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ARTICLE IN PRESS
Continental Shelf Research 29 (2009) 1896–1907

Contents lists available at ScienceDirect

Continental Shelf Research
journal homepage: www.elsevier.com/locate/csr

Biweekly periodic variation of the Kuroshio axis northeast of Taiwan asrevealed by ocean high-frequency radar
Daisuke Takahashi a,, Xinyu Guo b, Akihiko Morimoto a, Shoichiro Kojima caHydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japanc
Okinawa Subtropical Environment Remote Sensing Center, National Institute of Information and Communications Technology, 4484 Aza-Onna, Onna, Kunigami,
Okinawa 904-0411, Japanba r t i c l e in f o

a b s t r a c t

Article history:
Received 27 February 2009
Received in revised form
16 June 2009
Accepted 13 July 2009
Available online 21 July 2009

An analysis of surface current data obtained from 2002 to 2005 using long-range high-frequency radar
provides the ﬁrst evidence for the presence of biweekly (11–14 day) periodic variations of the Kuroshio
axis northeast of Taiwan. This analysis clariﬁes the spatiotemporal characteristics of these variations
and reveals that cyclonic/anticyclonic eddies propagating along the shelf slope from the vicinity of the
deep channel east of Taiwan induce these variations northeast of Taiwan. The behavior of the cyclonic/
anticyclonic eddies on the shelf slope is well explained by 2nd-mode interior shelf waves advected by
the Kuroshio’s mean ﬂow. Remote effects from the vicinity of the deep channel east of Taiwan, or fromoutside the East China Sea, are believed to play an important role in the generation of these biweekly
periodic variations of the Kuroshio axis northeast of Taiwan. Moreover, on the shelf slope, thesevariations cause an onshore current across the shelf slope, suggesting topographically controlled
upwelling. Therefore, the biweekly periodic variations of the Kuroshio axis northeast of Taiwan might
contribute not only to the onshore transport of Kuroshio surface water but also to transport nutrientrich Kuroshio subsurface water onto the shelf in the East China Sea.
& 2009 Elsevier Ltd. All rights reserved.

Keywords:
Short-term ﬂow ﬂuctuation
Kuroshio axis variation
Interior shelf wave
High-frequency radar
Northeast of Taiwan

1. Introduction
The Kuroshio is a western boundary current that is part of thesubtropical gyre in the North Paciﬁc. It enters the East China Sea(ECS) through a deep channel between Taiwan and Yonagunijima(hereafter, this channel is referred to as the East Taiwan Channel;
ETC). In the ECS, the Kuroshio ﬂows northeastward along the shelfslope to 301N at isobaths of 200–500 m. Near 301N, it veers
clockwise; exiting the ECS through the Tokara Strait and thenﬂowing eastward off the south coast of Japan (Fig. 1).
Short-term meandering variations of the Kuroshio axis in the
ECS have been reported in many observational datasets, including
those from short- and long-term mooring current meter records(Sugimoto et al., 1988; Feng et al., 2000; Nakamura et al., 2003),
mooring inverted echo sounder data (James et al., 1999), satellite
images of sea surface temperature (Qiu et al., 1990), andhydrographic data (Ichikawa and Beardsley, 1993). Prior studies
identiﬁed consistent characteristics of the Kuroshio axis meanders
in the ECS, such as their period (10–20 days), wavelength(100–375 km), and downstream phase speed (17–30 km day1).
Baroclinic instability has been proposed as a dominant factor in Corresponding author.

E-mail address: d-taka@hyarc.nagoya-u.ac.jp (D. Takahashi).
0278-4343/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.csr.2009.07.007

driving the short-term Kuroshio axis meanders on the shelf slope
across the PN line of the ECS (James et al., 1999). However, all ofthese studies are limited to the region from the middle of theKuroshio in the ECS to downstream areas (from the PN line to theTokara Strait; see Fig. 1 for geographic locations).
Zhang et al. (2001) reported strong meandering signals with aperiod of 10 days and 18 days from long-term mooring current
meter records within the ETC, which is the entrance of theKuroshio to the ECS. James et al. (1999) also indicated thepossibility of downstream propagation of short-term Kuroshio
axis meanders, with a period of 16 days, from the upstream area of
the PN line. These previous studies imply the presence of shortterm variations (or meanders) of the Kuroshio axis at the area
between the ETC and the PN line and suggest that it propagates tothe downstream area of the Kuroshio in the ECS. However, shortterm Kuroshio axis variations over a time scale of 10–20 days andtheir downstream propagation as well as the linkage of such asignal to upstream and downstream areas have not beenaddressed. Additionally, if the downstream propagation of shortterm variations along the Kuroshio from the ETC can be conﬁrmed,
the local effect of baroclinic instability on the shelf slope must not
be the only cause for short-term variations of the Kuroshio axis ona time scale of 10–20 days from the PN line to the Tokara Strait.
The region northeast of Taiwan, located between the ETC andthe PN line, is known to be an active upwelling area with nutrientAuthor's personal copy
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1897

2. Data and methods

Fig. 1. Bathymetry map of the East China Sea (ECS) and the area northeast of
Taiwan. ‘‘A’’ denotes A line, ‘‘PN’’ denotes the PN line; and ‘‘HF’’ indicates the area
measured by long-range high-frequency radar (HF radar). Solid circles represent
the location of HF radar sites at Yonagunijima and Ishigakijima; a shaded arrow
represents the Kuroshio. The channel between Taiwan and Yonagunijima isreferred to as the East Taiwan Channel (ETC) in this paper.

The National Institute of Information and Communications
Technology (NICT), Japan, developed a long-range ocean radarsystem for monitoring surface currents northeast of Taiwan(Fig. 1). The HF radar measures radial surface current components
with a 7 km spatial interval and a 0.5 h temporal interval. The
present study uses eastward and northward surface current
components from 2002 to 2005, as calculated and distributed
by NICT (Sato et al., 2004).
First, current data with speeds greater than 2.5 m s1 were
excluded. The remaining data were then interpolated linearly intime and to horizontal grids using a spline function to generate
gridded data. The interpolated data were then spatially smoothedusing a Gaussian function with an effective radius of 15 km andtemporally smoothed using a low-pass ﬁlter (Hanawa and
Mitsudera, 1985) to remove high-frequency ﬂuctuations such astides and inertial oscillations. Finally, relative vorticity (z ¼ @v/@x@u/@y) was estimated from the low-pass ﬁltered currents.

3. Statistical characteristics of variations in the surface ﬂow
ﬁeld northeast of Taiwan
3.1. Mean and ﬂuctuation ﬁelds in the surface current and relative
vorticity northeast of Taiwan

supply from the Kuroshio subsurface layer (Liu et al., 1992; Hsueh
et al., 1992; Hsueh, 1993, 2000; Sun and Su, 1994; Tang et al.,
1999; Qiao et al., 2005). Short- and relatively long-term current
variations in relation to the Kuroshio on time scales of a few days
or a few months on the shelf slope have been reported fromdiscontinuous hydrographic or short-term mooring current
observations (Chuang and Wu, 1991; Tang and Yang, 1993;
Chuang et al., 1993; Tang et al., 2000). In addition, upwelling ofKuroshio subsurface water on the shelf break is related tovariations of the Kuroshio axis (Hsueh et al., 1992; Hsueh, 1993;
Chuang and Liang, 1994; Liu et al., 1992). However, the presence ofshort-term variations of the Kuroshio axis on time scales of 10–20
days as well as the relationship between variations and upwellingof nutrient-rich Kuroshio subsurface water has not been addressed northeast of Taiwan.
Guo et al. (2006) estimated the total volume transport acrossthe 200 m isobath along the shelf break in the ECS using a 1/181
nested ocean model and found that the total cross-shelf volumetransport shows a clear short-term variation with a period of
about 17 days, suggesting that the short-term variation of theKuroshio axis can occur along the entire shelf break in the ECS and
can thereby induce cross-shelf ﬂows. This indicates a possible
contribution of short-term Kuroshio axis variations with a timescale of 10–20 days to the cross-shelf transport of surface andnutrient-rich subsurface waters originating from the Kuroshio
along the entire shelf break in the ECS, including the shelf breaknortheast of Taiwan. Therefore, it is very important from abiogeochemical point of view as well as from a physical point ofview to investigate the temporal-spatial characteristics anddynamics of the short-term variations of the Kuroshio axis
northeast of Taiwan.
In the present study, we present evidence for a short-term
variation of the Kuroshio axis northeast of Taiwan with a timescale of 10–20 days by describing its spatiotemporal characteristics via an analysis of sea surface currents obtained using longrange high-frequency radar (HF radar). Finally, we also discuss the
behavior and generation of this Kuroshio axis variation and its
relationship with cross-shelf transport of Kuroshio surface andsubsurface waters on the shelf slope northeast of Taiwan.

The mean and ﬂuctuation ﬁelds in the low-pass ﬁltered surface
current and the current’s relative vorticity northeast of Taiwan for
2002–2005 are shown in Figs. 2 and 3. In the ﬂow ﬁeld, strong
northeastward mean ﬂows with speeds greater than 80 cm s1 aredetected on the shelf slope around 25.0–25.751N. The mean ﬂows
also have high mean kinetic energy KE per unit mass, withmagnitudes greater than 3.0  103 cm2 s2 for each year of
2002–2005. The strong northeastward mean ﬂows veer
clockwise on the shelf slope around 25.751N, and then thiscurrent direction changes to eastward along the shelf slope at
isobaths of 200–1500 m (Fig. 2a–d). In the strong northeastward
mean ﬂow area, large ﬂow ﬂuctuations with magnitudes of
30–50 cm s1 are also detected for 2002–2005. The ellipses of thelarge ﬂow ﬂuctuations are ﬂattened, and their principal axes arenearly directed northeastward, namely, the ﬂow ﬂuctuations inthe strong mean ﬂow area dominate along the mean ﬂows. Theselarge ﬂow ﬂuctuations are characterized by relatively high eddy
kinetic energy KE0 per unit mass with magnitudes greater than
5.0  102 cm2 s2 for each year of 2002–2005. In particular, theﬂow ﬂuctuations become larger on the shelf slope around25.0–25.751N, and their ellipses become somewhat circular,
suggesting that there are ﬂow ﬂuctuations not only along themean ﬂow (along the northeast–southwestward direction) but
also across it (along the northwest–southeastward direction) in
this area (Fig. 2e–h).
The surface current northeast of Taiwan exhibits high KE andrelatively low KE0 ; therefore the ratios of eddy to mean kinetic
energy KE0 =KE there are less than 1, whereas the ratios on the shelfslope, around 25.0–25.751N where there are larger ﬂow ﬂuctuations, become somewhat larger for each year of 2002–2005. In
particular, the ratios are greater than 1 in 2002, 2003, and 2004,
indicative of larger ﬂow ﬂuctuations in comparison with the mean
ﬂows (Fig. 2i–l). Therefore, the Kuroshio northeast of Taiwan can
be characterized not only by stable northeastward ﬂows withspeeds greater than 80 cm s1, but also by the larger ﬂow
ﬂuctuations on its left side on the shelf slope.
In the mean relative vorticity ﬁeld, the Kuroshio is characterized by positive (z41.5  105 s1) and negative (z1.5  105s1) relative vorticity on its left and right sides over from

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D. Takahashi et al. / Continental Shelf Research 29 (2009) 1896–1907

122˚E

123˚E

124˚E

km
50

123˚E

124˚E

km

2002

26˚N
0

125˚E 122˚E

0

125˚E 122˚E

124˚E

km

2003

50

123˚E

0

125˚E 122˚E

124˚E

km

2004

50

123˚E

0

50

25˚N

a

100cm/s
24˚N
km
0

km

2002

26˚N
50

b

100cm/s

0

km

2003

50

c

100cm/s

0100cm/s
km

2004

50

0

125˚E
KE [cm2s-2]
2005
3000
2500
2000
1500
1000
500
0
d

h

KE’ [cm2s-2]
1000
900
800
700
600
500
400
300
200
100
0

2005

KE’/KE
101

2005

50

25˚N

e

100cm/s
24˚N
km

km

2002

0.5

0.5

0

50

1

0.5
1

50

0

0.5

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50

1

1

1

0.3
1

10 0

i

0.3

j

4

1

1

1

1

24˚N

km

0.5

0.2

1

100cm/s

2004

1

0.5

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0.3

25˚N

0

km

2003

0.5

g

100cm/s

1

26˚N

f

100cm/s

k

10-1

l

Fig. 2. (a–d) Horizontal distributions of mean ﬂow and mean kinetic energy KE½KE ¼ 0:5  ðu 2 þ v 2 Þ per unit mass for each year of 2002–2005. Here, u and v are eastward
and northward current components, respectively. (e–h) Horizontal distributions of the variance ellipse and eddy kinetic energy KE0 [KE0 ¼ 0.5 (su2+sv2)] per unit mass foreach year of 2002–2005. Here, su2 and sv2 are variances of u and v, respectively. (i–l) Horizontal distributions of the ratio of eddy kinetic energy to mean kinetic energy foreach year. Shaded areas in (i–l) represent locations where the ratio is greater than 1. Contour intervals for the ratios in (i–l) are 0.1 in the areas with magnitudes less than 1
and 1 in the areas with magnitudes greater than 1. Thin gray dashed lines in (a–l) represent the 200–500 m isobaths with contour intervals of 100 m, and thick gray dashedlines in (a–l) represent the 1000 and 1500 m isobaths. Areas enclosed by white lines in (e–h) represent locations where ﬂuctuations in relative vorticity are large for eachyear of 2002–2005, and the white lines indicate standard deviations of 2.0  105, 2.0  105, 1.75  105, and 1.65  105 s1 in the relative vorticity for each year of
2002–2005, respectively.

123˚E

124˚E

km

125˚E 122˚E

0

-1

50

1

0

0

0

0

km
0

1

50

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-1.5

km
0

1

1

km

2004

50

0

125˚E
[*10-5 s-1]
2005
2.0
1.5
1.0
0.5
0.0
-0.5
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-1.5
-2.0
d

2005
1

50

1.5
1.0

1

e

[*10-5 s-1]
2.5
2.0

1

1

25˚N

24˚N

0

c

2003
1

26˚N
2

0

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0

b

2002

50

-1

-1.5

a
km

0

1
-1

-1.5

0

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-1

-1

2

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km

2004

-1

-1.5

0

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0

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km

125˚E 122˚E

2

0

124˚E

1

1

123˚E

-1

50

125˚E 122˚E

2003

0

0

124˚E

km

2002

26˚N

123˚E

2

122˚E

f

0.5

g

h

0.0

Fig. 3. Horizontal distributions of (a–d) mean relative vorticity and (e–h) standard deviation of relative vorticity for each year of 2002–2005. Contour intervals for the mean
relative vorticity in (a–d) and for the standard deviation in (e–h) are 0.5  105 and 0.25  105 s1, respectively. Shaded areas in (a–d) represent negative values of relative
vorticity. Shaded areas in (e–h) represent locations where the standard deviation is greater than 1.0  105 s1. Thin gray dashed lines in (a–h) represent the 200–500 m
isobaths with contour intervals of 100 m, and thick gray dashed lines in (a–h) represent the 1000 and 1500 m isobaths. White dashed lines in (e–h) indicate values of
2.0  105, 2.0  105, 1.75  105, and 1.65  105 s1, respectively.

2002–2005, respectively, indicating that the Kuroshio has cyclonic(anticyclonic) circulation on its left (right) side (Fig. 3a–d). On theother hand, in the ﬂuctuation ﬁeld of relative vorticity, large relative
vorticity ﬂuctuations (41.5  105 s1) are also detected in thestrong northeastward mean ﬂow area. In particular, on the left sideof the Kuroshio on the shelf slope around 25.0–25.751N, there arelarger relative vorticity ﬂuctuations with magnitudes greater than
1.5  105 s1, and this area well corresponds to the area with larger
ﬂow ﬂuctuations for 2002–2005 (Figs. 2e–h and 3e–h).

3.2. Temporal variation of the ﬂow ﬁeld in the Kuroshio northeast of
TaiwanTo investigate the temporal variation of the ﬂow ﬁeld in the
Kuroshio northeast of Taiwan, the spatial-mean ﬂow and relative
vorticity on its left side, where larger ﬂow and relative vorticity
ﬂuctuations occur (areas enclosed by white lines and whitedashed lines in Figs. 2e–h and 3e–h, respectively), were estimatedfrom the low-pass ﬁltered surface current for 2002–2005, andAuthor's personal copy
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vector stick plots of the spatial-mean ﬂow and time series of thespatial-mean relative vorticity for 2002–2005 are shown in Fig. 4.
At the left side of the Kuroshio on the shelf slope, stablenortheastward ﬂows with speeds greater than 50 cm s1 areprominent over 2002–2005, whereas sudden changes in thecurrent direction are detected around positive peaks in therelative vorticity, suggesting that ﬂow and relative vorticity
ﬂuctuations occur together at the left side of the Kuroshio on
the shelf slope. These ﬂow and relative vorticity ﬂuctuations havetime scales from several days to several months, suggesting thepresence of not only long- but also short-term ﬂow and relative
vorticity ﬂuctuations in the Kuroshio northeast of Taiwan (Fig. 4).
To examine temporal variability in the spatial-mean ﬂow andrelative vorticity on the left side of the Kuroshio, we conducted aspectral analysis of the spatial-mean ﬂow and relative vorticity for
2002–2005. Here, for the spatial-mean ﬂow, the eddy kinetic
energy KE0 (f) in the frequency region was estimated. The eddy
kinetic energy KE0 (f) can be written using the following equation:
KE0 ðf Þ ¼

1
ðSu ðf Þ þ Sv ðf ÞÞ;2

ð1Þ

Rel. vor. [*10-5 s-1]

where f is the frequency, Su(f) and Sv(f) are the power spectra
density for the eastward and northward current components,

Rel. vor. [*10-5 s-1]

100cm/s

North

5.0
2.5
0.0
-2.5
-5.0

Period>48hours

Feb.

Mar.

Apr.

May

Jun. Jul.
2002

Aug.

Sep.

Oct.

50cm/sNov. Dec.

100cm/s

North

5.0
2.5
0.0
-2.5
-5.0

Period>48hours

Jan.

Rel. vor. [*10-5 s-1]

respectively. The estimated eddy kinetic energies for the spatialmean ﬂow and the power spectra for the spatial-mean relative
vorticity for 2002–2005 are shown in Fig. 5.
For periods longer than 30 days, speciﬁc spectral peaks are notdetected for a common frequency over 2002–2005, whereas
energy levels of both the eddy kinetic energy and the power
spectra are always higher for each year of 2002–2005. In
particular, higher energy levels with a period of 100 days or morein both the eddy kinetic energy and the power spectra are foundfor each year of 2002–2005 (Fig. 5), suggesting the presence ofKuroshio axis variations induced by propagating mesoscale eddies(Yang et al., 1999; Ichikawa, 2001; Zhang et al., 2001, Ichikawa
et al., 2008).
On the other hand, for periods shorter than 30 days, there aresome speciﬁc energetic peaks of eddy kinetic energy for the
spatial-mean ﬂow at 11–14 days (biweekly period), 7–9 days, and5–6 days for 2002–2005. Although the period of speciﬁc energetic
peaks of eddy kinetic energy at 11–14 days is somewhat different
for each year (12.6, 11.6, 13.5, and 13.8 day in 2002, 2003, 2004,
and 2005, respectively), these energetic peaks always occur
together with peaks in the power spectra of spatial-mean relative
vorticity over 2002–2005. By contrast, the energetic peaks of eddy
kinetic energy at 7–9 days and 5–6 days are not always

50cm/s

Jan.

Feb.

Mar.

Apr.

May

Jun. Jul.
2003

Aug.

Sep.

Oct.

50cm/sNov. Dec.

100cm/s

North

5.0
2.5
0.0
-2.5
-5.0

Period>48hours

Jan.

Rel. vor. [*10-5 s-1]

1899

Feb.

Mar.

Apr.

May

Jun. Jul.
2004

Aug.

Sep.

Oct.

50cm/sNov. Dec.

100cm/s

North

5.0
2.5
0.0
-2.5
-5.0

Period>48hours

Jan.

Feb.

Mar.

Apr.

May

Jun. Jul.
2005

Aug.

Sep.

Oct.

Nov. Dec.

Fig. 4. Vector stick plots of spatial-mean low-pass ﬁltered current vectors and time series of spatial-mean relative vorticity estimated from the low-pass ﬁltered current
vectors for each year of 2002–2005. The spatial-mean current vectors and relative vorticity were estimated in areas enclosed by white lines and white dashed lines in Figs.
2e–h and 3e–h, respectively.

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D. Takahashi et al. / Continental Shelf Research 29 (2009) 1896–1907

Fig. 5. Eddy kinetic energy KE0 (f) in the frequency region for spatial-mean low-pass ﬁltered current vectors and power spectral density for spatial-mean relative vorticity
estimated from the low-pass ﬁltered current vectors for each year of 2002–2005. Thick lines indicate the eddy kinetic energy for the spatial-mean low-pass ﬁltered current
vectors, and dashed lines indicate the power spectral density for the spatial-mean relative vorticity. Shaded areas represent a band with period 10.5–14.5 days. Numerals
are periods in days.

accompanied by peaks in the power spectra of spatial-mean
relative vorticity (Fig. 5). Therefore, the biweekly periodic ﬂow
ﬂuctuations occur together with ﬂuctuations in relative vorticity
on the left side of the Kuroshio on the shelf slope over the period
2002–2005, suggesting the biweekly periodic ﬂow ﬂuctuations
with eddy-like structures and the presence of short-term Kuroshio
axis variations with a time scale of 10–20 days northeast of
Taiwan. In contrast, the ﬂow ﬂuctuations with periods of 5–6 days
and 7–9 days appear to have weak or no-eddy-like structures. In
this study, we focus on the biweekly periodic ﬂow and relative
vorticity ﬂuctuations in the Kuroshio on the shelf slope northeastof Taiwan.

4. Biweekly periodic variation of the Kuroshio axis northeastof Taiwan
4.1. Biweekly periodic Kuroshio ﬂow ﬂuctuation on the shelf slope
and its inﬂuence on the Kuroshio axis northeast of Taiwan
To investigate the biweekly periodic Kuroshio ﬂow ﬂuctuationin detail, the 2002 data were extracted using a band-pass ﬁlter
with 280.3 and 353.5 h half-power points. The same band-passﬁlter was also used for the 2004 and 2005 data, whereas a bandpass ﬁlter with 243.5 and 289.3 h half-power points was used for2003 data. Vector stick plots of the 11–14 days band-pass ﬁlteredspatial-mean ﬂow and time series of the 11–14 days band-passﬁltered spatial-mean relative vorticity for 2002–2005 are shownin Fig. 6. The periodic ﬂow reversals are clearly seen, withamplitudes of 10–20 cm s1 that sometimes become larger for
2002–2005. The larger biweekly periodic ﬂow ﬂuctuations occur
together with larger relative vorticity ﬂuctuations, and these ﬂows
clearly rotate anticlockwise over that time period (Fig. 6).
In the case that the positive (negative) amplitude of the bandpass ﬁltered spatial-mean relative vorticity deviated from thetemporal mean by more than one standard deviation (shown bydashed lines in Fig. 6), it was categorized as a typical positive(negative) case of the biweekly periodic spatial-mean relative
vorticity for each year of 2002–2005. The band-pass ﬁlteredsurface currents in these typical positive/negative cases during
each year were then averaged to obtain representative annual
values. The representative values of the band-pass ﬁltered surface

current calculated from typical positive/negative cases of the
band-pass ﬁltered spatial-mean relative vorticity ﬂuctuation (seeFig. 6) for 2002–2005 are shown in Fig. 7.
In the positive pattern, a cyclonic eddy appears on the shelfslope around 25.0–25.751N for 2002–2005. In contrast, in thenegative case when the cyclonic eddy pattern disappears, ananticyclonic eddy appears on the shelf slope. The cyclonic/
anticyclonic eddies have approximately elliptical shapes, withminor axes of about 100 km in all years, whereas the lengths ofmajor axes are about 100, 150, 120, and 160 km in 2002, 2003,
2004, and 2005, respectively. In addition, the cyclonic/anticyclonic eddies are located in the Kuroshio and seem to be
trapped on the shelf slope at isobaths of 200–1500 m for all years,
indicating that the biweekly periodic ﬂow and relative vorticity
ﬂuctuations on the left side of the Kuroshio northeast of Taiwan
are caused by the appearance of cyclonic/anticyclonic eddies.
To examine inﬂuences of the biweekly periodic appearance of
cyclonic/anticyclonic eddies on the Kuroshio axis’ variation
northeast of Taiwan, the low-pass ﬁltered surface currents werecategorized as typical positive (negative) case of the band-passﬁltered spatial-mean relative vorticity ﬂuctuation (see Fig. 6). The
low-pass ﬁltered surface currents in these typical positive/
negative cases at each year were then averaged to obtainrepresentative values for each year. Additionally, we calculated
the difference in the low-pass ﬁltered surface current between thepositive and negative cases for each year. Fig. 8 presents therepresentative values of the low-pass ﬁltered surface current
calculated from the typical positive/negative cases of band-passﬁltered spatial-mean relative vorticity ﬂuctuation (see Fig. 6), andFig. 9 shows the differences among the typical positive andnegative cases.
In the typical positive pattern, the Kuroshio ﬂows northeastward with speeds greater than 60 cm s1 (shown by shaded
area in Fig. 8) for each year, but the Kuroshio appears to deﬂect
slightly offshore (southeastward) from the shelf slope around25.0–25.571N compared to the mean ﬂow ﬁeld (see Fig. 2a–d).
Cyclonic/anticyclonic circulations are detected on the left/rightsides of the Kuroshio, respectively, and the cyclonic circulation(z ¼ 1.5–4.0  105 s1) becomes somewhat stronger than the
anticyclonic circulation (z ¼ 1.02.0  105 s1) in all years.
Additionally, the cyclonic circulation is stronger than that in themean relative vorticity ﬁeld (z ¼ 1.0–2.0  105 s1; see Fig. 3a–d)

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Rel. vor. [*10-5 s-1]
Rel. vor. [*10-5 s-1]

1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5

Rel. vor. [*10-5 s-1]

North

10cm/s
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5

Rel. vor. [*10-5 s-1]

1901

Period=11.7-14.7days

Mar.

Apr.

May
2002

Jun.

Jul.
North

10cm/sPeriod=10.1-12.1days

Aug.

Sep.

Oct.
2003

Nov.

Dec.
North

10cm/s
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5

Period=11.7-14.7days

Jun.

Jul.

Aug.
2004

Sep.

Oct.
North

10cm/s
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5

Period=11.7-14.7days

Apr.

May

Jun.
2005

Jul.

Aug.

Fig. 6. Vector stick plots of spatial mean 11–14 day band-pass ﬁltered current vectors and time series of spatial mean 11–14 day band-pass ﬁltered relative vorticity for
2002–2005. Dashed lines represent one standard deviation for the spatial mean 11–14 day band-pass ﬁltered relative vorticity.

122°E
km
26°N

0

123°E

124°E

125°E 122°E
2002

Positive

km
0

50

123°E
Positive

124°E

125°E 122°E
2003

km
0

50

123°E
Positive

124°E

125°E 122°E
2004

km
0

50

123°E
Positive

124°E

125°E
2005

50

25°N
10cm/s10cm/s10cm/s10cm/s

24°N
km
26°N

0

2002

Negativekm
050

Negative

2003

km
0

50

Negative

2004

km
0

50

Negative

2005

50

25°N

24°N

10cm/s10cm/s10cm/s10cm/s[cm s-1]
0 5 10 15 20 25
Fig. 7. Horizontal distributions of 11–14 day band-pass ﬁltered surface currents corresponding to typical (a–d) positive and (e–h) negative patterns of 11–14 day band-passspatial-mean relative vorticity for 2002–2005. Shaded areas in (a–h) represent current speeds greater than 5 cm s1. Thin gray dashed lines in (a–h) represent the
200–500 m isobaths with contour intervals of 100 m, and thick gray dashed lines in (a–h) represent the 1000 and 1500 m isobaths.

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122°E

0

124°E

125°E 122°E
2002

Positive

km

26°N

123°E

124°E

125°E 122°E
2003

Positive

km
0

50

123°E

124°E

125°E 122°E
2004

Positive

km
0

50

123°E

km
0

50

123°E

124°E

125°E

Positive

2005

100cm/sd

Negative

2005

100cm/s

h

50

25°N
100cm/sa

Negative

2002

100cm/sb

Negative

2003

100cm/sc

Negative

2004

24°N
km

26°N

0

km
0

50

km
0

50

km
0

50

50

25°N

e

100cm/sf

100cm/sg100cm/s

24°N[cm s-1]
0 20 40 60 80 100
Fig. 8. Horizontal distributions of low-pass ﬁltered surface currents corresponding to typical (a–d) positive and (e–h) negative patterns of 11–14 day band-pass ﬁlteredspatial-mean relative vorticity for 2002–2005. Shaded areas in (a–h) represent locations where current speeds are greater than 60 cm s1. Thin gray dashed lines in (a–h)
represent the 200–500 m isobaths with contour intervals of 100 m, and thick gray dashed lines in (a–h) represent the 1000 and 1500 m isobaths.

26°N

0

122°E

123°E

124°E

km

Positive-Negative

125°E 122°E
2002

km
0

50

123°E

124°E

Positive-Negative

125°E 122°E
2003

km
0

50

123°E

124°E

Positive-Negative

125°E 122°E
2004

km
0

50

123°E

124°E

Positive-Negative

125°E
2005

50

25°N
20cm/s

20cm/s

20cm/s

20cm/s24°N[cm s-1]
0 5 10 15 20 25 30
Fig. 9. Horizontal distributions of low-pass ﬁltered surface current differences between the positive and negative patterns for 2002–2005 shown in Fig. 8. Shaded areas
represent locations where current speeds are greater than 10 cm s–1. Thin gray dashed lines represent the 200–500 m isobaths with contour intervals of 100 m, and thickgray dashed lines represent the 1000 and 1500 m isobaths.

for each year. Although the anticyclonic circulation on the rightside of the Kuroshio changes little over the period 2002–2005, thecyclonic circulation on its left side changes clearly. In particular,
the cyclonic circulations (z ¼ 2.5–4.0  105 s1) in 2002 and2003 are enhanced compared to those (z ¼ 1.5–2.0  105 s1) in
2004 and 2005, resulting in a small meander on the shelf slope
around 25.0–25.751N for 2002 and 2003. By contrast, in the
typical negative pattern, the cyclonic circulation (zo2.0  105s1) is clearly weakened on the left side of the Kuroshio for eachyear of 2002–2005 in comparison with the typical positive
pattern. This weakening of the cyclonic circulation results in an
onshore (northwestward) movement of the Kuroshio axis around25.0–25.751N for 2002–2005 (Fig. 8).
The difference ﬁelds between the positive and negative
patterns show that the cyclonic circulation is enhanced on the
shelf slope around 25.0–25.751N for 2002–2005 in the typical
positive pattern. The horizontal structure of the surface current in
the difference ﬁeld corresponds well with that in the 11–14 days
band-pass ﬁltered ﬂow ﬁeld of the typical positive pattern for
2002–2005 (see Fig. 7a–d), indicating that the enhancement of
cyclonic circulation is caused by the biweekly periodic appearances of the cyclonic eddies, and that the onshore–offshore(northwest–southeastward) movement of the Kuroshio axis isinduced by the appearance of cyclonic/anticyclonic eddies in theregion showing biweekly frequency on the shelf slope northeast of
Taiwan (Fig. 9).

4.2. Downstream propagation of the biweekly periodic ﬂow
ﬂuctuation along the Kuroshio on the shelf slope northeast of Taiwan
The biweekly periodic ﬂow ﬂuctuations on the left side of theKuroshio on the shelf slope rotate anticlockwise when thebiweekly periodic relative vorticity ﬂuctuations become larger(see Fig. 6), suggesting that in the region exhibiting biweekly
frequency the cyclonic/anticyclonic eddies propagate downstream
in the Kuroshio. Therefore, we examine the downstream propagation of the cyclonic/anticyclonic eddies in the region northeast of
Taiwan. Fig. 10 shows the horizontal distributions of band-passﬁltered and low-pass ﬁltered surface currents with 3.5 day
intervals from May 16 to May 23, 2002, when the large
biweekly periodic ﬂow and relative vorticity ﬂuctuations
occurred on the left side of the Kuroshio on the shelf slope (seeFig. 6a). As shown in the biweekly periodic ﬂow ﬂuctuation ﬁeld,
cyclonic/anticyclonic eddies with approximately elliptical shapes
and major axes of about 100 km in length appear alternately near

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122˚E

123˚E

124˚E

125˚E 122˚E

km Period=11.7-14.7days 2002

123˚E

124˚E

125˚E 122˚E

km Period=11.7-14.7days 2002

1903

123˚E

124˚E

125˚E

km Period=11.7-14.7days 2002

26˚N
0

50

0

50

0

50

25˚N

24˚N

May 16 00:00

a

May 19 12:00

b

May 23 00:00

c

km Period>48hours

2002

km Period>48hours

2002

km Period>48hours

2002

26˚N
0

50

0

50

0

50

25˚N

24˚N

May 16 00:00

d

May 19 12:00

e

May 23 00:00

f[cm s-1]
25
20
15
10
5
0
[cm s-1]
100
80
60
40
20
0

Fig. 10. Horizontal distributions of (a–c) 11–14 day band-pass ﬁltered surface currents, and (d–f) low-pass ﬁltered surface currents for 16–23 May 2002. The time interval
between panels is 3.5 days. Shaded areas in (a–c) represent locations where current speeds are greater than 5 cm s1. Shaded areas in (d–f) represent areas where current
speeds are greater than 50 cm s1. Thin gray dashed lines represent the 200–500 m isobaths with contour intervals of 100 m, and thick gray dashed lines represent the 1000
and 1500 m isobaths.

the ETC before propagating northeastward across the shelf breakat a phase speed of about 16 km day1 (Fig. 10a–c). When thecyclonic eddy appears on the shelf slope around 25.0–25.751N in
the region showing biweekly frequency, the Kuroshio axis
meander occurs on the shelf slope in the low-pass ﬁltered ﬂow
ﬁeld (Fig. 10a and d). By contrast, when the anticyclonic eddy
appears on the shelf slope around 25.0–25.751N in the region withbiweekly periodicity, the Kuroshio in the low-pass ﬁltered ﬂow
ﬁeld ﬂowed northward or northeastward along the shelf slope
without meandering (Fig. 10c and f). This shows that the
meanders of the Kuroshio axis northeast of Taiwan are closely
related to the arrival of cyclonic eddies from the vicinity of the
ETC. Similar downstream propagation of cyclonic/anticycloniceddies is also detected on the shelf slope at isobaths of
200–1500 m in other years (see Fig. 11). Thus, the anticlockwiserotating ﬂow ﬂuctuations in the region exhibiting biweekly
frequency (see Fig. 6) are caused by the downstream
propagation of cyclonic/anticyclonic eddies along the shelf slope,and the propagating cyclonic eddies induce not only offshore(southeastward) movements of the Kuroshio axis but also
sometimes Kuroshio meanders northeast of Taiwan.
To examine the downstream propagation of cyclonic/anticyclonic eddies in detail, time-distance diagrams of the band-passﬁltered northwestward current component approximately along
the shelf slope at isobaths of 200–1500 m (southwest–northeastward direction shown by dashed lines in Figs. 9 and 10a–c), where
the strong northeastward mean ﬂow of the Kuroshio exists withspeeds greater than 80 cm s1 (see Fig. 2a–d), are shown in Fig. 11.
As shown in Fig. 11, biweekly periodic ﬂow ﬂuctuations propagate
downstream along both the shelf slope and the Kuroshio’s mean
ﬂow over the period 2002–2005, whereas the downstream phasespeed is somewhat different for each year. This downstream
propagation has a nearly constant phase speed on the shelf slopefor 2002–2005. The downstream phase speed estimated from Fig.
11 using cross-correlation analysis (ﬁgures not shown) for eachyear is listed in Table 1. The biweekly periodic ﬂow ﬂuctuations
propagate with phase speeds of 16–29 km day1, and theirwavelength, which was estimated from the period of eddy

Fig. 11. Time-space diagrams of 11–14 day band-pass ﬁltered northwestward
current components along the shelf slope at isobaths of 200–1500 m (southwest–northeastward dashed line shown in Figs. 9 and 10a–c) for each year of
2002–2005. Shaded areas represent the southeastward current component.
Arrows represent the location of the shelf break. Contour interval is 5 cm s1.

kinetic energy peaks of spatial-mean ﬂow ﬂuctuations in theregion showing biweekly frequency (11–14 days; see Fig. 5) andfrom the downstream phase speed (16–29 km day1), is
197–353 km over the period of study. Interestingly, half theestimated wavelength closely corresponds with the major axis

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Table 1
Characteristics of the biweekly (11–14 day) periodic ﬂow ﬂuctuations along the Kuroshio on the shelf slope northeast of Taiwan for each year of 2002–2005.
Dates

Period (days)

Downstream phase speed (km day1)

Wavelength (km)18 March 2002–19 July 2002
10 August 2003–17 October 2003
23 June 2004–17 October 2004
30 April 2005–29 August 2005

12.6
11.6
13.5
13.8

16
29
19
26

197
333
253
353

T is period, C is downstream phase speed, and l ( ¼ C  T) is wavelength.

length of the propagating cyclonic/anticyclonic eddies for
2002–2005 (see Fig. 7), suggesting that these propagatingeddies have wavelike characteristics trapped on the shelf slope
northeast of Taiwan (Table 1). This indicates the possible presenceof a shelf wave on the shelf slope, although propagation of thebiweekly periodic ﬂow ﬂuctuations (cyclonic/anticyclonic eddies)occurs along the shelf slope on the left-hand side.

4.3. Behavior of the biweekly periodic ﬂow ﬂuctuations on the shelfslope northeast of Taiwan
In the region exhibiting biweekly frequency, the cyclonic/
anticyclonic eddies propagate in the Kuroshio’s mean ﬂow along
the shelf slope for 2002–2005, indicating that these eddies areadvected by the mean ﬂow. If the cyclonic/anticyclonic eddies arecompletely advected by the Kuroshio’s mean ﬂow, the downstream phase speed should correspond with the current speed ofthe mean ﬂow. However, the downstream phase speed of thecyclonic/anticyclonic eddies (16–29 km day1; see Table 1) ismuch smaller than the Kuroshio’s mean current speed (80–100
cm s1; see Fig. 2), indicating upstream propagation of thecyclonic/anticyclonic eddies in the mean ﬂow.
The phase speed C of the propagating cyclonic/anticycloniceddies in the Kuroshio’s mean ﬂow, where C is the phase speed
perceived by an observer at rest, can be written by the following
equation:
C ¼ C 0 þ U;

ð2Þ

where C0 is the phase speed of the propagating cyclonic/anticyclonic eddies relative to the mean ﬂow of the Kuroshio, and U is
the current speed of the Kuroshio’s mean ﬂow. The phase speed C0
is estimated from Eq. (2) using the phase speed C (16–29 kmday1; see Table 1) and the current speed U (80–100 cm s1; see
Fig. 2) of the Kuroshio’s mean ﬂow, giving C0 ¼ 4069 km
day1 over the period 2002–2005. The estimated C0 indicates thatthe cyclonic/anticyclonic eddies propagate upstream with phasespeeds of 40–69 km day1 in the Kuroshio’s mean ﬂow. Interestingly, the cyclonic/anticyclonic eddies propagate along the shelfslope at 200–1500 m isobaths on the right-hand side, suggestiveof the shelf wave trapped on the shelf slope (Gill, 1982).
The physical properties of the shelf wave generally change withthe bottom topography of the shelf slope and the density
structure. The inﬂuence of bottom topography is characterized
by the topographic b* ( ¼ fH1@H/@y), where f is the Coriolis
parameter, H is the mean depth around the shelf break, and @H/@yis the shelf slope. The topographic b* for the A line (see Fig. 1)
across the shelf slope northeast of Taiwan is estimated using
f ¼ 6.3  105 s1 at 25.51N, H ¼ 100 m, and @H/@y ¼ 1.5  102,
giving a topographic b* of 9.4  109 s1 m1 as a rough estimate.
To compare the topographic b* with the planetary b( ¼ 2.1 1011 s1 m1 near 25.51N) for A line, the topographic
b* is two orders of magnitudes larger than the planetary b,
indicating that the topographic b* is important for the upstream
propagation of the cyclonic/anticyclonic eddies on the shelf slope.

On the other hand, the inﬂuence of stratiﬁcation is characterized
by the stratiﬁcation parameter S ¼ N2H2/f2L2, where N is theBrunt–Väisälä frequency and L is the width of the shelf slope area.
For Sb1, the motion on the shelf slope is strongly baroclinic, while
for S51, it is mainly barotropic (Huthnance, 1978). The stratiﬁcation parameter S for A line was estimated using a typical
N of 1.0  10–6–1.0  103 s1 and L ¼ 100 km, giving S of
2.5  104–2.5  101 as rough estimate. The estimated S indicatesthat stratiﬁcation can be expected to have a moderately weakinﬂuence on shelf slope motion. This approximation is supported
by Ichikawa and Beardsley (1993), who implied that most of thevolume transport of the Kuroshio near the PN line is barotropic.
Additionally, we estimated the divergence parameter e ¼ f2L2/gH,
where g( ¼ 9.8 m s2) is gravitational acceleration, giving e ¼ 0.04
as a rough estimate. The estimated e indicates that the motion on
the shelf slope area is nearly horizontally nondivergent. Therefore,
for a ﬁrst approximation, we adopt the barotropic interior shelfwave model of Buchwald and Adams (1968) for the upstream
propagation of cyclonic/anticyclonic eddies in the Kuroshio’s
mean ﬂow on the shelf slope northeast of Taiwan.
The Buchwald–Adams theory is adopted for the case of broad
shelves and steep shelf slopes northeast of Taiwan. An idealized
shelf proﬁle used to simulate real bottom topography along A line
is shown in Fig. 12a. Based on the idealized shelf proﬁle, thedispersion curves for the ﬁrst three modes have been calculated asa function of wavenumber in accordance with the
Buchwald–Adams theory. The calculated dispersion curves areshown in Fig. 12b. Here, a circle, triangle, square, and diamondwith a bar represent the dispersion relations of the biweekly
periodic ﬂow ﬂuctuations in the Kuroshio’s mean ﬂow withspeeds of 80–100 cm s1 in 2002, 2003, 2004, and 2005,
respectively. As shown in Fig. 12b, the estimated normalized
frequencies (o/f) of the biweekly periodic ﬂow ﬂuctuations in the
Kuroshio’s mean ﬂow in 2002, 2003, 2004, and 2005 are
0.28–0.37, 0.14–0.20, 0.26–0.35, and 0.11–0.17, respectively,
indicating the possible presence of short-term periodic ﬂow
ﬂuctuations with periods of 3–10 days in the mean ﬂow of theKuroshio. The dispersion relations of the short-term periodic ﬂow
ﬂuctuations there correspond well with those of the 2nd-mode
interior shelf wave calculated under the Buchwald–Adams theory.
To examine the horizontal structure of the 2nd-mode interior
shelf waves on the shelf slope, the current ellipses of the 2ndmode interior shelf waves with normalized frequencies of 0.31
and 0.16 were calculated. The horizontal distribution of thesecurrent ellipses is shown in Fig. 12c. The 2nd-mode interior shelfwaves with normalized frequencies of 0.31 and 0.16 arecharacterized by an eddy-like structure on the shelf slope. The
eddy-like structure is remarkable on the shelf slope at isobaths of
150–1000 m (Fig. 12c), where the strong Kuroshio’s mean ﬂow
exists (see Fig. 2a–d). This indicates that the 2nd-mode interior
shelf waves on the shelf slope have the eddy-like structure
trapped on the shelf slope at isobaths of 150–1000 m. The
observed biweekly periodic ﬂow ﬂuctuations (cyclonic/
anticyclonic eddies) in the Kuroshio’s mean ﬂow also haveapproximately similar current structures as the 2nd-mode

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0.6

0

1st

200

0.5
Frequency ω / f

Depth [m]

2002
2003

400
600
800
1000

2004

0.4

2005

0.3

2nd

0.2
3rd

1200
0.1

1400
1600

0.0
0

50
100
Distance [km]

150

0

5
10 15 20
Wavenumber kl

Current ellipse

ω/f = 0.31

ω/f = 0.16

0

50
100
Distance [km]150

Fig. 12. (a) Bottom topography and an idealized shelf proﬁle (dashed line) used tosimulate the real bottom topography along A line shown in Fig. 1. (b) Dispersion
curves for interior shelf waves of the 1st-, 2nd-, and 3rd-modes, and observed
dispersion relations (a circle, triangle, square, and diamond with a bar) for
biweekly periodic ﬂow ﬂuctuations in the mean ﬂow of the Kuroshio with speedsof 80–100 cm s1 on the shelf slope. Here, o is angular velocity, f ( ¼ 6.3  105 s1
at 25.51N) is the Coriolis parameter, k is wavenumber, and l ( ¼ 92 km) is width of
the shelf slope. (c) Current ellipses for the interior shelf wave of the 2nd-mode
with normalized frequencies (o/f) of 0.31 and 0.16 along A line. Shaded ellipses in(c) represent clockwise components.

interior shelf waves (see Figs. 7 and 10), and their spatiotemporalcharacteristics (dispersion relations) correspond well with thoseof the 2nd-mode interior shelf waves. Thus, in the region showing
biweekly frequency, the northeastward propagation of cyclonic/
anticyclonic eddies on the shelf slope northeast of Taiwan can be
explained mainly by the 2nd-mode interior shelf waves advectedby the Kuroshio’s mean ﬂow. This also indicates that the biweekly
periodic variations of the Kuroshio axis are caused by the 2ndmode interior shelf waves advected by the Kuroshio’s mean ﬂow
northeast of Taiwan.

5. Discussion
5.1. Generation of the biweekly periodic ﬂow ﬂuctuations on the
shelf slope northeast of Taiwan
On the shelf slope northeast of Taiwan, the biweekly periodic
ﬂow ﬂuctuations that induce the variation of the Kuroshio axis are
generated by two main processes: indirect processes such as
frequency modulations of short-term (3–10 days) periodic ﬂow
ﬂuctuations by the Kuroshio’s mean ﬂow (see Section 4.3), and the
direct formation process of biweekly periodic ﬂow ﬂuctuations in
the Kuroshio’s mean ﬂow northeast of Taiwan.

1905

In the region showing biweekly frequency, northeastward
propagation of the cyclonic/anticyclonic eddies on the shelf slope
can be explained well by the short-term interior shelf wave
advected by the Kuroshio’s mean ﬂow, which has frequency
modulations similar to the short-term (3–10 days) periodic ﬂow
ﬂuctuations (see Fig. 12b). The short-term periodic ﬂow ﬂuctuations have a period of about 3.7 days in 2002 and 2004 and of
about 7.2 days in 2003 and 2005. Qiu and Imasato (1990) found,using a barotropic inﬂow–outﬂow model, that topographic Rossby
waves with a period of 2–4 days are induced by the Kuroshio
repeatedly crossing over the shelf slope northeast of Taiwan. Chuang
and Wu (1991) found from mooring observations that 2–4 day
periodic barotropic ﬂow ﬂuctuations with cross-shelf ﬂows are
induced by cold frontal passages northeast of Taiwan. These previous
results show the possibility that short-term ﬂow ﬂuctuations with aperiod of 2–4 days could form on the shelf slope northeast of Taiwan,
suggestive of local effects. Therefore, interior shelf waves with aperiod of about 3.7 days in 2002 and 2004 might be induced by the
Kuroshio’s crossing over the shelf or by cold frontal passages, andthen biweekly periodic ﬂow ﬂuctuations might be caused by thefrequency modulation due to the Kuroshio’s mean ﬂow. However,ﬂow ﬂuctuations with periods of about 7.2 days are not induced by
the Kuroshio’s crossing or by the cold frontal passages; namely, these
ﬂuctuations are not caused by the local effects, indicating thepossibility of remote effects from outside observed area northeast of
Taiwan. Thus, the biweekly periodic ﬂow ﬂuctuations in 2002 and2004 might be induced by the local effects, whereas at present thereason is uncertain why the ﬂuctuations in 2003 and 2005 occurred
as a result of indirect processes.
On the other hand, a direct process could generate thebiweekly periodic ﬂow ﬂuctuations in the mean ﬂow of theKuroshio northeast of Taiwan. As shown in Fig. 10, in the biweekly
period region, the observed cyclonic/anticyclonic eddies propagate from the vicinity of the ETC, indicating that the generation
area of the biweekly periodic ﬂow ﬂuctuations is not the observedarea, but the more upstream region. Zhang et al. (2001) foundfrom mooring observations that Kuroshio axis meanders with aperiod of 7–22 days exist in the ETC, suggesting that meanders
with a period of 7–22 days are generated near the ETC or outside
the ECS. The authors also separated eddies passing through the
ETC into two temporal bands (7–11 days and 12–20 days) andfound different spatial scales for the eddies in each band: eddies
in the 7–11 day band have spatial scales of 100 km, while thosein the 12–20 day band have spatial scales greater than 100 km.
The minor axes’ lengths of the observed cyclonic/anticycloniceddies in the region showing biweekly (11–14 day) frequency is100 km (see Fig. 7), which is similar to the spatial scale of theeddies in the 7–11 day band reported by Zhang et al. (2001), but
smaller than that of the eddies in the 12–20 day band. If weassume that the observed biweekly (11–14 day) periodic ﬂow
ﬂuctuations (cyclonic/anticyclonic eddies) propagate from theETC, their spatial scale must therefore decrease to 100 km during
passage from the ETC toward the area east of Taiwan. A possible
explanation for this phenomenon is the width of the ETC(100 km), which becomes the upper limit of the eddies’ spatial
scales as they pass through the ETC. This also raises the possibility
that the width of the ETC is a primary factor determining the
spatial scales of Kuroshio axis meanders northeast of Taiwan. The
horizontal spatial scale limitation (length of the minor axis) of theobserved cyclonic/anticyclonic eddies in the region showing
biweekly frequency leads us to the conclusion that remote effects
from the vicinity of the ETC or outside the ECS experiencing directprocesses is an important factor in generating the biweekly
periodic ﬂuctuations northeast of Taiwan.
Thus, both indirect and direct processes may contribute to thegeneration of the biweekly periodic ﬂow ﬂuctuations on the shelfAuthor's personal copy
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D. Takahashi et al. / Continental Shelf Research 29 (2009) 1896–1907

slope northeast of Taiwan. However, according to previous andpresent results, these ﬂuctuations on the shelf slope northeast of
Taiwan are more likely to be generated by remote effects fromdirect processes from the vicinity of the ETC or outside the ECS. In
this case, after the biweekly periodic ﬂow ﬂuctuations reach theshelf slope northeast of Taiwan, these ﬂuctuations will behave likethe 2nd-mode interior shelf waves advected by the Kuroshio’s
mean ﬂow.
5.2. Biweekly periodic variations of the Kuroshio axis on the shelfslope in the East China Sea
Previous results indicate that Kuroshio meanders in the ETC
and in the region between the PN line and the Tokara Strait haveperiods of 10–20 days, wavelengths of 100–375 km, and downstream phase speeds of about 17–30 km day1 (Sugimoto et al.,
1988; Qiu et al., 1990; Ichikawa and Beardsley, 1993; James et al.,
1999; Feng et al., 2000; Zhang et al., 2001; Nakamura et al., 2003).
The biweekly periodic variations of the Kuroshio axis northeast of
Taiwan described in the present study have similar spatiotemporal scales as those described in the aforementioned studies,suggesting that similar variations or meanders exist along theentire shelf break in the ECS. This is supported by the numericalresults of Guo et al. (2006), who estimated the total volumetransport across the 200 m isobath along the ECS shelf break using
a 1/181 nested ocean model and found that the total cross-shelfvolume transport varies over the short term with a period of about
17 days. Moreover, the downstream propagation of the variations
or meanders of the Kuroshio axis northeast of Taiwan and thesimilar variations or meanders in the ETC, northeast of Taiwan,
and from the PN line to the Tokara Strait suggest that the Kuroshio
axis variations in the ECS are caused not only by local baroclinic
instability effects on the shelf slope but also by remote effects
from upstream regions.
Moreover, the biweekly periodic variations of the Kuroshio axis
produce onshore cross-shelf ﬂows on the shelf slope northeast of
Taiwan (see Fig. 10), suggesting that topographically controlled
upwelling is induced by both the biweekly periodic Kuroshio axis
variations and the shelf slope. Considering the topographicallycontrolled upwelling and the onshore cross-shelf ﬂow, thebiweekly periodic Kuroshio axis variations northeast of Taiwan
might contribute not only to the onshore transport of Kuroshio
surface water, but also that of nutrient-rich Kuroshio subsurface
water onto the ECS shelf. This scenario might be supported by the
results of Chen and Wang (1999) and Guo et al. (2006). Chen andWang (1999) hydrographic observations indicated that upwellingof subsurface water is an important source of nutrients to the ECS.
According to Guo et al. (2006), biweekly periodic Kuroshio
variations can occur along the entire ECS shelf slope, including
the shelf slope northeast of Taiwan. This indicates that theonshore transport of the Kuroshio surface and nutrient-rich
subsurface waters could be induced by the biweekly periodic
Kuroshio axis variations on the shelf slope northeast of Taiwan.

6. Summary
We investigated the short-term variations of the Kuroshio axis
northeast of Taiwan occurring over a period of 10–20 days using
long-range high-frequency radar data from 2002–2005. The shortterm Kuroshio axis variations, with periods of 11–14 days, arecharacterized by onshore–offshore (northwest–southeastward)
movements of the Kuroshio axis on the shelf slope around25.0–25.751N over this period. The onshore–offshore movementsof the Kuroshio axis are induced by biweekly periodic ﬂow
ﬂuctuations, which are caused by the downstream (northeast-ward) propagation of cyclonic/anticyclonic eddies from the ETC
vicinity. This downstream propagation keeps wavelike characteristics trapped on the shelf slope, which are characterized bywavelengths along the Kuroshio of 197–353 km and phase speedsof 16–29 km day1, and which occur in the strong Kuroshio’s mean
ﬂow along the shelf slope northeast of Taiwan from 2002–2005.
The behaviors of the biweekly periodic ﬂow ﬂuctuations on the
shelf slope are explained well by the propagation of 2nd-mode
interior shelf waves advected by the mean ﬂow. It should be
noted, however, that the 2nd-mode interior shelf waves advectedby the Kuroshio’s mean ﬂow are considered on the assumptionthat the mean ﬂow is horizontally and vertically uniform and thatstratiﬁcation has a moderately weak inﬂuence. It is therefore
necessary to consider the effects of both horizontal and vertical
shears of the mean ﬂow and stratiﬁcation on shelf slope motion in
future work.
Moreover, remote effects from the vicinity of the ETC or fromoutside the ECS are believed to play an important role ingenerating the biweekly periodic ﬂow ﬂuctuations northeast of
Taiwan. On the shelf slope, an onshore current across the shelfslope is driven by the biweekly periodic Kuroshio axis variations,suggesting possible topographically controlled upwelling on the
shelf slope. Therefore, the biweekly periodic Kuroshio axis
variations northeast of Taiwan might contribute not only toonshore transport of the Kuroshio surface water but also to that ofnutrient-rich Kuroshio subsurface water onto the shelf in the East
China Sea. It should be noted, however, that these generation andonshore transport scenarios are subject to some assumptions. It istherefore necessary to conduct future observations and numericalmodeling to conﬁrm these scenarios.

Acknowledgments
The present study was supported in part by a Grant-in-Aid for
the Global COE Program in Ehime University from the Ministry of
Education, Culture, Sports, Science and Technology, Japan (MEXT),by the Japan Society for the Promotion of Science (JSPS), and by
Nihonseimei-zaidan, Japan. We thank Dr. H. Takeoka, Dr. A.
Kaneda, Dr. K. Ichikawa, Dr. P. H. Chang, and Dr. R. S. Balotro for
their useful suggestions. We wish to thank Dr. A. Masuda and Dr.
Y. Yoshikawa for their instructive comments in a meeting in the
Research Institute for Applied Mechanics, Kyushu University. We
also thank the National Institute of Information and Communication Technology (NICT) for providing surface current dataobtained using high-frequency radar (HF radar).
References
Buchwald, V.T., Adams, J.K., 1968. The propagation of continental shelf waves.
Proceedings of the Royal Society of London 305, 235–250.
Chen, -T.A.C., Wang, S.-L., 1999. Carbon, alkalinity and nutrient budgets on the East
China Sea continental shelf. Journal of Geophysical Research 104,
20675–20686.
Chuang, W.-S., Wu, C.-K., 1991. Slope-current ﬂuctuations northeast of Taiwan
winter 1990. Journal of the Oceanographical Society of Japan 47, 185–193.
Chuang, W.-S., Li, H.-W., Tang, T.Y., Wu, C.-K., 1993. Observations of the countercurrent on the inshore side of the Kuroshio northeast of Taiwan. Journal of
Oceanography 49, 581–592.
Chuang, W.-S., Liang, W.-D., 1994. Seasonal variability of intrusion of the Kuroshio
water across the continental shelf northeast of Taiwan. Journal of Oceanography 50, 531–542.
Feng, M., Mitsudera, H., Yoshikawa, Y., 2000. Structure and variability of theKuroshio current in Tokara Strait. Journal of Physical Oceanography 30,
2257–2276.
Gill, A.E., 1982. Atmosphere–Ocean Dynamics. Academic Press, London, p. 412.
Guo, X., Miyazawa, Y., Yamagata, T., 2006. The Kuroshio onshore intrusion along
the shelf break of the east China Sea: the origin of the Tsushima warm current.
Journal of Physical Oceanography 36, 2205–2231.
Hanawa, K., Mitsudera, H., 1985. On daily average of oceanographic data. Bulletin
on Coastal Oceanography 23, 79–87 (in Japanese).

Author's personal copy
ARTICLE IN PRESS
D. Takahashi et al. / Continental Shelf Research 29 (2009) 1896–1907

Hsueh, Y., 1993. Blocking of the Kuroshio by the continental shelf northeast of
Taiwan. Journal of Geophysical Research 98, 12351–12359.
Hsueh, Y., 2000. The Kuroshio in the East China Sea. Journal of Marine Systems 24,
131–139.
Hsueh, Y., Wang, J., Chern, C.S., 1992. The intrusion of the Kuroshio across thecontinental shelf northeast of Taiwan. Journal of Geophysical Research 97,
14323–14330.
Huthnance, J.M., 1978. On coastal trapped waves: analysis and numericalcalculation by inverse iteration. Journal of Physical Oceanography 8,
74–92.
Ichikawa, K., 2001. Variation of the Kuroshio in the Tokara Strait induced by mesoscale eddies. Journal of Oceanography 57, 55–68.
Ichikawa, K., Tokeshi, R., Kashima, M., Sato, K., Matsuoka, T., Kojima, S., Fujii, S.,
2008. Kuroshio variations in the upstream region as seen by HF radar andsatellite altimetry data. International Journal of Remote Sensing 29,
6417–6426.
Ichikawa, H., Beardsley, R.C., 1993. Temporal and spatial variability of volumetransport of the Kuroshio in the East China Sea. Deep Sea Research 40,
583–605.
James, C., Wimbush, M., Ichikawa, H., 1999. Kuroshio meanders in the East China
Sea. Journal of Physical Oceanography 29, 259–272.
Liu, K.-K., Gong, G.-C., Shyu, C.-Z., Pai, S.-C., Wei, C.-L., Chao, S.-Y., 1992. Response of
Kuroshio upwelling to the onset of the northeast monsoon in the sea north of
Taiwan: observation and a numerical simulation. Journal of GeophysicalResearch 97, 12511–12526.
Nakamura, H., Ichikawa, H., Nishina, A., 2003. Kuroshio path meander between thecontinental slope and the Tokara Strait in the East China Sea. Journal ofGeophysical Research 108 (C11), 3360, doi:10.1029/2002JC001450.

1907

Qiao, F., Zheng, Q., Ge, R., Yu, F., Fang, G., 2005. Cruse observations of a cold corering and b-spiral on the East China Sea continental shelf. Geophysical Research
Letters 30, L02601, doi:10.1029/2004GL021591.
Qiu, B., Imasato, N., 1990. A numerical study on the formation of the Kuroshio
counter current and the Kuroshio branch current in the East China Sea.
Continental Shelf Research 10, 165–184.
Qiu, B., Toda, T., Imasato, N., 1990. On Kuroshio front ﬂuctuations in the East China
Sea using satellite and in situ observational data. Journal of GeophysicalResearch 95, 18191–18204.
Sato, K., Matsuoka, T., Kojima, S., Fujii, S., 2004. Long-range ocean radar observationof Kuroshio upstream region in the East China Sea. Bulletin on Coastal
Oceanography 41, 119–127 (in Japanese with English abstract).
Sugimoto, T., Kimura, S., Miyaji, K., 1988. Meander of the Kuroshio front and current
variability in the East China Sea. Journal of the Oceanographical Society ofJapan 44, 125–135.
Sun, X., Su, Y., 1994. On the Variation of Kuroshio in East China Sea. Oceanology ofChina Sea 1, vol. 1. Kluwer Academic Publishers, London, pp. 49–58.
Tang, T.Y., Tai, J.H., Yang, Y.J., 2000. The ﬂow pattern north of Taiwan and themigration of the Kuroshio. Continental Shelf Research 20, 349–371.
Tang, T.Y., Hsueh, Y., Yang, Y.J., Ma, J.C., 1999. Continental slope ﬂow northeast of
Taiwan. Journal of Physical Oceanography 29, 1353–1362.
Tang, T.Y., Yang, Y.J., 1993. Low frequency current variability on the shelf breaknortheast of Taiwan. Journal of Oceanography 49, 193–210.
Yang, Y., Liu, C.–T., Hu, J.–H., Koga, M., 1999. Taiwan current (Kuroshio) and
impinging eddies. Journal of Oceanography 55, 609–617.
Zhang, D., Lee, T.N., Johns, W.E., Liu, C.–T., Zantopp, R., 2001. The Kuroshio east of
Taiwan: modes of variability and relationship to interior ocean mesoscaleeddies. Journal of Physical Oceanography 31, 1054–1074.