JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 490–501, doi:10.1029/2012JC008429, 2013
An analysis of the current deflection around Dongsha Islands
in the northern South China Sea
Dongxiao Wang,1 Qiang Wang,1,2 Weidong Zhou,1 Shuqun Cai,1 Li Li,3 and Bo Hong4
Received 3 August 2012; revised 21 November 2012; accepted 11 December 2012; published 31 January 2013.
[1] Based on the in situ data and ADCP observation in fall, it is found that a northeastward
current at inter-middle level flows on the Northern South China Sea (NSCS) continental
shelf. This current flows almost along the isobaths, and it deflects from the isobaths veering
toward deep water when flowing over the Dongsha Islands. Geographic currents derived
from the climatologic hydrography data (WOA01) and absolute dynamic topography
(ADT) data confirm the deflection of the northeastward current on NSCS continent. A fine
resolution regional ocean model which can well reproduce the large scale circulation in the
NSCS is used to analyze the dynamic about the deflection. The vorticity term balances
shows that JEBAR (Joint Effect of Baroclinicity and Relief) drives the water column to
depart from the isobaths. To the east of the Dongsha Islands, the isopycnal is almost
orthogonal to the isobaths. The joint effect of the topographic and the baroclinic effect
supplies negative vorticity and drives the water column to deflect from the isobaths and
veer to deeper water. Momentum analysis along the stream line shows that, when the sea
water flows around the Dongsha islands, the pressure gradient along the isobath pushes the
sea water to accelerate, and then the Coriolis force orthogonal to the isobath increases and
overcomes the corresponding pressure gradient, which drives the water deflected from the
isobath toward the deep sea.
Citation: Wang, D., Q. Wang, W. Zhou, S. Cai, L. Li, and B. Hong (2013), An analysis of the current deflection around
Dongsha Islands in the northern South China Sea, J. Geophys. Res. Oceans, 118, 490–501, doi:10.1029/2012JC008429.
1. Introduction
[2] The South China Sea (SCS) is a semi-enclosed
marginal sea in the western Pacific Ocean, with a broad
shelf and slope in its northern part. The continental shelf
in the northern SCS (NSCS) is in the northeast-southwest
orientation and 150–250 km wide, with Dongsha Islands
located about 200 km offshore on a plateau over the upper
continental slope.
[3] The large-scale circulation in the SCS is driven mainly
by Asian monsoons and the lateral influxes that intrude from
the Luzon Strait and Taiwan Strait in the NSCS and from the
Karimata Strait in the southwest SCS [e.g., Wyrtki, 1961;
Dong et al., 2010; Wang et al., 2011; Shu et al., 2011].
Driven by the prevailing northeasterly (southwesterly)
monsoon in winter (summer), the basin-scale cyclonic
(anticyclonic) circulation is typically formed flowing around
1
State Key Laboratory of Tropical Oceanography, South China Sea
Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China.
2
Graduate School of the Chinese Academy of Sciences, Beijing, China.
3
The Third Institute of Oceanography, State Oceanic Administration,
Xiamen, China.
4
Virginia Institute of Marine Science, College of William and Mary,
Virginia, USA.
Corresponding author: Q. Wang, State Key Laboratory of Tropical
Oceanography, South China Sea Institute of Oceanology, Chinese Academic of Sciences, Guangzhou, China. ([email protected])
©2012. American Geophysical Union. All Rights Reserved.
2169-9275/13/2012JC008429
the continental margin in the SCS. An analysis using various
observational data leads to the conclusion that JEBAR (Joint
Effect of Baroclinicity and Relief) is an important impact
factor on the SCS current, with the terrain effect superior
to the b effect (i.e., planetary voriticity changes with
latitudes ) [Wang et al., 2004, 2004; Liao et al., 2007,
2008; Yuan et al., 2008].
[4] In the NSCS, there are three adjacent band-like currents
with alternative directions in winter, namely the leeward
Guangdong coastal current, the SCS Warm Current (SCSWC),
and the southwestward slop current [Guo et al., 1985]. The
SCSWC consists of two distinct portions. The eastern portion
exits at the east of the Dongsha Islands and flows steadily
north-eastward; the western portion is at the west of the
Dongsha Islands and the flow path, width, salinity, and flow
velocity vary seasonally [Guan, 1985]. The eastern portion is
stronger, wider, and deeper than the western one. In addition,
the northeastward flow was observed in deeper waters east of
the Dongsha Islands [Guo et al., 1985]. In view of the existence
of the anticyclonic Kuroshio loop in this region, this deep
current is probably some combination of the SCSWC and the
return part of the anticyclonic loop [Hsueh and Zhong, 2004].
The north-east ward current discussed in this paper maybe have
some relationships with the east portion SCSWC.
[5] The north-east ward inter-middle level current, behaving
deflecting toward deep water, has been implied in some
researches. Liao et al. [2008] calculated the three-dimensional
(3-D) structure of the winter circulation in the SCS using a 3-D
diagnostic model. In their results, it can be found that the
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WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
current at 500 m on the NSCS continental shelf flows eastward
along the isobath, and then deflects isobath toward the deep
sea direction, which happens around Dongsha Islands. This
deflection current forms an eddy in the interior. The similar
structure on the NSCS continental shelf also appears on their
vertical integrated stream function distribution. Many
researches based on the hydrographic data show that eddies
occur near the Dongsha Island frequently [Liao et al., 2007,
2008; Yuan et al., 2007; Chow et al., 2008; Nan et al.,
2011]. And the mechanisms are often connected with the
shedding from the Kuroshio loop [Li et al., 1998; Jia and
Liu, 2005; Jia et al., 2004] and locally wind stress curl
[Metzger and Hurlburt, 2001; Wang et al., 2008]. Maybe the
instability generated by the deflection is another local mechanism, while in this paper we focus on the dynamic of the
deflection and discuss other related issues.
[6] The ocean current is expected to flow along the f/H
contours (where f is the Coriolis parameter, and H is water
depth) when the relative vorticity is much smaller than the
planetary vorticity. Current deflection is a feature in which
the current leaves the f/H contours that it is attached to and
overshoots to form an eddy in the interior. The offshore
deflection of the coastal jet from the coast of Vietnam is an
example of current deflection [Haidvogel et al., 1992; Gan
et al., 1997, 2004]. The deflection of the Gulf Stream from
the continental slope near Cape Hatteras also has attracted
much attention [Robinson and Niiler, 1967; Robinson
et al., 1975; Luyten, 1977]. The mechanism for current
deflection has been generally related to the formation of
the adverse pressure gradient force, induced mainly by the
intensified current (jet), wind forcing, coastal curvature,
rotation, and other factors in the boundary layer [Gan and
Qu, 2008]. Hsienwangou, Wilhelmus P.M. De Ruuter
(1985) used a two-layer model to examine the deflection of
an inertial boundary current from a curved coastline and
found that the deflection occurs where the coastline has a
large positive curvature (i.e., convex outward). Stern
(1998) used an inviscid, steady, finite-amplitude longwave
theory to study the deflection of a midlevel density current
from the bottom of a continental slope and found that, when
the cross-stream topographic slope increases gradually in the
downstream direction, the midlevel current deflects off the
bottom slope and onto the upper density interface.
[7] On the NSCS, the continental shelf is relatively
straight, and the Dongsha Islands embedding in the east
segment makes the continental shelf convex toward the deep
sea side. The kuroshio intrusion affects the baroclinic
temperature and the salinity field on the northeast
continental shelf. The special terrain and baroclinic field
complicate the NSCS oceanography background. The
behavior of the northeastward midlevel current on the NSCS
continental shelf must be related to the special terrain
characters and the baroclinic background.
[8] This paper shows the deflection characters of the NSCS
continental shelf current around Dongsha Islands and analyzes
the associated dynamics. An outline of the paper is as follows.
In section 2, the results of in situ CTD data and the velocity
observation will be shown, and the large-scale middle layer
current at the NSCS continental shelf will also be given. In
order to analyze the dynamic process in detail, a fine resolution
regional ocean model is applied in section 3. Discussions and
conclusions are given in sections 4 and 5.
2. Observation of the NSCS Continental Shelf
Current and the Deflection
2.1. NSCS Continental Shelf In Situ Observation
[9] In order to quantitatively give the current characteristics
on the NSCS continental shelf, the pressure distribution on the
continental shelf is calculated. The CTD data obtained
between 5 and 23 September in 2005 from the SCS Open
Cruise observation and the absolute dynamic topography
(ADT) data are a merged product from TOPEX/Poseidon,
ERS, and Jason-1 satellites. From the density and ADT data,
the pressure distribution at 300 and 500 m are calculated using
the hydrostatic equation.
[10] Pressure gradient is an important driving forcing in
ocean and determines the geostrophic current directly. In
Figure 1, each station’s pressure has been calculated, and
the comparison between two stations can give the
geostrophic current. The observation sections are almost
orthogonal to the isobath. From pressure distribution at the
300 and 500 m, it can be found that high pressure locates
at the outer side of the continental shelf, and the pressure
increases from the shallower water to the deep side. The
pressure gradient forcing points to the shallower water side
along all sections. Under the geostrophic constraint, the
large-scale current direction of the current is northeast.
23oN
22oN
a
0.1m/s
50
21oN
100
20oN
19oN
18oN
23oN
1.8649
1.8379
1.8077
1.777
1.7726
1.7445
1.7442
1.7431
1.6954
1.694
1.681
5001000
2000
b
o
22 N
21oN
3.0573
3.0373
3.0133
2.986
2.9813
2.9502
2.937
2.8839
2.8739
20oN
o
19 N
18oN o
112 E
o
114 E
o
116 E
o
118 E
o
120 E
Figure 1. The horizontal distribution of the pressure:
(a) 300 m and (b) 500 m. The vectors are the geostrophic
velocity calculated from the adjacent two points. The pressure
is calculated from the in situ CTD data acquired from SCS
open cruise between 5 and 23 September in 2005 and the
corresponding absolute dynamic topography (ADT) data
(T/P), and the pressure has been multiplied by 10!4 and then
subtracted 300 or 500.
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WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
2.2. Ship-Mounted ADCP
[11] The ship-mounted ADCP data were obtained during
the following three cruises: 5–23 September 2004, 11 August
to 2 September 2008, and 5–25 September 2010. In order
to remove the tides in the ship-mounted ADCP observation,
a simple but useful method has been applied [Candela
et al., 1992; Chen et al., 1994]. The fitted currents at
471.5 m are shown in Figure 2.
[12] It can be found that the current flows northeastward at
the west of the Dongsha Islands, which is consistent with the
pressure distribution, and almost along the isobath, while the
current will deflect from the isobath and veer to the deep sea
side, when flowing over the Dongsha islands. All the 3 year
observations show the similar deflection character. The
deflection occurs when the current flowing over the Dongsha
Islands where the special convex topography may push the
water to depart from the isobath (Hsienwangou, Wilhelmus
P.M. De Ruuter, 1985). The detailed dynamic analysis will
be shown later.
2.3. Mooring ADCP Observation
[13] The mooring station is located at (118" 24.4610 E,
20" 34.8510 N), east to the Dongsha Islands, and the water
depth is 2474 m. At each level of 1500 and 2000 m, an
Aanderaa current meter has been set, and sampling time is
30 min. The Aanderaa current meter at 1500 m level went
wrong after it was placed. And the velocity data have been
collected from 20 August 2000 to 17 March 2001. All the data
have been processed with low-pass filter (7 days) and then
averaged to daily mean. And the velocity vector has been
turned an angle to fit the isobaths, i.e., the longitude velocity
and latitude velocity have been replaced by cross-isobath
velocity and along-isobath velocity.
[14] Figure 3 shows the velocity vector. It can be seen that
the velocity component orthogonal to the isobaths is
dominant, and the direction toward deep sea is dominant.
Except October and late February, the direction toward the
deep sea almost did not change. From this observation, it
can be seen that the deflection not only occurs in fall but also
may occur in other seasons. We just focus on the fall in this
paper.
2.4. Climate Large-Scale Current on the NSCS
Continental Shelf
[15] In order to acquire the climate large-scale current on the
NSCS continental shelf, the geostrophic current is calculated
from the climatologic temperature and salinity data and ADT
data. To avoid the error brought by the selection of the
reference level, the geostrophic current is calculated from the
sea surface, and the sea surface geostrophic current is derived
from the ADT data [Ho et al., 2000; Li Li and Guo, 2000a,
2000b; Ge et al., 2012]. The temperature and salinity fields are
from the World Ocean Atlas 2001 (WOA01; Boyer et al.,
2005), with a space resolution of 0.25" # 0.25" , and ADT data
are the same as above except that it is the seasonal mean.
[16] The velocity at 100, 300, and 500 m are shown in
Figure 4. At the 100 m level, the southwest current
dominates the NSCS continental shelf. At the 300 and
500 m levels especially the 500 m level, there is a regular
northeast current on the continental shelf. The deflection
happens when the current flows over the Dongsha Islands,
which is consistent with the above discussion. The
consistency confirms that the deflection is not just an
occasional event, but a climate phenomenon.
3. Numerical Investigation of the Current
Deflection
3.1. Model and Forcing Function
[17] In order to investigate the deflection around Dongsha
Island, the Princeton Ocean Model (POM) [Blumberg and
o
a1
0.2m/s
0
30’
30
21 N
o
20 N
a2
500
1000
2000
30’
o
19 N
o
21 N
30’
b1
b2
c1
c2
o
20 N
30’
o
19 N
o
21 N
30’
o
20 N
30’
o
19 N o 30’
o
114 E 115 E
30’
o
116 E
30’
o
117 E
30’
o
118 E
30’
o
119 E
30’
o
114 E
30’
o
115 E
30’
o
116 E
30’
o
117 E
30’
o
118 E
30’
o
119 E
30’
Figure 2. Snapshot of the currents at 471.5 m around Dongsha Island, superimposed with the 300, 500,
1000, and 2000 m isobaths, the triangle marks Dongsha Islands. The ship-mounted ADCP current were
obtained between (a) 5 and 23 September in 2004, (b) 11 August and 2 September in 2008, and (c) 5
and 25 September in 2010. The left columns are fitted results and the right columns are the raw results.
492
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
relaxation scheme. Both the sea surface temperature and sea
surface salinity are relaxed toward the monthly mean
climatological data of WOA01, with a relaxation time scale
of 30 days for both fields.
[20] The lateral forcing of the model is provided by the
Simple Ocean Data Assimilation (SODA) reanalysis product
[Carton and Giese, 2008] which have been interpolated into
the model levels using a bilinear method, with the one-way
radiative nesting scheme proposed by Flather [1976] for the
normal component of the external mode barotropic velocity.
Non-gradient boundary condition is applied to the sea surface
high (SSH). The internal mode velocity at each level is free to
adjust geostrophically to the density field. Upstream scheme is
adopted for the open boundary conditions of both temperature
and salinity, which can best characterize the essence of the
hyperbolic equations.
[21] The model is integrated from a state of rest. Under the
January wind forcing, the model is integrated for 10 years,
and the velocity, temperature, salinity, and SSH all adjust
accordingly to reach a dynamic balance. After 10 years of
integration, the results reach an equilibrium state, as
indicated by the time series of the volume-averaged kinetic
energy. After that, the model is forced by seasonal winds.
Output from the 20th year is sampled daily and averaged over
30 days for the analysis.
5cm/s
01Mar
Along
Vertical (to deep)
01Feb
N
01Jan
Time(day)
22N
3 00
00
0
50 10 2000
01Dec
24
21
00
00
26
20
01Nov
116
117
118
119E
01Oct
01Sep
o
23 N
2000
Depth(m)
a
o
22 N
Figure 3. The velocity data are from mooring station ADCP
observation from 20 August 2000 to 17 March 2001, and
collected from Aanderaa current meter at 1500 and 2000 m.
“Along” means along the isobaths and “Vertical” means
vertical to the isobaths. Terrain distribution around Dongsha
Island is shown in the small box, with the mooring stations
marked by solid pentacle and Dongsha Islands marked by
solid triangle.
0.1m/s
50
100
1500
o
21 N
o
20 N
0
50
o
19 N
2000
1000
o
18 N
o
23 N
b
o
22 N
Mellor, 1987] which can well describe the topography has
been utilized. The POM is a hydrostatic, free-surface, sigmacoordinate, primitive equation model and is propitious for
studying nonlinear dynamics over a shallow, gently sloping
continental shelf.
[18] The model domain includes the whole SCS, southern
East China Sea, and part of the western Pacific Ocean, in
order to achieve a better simulation of the Kuroshio intrusion
through the Luzon Strait (Figure 5a). ETOPO2 [Marks and
Smith, 2006] is used to prescribe the bathymetry in the model
domain. The horizontal orthogonal curvilinear coordinates are
utilized to improve the resolution of the currents on the NSCS
for the coastline fitting and local refinement. The model grid
has a spatial scale ranging from 10 to 30 km with an average
of about 13 km. The vertical sigma-coordinate has 25 levels,
which are logarithmically distributed with higher resolution
near the surface and the bottom in order to better resolve the
surface and bottom Ekman layers.
[19] The surface momentum flux is calculated from the
monthly mean climatology based on 40 years of the ERA40
data set [Uppala et al., 2005]. The model is initialized by the
January climatological hydrographic data (WOA01) with
0.25" resolution. The total surface heat flux is replaced by a
o
21 N
o
20 N
o
19 N
o
18 N
o
23 N
c
o
22 N
o
21 N
o
20 N
o
19 N
o
18 N
o
110 E
o
112 E
o
114 E
o
116 E
o
118 E
Figure 4. The geostrophic velocity vectors at (a) 100 m,
(b) 300 m, and (c) 500 m in fall. The geostrophic velocity
is derived from the climatologic hydrography data (WOA01)
and absolute dynamic topography (ADT). Dongsha Islands
are marked by solid triangle.
493
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
o
o
30 N
24 N
a
b
China
o
24 N
200
0
500
o
22 N
100
2000
o
Vietnam
18 N
o
12 N
o
20 N
o
6N
0
3500
o
o
o
102 E
o
108 E
o
114 E
o
120 E
o
126 E
o
132 E
18 N
o
115 E
o
117 E
o
119 E
Figure 5. Model domain. (a) Regional forecast model domain and computational grids and (b) terrain
distribution around Dongsha Island, with the mooring stations marked by solid pentacle and the triangle
marks Dongsha Islands. The two dashed lines are the selected sections in Figure 10.
3.2. Validation of Model Results
[22] Observational data for the velocity field in the SCS
are very limited. First, the data from satellite remote sensing
have been used for qualitative extraction of the large-scale
circulation characteristics. The simulated sea surface height
anomaly (SSHA) and satellite altimetry data are compared
(Figure 6).
[23] Figure 6 shows that the simulated and observed
SSHAs well resemble each other. They both show the
negative/positive SSHA centers to the west of Philippines
and to the east off Vietnam in the winter/summer, which
denote the seasonal current transition characters. In fall, the
negative SSHAs occupy the deep ocean and the high SSHA
belt exists on the NSCS continental shelf in both fields.
These consistencies suggest that the model is able to
realistically capture the dynamic conditions in SCS.
[24] In order to further validate model results, we compare
the modeled and observed vertical structures of currents at a
mooring station (marked in Figure 5b by solid pentacle) east
to the Dongsha Islands. The observed data are then averaged
to monthly mean (Figure 7). From the comparison, it can be
seen that the model results to a little underestimate of the
current amplitude, while the direction is similar.
[25] Deflection is anther criterion of the model result
validation. The mean flow in fall at 500 m is shown in
Figure 8. The current on the NSCS continent west to the
Dongsha Islands almost flows along the isobath and deflects
from the isobath veering to the deeper sea side when flowing
over the Dongsha Islands. The current deflection characters
are well reproduced.
[26] Based on the agreement, the model results are used to
investigate the possible dynamic mechanism for the current
deflection when flowing over the Dongsha Islands.
3.3. Dynamical Analyses
[27] The ocean currents are expected to flow along the f/H
contours when the relative vorticity is much smaller than the
planetary vorticity. The north-south scale of the deflection
feature around the Dongsha Islands is very small, about
100 km, which means that the variability of the Coriolis
parameter is very small, so the f-plane can be used. Then,
the isobath is equivalent to the f/H contour in some extent.
From the earlier discussion, we saw that the current around
the Dongsha Islands does not follow the f/H contour, but
deflects from the f/H contour and veers toward the deep
water after flowing over the Dongsha Islands. Then, the
velocity appears to be perpendicular to the isobath and the
joint effect of bottom slope and density field arises. In order
to clarify the dynamic mechanism of the deflection, the
vorticity balance and momentum balance are investigated
in the following sections.
3.3.1. Vorticity Balance
[28] Cross-differentiate the vertically integrated momentum
equation will result in the vertically integrated vorticity
equation [Wang et al., 2010]
# ! "
! "$
#
! "
! "$
@ @ !v
@ !u
@ f
@ f
!
þ !v%
¼ ! !u%
@t @x D
@y D
@x D
@y D
a
!
" b
! "
! "
1
F
A
þ curl
! curl
þ J Φ;
D
D
D
c
d!
!
"
" e
ta
tb
! curl
þ curl
r0 D
r0 D
(3:1)
f
g
where ( !u , !v ) represents the vertically integrated velocity,
Z !
%
&
J Φ; D1 is the JEBAR term, Φ ¼
zgr=r0 dz is the
!H
potential energy, r is the density, ! is the surface elevation,
D = H + ! is the water column depth, (tx,ty,)a and (t
are
x,ty,)b !
!
the!surface and
bottom
stresses,
respectively,
and
F
¼
F
i
þ
x
!
!
!
Fy j and A ¼ Ax i þ Ay j are the vertically integrated
horizontal diffusion term and the nonlinear advection term,
respectively. The left-hand side of equation (3.1) includes
(a) the tendency term. The right-hand side of equation
494
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
a1
a2
b1
b2
c1
c2
d1
d2
3.3.2. JEBAR Effect on the Deflection
[31] Sakamoto and Yamagata [1996] first explicitly applied
the dynamic (i.e., time-dependent) aspects of JEBAR to the
seasonal variation of a wind-driven gyre. Generally, JEBAR
represents the torque exerted on the fluid column by the joint
action of density and topographic gradients to drive it from
following the f/H contour [e.g., Sarkisyan and Ivanov, 1971;
Holland, 1973; Mellor et al., 1982; Myers et al., 1996; Isobe,
2000; Guo et al., 2003]. Two sections along the isobath
(marketed in Figure 6b) have been selected to clarify the
relationship between the JEBAR and the deflection.
[32] The regions bounded between 900 and 1000 m and
between 1400 and 1500 m are selected for the analyses of
the vertical deflection structure. The vertical profile of the
across-shelf flow together with the vorticity terms is shown
in Figure 10. The velocity profile and the vorticity terms are
all averaged in the across-shelf direction in the two selected
ranges. Negative value denotes the current flowing toward
the deep sea, and the positive value is the opposite. It can be
seen that the vertical structure (except the surface and the
bottom) presents the same character that the direction is almost
consistent for both ranges. Each offshore (onshore) flow corresponds to the negative (positive) JEBAR. The JEBAR term is
the dominant one of such deflection in this area based on the
term-by-term analyses in Figure 9. The JEBAR term supplies
the negative vorticity, so there must be positive vorticity
transport in order to maintain the vorticity balance (i.e.,
offshore flow). On the west of the Dongsha Islands, the water
column on the shallower field which contains larger planet
potential vorticity (f/D) has to flow across the isobaths toward
the deeper water where the smaller planet potential vorticity
exists and induces the positive planet potential vorticity
advection to balance the JEBAR term. And then, the
deflection occurs.
20oN
16oN
12oN
8oN
4oN
24oN
20oN
16oN
12oN
8oN
4oN
24oN
20oN
16oN
12oN
8oN
4oN
24oN
20oN
16oN
12oN
8oN
o
4N
100oE104oE108oE112oE116oE120oE
0.1
0.05
100oE104oE108oE112oE116oE120oE
0
0.05
0.1
Figure 6. Mean sea surface height anomaly (m) from
model (left) and derived from T/P altimetry data from winter
to fall: (a) winter, (b) spring, (c) summer, and (d) fall.
(3.1) includes (b) advection of the planet potential vorticity
f/D (APV) term, (c) the JEBAR term, (d) the diffusion term,
(e) the advection term, (f) surface stress torque, and (g) bottom stress torque. All the terms are diagnosed using the
model results. In the quasi-steady state, the tendency term
should approximately be zero.
[29] The APV term denotes the cross-isobath transport of the
planetary potential vorticity. If the APV term is positive, the
transport is in the offshore direction and the current tends to
veer toward the deep ocean [Sakamoto and Yamagata, 1996;
Guo et al., 2003].
[30] Figure 9 displays the spatial distribution of each term in
equation (3.1) in fall, limited to the deflection domain. The
JEBAR and APV terms are dominant, and the advection term
is secondary. Comparing between the JEBAR distribution and
current deflection, it can be seen that the deflection occurs at
the negative JEBAR region. Negative JEBAR supplies
negative vorticity, and the current has to anticyclone bend
under the negative JEBAR.
3.3.3. Factors Influencing JEBAR Effect
[33] The magnitude and sign of the JEBAR term are
determined by both the topographic gradient and horizontal
density gradient. Figure 11 is the horizontal distribution of
the density from WOA01 and simulated results integrated
in the upper 500 m. Both present the same character that
the density isoline is almost orthogonal to the isobaths at
the east of the Dongsha Islands. This special structure is
the important factor that determines the JEBAR distribution.
The along-isobath density gradient interacts with the
topography and generates large negative JEBAR distribution
which makes the current direction turn to the deep side.
N
Model
Obs
0.02m/s
1500
Depth(m)
24oN
2000
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Time
Figure 7. Comparison between model result and observation. The station is marked in Figure 5b by solid pentacle.
495
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
o
22 N
0.1m/s
maintains the eastward current. The acceleration of the
current is negative, and the ageostrophic term is also
dominant, in which the Coriolis forcing is the active driver.
The deflection process can be described as that the along-isobath pressure gradient pushes the current to accelerate and the
Coriolis forcing vertical to the isobath increases gradually,
and then becomes larger than the cross-isobath pressure gradient forcing. So, the cross-isobath acceleration is negative, and
then the current is pushed to deflect from the isobath toward
the deep sea side, after enough time to accelerate.
50
o
21 N
100
o
20 N
500
o
19 N
1000
2000
o
b
3.4. Numerical Experiments
[37] From the above analysis, it can be seen that the
topography of the Dongsha Islands plays an important role in
the current deflection. In order to investigate the contribution
of the Dongsha Islands topography, a simple numerical
experiment has been designed.
[38] The forcing function is the same as the control run, but
the topography around the Dongsha Islands has been changed.
o
22 N
o
21 N
o
20 N
o
19 N
o
c
o
21 N
o
22 N
a
40’
21 N
20’
o
20 N
0
800
o
20 N
o
19 N
40’
21 N
o
o
110 E
o
112 E
o
114 E
o
116 E
o
118 E
b
40’
Figure 8. Velocity field at (a) 100 m, (b) 300, and
(c) 500 m in fall, superimposed with the 50, 100, 500,
1000, and 2000 m isobaths. The triangle marks Dongsha
Islands, and mooring stations marked by solid pentacle.
The red line is a stream line that we selected to analyze the
momentum balance in Figure 12.
20’
0
800
o
20 N
x 10
11
40’
o
21 N
3
c
40’
30
2
0
500
20’
3.3.4. Momentum Balance
[34] Here, we present the momentum balance to identify the
dynamic cause of the deflection. In order to analyze the current
deflection dynamic characteristics, the coordinate has been
transformed to the isobath coordinate and the momentum
balance is based on the Euler equations (see Appendix A).
We selected a stream line at 500 m level around the Dongsha
Island (see Figure 8) to analyze the momentum balance
(Figure 12).
[35] In both directions, the geostrophic balance dominates
the continental shelf current. The along-isobath acceleration is
positive during the deflection, and the ageostrophic term is
the main contribution, which demonstrates the along-isobath
pressure gradient is the active force that pushes the current to
accelerate. From the above discussion, it can be found that
the vertical integrated density gradient along the isobath at
the Dongsha Island is westward, and then the along-isobath
pressure gradient forcing is positive. The positive pressure gradient forcing is stronger than the Coriolis forcing and pushes
the current to accelerate.
[36] In the cross-isobath direction, the pressure gradient
forcing is positive which is consistent with the observation
(Figure 1). The high pressure at the continental shelf outer
30
500
10
0
15 0
00
o
18 N
30
500
o
1
10
0
15 0
00
18 N
o
23 N
800
0
o
1
20 N
2
40’
o
21 N
3
d
40’
30
0
500
20’
10
0
15 0
00
18 N
o
23 N
10
0
15 0
00
a
o
800
o
20 N
40’
o
21 N
e
40’
30
0
500
20’
10
0
15 0
00
23 N
800
o
20 N
40’
o
116 E
20’
40’
o
117 E
20’
40’
o
118 E
Figure 9. Spatial distribution of the vorticity terms in fall,
superimposed with the isobaths.(a) JEBAR, (b) advection of
the geostrophic potential vorticity, (c) advection plus diffusion, (d) surface stress torque, and (e) bottom stress torque.
496
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
a1
Depth(m)
200
a2
200
400
0.15
0.1
0.05
600
0
600
0.05
1000
800
0.1
Vorticity term balance(m/s2)
1400
3
0.15
b1
2
b2
1
0
1
2
3
116
116.5
117
117.5
jebar
118
beta
116
116.5
adv+dif
117
windcurl
117.5
118
bocurl
Figure 10. Vertical profile for across-isobaths velocity (m/s; upper) and vorticity (m/s2; lower) along the
continental slope near Dongsha Island. The velocity and vorticity terms are the results averaged in the
across-isobath direction (a) between 900 and 1000 m, or (b) between 1400 and 1500 m. The positive area
in the velocity profile denotes the onshore flow.
o
23 N
a
b
50
o
25
o
25.
21 N
25.4
500
o
20 N
1000
2000
.3
5
4
.4
100
25
22 N
5
.5
25
o
19 N o
114 E
5
25.4
25.5
o
116 E
o
o
118 E
25.2
o
120 E
25.3
25.4
o
114 E
25.5
25.6
116 E
25.7
o
118 E
o
120 E
25.8
Figure 11. Horizontal distribution of density (a) WOA01 and (b) model result integrated in the upper
500 m in fall, superimposed with the 50, 100, 500, 1000, and 2000 m isobaths.
The changed domain has been shown in Figure 13. In order to
remove the convex topography, five-point smooth has been
utilized.
[39] The numerical experiment currents have been shown in
Figure 14. It can be found that the northeastward midlevel
current still exists, while the deflection disappears. This current flows along the isobath and combines with the intrusion
of the Kuroshio at the southwest of Taiwan. This numerical
experiment explores the important role of the topography in
the deflection.
4. Discussion
[40] In the NSCS, the current on the continental shelf flows
along the isobath west to the Dongsha Islands, and then veers
toward the deep water when flowing over the islands,
deflecting from the isobath. Dynamic analysis shows that the
JEBAR term is an important balance term that derives the
current’s departure from the isobath toward the deep water
when flowing over the Dongsha Islands. We want to obtain a
clear picture about the JEBAR term’s contribution to the
current’s departure from the isobath. The linear vertical
integrated vorticity equation can be written as
! ¼ JEBAR þ WSR þ BSR
r%f u
(4:1)
! "
d f
ðJEBAR þ WSR þ BSRÞ
¼
dt H
H
(4:2)
! Z 0
"
Z
1
1 0
!¼
where u
udz;
vdz is the vertical inteH !H
H !H
grated velocity, and on the right-hand side the WSR is the
wind stress curl and BSR term is bottom stress curl. The
variation of the potential vorticity is mainly determined by
the change of the water depth. For the relative vorticity is
much smaller than the planetary vorticity, the variation of
x/H is much smaller than the f/H and the relative vorticity
has been omitted. Equation (4.1) can be rewritten as follows,
after the Boussinesq approximation has been used:
[41] Following the discussion in section 3.3, it has been
found that the JEBAR is the dominant term, and then the linear
vertical integrated vorticity equation can been simplified to
497
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
10
6
cor
x
prex
a1
cor
y
prey
b1
Momentum term balance(m/s2)
2
2
6
10
0.2
agex
ace
x
difh
x
difv
x
A
a2
0.1
agey
ace
y
difh
y
difv
y
A
b2
x
y
0
0.1
0.2
116.5
117
117.5
116.5
117
Longitude
117.5
Longitude
Figure 12. Momentum balance along the stream line at 500 m (shown in Figure 8). cor, Coriolis force;
pre, pressure gradient; age, ageostrophic term; ace, acceleration; difh, horizontal diffusion; difv, vertical
diffusion; A, terrain term (all multiplied by 106). x denotes the direction along the stream, and y denotes
the direction orthogonal to the stream.
o
24 N
a
b
o
22 N
o
20 N
200
200
500
500
00
10
00
10
2000
3500
0
200
3500
o
18 N
o
115 E
o
117 E
o
o
115 E
119 E
o
117 E
o
119 E
Figure 13. The distribution of the topography in (a) control experiment and in the (b) sensitive
experiment. The topography is changed in the rectangle domain.
! "
d f
JEBAR
¼
dt H
H
(4:3)
where Hf is the planet potential vorticity.
[42] Equation (4.3) is a simple but useful expression of the
linear vertical integrated vorticity equation to explain the
JEBAR effect on the current. It is known that the current
without any forcing is expected to flow along the isoline of
f þz
H , where z is the relative vorticity [Pedlosky, 1996]. In most
cases, z is very small compared to the planet vorticity, so Hf is a
good approximation. That is to say all the terms on the righthand side of equation (4.2) equal to zero, which gives the
potential vorticity conservation. However, when the JEBAR
terms are involved in equation (4.2), the current departs from
the isoline of Hf . JEBAR term can be interpreted as the bottom
torque, which is simply the curl of the horizontal force exerted
by the bottom on the fluid to change the stretching of the vortex tube, and also analogous as a transport-generating term in
that it is a baroclinic effect of flow across contours of f/H
[Mertz and Wright, 1992]. As in equation (4.2), JEBAR is
analogous to the curl of the surface and bottom torque, and
may play the same role of PV input and dissipation like the
wind stress and bottom friction which are in the form of curl
(t)/H (where t stands for the wind stress and bottom friction).
498
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
o
23 N
o
Islands. The along-isobath gradient of the density field interacts
with the topography that supplies negative potential vorticity
and drives the column-integrated flow away from the isobaths,
veering toward the deep water.
[47] The momentum balance shows that the along-isobath
pressure gradient forcing accelerates the current when
flowing over the Dongsha Islands, and then the cross-isobath
Coriolis forcing is increased (the sign is negative and the
absolute value is increased). The cross-isobath Coriolis
forcing becomes larger than the cross-isobath pressure
gradient forcing, and then pushes the current deflected from
the isobath and turned to the deep sea side.
[48] The along isobath density gradient at the east to the
Dongsha Islands is the mainly active factor that makes the
current deflect from the isobath toward the deep sea side.
However, the reason how the density gradient occurs is not
investigated in this paper, and maybe related to the Kuroshio
intrusion. The generation of the background density fields
needs further investigation.
a
0.1m/s
22 N
50
o
21 N
100
o
20 N
0
50
0
100
0
200
o
19 N
o
23 N
b
o
22 N
50
o
21 N
o
100
20 N
0
50
1000
0
200
o
19 N o
o
o
o
o
o
o
o
112 E 113 E 114 E 115 E 116 E 117 E 118 E 119 E
Appendix A
Figure 14. The current distribution at (a) 300 m and
(b) 500 m, from the sensitive experiment.
So, when the wind stress and bottom friction are omitted, the
JEBAR/H term serves as the PV input or dissipation term.
[43] The relative vorticity calculated from the model
output around the Dongsha Islands is smaller than the planet
vorticity by one order at least (Figure 16a, which is
calculated along the streamline marked in Figure 15), and the
nonlinear influence is also smaller than the geostrophic terms
(Figures 10 and 11). In order to diagnose the relationship
between the JEBAR/H and the variability of the planetary
potential vorticity, we give Figure 16b which is also calculated
along the streamline marked in Figure 15. It can be found that
the equation (4.3) is a good approximate. Based on the above
discussion that the JEBAR is the dominant term, equation
(4.3) is a good simplified model to explain the deflection east
of the Dongsha Islands. Near the Dongsha Islands, the f-plane
is a good approximation for the scale of the domain is so small,
and then the variation of topography is the dominant factor that
controls the planet potential vorticity.
[44] From the distribution of the JEBAR (Figures 10 and
11), it can be found that there is negative JEBAR at the east
of the Dongsha Islands; in responding to the negative JEBAR,
the current must flow toward to the field where smaller Hf occurs,
under the constraint of the potential vorticity balance of
equation (4.3). A schematic of the Dongsha Islands inter-middle
current deflection mechanism has been given (Figure 17).
[49] In order to reveal the relationship between the isobath
and the velocity, the momentum equations are rewritten in
the isobath-coordinate (Figure A1). The x and y directions
denote the eastward and the northward directions, respectively.
θ is the angle that the isobath deflects from the eastward. In
the x-y coordinate, the momentum equations can be written as
du
¼ Fx
dt
dv
¼ Fy
dt
where (u, v) denote the longitude and latitude velocity, and
the (Fx,Fy) denote the forcing terms. The forcing terms can
be written as [Wang et al., 2010]
Fx ¼ Am uxx þ fv ! Px þ ðKm us Þs
!
a
!b !
d
c !
(A2)
Fy ¼ Am vxx ! fu ! Py þ ðKm us Þs
!
a
!b !
c
!
d
o
23 N
o
22 N
o
21 N
5. Conclusion
o
[45] From the observation in fall, it is found that the
northeastward midlevel current on the NSCS continental shelf
deflected from the isobath veering to the deep sea side when
flowing over the Dongsha Islands. In order to investigate the dynamic mechanism, a fine resolution regional ocean model has
been used. Some conclusions can be summarized as follows.
[46] The vorticity balance analysis based on the ocean model
shows that the JEBAR term is the dominant balance term that
makes the current depart from the isobaths around the Dongsha
(A1)
20 N
0
50
2000
00
10
o
19 N o
114 E
o
115 E
o
116 E
o
117 E
o
118 E
o
119 E
o
120 E
Figure 15. The barotropic stream function distribution.
The black dash lines are isobath. The red dash line is
selected to analyze the relationship between the variability
of planetary potential vorticity and the JEBAR.
499
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
7
x 10
8
6
Relative PV
Planetary PV
6
Potential Vorticity (m 1s 1)
13
d(f/H)/dt
JEBAR/H
b
Variability of Potential Vorticity (m 1s 2)
a
x 10
5
4
3
2
1
0
1
116 116.5 117 117.5 118 118.5
4
2
0
2
4
6
8
116 116.5 117 117.5 118 118.5
Longitude
Longitude
Figure 16. The (a) distribution of the potential vorticity and the (b) relationship between the variability
of the planetary potential vorticity and JEBAR, which are selected along the barotropic stream line in
Figure 15.
where (a) horizontal diffusion (difh), (b) Coriolis (cor), (c)
pressure gradient (pre), and (d) vertical diffusion (difv).
[50] In isobath-coordinate, the momentum equations can
be rewritten as
du
dv
cosðθÞ þ sinðθÞ ¼ Ft
dt
dt
dv
du
cosðθÞ !
sinðθÞ ¼ Fn
dt
dt
where ug is the velocity that along the isobath and vg is vertical to the isobath. To simplify the momentum equations, it
can been given as
!
"
dug
@θ
@θ
vg
¼ Ft þ u þ v
@x
@y
dt
!
"
dvg
@θ
@θ
ug
¼ Fn ! u þ v
@x
@y
dt
(A3)
[51] The velocity in the isobath-coordinate can also been
given as
ug ¼ u cosðθÞ þ v sinðθÞ
vg ¼ !u sinðθÞ þ v cosðθÞ
(A4)
(A5)
[52] The second term on the right side of equation (A5) is
the joint effect of the velocity and the terrain, in this paper, it
is called terrain term.
[53] Acknowledgments. This work is supported by National Basic
Research Program of China (Grant No. 2011CB403504), Natural Science
1
ath
0.8
isob
Dongsha Islands
JEBAR
y
0.6
n
τ
0.4
d f
dt H
JEBAR
H
0.2
θ
Figure 17. The schematic of the deflection of the Dongsha
Islands inter-middle current. The red current denotes the deflection currents discussed in our paper, and the black vector at the
east to the Dongsha Islands along the isobath denotes the current from the Seismic reflectometer data [Shao et al., 2007].
0
0
0.2
0.4
0.6
0.8
1
x
Figure A1. Schematic diagram of the x-y coordinate and
the isobath-coordinate.
500
WANG ET AL.: DEFLECTION AROUND DONGSHA IN NSCS
Foundation of China (Grant No. 91228202), Knowledge Innovation Project
for Distinguished Young Scholar of the Chinese Academy of Sciences
(Grant No. KZCX2-EW-QN203) and National Natural Science Foundation
of China (Grant No. 40776009). We would like to thank two anonymous
reviewers for comments that helped to improve the manuscript.
References
Blumberg, A. F., and G. L. Mellor (1987), A description of a three-dimensional coastal ocean circulation model, in Three-Dimensional Coastal
Ocean Models, edited by N. Heaps, pp. 1–16, AGU, Washington, D. C.
Boyer, T., S. Levitus, H. Garcia, R. Locarnini, C. Stephens, and J. Antonov
(2005), Objective analyses of annual, seasonal, and monthly temperature
and salinity for the world ocean on a 0.25" grid, J. Climate, 25, 931–945.
Candela, J., R. C. Beardsley, and R. Limeburner (1992), Separation of tidal
and subtidal currents in ship-mounted acoustic Doppler current profiler
observations, J. Geophys. Res., 97(C1):769–788.
Carton, J. A., and B. S. Giese (2008), A reanalysis of ocean climate using
SODA, Mon. Weather Rev., 136, 2999–3017.
Chen, C., R. C. Beardsley, and R. Limeburner (1994), Variability of currents in late spring in the northern Great South Channel, Continent Shelf.
Res., 15(4/5):451–473.
Chow, C.-H., J.-H. Hu, L. R. Centurioni, and P. P. Niile (2008), Mesoscale
Dongsha cyclonic eddy in the northern South China Sea by drifter and
satellite observations, J. Geophys. Res., 113, C04018, doi:10.1029/
2007JC004542.
Dong, D. P., W. D. Zhou, Y. Yang, and Y. Du (2010), On outflow passages
in the South China Sea, Adv. Atmos. Sci, 27(1), 60–68.
Flather, R. A. (1976), A tidal model of the northwest European continental
shelf, Mem. Soc. Roy. Sci. Liege, Ser., 6(10), 141–164.
Gan, J. P., R. G. Ingram, and R. Greatbatch (1997), On the unsteady separation/intrusion of Gaspe Current and variability in Baiedes Chaleurs,
modelling studies, J. Geophys. Res., 102, 15,567–15,581.
Gan, J. P., R. G. Ingram, R. Greatbatch, and T. Baaren (2004), Variability
of circulation induced by the separation of Gaspe Current in Baie des
Chaleurs: Observational studies, Estuar. Coast. Shelf Sci., 61, 393–402.
Gan, J. P., and T. D. Qu (2008), Coastal jet separation and associated
flow variability in the southwest South China Sea, Deep Sea Res. I,
55, 1–19.
Ge, L. L., X. H. Cheng, Y. Q. Qi, and D. X. Wang (2012), Upper-layer
geostrophic volume, heat and salt transports across 18" N in the South
China Sea (in Chinese), J. Trop. Oceanogr., 1(31):10–17.
Guan, B. (1985), Some features of the temporal and spatial distributions
of the “counter-wind” current in northern South China Sea in winter
(in Chinese), Oceanol. Limnol. Sin., 16(6), 429–438.
Guo, X., H. Hukuda, Y. Miyazawa, and T. Yamagata (2003), A triply
nested ocean model for simulating the Kuroshio—Roles of horizontal
resolution on JEBAR, J. Phys. Oceanogr., 33, 146–169.
Guo, Z., T. Yang, and D. Qiu (1985), The South China Sea Warm Current
and the SW-ward current on its right side in winter (in Chinese), Trop.
Oceanol., 4(1), 1–9.
Li L., W. D. Nowlin Jr., and S. Jilan (1998), Anticyclonic rings from the
Kuroshio in the South China Sea, Deep Sea Res. I, 45:1469–1482.
Li L., R. H. Wu, and X. G. Guo (2000a), Seasonal circulation in the South
China Sea-a TOPEX/POSEIDON satellite altimetry study (in Chinese),
J. Oceanol. Sin., 22(6): 13–26.
Liao, G. H., Y. C. Yuan, and X. H. Xu (2007), Diagnostic calculation of the
circulation in the South China Sea during summer 1998, J. Oceanogr, 63,
161–178.
Liao, G. H., Y. C. Yuan, and X. H. Xu (2008), Three dimensional diagnostic study of the circulation in the South China Sea during winter 1998,
J. Oceanogr, 64, 803–814.
Li L., R. H. Wu, and X. G. Guo (2000b), Seasonal circulation in the South
China Sea-a TOPEX/POSEIDON satellite altimetry study (in Chinese), J.
Oceanol. Sin., 22(6), 13–26.
Luyten, J. R. (1977), Scales of motion in the deep Gulf Stream and across
the continental rise, J. Mar. Res., 35, 49–74.
Haidvogel, A. B., J. C. McWilliams, and P. R. Gent (1992), Boundary current separation in a quasigeostrophic eddy-resolving ocean circulation
model, J. Phys. Oceanogr., 22, 882–902.
Ho, C. R., N. J. Kuo, Q. Zheng, and Y. S. Soong (2000), Dynamically active areas in the South China Sea detected from TOPEX/POSEIDON satellite altimeter data, Rem. Sens., 71, 320–328.
Holland, W. R. (1973), Baroclinic and topographic influences on the
transport in western boundary currents, Geophys. Fluid Dynam., 4,
187–210.
Hsienwangou, Wilhelmus P.M. De Ruuter (1985), Separation of an
inertial boundary current from a curved coastline, J. Phys. Oceanogr.,
16, 280–289.
Hsueh, Y., and L. Zhong (2004), A pressure-driven South China Sea Warm
Current, J. Geophys. Res., 109, C09014, doi:10.1029/2004JC002374, 2.
Isobe, A. (2000), Two-layer model on the branching of the Kuroshio
Southwest of Kyushu, Japan, J. Phys. Oceanogr., 30, 2461–2476.
Jia, Y. L., and Q. Y. Liu (2005), Eddy shedding from the Kuroshio Bend at
Luzon Strait, J. Oceanogr, 60:1063–1069.
Jia, Y. L., Q. Y. Liu, and W. Wei (2004), Primary study of the mechanism
of eddy shedding from the Kuroshio Bend in Luzon Strait, J. Oceanogr,
61, 1017–1027.
Marks, K. M., and W. H. F. Smith (2006), An evaluation of publicity available global bathymetry grids, Mar. Geophys. Res., 27, 19–34.
Mellor, G. L., C. Mechoso, and E. Keto (1982), A diagnostic calculation
of the general circulation of the Atlantic Ocean, Deep Sea Res., 29,
1171–1192.
Mertz, G., and D. G. Wright (1992), Interpretations of the JEBAR term, J.
Phys. Oceanogr., 22, 301–305.
Metzger, E. J., and H. E. Hurlburt (2001), The nondeterministic nature of
Kuroshio penetration and eddy shedding in the South China Sea,
J. Phys. Oceanogr., 31, 1712–1732.
Myers, P. G., A. F. Fanning, and A. J. Weaver (1996), JEBAR, bottom pressure torque, and Gulf stream separation, J. Phys. Oceanogr., 26, 671–683.
Nan, F., H. Xue, P. Xiu, F. Chai, M. Shi, and P. Guo (2011), Oceanic eddy
formation and propagation southwest of Taiwan, J. Geophys. Res.,
doi:10.1029/2011JC007386.
Pedlosky, J. (1996), Ocean Circulation Theory, 453 pp., Spring-Verlag,
New York, NY.
Robinson, A. R., J. R. Luyten, and G. Flierl (1975), On the theory of thin
rotating jets: A quasi-geostrophic time dependent model, Geophys. Fluid
Dynam., 6, 211–244.
Robinson, A. R., and P. P. Niiler (1967), The theory of free inertial currents:
I, Path Struct. Tellus, 19(2), 269–291.
Sakamoto, T., and T. Yamagata (1996), Seasonal transport variations of the
wind-driven ocean circulation in a two-layer planetary geostrophic model
with a continental slope, J. Mar. Res., 54, 261–284.
Sarkisyan, A. S., and V. F. Ivanov (1971), Joint effect of baroclinicity and
bottom relief as an important factor in the dynamics of sea currents,
Izv. Acad. Sci., USSR Atmos. and Oceanic Phys. (English Translation)
7, 173–178.
Shao, L., X. J. Li, J. H. Geng, X. Pang, Y. C. Lei, P. J. Qiao, L. L. Wang,
and H. B. Wang (2007), Deep water bottom current deposition in the
northern South China Sea, Sci. China D: Earth Sci., 50(7), 1060–1066.
Shu Y. Q., D. X. Wang, J. Zhu, and S. Q. Peng (2011), The 4-D structure of
upwelling and Pearl River plume in the northern South China Sea
during summer 2008 revealed by a data assimilation model, Ocean
Model., 36, 228–241.
Stern, M. E. (1998), Separation of a midlevel density current from the
bottom of a continental slope, Proc. Natl. Acad. Sci. USA, 96, 1206–1211.
Uppala, S. M., et al. (2005), The ERA-40 re-analysis, Q. J. Roy. Meteorol.
Soc., 131, 2961–3012.
Wang, D. X., B. Hong, J. P. Gan, and H. Z. Xu (2010), Numerical investigation on propulsion of the counter-wind current in the northern South
China Sea in winter, Deep Sea Res., doi:10.1016/j.dsr.2102.06.07.
Wang, H. Q., Y. C. Yuan, W. B. Guan, R. Y. Lou, and K. S. Wang (2004a),
Circulation in the South China Sea during summer 2000 as obtained from
observations and a generalized topography-following ocean model,
J. Geophys. Res., 109, C07007, doi:10.1029/2003JC002134.
Wang, H., Y. Yuan, W. Guan, R. Lou, and K. Wang (2004b), Circulation in
the South China Sea during summer 2000 as obtained from observations
and a generalized topography-following ocean model, J. Geophys. Res.,
109, C07007.
Wang, G., D. Chen, and J. Su (2008), Winter eddy genesis in the eastern
South China Sea due to orographic wind jets, J. Phys. Oceanogr.,
38:726–732.
Wang, Q., Y. X. Wang, B. Hong, W. D. Zhou, and D. X. Wang (2011),
Different roles of Ekman pumping in the west and east segments of the
South China Sea Warm Current, J. Oceanol. Sin., 30(3), 1–13.
Wyrtki, K. (1961), Physical oceanography of the Southeast Asian waters,
scientific results of marine investigations of the South China Sea and
the Gulf of Thailand. NAGA Report, Scripps Institution of oceanography,
La Jolla, CA, 2, 195.
Yuan, Y.C., G. H. Liao, and X. H. Xu (2007), Three dimensional diagnostic
modeling study of the South China Sea circulation before onset of
summer monsoon in 1998, J. Oceanogr, 63, 77–100.
Yuan, Y. C., G. H. Liao, and C. G. Yang (2008), The Kuroshio near the
Luzon Strait and circulation in the Northern South China Sea during
August and September1994, J. Oceanogr, 64, 777–788.
Shu, Y. Q., D. X. Wang, J. Zhu, and S. Q. Peng (2011), The 4-D structure of
upwelling and Pearl River plume in the northern South China Sea during
summer 2008 revealed by a data assimilation model, Ocean Model., 36, 228–241.
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