Interannual Salinity Variations in the Tsushima Strait and...

681

Journal of Oceanography, Vol. 62, pp. 681 to 692, 2006

Keywords:⋅⋅⋅⋅⋅ Tsushima Strait,⋅⋅⋅⋅⋅ low salinity water,⋅⋅⋅⋅⋅ Changjiangdischarge,

⋅⋅⋅⋅⋅ Changjiang DilutedWater,

⋅⋅⋅⋅⋅ summer salinitycondition,

⋅⋅⋅⋅⋅ interannualvariability,

⋅⋅⋅⋅⋅ EOF analysis,⋅⋅⋅⋅⋅ Japan Sea,⋅⋅⋅⋅⋅ East China Sea.

* Corresponding author. E-mail: [email protected]

Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

Interannual Salinity Variations in the Tsushima Strait andIts Relation to the Changjiang Discharge

TOMOHARU SENJYU1*, HIROFUMI ENOMOTO2, TAKESHI MATSUNO1 and SIGEAKI MATSUI3

1Research Institute for Applied Mechanics, Kyushu University, Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan2Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan3Fukuoka Fisheries and Marine Technology Research Center, Imazu, Nishi-ku, Fukuoka 819-0165, Japan

(Received 26 December 2005; in revised form 19 May 2006; accepted 25 May 2006)

Interannual salinity variations in the Tsushima Strait are investigated on the basis ofhistorical hydrographic data. The EOF analysis revealed that the most dominant modeis the in-phase salinity variation between the eastern and western channels. The timecoefficients of the EOF first mode in summer show a negative correlation with theChangjiang discharge, which indicates that salinity in the Tsushima Strait tends todecrease over summer, related to a large discharge of the Changjiang. The eigenvectorsof the first mode are larger in the eastern channel than those in the western channel,though the low salinity water mainly flows through the western channel. This is be-cause the low salinity water spreads into the eastern channel as well as the westernchannel over summers with a large discharge of the Changjiang. The out-of-phasesalinity variation between the channels is extracted as the EOF second mode; this isthe predominant variation in the western channel. The time coefficients of the secondmode in summer show no significant correlations to the volume transports throughthe western channel and the transport differences between channels. A relationshipbetween the EOF second mode and variations in the wind stress over the East ChinaSea is suggested.

which is the entrance of materials into the Japan Sea. TheTsushima Islands separate the strait into eastern and west-ern channels; the width and maximum depth are 140 kmand 110 m in the eastern channel, and 40 km and 200 min the western channel, respectively. Hydrographically,the Tsushima Current, which is the strongest flow in theJapan Sea, originates from the strait, though more up-stream origins of the current have been suggested in theKuroshio (ex. Sverdrup et al., 1942; Lie and Cho, 1994)and/or in the Taiwan Strait (Fang et al., 1991; Isobe,1999a, b).

Recent comprehensive observations have revealedcharacteristic features of the current field in the TsushimaStrait. On the basis of long-term monitoring with anAcoustic Doppler Current Profiler (ADCP) mounted ona ferryboat, Takikawa et al. (2005) revealed that the vol-ume transport through the western channel is larger thanthat in the eastern channel: 1.54 and 1.10 × 106 m3s–1,respectively. In addition, the volume transport exhibitstwo maxima in spring and autumn; the autumn peak is

1. IntroductionLarge quantities of materials such as salt, freshwa-

ter, suspended organic and inorganic particles, phyto- andzoo-plankton, fish eggs and larvae are transported fromthe East China Sea to the Japan Sea by the Tsushima Cur-rent. For example, Isobe et al. (2002) has estimated thatthe annual average temperature and freshwater transportsfrom the East China Sea to the Japan Sea amount to 0.17PW and 3.3 × 104 m3s–1, respectively; the later is compa-rable to the total river discharge flowing into the Yellowand East China Seas. Recently, Onitsuka and Yanagi(2005) suggested that the water from the East China Seais one of the important sources of nutrients feeding pri-mary production in the southern Japan Sea.

The Tsushima Strait is a narrow, shallow strait con-necting the East China Sea with the Japan Sea (Fig. 1),

682 T. Senjyu et al.

more pronounced than the spring peak in the westernchannel, while an inverse relation is found in the easternchannel. A similar flow intensification in the westernchannel in autumn has been observed from bottom-in-stalled ADCP arrays (Jacobs et al., 2001; Teagu et al.,2002), and High Frequency (HF) radar measurements(Yoshikawa et al., 2005). However, properties of the wa-ter transported by the Tsushima Current in the strait arestill obscure.

One of the striking features of water characteristicsin the Tsushima Strait is a clear seasonal variation in sa-linity. Figure 2 shows time series of the channel-aver-aged salinity at depths of 0 and 30 m. Salinity in bothchannels show low values from summer to autumn andhigh ones from winter to spring. The decrease in salinityover summer is due to the advection of low salinity waterfrom the East China Sea (Moriyasu, 1972; Ogawa et al.,

1977; Ogawa, 1983). The low salinity water spreadswidely into the southern Japan Sea (Senjyu, 1999), andoccasionally causes abnormal low-salinity conditionsalong the Japanese coast (Kuroda and Hirai, 2000). Theprosperity and decay of the low salinity water are closelyrelated to the distribution and migration of post-larvalanchovy, Engraulis japonica, and juvenile red sea-bream,Chrysophrys major, in the southwestern Japan Sea(Ogawa et al., 1977).

The low salinity water is considered to have its ori-gin in the Changjiang (Yangtze River), which is the larg-est river in Asia (Fig. 1). However, few studies discussthe relationship between the Changjiang discharge andhydrographic conditions in the Tsushima Strait or the Ja-pan Sea. Kitani et al. (2003) showed a negative correla-tion between the maximum discharge of the Changjiangand minimum surface salinity in the eastern channel.

Fig. 1. Study area. Upper panel shows the location of Changjiang, the East China Sea, and the Japan Sea. The Tsushima Straitregion shown in the box in the upper panel is enlarged in the lower panel. Solid squares and circles in the eastern and westernchannels indicate hydrographic stations observed by FFMTRC and NFRDI, respectively.

Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge 683

However, the spatiotemporal variability in the salinityfield is still unknown, in particular in the western chan-nel. Based on a box model analysis, Nof (2001) suggestedthat the diversion of freshwater from the Changjiang (andYellow River) for agricultural use can induce frequentbottom water formation in the Japan Sea through changesin sea surface salinity. Yanagi (2002) also suggested thatthe changes in salinity in the East China Sea cause seri-ous changes in the nutrient condition in the upper 200 mof the Japan Sea. The Three-Gorges Dam, the largest hy-droelectric dam in the world, has been constructed in theupstream region of Changjiang from 1993, with comple-tion planned in 2009, and some impacts on the physicaland biochemical environments in waters adjacent to theChangjiang Estuary have to be considered (Zhu et al.,2003). To assess the dam’s effects correctly and evaluateits impacts on the Japan Sea, an understanding of the lowsalinity water transports into the Tsushima Strait is cru-cial.

In the next section, the salinity data used in this studyare described, together with a brief introduction of theChangjiang discharge. In Section 3 we show the seasonalchange of salinity in the Tsushima Strait; this is the strong-est signal in the salinity variation. A description of prin-cipal components of interannual variability in the salin-

ity field is provided in Section 4. Section 5 discusses theinterannual salinity variations in summer in relation tothe Changjiang discharge. Finally, conclusions and re-marks are given in Section 6.

2. Data

2.1 SalinityThe data analyzed consist of salinity measurements

at standard depths obtained at the stations shown in Fig.1. In the eastern channel, the Fukuoka Fisheries and Ma-rine Technology Research Center (FFMTRC) in Japan hasobserved monthly temperature and salinity at the stationsshown by solid squares in Fig. 1. On the other hand,hydrographic observations in the western channel havebeen carried out in even months by the National Fisher-ies Research and Development Institute (NFRDI) of Ko-rea (solid circles in Fig. 1). In order to discuss the con-current variations in both channels, we used bi-monthlysalinity data during the period from February 1971 toDecember 2000. These data were provided from FFMTRCand Korea Oceanographic Data Center.

The salinity obtained at irregular depths were lin-early interpolated into standard depths (0, 10, 20, 30, 50,75, 100, 125, 150 and 200 m), except for the bottomdepths. This procedure sets the data points in both thechannels to nearly the same number (70 and 72 points inthe eastern and western channels, respectively). Sincesalinity measurements in April 1974 and August 1976 atstations in the western channel were absent, they werelinearly interpolated from the observed values in Febru-ary and May in 1974 and June and September in 1976,respectively. This is the basic salinity dataset.

For the analysis of interannual variability, a datasetof salinity anomalies was prepared by subtracting theclimatological monthly means at each data point from thecorresponding original data. Since the basic dataset in-cludes no-data periods, we solved the lack of data in theanomaly dataset as follows. The salinity anomalies at Sta.E11 in the period from April 1997 to December 2000 wereestimated from those at Sta. E10, because relatively highcorrelations were found between them; the correlationcoefficients range from +0.84 at 10 m to +0.64 at 80 m.Observations at Stas. W01, W02, W06, and W07 had beeninterrupted since February 1999 as the new Japan-KoreaFishery Agreement went into effect in 1999. As for thesalinity anomalies at Sta. W02, a similar estimationmethod to that at Sta. E11 was applied using the values atSta. W03. (For the estimation at the bottom depth of 100m at Sta. W02, we used the anomalies at the bottom depthof 90 m at Sta. W03.) Salinity anomalies at Sta. W01 werefilled by the mean values of those at Stas. W03, W04,and W05 at each depth. Similarly, anomalies at Stas. W06and W07 were filled by the mean values of those at Stas.

Fig. 2. Time series of channel-averaged salinity at depths of 0m (dashed line) and 30 m (solid line) from February 1971to December 2000. According to the observation intervalin the western channel, the data in even months are plotted.The upper and lower panels are for the eastern and westernchannels, respectively.


W08, W09, and W10 at each depth. Anomalies below thedepth of 125 m at these stations were set to zero. For otherinterruptions, temporal interpolation with a cubic func-tion was applied: in the periods October 1974–February1975, June 1975, and October 1975–February 1976 atStas. E06–E11 and in December 1989, December 1991,and December 1992 at all stations in the western chan-nel.

2.2 Changjiang dischargeWe combined two datasets of monthly mean flow

rates at Datong in China (Fig. 1): Monthly Discharge Datafor World Rivers (Bodo, 2001) and Zhu (personal com-munication) for the periods 1971–1988 and 1989–2000,respectively. The discharges in 1991–1999 are also shownin Zhu et al. (2001). The flow rates at Datong are usuallyregarded as the discharges of the Changjiang, because thisis the lowest location without tidal effects (Minagawa,2003).

The time series of the Changjiang discharge in theperiod from January 1971 to December 2000 is shown inFig. 3 (upper panel). The discharge shows a clear sea-sonal variation. On average, the maximum discharge oc-curs in July (51,866.5 m3s–1), and the second and thirdlargest discharges appear in August (43,976.4 m3s–1) andJune (40,252.5 m3s–1) with large standard deviations

(lower panel in Fig. 3). On the other hand, the minimumdischarge is found in January (11,257.9 m3s–1) with thesmallest standard deviation.

The discharge shows a significant interannual varia-tion in summer, as shown by the largest standard devia-tions; from 77,100 m3s–1 in August 1998 to 32,800 m3s–1

in July 1972. The spectrum density of the Changjiangdischarge calculated with the maximum entropy method(MEM) is shown in Fig. 4. The three spectrum peaks at7.5, 2.5, and 1.5 years are recognizable in the periodslonger than the annual cycle. These spectrum peaks, ex-cept the one for 1.5 years, coincide with the primary-interannual periods in the Changjiang discharge, as re-ported by Chang and Isobe (2005).

3. Seasonal Salinity Changes in the Tsushima StraitBefore discussing the interannual salinity variations,

we briefly introduce the general distribution and seasonalchanges of salinity in the Tsushima Strait because this isthe strongest signal in the salinity field (Fig. 2). Figure 5shows the climatological monthly means and standarddeviations of the channel-averaged salinities in Fig. 2.

The salinity in the western channel is generally lowerthan that in the eastern channel throughout the year (Fig.5). This indicates that most of the low salinity water fromthe East China Sea flows into the western channel. Inaddition, the existence of salinity differences between thechannels in December and February implies that theadvection of low salinity water from the East China Seato the Japan Sea does not become zero, even in winter.

The decrease in salinity over summer is discernible

Fig. 3. Time series of the Changjiang discharge at Datong fromJanuary 1971 to December 2000 (upper panel) andclimatological monthly means with double standard devia-tion (lower panel).

Fig. 4. Spectrum density of the Changjiang discharge varia-tion calculated with MEM. Three interannual spectrumpeaks are indicated by arrows at 7.5, 2.5, and 1.5 years,respectively.


even at a depth of 30 m in both channels. On average, theminimum salinity occurs in August at the sea surface inboth channels, and at a depth of 30 m in the eastern chan-nel. However, there are many cases in which the mini-mum salinity appears in October, especially in layersdeeper than 10 m (Table 1). Indeed, the 30 m layer in thewestern channel exhibits a salinity minimum in October(Fig. 5). The large standard deviations in October at adepth of 30 m in both channels reflect this situation. Simi-lar salinity variations are reported in the northeasternTsushima Strait (or the southwestern Japan Sea), thoughthe minimum salinity appears in September at the seasurface (Ogawa, 1983).

The climatological salinity distributions over sum-mer (means of August and October) along the northernand southern observation lines in Fig. 1 are shown in Fig.6. The upper 20 m in both channels is covered with lowsalinity water (below 33.5 in practical salinity unit). In

particular, the low salinity water of less than 33.0 is foundin the top 20 m in the western channel. As a result, rela-tively strong stratifications (halocline) are formed in thedepth range of 10–50 m in the western channel, whilethey are found between 20–50 m in the eastern channel.

Generally, the salinity exhibits higher values in win-ter (December to April), and shows a maximum in Aprilat all depths in both channels (Fig. 5). This is broughtabout by vertical mixing with the higher salinity water inthe deeper layer due to the surface cooling, becausesalinities at depths between 0 and 30 m show almost thesame values in each channel. The salinity stratificationcommences in June and continues to October in the west-ern channel, while the stratification levels in June andOctober are rather weak in the eastern channel.

4. Principal Components of the Interannual Vari-ability in SalinitySince our interest is in the interannual variability in

salinity, the salinity anomalies from the climatological

Depth Eastern channel Western channel

August October August October

0 m 28 2 29 110 m 27 3 28 220 m 26 4 21 930 m 18 12 11 19

Fig. 5. Seasonal variation of the channel-averaged salinitiesshown in Fig. 2. Though the bi-monthly data are plotted,full months are shown on the horizontal axis to clarify theorder of months. The open symbols connected by dashedlines and solid symbols connected by bold lines indicateclimatological monthly mean salinities at depths of 0 and30 m, respectively. The square and circle symbols denotesalinities in the eastern and western channels, respectively.

Table 1. Frequency distribution of minimum salinity occur-rence during 1971–2000 in the time series of channel-aver-aged salinities.

33.5

N - Line

33.0

34.0 34.0

33.5

S - Line

34.0

33.5

34.0

33.5

33.0

Fig. 6. Mean salinity sections in summer (August and Octo-ber) along the northern (upper) and southern observationlines (lower) in Fig. 1. Contour interval is 0.1 practical sa-linity unit.


monthly means are examined. As examples, the station-time diagrams of salinity anomalies at depths of 0 and 30m along the southern line are shown in Fig. 7. Low andhigh salinity events with amplitudes of more than 1.0 arefound sporadically. Though the pattern of anomalies atthe sea surface resembles that at a depth of 30 m, it ishard to find systematic variations from the diagrams.Therefore, to extract principal components of variation,we carried out an empirical orthogonal functions (EOF)analysis of the salinity anomalies.

In order to obtain three-dimensionally coherent vari-ations, the EOF analysis is applied simultaneously to thesalinity anomalies at all depths rather than those at eachstandard depth. Therefore, we decomposed the salinityanomaly variation into 142 modes in accordance with thenumber of data points. In EOF analysis, the cross corre-lation matrix is decomposed instead of the covariancematrix, because the amplitude of anomalies tends to besmall with increasing depth, as shown in Fig. 7. There-fore, calculated eigenvectors and time coefficients foreach mode are non-dimensional. This method allocatesan equal chance of contribution to all data points (Emeryand Thomson, 1998). In this study, only the first and sec-ond modes are examined because these two leading modesaccount for about 45% of the total variance and each vari-

ance ratio for higher modes is less than 8%.The EOF first mode explains about 28.2% of the to-

tal variance. Figure 8 shows the eigenvector distributionof the first mode along the northern and southern lines.Almost all the data points in both channels show positiveeigenvectors; this indicates that the first mode exhibitsthe simultaneous salinity variation for the entire TsushimaStrait. However, the values of eigenvectors are generallylarger in the eastern channel than those in the westernone at similar levels. In particular, the eigenvectors be-low 50 m depth in the western channel are one or twoorders of magnitude smaller than those in the eastern one.In this sense, the first mode can be considered as repre-sentative of the variation in the eastern channel. In fact,if the EOF analysis is applied only to the anomalies inthe eastern channel, a very similar variation to the firstmode is obtained as the leading mode, with a varianceratio of 52.7%.

The time coefficients of the EOF first mode and theirspectrum density calculated with MEM are shown in Fig.9. Positive time coefficients with large amplitudes of morethan +10.0 are frequently found in the period from 1977to 1988. By contrast, negative time coefficients of lessthan –10.0 are often seen in the 1990s. The MEM spec-trum shows dominant interannual-periods of variation at

75

80

85

90

95

00

0m (S-Line)

75

80

85

90

95

00

30m (S-Line)

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

Fig. 7. Station-time diagram of the salinity anomalies along the southern line at depths of 0 m (left) and 30 m (right). Contourinterval is 0.5 and contours of –1.0, 0.0, and 1.0 are drawn by bold lines. Negative anomalies are shaded; light, medium, andheavy shadings indicate anomalies of >–0.5, from –0.5 to –1.0, and


7.4, 2.1, and 1.4 years, respectively.The eigenvectors of the EOF second mode, which

accounts for about 16.3% of the total variance, are shownin Fig. 10. The western channel is occupied by positiveeigenvectors, while negative eigenvectors are dominantin the eastern channel. This indicates that the second modeis the out-of-phase variation between the channels. How-ever, in contrast to the first mode, the eigenvectors of thesecond mode exhibit larger absolute values in the west-ern channel than in the eastern one. This fact indicatesthat the second mode is characteristic of the variation inthe western channel. Indeed, similar eigenvectors and timecoefficients to the second mode are obtained as the lead-ing mode with a variance ratio of 34.1%, when we ap-plied the EOF analysis only to the anomalies in the west-ern channel.

Extremely small eigenvectors are found at 0–75 mand 0–30 m layers at Stas. E04 and E05 on the southernline, respectively. By contrast, the largest eigenvectors

are found in the upper 50 m at Stas. W03 and W04. It isinteresting that these stations correspond to the surfaceflow axis in each channel (Yoshikawa et al., 2005). Stas.E09 and E10 on the northern line are also located in thestrongest flow region in the eastern channel.

The time coefficients of the second mode and theirspectrum density are shown in Fig. 11. The amplitude oftime coefficients is moderate compared with that of thefirst mode. The two dominant periods of variation arediscernible as spectrum peaks at 3.7 and 1.6 years.

5. DiscussionIn the previous section, the EOF first and second

modes were extracted as characteristic interannual salin-ity variations in the eastern and western channels, respec-tively. In this section we examine the physical meaningof these leading modes.

It is noteworthy that the dominant periods of varia-tion in the EOF first mode (Fig. 9) correspond to the threemain spectrum peaks in the Changjiang discharge (Fig.4). Since the minimum salinity occurs in August or Octo-ber in the Tsushima Strait, we compared the time coeffi-

N - Line

EOF - 1

S - Line

EOF - 1

Fig. 8. Eigenvector distribution for the EOF first mode alongthe northern (upper) and southern observation lines (lower).Circle size is proportional to the absolute value ofeigenvectors. Open and shaded symbols indicate positiveand negative values, respectively.

Fig. 9. Time coefficients of the EOF first mode (upper) andtheir spectrum density calculated with MEM (lower). Domi-nant periods of variation are shown by arrows at 7.4, 2.1,and 1.4 years in the lower panel.


cients of the first mode in the two months with theChangjiang discharges in summer. Figure 12 show thetime series of the averaged time coefficient in August andOctober (solid circles connected by solid lines) and themean discharge of Changjiang from June to August (opentriangles connected by dashed lines). A large dischargeof more than one standard deviation occurred six times,in 1973, 1983, 1995, 1996, 1998, and 1999. In these years,the time coefficients exhibit negative values, except for1996. By contrast, positive peaks of the time coefficients,such as 1978, 1988, and 1994, tend to occur in the yearswhen discharge smaller than usual. This negative corre-lation can be confirmed in Fig. 13. The correlation coef-ficient (r) is –0.46, but the correlation increases to r =–0.59 if the two exceptional years are excluded: 1972 inwhich the smallest discharge was recorded in summer(Fig. 3), and 1976 in which the smallest time coefficientwas found in October (Fig. 9). These facts indicate thatthe EOF first mode is associated with the Changjiang dis-charge variability, and the salinity in the Tsushima Straittends to decrease in the summer with a large discharge ofthe Changjiang.

According to the correlation diagram (Fig. 13), we

selected 1983, 1995, 1998, and 1999 as typical years withlow salinity and high discharge (LSHD), and 1978, 1979,and 1988 as typical years with high salinity and low dis-charge (HSLD). Figure 14 shows the salinity anomalydistributions in the summer of LSHD and HSLD; it showsthe deviations from the mean salinity distributions in Fig.6. Significant changes in the salinity field are recogniz-able; in the summer of LSHD, negative anomalies oc-cupy most of the eastern channel and the upper 50 m inthe western channel, in agreement with the eigenvectordistributions (Fig. 8), while positive anomalies are domi-nant in both the channels in the summer of HSLD. Therange of salinity anomaly change amounts to 1.33 and1.18 in the eastern and western channels, respectively.

It is notable that the negative anomalies in the LSHDsummer are distributed from the sea surface to near bot-tom in the eastern channel, while they are confined in theupper layer above the halocline in the western channel(Fig. 6). In addition, negative anomalies of less than –0.5appear in the upper 20 m layer in the eastern channel;such strong negative anomalies are not seen in the west-ern channel. From these facts we can conclude that thesalinity anomalies in the eastern channel are more sensi-

N - Line

EOF - 2

S - Line

EOF - 2

Fig. 10. As Fig. 8 except for the EOF second mode.

Fig. 11. As Fig. 9 except for the EOF second mode. Dominantperiods of variation are shown by arrows at 3.7 and 1.6 yearsin the lower panel.


Fig. 12. Inter-summer time coefficients of the EOF first mode (solid circles connected by solid lines) and summer discharges ofthe Changjiang (open triangles connected by dashed lines). Mean and standard deviations of the summer discharges areshown by solid and dashed thin lines, respectively.

Fig. 13. Correlation diagram between the summer time coeffi-cients of the EOF first mode and the summer discharges ofthe Changjiang, as shown in Fig. 12. Vertical thin line indi-cates mean summer discharge. Numerals near the pointsindicate the year of the data. Points in 1972 and 1976 areshown by crosses (see text). Correlation coefficient exclud-ing these two points is shown in the upper right corner.

tive to the Changjiang discharge variability than those inthe western channel, though the core of the low salinitywater is located in the western channel (Fig. 6). This situ-ation can be explained as follows. Most of the low salin-ity water flows in the western channel in standard sum-mers. However, in summers with a large Changjiang dis-charge, the low salinity water spreads not only into thewestern channel but also the eastern one. As a result, sa-linity in the eastern channel decreases, and strong nega-tive anomalies occur. Concurrently, salinity in the west-ern channel also decreases, but the magnitudes of salin-ity anomaly decrease are smaller than those in the east-ern channel because the mean salinity in the western chan-nel is originally lower than that in the eastern channel.The larger values of eigenvectors in the eastern channelreflect the larger amplitude of the salinity anomaly vari-ability (Fig. 8). This is consistent with the result of Kitaniet al. (2003).

A region with small eigenvectors of the first mode isfound below the depth of 50 m in the western channel(Fig. 8); this shows that the salinity variation in this re-gion is independent of the low salinity water from theEast China Sea. Lim and Chang (1969) and Cho and Kim(1998) reported that the intrusion of the bottom cold wa-ter from the Japan Sea occurs on the bottom slope in thewestern channel with a large interannual variability. Thismay be a reason why the strong signal associated withthe first mode does not appear in the deeper layer of thewestern channel, in contrast to the eastern channel.

The EOF second mode represents the salinityanomaly variation in the western channel. Takikawa etal. (2005) suggested a relationship between the autumnpeak in the volume transport through the western chan-nel and the low salinity water in summer. Therefore, timecoefficients of the second mode in summer (means ofAugust and October) are compared with the mean vol-ume transports from August to October in the westernchannel (Fig. 15(a)). In addition, considering the out-of-

phase character in the second mode, the transport differ-ences between the western and eastern channels are alsocompared (Fig. 15(b)); these volume transports are esti-mated from sea level differences across the channels(Takikawa and Yoon, 2005). Both the diagrams in Fig. 15show no significant correlations (r = –0.04 and –0.05,respectively), which indicates that the interannual salin-ity variation associated with the EOF second mode is notlikely to arise from the volume transport variation in thestrait, though they may link with each other in the sea-sonal timescale as suggested by Takikawa and Yoon(2005).

The corresponding phenomenon with the EOF sec-


0.0

0.0

0.0

N - Line

LSHD

-0.5

S - Line

LSHD

L

0.0

-0.5

N - Line

HSLD

0.0

0.5

0.0

S - Line

HSLD

0.50.5

0.5

0.0

Fig. 14. Distributions of mean salinity anomalies in the summers of LSHD (left) and HSLD (right) along the northern (upper) andsouthern observation lines (lower). Contour interval is 0.1. Negative anomalies are shaded; lighter and heavier shadingsindicate anomalies of >–0.5 and


ond mode is unclear at present. Since the second modeshows an out-of-phase variation between channels, thismode may represent the tendency of low salinity water toenter the western or eastern channels from the East ChinaSea. It is known that the behavior of the Changjiang Di-luted Water (CDW) in the East China Sea is subject tosurface winds, as shown by drifting buoy trajectories (Leeet al., 2003; Pang et al., 2004). In particular, the east/northeastward extension of CDW in summer is associ-ated with the Ekman transport induced by the southerlymonsoon (Bang and Lie, 1999). It is noteworthy that theinterannual spectrum peak at 3.7 years in the time coeffi-cients of the second mode (Fig. 11) corresponds to theprimary interannual period (3.6 years) in the wind stressover the Yellow and East China Seas (Chang and Isobe,2005). This fact suggests that the second mode is associ-ated with the CDW behavior in the East China Sea, af-fected by variations in the wind stress field.

6. Concluding RemarksWe have investigated the interannual variability of

salinity in the Tsushima Strait on the basis of the histori-cal hydrographic data. The EOF analysis revealed thatthe in-phase salinity variation between the eastern andwestern channels is the predominant mode. The time co-efficients of the EOF first mode in summer correlate nega-tively with the Changjiang discharges, which indicatesthat salinity in the Tsushima Strait tends to decrease insummer with a large Changjiang discharge. Theeigenvectors of the first mode are larger in the easternchannel than those in the western one, though most of thelow salinity water flows in the western channel. This isbecause the low salinity water spreads not only into thewestern channel, but also the eastern channel in summerswith a large Changjiang discharge.

The out-of-phase salinity variation between the chan-nels was extracted as the EOF second mode, which rep-resents the salinity anomaly variation in the western chan-nel. The time coefficients of the second mode in summershow no significant correlation with the volume trans-ports through the western channel nor the transport dif-ferences between channels. A relationship with theinterannual variability in the wind stress over the EastChina Sea is suggested, but more statistical discussionsare needed to confirm the physical meaning of the sec-ond mode.

In this study we examined only the conditions at the“entrance” and “exit” of freshwater flows in the EastChina Sea, though the low salinity water is modified inthe East China Sea by local precipitation and verticalmixing due to typhoons (Pang et al., 2003). In particular,the transport of low salinity water into the Tsushima Straitdepends on the behavior of CDW in the East China Sea(Chang and Isobe, 2003, 2005). Nevertheless, the EOFfirst mode showed a relationship between the Changjiang

discharge and salinity variation in the Tsushima Strait.Conversely, this implies that the spreading of the freshwater in the East China Sea depends on the Changjiangdischarge, as suggested by Ichikawa et al. (2001) andDelcroix and Murtugudde (2002).

This study revealed that the low salinity water fromthe East China Sea spreads into the eastern channel insummers with a large Changjiang discharge. This fact issignificant for the Japanese coastal environment in theJapan Sea, because the water through the eastern channelflows along the Japanese coast as the first branch of theTsushima Current (Ishii and Michida, 1996; Hase et al.,1999). This suggests that an extensive spreading of lowsalinity water along the Japanese coast occurs in sum-mers with a large Changjiang discharge. In fact, abnor-mally low saline water was reported in the Japan Sea in1998 when the largest discharge was recorded in August(Fig. 3) (Kuroda and Hirai, 2000). To confirm the rela-tionship between the behavior of the low salinity waterin the Japan Sea and the Changjiang discharge, quantita-tive studies are required of the variability in the influx offreshwater through the strait.

AcknowledgementsWe would like to thank Drs. P.-H. Chang of Ehime

University and J. Zhu of the East China Normal Univer-sity for the Changjiang discharge data used in this study.Dr. T. Takikawa of the National Fisheries University pro-vided the data of volume transport in the Tsushima Strait.Thanks are also due to colleagues in the Laboratory ofOcean Circulation Dynamics, RIAM, Kyushu Universityfor their comments and discussion. The GFD DENNOU,PSPLOT, and GMT Libraries were used for plotting fig-ures. This study was supported by Grants-in-Aid for Sci-entific Research (14204045 and 15540422) from the Ja-pan Society for the Promotion of Science.

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