Does the Taiwan Warm Current originate in the Taiwan Strait in wintertime?

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2005jc003281, 2006 Does the Taiwan Warm Current originate in the Taiwan Strait in wintertime? Chen-Tung Arthur Chen 1 and David D. Sheu 1 Received 6 September 2005; revised 12 December 2005; accepted 9 January 2006; published 13 April [1] There is no doubt that the Taiwan Warm Current (TWC) flows through the Taiwan Strait in summer and it reaches the East China Sea (ECS) proper, bringing with it a great deal of nutrients. Here satellite temperature, as well as hydrological, satellite-tracked drifter, and 18 O data, are used to show that in winter, warm waters from the South China Sea and a branch of the Kuroshio south of Taiwan reach only the southern part of the Taiwan Strait and that in no way do they flow up freely through the northern part of the Taiwan Strait. This is a clear sign that in winter most of the TWC must originate in the Kuroshio, which moves onto the ECS shelf northeast of Taiwan. Citation: Chen, C.-T. A., and D. D. Sheu (2006), Does the Taiwan Warm Current originate in the Taiwan Strait in wintertime?, J. Geophys. Res., 111,, doi: /2005jc Introduction [2] The Taiwan Warm Current (TWC) which flows to the north all year round between the 50-m and 100-m isobaths has an overwhelming influence on the western part of the East China Sea (ECS). Named for its characteristic high temperature [Mao et al., 1964] even during strong NE monsoons in winter, particularly when compared with the southward flowing coastal current [Su et al., 1990], the TWC has had its fair share of debate with regard to exactly where it originates in winter. Earlier hydrographic studies [Uda, 1934; Mao et al., 1964] and results from bottom drifters [Inoue, 1975] provided some evidence that the TWC is a branch of the Kuroshio and that it originates in the NE of Taiwan where the Kuroshio impinges upon and upwells onto the ECS continental shelf [Chern and Wang, 1990; Wong et al., 1991; Liu et al., 1992; Chen et al., 1995]. [3] Beardsley et al. [1985] and Chen et al. [1994], on the other hand, generally subscribed to the view of Niino and Emery [1961], Mao and Guan [1982] and Nitani [1972] that the TWC originates in the Taiwan Strait and flows northeastwardly. They also contended that unlike the Kuroshio which flows to the south of Japan, the TWC flows into the Sea of Japan through the Tsushima Straits between Korea and Japan. Guan [1983] pointed out that in summer, the upper part of the TWC must have its source in waters flowing through the Taiwan Strait. Yet, the picture would be incomplete without recognizing the work of Weng and Wang [1984] that advocated that in summer, the lower layer of the TWC is derived from the Kuroshio intrusion northeast of Taiwan, while the upper layer is a mixture of the intruding Kuroshio water flowing through the Taiwan Strait. Perhaps complicating the issue 1 Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan. Copyright 2006 by the American Geophysical Union /06/2005JC003281$09.00 further, Su and Pan [1987, 1990] and Yuan et al. [1987] advanced the notion that the TWC actually has two branches, one inshore that flows northward near the 50-m isobath and the other that turns toward the shelf break around 27 N. [4] Su et al. [1990, 1994] reported that in summer the TWC originates either in the Taiwan Strait or north of Taiwan because of the northward intrusion of the Kuroshio, a view that, at least partially, supports the results of Weng and Wang [1984]. Su et al. further explained that because the shelf water is denser than the Kuroshio water in winter, the intrusion of the Kuroshio Shelf Water (KSS) must take place while the Kuroshio Subsurface Water is upwelling onto the shelf and that most of the intruding KSS returns to the shelf break region, thus forming an offshore branch of the TWC. The rest, they claimed, flows northward east of the cold coastal waters, and is often strengthened by currents from the Taiwan Strait except during strong northerly winds. According to them, it is this that leads to the formation of the inshore branch of the TWC. [5] In their recent article, Zhu et al. [2004] have reported that there is no question about the existence of the TWC in summer. They have also made the case to argue that the TWC is present in winter too, but Zhu et al. [2004, paragraph 10] have concluded that it originates in... a warm water sourcing from the Taiwan Strait and flowing continuously to the submerged river valley off the Changjiang. Furthermore, Kim et al. [2005] have very recently studied the origin of the Tsushima Current using their oxygen isotope measurements near the Tsushima Straits as well as literature data for the Taiwan Strait and those for northern Taiwan. They have adopted the view of Beardsley et al. [1985] that the Taiwan Strait waters contribute to the Tsushima Current. The present study, however, hopefully resolves this much discussed issue, and it does so by employing satellite temperature data, field measurements of hydrography, plus satellite-tracked drifter and 18 O studies in the Taiwan Strait. In a nutshell, what the 1of8

2 results indicate is that, in all likelihood, the TWC does not originate in the Taiwan Strait in winter. 2. Methods [6] Discrete surface water samples at 2 m were collected along three transects across the Taiwan Strait during March 2000 on board the R/V Ocean Researcher III. The d 18 O analysis of water samples followed the procedures of Epstein and Mayeda [1953], and was first equilibrated with CO 2 at 40 ± 0.05 C using Micromass Multiprep system, then performed with a VG Optima isotopic ratio mass spectrometer. Results of isotopic measurement were expressed with the conventional d notation and reported in per mil (%) difference relative to the Vienna SMOW standard. The overall procedural error evaluated by triplicate water samples was approximately ±0.1%. 3. Satellite Data [7] Zhu et al. [2004] have used satellite temperature data for January 1986 and February 2001 to demonstrate the intrusion of the warm TWC. Pertinent here, however, is that while the composite NOAA sea surface temperatures (SST) for the months of January and February 2001 (Figures 1a 1f) do indeed show a tongue-like warmer water feature extending to an area off the Changjiang River estuary near 123 E, 31 N (Figures 1d and 1e), correctly interpreting the snapshot SST images can be tricky. More to the point, the same snapshots may very well be explained by both the southward flow of cold coastal waters to the west of the remnant warm waters from summer and, at the same time, by the southward flow of the cold Yellow Sea waters to the east of the remnant warm waters. Actually, the southward flow of cold waters may be substantiated by looking at a series of SST snapshots starting from October 2000 (Figure 1a) where the 24 C water (bright yellow) spreads to roughly 27 N along the coast. Offshore, the 24 C water flows in a downwind, southeastward direction from a point just south of the Changjiang River mouth and extends to about N at E. Between these colder waters is the 25 C water (light pink in Figure 1a) which extends northward to 30 N along a submerged river valley offshore of the Changjiang River mouth [Beardsley et al., 1985]. In November 2000, the 24 C water reaches not only Taiwan, but also the northern South China Sea (Figure 1b). In fact, the 20 C water (dark green in Figure 1b) reaches 24 N along the Chinese coast and the 25 C water previously found off the Changjiang River estuary has by now been forced to as far as 125 E. [8] In December 2000 the 18 C water (light green in Figure 1c) reaches 24 N in the western Taiwan Strait, while the 24 C water can no longer be found on the ECS shelf except off northeastern Taiwan (Figure 1c). The patch of cold water further extends southward from the Yellow Sea, and the 18 C water reaches about 31 N. Slightly warmer water exists between this cold patch and the coastal waters off the Changjiang River estuary. This warm water, or the remnant TWC, has not cooled very much by January 2001 unlike the cold waters on both sides which have cooled much more (Figure 1d). Of particular interest here is that in the Taiwan Strait the 24 C water only exists in the southeastern part. The entire northern part of the Taiwan Strait is now less than 20 C, but north of Taiwan, the intrusion of the 24 C water becomes obvious. The explanation for this is simple: It must have come from the Kuroshio. [9] In February 2001 the SST pattern remains the same (Figure 1e) as that in the previous month. Above all, the warmer water feature near 123 E and 31 N seems almost stationary, with colder waters on both sides. Zhu et al. [2004] have interpreted this as evidence of the northward flow of the TWC. In March 2001 the 24 C water in the Taiwan Strait finally reaches northwestern Taiwan, but it does not seem to flow all the way to the ECS proper (Figure 1f). [10] On the weight of the evidence from Figures 1a 1f, there is little doubt that the warm surface waters in all likelihood do not flow right up through the Taiwan Strait in winter. 4. Hydrography of the Taiwan Strait [11] The q/s diagram of the northern Taiwan Strait, shown in Figure 2, is very distinctive. Waters in the eastern part (stations A C) where the water depth is more than 75 m are those that have circumvented the northern tip of Taiwan. These waters probably originated in the South China Sea which accounts for the fact the q/s signature (longer line segment in Figure 2a) is similar to that of the South China Sea (SCS). Enhanced mixing on the ECS shelf, however, turns the originally curved SCS q/s plot [Chen and Huang, 1996; Chen and Wang, 1998] into a line [Chen et al., 1995; Chen and Hsing, 2005]. Waters in the western portion of the Taiwan Strait (stations D I) also have a linear q/s plot (shorter line segment in Figure 2a), and these waters are a mixture of the coastal waters and the waters that have flowed through the Taiwan Strait. Both q/s plots have a negative slope. [12] In winter, the waters in the eastern part of the northern Taiwan Strait have much lower temperatures but only slightly lower salinity, and the q/s plot still has a negative slope (shorter line segment in Figure 2b). Waters in the western part of the strait, however, have already been cooled a great deal along the Chinese coast, thereby changing the slope of the q/s plot to a positive one. The cold coastal waters are nutrient-rich and must have come from the north [Chen, 2003]. In plain terms, this means that a mixture of the SCS water and the Kuroshio branch is found in the entire Taiwan Strait in summer, but only in the southern end of the Taiwan Strait in winter. The reason for this is that it does not continue to flow right through to the northern part of the Taiwan Strait, let alone ever reach the Changjiang estuary. The hydrographical data and the numerical study of Jan et al. [2002] have similarly indicated Figure 1. Sea surface temperatures between October 2000 and March 2001 obtained from NOAA (provided by National Taiwan Ocean University). In the color bar code, each shade corresponds to sea surface temperatures indicated. Bar code is same in all figures. 2of8

3 Figure 1 3 of 8

4 Figure 2. The q/s plots for northern part of Taiwan Strait obtained in August and December Typical q/s plots of South China Sea and West Philippine Sea waters are represented by two thin lines (modified from Chen and Hsing [2005]). that only a small portion of the Kuroshio branch in the against the even stronger NE monsoon. Worth bearing in southern Taiwan Strait manages to flow to the northern end mind, after the drifter joins the Kuroshio, it flows onto the of the strait in winter. shelf before turning back toward the northeast. This is [13] Zhu et al. [2004] observed a northward flow on 5 6 highly consistent with the pattern of the satellite temperatures in early winter (Figure 1d). It should be no surprise March at a station where the water depth is 50 m. On the other hand, the acoustic Doppler current profiler (ADCP) then that Tang et al. [2000] reported that the intrusion of the results indicated that the flow was southward on 6 9 March Kuroshio onto the ECS shelf can block the flow from the at two shallower stations. True, Zhu et al. have correctly Taiwan Strait to the ECS. Along the same lines of evidence reported that southward winds prevailed during their February March 2001 survey, but they seemingly failed to notice that the strong NW winds relaxed on 5 March, and actually turned northward for two days when the currents were measured at the 50-m-deep station. In this regard, Chen [2003] reported that a sudden relaxation of the southward winds in the Taiwan Strait resulted in a sudden northward rush of the Taiwan Strait water which had previously been held back by the prevailing winds that had been blowing in the opposite direction. In this sense, the northward flowing TWC that Zhu et al. have claimed to observe may, in fact, only be a rare event in winter. 5. Drifter Data [14] Although it is argued above that in winter waters in the northern Taiwan Strait do not flow northward, in no way does it exclude the possibility that an intermittent northward flow does actually occur, especially when there is a relaxation in the NE monsoon. A northward flow, nevertheless, is probably an exception rather than the rule and therefore not continuous [Chen, 2003] and may very well be diverted back toward the south once the NE monsoon resumes. The bottom-mounted ADCP data coupled with the results from model calculations support just that [Ko et al., 2003; Wu and Hsin, 2005]. [15] Figure 3 shows the trajectory of each of two satellitetracked drifters afloat in the Taiwan Strait from 7 October to 29 November 1997 [Tseng and Shen, 2003]. It is apparent that although the drifters do flow northward through the Taiwan Strait in late October, one drifter turns somewhat abruptly toward the south, and then flows southeastwardly and eventually joins the Kuroshio in early November. The other drifter ceased to transmit data on 26 October. In winter, it would probably be even more difficult for a drifter, or the current for that matter, to flow northward Figure 3. Satellite-tracked drifter 1 (blue line) and drifter 2 (red line) trajectories from 7 October to 29 November Isobaths (in meters) in Taiwan Strait and in adjacent seas are also shown. Number beside each trajectory shows month and day, where 1026 means 26 October, for example. Direction of flow is indicated by arrow next to each trajectory. PHC is Peng-hu Channel; CYR is Chang-yuen Ridge; and KYD is Kuan-Yin Depression (Tseng and Shen [2003]; courtesy of R. S. Tseng). 4of8

5 Figure 4. Distribution of (a) surface temperature ( C), (b) salinity, (c) s t, and (d) d 18 O(%) in Taiwan Strait in March Black dots indicate sampling sites. too is the fact that a temperature front in the southern ECS also suggests that the northern Taiwan Strait water is separated from the ECS in winter [Hickox et al., 2000]. 6. The 18 O Data [16] The horizontal distribution of surface temperatures ( C), salinity, s t, and of d 18 O(%) in the Taiwan Strait in March 2000, given in Figure 4, provides yet another window on the issue here. It is immediately clear that the temperature, salinity, and s t contours in the northern Taiwan Strait are more or less horizontal, with colder, fresher but heavier waters located in the northern part. These contours indicate that the warmer, more saline but lighter waters in the southern Taiwan Strait do not flow through the strait. Besides this, the s t contours indicate that although the waters on the Chinese coast are cooled to 12 C, they are still relatively light on account of the low salinity when compared to waters northwest of Taiwan. [17] The d 18 O contours in the northern Taiwan Strait are also more or less horizontal. Particularly important here is that the 0.4% contour line extends across the entire northern Taiwan Strait. Against this, the waters in the southern Taiwan Strait have a d 18 O value of 0.1% or even higher. It is reasonable to conclude that the warmer, saltier and 18 O heavy waters in the southern Taiwan Strait do not flow to the northern part of the Taiwan Strait. On the contrary, the northern part of the Taiwan Strait is filled with colder, fresher, 18 O light waters in winter. The d 18 O values correlate well with salinity (d 18 O= S) with an r 2 value of The large negative interaction at S =0is indicative of riverine input, which is mostly contributed by the Chiangjiang (Yangtze River). Of note is that Kim et al. [2005] have assumed that some low- 18 O waters in their study area originated in the Taiwan Strait. In fact, the small, low- 18 O signals found in their study, as well as the large, low- 18 O signals detected in the northern Taiwan Strait in this study must have come from diluted water from the Chiangjiang. 7. Discussion and Conclusions [18] The satellite sea surface temperature as well as the hydrological and drifter data all substantiate the view that, in winter, waters do not flow freely through to the northern part of the Taiwan Strait, if at all. In fact, Wang and Chern [1988], using CTD data gathered in winter, identified a quasi-stationary zonal oceanic front in the central Taiwan Strait, thus blocking the northward intrusion of the Kuroshio branch water. Further, Lie and Cho [1994] concluded that the saline Kuroshio branch water is stagnant in the Taiwan Strait in winter and flows out only intermittently from winter to early spring. Using flow volume transports measured in the Peng-hu Channel (Figure 3), which is the major inflow region of the Taiwan Strait, Jan and Chao [2003] also concluded that there is almost no northward 5of8

6 Figure 4. (continued) Figure 4. (continued) 6of8

7 Figure 4. (continued) flowing warm current against the strong NE monsoon in winter. As a result, there is no connection between the socalled South China Sea Warm Current and the TWC. Finally, results from current measurements using four bottom-mounted ADCPs across the central Taiwan Strait indicated that there is no persistent northward flow in the Taiwan Strait in winter [Ko et al., 2003; Teague et al., 2003; Lin et al., 2005]. What all this means is that the TWC found in the ECS could not have originated in the Taiwan Strait, but instead must have come from a branch of the Kuroshio. On these grounds, it follows that the Taiwan Strait water most likely does not contribute much to the Tsushima Warm Current in winter. In fact, Teague et al. [2003] calculated fluxes on the basis of bottom-mounted ADCP data and concluded that the average volume transport was only 0.14 Sv through the Taiwan Strait between October and December 1999 compared with a remarkable 0.59 Sv through the Cheju Strait and an even greater 3.17 Sv through the Korea Strait. The main source of the Tsushima Current and its flow into the Sea of Japan is unquestionably the Kuroshio in the fall months. The contribution from the Taiwan Strait is expected to be even smaller in winter when the NE winds are stronger. [19] It is important to clearly understand the source of the TWC in that it affects the heat and salt balances of not only the Yellow and East China seas, but also the Sea of Japan. Beyond that, it has important implications as far as the transport of nutrients to the ECS goes. Since the Taiwan Strait is rather shallow, with a sill depth of only 60 m, any SCS or Kuroshio water that does manage to flow right up through the strait is confined to the surface layer and, as a rule, is low in nutrient concentrations. Hence if the TWC did originate in the Taiwan Strait it would not provide much nutrients to the ECS to support one of the largest fishing grounds in the world. The truth of the matter is that the subsurface waters of the Kuroshio are nutrient-rich to the point that when the Kuroshio moves onto the ECS shelf and forms the TWC, it may very well contain subsurface waters and transport a great deal of nutrients to the ECS proper [Chen, 1996, 2005; Chen and Wang, 1999]. [20] Acknowledgments. This research was supported by the National Science Council of Taiwan (NSC M , Z ) and the Aim for Top University Plan (95C 0302). S. Jan of National Central University, C. R. Wu of National Taiwan Normal University, and R. S. Tseng and C. C. Chang of National Sun Yat-sen University provided assistance. The National Center for Ocean Research of Taiwan supported the cruise under the project Taiwan Strait Nowcast Study. Two anonymous reviewers and J. Richman provided valuable comments which strengthened the manuscript. References Beardsley, R. C., R. Limeburner, H. Yu, and G. A. Cannon (1985), Discharge of the Changjiang (Yangtze River) into the East China Sea, Cont. Shelf Res., 4, Chen, C. S., R. C. Beardsley, R. Limeburner, and K. Kim (1994), Comparison of winter and summer hydrographic observations in the Yellow and East China seas and adjacent Kuroshio during 1986, Cont. Shelf Res., 14, Chen, C. T. A. (1996), The Kuroshio intermediate water is the major source of nutrients on the East China Sea continental shelf, Oceanol. Acta, 19, of8

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Chen, P. Ding, C. Li, and H. Lin (2004), Does the Taiwan Warm Current exist in winter?, Geophys. Res. Lett., 31, L12302, doi: / 2004GL C.-T. A. Chen and D. D. Sheu, Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung 804, Taiwan. (ctchen@mail.nsysu.edu.tw) 8of8