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

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1 Journal of Oceanography, Vol. 62, pp. 681 to 692, 2006 Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge TOMOHARU SENJYU 1 *, HIROFUMI ENOMOTO 2, TAKESHI MATSUNO 1 and SIGEAKI MATSUI 3 1 Research Institute for Applied Mechanics, Kyushu University, Kasuga-Koen, Kasuga, Fukuoka , Japan 2 Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-Koen, Kasuga, Fukuoka , Japan 3 Fukuoka Fisheries and Marine Technology Research Center, Imazu, Nishi-ku, Fukuoka , 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 of historical hydrographic data. The EOF analysis revealed that the most dominant mode is the in-phase salinity variation between the eastern and western channels. The time coefficients of the EOF first mode in summer show a negative correlation with the Changjiang discharge, which indicates that salinity in the Tsushima Strait tends to decrease over summer, related to a large discharge of the Changjiang. The eigenvectors of 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 because the low salinity water spreads into the eastern channel as well as the western channel over summers with a large discharge of the Changjiang. The out-of-phase salinity variation between the channels is extracted as the EOF second mode; this is the predominant variation in the western channel. The time coefficients of the second mode in summer show no significant correlations to the volume transports through the western channel and the transport differences between channels. A relationship between the EOF second mode and variations in the wind stress over the East China Sea is suggested. Keywords: Tsushima Strait, low salinity water, Changjiang discharge, Changjiang Diluted Water, summer salinity condition, interannual variability, EOF analysis, Japan Sea, East China Sea. 1. Introduction Large quantities of materials such as salt, freshwater, suspended organic and inorganic particles, phyto- and zoo-plankton, fish eggs and larvae are transported from the East China Sea to the Japan Sea by the Tsushima Current. For example, Isobe et al. (2002) has estimated that the annual average temperature and freshwater transports from the East China Sea to the Japan Sea amount to 0.17 PW and m 3 s 1, respectively; the later is comparable to the total river discharge flowing into the Yellow and East China Seas. Recently, Onitsuka and Yanagi (2005) suggested that the water from the East China Sea is one of the important sources of nutrients feeding primary production in the southern Japan Sea. The Tsushima Strait is a narrow, shallow strait connecting the East China Sea with the Japan Sea (Fig. 1), * Corresponding author. senjyu@riam.kyushu-u.ac.jp Copyright The Oceanographic Society of Japan/TERRAPUB/Springer which is the entrance of materials into the Japan Sea. The Tsushima Islands separate the strait into eastern and western channels; the width and maximum depth are 140 km and 110 m in the eastern channel, and 40 km and 200 m in the western channel, respectively. Hydrographically, the Tsushima Current, which is the strongest flow in the Japan Sea, originates from the strait, though more upstream origins of the current have been suggested in the Kuroshio (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 revealed characteristic features of the current field in the Tsushima Strait. On the basis of long-term monitoring with an Acoustic Doppler Current Profiler (ADCP) mounted on a ferryboat, Takikawa et al. (2005) revealed that the volume transport through the western channel is larger than that in the eastern channel: 1.54 and m 3 s 1, respectively. In addition, the volume transport exhibits two maxima in spring and autumn; the autumn peak is 681

2 Fig. 1. Study area. Upper panel shows the location of Changjiang, the East China Sea, and the Japan Sea. The Tsushima Strait region shown in the box in the upper panel is enlarged in the lower panel. Solid squares and circles in the eastern and western channels indicate hydrographic stations observed by FFMTRC and NFRDI, respectively. more pronounced than the spring peak in the western channel, while an inverse relation is found in the eastern channel. A similar flow intensification in the western channel in autumn has been observed from bottom-installed ADCP arrays (Jacobs et al., 2001; Teagu et al., 2002), and High Frequency (HF) radar measurements (Yoshikawa et al., 2005). However, properties of the water transported by the Tsushima Current in the strait are still obscure. One of the striking features of water characteristics in the Tsushima Strait is a clear seasonal variation in salinity. Figure 2 shows time series of the channel-averaged salinity at depths of 0 and 30 m. Salinity in both channels show low values from summer to autumn and high ones from winter to spring. The decrease in salinity over summer is due to the advection of low salinity water from the East China Sea (Moriyasu, 1972; Ogawa et al., 1977; Ogawa, 1983). The low salinity water spreads widely into the southern Japan Sea (Senjyu, 1999), and occasionally causes abnormal low-salinity conditions along the Japanese coast (Kuroda and Hirai, 2000). The prosperity and decay of the low salinity water are closely related to the distribution and migration of post-larval anchovy, 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 origin in the Changjiang (Yangtze River), which is the largest river in Asia (Fig. 1). However, few studies discuss the relationship between the Changjiang discharge and hydrographic conditions in the Tsushima Strait or the Japan Sea. Kitani et al. (2003) showed a negative correlation between the maximum discharge of the Changjiang and minimum surface salinity in the eastern channel. 682 T. Senjyu et al.

3 ity field is provided in Section 4. Section 5 discusses the interannual salinity variations in summer in relation to the Changjiang discharge. Finally, conclusions and remarks are given in Section Data Fig. 2. Time series of channel-averaged salinity at depths of 0 m (dashed line) and 30 m (solid line) from February 1971 to December According to the observation interval in the western channel, the data in even months are plotted. The upper and lower panels are for the eastern and western channels, respectively. However, the spatiotemporal variability in the salinity field is still unknown, in particular in the western channel. Based on a box model analysis, Nof (2001) suggested that the diversion of freshwater from the Changjiang (and Yellow River) for agricultural use can induce frequent bottom water formation in the Japan Sea through changes in sea surface salinity. Yanagi (2002) also suggested that the changes in salinity in the East China Sea cause serious changes in the nutrient condition in the upper 200 m of the Japan Sea. The Three-Gorges Dam, the largest hydroelectric dam in the world, has been constructed in the upstream region of Changjiang from 1993, with completion planned in 2009, and some impacts on the physical and biochemical environments in waters adjacent to the Changjiang Estuary have to be considered (Zhu et al., 2003). To assess the dam s effects correctly and evaluate its impacts on the Japan Sea, an understanding of the low salinity water transports into the Tsushima Strait is crucial. In the next section, the salinity data used in this study are described, together with a brief introduction of the Changjiang discharge. In Section 3 we show the seasonal change of salinity in the Tsushima Strait; this is the strongest signal in the salinity variation. A description of principal components of interannual variability in the salin- 2.1 Salinity The 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 Marine Technology Research Center (FFMTRC) in Japan has observed monthly temperature and salinity at the stations shown by solid squares in Fig. 1. On the other hand, hydrographic observations in the western channel have been carried out in even months by the National Fisheries Research and Development Institute (NFRDI) of Korea (solid circles in Fig. 1). In order to discuss the concurrent variations in both channels, we used bi-monthly salinity data during the period from February 1971 to December These data were provided from FFMTRC and Korea Oceanographic Data Center. The salinity obtained at irregular depths were linearly interpolated into standard depths (0, 10, 20, 30, 50, 75, 100, 125, 150 and 200 m), except for the bottom depths. This procedure sets the data points in both the channels to nearly the same number (70 and 72 points in the eastern and western channels, respectively). Since salinity measurements in April 1974 and August 1976 at stations in the western channel were absent, they were linearly interpolated from the observed values in February and May in 1974 and June and September in 1976, respectively. This is the basic salinity dataset. For the analysis of interannual variability, a dataset of salinity anomalies was prepared by subtracting the climatological monthly means at each data point from the corresponding original data. Since the basic dataset includes no-data periods, we solved the lack of data in the anomaly dataset as follows. The salinity anomalies at Sta. E11 in the period from April 1997 to December 2000 were estimated from those at Sta. E10, because relatively high correlations were found between them; the correlation coefficients range from at 10 m to at 80 m. Observations at Stas. W01, W02, W06, and W07 had been interrupted since February 1999 as the new Japan-Korea Fishery Agreement went into effect in As for the salinity anomalies at Sta. W02, a similar estimation method to that at Sta. E11 was applied using the values at Sta. W03. (For the estimation at the bottom depth of 100 m at Sta. W02, we used the anomalies at the bottom depth of 90 m at Sta. W03.) Salinity anomalies at Sta. W01 were filled by the mean values of those at Stas. W03, W04, and W05 at each depth. Similarly, anomalies at Stas. W06 and W07 were filled by the mean values of those at Stas. Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge 683

4 Fig. 4. Spectrum density of the Changjiang discharge variation calculated with MEM. Three interannual spectrum peaks are indicated by arrows at 7.5, 2.5, and 1.5 years, respectively. Fig. 3. Time series of the Changjiang discharge at Datong from January 1971 to December 2000 (upper panel) and climatological monthly means with double standard deviation (lower panel). W08, W09, and W10 at each depth. Anomalies below the depth of 125 m at these stations were set to zero. For other interruptions, temporal interpolation with a cubic function was applied: in the periods October 1974 February 1975, June 1975, and October 1975 February 1976 at Stas. E06 E11 and in December 1989, December 1991, and December 1992 at all stations in the western channel. 2.2 Changjiang discharge We combined two datasets of monthly mean flow rates at Datong in China (Fig. 1): Monthly Discharge Data for World Rivers (Bodo, 2001) and Zhu (personal communication) for the periods and , respectively. The discharges in are also shown in Zhu et al. (2001). The flow rates at Datong are usually regarded as the discharges of the Changjiang, because this is the lowest location without tidal effects (Minagawa, 2003). The time series of the Changjiang discharge in the period from January 1971 to December 2000 is shown in Fig. 3 (upper panel). The discharge shows a clear seasonal variation. On average, the maximum discharge occurs in July (51,866.5 m 3 s 1 ), and the second and third largest discharges appear in August (43,976.4 m 3 s 1 ) and June (40,252.5 m 3 s 1 ) with large standard deviations (lower panel in Fig. 3). On the other hand, the minimum discharge is found in January (11,257.9 m 3 s 1 ) with the smallest standard deviation. The discharge shows a significant interannual variation in summer, as shown by the largest standard deviations; from 77,100 m 3 s 1 in August 1998 to 32,800 m 3 s 1 in July The spectrum density of the Changjiang discharge calculated with the maximum entropy method (MEM) is shown in Fig. 4. The three spectrum peaks at 7.5, 2.5, and 1.5 years are recognizable in the periods longer than the annual cycle. These spectrum peaks, except the one for 1.5 years, coincide with the primaryinterannual periods in the Changjiang discharge, as reported by Chang and Isobe (2005). 3. Seasonal Salinity Changes in the Tsushima Strait Before discussing the interannual salinity variations, we briefly introduce the general distribution and seasonal changes of salinity in the Tsushima Strait because this is the strongest signal in the salinity field (Fig. 2). Figure 5 shows the climatological monthly means and standard deviations of the channel-averaged salinities in Fig. 2. The salinity in the western channel is generally lower than that in the eastern channel throughout the year (Fig. 5). This indicates that most of the low salinity water from the East China Sea flows into the western channel. In addition, the existence of salinity differences between the channels in December and February implies that the advection of low salinity water from the East China Sea to the Japan Sea does not become zero, even in winter. The decrease in salinity over summer is discernible 684 T. Senjyu et al.

5 N - Line Fig. 5. Seasonal variation of the channel-averaged salinities shown in Fig. 2. Though the bi-monthly data are plotted, full months are shown on the horizontal axis to clarify the order of months. The open symbols connected by dashed lines and solid symbols connected by bold lines indicate climatological monthly mean salinities at depths of 0 and 30 m, respectively. The square and circle symbols denote salinities in the eastern and western channels, respectively Table 1. Frequency distribution of minimum salinity occurrence during in the time series of channel-averaged salinities. Depth Eastern channel Western channel August October August October 0 m m m m even at a depth of 30 m in both channels. On average, the minimum salinity occurs in August at the sea surface in both channels, and at a depth of 30 m in the eastern channel. However, there are many cases in which the minimum salinity appears in October, especially in layers deeper than 10 m (Table 1). Indeed, the 30 m layer in the western channel exhibits a salinity minimum in October (Fig. 5). The large standard deviations in October at a depth of 30 m in both channels reflect this situation. Similar salinity variations are reported in the northeastern Tsushima Strait (or the southwestern Japan Sea), though the minimum salinity appears in September at the sea surface (Ogawa, 1983). The climatological salinity distributions over summer (means of August and October) along the northern and southern observation lines in Fig. 1 are shown in Fig. 6. The upper 20 m in both channels is covered with low salinity water (below 33.5 in practical salinity unit). In S - Line Fig. 6. Mean salinity sections in summer (August and October) along the northern (upper) and southern observation lines (lower) in Fig. 1. Contour interval is 0.1 practical salinity unit. particular, the low salinity water of less than 33.0 is found in the top 20 m in the western channel. As a result, relatively strong stratifications (halocline) are formed in the depth range of m in the western channel, while they are found between m in the eastern channel. Generally, the salinity exhibits higher values in winter (December to April), and shows a maximum in April at all depths in both channels (Fig. 5). This is brought about by vertical mixing with the higher salinity water in the deeper layer due to the surface cooling, because salinities at depths between 0 and 30 m show almost the same values in each channel. The salinity stratification commences in June and continues to October in the western channel, while the stratification levels in June and October are rather weak in the eastern channel. 4. Principal Components of the Interannual Variability in Salinity Since our interest is in the interannual variability in salinity, the salinity anomalies from the climatological Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge 685

6 0m (S-Line) 30m (S-Line) Fig. 7. Station-time diagram of the salinity anomalies along the southern line at depths of 0 m (left) and 30 m (right). Contour interval is 0.5 and contours of 1.0,, and 1.0 are drawn by bold lines. Negative anomalies are shaded; light, medium, and heavy shadings indicate anomalies of > 0.5, from 0.5 to 1.0, and < 1.0, respectively. monthly means are examined. As examples, the stationtime diagrams of salinity anomalies at depths of 0 and 30 m along the southern line are shown in Fig. 7. Low and high salinity events with amplitudes of more than 1.0 are found sporadically. Though the pattern of anomalies at the sea surface resembles that at a depth of 30 m, it is hard 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 variations, the EOF analysis is applied simultaneously to the salinity anomalies at all depths rather than those at each standard depth. Therefore, we decomposed the salinity anomaly variation into 142 modes in accordance with the number of data points. In EOF analysis, the cross correlation matrix is decomposed instead of the covariance matrix, because the amplitude of anomalies tends to be small with increasing depth, as shown in Fig. 7. Therefore, calculated eigenvectors and time coefficients for each mode are non-dimensional. This method allocates an equal chance of contribution to all data points (Emery and Thomson, 1998). In this study, only the first and second modes are examined because these two leading modes account for about 45% of the total variance and each variance ratio for higher modes is less than 8%. The EOF first mode explains about 28.2% of the total variance. Figure 8 shows the eigenvector distribution of the first mode along the northern and southern lines. Almost all the data points in both channels show positive eigenvectors; this indicates that the first mode exhibits the simultaneous salinity variation for the entire Tsushima Strait. However, the values of eigenvectors are generally larger in the eastern channel than those in the western one at similar levels. In particular, the eigenvectors below 50 m depth in the western channel are one or two orders of magnitude smaller than those in the eastern one. In this sense, the first mode can be considered as representative of the variation in the eastern channel. In fact, if the EOF analysis is applied only to the anomalies in the eastern channel, a very similar variation to the first mode is obtained as the leading mode, with a variance ratio of 52.7%. The time coefficients of the EOF first mode and their spectrum density calculated with MEM are shown in Fig. 9. Positive time coefficients with large amplitudes of more than +1 are frequently found in the period from 1977 to By contrast, negative time coefficients of less than 1 are often seen in the 1990s. The MEM spectrum shows dominant interannual-periods of variation at 686 T. Senjyu et al.

7 EOF - 1 N - Line EOF - 1 S - Line Fig. 8. Eigenvector distribution for the EOF first mode along the northern (upper) and southern observation lines (lower). Circle size is proportional to the absolute value of eigenvectors. Open and shaded symbols indicate positive and negative values, respectively. 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 shown in Fig. 10. The western channel is occupied by positive eigenvectors, while negative eigenvectors are dominant in the eastern channel. This indicates that the second mode is the out-of-phase variation between the channels. However, in contrast to the first mode, the eigenvectors of the second mode exhibit larger absolute values in the western channel than in the eastern one. This fact indicates that the second mode is characteristic of the variation in the western channel. Indeed, similar eigenvectors and time coefficients to the second mode are obtained as the leading mode with a variance ratio of 34.1%, when we applied the EOF analysis only to the anomalies in the western channel. Extremely small eigenvectors are found at 0 75 m and 0 30 m layers at Stas. E04 and E05 on the southern line, respectively. By contrast, the largest eigenvectors Fig. 9. Time coefficients of the EOF first mode (upper) and their spectrum density calculated with MEM (lower). Dominant periods of variation are shown by arrows at 7.4, 2.1, and 1.4 years in the lower panel. are found in the upper 50 m at Stas. W03 and W04. It is interesting that these stations correspond to the surface flow axis in each channel (Yoshikawa et al., 2005). Stas. E09 and E10 on the northern line are also located in the strongest flow region in the eastern channel. The time coefficients of the second mode and their spectrum density are shown in Fig. 11. The amplitude of time coefficients is moderate compared with that of the first mode. The two dominant periods of variation are discernible as spectrum peaks at 3.7 and 1.6 years. 5. Discussion In the previous section, the EOF first and second modes were extracted as characteristic interannual salinity variations in the eastern and western channels, respectively. In this section we examine the physical meaning of these leading modes. It is noteworthy that the dominant periods of variation in the EOF first mode (Fig. 9) correspond to the three main spectrum peaks in the Changjiang discharge (Fig. 4). Since the minimum salinity occurs in August or October in the Tsushima Strait, we compared the time coeffi- Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge 687

8 EOF - 2 N - Line EOF - 2 S - Line Fig. 10. As Fig. 8 except for the EOF second mode. Fig. 11. As Fig. 9 except for the EOF second mode. Dominant periods of variation are shown by arrows at 3.7 and 1.6 years in the lower panel. cients of the first mode in the two months with the Changjiang discharges in summer. Figure 12 show the time series of the averaged time coefficient in August and October (solid circles connected by solid lines) and the mean discharge of Changjiang from June to August (open triangles connected by dashed lines). A large discharge of more than one standard deviation occurred six times, in 1973, 1983, 1995, 1996, 1998, and In these years, the time coefficients exhibit negative values, except for By contrast, positive peaks of the time coefficients, such as 1978, 1988, and 1994, tend to occur in the years when discharge smaller than usual. This negative correlation can be confirmed in Fig. 13. The correlation coefficient (r) is 0.46, but the correlation increases to r = 0.59 if the two exceptional years are excluded: 1972 in which the smallest discharge was recorded in summer (Fig. 3), and 1976 in which the smallest time coefficient was found in October (Fig. 9). These facts indicate that the EOF first mode is associated with the Changjiang discharge variability, and the salinity in the Tsushima Strait tends to decrease in the summer with a large discharge of the Changjiang. According to the correlation diagram (Fig. 13), we selected 1983, 1995, 1998, and 1999 as typical years with low salinity and high discharge (LSHD), and 1978, 1979, and 1988 as typical years with high salinity and low discharge (HSLD). Figure 14 shows the salinity anomaly distributions in the summer of LSHD and HSLD; it shows the deviations from the mean salinity distributions in Fig. 6. Significant changes in the salinity field are recognizable; in the summer of LSHD, negative anomalies occupy most of the eastern channel and the upper 50 m in the western channel, in agreement with the eigenvector distributions (Fig. 8), while positive anomalies are dominant in both the channels in the summer of HSLD. The range of salinity anomaly change amounts to 1.33 and 1.18 in the eastern and western channels, respectively. It is notable that the negative anomalies in the LSHD summer are distributed from the sea surface to near bottom in the eastern channel, while they are confined in the upper layer above the halocline in the western channel (Fig. 6). In addition, negative anomalies of less than 0.5 appear in the upper 20 m layer in the eastern channel; such strong negative anomalies are not seen in the western channel. From these facts we can conclude that the salinity anomalies in the eastern channel are more sensi- 688 T. Senjyu et al.

9 Fig. 12. Inter-summer time coefficients of the EOF first mode (solid circles connected by solid lines) and summer discharges of the Changjiang (open triangles connected by dashed lines). Mean and standard deviations of the summer discharges are shown by solid and dashed thin lines, respectively. Fig. 13. Correlation diagram between the summer time coefficients of the EOF first mode and the summer discharges of the Changjiang, as shown in Fig. 12. Vertical thin line indicates mean summer discharge. Numerals near the points indicate the year of the data. Points in 1972 and 1976 are shown by crosses (see text). Correlation coefficient excluding these two points is shown in the upper right corner. tive to the Changjiang discharge variability than those in the western channel, though the core of the low salinity water is located in the western channel (Fig. 6). This situation can be explained as follows. Most of the low salinity water flows in the western channel in standard summers. However, in summers with a large Changjiang discharge, the low salinity water spreads not only into the western channel but also the eastern one. As a result, salinity in the eastern channel decreases, and strong negative anomalies occur. Concurrently, salinity in the western channel also decreases, but the magnitudes of salinity anomaly decrease are smaller than those in the eastern channel because the mean salinity in the western channel is originally lower than that in the eastern channel. The larger values of eigenvectors in the eastern channel reflect the larger amplitude of the salinity anomaly variability (Fig. 8). This is consistent with the result of Kitani et al. (2003). A region with small eigenvectors of the first mode is found below the depth of 50 m in the western channel (Fig. 8); this shows that the salinity variation in this region is independent of the low salinity water from the East China Sea. Lim and Chang (1969) and Cho and Kim (1998) reported that the intrusion of the bottom cold water from the Japan Sea occurs on the bottom slope in the western channel with a large interannual variability. This may be a reason why the strong signal associated with the first mode does not appear in the deeper layer of the western channel, in contrast to the eastern channel. The EOF second mode represents the salinity anomaly variation in the western channel. Takikawa et al. (2005) suggested a relationship between the autumn peak in the volume transport through the western channel and the low salinity water in summer. Therefore, time coefficients of the second mode in summer (means of August and October) are compared with the mean volume transports from August to October in the western channel (Fig. 15(a)). In addition, considering the out-ofphase character in the second mode, the transport differences between the western and eastern channels are also compared (Fig. 15(b)); these volume transports are estimated from sea level differences across the channels (Takikawa and Yoon, 2005). Both the diagrams in Fig. 15 show no significant correlations (r = 4 and 5, respectively), which indicates that the interannual salinity variation associated with the EOF second mode is not likely to arise from the volume transport variation in the strait, though they may link with each other in the seasonal timescale as suggested by Takikawa and Yoon (2005). The corresponding phenomenon with the EOF sec- Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge 689

10 LSHD N - Line HSLD N - Line L LSHD S - Line HSLD S - Line Fig. 14. Distributions of mean salinity anomalies in the summers of LSHD (left) and HSLD (right) along the northern (upper) and southern observation lines (lower). Contour interval is 0.1. Negative anomalies are shaded; lighter and heavier shadings indicate anomalies of > 0.5 and < 0.5, respectively. Fig. 15. Correlation diagrams for the summer time coefficients of the EOF second mode: (a) to the volume transports through the western channel, and (b) to the volume transport differences between channels (western minus eastern). Vertical thin line in (a) and (b) indicates the means of transports and transport differences, respectively. Numerals near the points indicate the year of the data. Correlation coefficients are shown in the lower left corner in each diagram. 690 T. Senjyu et al.

11 ond mode is unclear at present. Since the second mode shows an out-of-phase variation between channels, this mode may represent the tendency of low salinity water to enter the western or eastern channels from the East China Sea. It is known that the behavior of the Changjiang Diluted Water (CDW) in the East China Sea is subject to surface winds, as shown by drifting buoy trajectories (Lee et al., 2003; Pang et al., 2004). In particular, the east/ northeastward extension of CDW in summer is associated with the Ekman transport induced by the southerly monsoon (Bang and Lie, 1999). It is noteworthy that the interannual spectrum peak at 3.7 years in the time coefficients of the second mode (Fig. 11) corresponds to the primary interannual period (3.6 years) in the wind stress over the Yellow and East China Seas (Chang and Isobe, 2005). This fact suggests that the second mode is associated with the CDW behavior in the East China Sea, affected by variations in the wind stress field. 6. Concluding Remarks We have investigated the interannual variability of salinity in the Tsushima Strait on the basis of the historical hydrographic data. The EOF analysis revealed that the in-phase salinity variation between the eastern and western channels is the predominant mode. The time coefficients of the EOF first mode in summer correlate negatively with the Changjiang discharges, which indicates that salinity in the Tsushima Strait tends to decrease in summer with a large Changjiang discharge. The eigenvectors of the first mode are larger in the eastern channel than those in the western one, though most of the low salinity water flows in the western channel. This is because the low salinity water spreads not only into the western channel, but also the eastern channel in summers with a large Changjiang discharge. The out-of-phase salinity variation between the channels was extracted as the EOF second mode, which represents the salinity anomaly variation in the western channel. The time coefficients of the second mode in summer show no significant correlation with the volume transports through the western channel nor the transport differences between channels. A relationship with the interannual variability in the wind stress over the East China Sea is suggested, but more statistical discussions are needed to confirm the physical meaning of the second mode. In this study we examined only the conditions at the entrance and exit of freshwater flows in the East China Sea, though the low salinity water is modified in the East China Sea by local precipitation and vertical mixing due to typhoons (Pang et al., 2003). In particular, the transport of low salinity water into the Tsushima Strait depends on the behavior of CDW in the East China Sea (Chang and Isobe, 2003, 2005). Nevertheless, the EOF first mode showed a relationship between the Changjiang discharge and salinity variation in the Tsushima Strait. Conversely, this implies that the spreading of the fresh water in the East China Sea depends on the Changjiang discharge, as suggested by Ichikawa et al. (2001) and Delcroix and Murtugudde (2002). This study revealed that the low salinity water from the East China Sea spreads into the eastern channel in summers with a large Changjiang discharge. This fact is significant for the Japanese coastal environment in the Japan Sea, because the water through the eastern channel flows along the Japanese coast as the first branch of the Tsushima Current (Ishii and Michida, 1996; Hase et al., 1999). This suggests that an extensive spreading of low salinity water along the Japanese coast occurs in summers with a large Changjiang discharge. In fact, abnormally low saline water was reported in the Japan Sea in 1998 when the largest discharge was recorded in August (Fig. 3) (Kuroda and Hirai, 2000). To confirm the relationship between the behavior of the low salinity water in the Japan Sea and the Changjiang discharge, quantitative studies are required of the variability in the influx of freshwater through the strait. Acknowledgements We would like to thank Drs. P.-H. Chang of Ehime University and J. Zhu of the East China Normal University for the Changjiang discharge data used in this study. Dr. T. Takikawa of the National Fisheries University provided the data of volume transport in the Tsushima Strait. Thanks are also due to colleagues in the Laboratory of Ocean Circulation Dynamics, RIAM, Kyushu University for their comments and discussion. The GFD DENNOU, PSPLOT, and GMT Libraries were used for plotting figures. This study was supported by Grants-in-Aid for Scientific Research ( and ) from the Japan Society for the Promotion of Science. References Bang, I. and H.-J. Lie (1999): A numerical experiment on the dispersion of the Changjiang river plume. J. Korean Soc. Oceanogr., 34, Bodo, B. A. (2001): Annotations for monthly discharges data for world rivers (excluding former Soviet Union), 85 pp. [Available on-line from docs/] Chang, P.-H. and A. Isobe (2003): A numerical study on the Changjiang diluted water in the Yellow and East China Seas. J. Geophys. Res., 108(C9), 3299, doi: / 2002JC Chang, P.-H. and A. Isobe (2005): Interannual variation of freshwater in the Yellow and East China Seas: role of the Changjiang discharge and wind forcing. J. Oceanogr., 61, Cho, Y.-K. and K. Kim (1998): Structure of the Korea Strait Bottom Cold Water and its seasonal variation in Cont. Shelf Res., 18, Interannual Salinity Variations in the Tsushima Strait and Its Relation to the Changjiang Discharge 691

12 Delcroix, T. and R. Murtugudde (2002): Sea surface salinity changes in the East China Sea during : influence of the Yangtze River. J. Geophys. Res., 107(C12), 8008, doi: /2001jc Emery, W. J. and R. E. Thomson (1998): Data Analysis Methods in Physical Oceanography. Pergamon Press, Great Britain, 634 pp. Fang, G., B. Zhao and Y. Zhu (1991): Water volume transport through the Taiwan Strait and the continental shelf of the East China Sea measured with current meters. p In Oceanography of Asian Marginal Seas, ed. by K. Takano, Elsevier, Amsterdam. Hase, H., J.-H. Yoon and W. Koterayama (1999): The current structure of the Tsushima Warm Current along the Japanese coast. J. Oceanogr., 55, Ichikawa, H., H. Sakajiri, H. Nakamura and A. Nishina (2001): Year to year variation of sea surface salinity in the East China Sea. Proc. Extended Abstract Volume The 11th PAMS/ JECSS Workshop, p Ishii, H. and Y. Michida (1996): Tracking of the first branch of the Tsushima Warm Current with surface drifter. Rep. Hydrographic Res., 32, (in Japanese with English abstract). Isobe, A. (1999a): On the origin of the Tsushima Warm Current and its seasonality. Cont. Shelf Res., 19, Isobe, A. (1999b): The Taiwan-Tsushima warm current system: its path and the transformation of the water mass in the East China Sea. J. Oceanogr., 55, Isobe, A., M. Ando, T. Watanabe, T. Senjyu, S. Sugihara and A. Manda (2002): Freshwater and temperature transports through the Tsushima-Korea Straits. J. Geophys. Res., 107(C7), doi: /2000jc Jacobs, G. A., H. T. Perkins, W. J. Teague and P. J. Hogan (2001): Summer transport through the Korea-Tsushima Strait. J. Geophys. Res., 106(C4), Kitani, K., Y. Kiyomoto and H. Nagata (2003): The character of recent hydrographic variation in the East China Sea; on the behavior of the water originating from the deluge of the Changjiang River. Proc. the Symposium on the Deluge of the Changjiang River and Its Influence in Marine Environment of the East China Sea and Adjacent Seas, Seikai National Fisheries Research Institute, Fisheries Research Agency, p (in Japanese). Kuroda, K. and M. Hirai (2000): Abnormal low-salinity phenomenon occurred in the Japan Sea in JAMARC, Japan Marine Fishery Resources Research Center, 54, (in Japanese). Lee, D.-K., P. Niiler and H.-D. Jeong (2003): Characteristics of the East China Sea low salinity water in August. Proc. International Symposium on Oceanographic Environmental Change after Completion of the Changjiang (Yangtze River) Three Gorges Dam, p Lie, H.-J. and C.-H. Cho (1994): On the origin of the Tsushima Warm Current. J. Geophys. Res., 99(C12), Lim, D. B. and S.-D. Chang (1969): On the cold water mass in the Korea Strait. J. Oceanol. Soc. Korea, 4, Minagawa, M. (2003): On the recent precipitation and discharge of Changjiang in China. Proc. the Symposium on the Deluge of the Changjiang River and Its Influence in Marine Environment of the East China Sea and Adjacent Seas, Seikai National Fisheries Research Institute, Fisheries Research Agency, p (in Japanese). Moriyasu, S. (1972): The Tsushima Current. p In Kuroshio Its Physical Aspects, ed. by H. Stommel and K. Yoshida, Univ. of Tokyo Press, Tokyo. Nof, D. (2001): China s development could lead to Bottom Water formation in the Japan/East Sea. Bull. American Meteorol. Soc., 82, Ogawa, Y. (1983): Seasonal changes in temperature and salinity of water flowing into the Japan Sea through the Tsushima Straits. Bull. Japan Soc. Fish. Oceanogr., 43, 1 8 (in Japanese with English abstract). Ogawa, Y., T. Nakahara and R. Tanaka (1977): Hydrographic nature of coastal fishing grounds in the southwestern Japan Sea. Bull. Seikai Regional Fisheries Research Lab., 50, Onitsuka, G. and T. Yanagi (2005): Roles of the Tsushima Warm Current and upwelling along the east coast of Korea as nutrient sources in the Japan Sea. Proc. the Indonesia Ocean Forum 2005 and the 13th PAMS/JECSS Workshop (CD- ROM). Pang, I.-C., K.-H. Hyun and C.-S. Hong (2003): Changjiang coastal water around Cheju Island in summer. Proc. Extended Abstract Volume The 12th PAMS/JECSS Workshop, p Pang, I.-C., H.-J. Ko, J.-H. Moon, T. Matsuno, J.-H. Lee and D.-K. Lee (2004): Drifter observations for Changjiang Diluted Water (CDW) in 2003 and Proc. The 2nd International Symposium on PEACE, p Senjyu, T. (1999): The Japan Sea Intermediate Water; its characteristics and circulation. J. Oceanogr., 55, Sverdrup, H. U., M. W. Johnson and R. H. Fleming (1942): The Oceans, Their Physics, Chemistry, and General Biology. Printice-Hall, New York, 1087 pp. Takikawa, T. and J.-H. Yoon (2005): Volume transport through the Tsushima Straits estimated from sea level difference. J. Oceanogr., 61, Takikawa, T., J.-H. Yoon and K.-D. Cho (2005): The Tsushima Warm Current through Tsushima Straits estimated from ferryboat ADCP data. J. Phys. Oceanogr., 35, Teague, W. J., G. A. Jacobs, H. T. Perkins, J. W. Book, K.-I. Chang and M.-S. Suk (2002): Low-frequency current observations in the Korea/Tsushima Strait. J. Phys. Oceanogr., 32, Yanagi, T. (2002): Water, salt, phosphorus and nitrogen budgets of the Japan Sea. J. Oceanogr., 58, Yoshikawa, Y., A. Masuda, K. Marubayashi, M. Ishibashi and A. Okuno (2005): Current variability in the Tsushima Strait. Proc. the 54th Nat. Cong. of Theoretical and Applied Mechanics, p (in Japanese with English abstract). Zhu, J., P. Ding, S. Hu and L. Yang (2001): Observation of the Changjiang diluted water, plume front and upwelling off the Changjiang mouth during August in Proc. the 11th PAMS/JECSS Workshop (CD-ROM). Zhu, X., W. Xian and F. Miao (2003): Ecological modification of runoff to the estuarine ecosystem of Changjiang River. Proc. the Symposium on the Deluge of the Changjiang River and Its Influence in Marine Environment of the East China Sea and Adjacent Seas, Seikai National Fisheries Research Institute, Fisheries Research Agency, p T. Senjyu et al.