VOLUME 36

JOURNAL OF PHYSICAL OCEANOGRAPHYDECEMBER 2006

The Kuroshio Onshore Intrusion along the Shelf Break of the East China Sea:
The Origin of the Tsushima Warm Current
XINYU GUO
Center for Marine Environmental Studies, Ehime University, Matsuyama, and Frontier Research Center for Global Change/
JAMSTEC, Yokohama, JapanYASUMASA MIYAZAWA
Frontier Research Center for Global Change/JAMSTEC, Yokohama, JapanTOSHIO YAMAGATA
Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, and Frontier Research Center
for Global Change/JAMSTEC, Yokohama, Japan(Manuscript received 16 October 2005, in final form 10 May 2006)
ABSTRACTA 1⁄18° nested ocean model is used to determine locations, volume transports, and temporal variations ofKuroshio onshore fluxes across the shelf break of the East China Sea (ECS). The Kuroshio onshore flux
shows strong seasonality: maximum (3 Sv; 1 Sv  106 m3 s1) in autumn and minimum (0.5 Sv) insummer. Another short-term (17 days) variation due to Kuroshio meanders introduces large fluctuationsin the onshore fluxes but its seasonal average almost vanishes. The Kuroshio onshore fluxes have two major
sources, Kuroshio intrusion northeast of Taiwan and Kuroshio separation southwest of Kyushu; the former
provides larger onshore flux than the latter. Therefore, in addition to the waters from the Taiwan Strait andthe Kuroshio separation region southwest of Kyushu, the water due to the Kuroshio intrusion northeast of
Taiwan is also a major source of the Tsushima Warm Current. A vorticity equation is used to separate thecontribution of surface Ekman transport to the Kuroshio onshore fluxes in the ECS from that relating todensity fields. For the total Kuroshio onshore flux across the entire shelf break, its seasonal variation is
primarily controlled by the Ekman transport while the change in density field is secondary. For the Kuroshio onshore flux at a fixed location along the shelf break, its seasonal variation is primarily related to thechange in density field and the Ekman transport is secondary. Furthermore, the role of the Taiwan Strait
water and the Kuroshio water across the shelf break on material transport in the ECS is examined with
passive tracer experiments. In summer, about half of the tracer in the Tsushima Strait originates in the
Taiwan Strait, while the other half comes from the Kuroshio. From summer to winter, the ratio changes
dramatically; the contribution from the Taiwan Strait decreases to 20% and that from the Kuroshio increases up to 80%. The tracer originating in the Kuroshio water dominates the bottom layer of thecontinental shelf in the ECS throughout the year.

1. Introduction
The Tsushima Warm Current (TWC) carries volume,
heat, and salt from the East China Sea (ECS) to theJapan Sea and has a large impact on the surface circulation of the Japan Sea. In general, there are two views

Corresponding author address: Xinyu Guo, Center for Marine
Environmental Studies, Ehime University, 2-5 Bunkyo-Cho, Matsuyama 790-8577, Japan.
E-mail: guoxinyu@dpc.ehime-u.ac.jp© 2006 American Meteorological Society

JPO2976on the origin of the TWC. The first considers the TWC
as a northward branch of the Kuroshio as it separates
southwest of Kyushu (Nitani 1972). This view is supported by trajectories of satellite-tracked drifters (Lieand Cho 1994). The second view considers that the
TWC originates mainly in the Taiwan Strait (Beardsley
et al. 1985). This is supported by current meter dataover the continental shelf of the ECS (Fang et al. 1991).
The volume transport through the Tsushima Strait
has been estimated by several studies (Toba et al. 1982;
Isobe et al. 1994; Teague et al. 2002; Takikawa et al.

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JOURNAL OF PHYSICAL OCEANOGRAPHY2005). The most recent estimates, using long-term current profiles, give a mean volume transport of 2.7(Teague et al. 2002) and 2.64 Sv (1 Sv  106 m3 s1;
Takikawa et al. 2005), respectively. In comparison, thevolume transport through the Taiwan Strait is not wellstudied. Using current data, Fang et al. (1991) estimate
a mean volume transport of 2 Sv in the Taiwan Strait.
With eight cruises of shipboard ADCP data during
1999–2001 in the Penghu Channel, a deep channel inthe eastern part of the Taiwan Strait, Jan and Chao(2003) have reported a mean volume transport of 0.86
Sv. Thus, the mean volume transport through the
Tsushima Strait appears to be at least 1 Sv larger thanthat through the Taiwan Strait.
The onshore flux of the Kuroshio across the shelf
break should balance this difference. Using the volumetransports estimated from the current data in the Taiwan Strait and Tsushima Strait, Teague et al. (2003)
estimate the onshore flux of the Kuroshio across theshelf break to be 3 Sv for the period from October toDecember 1999.
The branch of the Kuroshio that separates southwestof Kyushu could be a source of the Kuroshio onshore
flux across the shelf break (Lie et al. 1998), particularly
during the autumn (Isobe 1999). The intrusion of Kuroshio water northeast of Taiwan (Tang and Yang 1993;
Chuang and Liang 1994), however, could also be asource of the Kuroshio onshore flux. Because the Kuroshio water intruding from the region northeast of Taiwan rejoins the main stream of the Kuroshio as theKuroshio Branch Current (Qiu and Imasato 1990), its
contribution to the total onshore flux of the Kuroshio isthought to be smaller than the northward branch of theKuroshio that separates southwest of Kyushu (Teague
et al. 2003). Therefore, it is generally assumed that theKuroshio intrusion off Taiwan is negligible to the TWC(Lie et al. 1998). However, there has actually been no
study that aimed to determine the specific location andthe corresponding flux of the Kuroshio intrusion along
the entire shelf break in the ECS. Therefore, the relative role of the separation southwest of Kyushu and the
intrusion northeast of Taiwan to the total onshore fluxof the Kuroshio across the shelf break remains unknown.
There have been several numerical studies on theorigin of the TWC. Isobe (1999) diagnoses the current
pattern in the ECS in four seasons from long-term hydrographic data and suggests that the TWC originates
near the Taiwan Strait except during the autumn, when
the Kuroshio branch that separates southwest of
Kyushu is the major source of the TWC. In a subsequent paper, Isobe (2000) explains this seasonality using a two-layer model. Hsueh et al. (1996) explain theo-

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retically the separation of the branch of the Kuroshio
southwest of Kyushu.
We note that the diagnostic model such as Isobe(1999) closely depends on the prescribed density fields
that are usually sparse, and hardly resolves the processes related to the steep shelf break. Furthermore,
the diagnostic model only produces a steady current
field and cannot consider the short-term variation. A
simple two-layer model or theory such as Hsueh et al.
(1996) and Isobe (2000) may be useful to understandthe intrusion of the Kuroshio but cannot give a quantitative description. Therefore a prognostic, primitive
equation model with a realistic bathymetry is necessary
and it is the sole approach for studying quantitatively
the intrusion of the Kuroshio along the entire shelf
break in the ECS and its relation to the TWC.
A prognostic model of the ECS must overcome atleast two difficulties. First, the model should resolve
the steep shelf break in the ECS along which the Kuroshio flows. Across the shelf break, the water depth varies from 200 m to more than 1000 m over a distance of
20  30 km. This requires the model resolution to beless than 10 km. Second, the model needs to resolve thevertical structure of the Kuroshio because the interaction between topography and baroclinicity is crucial tothe cross-isobath currents in the ECS (Isobe 2000; Guoet al. 2003). Improving the model’s horizontal resolution can address these two problems simultaneously(Guo et al. 2003).
From a viewpoint of high resolution, a local marginal
sea model rather than a global ocean model might bepreferred. However, the results of a local marginal sea
model depend closely on the artificial boundary conditions along the open boundary, where the vertical structures of current as well as their temporal variations areusually unknown. Only a global ocean model could
avoid the problem of open boundary conditions but thecomputer resources limit its resolution, which is crucial
to resolving the shelf break. A nested model, for which
the global coarse-grid model provides the boundary
conditions, can be a solution to the dilemma between
model resolution and model domain. Therefore, using
the one-way nested method (Guo et al. 2003), we havedeveloped a prognostic model with 1⁄18° (6 km) horizontal resolution for the eastern Asian marginal seas
covering the ECS, the southern part of the Japan Sea
and the western Pacific south of Japan (NEST3 area in
Fig. 1).
The rest of the article is divided as follows. After abrief description of our numerical model, we discuss the
distribution of volume transport at three layers and examine the water budget in the ECS. Then, we present a
detailed examination of the Kuroshio onshore flux

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FIG. 1. (a) Domain of triply nested models: NEST1, NEST2, and NEST3, and (b) model bathymetry of the NEST3 in the East China
Sea. Position of sections is shown in (b) for reference. TAS: Taiwan Strait; ET: East of Taiwan; RK-1, RK-2, RK-3, and RK-4: four
sections around Ryukyu Islands; TOS: Tokara Strait; OS: Osumi Strait; WK: West of Kyushu; EC: East of Cheju; CS: Cheju Strait; andTUS: Tsushima Strait. The contours with numbers are isobaths with unit in meters; the thick line with black dots along the shelf breakis called the 200-m isobath in text and is chosen to coincide with 200-m contour; the black dots are plotted every 30 grid points. The
volume transport through the 200-m isobath is defined as the Kuroshio onshore flux across the shelf break.

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across the 200-m isobath in the ECS, including its temporal variation and spatial distribution. Tracer experiments follow to emphasize the important role of Kuroshio onshore flux on material transport in the ECS, andto give a quantitative estimate on the origin of the material passing through the Tsushima Strait. A final summary is given after a discussion on factors controlling
the Kuroshio onshore flux across the shelf break in the
ECS.

2. Model configuration
The three nested models, called NEST1, NEST2, andNEST3 in order of increasing horizontal resolution, areshown in Fig. 1. The 1⁄18° model (NEST3) is embedded
in a 1⁄6° resolution model (NEST2) for the northwestern
Pacific Ocean, which is further embedded in a 1⁄2° resolution model (NEST1) for the Pacific Ocean. The baseof the three nested models is the Princeton Ocean
Model (POM; Mellor 1998). In the vertical direction, allof three models have the same 21 sigma levels (0.000,
0.002, 0.004, 0.010, 0.020, 0.040, 0.060,
0.080, 0.100, 0.120, 0.140, 0.170, 0.200,
0.300, 0.400, 0.500, 0.650, 0.800, 0.900,
0.950, 1.000), with finer grid spacing near the sea
surface.
To reduce the erroneous velocity due to pressure gradient error in a sigma-coordinate model, the bathymetry data were smoothed to meet the criteria given by
Mellor et al. (1994). Furthermore, a zero-forcing experiment was carried out, in which no external forcing
and no horizontal gradient of density are given but thevertical gradients of temperature and salinity are kept
(Beckman and Haidvogel 1993; Mellor et al. 1998). The
magnitude of velocity, which should be zero if themodel is perfect, is less than a few centimeters per second in our models. After the zero-forcing experiment,
the bathymetry is fixed (Fig. 1).
The embedded level-2.5 Mellor–Yamada turbulent
closure model in POM is adopted for vertical mixing;
for horizontal mixing, the embedded Smagorinsky formula along with the inverse Prandtl number is adopted.
The proportionality factor in the Smagorinsky formula
is set to 0.1 in each of the three models and the inverse
Prandtl number is set to 0.5. Details on the lateralboundary condition of the three models and the adjustment of normal external velocity along the interface
between the fine- and coarse-grid models may be foundin Guo et al. (2003).
All three models are driven by wind stresses, heat,and salt fluxes at the sea surface. Heat and salt fluxes
are of Haney type [see (26) in Barnier (1998)]. The
NEST1 and NEST2 models both start from rest withVOLUME 36

the annual mean temperature and salinity of the World
Ocean Atlas 1994 (WOA94; Levitus et al. 1994; Levitus
and Boyer 1994), while the NEST3 model starts froman initial condition interpolated from the spinup resultsof NEST2. The integration of models is divided into
two phases: spinup and hindcast. During the spinup
phase, NEST1, NEST2, and NEST3 are integrated for
20, 15, and 3 yr, respectively, driven by monthly windstresses (Hellerman and Rosenstein 1983), monthly
heat fluxes (da Silva et al. 1994), zero salinity fluxes,
monthly sea surface temperature (SST), and monthly
sea surface salinity of WOA94. During the hindcast
phase from September 1991 to December 1998, weekly
wind stresses from European Remote Sensing Satellite-1(ERS-1) and -2 (ERS-2) [Centre ERS d’Archivage etde Traitement (CERSAT); http://www.ifremer.fr/
cersat] and weekly SST data blended with ship, buoy,
and bias-corrected satellite data (Reynolds and Smith
1994) are used to replace monthly wind stresses andmonthly SST data. A detailed comparison of the resultsof NEST1, NEST2, and NEST3, all restricted to the
NEST3 domain, has been given in Guo et al. (2003). In
this paper, we focus on the results of NEST3 in the
ECS.

3. Current distribution in the East China Sea
a. General features of currents in the ECSThe current at the surface layer in the ECS (Fig. 2)
shows a clear seasonal variation. East of Taiwan, thenorthward Kuroshio is strong in summer (August) andweak in winter (February), showing a similar seasonalvariation to that observed (Tang et al. 2000). After impinging on the shelf break northeast of Taiwan, theKuroshio turns northeastward and flows along the shelf
break up to the area around 30°N, 129°E where it turns
eastward and then exits the ECS through the Tokara
Strait. The strength and width of the modeled Kuroshio
in the ECS are about 1 m s1 and 100 km, respectively.
The speed of the Kuroshio along the shelf break increases from winter to summer and decreases fromsummer to winter in response to the Kuroshio east of
Taiwan. This can be inferred from the shaded area in
Fig. 2.
Before further describing Fig. 2, we compare themodel results and the observed data along a section
across the Kuroshio to examine its modeled verticalstructure. For the comparison, we choose the PN line
(see Fig. 1b for its position), where long-term hydrographic data are available (Oka 2000). There is very
good agreement between the observed annual mean
absolute geostrophic current (Fig. 3a) and the modeledcurrent (Fig. 3d) in terms of current strength, location

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FIG. 2. Monthly current distribution in the East China Sea at 5-m depth in (a) February, (b) May, (c) August,and (d) November. The region where the current speed exceeds 1 m s1 is shaded. The monthly average iscalculated from the daily results between 1994 and 1998.

of the strongest current, and horizontal and verticalgradients of current. Also, there is very good agreement
between the observed and modeled mean potential water temperature (Figs. 3b,e) and salinity (Figs. 3c,f).
The mean volume transport through the PN line is

24.44 Sv, which is also consistent with the value of 25.4
Sv calculated from the geostrophic velocity referred to
700 m (Hinada 1996). The appropriate representationof the vertical structure of the Kuroshio as well as of thebathymetry (Fig. 1) ensures a reliable cross-isobath in-

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FIG. 3. (top) Observed data at the PN line for along-shelf geostrophic velocity referred to 100-m ADCP current data, potential
temperature, and salinity [see Oka (2000) for details of geostrophic velocity calculation], and (bottom) the model results of NEST3. The
observed potential temperature and salinity data are averaged from 1973 to 1993; the model results are from 1994 to 1998. Regions withnegative values of along-shelf velocity are shaded.

trusion of the Kuroshio in the model as discussed by
Guo et al. (2003).
Returning to Fig. 2, we find that the current from the
Taiwan Strait also flows strongly into the ECS. We refer to it as the Taiwan Warm Current, which flows in anortheastward direction initially but turns eastward
around 28°N, 123°E. After further flowing eastward to
125°E, it turns northeastward following the local
bathymetry and finally enters the Tsushima Strait. The
Taiwan Warm Current is clearly strong in summer andweak in winter, which is consistent with the volumetransport variation through the Taiwan Strait estimatedby Fang et al. (1991).
The Taiwan Warm Current meets the Kuroshio water north of Taiwan (Fig. 2). During winter, the Taiwan
Warm Current is weak and the intruded Kuroshio water dominates. From winter to summer, the Taiwan
Warm Current intensifies gradually while the Kuroshio
intrusion weakens. This results in the Taiwan Warm
Current dominating north of Taiwan in summer. Fromsummer to winter, the Taiwan Warm Current gradually
weakens and the Kuroshio intrusion intensifies. Thismodeled seasonal variation of the onshore intrusion of
the Kuroshio northeast of Taiwan is consistent with theobservations of current (Tang and Yang 1993; Chuang
and Liang 1994) and watermass distribution diagrams
based on satellite infrared images (Chen et al. 1996).
In addition to the Kuroshio and the Taiwan Warm

Current, a variety of interesting currents such as theKuroshio countercurrent, the China Coastal Current,
the West Korea Coastal Current, and the southwardcurrent along the western coast of Kyushu are found in
Fig. 2. Among them, the China Coastal Current andWest Korea Coastal Current have a close relation to
the prevailed northerly wind in autumn and winter inthe ECS and Yellow Sea (Guan 1994). Since these currents have little relation with the Kuroshio onshore intrusion across the shelf break, we leave discussions ofthem for future research.
The TWC, that is, the strong northeastward current
in the Tsushima Strait, is clearly reproduced by model
results (Fig. 2). Tsushima Island, located in the centerof Tsushima Strait, divides the current into two
branches. After passing the Tsushima Strait, the western branch of the TWC flows along the eastern coast of
Korea and goes northward to around 37°N, while theeastern branch flows along the coast of Japan.

b. Distribution of volume transport in three layers
From a viewpoint of water budget, when discussing
the origin of the TWC, it is better to use volume transport rather than velocity distribution at a given depth.
For this purpose, we have calculated the volume transport through the lateral section of each grid point inthree layers: the upper layer (from the surface to50 m), the middle layer (50–100 m), and the lower layer

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FIG. 4. Monthly volume transport distribution for the surface layer (0–50 m) in (a) February, (b) May, (c) August,and (d) November. The contour lines denote the isobaths of 50, 100, and 200 m; values larger than 0.10 Sv havethe same tone; values larger than 0.20 Sv are not shown by arrows.

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FIG. 5. The same as Fig. 4 but for the middle layer (50–100 m).

(100–150 m) and present the seasonal variation of eachlayer in Figs. 4, 5, and 6. In the upper layer (Fig. 4), theKuroshio and Taiwan Warm Current are clearly distinguished. In winter, the Taiwan Warm Current has asmall volume transport and exists only in the easternpart of Taiwan Strait while the Kuroshio intrusion
northeast of Taiwan is noticeable. After merging northof Taiwan, the intruded Kuroshio water and the waterDECEMBER 2006

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FIG. 6. The same as Fig. 4 but for the lower layer (100–150 m).

from the Taiwan Strait form a northeastward current
over the continental shelf, nearly parallel to the Kuroshio. This current turns northward as it arrives at
30°N and forms a northward current west of Kyushu.

The majority of the northward current west of Kyushu
flows into the Tsushima Strait while a small proportion
flows into the Yellow Sea. The volume transport of thenorthward current west of Kyushu is smaller than the2214

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eastward current around 28°N, 123°E indicating that apart of the intruded Kuroshio water northeast of Taiwan rejoins the Kuroshio main stream, or is absorbed in
the middle layer.
From winter to summer, the volume transport of the
Taiwan Warm Current increases as the current itself
becomes stronger and wider. It reaches as far as 30°N
over the continental shelf in summer before turning
eastward. On the other hand, the intrusion of the Kuroshio northeast of Taiwan becomes weaker and an areawith very weak transport appears between the Taiwan
Warm Current and the Kuroshio. The weakened Kuroshio intrusion northeast of Taiwan suggests that the
Taiwan Warm Current supplies most of the northwardcurrent west of Kyushu in the surface layer in summer.
In autumn, although the volume transport of the Taiwan Warm Current decreases greatly, there is no clear
decrease in volume transport of the northward current
west of Kyushu and that of the TWC. Therefore, theKuroshio onshore intrusion along the shelf break must
be associated with a volume supply toward the continental shelf region in autumn.
The seasonality of the Taiwan Warm Current has
been explained by the combination of monsoonal wind
and topography forcing (Jan et al. 2002). The annual
mean current in the Taiwan Strait is northward(Chuang 1985, 1986). The northerly wind in wintertends to produce a southward current in the strait (Koet al. 2003), while the southerly wind in summer tends
to produce a northward current in the strait. As a result,the monsoonal wind weakens (reinforces) the annual
mean northward current in winter (summer). On theother hand, the blocking of a bottom ridge in the strait
is stronger in winter than that in summer because of thestrong barotropic structure of the current in winter andthe strong baroclinic structure in summer (Jan et al.
2002).
In the middle layer (Fig. 5), seasonal variations are
small. The Kuroshio intrusion northeast of Taiwan features a northward current west of Kyushu with thesame strength as that in the upper layer. In the lower
layer (Fig. 6), seasonal variations are unclear. The
northward current west of Kyushu can still be identified
and is fed likely by the separated northward branch of
the Kuroshio southwest of Kyushu.
The volume transport distributions in the upper,
middle, and lower layers suggest that the TWC is directly related to the northward current west of Kyushu.
These distributions also suggest that the Taiwan Warm
Current, the intruded Kuroshio water northeast of Taiwan, and the separated Kuroshio water southwest of
Kyushu are three important sources of the northwardcurrent west of Kyushu. Because these three sources

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of water merge into one in the region southwest of
Kyushu, local observations west or southwest of
Kyushu cannot distinguish the three sources.
The contribution of waters in the middle and lower
layers to the TWC is shown explicitly in Figs. 5 and 6
because it was not addressed in sufficient detail in previous studies (Fang et al. 1991; Lie and Cho 1994). The
role of each layer can be seen clearly in the time seriesof volume transport through each layer of section EC
between Cheju Island and Kyushu (Fig. 7). The mean
volume transport through the middle layer of section
EC is 0.93 Sv (Fig. 7b) and it is larger or at least thesame order as the volume transport of 0.82 Sv throughthe upper layer (Fig. 7a). The lower-layer transport is
0.58 Sv (Fig. 7c). Thus, the volume transport throughthe middle and lower layers (1.51 Sv) is definitely larger
than that through the upper layer.
The variation of volume transport in the three layersof section EC decreases with depth (Fig. 7). The shortterm variation is stronger in the upper layer than the
middle and lower layers. The seasonal variation patternin the upper layer is different from that in the middle
and lower layers. The volume transport through theupper layer increases from 0.5 to 1 Sv from Marchthrough September, decreases after September, andreaches its minimum (0.5 Sv) in January (Fig. 7a). The
volume transports in the middle and lower layers show
similar seasonality with the maximum in October andthe minimum from winter to spring (Figs. 7b,c). During
summer, the lower layer keeps a stable low transport
but the middle layer shows a gradual increase in transport just like the upper layer.
The mean volume transport through the Cheju Strait
is 0.69 Sv (Fig. 7d), which is not much different from thevalue of 0.59 Sv estimated by Teague et al. (2003) fromADCP data. Its maximum appears in October, almost
at the same time as the middle and lower layers ofsection EC but about a month after the maximum of
the upper layer of section EC.

c. Volume transport variations in the ECS
The water exchange in the ECS takes place in the
Taiwan Strait (TAS), the section east of Taiwan (ET),the four sections along Ryukyu Islands (RK-1 to RK4), the Tokara Strait (TOS), the Osumi Strait (OS), andthe Tsushima Strait (TUS) (see Fig. 1 for location ofthese sections). The volume transport through these
sections and their spectrums are shown in Figs. 8 and 9,
respectively.
The time-averaged volume transport through the
Taiwan Strait is 1.71 Sv and its standard deviation is
0.99 Sv (Fig. 8a). The seasonal variation is significant
with a maximum (3 Sv) in July and a minimum (0.5

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FIG. 7. Time series of daily volume transport (thin line) and 90-day running mean (thick line) through threelayers of section EC and the entire water depth of Cheju Strait. The first and second numbers inside the parentheses in each panel are the temporal average and standard deviation of daily volume transport, respectively.

Sv) in January. This is consistent with the variability
reported by Fang et al. (1991) on the basis of current
meter data. But the minimum value (0.5 Sv) in winteris larger than a recent estimate (0.14 Sv) by Teague
et al. (2003) on the basis of data from four moored
ADCPs from October through December 1999. The
difference might suggest the interannual variation of
the throughflow. In addition to the seasonal and interannual variations, the short-term variations between 10
and 100 days also exist (Fig. 9a).
The time-averaged volume transport through section
ET (i.e., east of Taiwan) is 23.83 Sv and the standard
deviation is 2.73 Sv (Fig. 8b). The seasonal variation is
weak. Instead, the short-term variation (10–100 days)
and interannual variations are prominent in this section. The calculated mean transport corresponds to thegenerally accepted value of the Kuroshio mean volumetransport into the ECS (Hsueh et al. 1992; Ichikawa andBeardsley 1993; Johns et al. 2001). The prominent
short-term variations (10–100 days, Fig. 9b) in the
Kuroshio transport east of Taiwan are consistent withthe results of a 20-month moored current meter dataset(Zhang et al. 2001).

The time-averaged volume transports (and theirstandard deviation) through the four sections aroundthe Ryukyu Islands are 0.73 (0.42) Sv for RK-1, 0.49(1.63) Sv for RK-2, 0.97 (0.83) Sv for RK-3, and 0.62(0.34) Sv for RK-4 (Fig. 8c–f), where the positive volume transport denotes an outward flow from the ECS.
Among the four sections, RK-2 and RK-3 have thelargest variation. Those are due to active mesoscaleeddies around the Ryukyu Islands, which were reported by Ichikawa (2001) and reproduced in thepresent model.
The time-averaged volume transport through theTokara Strait is 19.47 Sv and the standard deviation is
2.06 Sv (Fig. 8g). The mean transport is close to theclimatological value of 20 Sv used by Teague et al.
(2003), but it is a little smaller than the value of 23.4 Sv
given by Feng et al. (2000). Like the volume transport
through section ET, short-term and interannual variations are prominent in the volume transport throughthe Tokara Strait. The significant periods of 70 and 20
days in the spectrum of the volume transport (Fig. 9g)
are consistent with the observations (Feng et al. 2000).
There is a clear correspondence between the volume

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FIG. 8. Time series of daily volume transport (thin line) and 90-day running mean (thick line) through Taiwan Strait, East of Taiwan,
RK-1 to RK-4, Tokara Strait, Osumi Strait, and Tsushima Strait. The first and second numbers inside the parentheses in each panel are
the temporal average and standard deviation of daily volume transport, respectively.

transports through sections ET, RK-2, RK-3, and theTokara Strait particularly for the interannual variations. Ichikawa (2001) has reported the correlation between the mesoscale eddies around the Ryukyu Islands
and the sea level variations at the Tokara Strait fromobserved data.
The time-averaged volume transport through theOsumi Strait is 2.46 Sv and the standard deviation is0.49 Sv (Fig. 8h). Short-term variations of periods ofabout 70, 50, and 20 days are significant here (Fig. 9h),but interannual variations are weak (Fig. 8h).
The time-averaged volume transport through the
Tsushima Strait is 3.03 Sv and the standard deviation is
0.59 Sv (Fig. 8i). The seasonal variability is the strongest among other signals, while the short-term variationof 20 days also exists (Fig. 9i). We note that the volumetransport increases gradually from January to October.
After attaining to its maximum in October, it starts todecrease and reaches its minimum in the following

January. Although the mean transport of 3.03 Sv isslightly larger than the 2.7 Sv estimated by Teague et al.
(2002), the seasonal march is consistent with the observation.
To summarize the above results, the averaged volume transports across some selected sections in the
ECS are shown in Fig. 10. The volume transports
through the channels that enclose the ECS are balanced. Since the volume transport through the
Tsushima Strait is larger than that through the Taiwan
Strait by 1.32 Sv, the residual must be supplied by theonshore flux across the shelf break. In Fig. 10, we have
1.46 Sv as the onshore volume transport across the
200-m isobath and a 0.14 Sv southward volume transport through a section west of Kyushu (WK in Fig. 1).
As expected, the difference of these two values (1.46
minus 0.14 Sv) just explains the shortage of the volumetransports between the Tsushima Strait and the Taiwan
Strait.

DECEMBER 2006

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FIG. 9. Spectrums of the volume transport shown in Fig. 8.

d. Kuroshio onshore flux across 200-m isobath
Seasonal variability is the most significant signal in
the time series of Kuroshio onshore flux across the
200-m isobath (Fig. 11). From October to November,
the Kuroshio onshore flux reaches its maximum of
about 3 Sv and then it begins to decrease gradually until
reaching its minimum (0.5 Sv) from June to July. The
mean value is 1.46 Sv and the standard deviation is 1.17
Sv. It must be noted that the Kuroshio onshore flux
across the 200-m isobath calculated from the results ofzero-forcing experiment is around 0.02 Sv. Therefore,
the results shown in Fig. 11 contain 0.02 Sv of uncertainty.
In Fig. 11, the daily difference between the volumetransport through the Tsushima Strait and that throughthe Taiwan Strait is also shown, and is almost exactly
the same as the daily Kuroshio onshore flux across the

200-m isobath. Teague et al. (2003), based on the difference between the observed volume transport
through the Taiwan Strait and that through the
Tsushima Strait, suggest a value of 3 Sv for the Kuroshio onshore flux across the shelf break of the ECS
during October–December 1999. Our model result supports the result as well as the method.
In addition to the seasonal variability, short-term
variations are also prominent in the time series of thedaily Kuroshio onshore flux across the 200-m isobath(Fig. 11). The spectrum of the daily Kuroshio onshore
flux across the 200-m isobath (not shown here) showsan energy peak at 17 days, which is consistent with theperiod of Kuroshio meanders in the ECS (Sugimoto etal. 1988; Qiu et al. 1990; James et al. 1999). Therefore
we have plotted the daily Kuroshio path in the ECS andhave confirmed the existence of the short-term Kuroshio meanders in NEST3. According to Fig. 11, the2218

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FIG. 10. Mean state water budgets in the ECS. The number shows volume transport (Sv)through each section; the arrow indicates the direction of volume transport.

Kuroshio meanders cause transport variations of about
1 Sv across the 200-m isobath and results in a standard
variation of 0.37 Sv. Because the standard deviation
caused by the seasonal variation is 0.90 Sv, we conclude
safely that the seasonal variation of Kuroshio intrusions
is the most important factor controlling the Kuroshio
onshore flux across the shelf break in the ECS.
The spatial distribution of the time-averaged volumetransport across the 200-m isobath (Fig. 12) shows that
the Kuroshio intrusion northeast of Taiwan is the major
source of the mean Kuroshio onshore flux to the ECS.
This is seen more clearly in the spatially integrated onshore flux starting from the coast of Taiwan (Fig. 12).
The first 40 grid points (northeast of Taiwan) and thegrid point 210 to 240 (southwest of Kyushu) are two
major sections providing the onshore volume transport(see Fig. 1b for the location of grid point). The former
section provides 5.6 Sv of the onshore flux and thelatter provides about 1 Sv. We note that 4.6 Sv leaves as
the offshore flux between the grid points 40 and 210,
and 0.5 Sv leaves north of grid point 240. Thus, the total
mean Kuroshio onshore flux is maintained at 1.5 Sv.
We use the standard deviation at each grid point toexamine the spatial structure of the temporal variationof the Kuroshio onshore flux (Fig. 12). The standard
deviation of the long-term variation (SD-L) in the first
80 grid points (northeast of Taiwan) and the grid points
after 240 (southwest of Kyushu) is larger than that of

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FIG. 11. Daily Kuroshio onshore flux across the 200-m isobath (thin line); its 90-day runningmean (upper thick line); difference between daily volume transports through Tsushima Strait
and Taiwan Strait (broken thin line); and difference between daily Kuroshio onshore flux andits 90-day running mean (lower thick line). The temporal averages of daily Kuroshio onshore
flux, its 90-day running mean, and their difference are 1.46, 1.39, and 0.01 Sv, respectively;
the standard deviation of daily Kuroshio onshore flux, its 90-day running mean, and theirdifference is 1.17, 0.90, and 0.37 Sv, respectively.

the intermediate area, suggesting that the seasonalvariation takes place mainly in these two areas. The
short-term variation (SD-S) has the same order of magnitude and the same spatial distribution as the seasonalvariation in these two areas, implying that the shortterm and the seasonal variations are both strong there.
However, in the intermediate area (i.e., shelf break),
only the short-term variations are very strong and thelong-term variations are weak. This indicates that theKuroshio is stable without the seasonal variation along
the shelf break, but the active frontal eddies perturb theKuroshio frequently, causing Kuroshio meandering.
To clarify when and where the seasonal variation of
the Kuroshio onshore flux takes place, we have calculated the monthly anomaly of the volume transport
across the 200-m isobath at each grid point (Fig. 13). As
expected from the distribution of the standard deviation (Fig. 12), the first 80 grid points show remarkableseasonal variation. These grid points can be further divided into three subgroups according to the seasonalvariation pattern. The first 20 grid points and grid
points 40 to 80 have similar seasonal variation but grid
points 20 to 40 show an opposite seasonal variation.
The former two subgroups show an onshore flux
anomaly from October to February with a maximum in
November and an offshore flux anomaly from March to
September with a maximum in July. The same patternof variability is found for the grid points beyond point210. Grid points 20 to 40 show an onshore flux anomaly
from April to October with a maximum in July and an
offshore flux anomaly from November to March with amaximum in January.

e. Depth dependence of the temporal variation of
the Kuroshio onshore flux
To study the vertical distribution of the Kuroshio
onshore flux, we divide the 200-m isobath fluxes into
four layers: a surface layer (0–50 m), two middle layers(50–100 and 100–150 m), and a lower layer (150 m tobottom) and show the time series of volume transportin each layer in Fig. 14.
The volume transport from the surface and lower
layers contributes more than 80% of the mean Kuroshio onshore flux. Among the 1.46 Sv of the mean
Kuroshio onshore flux, the surface layer contributes
0.68 Sv and the lower layer 0.52 Sv. The surface layer
has the largest variation and its standard deviation isabout 3 times larger than that in the other three layers.
The increase of onshore flux from October to November is found in all four layers, while the decreasingphase is slightly different among four layers. In the surface layer, a negative onshore flux (i.e., an offshoreflux) appears in summer although it is modified frequently by short-term variations. In contrast, the lower

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FIG. 12. Time-averaged Kuroshio onshore flux at each grid point of the 200-m isobath(VOL, unit is 0.2 Sv); its spatial integration from Taiwan coast (first grid point) to a given grid
point of the 200-m isobath (IVOL; unit is 1 Sv); std dev of daily Kuroshio onshore flux at eachgrid point (SD-D, unit is 0.02 Sv); std dev of 90-day running mean of daily Kuroshio onshore
flux at each grid point (SD-L, unit is 0.02 Sv); and std dev of the difference between daily
Kuroshio onshore flux at each grid point and its 90-day running mean (SD-S, unit is 0.02 Sv).
The positive value of Kuroshio onshore flux is defined as that onto the continental shelf of the
ECS.

layer has little variation and provides almost 0.5 Sv ofonshore volume transport throughout the year. The
mean onshore flux in the two middle layers is small.
Further examination shows that 1.6 and 1.0 Sv of onshore fluxes exist in the first 40 grids of the upper andlower middle layers, respectively, while 1.5 and 1.0 Svof offshore fluxes exist from grid points 40 to 210 in theupper and lower middle layers, respectively. Thus, thenet onshore fluxes through two middle layers become
small as seen in Fig. 14.

4. Tracer experiments
To confirm the origin of TWC, we also need to discuss the transport of materials such as heat, salt, andnutrient. Here we adopt a passive tracer to represent
the material. After releasing different passive tracers atseveral locations, we examine the distribution of eachtracer and thus determine the contribution from eachlocation to the water property of the TWC.
We choose three sections for releasing the passive
tracers. Those are the Taiwan Strait, a section east of
Taiwan, and a section extending from the western coastto the Yellow Sea (Fig. 15). This selection is to examine

contributions from the Taiwan Strait, the Kuroshio, andthe Yellow Sea. The calculation for each of the tracers
starts from 1993 in the domain shown in Fig. 15. During
the integration up to 1998, the tracer concentration ateach release location is fixed to 1 while those at otherboundaries are fixed to zero. The velocity field used intracer calculations is from daily results of NEST3 andthe diffusion coefficients are calculated from daily velocity and density fields of NEST3 using embedded
level-2.5 Mellor–Yamada turbulent closure model andthe Smagorinsky formula in POM.
As a response to the seasonal variation of Kuroshio
onshore flux across the shelf break, the tracer from theKuroshio shows a completely different behavior in winter and in summer in the surface layer over the continental shelf in the ECS and within the Tsushima Strait(Fig. 16a). In January, the tracer from the Kuroshio isclearly seen in the Tsushima Strait, which is transported
from east of Taiwan in two ways. One is the local separation from the Kuroshio southwest of Kyushu; a highconcentration branch extends northward toward theTsushima Strait. The other is from the elongated continental shelf along the inshore side of the Kuroshio.
From March to May, the northward branch separating

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FIG. 13. Monthly anomaly of Kuroshio onshore flux at each grid point of the 200-m isobath(January–December, unit is 0.1 Sv, origin is shifted to the corresponding number of themonth); the temporal average of Kuroshio onshore flux shown in Fig. 11 is also presented forreference (Ann, unit is 0.2 Sv).

from the Kuroshio southwest of Kyushu gradually disappears and the concentration of the tracer from theKuroshio in the Tsushima Strait decreases. In July and
September, the tracer from the Kuroshio almost vanishes in the Tsushima Strait and only a small part of the
tracer from the Kuroshio is transported from the offshore area of the Changjiang River mouth to the
Tsushima Strait. In November, the intrusion of theKuroshio northeast of Taiwan and southwest of Kyushu
appears again.
The tracer released at the Taiwan Strait contributes
only a small fraction of the winter distribution at the
Tsushima Strait (Fig. 16b). From winter to summer,
however, its ratio increases gradually. The maximum isreached in summer, when the tracer from the Taiwan
Strait prevails in the surface layer over the continental
shelf. In late autumn, the tracer concentration decreases over the continental shelf but covers a wide
area of ECS.
The mean concentration of tracer in a check box at
the Tsushima Strait (see Fig. 15 for the location) is
shown in Fig. 17. The two time series of the tracers fromthe Kuroshio and the Taiwan Strait (Fig. 17) show aremarkable seasonal variation. In summer, about halfof the tracer in the Tsushima Strait comes from the
Taiwan Strait and the other half from the Kuroshio. In
winter, the contribution from the Kuroshio reaches amaximum of about 80% of the total amount, while thatfrom the Taiwan Strait shows a minimum of less than
20% of the total amount. The contribution from the
Yellow Sea is negligible throughout the year.
The opposite seasonal contributions from the Kuroshio region and from the Taiwan Strait are direct consequences of the seasonal variation of the Taiwan
Warm Current and the Kuroshio onshore flux acrossthe shelf break. The contribution from the Kuroshio insummer is kept to a level of one-half of that in winterdespite the fact that the Kuroshio onshore flux in summer is much smaller than the volume transport throughthe Taiwan Strait. This is due to the contribution fromthe lower layer, as the intrusion of the Kuroshio takes
place there throughout the year.
In Fig. 18, we show the distribution of tracers fromthe Kuroshio and from the Taiwan Strait at a depth of
10 m above the sea bottom. The time interval is thesame as in Fig. 16. Obviously, the tracer from east of
Taiwan occupies the entire lower layer of the continental shelf and the Tsushima Strait. The highest concentration is seen northeast of Taiwan and it extends
northeastward along the shelf toward the Tsushima
Strait. The area of concentration above 0.9 attains to a
maximum in late autumn, which agrees with the variation of the Kuroshio onshore flux. We confirm againthat the intruded Kuroshio water flows toward theTsushima Strait.
The tracer from the Kuroshio expands northward

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FIG. 14. Time series of daily Kuroshio onshore flux across the 200-m isobath (thin line) and its 90-day runningmean (thick line) through the (a) upper layer, (b), (c) two middle layers, and (d) lower layer. The first number
inside the parentheses in each panel is the temporal average and the second number is the std dev.

from March to November in the bottom layer (Fig.
18a). This explains the presence of the tracer from theKuroshio in the offshore area of the Changjiang Rivermouth at a depth of 10 m in summer (Fig. 16a). A
detailed analysis shows that a bottom Ekman layer
forms below the Taiwan Warm Current in summer andtransports the intruded Kuroshio water northwestward
to the Chinese coastal area. Then, summer coastal upwelling brings the tracer to the surface layer, as reported by Jacobs et al. (2000).

5. Discussions
a. Remote and local effects on the seasonality ofTsushima Warm Current and Kuroshio onshoreflux
As suggested by Minato and Kimura (1980) and Ohshima (1994), the volume transport of TWC depends
primarily on the sea level difference between the
Tsushima Strait and Tsugaru Strait, which is determined by the large-scale wind-driven Sverdrup dynamics in the Pacific. Further, Ohshima (1994) suggested
that the TWC also influences the northeastward currents along the shelves in the ECS and South China

Sea. Following this notion, the sea level difference between the Tsushima Strait and Tsugaru Strait is expected to affect the volume transport through the Taiwan Strait and the Kuroshio onshore flux. However,this expectation is not supported by the results from thepresent study.
From January to September, the transport throughTsushima Strait (Fig. 19a) is in phase with the sea level
difference between the Tsushima Strait and Tsugaru
Strait (Fig. 19b). Both of them increase in general andboth of them decrease in January, June, and September. This relation is broken after September. The sea
level difference decreases largely from September toNovember but the transport still keeps increasing andattains a maximum in November. The inconsistency
also happens in December when the sea level difference increases but the transport decreases. Therefore,
although a positive sea level difference between the
Tsushima Strait and Tsugaru Strait is kept throughout
the year (Fig. 19b) and drives an annually positivetransport from the ECS to Japan Sea through the
Tsushima Strait, which is consistent with the results of
Minato and Kimura (1980) and Ohshima (1994), theproportional relation of the transport of TWC to theDECEMBER 2006

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FIG. 15. Model domain for tracer experiments. The calculation starts with zero initial value over the entire model
domain. During the calculation, the value of tracer is fixed to 1.0 at three places denoted by Taiwan Strait, east of
Taiwan, and Yellow Sea. The mean value of each tracer in the check box located in the Tsushima Strait represents
the proportion of that tracer.

sea level difference between the Tsushima Strait andTsugaru Strait is not completely held in the seasonalvariation.
Further examination of the transport through the
Taiwan Strait and the total Kuroshio onshore flux
across the 200-m isobath (Fig. 19a) shows that the transport through the Taiwan Strait is generally in phasewith the sea level difference between the Tsushima
Strait and Tsugaru Strait (Fig. 19b) but the time lag isclear. The transport through Taiwan Strait decreases
after July while the sea level difference between the
Tsushima Strait and Tsugaru Strait decreases after August. On the other hand, the Kuroshio onshore flux
across 200-m isobath is generally out of phase with thesea level difference between the Tsushima Strait andTsugaru Strait. The former decreases from January toJune and increases from June to November while thelatter increases from February to August and decreases
from August to November.
The above comparison suggests that the sea level difference between the Tsushima Strait and Tsugaru Strait
is difficult to be the primary cause for the seasonalvariation of the volume transport through the Taiwan
Strait and that of the Kuroshio onshore flux across the
200-m isobath in the ECS. It also suggests that somelocal processes in the ECS, which were not consideredby Minato and Kimura (1980) and Ohshima (1994),
have modified the dependence of the TWC on the sea
level difference between the Tsushima Strait andTsugaru Strait. One candidate for the local processes is
the seasonal variation of winds over the ECS. The
monthly Ekman transport across the 200-m isobath isshown in Fig. 19a, which produces 1.3 Sv of onshoretransport in January, 0.5 Sv of offshore transport inJune, and 1.5 Sv of onshore transport in November.
The 1.5 Sv of onshore transport in November apparently contributes to the TWC.

b. Dynamical interpretation on the seasonality of
total Kuroshio onshore flux in the ECSThe Ekman transport across the 200-m isobath is apart of total Kuroshio onshore flux in the ECS and its
seasonal variation is generally in phase with the Kuroshio onshore flux (Fig. 19a). The difference between
them is minimum in March, increases gradually fromMarch to September, becomes maximum in October,and decreases from November to next March (Fig.
19a). Among the total Kuroshio onshore flux, the Ekman transport has an annual variation range of 2 Sv,
and the residual part has the same order of annual
variation range. Therefore, in addition to the local
winds, other unknown factors are also important to thetotal Kuroshio onshore flux.
Mertz and Wright (1992) gave an equation to calculate the transport normal to f /H contours from specified wind and density fields [see (19) in Mertz andWright (1992)], where f is the Coriolis parameter and H

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FIG. 16. Distribution of the tracer value at 10-m depth: (a) the distribution of the tracer from east of Taiwan; (b)those of the tracer from Taiwan Strait. The number in each panel means the date with the format of yymmdd, where
yy denotes the year, mm the month, and dd the day. Time interval between each panel is 60 days.

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GUO ET AL.

FIG. 17. Time series of mean values of the tracer in the
Tsushima Strait from different origins.

is water depth. The equation separates the contributionof local winds to the transport normal to f /H contours
from that of density field; the latter is expressed by
the so-called joint effect of baroclinicity and relief(JEBAR) term. Because f changes little in the ECS, the200-m isobath along the shelf break corresponds
roughly to a contour of f /H. Therefore, by following theidea in Mertz and Wright (1992), we can examine thedifferent role of local winds and density field in theseasonal variation of Kuroshio onshore flux across theshelf break in the ECS.
It must be noted that the equation involving the
JEBAR term is only a diagnostic tool of specified冉冊冉冊

flow and density fields. In fact, JEBAR was initiallyused to interpret the results of a diagnostic model that
calculates the current field from a given density field(Sarkisyan and Ivanov 1971; Holland and Hirschman1972). Because the density field is fixed in the diagnostic model, this concept is useful to consider the relation between the calculated current field and the given
density field. However, in a prognostic model like themodel we used, the density field is not fixed and canbe changed by many factors. Among these factors,some changes the density field directly, while somechanges the flow field directly and as a result of thechange in the flow field, the density field is changed.
The equation with the JEBAR term reflects the combined effects of these factors and cannot clarify thecausal relationship between the flow field and the density field.
Our numerical calculation, however, is not designed
to clarify the causal relationship between the Kuroshio
onshore flow and density field. This will be the second
step. As the first step, we want to confirm whether thechange in density field is important to the Kuroshio
onshore flux, by comparing the JEBAR term with thecontribution from local winds.
After vertically averaging the primitive momentum
equations and then cross differentiating to eliminate
the unnecessary terms, the following vorticity equation
can be obtained [see Mertz and Wright (1992) or Mellor (1996, chapter 8) for details]:

冉 冊

冉 冊

冉冊

冉冊

⭸
M
f
␶a
␶b
A
D
curlz
M⭈⵱
 J␹, H1  curlz
 curlz curlz curlz
⭸t
H
H
␳0H
␳0H
H
H
In (1), two operators are defined as J(A, B)  A/ x B/
y  A/ y B/ x and curlz(C)  Cy / x  Cx / y;M  0H(u i  j) dz is a volume transport vector withu and being the eastward and northward componentof water velocity; J( , H1) is the JEBAR term with 0Hzg / 0 dz being the potential energy; ␶a is thewind stress; and ␶b is the bottom stress. The wind stressterm can be further separated into two terms: ( 0H)1
curlz(␶a), the vorticity directly from wind stress, and⵱( 0H)1  ␶a, the vorticity induced by the surface
Ekman transport across isobaths. The bottom stressterm can also be similarly separated into two terms, andone of them is from the bottom Ekman transport. Twoother vectors, A  Axi  Ay j and D  Dx i  Dy j, arethe nonlinear advection term and the vertically integrated horizontal diffusion term, respectively.
In a quasi steady state such as monthly mean or annual mean state, the first term in the left-hand side of

1(1) is negligible and the second term M•⵱( fH1) is balanced by the five terms in the right-hand side of (1). A
positive M•⵱( fH1) [hereinafter referred to as APV(advection of the geostrophic potential vorticity)] corresponds to an onshore flux in the shelf break of ECS.
Among the five terms in the right-hand side of (1), theJEBAR term is the only term related to the density
field.
The monthly APV as well as the five terms in theright-hand side of (1) are integrated along the 200-m
isobath in the ECS and shown in Fig. 20a. As expected,
the APV corresponds well with the total onshore flux
across the 200-m isobath (Fig. 19a). Both of them havea clear seasonal variation showing a minimum in Juneand a maximum in November. Most of the seasonalvariation of the total onshore flux across the 200-m
isobath occurs in the surface layer (0–50 m) (Fig. 20b).
The seasonal variations in the two middle layers (50–2226

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FIG. 18. The same as Fig. 16 but for the depth of 10 m above the sea bottom.

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FIG. 19. (a) Monthly transport through the Tsushima Strait(thick solid line denoted by Tsushima), monthly transport throughthe Taiwan Strait (thin dashed line denoted by Taiwan), monthly
Kuroshio onshore flux across the 200-m isobath (thin dashed line
denoted by 200 m), monthly Ekman transport across the 200-m
isobath (thick solid line denoted by Ekman), and monthly difference between Kuroshio onshore flux and Ekman transport (thicksolid line denoted by 200-m Ekman, which has been shifted downby 2 Sv for visual purpose). (b) Monthly sea level difference between the Tsushima Strait and Tsugaru Strait. The sea level at the
Tsushima Strait is the value averaged over an area of 33.5°–34°N,
126.5°–130.5°E while the sea level at the Tsugaru Strait is thevalue averaged over an area of 40°–43°N, 141.5°–142°E. The
reader can refer to Fig. 5 of Guo et al. (2003) for the distributionof annual mean sea level in whole model domain of NEST3.

100 and 100–150 m) and in the bottom layer (150 m–bottom) are small (Fig. 20b) but have an apparent increasing from September to November.
The annual averaged APV—that is, the annual averaged Kuroshio onshore flux—is primarily balanced by
the JEBAR term (Fig. 20a). In Fig. 20a, the annual
mean of the APV is 0.77 m2 s2, while the JEBAR term
is 0.50 m2 s2. This, however, just indicates that theKuroshio onshore flow across the 200-m isobath in the
ECS is in a basically geostrophic state. The bottom
stress and the wind stress produce an annual mean of
0.21 and 0.17 m2 s2 in Fig. 20a, respectively, and become the secondary and third factors to balance the2227

FIG. 20. (a) Monthly value of the six terms in (1), in which WS
denotes wind stress term, BS the bottom stress term, DIF thehorizontal diffusion term, and ADV the advection term. (b)Monthly Kuroshio onshore flux across the 200-m isobath in the
ECS in four different layers.

annual averaged APV. The contribution from residual
two terms, the horizontal diffusion term (0.07 m2 s2)
and the nonlinear advection term (0.09 m2 s2), areboth small.
On the other hand, the seasonality of the Kuroshio
onshore flux is primarily balanced by the wind stressterm and the JEBAR term is secondary. Among thefive terms in the right-hand side of (1), the wind stressterm has almost the same seasonal variation as theAPV (Fig. 20a). They both have a minimum in Juneand increases from June to November. After November, the APV decreases until next June, while the windstress term keeps increasing to December and then also
decreases until next June. This result is consistent withthe comparison of the Kuroshio onshore flux and thesurface Ekman transport in Fig. 19a. It is worthy ofnoting that nearly 95% of the wind stress term shown in
Fig. 20a is from the surface Ekman transport across the
200-m isobath and the residual is from wind stress curl.
The change in density field—that is, the JEBAR
term—and the nonlinear advection term in Fig. 20a can

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be related to the difference between total Kuroshio onshore flux and the Ekman transport across the 200-m
isobath shown in Fig. 19a. The JEBAR term shows aminimum in March, increases from March to August,keeps a maximum from August to November, and decreases to the minimum in next March; the nonlinear
advection term keeps almost constant through a year
except for a decrease from July to August and an increase from September to October (Fig. 20a). The combination of these two terms explains well the maximum
in the difference between total Kuroshio onshore flux
and the Ekman transport across the 200-m isobath inautumn (Fig. 19a).
As depicted in Fig. 19, the dependence of the TWC
on the sea level difference between the Tsushima Strait
and Tsugaru Strait is not held from September to December. According to Fig. 20a, the wind-induced surface Ekman transport across the shelf break in the ECS
and the change in the density field in the ECS are probably the most possible explanation on the discrepancy.

c. Dynamical interpretation on the locality ofKuroshio onshore flux along the shelf break of
the ECS and its seasonality
The locality of the Kuroshio onshore flux along the
200-m isobath and its seasonal variation has beenshown in Fig. 13. To know the major factor controllingthe seasonal variation of the onshore flux at a given
location, we show the distribution of the monthly
anomaly of APV, the JEBAR term, and the wind stressterm along the 200-m isobath in Fig. 21.
The magnitude of the terms shown in Fig. 21 suggeststhat the change in the density field affects much morethan the wind stress on the seasonal variation of theonshore flux at a given location. Differing from thewind, whose surface Ekman transport has the same sign
almost all the way along the shelf break, the JEBAR
term changes sign along the shelf break. Therefore, although the monthly anomaly of the JEBAR term at agiven point is larger than that of the wind stress term by
one order, the total JEBAR term contributes less to thetotal onshore flux across the shelf break than the total
wind stress term.
Figure 21 can be linked to the observational results
northeast of Taiwan. The abrupt intrusion of the Kuroshio northeast of Taiwan in mid-October has been captured by a buoy-mounted ADCP (Tang and Yang 1993)
and by moored current meters (Chuang and Liang1994), and in satellite images (Chen et al. 1996). The
observations in other months suggest seasonal variations of the intrusion there. Hsueh et al. (1992) reported the Kuroshio intrusion in spring based on hydrographic data in April 1989. In a subsequent paper,

FIG. 21. Distribution of monthly anomaly of the (a) APV term,(b) JEBAR term, and (c) wind stress term along the 200-m isobath.

Hsueh et al. (1993) reported a blocking of the Kuroshio
there in August 1991. Numerical models (Chao 1990;
Chen et al. 1996) also show the same seasonal variationof the Kuroshio onshore intrusion northeast of Taiwan.
The local wind effect suggested by numerical model
results (Chao 1990; Chen et al. 1996) is not obvious in
observed current data (Tang and Yang 1993; Chuang
and Liang 1994). Instead, Tang and Yang (1993) suggested that nonlocal factors induced the abrupt intrusion of Kuroshio in mid-October. Chuang and Liang(1994) suggested that cooling should be the major cause
for triggering the massive intrusion event in midOctober.
Figure 21 suggests that the local wind really contributes to the Kuroshio intrusion northeast of Taiwan but

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GUO ET AL.

it is not the major factor. The major factor is in thechange of density field. The cooling suggested by
Chuang and Liang (1994) could be one possible causechanging the local density field northeast of Taiwan.
On the other hand, a seasonal change in the basin-scaleatmospheric forcing can alter the density and current
structures of the Kuroshio and then affect the intensityof impinging of the Kuroshio on the shelf break northeast of Taiwan, which can alter the local density fieldnortheast of Taiwan too. Therefore, to clarify the real
physical process beneath the JEBAR term, more numerical experiments are necessary.

6. Summary
Using a high-resolution model, we have discussed thelocation and the temporal variability of the Kuroshio
onshore flux across the shelf break of the ECS. The
Kuroshio intrusion northeast of Taiwan and the Kuroshio water that separates southwest of Kyushu arefound to be two major sources of onshore flux. The
spatial distribution of the Kuroshio onshore flux indicates that the intrusion northeast of Taiwan provides alarger onshore flux than the separation southwest of
Kyushu. The Kuroshio onshore flux shows a strong seasonal variation with a maximum (3 Sv) in autumn anda minimum (0.5 Sv) in summer. The maximum valueof 3 Sv during the autumn is the same as that estimatedby Teague et al. (2003) from observed data in the same
season. Short-term Kuroshio meanders can induce
large fluctuations of the Kuroshio onshore flux but thetemporal average is almost negligible.
We have also shown that the Kuroshio intrusion
northeast of Taiwan is an important contributor to the
TWC. So far, based on surface drifter data, the local
separation of the Kuroshio southwest of Kyushu has
been emphasized as the origin of the northward current
west of Kyushu (Lie and Cho 1994; Lie et al. 1998).
However, local hydrographic observations southwest or
west of Kyushu and surface drifter data in the ECS
cannot distinguish the contribution of the Kuroshio water locally separated southwest of Kyushu from that of
the Kuroshio water intruding farther upstream. On theother hand, the current meter data over the continental
shelf (Fang et al. 1991), which supports the origin in the
Taiwan Warm Current, has the same weakness. In fact,
the Kuroshio water intruded northeast of Taiwan is alsoa part of the current flowing northeastward over thecontinental shelf of the ECS.
Based on the present study, we suggest that the TWC
is composed of three water masses. One is the waterfrom the Taiwan Strait, which has the maximum (minimum) influence in summer (winter). Another is theKuroshio water intruded northeast of Taiwan, which
has the maximum (minimum) effect in autumn (summer). The other is the Kuroshio water separated southwest of Kyushu, which also has the maximum (minimum) influence in autumn (summer). In total, the volume transport of the TWC shows the maximum(minimum) in autumn (winter), as observed by Teague
et al. (2002).
It must be noted that the maximum Kuroshio onshore flux in autumn modifies the proportional relationof the transport of TWC to the sea level difference
between the Tsushima Strait and Tsugaru Strait suggested by Minato and Kimura (1980) and Ohshima(1994). The seasonal variation of the total Kuroshio
onshore flux across the whole 200-m isobath is mainly
related to the monsoonal wind but its dependence on
the wind is obviously modified by the change in thedensity field. The seasonal variation of Kuroshio onshore flux at a locally fixed location is, however, mainly
related to the change in the density field.
The Kuroshio onshore flux is important from theviewpoint of material transport through the Tsushima
Strait. Tracer experiments show that, in summer, about
half of the water mass in the Tsushima Strait originatesfrom the Kuroshio and the other half originates fromthe Taiwan Strait. During the winter, the ratio changes
to 80% for the Kuroshio origin and 20% for the Taiwan
Strait origin. The bottom layer of the continental shelfin the ECS is occupied throughout the year by thetracer from the Kuroshio. The present results mostly
based on a high-resolution numerical model suggestthat the Kuroshio onshore flux and its seasonal variation are crucial to the material transport in the ECS.
Acknowledgments. The present work was partly conducted as a part of the Japanese Coastal Ocean Predictability Experiment (JCOPE) activities of FRSGC
under Japan Marine Science and Technology Center.
Guo thanks the financial support by Grant-in-Aid forYoung Scientists (B) from Monbu Kagakusho (Grant
17710014). We thank Drs. H. Takeoka, Y. Sugimoto,and T. Kagimoto for their helpful suggestions, Dr. E.
Oka for providing the figure of observation data along
the PN line, and Dr. D. Takahashi for drawing Fig. 9.
We thank Drs. Arnoldo Valle-Levinson and Andres
Tejada-Martinez for their comments on the manuscript.
The comments of anonymous reviewers were helpful inimproving the original manuscript.
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