The Taiwan-Tsushima Warm Current System: Its Path and the Transformation of the Water Mass in the East China Sea

Transcription

1 Journal of Oceanography, Vol. 55, pp. 185 to The Taiwan-Tsushima Warm Current System: Its Path and the Transformation of the Water Mass in the East China Sea ATSUHIKO ISOBE Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-Koen, Kasuga , Japan (Received 30 September 1998; in revised form 29 October 1998; accepted 13 November 1998) Using a temperature data set from 1961 to 1990, we estimated the monthly distribution of the vertically integrated heat content in the East China Sea. We then drew the monthly map of the horizontal heat transport, which is obtained as the difference between the vertically integrated heat content and the surface heat flux. We anticipate that its distribution pattern is determined mainly due to the advection by the ocean current if it exists stably in the East China Sea. The monthly map of the horizontal heat transport showed the existence of the Taiwan-Tsushima Warm Current System (TTWCS) at least from April to August. The T-S (temperature-salinity) analysis along the path of TTWCS indicated that the TTWCS changes its T-S property as it flows in the East China Sea forming the Tsushima Warm Current water. The end members of the Tsushima Warm Current water detected in this study are water masses in the Taiwan Strait and the Kuroshio surface layer, the fresh water from the mainland of China, and the southern tip of the Yellow Sea Cold Water extending in the northern part of the East China Sea. Keywords: Taiwan-Tsushima Warm Current System, East China Sea. 1. Introduction There are two different schools of thought regarding the origin of the Tsushima Warm Current. One believes that it originates around the Taiwan Strait (Fang et al., 1991). They regard the Tsushima Warm Current as a part of the so-called Taiwan-Tsushima Warm Current System (hereafter referred to as the TTWCS). Isobe (1999) supported this idea through a diagnostic model of the vertically averaged vorticity equation with the long-term observed hydrographic and wind data. According to his study, the TTWCS flows along the isobath in the East China Sea. The wind-induced and the JEBAR terms are too small to break this flow pattern. However, he found that a large positive JEBAR term is evaluated along the shelf edge during autumn (from October to December) southwest of Kyushu. Isobe (1999) insists that the TTWCS breaks down in autumn, and that the Tsushima Warm Current bifurcates from the Kuroshio southwest of Kyushu during this season. The other school believes that the current always comes from southwest of Kyushu as a branch of the Kuroshio (Lie and Cho, 1994; Hsueh et al., 1996). Huh (1982), using satellite images, suggested that the Tsushima Warm Current included a repeatedly detached warm eddy from the Kuroshio southwest of Kyushu. This idea should be contained in the latter one. The above thoughts-except for Isobe (1999)-were derived from shortterm observations, e.g., data set of one-day current meter moorings (Fang et al., 1991), trajectories of the satellite- tracked drifters with the hydrographic data (Lie and Cho, 1994), and several infrared images (Huh, 1982). Therefore, we need to confirm the origin of the Tsushima Warm Current using a long-term observational data set. In this study, we try to reconfirm the existence of the TTWCS using a different procedure from that used in Isobe (1999). The water temperature is the most abundant data set in the ocean, so we use temperature data to detect the path of the ocean current in this study. We first obtain the horizontal distributions of the vertically integrated heat content using the temperature data set historically obtained in the East China Sea. The distribution of the heat content is largely influenced by the sea surface heat flux, especially in the shallow shelf region, so that it is difficult to detect the current path using the heat content pattern as it is. Therefore, we have to remove the contribution of the surface heat flux from the heat content, and obtain the distribution of the horizontal heat transport. Although the horizontal heat transport contains the effect of the horizontal diffusion, we anticipate that its distribution pattern is determined mainly due to the advection by the ocean current if it exists in the East China Sea stably. The ocean current changes its T-S property as it flows in the East China Sea because of the addition of the fresh water from the mainland of China, exchange of the water mass with the Kuroshio and so on. After detecting the current path in the East China Sea, we compare the T-S Copyright The Oceanographic Society of Japan. 185

2 diagrams along the current path in order to trace the transformation of the water mass. This analysis elucidates the water masses that make up the Tsushima Warm Current water. 2. Data and Method Figure 1 shows the study area, which is surrounded by the thick line. We divided it into 62 boxes whose resolution is 1 in both latitude and longitude. The conservation equation of the heat content at arbitrary depth is; ( cρt) t ( ) + u x ( cρt)= K H 2 ( 2 cρt cρt)+ K Z z 2 1 where c is the specific heat, ρ the water density, T is the water temperature, u is the current vectors, and K H, K Z are the horizontal and vertical diffusivities, respectively. We vertically integrate Eq. (1) from surface (z = 0) to the bottom (z = H); () q t = q h + q s, 2 ( ) where q is the vertically integrated heat content of unit area, and is expressed as follows. 0 q = cρ Tdz ( 3) where T is to be regarded as a representative temperature profile in each box. In this study, c and ρ are assumed to be constant. q h is the horizontal heat transport which is expressed by; H 0 q h = cρ ut x + vt + K H 2 T dz, ( 4) H y and q s is the heat flux through the sea surface. Using the temperature data from 1961 to 1990, supplied by the Japan Oceanographic Data Center (JODC), we converted these data to the vertically integrated heat content (q) of unit area in each box. We omitted the water temperature data that exceed three times the standard deviation from its average value at the same depth in each box. Also we omitted the temperature profile whose measured depth range (maximum measured depth minus minimum one) is less than 70 percent of the mean depth of the box. We used the temperature data above 400 m depth because the data below this depth are scarce. In the case that the depth of the box is more than 400 m, we neglected the vertical heat transport Fig. 1. The study area in the East China Sea with the divided boxes. Also shown is the isobath in meters. 186 A. Isobe

3 Fig. 2. Upper panel shows the locations at which we show the vertically integrated heat content and each term in Eq. (2). In lower four panels, small dots with the thin line show the vertically integrated heat content (q, cal/cm 2 ) in each box. The broken line with the open square indicates the temporal differentiation of q (q t, W/m 2 ). The doted line with the closed square shows the heat flux through the sea surface (q s, W/m 2 ). The thick line with the closed circle indicates the horizontal heat transport (q h, W/m 2 ) defined by Eq. (4). The Taiwan-Tsushima Warm Current System 187

4 across this depth. This assumption is considered to be valid because the horizontal heat transport prevails in the deep offshore region where the Kuroshio flows. Using the all quality controlled data of q, we drew the scatter plot that denotes the annual variation of q in each box. Figure 2 shows the four examples (boxes a, b, c and d) of the scatter plot of q. The annual variation of q was fitted by sinusoidal curves using the least square method. In this study, we used a function that is expressed by; 3 qt ()= q 0 + a n sin 2nπ τ t K n 5 n=1 ( ) where q 0 is a constant, a n and K n are the amplitude and the phase lag of the n-th signal, respectively. τ is one year, which is the longest period. Through trial and error, we determined one-third year as the shortest signal to follow the temporal variation of the heat content in a box. If we chose a much higher frequency, the fitted curve would be overly complicated. The thin line in Fig. 2 shows the fitted curves. Differentiating Eq. (5), we evaluated the left-hand side of Eq. (2), which is denoted by the broken line with open squares in Fig. 2. In this study, the time increment was selected to be one month. Hirose et al. (1998) supplied the monthly net heat flux through the sea surface in each 1 latitude and longitude box in the East China Sea (the dotted line with closed squares in Fig. 2). They evaluated the net heat flux using the meteorological and oceanographic data set from 1960 to 1990 (see their text for details). Their analyzed period is nearly same as ours. Using their data set, we obtained the monthly distribution of the horizontal heat transport (q h ) in the East China Sea as a difference between q/ t and q s. In Fig. 2, q h is shown by the thick line with the closed circles. 3. The Distribution of the Horizontal Heat Transport in the East China Sea In Fig. 2, the boxes a and b are located in the southwestern and northeastern side of the East China Sea, while the boxes c and d are located in the upstream and downstream region of the Kuroshio, respectively. The surface heat flux (q s ) in all boxes shows a sinusoidal curve with its maximum from June to August. Apparently, its annual average has a negative value. On the other hand, the annual average of the horizontal heat transport (q h ) seems to be positive. This means that the heat loss due to the surface heat flux is, on average, compensated by the horizontal heat transport in the study area. It is found that the q h shows its maximum in spring along the Kuroshio path (c and d). The peak of q h at box d is delayed for 1 2 months compared to that at box c. This suggests that the warm water moves northeastward as the Kuroshio flows. The peak of q h in the shelf region of the East China Sea varies from summer (a) to autumn (b) with 2 months delay. This also implies the existence of the northeastward flow that transports the warm water mass. Figure 3 shows the monthly distribution of the horizontal heat transport in the East China Sea. It is found that the area where the heat transport is greater than 300 W/m 2 (hereafter referred to as the warming region) moves northeastward in the offshore region from March to May (see the shaded area given by arrows with a closed circle). Such a warming region corresponds to the area where the temporal variation of the heat content is large due to the horizontal heat transport. We consider that the appearance of such a warming region is due to the advection of the warm water mass by the ambient mean flow. The location of the warming region is considered to correspond to the edge of the warm water mass that moves with the Kuroshio. So it is considered that successive change of the location of the warming region indicates the Kuroshio path, which seems to be along the shelf edge and end at the Tokara Strait in Fig. 3. The detected Kuroshio path is consistent with other studies (e.g., Yamashio et al., 1993). We find, too, that the other warming region moves northeastward in the shallow shelf region from April to August (see the arrows with an open circle). The propagation speed is apparently slower than that along the Kuroshio path, so it is considered that such a movement of the warming region is effected by the weak northeastward flow compared to the Kuroshio. This warming region moves along the isobath of around 100 m, that is, the same path as TTWCS (Fang et al., 1991). Isobe (1999) also use a diagnostic model to show that the TTWCS flows along the isobath in the East China and Yellow Seas, and that wind-induced and JEBAR terms are too small to disturb this flow path. The propagation speed is estimated to be around 10 cm/sec (800 km/ 3 months) along the 100 m isobath, which is the same order as the transport density (volume transport in the unit width) of TTWCS (~10 m 2 /sec, Fang et al., 1991). So we consider that the movement of the warming region from April to August is mainly due to the advection by TTWCS that flows along the isobath in the East China Sea. If the Tsushima Warm Current originates around the southwest of Kyushu crossing the steep shelf edge, crossshelf heat transport should be dominant there. Such a distribution pattern is found at least in November and December. Isobe (1999) shows that the TTWCS breaks down in autumn, and that the cross-shelf transport prevails southwest of Kyushu as an origin of the Tsushima Warm Current in autumn. The distribution pattern shown in Fig. 3 is consistent with the seasonality of the TTWCS. In winter (from January to March), the horizontal heat transport has a large value along the Kuroshio path. However, it is difficult to describe a current path clearly in the shelf region. The intense surface cooling during mid-winter makes the shelf region homogeneous. Therefore, the horizontal heat transport does not show the clear spatial pattern 188 A. Isobe

5 The Taiwan-Tsushima Warm Current System 189 Fig. 3. Monthly distribution of the horizontal heat transport in the East China Sea. The contour interval is 100 W/m 2. The region with the positive value is shaded: double intensity denoting more than 300 W/m 2, i.e. warming region. See the text for the meaning of arrows with open and closed circles. Also shown is the depth in meters.

6 Fig. 4. T-S diagrams at the boxes that are shown in the upper panel. The warming regions are also indicated in the upper panel by broken lines. In the lower panels, the observed T-S plots are divided into three layers, which are shown in the T-S diagram of May. The dotted rectangular in T-S diagrams in July and August indicates the range of the cold water (see text for detail). 190 A. Isobe

7 from January to March, although the northeastward movement of the positive region from January to March implies the existence of the TTWCS. 4. Transformation of the Water Mass along the Path of TTWCS The TTWCS may be greatly altered in terms of its T-S (temperature-salinity) property as it flows in the East China Sea due to the mixing process there. Now we show the T-S diagram within a warming region from May to August. We then describe the transformation of the water mass of the TTWCS. The salinity data used here are also supplied by JODC. As shown later in this section, we also use some T- S data outside of our study area, which are obtained from JODC. Figure 4 shows the T-S diagram in May, June, July and August within a box in each warming region. The locations of each box are indicated in an upper panel. In May, we are able to see that the temperature of the TTWCS ranges between 16 and 26 C, and the salinity ranges between 31.5 and 34.7 psu. In order to find the origin of this water mass, we compare this T-S diagram with those in the Taiwan Strait and the Kuroshio surface layer (0 100 m) in May (Fig. 5). The water types of the Taiwan Strait and the Kuroshio surface layer resemble each other, at least, in this month. These two water types are also similar to that of TTWCS when entering the East China Sea. It is considered that the water passing through the Taiwan Strait and/or the Kuroshio surface layer originally composes the TTWCS. As the TTWCS flows northeastward, less saline water is added in the surface layer (0 30 m) in June, July and August. The temperature of the less saline water increases from June to July because of the heating through the sea surface. In July and August, the cold water is also added in the middle (30 50 m) and lower (>50 m) layers, which is surrounded by the dotted line in Fig. 4. The temperature of this cold water ranges between 12 to 16 C, while the salinity ranges between 32 and 34.7 psu. The temperature and salinity of the cold water in August are slightly higher than those in July. This implies that the temperature and the salinity of the cold water increase as the TTWCS flows northeastward. The T- S property in August is nearly same as that of the Tsushima Warm Current found in the Tsushima/Korea Strait in sum- Fig. 5. T-S diagram at boxes A (Taiwan Strait) and B (Kuroshio surface layer). The observed T-S plots are divided into three layers, which are shown in the each T-S diagram. The Taiwan-Tsushima Warm Current System 191

8 mer (see Fig. 2 in Ogawa, 1983). We therefore conclude that the transformation processes of the TTWCS water are complete around the location of the box in August. Now we consider the origins of the less saline water added to the surface layer of the TTWCS, and the cold water added to the middle and lower layers. Figure 6 shows the horizontal distribution of the surface salinity. The less saline water extends to the south or east from the river mouth of the Changjiang. It is found that the warming region intersects that low salinity region. The intersecting area increases from May to July corresponding to the increase of the less saline water in the T-S diagram. So we conclude that the origin of the less saline water added to the TTWCS is the mainland of China, especially the river mouth of Changjiang. Figure 7 shows the horizontal distribution of the temperature at 50 m depth. We can see that the warming region intersects the cold water mass that is located in the northern part of the study area. The intersection seems to start from July. This is consistent with the T-S diagram in which the cold water below 16 C starts to appear from July. So we conclude that addition of the cold water to the TTWCS is mainly due to mixing with the cold water mass extending in the northern part of the study area. Now we consider why the cold water below 16 C appears in the northern part of the study area. Figure 8 shows the monthly mean vertical distribution of the temperature in July along a line A-A (see the upper panel of Fig. 8 for its location). Also shown is the vertical section in February for reference. In February, we can see the cold water below 10 C in the Yellow Sea, which is named the Yellow Sea Cold Water (hereafter YSCW, Nakao, 1977). This water mass forms due to intense cooling in midwinter, and remains below the thermocline until October, reducing its thickness Fig. 6. The horizontal distributions of the surface salinity in May, June, July and August within the study area. Contour interval is 0.5 psu. Shading has been chosen to emphasize the less saline water: single, double intensity denoting, respectively, less than 34 and 32 psu. Also shown are the locations of the warming region (broken line). 192 A. Isobe

9 due to the mixture with the warm upper layer (Tawara and Yamagata, 1991). In Fig. 8, we can find the YSCW in the lower layer in the Yellow Sea in July, too. Although some studies (e.g., Uda, 1934) have pointed out the southward movement of the YSCW in spring and summer, Tawara and Yamagata (1991) show that the YSCW keeps its location throughout a year on the basis of 10-year mean temperature fields in the Yellow Sea. Figure 8 also shows that the YSCW in July stays in its generation area. In Fig. 8, we also find the cold water below 16 C in the East China Sea, which is the same water mass extending in the northern part of our study area. The seasonal variation of the distribution pattern of this cold water mass is very similar to the YSCW, so we consider that the generation process of this cold water below 16 C is same as that of the YSCW. We can regard this cold water as the southern tip of the YSCW. The temperature and salinity of the added cold water increase as TTWCS flows northeastward, as mentioned previously. The upper panel of Fig. 9 shows the location at which added cold water (12 T ( C) 16, 32 S (psu) 34.7) has been observed in July in the lower layer ( m). The cold water extends from the northern part of the East China Sea to the Tsushima/Korea Strait. We set the boxes A, B, C and D within the study area in order to investigate the change of the T-S property of the cold water as we go northeast. The lower panel of Fig. 9 shows the T-S diagram of the cold water (same range as the dotted rectangular in Fig. 4) at four boxes. We find that the dots move from the lower left area to the upper right as we go northeast. The temperature and salinity of the box D are close to those of the original TTWCS water (see Fig. 4). Thus we consider that the mixture with the original TTWCS water mainly causes the transformation of the cold water added to the TTWCS. The temperature and salinity of the box D are close to the T- Fig. 7. The horizontal distributions of the temperature at 50 m depth in May, June, July and August. Contour interval is 1 C. Shading has been chosen to emphasize the cold water less than 16 C. Also shown are the locations of the warming region (broken line). The Taiwan-Tsushima Warm Current System 193

10 Fig. 8. The monthly mean vertical distribution of the temperature in February and July along the line A-A. Shading has been chosen to emphasize the cold water: single, double intensity denoting, respectively, less than 16 C and 10 C. S property of the Kuroshio surface water as well (see Fig. 5). The Kuroshio surface water that exists around the shelf edge west of Kyushu may also be responsible for the transformation of the T-S property of the added cold water. 5. Conclusions The monthly distributions of the horizontal heat transport show a path of the TTWCS, at least from April to Fig. 9. The upper panel shows the locations where the cold water defined in this study has been observed in July in the lower layer ( m). Also shown are the locations of boxes A, B, C and D at which we show the T-S diagram in the lower panel. Note that the range of the T-S diagram is different from others (Figs. 4 and 5) in order to enlarge the T-S area of the cold water. August. The present study indicates that the TTWCS originates around Taiwan, i.e., Taiwan Strait and the Kuroshio region east of Taiwan. Then, TTWCS greatly changes its T- S property due to the mixing process with the surrounding water masses as it flows in the East China Sea. We trace its transformation process using the T-S analysis along the path of the TTWCS from April to August. The water mass of the 194 A. Isobe

11 TTWCS entering the East China Sea has the same T-S property as those around the Taiwan Strait and the Kuroshio surface layer. Then the water mass of TTWCS changes due to admixture with the freshwater from the mainland of China, and with the southern tip of YSCW extending in the northern part of the East China Sea. The horizontal mixture with the Kuroshio surface water across the shelf edge may also be responsible for the formation of the Tsushima Warm Current water. However, it is difficult to pick up this process in the T-S diagram, because TTWCS originally has the same T-S property as that of the Kuroshio surface water, as shown in Fig. 5. The water masses that we have mentioned above should be considered as the end members composing the Tsushima Warm Current water, although our discussion has been qualitative. A quantitative discussion should be accomplished in the near future. Also, we have to clarify the reason for the appearance of the warming region in the East China Sea only during May-August, which has remained obscure in this study. Acknowledgements Data supplies by JODC (Data Online Service System, J-DOSS) and Dr. Hirose, Kyushu University are gratefully acknowledged. Thanks are also extended to two anonymous reviewers. References 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. Hirose, N., H.-C. Lee and J.-H. Yoon (1998): Surface heat flux in the East China Sea and the Yellow Sea. J. Phys. Oceanogr. (in press). Hsueh, Y., H.-J. Lie and H. Ichikawa (1996): On the branching of the Kuroshio west of Kyushu. J. Geophys. Res., 101, Huh, O. K. (1982): Spring season flow of the Tsushima Current and its separation from the Kuroshio: Satellite evidence. J. Geophys. Res., 87, Isobe, A. (1999): On the origin of the Tsushima Warm Current and its seasonality. Cont. Shelf Res., 19, Lie, H.-J. and C.-H. Cho (1994): On the origin of the Tsushima Warm Current. J. Geophys. Res., 99, Nakao, T. (1977): Oceanic variability in relation to fisheries in the East China Sea and the Yellow Sea. J. Fac. Mar. Sci. Technol., Tokai Univ., Special Number, Ogawa, Y. (1983): Seasonal changes in temperature and salinity of water flowing into the Japan Sea through the Tsushima Strait. Bull. Japan. Soc. Fish. Oceanogr., 43, 1 8 (in Japanese). Tawara, S. and T. Yamagata (1991): Seasonal formation of bottom water in the Yellow Sea and its interannual variability. Umi to Sora (Sea and Sky), 66, (in Japanese). Uda, M. (1934): Hydrographical researches on the normal monthly conditions in the Japan Sea, the Yellow Sea, and the Okhotsk Sea. J. Imp. Fish. Exp. Sta., 5, (in Japanese). Yamashio, T., A. Maeda and M. Sakurai (1993): Mean position and deviation of the Kuroshio axis in the East China Sea. Umi to Sora (Sea and Sky), 69, (in Japanese). The Taiwan-Tsushima Warm Current System 195