IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 9, NO. 3, MAY 2012 517

Kuroshio Front in the East China Sea from SatelliteSST and Remote Sensing Data

Ze Liu and Yijun Hou

Abstract—Based on the Moderate-Resolution Imaging Spec-troradiometer (MODIS) sea surface temperature (SST) of tenyears (2000–2009) and Sea-viewing Wide Field-of-view Sensor(SeaWiFS) ocean color data of 13 years (1998–2010), the Kuroshiooceanic front exists between two entirely different water massesin the East China Sea, namely, the Kuroshio and continental shelfwaters. The Kuroshio thermal front (derived from the SST data)disappears in summer due to the spatially uniform surface heating,whereas the Kuroshio ocean color front in summer is distinct andexhibits the same spatial pattern along the shelf break of the deepOkinawa Trough as in winter. The Kuroshio thermal front hasone bifurcation at about (126◦ E, 28◦ N) and extends northwardtoward the Cheju Island lasting from December to March, but thiskind of branch is absent in the ocean color data.

Index Terms—East China Sea (ECS), Kuroshio front, oceancolor, sea surface temperature (SST).

I. INTRODUCTION

THE East China Sea (ECS) includes a broad continentalshelf to the northwest and a narrow deep component of

Okinawa Trough to the southeast. The ECS is connected to theSouth China Sea (SCS), Japan Sea, and Northwestern Pacificthrough the Taiwan Strait, Tsushima Strait, and Tokara Strait,respectively (Fig. 1). As the western boundary current in theNorth Pacific subtropical gyre, the Kuroshio is the main sourceof heat and oceanic materials of open-ocean origin and playsan important role in the ECS transports and exchanges of massand heat [1]–[4].

Except for the coastal zones (shallower than 50 m), the ECSmainly contains the shelf water and the Kuroshio water [5].The oceanic fronts separating these two water masses withdifferent physical and bio-optical parameters develop over theshelf break, an area where the depths range between 200-and 1000-m isobaths. Based on satellite-derived sea surfacetemperature (SST) images in thermal infrared bands, seasonal

Manuscript received July 15, 2011; revised September 18, 2011; acceptedOctober 9, 2011. Date of publication December 2, 2011; date of current versionMarch 7, 2012. This work was supported in part by the National NaturalScience Foundation of China under Grant 41030855 and in part by the Knowl-edge Innovation Program of the Chinese Academy of Sciences under GrantKZCX1-YW-12.

Z. Liu is with the Key Laboratory of Ocean Circulation and Waves, Instituteof Oceanology, Chinese Academy of Sciences, Qingdao 266071, China, andalso with the Graduate University of Chinese Academy of Sciences, Beijing100039, China.

Y. Hou is with the Key Laboratory of Ocean Circulation and Waves, Instituteof Oceanology, Chinese Academy of Sciences, Qingdao 266071, China (e-mail:yjhou@qdio.ac.cn).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LGRS.2011.2173289

Fig. 1. Chart of the ECS with bottom bathymetry from ETOP02 digitaltopography.

variability of thermal fronts has been studied in the ECS [6]and in the northern SCS [7]. Examining ocean color images ofchlorophyll-a concentration (CHL), Takahashi and Kawamura[8] proposed a new method to detect the Kuroshio front pathto avoid the heating influence by solar radiation that makes thewhole region more uniform and demonstrated its applicabilityin the ocean south of Japan in summer.

This study adds the previous literature in several aspects.First, long-term annual mean maps of SST, ocean colors, andtheir gradient magnitude distributions are made for the ECSarea from ten-year (2000–2009) Moderate-Resolution ImagingSpectroradiometer (MODIS) SST and 13-year (1998–2010)Sea-viewing Wide Field-of-view Sensor (SeaWiFS) oceancolor data. Second, two kinds of Kuroshio fronts are detectedbased on a combination of existing algorithms, which doesnot need any objective gradient thresholds for satellite data.In addition, one branch splits from the Kuroshio thermal frontonly in wintertime, extending along northwest of Ryukyu Islandarc chain and coastwise southwest of Kyushu. However, theocean color front does not have this kind of branch at thesame time.

II. DATA AND METHODS

The data used in this study have been obtained byMODIS/Terra since 2000 and by the SeaWiFS/SeaStar since1998. Although MODIS measures the SST by two bands atthermal infrared with 11 μm and mid-infrared (mid-IR) with4 μm, the differences are quite indistinctive to the study, and we

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518 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 9, NO. 3, MAY 2012

Fig. 2. (Top row) Long-term annual mean original distributions and (bottom row) gradient magnitude maps. Left column indicates SST from MODIS for the2000–2009 period; mid and right columns show the CHL and Kd490 distributions of SeaWiFS data averaged from 1998 to 2010, respectively. Black contours areisolines of SST, CHL, and Kd490, respectively; white contours delineate the 100- and 200-m isobaths.

only choose the 4-μm SST product. The monthly SST data aregridded with global spatial coverage of nearly 4-km resolution,acquired from the Goddard Distributed Active Archive Centerof National Aeronautics and Space Administration (NASA).The MODIS mid-IR detectors measure SST in the 3.8–4.1-μmranges and have higher sensitivity and lower noise-to-signalcharacteristics compared with previous radiometers, which isparticularly useful in the high-water-vapor low-latitude regionssuch as the ECS. The SeaWiFS-derived CHL and diffuseattenuation parameter (Kd) are obtained from the SeaWiFS(version 5.1) level 3 mapped ocean color products which wereprocessed by the NASA Ocean Biology Processing Group. Thehorizontal and time resolutions of the data are about 9.28 kmand one month, respectively. The commonly used quantity inocean color remote sensing is the vertically averaged value thatintegrates data not only from the surface, like SST, but alsothroughout the euphotic zone. The Kd mainly represents theturbidity of oceanic and coastal waters; in this letter, the Kd490(Kd at λ = 490 nm, where λ is the light wavelength in freespace) data are used to study the Kuroshio front in the ECS forthe first time.

According to the contextual median filter method [9], all dataare first low pass filtered to remove isolated noise; then, the

gradient vector is computed by the Sobel operator consisting oftwo 3 × 3 convolution masks GX and GY , which is known asan effective way to improve visibility of edges in digital images.GX = [−1 0 1;−2 0 2;−1 0 1], and GY simply turns GX90◦ counterclockwise, containing approximations for derivatesin X- and Y -directions, respectively. Bio-optical variables suchas CHL and Kd490 are shown to be distributed lognormally ata variety of spatiotemporal scales [10], so that the ocean colorgradient is calculated from the lognormally transformed datawhich are the natural logarithm of the ratio of adjacent CHLor Kd490 values at every pixel. Finally, we utilize the Cannyedge detector [11] to identify the Kuroshio front in the ECS.Edges are marked at local maxima in gradient magnitude basedon a smoothed image, where thermal front path is defined inSST fields, and ocean color front path is identified by remotesensing fields.

Of the total 119 monthly SST fields from MODIS and 157monthly ocean color fields from SeaWiFS, 18 SST maps and49 ocean color maps are heavily covered by clouds or badretrievals over the ECS, particularly in the vicinity of Kuroshio.Therefore, interpretation of the frontal distribution from in-dividual images is impractical, and climatological month issuitable as temporal interval in this study.

LIU AND HOU: KUROSHIO FRONT IN THE EAST CHINA SEA 519

Fig. 3. Kuroshio front path patterns of monthly climatology in the ECS, including (black solid lines) SST fronts, (black dashed lines) SST front branches, (reddashed lines) CHL fronts, and (blue dashed lines) Kd490 fronts.

III. RESULTS

A. Annual Mean Composite Front Patterns

In Fig. 2(a), the Kuroshio path in the ECS is clearly identifiedby a warm tongue-shape feature in the Okinawa Trough extend-ing northeastward from the east coast of Taiwan. The bound-ary between the Kuroshio and the continental shelf waters isindicated by 25 ◦C–27 ◦C contours along the approximately200-m isobath. Line features with relatively low values of CHLor Kd490 along the bottom topography are prominent [Fig. 2(b)and (c)] instead of the tongue-shape protrusion in the Kuroshioregion. The 0.2-mg · m−3 CHL and 0.05-m−1 Kd490 isolinesfollow the 200-m isobath and outflow the ECS at the TokaraStrait, while the 0.5-mg · m−3 CHL and 0.07-m−1 Kd490isolines wander along the 100-m isobath until reaching theTsushima Strait. The Kd490 values lower than 0.2 m−1 in theECS encompass most of the shelf and Kuroshio regions, whichmeans that the systematic underestimation produced by theempirical algorithms is not significant [12].

The SST gradient magnitude distribution in Fig. 2(d) agreeswell with the thermal frontal probability map for the period1985–1996 [6]. The front of the Kuroshio main stream is alwaysstronger than 0.03 ◦C · km−1 and even reaches 0.05 ◦C · km−1

off northeastern Taiwan caused by the branch current [13]or by the shoreward intrusion of Kuroshio [14]. A frontalbranch originates from the Kuroshio in the central ECS andstretches northward toward the Tsushima Strait along the 100-misobath with maximum values ranging from 0.03 ◦C · km−1 to0.04 ◦C · km−1. The Kuroshio front also appears in the CHLand Kd490 maps [Fig. 2(e) and (f)] with features similar to thecorresponding SST maps. The large gradient values with highbright signals in the CHL and Kd490 maps emerge northeastof Taiwan; however, bifurcation southwest of Kyushu is tooblurred to be identified.

B. Monthly Variation of the Kuroshio Front Path

To study the seasonal cycle of the Kuroshio front, we plot-ted local maximum points of gradient magnitude across theKuroshio front along meridional sections. First, the annualmean gradient magnitude is used to determine the locationsof Kuroshio front. Then, using the monthly climatology dataand Canny method [11], the mean monthly frontal paths of theKuroshio in the ECS are calculated. This gives long-term meanmonthly estimates of the Kuroshio front path.

Fig. 3 shows the seasonal variability of the Kuroshio front.The Kuroshio thermal front does not overlap with the Kuroshioocean color front completely, with the former protruding closeronshore than the latter, particularly in the upper and middle

reaches in some months. The thermal front twists north ofTaiwan and extends to the northeast along the shelf break inwintertime, whereas it becomes a straight short line in summer-time, apparently because of the seasonal homogeneous surfacewarming. The northward branch splits from the main front atabout (126◦ E, 28◦ N) during the period from December toMarch and is collocated with the Tsushima Warm Current [15]–[17] in the surface layer. Both CHL and Kd490 parametersshow that the front path wriggles without a distinct seasonalpattern, which means that the boundary between the shelf waterand the Kuroshio water persists even in summer. Nevertheless,there is no branch originating from the Kuroshio main frontderived from ocean color data in any season. This phenomenonis inconsistent with the thermal frontal branch in SST fieldsand may result from the baroclinicity or vertically varyingcomponent of the Tsushima Warm Current [18].

IV. CONCLUSION

The ten-year (2000–2009) MODIS SST and 13-year(1998–2010) SeaWiFS ocean color data have been applied hereto produce the first comprehensive year-round climatology ofthe oceanic thermal front path patterns separating shelf waterfrom Kuroshio water in the ECS. A newly found front derivedfrom ocean color data is also described. All of the Kuroshiofronts in the ECS are seasonally persistent, following the 200-misobath. It is noteworthy that, during summertime, signaturesof thermal front fade and disappear due to heating, whilethe signatures of missing features can be derived from oceancolor images. A thermal frontal branch emerges in wintertime,suggestive of an important role of the Tsushima Warm Currentforcing on the surface temperature evolution in this region.

ACKNOWLEDGMENT

The authors would like to thank the editor and five anony-mous reviewers for comments that helped to improve the earlierversion of this letter. The authors would also like to thankDr. I. Belkin for helping to perfect the presentation and structureof this paper.

REFERENCES

[1] C.-S. Chern, J. Wang, and D.-P. Wang, “The exchange of Kuroshio andEast China Sea shelf water,” J. Geophys. Res., vol. 95, no. C9, pp. 16 017–16 023, Sep. 1990.

[2] H. Ichikawa and R. C. Beardsley, “Temporal and spatial variability ofvolume transport of the Kuroshio in the East China Sea,” Deep-Sea Res.,Part I, Oceanogr. Res. Papers, vol. 40, no. 3, pp. 583–605, 1993.

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[3] H.-J. Lie and C.-H. Cho, “Recent advances in understanding the circula-tion and hydrography of the East China Sea,” Fish. Oceanogr., vol. 11,no. 6, pp. 318–328, Nov. 2002.

[4] G.-C. Gong, Y.-H. Wen, B.-W. Wang, and G.-J. Liu, “Seasonal vari-ation of chlorophyll a concentration, primary production and environ-mental conditions in the subtropical East China Sea,” Deep-Sea Res.,Part II, Topical Studies Oceanogr., vol. 50, no. 6/7, pp. 1219–1236,Mar./Apr. 2003.

[5] G. T. F. Wong, S.-Y. Chao, Y.-H. Li, and F.-K. Shiah, “The Kuroshioedge exchange processes (KEEP) study—An introduction to hypothesesand highlights,” Continental Shelf Res., vol. 20, no. 4/5, pp. 335–347,Mar. 2000.

[6] R. Hickox, I. Belkin, P. Cornillon, and Z. Shan, “Climatology and seasonalvariability of ocean fronts in the East China, Yellow and Bohai seas fromsatellite SST data,” Geophys. Res. Lett., vol. 27, no. 18, pp. 2945–2948,Sep. 2000.

[7] D. Wang, Y. Liu, Y. Qi, and P. Shi, “Seasonal variability of thermal frontsin the northern South China Sea from satellite data,” Geophys. Res. Lett.,vol. 28, no. 20, pp. 3963–3966, Oct. 2001.

[8] W. Takahashi and H. Kawamura, “Detecting method of the Kuroshio frontusing the satellite-derived chlorophyll-a images,” Remote Sens. Environ.,vol. 97, no. 1, pp. 83–91, 2005.

[9] I. M. Belkin and J. E. O’Reilly, “An algorithm for oceanic front detectionin chlorophyll and SST satellite imagery,” J. Marine Syst., vol. 78, no. 3,pp. 319–326, Oct. 2009.

[10] J. W. Campbell, “The lognormal distribution as a model for bio-opticalvariability in the sea,” J. Geophys. Res., vol. 100, no. C7, pp. 13 237–13 254, Jul. 1995.

[11] J. Canny, “A computational approach to edge detection,” IEEETrans. Pattern Anal. Mach. Intell., vol. PAMI-8, no. 6, pp. 679–698,Nov. 1986.

[12] Z.-P. Lee, M. Darecki, K. L. Carder, C. O. Davis, D. Stramski, andW. J. Rhea, “Diffuse attenuation coefficient of downwelling irradiance:An evaluation of remote sensing methods,” J. Geophys. Res., vol. 110,no. C02 017, Feb. 2005. doi:10.1029/2004JC002573.

[13] B. Qiu and N. Imasato, “A numerical study on the formation ofthe Kuroshio counter current and the Kuroshio branch current in theEast China Sea,” Continental Shelf Res., vol. 10, no. 2, pp. 165–184,Feb. 1990.

[14] Y. Hsueh, J. Wang, and C.-S. Chern, “The intrusion of the Kuroshio acrossthe continental shelf northeast of Taiwan,” J. Geophys. Res., vol. 97,no. C9, pp. 14 323–14 330, Sep. 1992.

[15] O. K. Huh, “Spring season flow of the Tsushima current and its separationfrom the Kuroshio: Satellite evidence,” J. Geophys. Res., vol. 87, no. C12,pp. 9687–9693, Nov. 1982.

[16] H.-J. Lie and C.-H. Cho, “On the origin of the Tsushima Warm Current,”J. Geophys. Res., vol. 99, no. C12, pp. 25 081–25 091, Dec. 1994.

[17] Y. Hsueh, H.-J. Lie, and H. Ichikawa, “On the branching of the Kuroshiowest of Kyushu,” J. Geophys. Res., vol. 101, no. C2, pp. 3851–3857,Feb. 1996.

[18] A. Isobe, “Seasonal variability of the barotropic and baroclinic motion inthe Tsushima–Korea Strait,” J. Oceanogr., vol. 50, no. 2, pp. 223–238,Mar. 1994.