GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L23603, doi:10.1029/2006GL027415, 2006
Wintertime sea surface temperature fronts in the Taiwan Strait
Yi Chang,1 Teruhisa Shimada,2 Ming-An Lee,1,3 Hsueh-Jung Lu,1 Futoki Sakaida,2 and
Hiroshi Kawamura2
Received 30 June 2006; revised 8 September 2006; accepted 18 October 2006; published 5 December 2006.
[1] We present wintertime variations and distributions of
sea surface temperature (SST) fronts in the Taiwan Strait
by applying an entropy-based edge detection method to
10-year (1996 –2005) satellite SST images with grid size
of 0.01°. From climatological monthly mean maps of
SST gradient magnitude in winter, we identify four
significant SST fronts in the Taiwan Strait. The Mainland
China Coastal Front is a long frontal band along the 50-m
isobath near the Chinese coast. The sharp Peng-Chang
Front appears along the Peng-Hu Channel and extends
northward around the Chang-Yuen Ridge. The Taiwan Bank
Front evolves in early winter. As the winter progresses, the
front becomes broad and moves toward the Chinese coast,
connecting to the Mainland China Coastal Front. The
Kuroshio Front extends northeastward from the northeastern
tip of Taiwan with a semicircle-shape curving along the
100-m isobath. Citation: Chang, Y., T. Shimada, M.-A. Lee,
prehensive vision of SST fronts in TS has not been available
yet. That is because detection of finer structure of SST
fronts in the strait requires higher resolution data and more
sophisticated front detection methods.
[4] In this study, we apply a front detection method
developed by Shimada et al. [2005] to 10-year NOAA
AVHRR (Advanced Very High Resolution Radiometer)
SST images with a grid size of 0.01° and present wintertime
SST frontal views in TS. This study focuses on wintertime
only, but gives the first full description of inter-seasonal
SST front evolution and distribution in TS together with
discussion of relation with the results of comparable studies
in the surrounding seas [Hickox et al., 2000; Wang et al.,
2001].
H.-J. Lu, F. Sakaida, and H. Kawamura (2006), Wintertime sea
surface temperature fronts in the Taiwan Strait, Geophys. Res.
Lett., 33, L23603, doi:10.1029/2006GL027415.
[ 5 ] We use 10-year (1996 – 2005) NOAA AVHRR
SST images with a grid size of 0.01° produced by the AHIGHERS system [Sakaida et al., 2000] from regional
HRPT data library at Tohoku University and National
Taiwan Ocean University.
[6] We adopt an entropy-based edge detection method
proposed by Shimada et al. [2005]. The methodology is
independent of seasonally/regionally varying geophysical
parameters, and can detect and retain finer-scale SST fronts.
The effectiveness is proven especially for detecting fronts in
coastal seas [Shimada et al., 2005].
[7] Among the several kinds of composite front maps
[e.g., Shimada et al., 2005], this study uses climatological
monthly mean maps of frontal SST gradient magnitude
(GM). The GM is defined as below:
1. Introduction
[2] The Taiwan Strait (TS) is a shallow (60 m) channel
with 180-km wide and 350-km long and connecting the East
China Sea (ECS) and the South China Sea (SCS). It has
complex bathymetric features such as shallow (<50 m) shelf
along allover the Chinese coast, the Taiwan Bank (<50 m),
the Chang-Yuen Ridge (CYR) (<50 m), and Peng-Hu
Channel (PHC) (100 m) as shown in Figure 1. Coastal
and oceanic waters flowing from ECS, SCS, and the
northwestern Pacific Ocean are present in the strait. These
features contribute to the formation of thermal fronts
between the water masses [e.g., Jan et al., 2002].
[3] As for sea surface temperature (SST) fronts in adjoining ECS and SCS, Hickox et al. [2000] and Wang et al.
[2001] successfully derived SST front schematics by applying histogram-based [Cayula and Cornillon, 1995] and
gradient-based front detection methods to long-term NOAA
(National Oceanic and Atmospheric Administration)/NASA
(National Aeronautics and Space Administration) Pathfinder SST images with 9.28-km resolution. They identify
ten predominant fronts in ECS and six in SCS, and discuss
their climatological seasonal variations. However, a com1
Department of Environmental Biology and Fisheries Science, National
Taiwan Ocean University, Keelung, Taiwan.
2
Ocean Environment Group, Center for Atmospheric and Oceanic
Studies, Graduate School of Science, Tohoku University, Sendai, Japan.
3
Also at Ocean Data Bank, National Center for Ocean Research, Taipei,
Taiwan.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL027415
2. Data and Method
GM ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð@T =@xÞ2 þð@T =@yÞ2 ð C=kmÞ
ð1Þ
where T is SST, and x and y axes are directed toward east
and north, respectively. The GM is computed at all the
frontal pixels for each image, and then monthly mean GM is
computed pixel by pixel. This map enhances frontal patterns
and reveals details of frontal systems. The GM itself is also
important for describing the front characteristics.
3. Climatological Monthly Maps of SST Fronts in
Winter
[8] Figure 2 shows the climatological monthly mean
maps of frontal SST GM in winter (December – February).
The close-up views in January and February are shown in
Figure 3 with a focus on the west and north of the Taiwan.
We identify four significant SST fronts, and describe their
distributions and evolutions one by one. First we identify a
long frontal band with larger SST GM along the 50-m
L23603
1 of 4
L23603
CHANG ET AL.: WINTERTIME SST FRONTS IN THE TAIWAN STRAIT
Figure 1. Map of bottom topography and geographical
names referred to in this paper. The black contours indicate
coastal line and bottom depth in meter. CYR, PHI and PHC
represent the location of Chang-Yuen Ridge, Peng-Hu
Islands and Peng-Hu Channel, respectively. ECS and SCS
are the East China Sea and the South China Sea.
isobath near the Chinese coast. Hereafter this front is
referred to as the Mainland China Coastal Front. The SST
GMs in the front increase as winter progresses from
December (0.2°C/km) to February (up to 0.3°C/km),
along with growth of the area with large SST GM. The
frontal band has three zones with particularly large SST GM
L23603
(>0.2°C/km), which are located at latitudes of 24.0– 25.0°N,
26.5– 27.0°N and 27.5– 28.5°N. In January and February
(Figures 2b, 2c and 3a), this frontal band extends southwestward from 24.0°N to reach the northern edge of the
Taiwan Bank. This extension has SST GM of 0.2°C/km.
[9] From the both sides of the PHC between Peng-Hu
Islands (PHI) and the southwest coast of Taiwan, two sharp
frontal bands with large GMs (>0.1°C/km) are formed
(Figures 2 and 3a). One extends northwestward from the
east side of PHI along the southern edge of the CYR. The
other is located along the Taiwanese coast. The SST GMs
of these fronts are largest up to 0.2°C/km in January
(Figures 2b and 3a). Meanwhile, a weak front is seen on
the northern side of CYR (Figure 3a). At the most developed stage in January (Figure 3a), the front is formed on the
shallower side of the 50-m isobath. These fronts are
collectively called the Peng-Chang Front.
[10] There is a broad frontal band with GMs of 0.1–
0.2°C/km in December (Figure 2a) along the southern steep
edge of the Taiwan Bank, right inshore of the 50-m isobath.
Then, the core of the front moves northward and the frontal
band becomes blurred and scatters widely over the bank in
January and February (Figures 2b, 2c and 3a). Additionally,
this broad frontal band over the Taiwan Bank connects to
the southward extension of the Mainland China Coastal
Front (Figure 3b). This is quite a contrast to separated
structures in December (Figure 2a). This front is called
the Taiwan Bank Front.
[11] A frontal band composed of many lines leaves from
the northeastern tip of the Taiwanese coast. It is a semicircular curve roughly along the 100-m isobath on the deepwater side (Figures 2 and 3b). Then, it deviates eastward
from the isobath at around 26.4°N/122.0°E. The SST GM of
this frontal band becomes larger from 0.1°C/km in December to 0.2°C/km in February. Throughout the winter,
the largest SST GM (>0.2°C/km) is found in the region
where the band leaves the coast. Because this front is
Figure 2. Climatological monthly mean maps of frontal SST GM in winter: (a) December; (b) January; and (c) February.
2 of 4
L23603
CHANG ET AL.: WINTERTIME SST FRONTS IN THE TAIWAN STRAIT
L23603
Figure 3. Close-up views of climatological monthly mean maps of frontal SST GM in (a) January and (b) February, with
focus on west and north of Taiwan.
consistent with the Kuroshio path, we labeled it as the
Kuroshio Front.
4. Discussion
[12] In this section, we discuss the relation of the present
results with the results of comparable studies in the ECS
[Hickox et al., 2000] and SCS [Wang et al., 2001]. Figure 4
illustrates a schematic of the four SST fronts described in
section 3 together with parts of schematic diagrams derived
by those studies.
[13] From Figure 4, we can say that the Mainland China
Coastal Front is a southward extension of the ZhejiangFujian Front defined by Hickox et al. [2000]. The ZhejiangFujian Front extends northward along the Chinese coast and
connects to the Jiangsu Front [Hickox et al., 2000]. On the
other hand, Wang et al. [2001] have shown that the FujianGuangdong Coastal and the Taiwan Bank Front merge in
winter and extend westward to the Pearl River Estuary.
Synthesis of these two studies and this study proves the
continuity of these fronts. Additionally, the present study
clarifies the wintertime evolution and local characteristics of
the Mainland China Coastal Front, including its merging
with the Taiwan Bank Front and regional difference of SST
GM along the fronts. Now it can be concluded that we have
presented a complete wintertime picture of the front along
the Chinese coast (i.e., 50-m isobath) between 22 –35°N
through the TS.
[14] The shelf fronts develop in shallow seas due to
intense cooling by the East Asian winter monsoon. They
are boundaries between relatively cool and fresh coastal
water and warm and saline shelf water [e.g., Jan et al.,
2002]. The Fujian-Guangdong Front is the boundary between the Fujian coastal water and the SCS open water
[Wang et al., 2001], and Zhejiang-Fujian and Jiangsu Fronts
are the boundaries of the coastal water and ECS open water.
The Mainland China Coastal Front is a boundary of cold
coastal water and waters in TS. Meanwhile it is also pointed
out that the Mainland China Coastal Front is associated with
the Mainland China Coastal Current flowing southwestward
near the Chinese coast in winter [e.g., Guan and Fang,
2006].
[15] This study takes advantage of the high-resolution
data and the sophisticated edge detection method to detect
fronts around CYR and PHC. This study clearly shows the
detail frontal structures, compared with the results of Wang
Figure 4. A schematic map of the four representative SST
fronts in the Taiwan Strait. Each colored solid line marks
cores of the frontal bands with large SST GM: (1) Mainland
China Coast Front; (2) Peng-Chang Front; (3) Taiwan Bank
Front; and (4) Kuroshio Front. Parts of SST front
schematics derived by Hickox et al. [2000] and Wang et
al. [2001] are also depicted by broad gray lines: (5) southern
part of Jiangsu Front; (6) Yangtze Bank Ring Front;
(7) Zhejiang-Fujian Front; (8) Fujian-Guangdong Coastal
and Taiwan Bank fronts; and (9) Kuroshio Front.
3 of 4
L23603
CHANG ET AL.: WINTERTIME SST FRONTS IN THE TAIWAN STRAIT
et al. [2001]. The two fronts extending from the PHC
indicate inflows of the Kuroshio and SCS waters. The
inflows into the eastern TS through PHC are clearly
ascertained by satellite SST images [e.g., Zhu et al., 2004;
Shang et al., 2005; Chen and Sheu, 2006]. The PHC guides
the intrusions of Kuroshio and SCS waters. The frontal
structures around CYR also indicate watercourses of the
Kuroshio and SCS waters and the boundaries between these
and coastal waters in the TS. The CYR partially blocks and
diverts the water transport.
[16] We have showed that the Taiwan Bank Front forms a
part of the front along the Chinese coast. Over the bank,
cold SST dominates due to winter cooling. Moreover, the
Fujian coastal cold water [Li et al., 2000] and the cold water
associated with the Mainland China Coastal Current [e.g.,
Jan et al., 2002; Lee et al., 2003] flow eastward and
southward, and reach the bank, respectively. On the other
hand, the SCS warm water flows into TS along the steep
southern edge of the Taiwan Bank. The Taiwan Bank Front
is a boundary between these coastal waters and the SCS
warm water [Li et al., 2000]. It is pointed out by Lin et al.
[2005] that the complex current system over the bank
facilitates the tidal mixing, which may be one of the reasons
why the front positions scatter widely over the bank.
[17] As for the Kuroshio Front on the northeast of
Taiwan, Hickox et al. [2000] have also detected and
represented the front at the same location. The width of
the frontal band reflects large variability of the Kuroshio
path [e.g., Tang et al., 2000]. At the same time, our study
manifests the Kuroshio Front as a composition of several
sharp thin frontal features (Figure 3b). In winter, the
Kuroshio axis shifts westward after leaving the Taiwanese
coast, and has the largest curvature accompanying the width
increase and core speed decrease (less than 100 cm/s).
Further studies are required for an investigation of the
mechanism of the Kuroshio Front formation.
5. Summary
[18] We have presented wintertime monthly climatology
of SST fronts in the Taiwan Strait. They are derived from
high-resolution SST images for 10 years by applying an
edge detection method developed by Shimada et al. [2005].
The monthly maps of SST fronts reveal unprecedented fine
structures of SST field, and four significant SST fronts are
identified in the TS. This study wraps up an SST front
schematic ranging over the coastal seas from ECS to SCS
through the TS.
[19] 1. The Mainland China Coastal Front is a long
frontal band with larger SST GM (>0.2°C/km) along the
50-m isobath near the Chinese coast. It is a southward
extension of the Zhejiang-Fujian Front defined by Hickox et
al. [2000]. On the other hand, this front reaches the northern
edge of the Taiwan Bank and merges with the Taiwan Bank
Front in January and February.
[20] 2. Sharp SST fronts with SST GM up to 0.2°C/km
develop on the both sides of the PHC as the winter
progresses. One front extends along the Taiwanese coast
and the other extends northwestward parallel to the edge of
CYR. There is a weak fronts with moderate SST GM
(<0.15°C/km) along the northern edge of the CYR.
L23603
[21] 3. The Taiwan Bank Front is a broad frontal band
with SST GMs less than 0.2°C/km. It evolves along the
southern edge of the bank (i.e., the 50-m isobath) in
December. In January and February, the frontal band
scatters widely over the bank and connects to the southward
extension of the Mainland China Coastal Front along the
50-m isobath. On the other hand, this front also extends
southwestward and connects to the Fujian-Guangdong
Front [Wang et al., 2001].
[22] 4. The Kuroshio Front extends northeastward from
the northeastern tip of the Taiwanese coast. The front has a
semicircle-shape curve along the 100-m isobath and deviates eastward from the isobath at around 26.4°N.
[23] Acknowledgments. This study was conducted during the
3-month stay of the first author in Tohoku University, Japan, under the
agreement on academic exchange between Tohoku University in Japan and
the National Taiwan Ocean University (NTOU) in Taiwan. His stay was
supported by the special educational funding of the Center for Marine
Bioscience and Biotechnology, NTOU. This research is partly supported by
Special Coordination Fund for Promoting Science and Technology
‘‘New Generation SST’’ of MEXT, Japan, and the research funding
(NSC93-2611-M-019-007) of National Science Council, Taiwan. Bathymetric data were obtained from the Center for Ocean Research, Taiwan. The
authors thank three anonymous reviewers for their constructive suggestions
for improvement.
References
Cayula, J.-F., and P. Cornillon (1995), Multi-image edge detection of SST
images, J. Atmos. Oceanic Technol., 12, 821 – 829.
Chen, C.-T. A., and D. D. Sheu (2006), Does the Taiwan Warm Current
originate in the Taiwan Strait in wintertime?, J. Geophys. Res., 111,
C04005, doi:10.1029/2005JC003281.
Guan, B., and G. Fang (2006), Winter counter-wind currents off the southeastern China coast: A review, J. Oceanogr., 62, 1 – 24.
Hickox, R., I. Belkin, P. Cornillon, and Z. Shan (2000), Climatology and
seasonal variability of ocean fronts in the East China, Yellow and Bohai
Seas from satellite SST data, Geophys. Res. Lett., 27(18), 2945 – 2948.
Jan, S., J. Wang, C. S. Chen, and S. Y. Chao (2002), Seasonal variation of
the circulation in the Taiwan Strait, J. Mar. Syst., 35, 249 – 268.
Lee, M. A., C. D. Yeah, C. H. Cheng, J. W. Chan, and K. T. Lee (2003),
Empirical orthogonal function analysis of AVHRR sea surface temperature patterns in Taiwan Strait, J. Mar. Sci. Technol., 11(1), 1 – 7.
Li, L., X. Guo, and R. Wu (2000), Oceanic fronts in the southern Taiwan
Strait, J. Oceanogr. Taiwan Strait, 19(2), 147 – 156.
Lin, S. F., T. Y. Tang, S. Jan, and C. J. Chen (2005), Taiwan Strait current in
winter, Cont. Shelf Res., 25, 1023 – 1042.
Sakaida, F., J. Kudoh, and H. Kawamura (2000), A-HIGERS—The system
to produce the high spatial resolution sea surface temperature maps of
the western North Pacific using the AVHRR/NOAA, J. Oceanogr., 56,
707 – 716.
Shang, S., C. Zhang, H. Hong, Q. Liu, G. T. F. Wong, C. Hu, and B. Huang
(2005), Hydrographic and biological changes in the Taiwan Strait during
the 1997 – 1998 El Niño winter, Geophys. Res. Lett., 32, L11601,
doi:10.1029/2005GL022578.
Shimada, T., F. Sakaida, H. Kawamura, and T. Okumura (2005), Application of an edge detection method to satellite images for distinguishing
sea surface temperature fronts near the Japanese coast, Remote Sens.
Environ., 98, 21 – 34.
Tang, T. Y., J. H. Tai, and Y. J. Yang (2000), The flow pattern north of
Taiwan and the migration of the Kuroshio, Cont. Shelf Res., 20, 349 – 371.
Wang, D., Y. Liu, Y. Qi, and P. Shi (2001), Seasonal variability of thermal
fronts in the northern South China Sea from satellite data, Geophys. Res.
Lett., 28(20), 3963 – 3966.
Zhu, J., C. Chen, P. Ding, C. Li, and H. Lin (2004), Does the Taiwan warm
current exist in winter?, Geophys. Res. Lett., 31, L12302, doi:10.1029/
2004GL019997.
Y. Chang, M.-A. Lee, and H.-J. Lu, Department of Environmental
Biology and Fisheries Science, National Taiwan Ocean University, PeiNing Road, Keelung 20224, Taiwan. ([email protected])
H. Kawamura, F. Sakaida, and T. Shimada, Ocean Environment Group,
Center for Atmospheric and Oceanic Studies, Graduate School of Science,
Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai 980-8578, Japan.
4 of 4