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Water Exchange across Isobaths over the Continental Shelf of the East China Sea
JING ZHANG
Key Laboratory of Physical Oceanography, Ocean University of China, Ministry of Education China, Qingdao,
China, and Center for Marine Environmental Studies, Ehime University, Matsuyama, JapanXINYU GUO
Center for Marine Environmental Studies, Ehime University, Matsuyama, and Application Laboratory, Japan
Agency for Marine-Earth Science and Technology, Yokohama, Japan, and Key Laboratory of Marine
Environment and Ecology, Ocean University of China, Ministry of Education, Qingdao, China,

LIANG ZHAO
College of Marine and Environmental Sciences, Tianjin University of Science and Technology, Tianjin, China

YASUMASA MIYAZAWA
Application Laboratory, Japan Agency for Marine-Earth Science and Technology, Yokohama, JapanQUN SUN
College of Marine and Environmental Sciences, Tianjin University of Science and Technology, Tianjin, China(Manuscript received 16 October 2016, in final form 28 January 2017)
ABSTRACT
Onshore and offshore currents and the associated volume transport across three isobaths (50, 100, and 200 m)
over the continental shelf of the East China Sea were examined using daily reanalysis data in 1993–2012. Afterbeing averaged along the isobaths, the velocities across 100 and 50 m are onshore in the bottom layer but
offshore in the surface layer. In contrast, those across the 200-m isobath are onshore in the surface and bottom
layers but without a clear direction in the midlayer, suggesting a three-layer structure. The surface offshore
current across the 100-m isobath mainly arises from the Taiwan Strait Current, while the surface onshore current
across the 200-m isobath mainly arises from the Kuroshio, both of which converge in the area between the 100and 200-m isobaths and flow toward the Tsushima Strait. The control of bottom Ekman dynamics on the onshore bottom currents is important at the 100-m isobath, partly important at the 200-m isobath, and slightly
important at the 50-m isobath. The seasonal variations of onshore and offshore currents in the surface layers
across the three isobaths are likely caused by local winds, the Taiwan Strait Current, and the Changjiang discharge, while those in midlayer across the 200-m isobath demonstrate a strong geostrophic control and can beinterpreted from a traditional viewpoint on the Kuroshio intrusion over the entire water column across the shelfslope. The close connection of bottom onshore currents across the three isobaths suggests that the bottom layer
is an important pathway for water exchange of shelf water and the open sea.

1. Introduction
The East China Sea (ECS) is a marginal sea of thenorthwest Pacific Ocean and has one of the largest
continental shelves in the world. The Kuroshio, a western boundary current in the North Pacific Ocean, enters

Corresponding author e-mail: Xinyu Guo, guoxinyu@sci.
ehime-u.ac.jp

the ECS from the northeast of Taiwan. The main flow
extends northeastward along the shelf break and a portion is diverted from the constraint of steep topography
and intrudes onto the ECS shelf. Through exchanges withshelf water along the shelf break, which is usually denotedby the 200-m isobath, the Kuroshio transports large
amounts of heat, salt, and nutrients onto the shelf (Chenand Wang 1999; Hsueh 2000; Fang et al. 2003; Zhang et al.
2007). The exchange between Kuroshio and ECS shelf

DOI: 10.1175/JPO-D-16-0231.1
Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright
Policy (www.ametsoc.org/PUBSReuseLicenses).

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water plays a large role in continental shelf circulation(Hsueh et al. 1997; Ma et al. 2010) and the associatedecosystems (Zhao and Guo 2011) in the ECS.
The Kuroshio intrudes onto the shelf mainly at two
locations: northeast of Taiwan and southwest of Kyushu.
In the area northeast of Taiwan, the Kuroshio loses the
support of the Taiwan Island and intrudes onto the shelfbecause of the geostrophic adjustment (Su 2001). Someof the intruded water, which is considered the Kuroshio
Branch Current north of Taiwan (KBCNT), returns to
the main stream because of the bottom friction and the
topographic beta effect (Qiu and Imasato 1990; Su
2001). The other intruded water goes farther into the
ECS shelf. Yang et al. (2011, 2012) proposed that anearshore Kuroshio bottom branch current could upwell
gradually from the shelf break to the continental shelfoffshore of the Changjiang estuary in summer.
When the Kuroshio main stream reaches the southwest of Kyushu, it turns clockwise to the east and leaves
the ECS through the Tokara Strait. As a result of theconservation of potential vorticity, another Kuroshio
Branch Current west of Kyushu (KBCWK) forms andintrudes shoreward (Hsueh et al. 1996; Lie et al. 1998;
Hsueh 2000; Isobe 2000).
Besides the KBCNT and KBCWK, the onshore intrusion of the Kuroshio along the entire shelf break has
been gaining more attention recently through both observations and numerical models. Teague et al. (2003)
computed volume transports across the Taiwan andTsushima Straits through direct velocity measurements.
Based on the difference between the two straits, they
estimated that the volume transport across the entire
shelf break was about 3 Sv (1 Sv [ 106 m3 s21) onshore
from October to December. Guo et al. (2006) used anested ocean model and obtained a mean Kuroshio
onshore transport of 1.46 Sv across the shelf break of the
ECS (denoted by the 200-m isobath). Isobe (2008) reported an annual-mean transport of 1.4 Sv based on thesummation of all available volume transport values
through the Taiwan Strait and Tsushima Strait in theliteratures.
The Kuroshio onshore transport has a strong seasonalvariation, showing a maximum in autumn and a minimum in summer (Teague et al. 2003; Guo et al. 2006;
Isobe 2008; Oey et al. 2010; Wu et al. 2014). Strong
seasonal variation in the Kuroshio onshore transport isassociated with the monsoon and the changes in thedensity field (Chao 1990; Guo et al. 2006) as well as volume transport through the Taiwan Strait and Kuroshiotransport east of Taiwan (Lee and Matsuno 2007).
Changes in the density field are further linked to the
combined effects of surface heat flux and the monsoon(Oey et al. 2010; Wu et al. 2014).

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In addition to seasonal fluctuations, the long-term
variability of Kuroshio onshore intrusion has also beenstudied (Liu et al. 2014; Wu et al. 2014). Based on model
results, Liu et al. (2014) reported that the interannual
variation of Kuroshio onshore intrusion was as prominent as its seasonal variation and had a period of 4 yrthat also appears in the volume transport of Kuroshio
itself. Based on multisatellite geostrophic velocity data,
Wu et al. (2014) reported that the interannual variationsof the Kuroshio intrusion northeast of Taiwan in winterwas related to the surface heat flux gradient (positively
correlated) and upstream Kuroshio transport (negatively correlated) rather than the local winds.
Although the Kuroshio onshore transport was emphasized in previous studies, shelf water is also known tobe present in the main stream of the Kuroshio. Isobeet al. (2004) observed less saline shelf water in the
Kuroshio subsurface layer (;100-m depth) around theshelf break of the ECS. Therefore, the water movement
across the shelf break of the ECS must occur in two
directions: onshore and offshore. The Kuroshio onshoretransport reported in previous studies is actually the
combined result of onshore and offshore transport. For
this reason, it is a net volume transport across theshelf break.
Net volume transport alone cannot fully account for
the actual water exchange process that occurs essentiallyin two directions, although it can be evaluated by thedifferences in volume transport through the Taiwan
Strait and Tsushima Strait (Teague et al. 2003). On theother hand, the onshore and offshore water movements
occur not only at the shelf break (usually at the 200-m
isobath in previous studies) but also over the ECS shelfwhere the water depth is less than 200 m. Therefore, we
also need to examine the water exchange processes between the inner shelf (,50 m) and midshelf (50–100 m)
and between the midshelf and outer shelf (100–200 m) in
the ECS.
The documentation of onshore and offshore transports is also of great significance to understand thematerial transport between the open ocean and shelfseas. Material concentration in water flowing onshore isusually different from that in water flowing offshore. For
example, nutrient concentrations have an apparent
horizontal gradient from coastal water to the Kuroshio
in the ECS (Zhang et al. 2007). Consequently, the meandissolved inorganic nitrogen concentration in the waterflowing onshore across the 200-m isobath was smaller(larger) than that in the water flowing offshore from
September to the following March (from April to September; Zhao and Guo 2011). Since the onshore andoffshore volume transports across the 200-m isobath in
the ECS are larger than the net volume transport by one

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order of magnitude, the difference in the nutrient concentration in the water flowing onshore and offshore isimportant to the nutrient budget in the ECS. Furthermore, the documentation of the offshore transport is
helpful to understand the transport of terrestrial material to the open sea. Based on measurements in the ECS,
Ren et al. (2015) suggested that the offshore transport ofterrestrial material contributed to the northward increase of dissolved aluminum in the upper regions of the
100-m isobath along the shelf break.
In this study, we focus on the dynamics related towater exchange between the inner shelf (,50 m) andmidshelf (50–100 m), between the midshelf and outer
shelf (100–200 m), and between the outer shelf and theKuroshio region (.200 m) over the continental shelf ofthe ECS. Based on reanalysis data, we calculated net,
onshore, and offshore volume transports across the 200-,
100- and 50-m isobaths in the ECS with special attention
to spatial and temporal variations in the cross-isobath
currents. We also discuss the controlling factors in theseasonal variations of net volume transports in different
vertical layers. Section 2 gives a brief description andvalidation of the reanalysis data as well as the calculation method of volume transport across three isobaths(50, 100, and 200 m). Section 3 presents the mean statesof currents and volume transports across the three isobaths as well as spatial and temporal variations. Section
4 gives an analysis on the mechanisms controlling theseasonal variations in volume transport at different
water depths. Section 5 discusses the connections among
the three isobaths and summarizes this study.

2. Reanalysis data
The reanalysis data were obtained by a data assimilative ocean model developed by the Japan Coastal
Ocean Predictability Experiment (JCOPE2; Miyazawa
et al. 2009). The ocean model in the JCOPE2 system isbased on the Princeton Ocean Model with a generalized
sigma coordinate (POMgcs; Mellor et al. 2002). A highresolution regional model (10.58–628N, 1088–1808E)
with spatial grid resolution of 1/ 128 and 46 vertical levels
was embedded in a low-resolution model (308S–628N,
1008E–908W) with a spatial grid resolution of approximately 1/ 48 and 21 sigma levels, covering almost theentire Pacific region, using the one-way nesting
method.
The model was driven by wind stresses and heat andsalt fluxes. Wind stress and heat flux fields were calculated from the 6-hourly National Centers for Environmental Prediction–National Center for AtmosphericResearch (NCEP–NCAR) reanalysis data (Kalnay et al.
1996) using bulk formulas (Kagimoto et al. 2008). The

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wind stress was multiplied by a factor of 0.7 to adjust theKuroshio transport south of Japan (Hirose 2011). Runoff of Changjiang was included as the only river discharge into the model domain. It is worth mentioningthat a tidal mixing scheme based on the amplitude oftidal currents (Lee and Matsuno 2007) was added whencalculating the eddy viscosity in the model in order toimprove the simulation of the circulation in the ECS,
where tide plays a certain role in the formation andmovement of water mass.
The following observation data were used for data
assimilation in the JCOPE2 model: along-track sea
surface height anomaly (SSHA) obtained from the
TOPEX/Poseidon and ERS-1 and ERS-2 satellites
during the period December 1992 to January 2002 andfrom the Jason-1 and Geosat Follow-On during the period January 2002 to January 2008; sea surface temperature (SST) obtained from the Advanced Very High
Resolution Radiometer (AVHRR)/multichannel sea
surface temperature (MCSST) products; and verticalprofiles of temperature and salinity obtained from thedata archive of the Global Temperature and Salinity
Profile Program (GTSPP).
The JCOPE2 used three-dimensional variational data
assimilation (3DVAR) to construct its data assimilation
system that is mainly designed for the Kuroshio regionand subtropical ocean [more details on the assimilation
method can be found in Miyazawa et al. (2009)]. In the
ECS, besides introducing Changjiang runoff and tidal
mixing effects, the JCOPE2 assimilates SST over it.
Since the JCOPE2 assimilates the data of water temperature and salinity but solves the momentum equation
without assimilation, the dynamics in the momentum
equation are physical consistence.
The low-resolution and high-resolution models werespun up for 15 and 5 yr, respectively. Reanalysis calculations from November 1992 were carried out for bothmodels with the data assimilation and the external
forcing. The daily mean of SSH, two horizontal velocity
components, water temperature, and salinity from 1993
to 2012 were provided for our study.
The bathymetry of JCOPE2 in the ECS is shown in
Fig. 1. We designated three isobaths for examining water exchange between the inner and midshelf by using
the 50-m isobath, between the mid- and outer shelf byusing the 100-m isobath, and between the outer shelf andthe Kuroshio by using the 200-m isobath. The grid pointsof the 100- and 200-m isobaths generally follow theirwater depths. For purposes of easily considering water
budget, we extended the grid points of the 50-m isobath
to the coastline. The four seasons were given according
to the common usage in the ECS: spring from March toMay, summer from June to August, autumn from

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FIG. 2. Seasonal variation of geostrophic volume transport acrossthe PN line. The solid line is averaged from the values for eachseason in Wei et al. (2013); the dashed line is from the reanalysis
data of JCOPE2. The period of average for the two lines is from1993 to 2009.
FIG. 1. Model bathymetry in the ECS. Regions are designated asfollows: Taiwan Strait (TAS), east of Taiwan (ET), Tsushima Strait(TSU), Changjiang (CJ), and PN line (PN). Triangles represent
observation points along the PN line. Contour lines in gray represent the real 20-, 50-, 100-, and 200-m isobaths. Contour lines inblack represent the 50-, 100-, and 200-m isobaths designated inthis study. Between each set of adjacent black dots on the blacklines, there are 20 grid points. The grid points are counted fromsouth to north. Arrows across the black contour lines represent
volume transport across the contour with the indicated magnitude (Sv).

September to November, and winter from December to
February.
The inflow volume transport (IVT), outflow volumetransport (OVT) and net volume transport (NVT) were
calculated as follows:
ðð
(1)
IVT 5
u1 (x, z) dx dz,
ððOVT 5
u2 (x, z) dx dz, and
(2)
ðð
NVT 5
u(x, z) dx dz.
(3)Here, u(x, z) is the daily velocity component normal
to the isobath, while subscripts 1 and 2 denote the directions of onshore and offshore, respectively. The term
x is the horizontal coordinate along the isobaths, and dx
is the horizontal interval of grid points. The term z is thevertical coordinate and dz is the vertical interval ofgrid points.
We compared the reanalysis data with observations at
the PN line (see Fig. 1 for its position), where long-term
routine hydrographic measurements were carried out by
the Japan Meteorological Agency. The volume transport across the PN line was often referred to as theKuroshio volume transport in the ECS (Ichikawa andBeardsley 2002). Using observed hydrographic data forthe years from 1955 to 2010 and the inverse method, Weiet al. (2013) provided quarterly geostrophic volumetransport across the PN line. The mean geostrophic
volume transport averaged from 1993 to 2009 [we discard 2010 because there is only a value for winter of2010 in Wei et al. (2013)] was 22.67 Sv with a maximum
in winter (23.59 Sv) and a minimum in autumn (21.29 Sv;
Fig. 2).
For comparison, we calculated the quarterly geostrophic velocity along the PN line using the densityfrom JCOPE2 reanalysis data at the same locations as
the observation and assuming a level of no motion at
700-m depth. The mean geostrophic volume transport
across the PN line was estimated to be 21.47 Sv, which
was a little smaller than the observed value of 22.67 Sv(Fig. 2). The difference in two mean transports is likely
attributed to the difference of dynamic method we used
and the inverse method Wei et al. (2013) used. As
demonstrated by Wei et al. (2013), the geostrophic
volume transport at the PN line calculated by the dynamic method was usually smaller than that by the inverse method by about 1 Sv.
The geostrophic volume transport obtained from thereanalysis showed a similar seasonal variation as theobservation: maximum in winter and minimum in autumn (Fig. 2). The transport from the reanalysis insummer was comparable with that in winter, which is alittle different from that in the observations where thetransport is slightly smaller in summer than in winter.
Considering the uncertainty of seasonal variationsin the transport from observations (four times a year)
at the PN line, the slight difference in summer isprobably acceptable. Soeyanto et al. (2014) also made a

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comparison of the volume transport through the PN
line between Wei et al. (2013) and the JCOPE2 reanalysis data but for the interannual variations andreported a significant correlation between the two data
sources. Therefore, we consider that the JCOPE2 reanalysis well represents the seasonal and interannual
variations of the Kuroshio in the ECS.

3. Analysis results
We divided our analysis into two parts: mean state andtemporal (mainly seasonal) variation. The mean state
results were averaged over the entire 20-yr reanalysis
period. The seasonal variations were examined frommonthly means over 20 yr for the same month.

a. Mean state of currents across isobaths
1) MEAN VOLUME TRANSPORTS ACROSSISOBATHS

The mean volume transports across the three isobathsin the ECS differed greatly (Fig. 1). The NVT acrossthe 200-m isobath (1.27 Sv in the direction from theKuroshio to outer shelf) agrees well with the previous
observations [3 Sv averaged from October to December
in Teague et al. (2003); 1.4 Sv in Isobe (2008)] and model
results [e.g., 1.46 Sv in Guo et al. (2006); 1.17 Sv in Zhou
et al. (2015)]. The IVT and OVT across the 200-m isobath of 31.82 and 230.55 Sv, respectively, are consistent
with the results from a different model (29.06
and 227.70 Sv in Zhao and Guo 2011) and much larger
than the corresponding NVT. The NVT across the 100-m
isobath was 20.86 Sv, indicating movement in an offshore direction from midshelf to outer shelf. The IVT(5.30 Sv) and OVT (26.16 Sv) across the 100-m isobath
are much larger than the NVT (20.86 Sv). The 50-m
isobath and coastal line round up a closed area, into
which the Changjiang is the only source of water. The
annual-mean discharge from the Changjiang is approximately 0.03 Sv (averaged over 1993 to 2012). Consequently, we obtained a very small NVT across the 50-m
isobath (20.03 Sv). The IVT and OVT across the 50-m
isobaths were 1.98 and 22.01 Sv, respectively, which areboth larger than the NVT.
The mean volume transport through the Taiwan Strait
was 1.47 Sv in the JCOPE2 reanalysis (Fig. 1). From theperspective of the water budget in the ECS, water fromthe Taiwan Strait has two exits. About 60% goes acrossthe 100-m isobath into the outer shelf and contributes
0.86 Sv to the NVT of the 100-m isobath. The other 40%
goes through the Jeju Strait and then heads to the
Tsushima Strait. The offshore shelf water across the
100-m isobath (0.86 Sv) and the onshore Kuroshio water

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across the 200-m isobath (1.27 Sv) come together in theouter shelf and exit the ECS from east of Jeju toward theTsushima Strait.

2) HORIZONTAL DISTRIBUTION OF THE MEAN
CURRENTS ACROSS THE ISOBATHSTaking a closer look at the cross-isobath volumetransport over the continental shelf (Fig. 1), we examined the spatial distribution of the cross-isobath currents. The mean currents at the depth of 5 m, the middleof the depth, and 10 m above the sea bottom are shownin Fig. 3.
Many characteristic currents such as Kuroshio Branch
Current, Taiwan Strait Current, and the China Coastal
Current can be identified in the distribution of currents
at the depth of 5 m (Fig. 3a). The KBCNT intrudes onshore and soon returns to the Kuroshio main stream,
which is responsible for the pair of onshore and offshore
currents off the northeast of Taiwan Island as denotedby the red and blue thick lines along the 200-m isobath(Fig. 3a). The Kuroshio main stream flows northeastward along the 200-m isobath before reaching the areaaround 308N where it turns to the east and leaves the
ECS. The KBCWK appears around 308N and flows toward the Tsushima Strait (Fig. 3a). The Taiwan Strait
Current flows northeastward from the Taiwan Strait
toward 288N, 122.78E and splits into two branches: theoffshore branch flows northeastward through theladder-shaped, the 100-m isobath south of 29.38N, resulting in the surface current alternatively changing directions (onshore or offshore); the onshore branch of
the Taiwan Strait Current goes straight to the south of
Changjiang estuary at about 29.58N, where a small
portion of it intrudes across the 50-m isobath between
grid points 75 and 95, but it soon turns eastward. In the
coastal region south of the Changjiang estuary, thecoastal currents flow southward and cross the 50-m isobath offshore.
Currents at 10 m above the sea bottom are almost
oriented in an onshore direction (black arrows in
Fig. 3b), especially in the areas northeast of Taiwan,
the middle of the shelf break, and southwest of Kyushu.
In the area northeast of Taiwan (258–288N, 1228–
1238E), the onshore bottom current becomes apparent between the 100- and 200-m isobaths and flows
northward across the 50-m isobath to the offshore
Changjiang area; in the middle area of the shelf break(278–298N, 1258–1268E), the bottom current flows northward from the 200- to 100-m isobaths and continues
northward along the 100-m isobath to the west of Jeju
Island where it turns northwestward across the 50-m
isobath; in the area southwest of Kyushu (308–338N,
1278–1288E), the bottom current flows northwestward

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FIG. 3. Annual-mean current distribution (a) at depth of 5 m, and (b) at middepth (brown arrows) and at 10 m
above the sea bottom (black arrows). Current in the areas deeper than 300 m is not shown in (b). Red (blue) thicklines along the isobaths indicate the presence of an onshore (offshore) current component.

between the 100- and 200-m isobaths and turns northeastward to the Tsushima Strait.
The Taiwan Strait Current in the bottom (Fig. 3b)
likely bifurcates as it does in the surface (Fig. 3a). The
onshore branch flows close to the coast and intrudes

onshore across the 50-m isobath up to 308N. The offshore branch heads northeastward and goes offshore
across the 100-m isobath at 288 and at 308N, respectively,
and goes onshore between them. Consequently, thecross-isobath current at the bottom demonstrates an

FIG. 4. (a) Vertical distribution of the current component across the 200-m isobath for theannual-mean flow field. (b) Vertical average of the current component in (a). The x axis shows
the grid points along the 200-m isobath starting from the south end (Fig. 1). (c) Horizontal
average of the current component in (a). Positive (negative) values indicate the current component is flowing in an onshore (offshore) direction.

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alternation in the current direction of onshore and offshore along the 100-m isobath.

3) VERTICAL DISTRIBUTION OF THE MEAN
CURRENTS ACROSS THE ISOBATHS

Along the 200-m isobath for which the gridpoint
numbers are given as the horizontal coordinates, thecross-isobath currents show a stripelike distribution,
indicating that the variation of velocity is smaller in thevertical direction than in the horizontal direction (Fig. 4a).
Based on direction, the cross-isobath velocities can bedivided into several water columns along the 200-m
isobath. The velocities are in the onshore direction
from the sea surface to bottom in some water columns(grid points 5–36, 60–78, 85–103, and 150–170) but are in
the offshore direction from the sea surface to a depth of;160 m and in the onshore direction from ;160 m to thesea bottom in the other columns (grid points 45–60,
78–85, and 103–130). The onshore and offshore velocities are largest at the surface of grid points 30 and 52, respectively. The maximum onshore velocity (25 cm s21) islarger than the offshore one (13 cm s21). Both the largest
onshore and offshore velocities appear northeast of
Taiwan, where the KBCNT intrudes onto the shelf, andsome of it returns back to the shelf break, forming thepair of strong onshore velocity and less strong offshore
velocity at grid points 30 and 52, respectively.
The vertical average of cross-isobath velocities suggests that there are three main onshore intrusion locations: northeast of Taiwan (grid points 1–40), middle ofthe shelf break (grid points 85–100), and southwest of
Kyushu (grid points 140–170; Fig. 4b). All have beenreported in previous studies (Guo et al. 2006; Liu et al.
2014; Zhou et al. 2015). In general, the vertically averaged cross-isobath velocity along the 200-m isobath
has a magnitude less than 5 cm s21. The largest is
14 cm s21, appearing northeast of Taiwan in an onshore
direction.
The horizontal average of the cross-isobath velocities
can be divided into three layers according to flow direction (Fig. 4c): surface layer (0–60 m), midlayer(60–140 m), and bottom layer (140–200 m). In the surface
layer, the direction is onshore with a magnitude between
0.5 to 1.0 cm s21; in the midlayer, the direction is unstable with a magnitude less than 0.5 cm s21; and in thebottom layer, the direction is onshore, which is the same
as for the surface layer but with a slightly larger magnitude (close to 1.0 cm s21). In the surface layer, thecross-isobath velocity at each grid point is strong but thedirection changes frequently along the isobath (Fig. 4a),resulting in a small, horizontal, averaged flow (Fig. 4c).
In the bottom layer, the cross-isobath velocity at eachgrid point is almost in the onshore direction, and the1049

cancellation of positive and negative velocities does notoccur during horizontal averaging, producing a stronger,
horizontally averaged, cross-isobath velocity in thebottom layer than in the surface layer (Fig. 4c).
The cross-isobath velocities along the 100-m isobath(Fig. 5a) show the same stripelike distribution as thosealong the 200-m isobath but with smaller magnitudes(,10 cm s21) than those along the 200-m isobath. The
onshore currents appear mainly at grid points 1–20, 42–75, and 100–128. At grid points 1–20 and 42–75, thevelocities reach maxima (;7 cm s21) in the bottom
layer, which is different from the case along the 200-m
isobath, where the largest onshore velocity appears inthe surface layer. At grid points 100–128, the velocities
are in an onshore direction from the surface to thebottom with little change. The offshore cross-isobath
velocities along the 100-m isobath are mainly in thesurface layers at grid points 20–40, 75–100, and 128–173
where onshore cross-isobath velocities are still evident
in the bottom layer at grid points 20–40 and 128–173.
Taking the vertically averaged cross-isobath velocities
for the 100-m isobath (Fig. 5b), the maximum offshore
velocity is larger than the onshore one, and there are
more areas where the velocities are in the offshore direction than in the onshore direction. The horizontally
averaged velocities show a two-layer structure: offshorein the surface layer (0–80 m) and onshore in the bottom
layer (80–100 m; Fig. 5c). In the upper 80-m layer, themagnitude changes little with depth. However, the velocities are in the opposite direction in the bottom layer
with a maximum velocity over 2.5 cm s21, which is larger
than that at the 200-m isobath (Fig. 4c). The thickness ofthe bottom onshore velocity layer at the 100-m isobath(20 m: 80–100-m depth) is thinner than that at the 200-m
isobath (60 m: 140–200-m depth).
For the 50-m isobath, except for at the ends, themagnitude of the velocity is under 3 cm s21, which is
weaker than the other two isobaths, but the stripe-type
distribution is maintained (Fig. 6a). The cross-isobath
velocities change little in the vertical direction at somepositions (grid points 22–30, 70–92, and 140–152) but
show a two-layer vertical structure at other positions(grid points 30–70 and 95–140) where the cross-isobath
current flows shoreward from a depth of 35 m to the sea
bottom. The largest onshore and offshore velocities
appear at two ends of the 50-m isobath, where theTaiwan Strait Current and the West Korea Coastal
Current, respectively, flow and result in the large
cross-isobath velocities.
The vertically averaged cross-isobath current along
the 50-m isobath is in the onshore direction at grid points
65–92 and 136–154 with a magnitude around 3.0 cm s21
and is in the offshore direction at some other grid points

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FIG. 5. As in Fig. 4, but for the 100-m isobath.

(Fig. 6b). The horizontally averaged cross-isobath current along the 50-m isobath presents an apparent twolayer structure (Fig. 6c): in the offshore direction from0- to 34-m depth with a maximum speed of 1.7 cm s21 at
the surface and in the onshore direction from 34 m to thebottom with a velocity close to 1.0 cm s21.
To obtain a general idea of the water exchange overthe continental shelf in the ECS, we make a comparisonof the horizontally averaged cross-isobath velocities at
the three isobaths. The horizontally averaged crossisobath velocities show a two-layer structure at the 50and 100-m isobaths (onshore in the bottom layer but
offshore in the upper layer) and a three-layer structure
at the 200-m isobath (onshore in both the surface andbottom layers but unstable in the midlayer). The common feature among the three isobaths is that the velocity

FIG. 6. As in Fig. 4, but for the 50-m isobath.

MAY 2017

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1051

is in the onshore direction in the bottom layer. The
thickness of the onshore bottom layer is 60, 20, and 16 m
at the 200-, 100- and 50-m isobaths, respectively; themaximum onshore velocity is approximately 1, 2.5,
and 1 cm s21 at the 200-, 100-, and 50-m isobaths, respectively. As shown in Fig. 3b, the bottom currents veer
left against the currents in the middepth. This feature of
the bottom Ekman dynamics has been recognized in thecurrent mooring data (Hu 1994; Yoshikawa et al. 2010)
and in the numerical model results (Jacobs et al. 2000;
Guo et al. 2006).

b. Seasonal variations of currents across isobaths
1) SEASONAL VARIATIONS OF IVT, OVT, AND
NVT ACROSS ISOBATHSThe IVT across the 200-m isobath has a maximum in
November (32.90 Sv), while the OVT peaks at
31.50 Sv in April (Fig. 7a). Both IVT and OVT across the
200-m isobath are lowest in December and Januarywith a minimum value of 30.50 and 29.20 Sv, respectively. Although the IVT and OVT show similarly
weak seasonal patterns, the difference (NVT) shows
significant seasonal variation. The NVT across the 200-m
isobath is positive throughout the year but is highest in
October (1.90 Sv) and lowest in April (0.75 Sv).
The IVT across the 100-m isobath has a maximum of
5.73 Sv in April and a minimum of 5.04 Sv in July(Fig. 7b). The OVT across the 100-m isobath reaches amaximum of 6.86 Sv in April and a minimum of
5.50 Sv in October. The range of seasonal variation issmaller for the IVT than for the OVT, and the NVT isnegative throughout the year across the 100-m isobath.
The NVT is in the offshore direction and the strongest(21.39 Sv) and weakest (20.29 Sv) values are in summer
and autumn, respectively.
The IVT and OVT across the 50-m isobath maintain abalance throughout the year. The maxima (;2.87 Sv)
and minima (;1.35 Sv) appear in summer and winter,
respectively (Fig. 7c). The NVT across the 50-m isobathis largest in October and smallest in January.

2) SEASONAL VARIATIONS OF HORIZONTALLYAVERAGED CROSS-ISOBATH CURRENTS
The horizontal averages of the cross-isobath currents(Figs. 4c, 5c, and 6c) show a clear two-layer or threelayer structure. Their seasonal variations are also important to our analysis from the viewpoint of waterexchange between the shelf and the Kuroshio region.
The horizontal average of currents across the 200-m
isobath is divided into three layers according to the direction of flow: surface layer (0–60 m) and bottom layer(140 m–bottom) have an onshore direction, and the

FIG. 7. Monthly volume transport of IVT (solid blue), OVT(dashed blue), and NVT (red) across the (a) 200-, (b) 100-, and(c) 50-m isobaths. The OVT is by definition a negative value, but
for easy comparison with IVT, we present the absolute value of
OVT. The red bar indicates the difference between IVT and OVT,
namely, NVT.

midlayer (60–140 m) has an unstable direction (Fig. 4c).
The horizontally averaged currents in these three layers
have diverse seasonal variations (Fig. 8a). In the surface
layer, the horizontal average of cross-isobath currents isin the onshore direction in autumn and winter and in theoffshore direction in summer (from May to August). In
the midlayer, the horizontal average of cross-isobath
currents is in the onshore direction in summer and in theoffshore direction in winter. In the bottom layer, thehorizontally averaged currents are always in the onshore
direction with a weak seasonality as shown by contours
in Fig. 8a. Apparently, the seasonal variations occur
mainly in the surface and midlayers. The seasonal variations in these two layers are generally opposite, especially in summer and winter, and the pattern in thesurface layer precedes that in the midlayer by about
1–2 months.
Both the horizontal averages of currents across the100- and 50-m isobaths show a two-layer structure(Figs. 5c, 6c): offshore in the surface layer and onshore in
the bottom layer. At the 100-m isobath (Fig. 8b), except

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FIG. 8. Monthly mean (contours) and anomaly (colors) of the horizontally averaged currents across the (a) 200-, (b) 100-, and (c) 50-m
isobaths. The solid (dashed) contours indicate currents are in the onshore (offshore) direction; thick black lines denote zero value. The
anomaly is the monthly mean of the horizontally averaged currents across the isobath minus the annual-mean one. The negative anomaly
of offshore current means that the offshore current is stronger than the mean offshore one.

for the upper 20-m layer in autumn, the horizontal average of the cross-isobath currents in the surface layer(0–80 m) is in the offshore direction, and the maximum
offshore velocity occurs in summer; that in the bottom
layer (80–100 m) is in the onshore direction throughout
the year and peaks at about 2.9 cm s21 in summer. At the
50-m isobath (Fig. 8c), the horizontal average of thecross-isobath currents in the surface layer (0–34 m) isalways in the offshore direction except for in the upper
5 m in September and reaches its maximum in summer;that in the bottom layer (34–50 m) is in the onshore direction throughout the year and reaches its maximum insummer. Therefore, the two-layer structure at the 100and 50-m isobaths shows the same seasonal variation:strong in summer and weak in winter.
The volume transport of the cross-isobath current ineach layer of the two- or three-layer structure at the threeisobaths presents a quantitative view to this issue (Fig. 9).
At the 200-m isobath, the mean NVT in the midlayer isvery small (0.1 Sv), which is only a quarter of the NVT in
the surface layer (0.38 Sv) and an eighth of the NVT in thebottom layer (0.79 Sv; Fig. 9a). The seasonal variations of
the NVT in the surface and midlayers at the 200-m isobath
are apparent, but the pattern is the opposite with similar
amplitude (standard deviation of 0.41 Sv in the surface
layer and 0.31 Sv in the midlayer). From a viewpoint of theintensity of onshore intrusion, the NVT in the surface
layer of the 200-m isobath is minimum in summer andmaximum in autumn and winter, while that in the midlayer
is maximum in summer and minimum in winter. The
seasonality of NVT in the bottom layer is weak with astandard deviation of only 0.04 Sv, which is one order ofmagnitude smaller than those in the surface and midlayers.

At the 100-m isobath (Fig. 9b), the mean NVT is
1.42 Sv offshore in the surface layer and 0.55 Sv onshorein the bottom layer. The seasonal variation of NVT acrossthe 100-m isobath occurs mainly in the surface layer
where the offshore NVT is strongest in summer (.2 Sv)
and weakest in winter (;1 Sv). The seasonal variation of
NVT across the 100-m isobath in the bottom layer ismuch weaker than that in the surface layer but it still
exists; it increases from February to July and then decreases gradually until reaching its minimum in January.
At the 50-m isobath (Fig. 9c), the mean NVT
is 20.34 Sv offshore in the surface layer and 0.31 Sv onshorein the bottom layer. The seasonal fluctuation in the twolayers shows almost the same pattern.
In this study, we do not give details on the interannual
variation of cross-isobath currents but report this variation in the context of a comparison with seasonal variation. The bars in Fig. 9 indicate the standard deviationof the interannual variation for each month. The interannual variations of NVTs in all layers of the threeisobaths, except for the bottom layer of the 200-m isobath, are generally weaker than the corresponding seasonal variations and the range of interannual variations
is 50%–60% of the seasonal ranges. Therefore, the water exchange over the shelf in the direction of the crossisobath flow is dominated by seasonal variation, andinterannual variation does not qualitatively change thepattern we report here for seasonal variation.

4. Mechanisms
In section 3, we showed the vertical structures andseasonal variations of currents in different layers acrossMAY 2017

ZHANG ET AL.

FIG. 9. Seasonal variation of NVT in (a) three layers of the 200-m
isobath and two layers of the (b) 100- and (c) 50-m isobaths. Bluelines denote the NVT in the surface layer; red lines denote the NVT
in the bottom layer; and black line denotes the NVT in the midlayerof the 200-m isobath. The depth ranges of surface, mid-, and bottom layer of the 200-m isobath are 0–60, 60–140, and 140–200 m,
respectively. The depth ranges of surface and bottom layers of the
100-m isobath are 0–80 and 80–100 m. The depth ranges of surface
and bottom layers of the 50-m isobath are 0–34 and 34–50 m. The
annual mean plus or minus standard deviation (Sv) are given next
to the line. The bar inside the line denotes the standard deviation ofinterannual variation in each month calculated from the anomaly
in the same month during a period of 1993 to 2012.

the three isobaths. Next, we examine mechanisms thatlikely control the seasonal variations of NVTs in
those layers.

a. Surface layer of all three isobaths
The surface layer of the 200-m isobath ranges in
depth from 0 to 60 m and has an NVT in the onshore
direction in autumn and winter and in the offshore direction in summer, showing the most prominent seasonal variation among the three layers of the 200-m
isobath. This result is consistent with the report by Guo
et al. (2006). Guo et al. (2006) proposed that the windinduced Ekman transport accounts for the seasonalvariation of NVT in the surface layer of the 200-m
isobath. Following this idea, we calculated the Ekman1053Ðtransport VTS as follows with VTS 5 t s /(r0 f ) dx,
where t s is wind stress in the direction along the isobath, r0 is water density, f is the Coriolis parameter, anddx is the horizontal interval of the grid points along theisobath. Surface Ekman transport across the threeisobaths shows similar seasonal variations with a weak
offshore transport in summer and strong onshoretransport in winter (Fig. 10).
Surface Ekman transport across the 200-m isobath(red line in Fig. 10) agrees well with the NVT in thesurface layer of the 200-m isobath (blue line in Fig. 9a).
Both reach a minimum in June at about 20.24 Sv and amaximum in November at 0.95 Sv for NVT and in December at 1.04 Sv for the Ekman transport. The slightdifference between the values of these two in winterindicates the possible presence of other mechanisms.
However, the local wind is likely the major factor controlling the seasonal variation of the NVT in the surface
layer of the 200-m isobath.
It is needed to emphasize that the NVT in the surface
layer of the 200-m isobath is the total volume transport
integrated over the entire shelf break. Both Oey et al.
(2010) and Wu et al. (2014) proposed that in winter the
local wind is not responsible for the Kuroshio intrusion
northeast of Taiwan. From the observations, Tang andYang (1993) and Chuang and Liang (1994) found that thecurrents northeast of Taiwan were not well correlatedwith the local wind; onshore intrusion of Kuroshio water
occurred 1 month after the starting of the northeast
monsoonal wind during the fall to winter season. Thisconclusion is also true in our reanalysis data. If we calculated the onshore volume transport across the 200-m
isobath only northeast of Taiwan, we also need a 1-monthlag to obtain a good correlation between the Ekman
transport and the onshore transport (figures not shownhere). It is only the surface transport across the entire
200-m isobath from northeast of Taiwan to southwest of
Kyushu that shows a good correlation with the corresponding surface Ekman transport without a time lag.
Regarding the above difference between northeast of
Taiwan and the entire shelf break, Guo et al. (2006)
demonstrated that for the Kuroshio onshore flux acrossthe entire shelf break, its seasonal variation was primarily controlled by the Ekman transport while the change
in the density field was secondary. For the Kuroshio
onshore flux at a fixed location along the shelf break, itsseasonal variation was primarily related to the change indensity field and the Ekman transport was secondary.
Zhou et al. (2015) also indicated that the Ekman effects
to some extent are responsible for variability in theupper-layer exchanges. Therefore, our results from the
JCOPE reanalysis data are consistent with the previous
studies.

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FIG. 10. Monthly volume transport through the Taiwan Strait(TAS; black line) and surface Ekman transport across the 200(red), 100- (blue), and 50-m (green) isobaths.

Similarly, surface Ekman dynamics can also be applied to the cross-isobath current at the 100- and 50-m
isobaths. However, wind-driven Ekman transport alone
cannot explain the NVT across the 100- and 50-m isobaths. The horizontally averaged cross-isobath currents
in the surface layer of the 100- and 50-m isobaths are in
the offshore direction throughout the year (blue lines in
Figs. 9b and 9c), which contrasts with onshore surface
Ekman transport in autumn, winter, and spring (blueand green lines in Fig. 10).
The horizontal distribution of currents at a depth of
5 m in January and July suggests that the Taiwan Strait
water is the main source of the offshore NVT in thesurface layer of the 100-m isobath (Fig. 11). In July, thevolume transport of the Taiwan Strait Current becomeslarge (black line in Fig. 10), and the currents across the
100-m isobath also become strong, especially at about

VOLUME 47

288N and south of Jeju Island (Fig. 11). At this moment,
the NVT in the surface layer of the 100-m isobath(0–80 m) is about 2 Sv (Fig. 9b), which is comparable tothe volume transport through the Taiwan Strait (1.90 Sv;
black line in Fig. 10). Both of them become weak inautumn and winter (about 1 Sv). Since the surface
Ekman transport across the 100-m isobath in July is
only 20.28 Sv, it cannot even explain the NVT in theupper 20 m of the 100-m isobath (20.90 Sv). Therefore,
the Taiwan Strait Current must be the major controlling
factor for the seasonal variations of NVT in the surface
layer of the 100-m isobath.
The main factor determining the NVT in the surface
layer of the 50-m isobath is likely from the Changjiang(Fig. 11). Since the annual mean of the Changjiang discharge is 0.03 Sv, which is one order smaller than the
NVT in the surface layer of the 50-m isobath, the riverwater itself contributes little to the NVT in the surface
layer of the 50-m isobath. However, the buoyancy effectfrom the Changjiang water is an important driving forceof circulation in the ECS (Naimie et al. 2001). The increase of river discharge results in the increase of
buoyancy, and more shelf water moves seaward fromthe surface layer, which should be compensated bybottom landward flow. In general, the density-driven
current in an estuary region is larger than the river discharge by about one order of magnitude (Dyer 1979;
MacCready and Geyer 2010). It follows that the NVT in
the surface layer (0.34 Sv) and the bottom layer (0.31 Sv)of the 50-m isobath can theoretically result from thebuoyancy effect of Changjiang water.
Changjiang discharge is high in summer and low in
winter. This is similar to the NVT in the surface layer of

FIG. 11. Mean current distribution at 5-m depth in (a) January and (b) July. The arrows denote the current direction
and the color denotes the velocity magnitude (cm s21).

MAY 2017

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1055

the 50-m isobath (Fig. 9c). The surface Ekman transport
across the 50-m isobath in summer is in the offshore
direction at about 0.3 Sv (Fig. 10). But in reality, thisvalue should be smaller than 0.3 Sv because of the limited water depth. Therefore, the strongest offshore NVT
in the surface layer of the 50-m isobath in summer(about 0.5 Sv) is probably the result of a combination of
the wind and Changjiang water (e.g., Chang and Isobe2003). In winter, the surface Ekman transport across the
50-m isobath is in the onshore direction but the NVT in
the surface layer of the 50-m isobath remains in theoffshore direction. A possible reason for this differenceis that the southward current from the Changjiang
mouth along China coast in winter contributes to theoffshore transport across the 50-m isobath (Fig. 11a).
This southward current has been observed recently by
Wu et al. (2013).

b. Midlayer of the 200-m isobath
The NVT in the midlayer of the 200-m isobath shows
prominent seasonal variation despite that the midlayer
is away from the surface layer (black line in Fig. 9a). The
NVT in the midlayer of the 200-m isobath is in the onshore direction in summer and autumn and in the offshore direction in winter and spring, which is opposite tothat of the surface layer. Therefore, it is difficult to associate the wind with the seasonal variation of the NVT
in the midlayer of the 200-m isobath.
The monthly anomaly of Coriolis force in the alongisobath direction due to the cross-isobath current shows
almost the same seasonal variation as the monthly
anomaly of the pressure gradient in the along-isobath
direction (Fig. 12). Apparently, the seasonal variation in
the current across the 200-m isobath at the midlayer isgenerally in a geostrophic balance. Many previous
studies have examined the vertically integrated currents
across the shelf slope of ECS using a concept of joint
effect of baroclinicity and relief (JEBAR; Guo et al. 2003,
2006; Oey et al. 2010; Liu et al. 2014; Wang and Oey2016). Theoretically, the balance between the JEBAR
and advection of the geostrophic potential along an
isobath is equivalent to a geostrophic balance in the
direction along the isobath (Oey et al. 2010). Since themidlayer (60–140-m depth) of the 200-m isobath is beyond the control of surface and bottom boundary layers,
the dominant role of geostrophic currents there is likely
reasonable (Zhou et al. 2015; Ding et al. 2016).
To illustrate the seasonal variation of the density
gradient along the isobath, we show the vertical distribution of water temperature along the 200-m isobath in
winter (February) and summer (August) in Fig. 13.
Because of the weak contribution of the salinity difference, the distribution of the water temperature along the

FIG. 12. Monthly anomaly (difference of monthly mean from theannual mean) of (a) the along-isobath Coriolis force and (b) thealong-isobath pressure gradient force. The anomalies are verticallyaveraged from 60- to 140-m depth along the 200-m isobath. A
positive anomaly means intensification of an onshore flow or
weakening of an offshore flow. The abscissa is the grid number
along the 200-m isobath starting from northeast of Taiwan andending southwest of Kyushu.

200-m isobath is similar to that of the density. As an
example of geostrophic balance, we focus on the areaaround grid point 105, the location with the mostprominent seasonal variation. According to Fig. 4a, thecross-isobath currents in the midlayer of the 200-m isobath are onshore (offshore) at the left (right) side of grid
point 105. This feature is consistent with the spatial
variation of water temperature whose isotherms steepen
northward (southward) on the left (right) side of grid
point 105. In summer, the stratification is intensified andthe isotherms become dense, indicating the increases ofalongshore gradient and onshore intrusion on the left
side of grid point 105. However, on its right side, theisotherms are less slanted, resulting in a smaller alongshore gradient and a weaker offshore current there insummer than in winter.
As shown by Oey et al. (2010), spatial variation in
local heat flux is a possible cause for the seasonal change
in density gradient along the isobaths in the ECS. In
addition, the onshore and offshore shift of Kuroshio axis
associated with seasonal variation in the vertical current
structure of Kuroshio (Guo et al. 2003), the change inthe Kuroshio current speed associated with basin-scalechange in wind field and arrival of mesoscale eddies(Soeyanto et al. 2014), and the seasonal variation in the
Kuroshio density structure associated with basin-scaleheat flux are also expected to affect the onshore andoffshore flow in the midlayer of the 200-m isobath from a1056

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FIG. 13. Temperature distribution along the 200-m isobath in(a) February and (b) August. The grid points are counted fromsouth to north.

viewpoint of geostrophic balance. To completely understand the contribution of these processes to thecross-isobath current in the ECS, a series of numericalexperiments designed for isolating the contribution ofeach process similar to that in Oey et al. (2010) arenecessary.

c. Bottom layer of the three isobaths
To examine the year-round onshore transport in thebottom layer of the three isobaths, we planned to investigate the effects of the bottom Ekman (Hu 1994;
Jacobs et al. 2000; Yoshikawa et al. 2010) from two aspects: the bottom Ekman transport and bottom Ekman
layer thickness. The bottom Ekman
transport is typiÐ
cally pcalculated
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ by VTB 5 t b /(r0 f ) dx and t b 5
r0 Cd (u2b 1 y 2b )ub , where t b is the component of bottom
stress along the isobath, y b is velocity across the isobath,
ub is velocity along the isobath, Cd is the bottom friction
coefficient, and dx is the horizontal interval of the grid
points along the isobaths. The bottom Ekman
layer
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
thickness is typically calculated by D 5 Ay /(0:5f ),where Ay is vertical eddy viscosity. Since the reanalysis
data of JCOPE2 provides us only daily averaged velocities, the nonlinear feature in the calculation for thebottom stresses and vertical eddy viscosity prevents us
from obtaining an accurate value for the two variables.
As an alternative, we estimated the bottom Ekman layer
thickness through the veering rate based on the leftveering feature of the bottom Ekman currents in the
Northern Hemisphere.
For stratified water over a sloping bottom, it has beenreported that the veering rate of the horizontal bottom
Ekman current is largest at the top of the bottom

VOLUME 47

FIG. 14. Distribution of mean bottom Ekman thickness in the
ECS. Bottom Ekman thickness in the areas deeper than 250-m
depth is not shown.

boundary layer (Weatherly and Martin 1978; Taylor andSarkar 2008). We therefore define the bottom Ekman
layer as the point where the first peak of the veering rate
comes up from the bottom and set its upper limit as halfof the water depth to avoid the case that there is noveering rate peak within a water column or the veering
rate peak is found in the surface Ekman layer. We presented the distribution of bottom Ekman thickness overthe ECS in Fig. 14. The averaged bottom Ekman
thicknesses at the 200-, 100-, and 50-m isobaths are 17.9,
10.9, and 5.1 m, respectively, with a decreasing trend
from the deep ocean to the shallow shelf sea.
Yoshikawa et al. (2010) estimated the vertical eddy
viscosity through velocity profiles observed at two stations in the ECS. The water depths at the two stations
are 128 and 53 m, respectively. The estimated verticaleddy viscosity peaked at 2–3 3 1023 m2 s21 at around
5 m abovethe sea bottom. By substituting this value into
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
D 5 Ay /(0:5f ), we obtained a bottom Ekman layer
thickness of about 8 m. We also estimated the bottom
Ekman layer thicknesses at the same two stations
through the veering rate method that gives values of 16
and 5 m for the 128- and 53-m depth stations, respectively. Although the results from the two methods
are not identical, they are very similar.
Based on the mooring velocity data at the 128-m
depth station, Yoshikawa et al. (2010) estimated theonshore transport per unit area as 0.33 m2 s21 for 11–
16 October 2007 and 0.13 m2 s21 for 19 August–17 October 2008. Since the bottom onshore cross-isobath
currents distribute over the entire isobaths (Figs. 4–6),
we multiplied the onshore transport per unit area reported by Yoshikawa et al. (2010) with the length of theMAY 2017

ZHANG ET AL.

three isobaths (about 1600 km) and obtained a value of0.53 or 0.21 Sv as the first-order approximation of thebottom onshore Ekman transport.
We next estimated the cross-isobath component of thetotal bottom Ekman
transport with the followingÐequation: VT0B 5 0:5DU dx (Pedlosky 1987), where U
is the geostrophic current, and D is the bottom Ekman
layer thickness. The mean VT0B at the 200-, 100- and 50-m
isobaths are 0.60, 0.66, and 0.13 Sv, respectively, using
the bottom Ekman layer thickness obtained by theveering rate method as D and the along-isobath current
velocity above the bottom Ekman layer as U. The VT0Bof the 100-m isobath is a little larger than the NVT in thebottom layer of the 100-m isobath (0.55 Sv), while thoseof the 200- and 50-m isobaths are a little smaller than thecorresponding NVT in the bottom layers of the 200-m
isobath (0.79 Sv) and the 50-m isobath (0.32 Sv).
The above estimation suggests that the bottom
Ekman effect is a very important process in the formation of NVT in the bottom layer of the 100-m isobath. At
the 200-m isobath, the bottom Ekman effect explains
only part of the NVT in the bottom layer. The densitychange along the isobath must also contribute to the
NVT in the bottom layer of the 200-m isobath. At the
50-m isobath, the bottom Ekman effect explains only asmall part of the NVT in its bottom layer. The responseof the bottom layer along the 50-m isobath to thebuoyancy effects of the Changjiang water is likely more
important than the bottom Ekman effect to the NVT inthe bottom layer of the 50-m isobath.

5. Discussion and conclusions
Using JCOPE2 reanalysis data, we presented the annual mean and seasonal variation in current velocity andvolume transport across the 200-, 100-, and 50-m isobaths of the ECS. For all isobaths, the IVT and OVTshow a larger magnitude than the corresponding NVT.
The NVT across the 200-m isobath is 1.27 Sv in theonshore direction. The NVT across the 100-m isobathis 20.86 Sv in the offshore direction. When it comes tothe 50-m isobath, the NVT is small but the IVT andOVT are about two orders of magnitude larger thanthat of the NVT. The seasonal variation of NVTs
weakens from outer shelf to the inner shelf, and the IVT
and OVT values roughly change in phase, except for the
100-m isobath.
Currents across the isobaths appear as stripelike vertical structures. At the 200-m isobath, the maximum
cross-isobath velocity appears in the surface layer. The
horizontal average of the cross-isobath is divided into
three layers with onshore transport identified in thesurface and bottom layers while the midlayer has an

1057

unstable current direction. At the 100- and 50-m isobaths, the maximum cross-isobath current occurs near
the bottom. The horizontal average of the cross-isobath
current is in the offshore direction in the surface layer
and in the onshore direction in the bottom layer,
representing a two-layer structure.
The NVTs in the vertical layers of the three isobaths
are likely related to each other to a certain degree(Fig. 15). As a mean state (mean in Fig. 15), ;0.34-Sv
water moves from the inner shelf to the midshelf in thesurface layer of the 50-m isobath, and 1.42-Sv water
moves from the midshelf to the outer shelf in the surface
layer of the 100-m isobath; ;0.38- and 0.1-Sv water flowfrom the open ocean to the outer shelf in the surface andmidlayer of the 200-m isobath, respectively, which are in
the opposite direction of the cross-isobath current in thesurface layers of the 50- and 100-m isobaths. In thebottom layers, all NVTs indicate the same onshore direction with values of 0.79, 0.55, and 0.31 Sv at the 200-,
100-, and 50-m isobaths, respectively. The mean NVTs
in the bottom layers of the 100- and 50-m isobaths are
70% and 40% of that across the 200-m isobath,showing a decreasing trend from the 200- to the 50-m
isobath. The relationship of the cross-isobath current in
the bottom layers of three isobaths suggests that thebottom layer over the shelf is likely an important pathway for water exchange between the Kuroshio andcontinental shelf of the ECS.
The seasonal variations of the NVTs in the surface
layers of three isobaths are all remarkable and similar(summer and winter in Fig. 15). In winter (summer), theoffshore cross-isobath transports in the surface layers of
the 50- and 100-m isobaths are weakened (intensified)
while the onshore cross-isobath transport in the surface
layer of the 200-m isobath is intensified (reversed to a
weak offshore transport). In the bottom layers, theNVTs across the 100- and 50-m isobaths have moremarked seasonality than that across the 200-m isobath.
The NVT in the midlayer of the 200-m isobath showsnearly opposite seasonal variation to that in the surface
layer of the 200-m isobath but nearly the same seasonalvariation as in the bottom layer of the 50- and 100-m
isobaths.
The controlling factor of seasonal variations varies fordifferent vertical layers and for different isobaths. The
surface Ekman transport contributes to the surface
cross-isobath transport integrated over the three isobaths. The agreement of phase and magnitude between
the NVT in the surface layer of the 200-m isobath andthe surface Ekman transport suggests the dominant roleof the winds there. However, the phase disagreement
between the NVT in the surface layer of the 100- and 50-m
isobaths and the surface Ekman transport suggests a

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FIG. 15. Schematic diagram of water exchange across isobaths over the continental shelf ofthe ECS. The three isobaths are denoted by light blue sections. The horizontally averaged
cross-section currents at the 50- and 100-m isobaths represent a two-layer structure and thecurrents at the 200-m isobath represent a three-layer structure. The depth ranges of surface,
mid-, and bottom layer of the 200-m isobath are 0–60 , 60–140, and 140–200 m, respectively. The
depth ranges of surface and bottom layers of the 100-m isobath are 0–80 and 80–100 m. The
depth ranges of surface and bottom layers of the 50-m isobath are 0–34 and 34–50 m. The pinkarrow represents the direction and magnitude of annual-mean NVT in each layer. The green
and yellow arrows represent them in summer and winter respectively. Numbers near arrows
denote the value of NVT (Sv).

more important contribution from other processes.
The NVT in the surface layer of the 100-m isobath islikely affected more by the Taiwan Strait Current thanby the surface winds and that of the 50-m isobathdevelops more due to the Changjiang discharge thandue to the surface winds. The seasonal variation of
NVT in the midlayer of the 200-m isobath is likely
under a strong geostrophic balance. The bottom
Ekman effect on the onshore NVT in the bottom layer
is important at the 100-m isobath, partly important at
the 200-m isobath, and slightly important at the 50-m
isobath. The geostrophic control by the densitychange along the isobath also contributes to the NVT
in the bottom layer of the 200-m isobath, while theresponse to the buoyancy effect of Changjiang discharge is important to the NVT in the bottom layer ofthe 50-m isobath.
The cross-shelf exchange in the ECS is a complexhydrodynamic system and is controlled by a combination of many processes. In this study, we present somemechanisms responsible for the cross-shelf currents. It is
possible for us to underestimate some other mechanisms. A quantitative estimation on the contributionof each potential mechanism to the cross-shelfcurrent needs more numerical experiments and fieldobservations.

Acknowledgments. This study was supported by JSPS
KAKENHI (26241009/26287116/H05821). J. Zhang
thanks the China Scholarship Council (CSC) for supporting her stay in Japan. X. Guo thanks support fromthe National Natural Science Foundation of China(NSFC 41576010) and the Fundamental Research Funds
for Central Universities of the Ministry of Education ofChina (201512004). L. Zhao thanks support from theStrategic Priority Research Program of the Chinese
Academy of Sciences (XDA11020305), the NSFC
(41276016), and the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT) to a
project on Joint Usage/Research Center, Leading Academia in Marine and Environmental Research (LaMer).
Q. Sun thanks support from the NSFC (41376006).
Comments from two anonymous reviewers were helpful
in improving the original manuscript.
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