Chemical and physical fronts in the Bohai, Yellow and East China seas

Journal of Marine Systems 78 (2009) 394–410

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Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s

Chemical and physical fronts in the Bohai, Yellow and East China seas Chen-Tung Arthur Chen ⁎ Institute of Marine Geology and Chemistry, National Sun Yat-sen University, 80441 Kaohsiung, Taiwan, Republic of China

a r t i c l e

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Article history: Received 31 August 2006 Received in revised form 17 June 2008 Accepted 10 November 2008 Available online 20 February 2009 Keywords: Fronts East China Sea Yellow Sea Bohai Sea Kuroshio

a b s t r a c t Associated with strong mixing and stirring, as well as enhanced bioproductivity and ecotones, oceanic fronts have garnered worldwide attention in recent years. Research into oceanic fronts, especially thermal fronts, has gained momentum since the advent of satellites and their increased accessibility. Yet, studies of salinity and nutrient fronts —particularly those that are subsurface are few and far between. This study reviews the most widely accepted facts about surface and subsurface temperature and salinity fronts in the Bohai, Yellow and East China seas and their seasonal variations. The distribution of nutrients in the surface and bottom waters are mapped and nutrient fronts, for the first time, are identified systematically. These fronts are generally strongest in winter when southward flowing coastal currents are influenced most by winter monsoons, and the contrasts between these cold, fresh, nutrient-rich currents and the northward flowing warm, saline but nutrient-poor Kuroshio are strongest. Surface fronts are generally weakest in summer when coastal currents may be weaker and temperature, salinity and nutrient contrasts are diminished. The existence of fronts and why some are disconnected are mainly related to oceanic features such as topography, boundaries between water masses and current flow patterns. Three latitudinal temperature and nutrient fronts in the southern East China Sea in winter may suggest eastward flowing currents. These currents have not been described previously. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The study area, i.e., the Bohai, Yellow and East China seas, is bordered by China, Korea, Kyushu, the Ryukyu Islands and Taiwan (Fig. 1). The almost enclosed Bohai Sea (BS), with a surface area of 77 × 103 km2, is the smallest and shallowest of the three seas with an average depth of only 18 m; maximum depth is 83 m and volume is 1.39 × 103 km3. The Huanghe (Yellow River) is a major source of freshwater whereas the Yellow Sea (YS) is the source of salt through the Bohai Strait. The semi-enclosed YS has a surface area of 380 × 103 km2, an average depth of 44 m and total volume of 16.7 × 103 km3. Maximum depth of the YS is only 140 m, and like the BS, it sits entirely on the continental shelf. The Changjiang (Yangtze River) at the southwest corner of the YS is the major source of freshwater for the YS and East China Sea (ECS) to the south. The more open ECS, at 770 × 103 km2, is the largest of the three seas, with an average depth of 370 m and volume of 285 × 103 km3. The western three quarters of the ECS is occupied by the continental shelf, while the eastern part is deep; for example, the Okinawa Trough reaches a depth of 2,719 m (Zhang and Su, 2006). The near-surface waters of the ECS exchange with surface waters of the South China Sea (SCS) through the Taiwan Strait, with the West Philippine Sea (WPS) through several passages in the Ryukyu Islands, and with the Sea of

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Japan through the Tsushima Strait. Subsurface tropical (Smax) and intermediate (Smin) SCS and WPS waters only exchange with the ECS through the Okinawa Trough. These relatively shallow seas are strongly affected by monsoons, massive freshwater outflows, tides and the Kuroshio. In winter, strong northerly winds carrying cold, dry air prevail over northeastern Asia and adjacent seas. River discharge in winter is typically low. In summer, mild southerly winds carrying warm, moist air prevail and lead to high river discharge as well as numerous estuarine and coastal fronts. Interactions among monsoon winds, cold air, bottom topography, large freshwater discharge and the Kuroshio intrusion result in various circulation regimes and different water masses. Generally, coastal waters flow southward, while offshore waters, especially those influenced by the Kuroshio, flow northward in winter. In summer, currents tend to flow northward except for the currents off the western coast of Korea flowing southward and the Changjiang Diluted Water (CDW) flowing northeastward (e.g., Yanagi et al.,1998; Ichikawa and Chaen, 2000; Chen et al., 2001; Lee et al., 2002; Su and Yuan, 2005). Oceanic fronts virtually function as boundaries separating two different water masses, where oceanographic parameters, such as temperature and salinity, change rapidly. For instance, a salinity front is formed when the warm Kuroshio water meets the fresher waters of the YS and ECS near the continental shelf break. In winter, the ECS water is markedly colder than the Kuroshio water; thus, the salinity front coincides with a thermal front (Chern and Wang, 1990; Qui et al., 1990; Oka and Kawabe, 1998; Lee and Chao, 2003; Chu et al., 2005; Chen and Wang, 2006). Close to the coastline, salinity fronts also form

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Fig. 1. Various water masses, currents, as well as seasonal probability maps of the thermal fronts in the Bohai, East China and Yellow Seas between 1985 and 1996 (Hickox et al., 2000; courtesy of I. Belkin; the colored bar code represents the seasonal frequency of appearance). The temperature fronts by number, as designated by Hickox et al., are also shown. Solid white lines show, schematically, major surface currents while yellow dashed lines show major subsurface currents. Fronts 1a, 1b and 1c were undesignated by Hickox et al. while Front 1d indicates the southernmost part of Front 1.

where low-salinity water is affected by land runoff meeting highsalinity water from the sea (Beardsley et al., 1983, 1985; Lie et al., 2003; Lee et al., 2004). In addition, subsurface cold water vertically

mixed by bottom tidal mixing forms tidal fronts that are often coincident with thermal fronts (Zhao, 1987; Lie, 1989; Bi and Zhao, 1993; Yanagi, 1999). These fronts are mostly characterized by strong

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mixing, stirring, enhanced bioproductivity and ecotones, and have been at the core of a plethora of studies (e.g., Zheng and Klemas, 1982; Choi, 1991; He et al., 1995; Hickox et al., 2000; Tang and Su, 2000; Belkin, 2002; Kasai et al., 2002; Wei et al., 2003; Park et al., 2004; Park and Chu, 2006). Largely because coastal and estuarine fronts are easy to study and because of their immediate impact, these fronts have been studied most. For coastal fronts, when stratification is involved, strong tidalinduced vertical mixing creates well-defined tidally-mixed fronts in shallow waters near the coast (water depth b100 m). Estuarine fronts, on the other hand, form at the perimeter of freshwater spread above saline coastal water (Yanagi, 1999). During winter, tidally-mixed coastal fronts disappear as strong wind mixing and surface cooling homogenize water columns (Chen and Beardsley, 2002). Now that satellite images are available and easily accessible, oceanic fronts, especially surface thermal fronts, can be studied with relative ease. However, few studies have analyzed other fronts, such as salinity and nutrient fronts, particularly those in subsurface waters. In fact, to the best of the present author's knowledge, no comprehensive study of nutrient fronts, surface or subsurface, exists for this study area. The following is a review of what is known about large-scale temperature, salinity and nutrient fronts in the BS, YS and ECS and their seasonal variations. Distributions of temperature, salinity and nutrients in surface and bottom waters are mapped based on published data as well as data in various atlases. Small-scale fronts, such as those off estuaries, are not included. Since these fronts are associated with other oceanic features, such as boundaries between water masses, current flow patterns and nutrient distributions, water mass distributions are described first. Furthermore, because the study area is in the Kuroshio and monsoon-forcing regimes, fronts in these three seas are related in terms of their genesis and characteristics, and often extend from one sea to another. Furthermore, most fronts are most visible in winter, whereas some wane in summer before regaining strength in autumn. Consequently, fronts are generally described in terms of seasons starting with winter. For each season, the description sequence moves from north to south, and from the continental shelves to regions influenced by the Kuroshio. Surface fronts are described first, followed by subsurface fronts, if any. The previously undisclosed latitudinal offshore flowing currents in the southern ECS in winter are inferred from three temperature, salinity and nutrient fronts. A list of acronyms is given in Appendix A. 2. Water masses Since oceanic fronts typically form between water masses, the main water masses in and around the BS, YS and ECS are described first. Because numerous water masses are referred to in this study, the use of acronyms is necessary, and start with place names such as the BS. “Co” stands for “coastal” and “C” stands for “central” or “cold,” and “W” is either “warm” or “water.” Thus, the Bohai Sea Coastal Water becomes BSCoW. The word “mass,” sometimes used at the end of a name for a water mass, is omitted for simplicity. Coastal waters, incontestably the most complicated, are defined here as waters with a salinity b31. Fig. 1a–d show the approximate locations of these waters in various seasons. From north to south, these waters include the BS Coastal Water (BSCoW) (Su and Weng, 1994), YS Coastal Water (YSCoW) (Pan and Pang, 1997), Western Korea Coastal Water (WKCoW) (Guan, 1994), Changjiang Diluted Water (CDW) (Su and Weng, 1994) and ECS Coastal Water (ECSCoW) (Su and Weng, 1994). With the exception of the CDW in summer, these waters are generally confined to coastal regions where water depth is b50 m. Of note is that due to the freshwater outflow of the Changjiang River, ranked fourth worldwide in terms of discharge, the CDW is prominent on shallow continental shelves. Additionally, because of the shallow water depth, these coastal waters are considerably cooler in winter (Fig. 2a) than in summer (Fig. 3a).

Further offshore, also from north to south, are the BS Central Water (BSCW) (Su and Weng, 1994), YS Cold and Warm Waters (YSCW) and (YSWW) (Su and Weng, 1994), ECS Dense (ECSDW) and Shelf Surface Waters (ECSSSW) (Su and Weng, 1994; Chen et al., 1995), the CDW in summer and the Taiwan Strait Water (TSW) (Chen et al., 1995; Chen, 2003). Near the continental shelf break are the Tsushima Warm Water (TWW) (Guan, 1994), Kuroshio Surface Water (KSW) (Chen et al., 1995), subsurface Kuroshio Tropical Water (KTW) (Chen et al., 1995) and Kuroshio Intermediate Water (KIW) (Chen et al., 1995). These offshore water masses are generally warmer, saltier and poorer in nutrients than the coastal waters (Fig. 1a–d). Tables 1 and 2 present the temperatures, salinity, and nitrate, phosphate and silicate concentrations in these water masses in the surface waters and at the bottom, respectively. 3. Fronts Not all fronts exist throughout all four seasons. Moreover, fronts generally meander in different seasons. Since temperature fronts have been analyzed most, notably by Hickox et al. (2000), their long-term seasonal composite frontal probability (i.e., the percentage of total time a front resides in a given location) maps for 1985–1996 are the basis of the following discussion (Fig. 1a–d). Hickox et al. (2000) identified ten temperature fronts: 1) the Kuroshio Front; 2) Zhejiang-Fujian Front; 3) Jiangsu Front; 4) Shandong Peninsula Front; 5) BS Front; 6) Seohan Bay Front; 7) Kyunggi Bay Front; 8) Western Jeju (Cheju) Island Front; 9) Eastern Jeju Island Front; and,10) Yangtze Bank Ring Front. These fronts are seasonally persistent, i.e., they exist in the same seasons in different years. Furthermore, three latitudinal fronts in the central and southern ECS (marked 1a–c in Fig. 1a) can be identified. Water masses, salinity and nutrient fronts are related to the following temperature fronts. The temperature fronts, especially surface fronts, are most easily observed from November to April. In late spring, say, early May, and in summer, warming of the surface seawater smoothes sharp contrasts in temperature, causing these thermal fronts to dissipate. Accordingly, fronts are described based on their seasonal characteristics, as summarized below. Surface fronts in the BS are described first, followed by those in the YS and ECS. Subsequently subsurface fronts are discussed, and finally the influence of the Kuroshio. Since thermal fronts are discussed based on the work by Hickox et al. (2000), their seasonal definitions, i.e., January to March for winter, are utilized. Indeed, in the study area, the coldest month is February and the warmest month is August (e.g., Chen, 1992; Bao et al., 2001). Winter data are discussed first when fronts are the most clear. 3.1. Winter Surface fronts are discussed first and the discussion generally progresses from the BS southward. Subsurface fronts and the Kuroshio influence follow. 3.1.1. Surface fronts on continental shelves In winter (January–March), the study area is subject to a strong NE monsoon. Cold coastal waters with temperatures b1 °C in the BS and the northern YS can be identified clearly (Fig. 2a). The temperatures of coastal waters increase to roughly 5 °C in the southern YS, and to about 15 °C in the southern ECS and Taiwan Strait. The contrast between cold (T b 17 °C), fresh (S b 31), nutrient-rich (NO3 up to 25 µmol/dm3, PO4 up to 1.6 µmol/dm3, SiO2 up to 30 µmol/dm3) (Figs. 1a, and 2 and Table 1) coastal currents and the warm (T = 20–24 °C), saline (S = 33.5–34.5), nutrient-poor (NO3 b 2 µmol/dm3, PO4 b 0.2 µmol/dm3, SiO2 = 3– 5 µmol/dm3) (Figs. 1a, and 2 and Table 1) Kuroshio waters is strongest (Figs. 1a and 2). In the surface layer, warmer, more saline and nutrient-poor YS Warm Water (YSWW) is well developed and enters the BS through the northern Bohai Strait, thereby resulting in the formation of the BSCW

C.-T.A. Chen / Journal of Marine Systems 78 (2009) 394–410 Fig. 2. Distribution of surface water (a) temperature, (b) salinity; and (c) nitrate, (d) phosphate, and (e) silicate concentrations as well as (f) the current flow patterns (1a, 1b and 1c refer to the thermal fronts shown in Fig. 1a) in the Bohai, East China and Yellow Seas in February (T, S and nutrient data taken from KORDI, 1987; Wong et al., 1991; Chen, 1992; KORDI, 1993; Chen et al., 1995, 1996; Chen, 1996; Wang and Chen, 1998; Lee et al., 1998; Fujien Oceanological Institute, 1998; Hu et al., 1999c; Chen, 2003; Guo et al., 2003; current flow patterns courtesy of X.Y. Guo).

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Fig. 2 (continued ).

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C.-T.A. Chen / Journal of Marine Systems 78 (2009) 394–410 Fig. 3. Distribution of bottom water in terms of (a) nitrate, (b) phosphate, and (c) silicate concentrations in the Bohai, East China and Yellow Seas in February (data taken from KORDI, 1987; Wong et al., 1991; Wang, 1991; KORDI, 1993; Hong and Dai, 1994; Chen et al., 1995, 1996; Chen, 1996; Lee et al., 1998; Wang and Chen, 1998; Fujien Oceanological Institute, 1998; Hu et al., 1999c; Chen, 2003; Guo et al., 2003).

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Table 1 Temperature, salinity and nutrients of main surface water masses in winter and summer. T, °C BSCW BSCoW WKCoW YSCoW YSCW YSWW JWC TWW CDW ECSDW ECSCoW TWC TSW KSW

NO3, µmol/dm3

S

PO4, µmol/dm3

SiO2, µmol/dm3

Winter

Summer

Winter

Summer

Winter

Summer

Winter

Summer

Winter

Summer

b1 b3 2–5 b4 5–10 5–12 10–15 12–15 6–17 9–13 8–17 13–18 18–23 20–24

25–27 24–26 20–26 24–26 8–10 25–27 25–28 26–28 27–29 27–28 26–28 28–29 28–29 28–29

b 31 30–31 31.5–32.5 31–31.5 32–33.5 32–34 33.5–34.5 34–34.75 20–31 32–34 25–31 33–34.5 33.5–34.5 33.5–34.75

b 30 b 31.5 b 30 b 31 30.5–31.5 30.5–32 31–33 32–33 b 32 31.5–33.5 b 33.5 32–34 33.5–34 33.5–34.5

1–10 3–5 3–8 1–3 1–3 1–4 1–3 ~1 10–20 5–12 10–25 2–6 1–4 b2

b1 b1 b1–N 2 b1 b0.5–1 b0.5–b1 b0.5–b1 b1 b1–30 b1–2 b1–5 b1 b1 ~ 0.2

0.4–0.8 ~ 0.4 N 0.6 0.2–0.4 0.4–0.6 0.2–0.6 0.2–0.4 0.2–0.4 0.6–1.6 0.4–0.6 0.4–1.6 0.2–0.4 0.1–0.4 b 0.2

~ 0.2 ~ 0.2 0.2–N 0.4 b0.2 b0.2 b0.2 b0.2 b0.2 b0.2–N1 b0.2 b0.2–0.8 b0.1–0.2 b0.1–0.2 b0.1

5–10 5–10 N10 5–10 b5–10 b5–10 b5–10 b5–10 10–50 5–20 5–30 5–15 b5–10 3–5

N 15 3–10 15–20 3–10 3–10 3–10 3–5 3–5 5–50 5–20 5–20 3–5 3–5 3–5

Some waters like the YSCW and ECSDW are limited to the bottom layer especially in summer. The properties listed refer to the surface water above YSCW and ECSDW. Data taken from references used for Figs. 2 and 4.

Off Korea, between the YSCoW, its extensions (i.e., the Western Korea Coastal Water) and YSWW, thermal and salinity fronts also exist. Hickox et al. identified two thermal fronts, No. 6 and No. 7. Front 7 separates the warm, saline, nutrient-poor YSWW from the cold, fresh, nutrient-rich WKCoW. Front 6 separates the warm, saline YSWW from the cold, fresh YSCoW branch (Figs.1a, 2a, and b). However, the YSCoW north of Front 6 has lower nutrient content than the YSWW south of the front. Nitrate data demonstrate that these two fronts are separated by a high nitrate ridge (NO3 N 3 µmol/dm3) (Fig. 2c). Since temperature and salinity data do not support a flow of high nitrate water from the BS eastward toward Korea, the high nitrate ridge is probably remnant water from the previous season. This is discussed in detail in Section 3.3.1. The strong NE monsoon winds push the CDW to flow southward, and the ECSCoW is likely connected with the YSCoW. The temperature and salinity front (Front No. 2) (Qiao et al., 2006; Chen et al., 2006) separates the cold, fresh, nutrient-rich CDW and ECSCoW (T =9–17 °C, S b 31, NO3 up to 25 µmol/dm3, PO4 up to 1.6 µmol/dm3, SiO2 up to 50 µmol/dm3) (Figs. 1a, 2 and Table 1) from the warm, salty, nutrient-poor waters (the Taiwan Warm Current (TWC) and TSW) influenced by the Kuroshio (T = 13–24 °C, S =33–34.75, NO3 b 6 µmol/dm3, PO4 b 0.4 µmol/dm3, SiO2 =3–5 µmol/dm3) (Figs. 1a and 2 and Table 1). Notably, Fronts 3 and 2 are not connected as a branch of the cold, fresh YSCoW flows southeastward (Fig. 2a and b) resulting in a tongue of low temperature and salinity and high-nutrient in these figures. Between this southeastward extending tongue and the CDW and ECSCoW lies the northern

and causes the cold, less saline, nutrient-rich BSCoW to exit through the southern Bohai Strait (Figs.1 and 2a–e) (Guan,1994; Hu, 1994; Bao et al., 2001). A meridional temperature front exists just west of the Bohai Strait (Temperature Front No. 5, as designated by Hickox et al., 2000) (Fig. 1a). The relatively cold, fresh, nutrient-rich YSCoW occupies the western, northern and eastern sides of the YS (Figs. 1a, and 2a–e); this water flows southward along the Shandong and Korean Peninsulas. The western branch of the YSCoW is a continuation of the BSCoW, while the eastern branch forms the WKCoW. The relatively warmer, saltier, nutrient-poor YSWW occupies the central deep region of the YS (Figs. 1a and 2a–e), flows northward and compensates for the southward flowing cold coastal waters (Guan, 1994). Meridional thermal and salinity fronts are very distinct between the YSCoW and YSWW off the Shandong Peninsula and further south (Fig. 2a, b) (Tang and Su, 2000). Notably, Fronts 4 and 3 are not connected (Fig. 1a). This is because a branch of the warm, saltier, nutrient-poorer YSWW flows from the interior of the YS westward toward the coast (Fig. 2). Indeed, near 35°N, south of the Shandong Peninsula temperature and salinity are maximal and nutrient minimal, clearly indicating the YSWW influence. Nevertheless, the YSWW-influenced YSCoW continues southward and the thermal (Front No. 3) and nutrient fronts (Fig. 2c, d) separate it from the YSWW. The nutrient concentrations west of Front No. 3 (NO3 N 9 µmol/ dm3, PO4 N1 µmol/ dm3, SiO2 N10 µmol/dm3) (Table 1), however, are higher than concentrations further north. This phenomenon is likely due to the contribution of the Changjiang River.

Table 2 Temperature, salinity and nutrients of main bottom water masses in winter and summer. T, °C BSCW BSCoW WKCoW YSCoW YSCW YSWW JWC TWW CDW ECSDW ECSCoW TWC TSW KSW

NO3, µmol/dm3

S

PO4, µmol/dm3

SiO2, µmol/dm3

Winter

Summer

Winter

Summer

Winter

Summer

Winter

Summer

Winter

Summer

b1 2–4 2–6 0–5 5–8 5–10 10–13 12–15 5–15 9–15 8–15 11–18 15–22 18–22

20–24 18–20 14–18 18–26 8–13 8–15 12–16 14–17 23–26 20–23 23–26 20–23 23–27 b16

b 31 30–31.5 31.5–32.5 31–31.5 32–33.5 32–34 33.5–34.5 34–34.75 20–31 32–34 25–31 33–34.5 33.5–34.5 34.5–34.6

b 30 b 31.5 b 32 b 31 32.5–33.5 32.5–33.5 33.5–34 33.5–34.5 b 33.5 32–34 b 33.5 34–34.5 34–34.5 34–34.6

2–10 3–5 3–8 1–3 1–3 1–4 3–5 3–10 10–20 5–14 5–25 2–6 2–5 5–40

~1 b1 1–3 b1 1–9 1–9 9–12 3–10 3–30 3–10 2–10 2–10 1–5 7–40

0.4–0.8 ~ 0.4 0.6–N0.8 0.2–0.4 0.4–0.6 0.4–0.6 0.2–0.4 0.2–0.6 0.6–1.6 0.4–0.8 0.4–1.6 0.2–0.4 0.2–0.4 0.2–2.0

0.2–0.4 0.2–0.4 N0.4 ~ 0.2 0.6–1.0 0..6–1.8 0.4–0.6 0.2–0.4 0.4–N 1 0.4–0.6 0.2–0.8 0.2–0.6 0.2–0.4 0.2–3

N10 5–10 N10 5–10 5–10 5–10 5–10 10–25 15–N 25 5–15 5–N25 5–10 5–10 10–100

10–15 b 5–10 15–20 5–10 5–20 5–20 10–20 15–20 10–N40 5–15 5–N 40 5–15 5–10 15–100

Some waters like the YSWW, CDW and KSW are limited to the surface layer especially in summer. The properties listed refer to the bottom water below YSWW, CDW and KSW. Data taken from references used for Figs. 3 and 5.

C.-T.A. Chen / Journal of Marine Systems 78 (2009) 394–410 Fig. 4. Distribution of surface water in terms of (a) temperature, (b) salinity; and (c) nitrate, (d) phosphate, and (e) silicate concentrations in the Bohai, East China and Yellow Seas in August (data taken from KORDI, 1987; Wong et al., 1991; Chen, 1992; KORDI, 1993; Chen, 1996; Chen et al., 1995, 1996; Wang and Chen, 1998; Lee et al., 1998; Fujien Oceanological Institute, 1998; Hu et al., 1999a,b; Wang et al., 2000; Chen, 2003). Stations given in (a) are discussed in Figs. 6 and 7.

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Fig. 4 (continued ).

tip of the TWC. Front No. 2 separates the warm tongue from the ECSCoW, and the southwest part of Front No. 10 separates the warm tongue from the cold offshore tongue of the YSCoW (Fig. 1a). The nutrient concentrations are high (Fig. 2c–e) inside the Yangtze Bank Ring Front (No. 10).

3.1.2. Subsurface fronts on continental shelves Beneath the YSWW is the YSCW, also called the Yellow Sea Bottom Cold Water, and is surrounded by Fronts 7, 8, 10, 3 and 4 (Fig. 1a). The YSCW retains its position in the YS basin, and is most prominent in spring and summer and preserves its winter characteristics throughout the year beneath the thermocline that forms in spring. Although temperature and nutrient concentrations increase slightly, salinity remains stable (Uda, 1950; Su and Weng, 1994; Pan and Pang, 1997; Lie et al., 1999). This water is low in nutrients (NO3 b1 µmol/dm3, PO4 b 0.2 µmol/dm3, SiO2 b10 µmol/dm3) (Fig. 3a–c and Table 2) due to vertical mixing with surface waters depleted of nutrients in warm seasons when the water column is stratified (Fig. 4 and Table 1). Most researchers believe that the nutrient-poor YSCW and nutrientrich ECSDW, also called the ECS Bottom Dense Water, form in winter when the water column is mixed vertically with minimal stratification due to cooling and strong winds. As a result, the bottom features on shelves (Fig. 3) are almost identical to the surface features in the BS and YS, especially in terms of temperature and salinity (Tables 1 and 2). Only the bottom water in the southern ECS may be 1–2 °C warmer than bottom waters further north (Chen, 1992). The ECSDW is likely associated with the Yangtze Bank and is surrounded by Fronts 10 and 2 (Fig. 1a). The ECSDW however moves along the 50- to 100-m isobaths toward Jeju Island and ultimately loses its characteristics by late summer or early fall (Mao et al., 1964; Inoue, 1975; Pan et al., 1991; Wang and Chen, 1998; Hur et al., 1999). Because of strong vertical mixing, nutrient concentrations also show a local maximum in the deep waters (NO3 N12 µmol/dm3, PO4 N 0.8 µmol/

dm3, SiO2 up to 20 µmol/dm3) (Fig. 3a–c and Table 2) within the ECSDW, which is surrounded by the Yangtze Bank Ring Front (No. 10). A latitudinal temperature front exists at approximately 30°N, which is hereby designated as Front 1a (Fig. 1a). The waters south of the front are warmer, saltier and contain less nutrients (Fig. 2c–e) than waters to the north. These waters are clearly impacted by the Kuroshio or the TWC. Apparently, a branch of the TWC turns eastward south of Front 1a as the rest of the TWC continues northward. Consequently, Front 1a does not extend all the way westward to join Front 2. Front 1a is discussed in further detail in Section 3.1.4.

3.1.3. The Kuroshio influence The Kuroshio moves inland near the shelf break of the ECS. It originates in the westward flowing North Equatorial Current, turns northward east of Luzon and flows past Taiwan into the ECS. Like its counterpart in the South Atlantic Bight, the Gulf Stream, the main stream of the Kuroshio in the ECS typically flows along the steep continental slope. The western edge of this strong current generally follows the 200-m isobath in the ECS and leaves the continental slope around 128–129°E and 30°N. The Kuroshio then flows out of the ECS through the Tokara Strait south of Kyushu Island. As a result of the Kuroshio impinging on the ECS shelf northeast of Taiwan and south of Kyushu, portions of the Kuroshio branch out onto the continental shelf (Guan, 1983; Sugimoto et al., 1988; Chern and Wang, 1990; Yuan et al., 1994; Su et al., 1994; Yuan and Su, 2000; Hsueh, 2000), and consequently, Front 1 (Fig.1a), as one example, runs largely in the NE–SW direction and becomes longitudinal northeast of Taiwan. North of about 28°N, blocking the Kuroshio by Kyushu forces the current to turn eastward while part of the left side of Kuroshio moves onto the shelf. This separation gives rise the Tsushima Current, Jeju Warm Current and the YS Warm Current (Hsueh et al., 1996; Hsueh, 2000). Often associated with the western edge of the warm Kuroshio are Temperature Fronts.1, 8 and 9, as well as the eastern part of Front.10 (Fig.1a–d) (Guan,

C.-T.A. Chen / Journal of Marine Systems 78 (2009) 394–410 Fig. 5. Distribution of bottom water in terms of (a) temperature, (b) salinity; and (c) nitrate, (d) phosphate and (e) silicate concentrations in the Bohai, East China and Yellow Seas in August (data taken from KORDI, 1987; Wong et al., 1991; Wang, 1991; KORDI, 1993; Hong and Dai, 1994; Chen et al., 1995, 1996; Chen, 1996; Lee et al., 1998; Wang and Chen, 1998; Fujien Oceanological Institute, 1998; Hu et al., 1999a,b; Wang et al., 2000; Chen, 2003).

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Fig. 5 (continued ).

1984; Nagata and Takeshita, 1985; Su et al., 1990; Qui et al., 1990). Eddies in the Kuroshio margins affect the exchange of the Kuroshio with continental shelf waters (Guo and Ge, 1997; Yanagi et al., 1998; Ichikawa and Chaen, 2000; Lee et al., 2002); however, these short-lived features cannot be represented in the figures presented. Of particular note is that the Kuroshio moves onshore in the nutrient-poor surface layer (NO3 b 2 µmol/dm3, PO4 b 0.2 µmol/dm3, SiO2 b 5 µmol/dm3) (Fig. 2c–e and Table 1) and in the nutrient-rich subsurface layer (Liu et al., 1992, 2000; Guo et al., 2006; Lee and Matsuno, 2007). Consequently, the Kuroshio Front is also a nutrient front at the sea bottom, with the Kuroshio subsurface waters containing substantially higher values of nutrients (NO3 N10 µmol/dm3, PO4 N1 µmol/dm3, SiO2 N10 µmol/dm3) (Fig. 3a–c and Table 2). The TWC which flows northward all year was named for its characteristic high temperature (Mao et al., 1964) even during strong northeast monsoons in winter, particularly when compared with the southward flowing, cold CDW and ECSCoW (Su et al., 1990). The origin of the TWC has been the subject of intense debate; however, the present author holds the view that the continuation of the Kuroshio branch forms the TWC, which is likely unrelated to the TSW (Chen and Sheu, 2006). A thermohaline front (Front 2) (Fig. 1a) exists between the ECSCoW and the TSW and TWC. This front is located roughly in the middle of the Taiwan Strait, but is close to China, and extends to the SCS. The slight bulge at 120°E, 25°N, however, extends in some instances southeastward toward Taiwan's coast, as denoted by the current marked “a” in Fig. 1a (Chang et al., 2006). A nutrient front coincides with the thermohaline front, with the nutrient-rich ECSCoW flowing southward from the surface to the bottom along the ZhejiangFujian coast; all the while, the TSW, which largely maintains its position in the eastern Taiwan Strait, has lower nutrient content from the surface to the bottom (Figs. 2c–e and 3a–c and Tables 1 and 2). Chen and Sheu (2006) argued that the TSW does not flow continuously through the Taiwan Strait in winter.

The impingement of the Kuroshio south of Kyushu is very complex and results in powerful temperature, salinity and nutrient fronts in winter (Fronts 8, 9 and 10) (Figs. 1a, 2c–e and 3a–c) (Qui et al., 1990; Wong et al., 1991). The Eastern Jeju Island Front (Front 9) (Fig. 1a) is observed west of Kyushu and south of Korea with the warm weak Tsushima Current (TsC) pushing the Tsushima Warm Water (TWW) toward Jeju Island (Fig. 1a) (Liu and Yuan, 1999a,b) and reaches 34°N, 124°E. The shoreward movement of warm Kuroshio waters (low in nutrients on the surface and high in nutrients in deep layers) (Figs. 2c–e and 3a–c) results in strong frontal areas northeast and southwest of Jeju Island (Fronts 8 and 10, respectively), both of which are important fishing grounds (Tang and Su, 2000). The Jeju Warm Current (JWC) south of Jeju Island comprises the western branch of the Kuroshio water moving shoreward. Important here is that the western part of Front 8 and the northern portion of Front 10 seem to be linked (Fig. 1a); however, as Lie et al. (2001) noted, this connection may intermittently collapse, thus flushing the JWC into the southwestern YS to form Yellow Sea Warm Water. 3.1.4. Three latitudinal fronts in the East China Sea Latitudinal temperature fronts exist in the southern ECS, and can be clearly seen on the map developed by Hickox et al. but unnamed by them. These fronts are numbered Fronts 1a–c (Fig. 1a). Front 1a is generally the southern boundary (S = 34, Fig. 2b) of the low salinity tongue extending southeastward. A nutrient front exists here at the surface (Fig. 2c) and bottom (Fig. 3a). It is tempting to suggest that this front is a result of the northward moving warm, saline and nutrient-poor TWC and the southward moving branch of the cold, fresh and nutrientrich Yellow Sea Cold Coastal Water. Partial collision of these two currents may have resulted in an eastward flowing current. To the best knowledge of the present author no such latitudinal currents have been reported previously. On the other hand, the recent simulated flow pattern presented, but not discussed, by Guo et al. (2006) suggested that latitudinal flows exists where Front 1a is found (Fig. 2f).

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The cause of Front 1b is uncertain but probably relates to the intrusion of salty nutrient-rich Kuroshio Subsurface Water. A nutrient front, this time with high concentrations in the south (Figs. 2c and 3a), exists where the thermal front is located. The flow chart developed by Guo et al. (2006) shows a strong meridional current flowing eastward (Fig. 2f). A relatively less prominent thermal front, Front 1c, resides between Fronts 1a and 1b. Front 1c can be seen on the map generated by Hickox et al. (2000) and may be related to the southeastward extending high NO3 bulge (Fig. 2c). Fig. 2f shows an eastward flowing current.

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flowing through the Taiwan Strait and joins with the TWC. The Kuroshio Front (Front 1 in Fig. 1b) is now wider than in winter (Fig. 1a) and combines with the Zhejiang-Fujian Front NNE of Taiwan. Fronts 1a and 1b, which exist in winter, now almost disappear; however, Front 1c can still be seen, albeit barely (Fig. 1b). It should be cautioned that in spring and fall, ocean circulation in the study area is subject to large transitions and, thus, average seasonal patterns may not be robust. 3.3. Summer

3.2. Spring The NE monsoons weaken in spring, as does the intensity of fronts. In this section, fronts on continental shelves are discussed first, starting with those in the BS. Fronts in the YS and ECS follow. This section ends with a discussion of the Kuroshio influence. 3.2.1. Fronts on the continental shelves In spring (April–June), the YSWW still enters the northern end of the Bohai Strait, whereas at the southern end, the BSCoW still exits. The temperature front (Front 5 in Fig. 1a) west of the strait weakens because of weakened NE monsoons and increased warming from the BSCW (Fig. 1b). The eastward flowing BSCoW circumvents the Shandong Peninsula and becomes part of the YSCoW, which flows southwestward (Pan et al., 1991). The Shandong Peninsula Front (Front 4 in Fig. 1b), however, dwindles away because the southward flowing YSCoW starts weakening. Off the Korean Peninsula, part of the cold YSCoW also flows southward along the coast. The warm YSWW, now facing weakened NE monsoons, flows northward in the central part of the YS. Other thermal fronts exist in the YS, with the YSWW at the center and the YSCoW on both sides (Fronts 3, 4, 6 and 7 in Fig. 1b). However, the Jiangsu Front (Front 3) is now separated from the Shandong Peninsula Front (Front 4), while the Kyunggi Bay Front (Front 7) retreats slightly to the north. In fact, as the CDW starts strengthening (Chen et al., 2001) and turns eastward, the Jiangsu Front merges with remnants of the Yangtze Bank Ring Front (Front 10) and forms a horseshoe-shaped front across the ECS basin (Hickox et al., 2000) (Fig. 1b). The water column begins stratifying; beneath the warm YSWW is the cold, remnant of the YSCW (yellow in Fig. 1b). 3.2.2. The Kuroshio influence West of Kyushu, the Kuroshio still upwells onto the shelf and contributes to the Tsushima Current (also called the Tsushima Warm Current), which enters the Sea of Japan. A thermohaline front exists between the Kuroshio and coastal waters south of Korea (Guo and Ge, 1997) (Front 9 in Fig. 1b). Part of the upwelled Kuroshio forms the Jeju Warm Current, extends westward toward Jeju Island and creates thermal fronts west and southwest of the island (Lie et al., 1999, 2001; Tang and Su, 2000) (Fronts 8, 9 and the eastern portion of 10 in Fig. 1b). Lie et al. (2000) demonstrated that the eastern branch of the YSCoW flows southward along the Korean coast, turns eastward south of the Korean Peninsula, and exists the YS north of Jeju Island. Fronts 8 and 9 separate the outflowing cold water from the outflowing branch of the warm Jeju and Tsushima Currents. The CDW continues flowing southward initially and starts turning eastward. As the season progresses and the NE monsoons weaken, the ECSCoW loses strength. The Zhejiang-Fujian thermohaline and density Front exists between the CDW, ECSSSW and the warm, saline, now northward flowing TSW, which is composed of water from the SCS and the Kuroshio Branch (Front 2 in Fig. 1b). In spring, the Zhejiang-Fujian Front is separated from the Jiangsu Front (Front 3 in Fig. 1b). Due to the weakened NE Monsoons and increased Changjiang outflow, the Zhejiang-Fujian Front widens and is pushed farther north along with the longitudinal section of the Kuroshio Front located north of Taiwan. Additionally, as a result of the weakened NE monsoons, the TSW starts

Monsoon winds turn southwest in summer when temperature is highest and front intensity is weakest. Surface fronts in the BS are discussed first, followed by those in the YS and ECS. Subsurface fronts are then characterized. This section ends with a discussion of the Kuroshio influence. 3.3.1. Fronts on continental shelves In summer (July–September), given that the contrast in density between the YSCW and coastal waters in the southern BS is markedly intensified, the current flowing westward through the northern Bohai Strait is even greater. Nevertheless, surface temperature is homogeneous and as a consequence, the thermal front west of the Bohai Strait that exists in winter (Front 5 in Figs. 1a and 4a) all but disappears in summer (Guan, 1994). The fresh, nutrient-rich BSCoW still flows out of the southern Bohai Strait circumventing the Shandong Peninsula (Figs. 1c and 4b–e); however, the YSCoW is generally weakened and no longer reaches the ECS. Although the thermohaline fronts west of Korea still exist (Fronts 6 and 7 in Fig. 1c), they are considerably weakened. Waters north of Front 6 and east of Front 7 are cooler, fresher and perhaps contain more nutrients (Fig. 4a–e) than waters south of the front. South of Shandong Peninsula, the thermohaline fronts between the BSCoW and the YSWW (Fronts 3 and 4 in Fig. 1c) further weaken; however, these waters are easily distinguishable from fresh, nutrientrich BSCoW on the coast (Fig. 4b–e). Because of the northeastward flowing CDW, the Kuroshio branch no longer penetrates freely to the YS. West of Kyushu, the surface thermal fronts that exist in winter and spring (Front Nos. 8 and 9 in Fig. 1a and b) almost totally disappear; however, a subsurface front exists between the Kuroshio branch and the YSCW (Fig. 5) (Tang and Su, 2000). As nutrients have been regenerated in the stratified deep waters since spring, the YSCW, earlier identified as having a minimum amount of nitrate in winter (e.g. NO3 b 2 µmol/dm3) (Fig. 3a), now has a maximum amount of nitrate (NO3 N 9 µmol/dm3) (Fig. 5c). Part of this high nitrate water likely remains for several months and contributes to the high-nitrate ridge existing between the Shandong Peninsula and Korea in winter (Fig. 2c). The CDW that flows southward through the Taiwan Strait in winter (Fig. 1a), starts reversing its direction in spring (Fig. 1b), and flows northeastward in summer. The thermal Fronts 2 and 10, which are clearly recognizable in winter, remain identifiable in spring, and completely disappear in summer (Fig. 1c). Conversely, the surface salinity plot (Fig. 4b) clearly shows the plume of the CDW extending toward Jeju Island. A snapshot of Moderate Resolution Imaging Spectroradiometer (MODIS) image of chlorophyll concentration identified multiple fronts within the CDW tongue on July 24, 2002 (Yuan et al., 2005). In addition to the multiple chlorophyll fronts, Jiao et al. (2007) identified multiple transparency and particulate organic carbon fronts in September 2002 and 2003. These multiple fronts, however, were not detected in Fig. 4 using multi-year averages. 3.3.2. Subsurface fronts on continental shelves The predominant subsurface feature in the YS in summer is the YSCW (Fig. 1c), which is cold (T b 13 °C, Fig. 5a) and high in salinity (S N 32.5, Fig. 5b) and nutrients (NO3 as high as 9 µmol/dm3, PO4 as high as 1 µmol/dm3, SiO2 as high as 10 µmol/dm3) (Fig. 5c–e). Table 2

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Fig. 6. θ/S plots for (a) ORII 806 and (b) ORI 631 cruises in the northern Taiwan Strait in summer and late fall, respectively. The thin lines represent typical South China Sea or West Philippine Sea trends (taken from Chen and Hsing, 2005 and Chen and Wang, 2006; station locations are given in Fig. 4a. The locations of Stns. B1–H1 are similar to Stns. B–H).

presents ranges in temperature, salinity and nutrients. Very sharp temperature and nutrient fronts exist around this water mass, which is clearly the remnants of winter water; that is, temperature, salinity and nutrient values are similar to those in winter. Additionally, a prominent subsurface feature exists in the ECS, namely, the ECSDW (Fig. 1c). In contrast to the YSCW, the ECSDW is relatively warm (T N 20 °C), salty (S N 32) and nutrient-poor (NO3 b10 µmol/dm3, PO4 b 0.6 µmol/dm3, SiO2 b15 µmol/dm3) (Fig. 5 and Table 2). The temperature and salinity fronts surrounding this water mass are weak; however, strong nutrient fronts exist. This water mass is also remnant winter water that is low in nutrients (Fig. 3a–c) (Wang and Chen, 1998). The temperature is roughly 10 °C higher than in winter; however, salinity and nutrients remain essentially unchanged.

3.3.3. The Kuroshio influence The TWC flows freely through the Taiwan Strait in summer, and in response to SW monsoons, relatively fresher, more nutrient-rich CDW switches direction and flows toward Jeju Island even though there are still high nutrient and chlorophyll waters hugging the coast of China (Fig. 4b–e) (Weng and Wang, 1984; Yuan et al., 2005). The salty, nutrient-poor Kuroshio moves farther offshore (Fig. 4b–e), especially in areas off of NE Taiwan. On the other hand, the Kuroshio thermal Front, present in other seasons (Front 1 in Fig. 1a–c), disappears. However, a distinct salinity front exists northwest of Taiwan (Fig. 6a). Roughly the eastern quarter (Stations A–C in Fig. 4a) of the northern TSW is occupied by well-mixed, salty, SCS-influenced waters (the straight θ/S line on the right side in Fig. 6a); in contrast, the western

Fig. 7. (a)θ/S; (b)θ/AOU; (c)θ/CFC 11; and (d)θ/CFC 12 plots at a cross-section perpendicular to the ECS continental shelf based on the data of Swift et al. (1990) in and near the central Okinawa Trough. The θ/S plot of ORI-462 Stn. CM6 near the ridge northeast of Taiwan is also given in (a). Station locations are given in Fig. 4a (taken from Chen, 2005).

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three-quarters (Stations O–I in Fig. 4a) is occupied by well-mixed, fresh ECS-influenced waters (the straight θ/S line on the left side in Fig. 6a). By way of comparison, the θ/S plot in late fall has an entirely different shape (Fig. 6b); this is discussed in detail later. Below the surface, the Kuroshio Tropical Water (KTW) and Kuroshio Intermediate Water (KIW) continue move inland (Guo et al., 2006; Lee and Matsuno, 2007), penetrate along the 50-m isobath along the western edge of the Okinawa Trough and form a strong subsurface thermohaline and nutrient front (Figs. 1c and 5c–e). The KTW and KIW most likely upwell northeast of Taiwan, and a subsurface salinity front between the KTW, KIW and shelf water on the ECS can be traced from about 25°N northwest of Taiwan to 30°N along the 80-m isobath (Chen and Wang, 2006). The combined effects of the CDW and upwelling of the KTW and KIW strongly impact fisheries (Lin et al., 1995; Chen, 2000). Importantly, the upwelled KTW and KIW likely mix with the SCS Tropical Waters (SCSTWs) and Intermediate Waters (SCSIWs), respectively, with the typical θ/S signature shown in Fig. 7, (panel a) as a distinct subsurface front exists between the SCS-influenced waters and pure Kuroshio Waters (Chen and Huang, 1996). For example, Fig. 7 clearly shows that the SCS-influenced waters are salty, and old as they have high apparent oxygen utilization (AOU) values (panel 7b; with station locations in Fig. 4a) and contain a small amount of chlorofluorocarbons (CFCs in panels 7c and 7d; Chen, 2005). The subsurface AOU, CFC 11 and CFC 12 data for stations east of Station 374 all fall on the same line on the right, while data for stations west of Station 376 all fall on a line on the left. Subsurface AOU and CFC fronts clearly exist although no temperature or salinity fronts are present. Chen (2005), in fact, traced SCS-influenced Tropical and Intermediate Waters to areas south of Japan. This warm salty water also upwells into the YS

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shelf south of Jeju Island, forming nutrient fronts along the edges of the warm water tongue (Fig. 5). 3.4. Fall The NE monsoon winds prevail in the fall and fronts intensify. Again, fronts in the BS are discussed first, followed by those in the YS and ECS. The Kuroshio influence is also briefly discussed. 3.4.1. Fronts on continental shelves The YSWW, originating from the branch of Kuroshio, is still predominant in the circulation linking the BS and YS in the fall (October–December). The YSWW enters the northern Bohai Strait, mixes with the BSCW and exits the southern strait as BSCoW. However, in fall, the clear thermal front west of the Strait that is found in winter (Front 5 in Fig. 1a) has not yet developed (Fig. 1d) (Guan, 1994). On the other hand, the thermohaline front southeast of the Shandong Peninsula (Front 4 in Fig.1d) intensifies because the YSCoW, driven by NE monsoon winds, intensifies. However, fronts west of Korea are not as recognizable as in other seasons (Fronts 6 and 7 in Fig. 1d). Between Korea and Jeju Island, the thermal front between the Kuroshio branch and the continuation of the WKCoW intensifies, making the frontal area an important fishing ground (Lie et al., 1999) (Front 8 in Fig. 1d). The TsC strengthens, and the front south of Korea (Front 9 in Fig. 1d) also intensifies (Liu and Yuan, 1999a,b). In the western YS and the ECS, salinity and temperature fronts also intensify (Tang and Su, 2000; Fronts 3 and 2 in Fig. 1d) because the CDW and ECSCoW start turning southward along the coast. Coastal temperature and salinity fronts reappear (Chen et al., 2006; Qiao et al.,

Fig. 8. Trajectory of satellite-tracked drifter 1 (blue line) and that of drifter 2 (red line) from October 7 to November 29, 1997. The isobaths (in meters) in the Taiwan Strait and in the adjacent seas are also shown. The number beside each trajectory shows the month and day, where 1026 means October 26, for example. The direction of flow is indicated by the arrow next to each trajectory. PHC is the Peng-hu Channel; CYR is the Chang-yuen Ridge; and KYD is the Kuan-Yin Depression (Tseng and Shen, 2003; courtesy of R.S. Tseng). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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2006). Snapshots of the MODIS images show a CDW plume extending southeastward to about 125°E, 28°N on Oct. 11, 2000 (Yuan et al., 2005). This plume likely exists before the CDW turned completely southward. Against the southward flowing CDW, the northward flowing TSW weakens under the force of the NE monsoons. In fact, drifter data demonstrate that even though the eastern part of the Taiwan Strait is still occupied by the TSW flowing northward, the northward flowing TSW may be forced to turn back north of Taiwan (Fig. 8) and continue until it joins the Kuroshio. Equally interesting is the trajectory of drifters north of Taiwan follows that of temperature fronts (seen clearly in Fig. 1a). Drifter 2 (red line) continues flowing northward after it exits the Taiwan Strait, and then turns 180° at about 26°30'N and flows southward along the western edge of the southernmost part of Front 1 (1d in Fig. 1a). Drifter 2 then turns eastward before turning northward again along the eastern edge of Front 1d. Finally, it flows eastward along the southern edge of Front 1b, which is coincident with the current flow pattern in Fig. 2f. In fall, the θ/S diagrams for stations across the northern Taiwan Strait (Fig. 6b) again exhibit two straight lines —very similar to those in summer (Fig. 6a). The saltiest mixing line on the right is fresher in fall than in summer. Strikingly, however, is the fresh mixing line on the left (Fig. 6b). Rather than a negative slope for the θ/S correlations commonly found in the western Pacific, the slope is positive; restated, surface waters are fresh and cold, whereas subsurface waters are warm and salty, characteristics that implicate the influence of the southward flowing CDW. 3.4.2. The Kuroshio influence The Kuroshio intrudes onto the ECS shelf, both in surface and subsurface layers, and the Kuroshio front reappears. With these phenomena generate an emerging temperature front that is typically found on the shoreward edge of the Kuroshio (Front 1 in Fig. 1d) (Nagata and Takeshita, 1985) in fall. A branch of Kuroshio south of Taiwan also contributes to the northward flow of TSW, which again links up with Kuroshio north of Taiwan. Front 2 (Fig. 1d) separates the TSW flowing northward from the CDW flowing southward. 4. Conclusion Oceanic fronts are associated with strong mixing and stirring, as well as enhanced bioproductivity and ecotones. Based on these very reasons, oceanic fronts have attracted considerable attention recently. As satellite images have become increasingly available, studies of oceanic fronts, especially thermal fronts, have become relatively easy. However, salinity and nutrient fronts, particularly those that are subsurface, are studied less frequently. This study prepared maps of nutrient distributions and, systematically identified nutrient fronts in the BS, YS and ECS for the first time. Furthermore, known temperature and salinity fronts in these seas and their seasonal variations have been reviewed and compared with the newly identified nutrient fronts. These fronts, both in the surface and subsurface layers, are generally strongest in winter and weakest in summer, and relate primarily to monsoonal winds and other oceanic characteristics such as topography, boundaries between water masses and current flow patterns. Three heretofore unidentified latitudinal fronts in the southern ECS are suggestive of currents flowing off the continental shelf, and deserve further investigation. Acknowledgments The author would like to thank the National Science Council of the Republic of China, Taiwan, (Contract Nos. NSC 95-2611-M-110-001 and 94-2621-Z-110-001) and the Aim for the Top University Plan (Contract No. 95C 0312) for financially supporting this research. The valuable comments of I. Belkin and six anonymous reviewers have greatly strengthened the manuscript quality.

Appendix A List of acronyms BSCW BSCoW CDW ECS ECSCoW ECSDW

Bohai Sea Central Water Bohai Sea Coastal Water Changjiang Diluted Water East China Sea East China Sea Coastal Water East China Sea Dense Water, sometimes called East China Sea Bottom Dense Water ECSSSW East China Sea Shelf Surface Water JWC Jeju Warm Current KIW Kuroshio Intermediate Water KSW Kuroshio Surface Water KTW Kuroshio Tropical Water SCS South China Sea SCSIW South China Sea Intermediate Water SCSTW South China Sea Tropical Water TSW Taiwan Strait Water TWC Taiwan Warm Current TsC Tsushima Current TWW Tsushima Warm Water WKCC Western Korea Cold Current WKCoW Western Korea Coastal Water WPS West Philippine Sea YSCoW Yellow Sea Coastal Water YSCW Yellow Sea Cold Water, sometimes called Yellow Sea Bottom Cold Water YSWC Yellow Sea Warm Current YSWW Yellow Sea Warm Water

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