Journal of Oceanography, Vol. 55, pp. 185 to 195. 1999
The Taiwan-Tsushima Warm Current System: Its Path and
the Transformation of the Water Mass in the East China Sea
ATSUHIKO ISOBE
Department of Earth System Science and Technology, Interdisciplinary Graduate School of
Engineering Sciences, Kyushu University, 6-1 Kasuga-Koen, Kasuga 816-8580, Japan
(Received 30 September 1998; in revised form 29 October 1998; accepted 13 November 1998)
Using a temperature data set from 1961 to 1990, we estimated the monthly distribution
of the vertically integrated heat content in the East China Sea. We then drew the monthly
map of the horizontal heat transport, which is obtained as the difference between the
vertically integrated heat content and the surface heat flux. We anticipate that its
distribution pattern is determined mainly due to the advection by the ocean current if it
exists stably in the East China Sea. The monthly map of the horizontal heat transport
showed the existence of the Taiwan-Tsushima Warm Current System (TTWCS) at least
from April to August. The T-S (temperature-salinity) analysis along the path of TTWCS
indicated that the TTWCS changes its T-S property as it flows in the East China Sea
forming the Tsushima Warm Current water. The end members of the Tsushima Warm
Current water detected in this study are water masses in the Taiwan Strait and the
Kuroshio surface layer, the fresh water from the mainland of China, and the southern tip
of the Yellow Sea Cold Water extending in the northern part of the East China Sea.
tracked drifters with the hydrographic data (Lie and Cho,
1994), and several infrared images (Huh, 1982). Therefore,
we need to confirm the origin of the Tsushima Warm
Current using a long-term observational data set. In this
study, we try to reconfirm the existence of the TTWCS using
a different procedure from that used in Isobe (1999).
The water temperature is the most abundant data set in
the ocean, so we use temperature data to detect the path of
the ocean current in this study. We first obtain the horizontal
distributions of the vertically integrated heat content using
the temperature data set historically obtained in the East
China Sea. The distribution of the heat content is largely
influenced by the sea surface heat flux, especially in the
shallow shelf region, so that it is difficult to detect the
current path using the heat content pattern as it is. Therefore,
we have to remove the contribution of the surface heat flux
from the heat content, and obtain the distribution of the
horizontal heat transport. Although the horizontal heat
transport contains the effect of the horizontal diffusion, we
anticipate that its distribution pattern is determined mainly
due to the advection by the ocean current if it exists in the
East China Sea stably.
The ocean current changes its T-S property as it flows
in the East China Sea because of the addition of the fresh
water from the mainland of China, exchange of the water
mass with the Kuroshio and so on. After detecting the
current path in the East China Sea, we compare the T-S
1. Introduction
There are two different schools of thought regarding
the origin of the Tsushima Warm Current. One believes that
it originates around the Taiwan Strait (Fang et al., 1991). They
regard the Tsushima Warm Current as a part of the so-called
Taiwan-Tsushima Warm Current System (hereafter referred
to as the TTWCS). Isobe (1999) supported this idea through
a diagnostic model of the vertically averaged vorticity
equation with the long-term observed hydrographic and
wind data. According to his study, the TTWCS flows along
the isobath in the East China Sea. The wind-induced and the
JEBAR terms are too small to break this flow pattern.
However, he found that a large positive JEBAR term is
evaluated along the shelf edge during autumn (from October
to December) southwest of Kyushu. Isobe (1999) insists that
the TTWCS breaks down in autumn, and that the Tsushima
Warm Current bifurcates from the Kuroshio southwest of
Kyushu during this season. The other school believes that
the current always comes from southwest of Kyushu as a
branch of the Kuroshio (Lie and Cho, 1994; Hsueh et al.,
1996). Huh (1982), using satellite images, suggested that the
Tsushima Warm Current included a repeatedly detached
warm eddy from the Kuroshio southwest of Kyushu. This
idea should be contained in the latter one. The above
thoughts-except for Isobe (1999)-were derived from shortterm observations, e.g., data set of one-day current meter
moorings (Fang et al., 1991), trajectories of the satellite-
185
Copyright  The Oceanographic Society of Japan.
Keywords:
⋅ Taiwan-Tsushima
Warm Current
System,
⋅ East China Sea.
diagrams along the current path in order to trace the transformation of the water mass. This analysis elucidates the water
masses that make up the Tsushima Warm Current water.
2. Data and Method
Figure 1 shows the study area, which is surrounded by
the thick line. We divided it into 62 boxes whose resolution
is 1° in both latitude and longitude.
The conservation equation of the heat content at arbitrary depth is;
∂ ( cρT )  ∂ 
∂ ( cρT )
+  u⋅  ( cρT ) = K H ∇ 2 ( cρT ) + K Z
(1)
 ∂x
∂t
∂z 2
2
where c is the specific heat, ρ the water density, T is the water
temperature, u is the current vectors, and KH, KZ are the
horizontal and vertical diffusivities, respectively. We vertically integrate Eq. (1) from surface (z = 0) to the bottom
(z = –H);
∂q
= qh + q s ,
∂t
(2)
where q is the vertically integrated heat content of unit area,
and is expressed as follows.
0
q = cρ ∫ Tdz
−H
where T is to be regarded as a representative temperature
profile in each box. In this study, c and ρ are assumed to be
constant.
qh is the horizontal heat transport which is expressed
by;
qh = cρ ∫
0
−H
  ∂ uT ∂ vT 
2 
+
− 
 + K H ∇ T  dz,
∂y 
  ∂x

A. Isobe
( 4)
and qs is the heat flux through the sea surface.
Using the temperature data from 1961 to 1990, supplied
by the Japan Oceanographic Data Center (JODC), we converted these data to the vertically integrated heat content (q)
of unit area in each box. We omitted the water temperature
data that exceed three times the standard deviation from its
average value at the same depth in each box. Also we
omitted the temperature profile whose measured depth range
(maximum measured depth minus minimum one) is less
than 70 percent of the mean depth of the box. We used the
temperature data above 400 m depth because the data below
this depth are scarce. In the case that the depth of the box is
more than 400 m, we neglected the vertical heat transport
Fig. 1. The study area in the East China Sea with the divided boxes. Also shown is the isobath in meters.
186
( 3)
Fig. 2. Upper panel shows the locations at which we show the vertically integrated heat content and each term in Eq. (2). In lower four
panels, small dots with the thin line show the vertically integrated heat content (q, cal/cm2) in each box. The broken line with the
open square indicates the temporal differentiation of q (qt, W/m2). The doted line with the closed square shows the heat flux through
the sea surface (qs, W/m2). The thick line with the closed circle indicates the horizontal heat transport (qh , W/m2 ) defined by Eq.
(4).
The Taiwan-Tsushima Warm Current System
187
across this depth. This assumption is considered to be valid
because the horizontal heat transport prevails in the deep
offshore region where the Kuroshio flows.
Using the all quality controlled data of q, we drew the
scatter plot that denotes the annual variation of q in each box.
Figure 2 shows the four examples (boxes a, b, c and d) of the
scatter plot of q. The annual variation of q was fitted by
sinusoidal curves using the least square method. In this
study, we used a function that is expressed by;
3
2nπ
q (t ) = q0 + ∑ an sin 
t − Kn 


τ
n =1
( 5)
where q0 is a constant, an and Kn are the amplitude and the
phase lag of the n-th signal, respectively. τ is one year, which
is the longest period. Through trial and error, we determined
one-third year as the shortest signal to follow the temporal
variation of the heat content in a box. If we chose a much
higher frequency, the fitted curve would be overly complicated. The thin line in Fig. 2 shows the fitted curves.
Differentiating Eq. (5), we evaluated the left-hand side
of Eq. (2), which is denoted by the broken line with open
squares in Fig. 2. In this study, the time increment was
selected to be one month. Hirose et al. (1998) supplied the
monthly net heat flux through the sea surface in each 1°
latitude and longitude box in the East China Sea (the dotted
line with closed squares in Fig. 2). They evaluated the net
heat flux using the meteorological and oceanographic data
set from 1960 to 1990 (see their text for details). Their
analyzed period is nearly same as ours. Using their data set,
we obtained the monthly distribution of the horizontal heat
transport (qh) in the East China Sea as a difference between
∂q/∂t and qs. In Fig. 2, qh is shown by the thick line with the
closed circles.
3.
The Distribution of the Horizontal Heat Transport
in the East China Sea
In Fig. 2, the boxes a and b are located in the southwestern and northeastern side of the East China Sea, while the
boxes c and d are located in the upstream and downstream
region of the Kuroshio, respectively. The surface heat flux
(qs) in all boxes shows a sinusoidal curve with its maximum
from June to August. Apparently, its annual average has a
negative value. On the other hand, the annual average of the
horizontal heat transport (qh) seems to be positive. This
means that the heat loss due to the surface heat flux is, on
average, compensated by the horizontal heat transport in the
study area. It is found that the qh shows its maximum in spring
along the Kuroshio path (c and d). The peak of qh at box d
is delayed for 1–2 months compared to that at box c. This
suggests that the warm water moves northeastward as the
Kuroshio flows. The peak of qh in the shelf region of the East
China Sea varies from summer (a) to autumn (b) with 2
188
A. Isobe
months delay. This also implies the existence of the
northeastward flow that transports the warm water mass.
Figure 3 shows the monthly distribution of the horizontal heat transport in the East China Sea. It is found that the
area where the heat transport is greater than 300 W/m2
(hereafter referred to as the warming region) moves
northeastward in the offshore region from March to May
(see the shaded area given by arrows with a closed circle).
Such a warming region corresponds to the area where the
temporal variation of the heat content is large due to the
horizontal heat transport. We consider that the appearance
of such a warming region is due to the advection of the warm
water mass by the ambient mean flow. The location of the
warming region is considered to correspond to the edge of
the warm water mass that moves with the Kuroshio. So it is
considered that successive change of the location of the
warming region indicates the Kuroshio path, which seems to
be along the shelf edge and end at the Tokara Strait in Fig.
3. The detected Kuroshio path is consistent with other
studies (e.g., Yamashio et al., 1993). We find, too, that the
other warming region moves northeastward in the shallow
shelf region from April to August (see the arrows with an
open circle). The propagation speed is apparently slower
than that along the Kuroshio path, so it is considered that
such a movement of the warming region is effected by the
weak northeastward flow compared to the Kuroshio. This
warming region moves along the isobath of around 100 m,
that is, the same path as TTWCS (Fang et al., 1991). Isobe
(1999) also use a diagnostic model to show that the TTWCS
flows along the isobath in the East China and Yellow Seas,
and that wind-induced and JEBAR terms are too small to
disturb this flow path. The propagation speed is estimated to
be around 10 cm/sec (800 km/ 3 months) along the 100 m
isobath, which is the same order as the transport density
(volume transport in the unit width) of TTWCS (~10 m2/sec,
Fang et al., 1991). So we consider that the movement of the
warming region from April to August is mainly due to the
advection by TTWCS that flows along the isobath in the
East China Sea.
If the Tsushima Warm Current originates around the
southwest of Kyushu crossing the steep shelf edge, crossshelf heat transport should be dominant there. Such a distribution pattern is found at least in November and December.
Isobe (1999) shows that the TTWCS breaks down in autumn, and that the cross-shelf transport prevails southwest
of Kyushu as an origin of the Tsushima Warm Current in
autumn. The distribution pattern shown in Fig. 3 is consistent with the seasonality of the TTWCS.
In winter (from January to March), the horizontal heat
transport has a large value along the Kuroshio path. However, it is difficult to describe a current path clearly in the
shelf region. The intense surface cooling during mid-winter
makes the shelf region homogeneous. Therefore, the horizontal heat transport does not show the clear spatial pattern
The Taiwan-Tsushima Warm Current System
189
Fig. 3. Monthly distribution of the horizontal heat transport in the East China Sea. The contour interval is 100 W/m2 . The region with
the positive value is shaded: double intensity denoting more than 300 W/m2, i.e. ‘warming region’. See the text for the meaning of
arrows with open and closed circles. Also shown is the depth in meters.
Fig. 4. T-S diagrams at the boxes that are shown in the upper panel. The warming regions are also indicated in the upper panel by broken
lines. In the lower panels, the observed T-S plots are divided into three layers, which are shown in the T-S diagram of May. The
dotted rectangular in T-S diagrams in July and August indicates the range of the cold water (see text for detail).
190
A. Isobe
from January to March, although the northeastward movement of the positive region from January to March implies
the existence of the TTWCS.
4.
Transformation of the Water Mass along the Path
of TTWCS
The TTWCS may be greatly altered in terms of its T-S
(temperature-salinity) property as it flows in the East China
Sea due to the mixing process there. Now we show the T-S
diagram within a warming region from May to August. We
then describe the transformation of the water mass of the
TTWCS. The salinity data used here are also supplied by
JODC. As shown later in this section, we also use some TS data outside of our study area, which are obtained from
JODC. Figure 4 shows the T-S diagram in May, June, July
and August within a box in each warming region. The
locations of each box are indicated in an upper panel. In
May, we are able to see that the temperature of the TTWCS
ranges between 16 and 26°C, and the salinity ranges between
31.5 and 34.7 psu. In order to find the origin of this water
mass, we compare this T-S diagram with those in the Taiwan
Strait and the Kuroshio surface layer (0–100 m) in May (Fig.
5). The water types of the Taiwan Strait and the Kuroshio
surface layer resemble each other, at least, in this month.
These two water types are also similar to that of TTWCS
when entering the East China Sea. It is considered that the
water passing through the Taiwan Strait and/or the Kuroshio
surface layer originally composes the TTWCS. As the
TTWCS flows northeastward, less saline water is added in
the surface layer (0–30 m) in June, July and August. The
temperature of the less saline water increases from June to
July because of the heating through the sea surface. In July
and August, the cold water is also added in the middle (30–
50 m) and lower (>50 m) layers, which is surrounded by the
dotted line in Fig. 4. The temperature of this cold water
ranges between 12 to 16°C, while the salinity ranges between
32 and 34.7 psu. The temperature and salinity of the cold
water in August are slightly higher than those in July. This
implies that the temperature and the salinity of the cold
water increase as the TTWCS flows northeastward. The TS property in August is nearly same as that of the Tsushima
Warm Current found in the Tsushima/Korea Strait in sum-
Fig. 5. T-S diagram at boxes A (Taiwan Strait) and B (Kuroshio surface layer). The observed T-S plots are divided into three layers,
which are shown in the each T-S diagram.
The Taiwan-Tsushima Warm Current System
191
mer (see Fig. 2 in Ogawa, 1983). We therefore conclude that
the transformation processes of the TTWCS water are
complete around the location of the box in August.
Now we consider the origins of the less saline water
added to the surface layer of the TTWCS, and the cold water
added to the middle and lower layers.
Figure 6 shows the horizontal distribution of the surface salinity. The less saline water extends to the south or
east from the river mouth of the Changjiang. It is found that
the warming region intersects that low salinity region. The
intersecting area increases from May to July corresponding
to the increase of the less saline water in the T-S diagram. So
we conclude that the origin of the less saline water added to
the TTWCS is the mainland of China, especially the river
mouth of Changjiang.
Figure 7 shows the horizontal distribution of the temperature at 50 m depth. We can see that the warming region
intersects the cold water mass that is located in the northern
part of the study area. The intersection seems to start from
July. This is consistent with the T-S diagram in which the
cold water below 16°C starts to appear from July. So we
conclude that addition of the cold water to the TTWCS is
mainly due to mixing with the cold water mass extending in
the northern part of the study area.
Now we consider why the cold water below 16°C
appears in the northern part of the study area. Figure 8 shows
the monthly mean vertical distribution of the temperature in
July along a line A-A’ (see the upper panel of Fig. 8 for its
location). Also shown is the vertical section in February for
reference. In February, we can see the cold water below
10°C in the Yellow Sea, which is named the Yellow Sea
Cold Water (hereafter YSCW, Nakao, 1977). This water
mass forms due to intense cooling in midwinter, and remains
below the thermocline until October, reducing its thickness
Fig. 6. The horizontal distributions of the surface salinity in May, June, July and August within the study area. Contour interval is 0.5
psu. Shading has been chosen to emphasize the less saline water: single, double intensity denoting, respectively, less than 34 and
32 psu. Also shown are the locations of the warming region (broken line).
192
A. Isobe
due to the mixture with the warm upper layer (Tawara and
Yamagata, 1991). In Fig. 8, we can find the YSCW in the
lower layer in the Yellow Sea in July, too. Although some
studies (e.g., Uda, 1934) have pointed out the southward
movement of the YSCW in spring and summer, Tawara and
Yamagata (1991) show that the YSCW keeps its location
throughout a year on the basis of 10-year mean temperature
fields in the Yellow Sea. Figure 8 also shows that the YSCW
in July stays in its generation area. In Fig. 8, we also find the
cold water below 16°C in the East China Sea, which is the
same water mass extending in the northern part of our study
area. The seasonal variation of the distribution pattern of
this cold water mass is very similar to the YSCW, so we
consider that the generation process of this cold water below
16°C is same as that of the YSCW. We can regard this cold
water as the southern tip of the YSCW.
The temperature and salinity of the added cold water
increase as TTWCS flows northeastward, as mentioned
previously. The upper panel of Fig. 9 shows the location at
which added cold water (12 ≤ T (°C) ≤ 16, 32 ≤ S (psu) ≤
34.7) has been observed in July in the lower layer (50–100
m). The cold water extends from the northern part of the East
China Sea to the Tsushima/Korea Strait. We set the boxes A,
B, C and D within the study area in order to investigate the
change of the T-S property of the cold water as we go
northeast. The lower panel of Fig. 9 shows the T-S diagram
of the cold water (same range as the dotted rectangular in
Fig. 4) at four boxes. We find that the dots move from the
lower left area to the upper right as we go northeast. The
temperature and salinity of the box D are close to those of the
original TTWCS water (see Fig. 4). Thus we consider that
the mixture with the original TTWCS water mainly causes
the transformation of the cold water added to the TTWCS.
The temperature and salinity of the box D are close to the T-
Fig. 7. The horizontal distributions of the temperature at 50 m depth in May, June, July and August. Contour interval is 1°C. Shading
has been chosen to emphasize the cold water less than 16°C. Also shown are the locations of the warming region (broken line).
The Taiwan-Tsushima Warm Current System
193
Fig. 8. The monthly mean vertical distribution of the temperature
in February and July along the line A-A’. Shading has been
chosen to emphasize the cold water: single, double intensity
denoting, respectively, less than 16°C and 10°C.
S property of the Kuroshio surface water as well (see Fig. 5).
The Kuroshio surface water that exists around the shelf edge
west of Kyushu may also be responsible for the transformation of the T-S property of the added cold water.
5. Conclusions
The monthly distributions of the horizontal heat
transport show a path of the TTWCS, at least from April to
194
A. Isobe
Fig. 9. The upper panel shows the locations where the cold water
defined in this study has been observed in July in the lower
layer (50–100 m). Also shown are the locations of boxes A, B,
C and D at which we show the T-S diagram in the lower panel.
Note that the range of the T-S diagram is different from others
(Figs. 4 and 5) in order to enlarge the T-S area of the cold
water.
August. The present study indicates that the TTWCS originates around Taiwan, i.e., Taiwan Strait and the Kuroshio
region east of Taiwan. Then, TTWCS greatly changes its TS property due to the mixing process with the surrounding
water masses as it flows in the East China Sea. We trace its
transformation process using the T-S analysis along the path
of the TTWCS from April to August. The water mass of the
TTWCS entering the East China Sea has the same T-S
property as those around the Taiwan Strait and the Kuroshio
surface layer. Then the water mass of TTWCS changes due
to admixture with the freshwater from the mainland of
China, and with the southern tip of YSCW extending in the
northern part of the East China Sea. The horizontal mixture
with the Kuroshio surface water across the shelf edge may
also be responsible for the formation of the Tsushima Warm
Current water. However, it is difficult to pick up this process
in the T-S diagram, because TTWCS originally has the same
T-S property as that of the Kuroshio surface water, as shown
in Fig. 5.
The water masses that we have mentioned above should
be considered as the end members composing the Tsushima
Warm Current water, although our discussion has been
qualitative. A quantitative discussion should be accomplished
in the near future. Also, we have to clarify the reason for the
appearance of the warming region in the East China Sea only
during May-August, which has remained obscure in this
study.
Acknowledgements
Data supplies by JODC (Data Online Service System,
J-DOSS) and Dr. Hirose, Kyushu University are gratefully
acknowledged. Thanks are also extended to two anonymous
reviewers.
References
Fang, G., B. Zhao and Y. Zhu (1991): Water volume transport
through the Taiwan Strait and the continental shelf of the East
China Sea measured with current meters. p. 345–358. In
Oceanography of Asian Marginal Seas, ed. by K. Takano,
Elsevier, Amsterdam.
Hirose, N., H.-C. Lee and J.-H. Yoon (1998): Surface heat flux in
the East China Sea and the Yellow Sea. J. Phys. Oceanogr. (in
press).
Hsueh, Y., H.-J. Lie and H. Ichikawa (1996): On the branching of
the Kuroshio west of Kyushu. J. Geophys. Res., 101, 3851–
3857.
Huh, O. K. (1982): Spring season flow of the Tsushima Current
and its separation from the Kuroshio: Satellite evidence. J.
Geophys. Res., 87, 9687–9693.
Isobe, A. (1999): On the origin of the Tsushima Warm Current and
its seasonality. Cont. Shelf Res., 19, 117–133.
Lie, H.-J. and C.-H. Cho (1994): On the origin of the Tsushima
Warm Current. J. Geophys. Res., 99, 25081–25091.
Nakao, T. (1977): Oceanic variability in relation to fisheries in the
East China Sea and the Yellow Sea. J. Fac. Mar. Sci. Technol.,
Tokai Univ., Special Number, 199–367.
Ogawa, Y. (1983): Seasonal changes in temperature and salinity
of water flowing into the Japan Sea through the Tsushima
Strait. Bull. Japan. Soc. Fish. Oceanogr., 43, 1–8 (in Japanese).
Tawara, S. and T. Yamagata (1991): Seasonal formation of
bottom water in the Yellow Sea and its interannual variability.
Umi to Sora (Sea and Sky), 66, 273–282 (in Japanese).
Uda, M. (1934): Hydrographical researches on the normal monthly
conditions in the Japan Sea, the Yellow Sea, and the Okhotsk
Sea. J. Imp. Fish. Exp. Sta., 5, 191–236 (in Japanese).
Yamashio, T., A. Maeda and M. Sakurai (1993): Mean position
and deviation of the Kuroshio axis in the East China Sea. Umi
to Sora (Sea and Sky), 69, 125–134 (in Japanese).
The Taiwan-Tsushima Warm Current System
195