of relatively high potential vorticity as a countercurrent,
which flows seawards across the slope.
Ichikawa et al. (2001) examined correlations of the
vertically-averaged salinity in the top 30-m layer in autumn at each of eight stations along section PN (Fig. 2)
with the annual mean discharge of the Changjiang River
during 1972 to 1988 reported by Yanagi (1994). They
found a significant negative correlation at two stations,
one at 280-m water depth in the left-hand edge of the
Kuroshio and another at 100-m water depth in the midshelf region, with lower mean salinities for larger river
discharge during years of smaller annual mean river discharge than 0.030 Sv. On the other hand, the correlations
are not significant at all stations during years with larger
river discharge greater than 0.030 Sv. These results suggest that the path of CRDW has large interannual variations depending on the Changjiang River discharge.
The seaward countercurrent of KBCNT suggested by
Stern and Austin (1995) may explain the negative corre-
broad Chinese Coastal Current flowing southward along
the Chinese coast in the Yellow Sea, the southward current west of the Korean coast, the southward current west
of the Kyushu coast, and the southwestward current west
of the Ryukyu Islands.
It should be noted that Kondo (1985) did not adopt
one unique mechanism but two alternative mechanisms
to derive each of the currents shown in Fig. 6, i.e., he
showed one current crossing isotherms or isohalines assuming that the effect of diffusion or mixing is more dominant than advection, and another current flowing along
isotherms or isohalines assuming that the effect of
advection by a geostrophic current is more dominant than
diffusion or mixing. As there is no evidence for adopting
either of these two alternative mechanisms, we can conclude that while Fig. 5 may represent well the general
features of the distribution pattern of water masses in the
Yellow Sea and ECS, the seasonal circulation patterns
shown in Fig. 6 may be misleading in some regions.
While Kondo suggested the possibility that the summer southward spreading of less saline CRDW in the surface layer may be compensated by northward intrusion
of more saline Kuroshio water in the lower layer, it is
generally accepted at present that the CRDW has a bimodal pattern of a southward coastal jet and a northeastward
spreading during the summer monsoon, and is transported
to the south along the Chinese coast during the winter
(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
the Kuroshio and Changjiang River discharge, i.e., part
of the KBCNT transporting a mixture of Kuroshio Water
and CRDW may flow seaward over the shelf as a countercurrent and may be entrained into the left-hand edge
of the Kuroshio main stream. The insignificant correlation at all stations during years with larger river discharge
may indicate that the CRDW disperses intermittently in
the shelf region during these years.
3.
Circulation Pattern Derived from Historical Current Data
3.1 Annual mean current pattern
Since currents with periods from 12 hours to several
days caused by tide and wind are very large in the Yellow
Sea and ECS, it is necessary to average current data over
a long period to remove these higher frequency components from the weaker low frequency current system in
the Yellow Sea and ECS. Qiu and Imasato (1990) constructed a map of the annual mean surface current in the
ECS by 1/5° × 1/5° areal averaging of current data measured by GEK from 1953 to 1984 (Fig. 7). Lie et al.
(1998a, b) derived another annual mean surface current
by 1/3° × 1/3° areal averaging of current data estimated
from trajectories of surface drifters from 1989 to 1996
(Fig. 8). Note that Figs. 7 and 8 represent the annual mean
84
H. Ichikawa and R. C. Beardsley
circulation pattern just in the surface layer while Fig. 6 is
the seasonal circulation pattern near 50-m depth.
The KBCWK is not significant in Fig. 7 but is in
Fig. 8. The discrepancy between the mean current fields
west of Kyushu shown in Figs. 7 and 8 can be attributed
to the different features of the Eulerian and Lagrangian
current measurements (Uchida et al., 1998). The northward surface current with speeds of 30 cm sec –1 originates from near 30°N, 127.5°E in the shelf region southwest of Kyushu in Fig. 8. This is direct evidence of the
existence of the KBCWK suggested by Kondo (1985). It
should be noted that the KBCWK does not flow over the
western flank of the northern Okinawa Trough shown in
Fig. 2, but over the shelf region with water depths of 100–
200 m in Fig. 8. This indicates that it is not the Kuroshio
main stream flowing along the 200–1000 m isobath but a
part of the current on the left-hand side of the Kuroshio
main stream that separates to become the KBCWK.
The southward current west of the Kyushu coast suggested in Fig. 6 can be seen in both Figs. 7 and 8. Hsueh
et al. (1996) examined theoretically the driving mechanism of the KBCWK together with the southward current
west of the Kyushu coast, and concluded that the KBCWK
may, indeed, be a byproduct of the turning of the
Kuroshio, forced by the shoaling topography of the continental shelf southwest of Kyushu.
According to Kondo (1985), the KBCNT exists
Fig. 7. The annual mean pattern of surface current derived from
GEK data from 1953 to 1984 (Qiu and Imasato, 1990).
throughout the year but the TWWC is dominant only in
spring and summer, and a part of the KBCNT flows northward and another flows eastward during both winter and
summer. The KBCNT and southeastward current can be
seen in Figs. 7 and 8 as an intrusion of the inflow Kuroshio
and a wide weak seaward current over the continental
shelf northeast of Taiwan. The TWWC can be identified
in Fig. 8 but not in Fig. 7 due to lack of data used in
deriving Fig. 7.
The northwestward current shown in Fig. 8 to the
south of Cheju Island does not appear in Fig. 7. As no
surface current has been observed to flow into the Yellow Sea from south of Cheju Island but a current flowing
around Cheju Island is dominant, Lie et al. (2000) proposed to name the mean surface current turning clockwise around Cheju Island in the northern ESC the Cheju
Warm Current (CJWC).
3.2 Seasonal mean current patterns
As the surface current is largely affected by seasonal
and interannual variations in wind forcing and river discharge, we should keep in mind that Figs. 7 and 8 may
represent the combined features of currents that are dominant throughout the year with other currents that are strong
only in one season. A current having the same magnitude
but opposite direction in different seasons may not appear in the annual mean. Therefore, using moored cur-
Fig. 8. The annual mean pattern of surface current derived from
trajectories of surface drifters from 1989 to 1996 (Lie et
al., 1998a, b), together with the 100-, 200-, 500- and 1000m isobaths.
rent-meter measurements made for periods longer than
several days, 25-hour current measurements made from
anchored ships, and surface current measurements obtained with satellite-tracked surface drifters between 1970
and 2000, Lin et al. (2001) examined the surface circulation patterns in the warm half-year (May–October) and
the cold half-year (November–April). Although their data
are rather sporadic, one of their most important conclusions is that the surface currents south of Cheju Island,
north of Taiwan, in the central Yellow Sea, and over the
shelf area west of where the Kuroshio turns eastward towards the Tokara Strait are variable and produce eddies.
These variable current regions are the key areas for understanding the water mass distributions shown in Fig. 5
and the driving mechanisms for the circulation pattern
shown in Fig. 6 as described below.
Eastern ECS
From surface current data collected in the eastern
ECS during 1900–1992 (GEK data in 1953–1992, ship
drift data in 1900–1974, and ADCP data in 1985–1992),
Isobe (2000) estimated the monthly mean surface current
vector at each 1/3° × 1/3° grid point using a Gaussian
filter with an e-folding scale of 20 km (Fig. 9). South of
32°N in Fig. 9, the current along 128°E is northward with
a speed of 15 cm sec–1 during July to October, but turns
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 1900–1992
(GEK data in 1953–1992, ship-drift data in 1900–1974, and ADCP data in 1985–1992), together with the 100-m, 200-m, and
1000-m isobaths.
and northeastward currents nearly correspond respectively
with the winter and summer patterns of KBCWK shown
in Fig. 6. The difference in the average speed of the
KBCWK shown in Fig. 8 from that in Fig. 9 can be attributed to the differences between their data source. The
southward current west of the Kyushu coast suggested in
Fig. 6 is clear also in the January mean surface current
field but not in April, July, and October in Fig. 9.
Southwestern ECS
Chao (1990) gave a comprehensive description of
the current system in the southwestern ECS revealed by
various observational results reported in many papers.
Chao (1990) and Lin et al. (2001) did not divide the
KBCNT from the TWWC, which may cause some confusion in understanding the effect of the Kuroshio on the
circulation in the Yellow Sea and ECS. The conclusion
of Lin et al. (2001) can be rewritten in that the surface
TWWC is dominant in the warm half-year while the surface KBCNT is dominant in the cold half-year, and that
during the cold half-year, a part of the surface KBCNT
flows northward along the 60-m isobath, and another
flows eastward along the 90-m isobath to merge with the
86
H. Ichikawa and R. C. Beardsley
Kuroshio. Kondo (1985) suggested that this bifurcation
of the KBCNT occurs during both winter and summer.
Katoh et al. (2000) conducted ADCP measurements along
two lines in the shelf region northeast of Taiwan in July
1995 when the Changjiang River discharge was large (Zhu
et al., 2001). They concluded from their diurnally-averaged current pattern at 20-m depth that the main portion
of the KBCNT flows northeastward along the 100-m
isobath and is clearly separated from the TWWC by a
region of very weak or southward current. Their result is
consistent with Kondo’s suggestion on the existence of
summer KBCNT.
Path of CRDW
Lin et al. (2001) showed that in the warm half-year,
CRDW flows along two paths towards the southeast and
northeast just off the Changjiang River mouth as suggested by Beardsley et al. (1985), with most of its discharge flowing directly towards Cheju Island after leaving the river mouth area. They observed two surface drifters released off the Changjiang River mouth to move
northward to Cheju Island with a mean speed of 21.2
cm s –1 after making many circle-like trajectories.
South of Cheju Island
The summer existence of a northwestward current
south of Cheju Island is suggested by a short current arrow in Fig. 6. This current can be found only in the October current map in Fig. 9. Lin et al. (2001) showed that
the current south of Cheju Island is variable and exhibits
significant eddy motion. The winter existence of a
northeastward current to the north of Cheju Island is suggested by a short current arrow west of Cheju Island in
Fig. 6. Lin et al. (2001) confirmed the existence of an
annual mean CJWC from moored current-meter measurements, and concluded that there is no branch current flowing westward into the Yellow Sea from the KBCWK.
Yellow Sea
An overview of circulation studies in the Yellow Sea
up to the mid-1990s is presented by Naimie et al. (2001).
Lin et al. (2001) found that in the warm half-year (May–
October), there is a basin-scale large cyclonic circulation
in the Yellow Sea with several small cyclonic or anticyclonic eddies in its central part and a northward current
in its northern part. They could not confirm the existence
of the southward current west of the Korean coast suggested in Fig. 6. Based on the trajectories of two surface
drifters and one residual current vector, they suggested
that the YSWC flows northward at 124°E, 34°N, near the
western flank of the central trough of Yellow Sea but did
not identify its origin. They also suggested the existence
of southward coastal currents on both sides of the Yellow
Sea in the cold half-year (November–April) based on the
trajectories of two surface drifters and two residual current vectors as suggested in Fig. 6.
4.
Current Patterns Derived from Numerical Model
Studies
Hsueh et al. (1997) calculated the annual mean circulation in the ECS using a high-resolution (1/6° × 1/6°),
three-dimensional (30 levels in 5000-m deep water), limited-area Kuroshio flow-through model based upon the
Bryan-Cox code. The model reproduced well not only the
Kuroshio main stream but also the cyclonic eddy north
of Taiwan, the KBCNT, and the KBCWK. While Hsueh
et al. (1997) concluded that the Kuroshio flow-through is
the dominant driving source of the mean circulation pattern in the Yellow Sea and ECS, they did not look at
changes in the circulation associated with the large seasonal variations in the water property distributions suggested by Kondo (1985).
Guo et al. (2001) developed a fully prognostic oneway nested model with horizontal resolution of 1/18° ×
1/18° and 21 sigma-levels in the vertical. This model is
based on the Princeton Ocean Model (POM), and is embedded in a 1/6° × 1/6° resolution model that covers the
northwest Pacific Ocean, which is further embedded in a
1/2° × 1/2° resolution model that covers the entire Pa-
cific Ocean. Weekly satellite winds, weekly sea surface
temperature, and the climatological monthly mean sea
surface salinity were used in a hindcast simulation for
the period September 1991 to December 1998. The model
reproduced well the Kuroshio and the TSWC, KBCWK,
KBCNT, and other weak currents shown in Fig. 6. The
seasonal variations of the TWWC and YSWC were also
reproduced. At the present time, the results of their model
seem to explain most well the observational results and
suggestions given by Kondo (1985). However, their model
does not include the Changjiang River discharge and tidal
forcing, both of which can strongly influence the surface
circulation near the Chinese coast and central ECS shelf
region. Lee and Beardsley (1999) examined the M2 tide
and residual current generation in the Yellow Sea using
the Blumberg and Mellor (1987) numerical coastal ocean
circulation model. They found that stratified tidal rectification intensifies the residual currents at the front and at
the top of the bottom boundary layer over the sloping
bottom, and the residual currents in the surface layer reach
about 40% of the observed mean currents from satellitetracked drifters.
Naimie et al. (2001) computed the three-dimensional
climatological circulation in the Bohai, Yellow and East
China Seas using a nonlinear, tide-resolving, baroclinic
numerical coastal ocean circulation model (Dartmouth
Circulation Model) for the shelf region shallower than
200 m with data inputs of seasonal hydrography, seasonal
mean wind and river input, and oceanic tides. The seaward boundary condition was given by the sea surface
elevation obtained from simulations on a larger-scale
domain. They concluded that the winter and summer circulations in the Bohai and Yellow Seas are partitioned
dynamically among tidal rectification, baroclinic pressure
gradients, wind response, and river input from the
Changjiang River. As the Kuroshio main stream and
KBCNT are not reproduced in their model results, we can
not use their work to evaluate the role of the Kuroshio in
driving the observed circulation in the Yellow Sea and
ECS.
Park and Oh (2001) constructed a three-dimensional
numerical model (based on POM) to study the dispersion
of the CRDW with realistic geometry and bottom topography. Park and Oh’s model features a 1/6° × 1/6° horizontal grid with 12 sigma levels in the vertical, and includes M2 tidal forcing and monthly mean wind forcing
and discharge of the Changjiang River. Their model results indicate that in summer, the CRDW flowing northward into the Yellow Sea is turned clockwise by the tidal
currents and disperses directly northeastward due to the
northward winds. In winter, the CRDW is confined to the
Chinese coast due to the southward winds. When the
northward winds are weak or the southward winds are
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 combination of wind and tidal forcing, is entrained into the
Kuroshio, and advected by the KBCWK to Cheju Island
and the Tsushima Strait.
5. The Origin of the Tsushima Warm Current
Against the traditional idea on the origin of the
TSWC shown in Fig. 2, Beardsley et al. (1985) proposed
that the origin of the TSWC is the continuation of the
TWWC flowing over the mid-shelf (with water depths of
50–100 m) towards the Tsushima Strait. Based on the
volume transport per unit width calculated by vertical
integration of 24-hour current records at 138 locations in
the ECS shelf region, Fang et al. (1991) concluded that
the TWWC is the primary contributor to the formation of
the TSWC, since the volume transports of the TWWC
and TSWC are comparable to each other, and the vertically-averaged current in the ECS shelf region flows
mostly towards the northeast. Fang et al. (1991) suggested
that the main driving force for this ECS through-flow is
the sea level difference between the northeastern South
China Sea and the Tsushima Strait.
Since this second idea was proposed, there has been
some debate over which idea is more accurate and what
is the structure of the actual current system. Lin et al.
(2001) concluded from their surface current data that in
the cold half-year, the KBCWK is the sole source of the
TSWC. In the warm half-year, the TSWC has multiple
sources such as the KBCWK, the TWWC including the
KBCNT, and the currents transporting CRDW and mixed
water in the northern ECS. Guo et al. (2001) found in
their numerical model solutions without tidal forcing and
Changjiang River discharge that the TSWC is supplied
by three sources: the TWWC, KBCNT, and KBCWK. In
the upper layer (0–50 m), the TWWC prevails over the
Kuroshio (KBCNT and KBCWK from the East Taiwan
Strait) in summer, but the Kuroshio prevails over the
TWWC in winter. In the middle layer (50–100 m), the
TWWC has a small proportion and the KBCNT is the main
source of the TSWC. In the lower layer (100–150 m), the
KBCWK is the main source of the TSWC.
From these results, the difference between these two
ideas about the origin of the TSWC can be said to have
come from the lack of observational or numerical model
results on the year-round existence of the KBCWK, the
seasonal and vertical change of horizontal circulation
pattern in the central ECS shelf region, and the confusion
about the northeastward current in the southwest ECS
shelf region whether it contains only TWWC or both
TWWC and KBCNT. It should be mentioned that Isobe
(2000) concluded that drifters flowing northward west of
Kyushu do not represent the existence of a stable separation branch crossing the steep shelf edge, since almost
all of the drifters were released over the shallow shelf
88
H. Ichikawa and R. C. Beardsley
region. However, we emphasize here that the KBCWK is
not a direct result of the bifurcation of the Kuroshio main
stream itself, but represents the separation of a part of
the current on the left-hand side of the main stream of the
Kuroshio. The numerical model results of Guo et al.
(2001) suggest that the vertically-integrated annual mean
circulation pattern in the ECS shelf region may be inappropriate to examine the origin of the TSWC. When we
regard the KBCNT as a part of the TWWC, we may be
able to conclude both the winter circulation pattern and
the vertically-averaged summer circulation pattern to be
only weakly influenced by the Kuroshio main stream in
the central ECS shelf region as shown by Fang et al.
(1991).
The ECS shelf water (ECSSW) is formed to the north
of Taiwan by mixing KBCNT water with CRDW in the
entire water column in winter and only in the upper layer
in summer. Katoh et al. (1996) showed using diurnallyaveraged currents at 20-m depth along many transects in
the ECS shelf region in summers from 1991 to 1994 that
the ECSSW flows northeastward along the 100-m isobath,
the low salinity water originating in the TWWC flows
over the bottom shallower than 90-m depth, and Kuroshio
water does not generally intrude over the continental shelf
near 28°N northwest of Okinawa. They concluded that
the summer surface TSWC is formed through the confluence of the TWWC, the KBCWK, and the flow transporting the ECSSW. It should be noted that, owing to Zhu et
al. (2001), among the years of current measurements conducted by Katoh et al., the volume transports of the
Changjiang River discharge in 1991–1993 were larger
than normal while that in 1994 was smaller. From the
observations of hydrography and water movements in the
ECS, Hsueh (2000) concluded that the surface flow along
the ECS shelf margin along which the Kuroshio flows is
marked by a convergence south of 28°N and a divergence
north of 28°N. The surface convergent flow in the south
is the KBCNT countercurrent suggested by Stern and
Austin (1995). By this countercurrent, some ECSSW is
transported to the left side of the Kuroshio main stream
and finally into the Tsushima Strait by the KBCWK, the
divergence in the north.
The above-mentioned results point to the following
interpretation about the origin of the TSWC. In winter,
the primary source of the TSWC is the KBCWK transporting ECSSW formed by mixing of KBCNT water with
CRDW. The TWWC water and much of the ECSSW
should flow northeastward along isobaths in the wide shelf
region due to the vertically-uniform density field, and may
flow not into the Tsushima Strait but into the Yellow Sea.
In summer, the sources of the TSWC in the surface 50-m
layer are the CRDW, the ECSSW, and the TWWC water.
In years of large Changjiang River discharge, the CRDW
flows directly to the Tsushima Strait, the TWWC water
Fig. 10. Schematics of the surface current pattern in summer when the Changjiang River discharge is large (a) or small-tomedium 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,
most of the ECSSW flows northeastward along roughly
the 100-m isobath, and a part of the ECSSW flows southeastward to the left side of the Kuroshio main stream to
become the KBCWK. In years of small or medium
Changjiang River discharge, the CRDW flows southward
along the Chinese coast and affects largely the salinity of
ECSSW, the current transporting ECSSW is dominant,
but the TWWC is variable or weak in the central ESC
shelf region. At depths greater than 100 m, only the
ECSSW flows northeastward along the Kuroshio to reach
the Tsushima Strait through the KBCWK. A schematic of
this summertime surface current pattern is shown in Fig.
10.
6. Concluding Remarks
This paper attempts to summarize recent developments in the study of the Yellow and East China Seas
surface current system based on new observational and
numerical model results and the analysis of historical data.
One of the most important observational findings in this
decade is the direct confirmation of the Kuroshio Branch
Current in the west of Kyushu using satellite-tracked surface drifters made by Lie et al. (1998b). The recent application of regional high-resolution three-dimensional
numerical circulation models featuring realistic model
topography and boundary forcing has also provided new
insights about the seasonal changes in the Yellow and East
China Seas circulation since these models can reproduce
seasonal variations in the Kuroshio, Taiwan Warm Cur-
rent, Tsushima Warm Current, Kuroshio Branch Current
in the west of Kyushu, Kuroshio Branch Current to the
North of Taiwan, and other currents in the Yellow and
East China Seas.
Owing to these developments, our understanding on
the current system in the Yellow and East China Seas,
especially on the origin of the Tsushima Warm Current,
has grown significantly. We conclude that the difference
between the two schools of thought on the origin of the
Tsushima Warm Current has come primarily from the lack
of observational and numerical model results on the yearround existence of the Kuroshio Branch Current in the
west of Kyushu and the Kuroshio Branch Current to the
North of Taiwan. 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
Water 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 in the west of Kyushu. The summer surface circulation pattern in the East China Sea shelf region changes year-to-year corresponding to the
interannual variations of Changjiang River discharge.
In this last decade, the current system schematics
proposed by Kondo (1985) were useful to help guide observational and numerical model studies to explore the
current system and attempt to simulate its features through
hindcasting. These recent studies indicate that the summer 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
interannual variation in Changjiang River discharge. This
result suggests the necessity for composite analysis for
different Changjiang River discharge values. In the coming decade, we suggest that observational studies should
focus on exploring and documenting the subsurface currents predicted by the numerical model studies, model
studies to focus on improved model physics (especially
in the parameterization of turbulent mixing) and more
realistic forcing in order to provide more realistic
simulations of the observed (and yet to be observed) currents, and theoretical studies to elucidate the dynamics
dominating the main features of the established current
system. We list next some of the more important questions that need to be addressed to make the next major
step in understanding circulation in the Yellow and East
China Sea.
1) The Yellow Sea Warm Current
Although the tongue-like distribution of water properties in winter in the Yellow Sea suggests strongly the
existence of a coherent Yellow Sea Warm Current flowing northwestward from south of Cheju Island and some
numerical models produced such a flow, the trajectories
of surface drifters and moored current-meter data do not
show such a Yellow Sea Warm Current. In order to better
investigate and identify those processes that produce the
observed water property distributions in the Yellow Sea,
new observational and model studies are required. One
hypothesis is that some combination of wind-forced episodic northward flow with tidal mixing and rectification
could create the observed tongue-like distribution.
2) The Chinese Coastal Current
Chinese coastal water occupies more than one third
of the entire Yellow and East China Seas area, and dominates the distribution of surface water properties in the
Yellow and East China Seas, yet there are few direct measurements of the currents in the western Yellow Sea that
can be used to document and understand the primary current patterns. Due to the intense fishing activities near
the Chinese coast, we encourage a new effort to use satellite-tracked surface drifters to study the surface currents
along the Chinese coast. This effort could use drifters
equipped with Global Positioning System (GPS)-tracking so that both tidal and subtidal currents can be measured accurately, and feature drifter releases made along
specific cross-isobath transects sufficient times each year
to resolve seasonal variability. Having simultaneous direct measurements of tidal and subtidal currents will allow direct comparisons of both components with results
from numerical model simulations that include tidal forcing and stratified tidal rectification.
3) The Kuroshio Branch Current west of Kyushu
The following features—the eastward path of the
Kuroshio from the shelf/slope region southwest of
90
H. Ichikawa and R. C. Beardsley
Kyushu, the separation of the Kuroshio Branch Current
west of Kyushu from the left side of the Kuroshio main
stream, the subsurface cyclonic eddy that accompanies
the slope counter current in the northern Okinawa Trough,
and the surface anticyclonic eddy associated with the
southward current west of the Kyushu coast—should have
a close relationship to each other. However, there does
not appear to be any generally acceptable theory that explains these features. Detailed comparisons of the results
of numerical model studies with observational results are
urgent tasks to further understanding of the dynamics associated with these features and their interconnections,
and to develop new observational efforts.
4) The deep circulation in the Okinawa Trough
The properties of deep water in the Okinawa Trough
differ from those to the east of the Ryukyu Islands. To
help examine which processes set the deep water properties and drive the deep circulation in the Okinawa Trough,
direct current measurements of the exchange through the
Kerama Gap and the deep flow in the southern section of
the Trough should be made with sufficient duration to
identify both seasonal and annual mean components. The
relationship of the slope counter current southwest of
Kyushu in the northern Okinawa Trough with that northeast of Taiwan in the southern Okinawa Trough also needs
to be determined using both observational and theoretical approaches.
Finally, we want to mention that some of the recent
results cited in this review paper were presented in the
11th Pacific Asian Marginal Seas (PAMS)/Japan and East
China Seas Study (JECSS) Workshop, held on April 11–
13, 2001, at Cheju Island, Korea. The PAMS/JECSS
Workshops, held once every two years, is one of the best
international meetings to get information about recent
developments in the study of the oceanography of the
Yellow Sea, the East China Sea, and other Asian marginal seas. Readers can get copies of the extended abstracts and proceeding papers at the web site http://
bada.skku.ac.kr/pams2001, or from Prof. Byung Ho Choi
([email protected]), Chair of the Local Organizing Committee for the 11th PAMS/JECSS Workshop.
Acknowledgements
This review paper is dedicated to Prof. Kenzo Takano
of the University of Tsukuba and the late Prof. Takashi
Ichiye of Texas A&M University. These two scientists
developed and promoted the international Japan and East
China Seas Study (JECSS) Workshops in the 1980s during a period when the strained political relations between
countries around the Yellow and East China Seas prevented much scientific collaboration. Many of the results
described in this paper might not have come about without their efforts and long-term support for international
research on the oceanography of the Yellow Sea and East
China Sea. Support for HI for preparation of this paper
was provided in part by the Grant in Aid (No. 11205203)
for Scientific Research of Priority Area (B): Physical,
Chemical and Biological Studies on Monitoring of Marginal Seas for Ocean Forecasting, sponsored by Ministry
of Education, Culture, Sports, Science and Technology
(MEXT), Japan, and by the research fund for CREST
(Core Research for Evolutional Science and Technology):
the Experimental Study on the Variability and Predictability of the Kuroshio, sponsored by the Japan Science
and Technology Corporation (JST). Support for RB was
provided by the Office of Naval Research through Grant
N00014-98-1-0345.
CJWC
CRDW
ECS
ECSSW
KBCNT
KBCWK
TSWC
TWWC
YSWC
Abbreviations
Cheju Warm Current
Changjiang River Diluted Water
East China Sea
East China Sea Shelf Water
Kuroshio Branch Current to the North of Taiwan
Kuroshio Branch Current West of Kyushu
Tsushima Warm Current
Taiwan Warm Current
Yellow Sea Warm Current
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