www.elsevier.com/locate/csr
0278-4343/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.csr.2006.05.003
Corresponding author. Tel.:+886 7 5255136;
fax:+886 7 5255346.
E-mail addresses:[email protected]
(C.-T. Arthur Chen),[email protected] (S.-L. Wang).
this front are occupied by fresher, cooler, more nutrient-rich and more productive water than those east of the front (Fig. 2; Chen, 2003). In late spring and summer, on the other hand, the surface waters on both sides are heated to such an extent that warm waters prevail, and the thermal front extending to the Taiwan Strait all but disappears.
The objectives of this paper are to demonstrate with field observations that in summer, a salinity front exists in the southern ECS and in the northern end part of the Taiwan Strait but not in the central or southern parts of the Taiwan Strait, and that this front separates the northward-flowing Taiwan Strait waters from the intruding subsurface Kuroshio waters which have earlier circumvented the north-ern tip of Taiwan. And besides this, it is proposed that the front is formed by the vertical mixing of downwelled subsurface Kuroshio waters which have previously upwelled near the shelf break northeast of Taiwan.
2. Sampling locations and methods
The present authors conducted research on five cruises in the vicinity of the Taiwan Strait roughly over a span of 13 years: the Ocean Researcher I-179 (ORI-179) cruise between 11 and 14 September 1988, the ORI-418B cruise from 8 to 17 May 1994, the ORI-508 cruise from 15 to 26, November 1997, the ORII-806 cruise from 27-28 August, 2001, as well as the ORIII-721 cruise from 6 to 7 August 2001. The relevant station locations are given in Fig. 3.
The shipboard temperature (T) and salinity (S) were determined with an SBE 911 plus Conducti-vity–Temperature–Depth Pressure (CTD) units manufactured by Sea-Bird, while surface T and S were continuously measured with an autosalino-graph. Discrete samples were collected at various depths with a Rosette sampler fitted with 2.5-1 Niskin bottles that were mounted on the Sea-Bird CTD unit for the determination of S, dissolved oxygen (DO), nitrite (NO2
), nitrate (NO3), phos-phate ðPO34 Þ and silicate (SiO2). Data from the sensors on the CTD unit were obtained during both the downcast and the upcast periods. Discrete water samples were picked up during the upcast. The CTD unit was lowered as well as raised at a rate of about 1.0 m/s.
Salinity in the discrete samples was determined by measuring conductivity with an AUTOSAL
salin-ometer, which was calibrated with IAPSO standard seawater (batch No. P128) with a precision of 0.003.
Dissolved oxygen in the discrete samples was measured by direct spectrophotometry (Pai et al., 1993), with a precision of about 0.32% at the 190 mmol kg1 level. Nitrate plus nitrite was mea-sured by reducing nitrate to nitrite, and then identifying nitrite by means of the pink azo dye method (Strickland and Parsons, 1972) using a flow injection analyzer with an on-line Cd coil. The precision of this method was about 1% at 35 mmol kg1, and 3% at 1 mmol kg1. Nitrite was determined following the pink azo dye method (Strickland and Parsons, 1972; Pai et al., 1990a) with a flow-injection analyzer with a precision of 0.02 mmol kg1. Phosphate was studied under the molybdenum blue method (Murphy and Riley, 1962;Pai et al., 1990b) again using a flow-injection analyzer with a precision of about 0.5% at 2.8 mmol kg1 and 3% at 0.1 mmol kg1. Silicate was measured in accordance with the silicomolyb-denum blue method (Fanning and Pilson, 1973) once again with a flow-injection analyzer with a precision of about 0.6% at 150 mmol kg1and 2%
at 5 mmol kg1.
pH was measured at 2570.05 1C with a Radio-meter PHM-85 pH Radio-meter and a GK 2401C combination electrode. The electrode and the electrode shift were, respectively, calibrated using a TRIS seawater buffer and IAPSO Standard seawater as the running standard. Precision was better than70.003 pH units. Total alkalinity (TA) was measured using a titration system composed of a Radiometer PHM-84 pH meter, a PHC-2401 combination electrode, an ABU- 80 autoburet, a 150-ml titration cell and a temperature-controlled water bath set at 2570.05 1C. For the TA values, until 1997, the end points had been determined from the Gran function with a precision of73 mmol kg1 (Chen et al., 1996), but thereafter, they were determined following the mass and charge balance method, as pioneered by Butler (1992), and again, with a precision of 73 mmol kg1. Total CO2
(dissolved inorganic carbon, DIC) was measured by employing the coulometric method with a precision of 70.05%. CO2gas was extracted from acidified seawater by using a single operator multi-parameter metabolic analyzer (SOMMA) system;
then the CO2gas was measured with a coulometric detector (model 5011 from UIC, Coulometrics, Inc.) (Dickson and Goyet, 1994). For calibration, the reference material prepared by A. Dickson was
ARTICLE IN PRESS
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1637
ARTICLE IN PRESS
Fig. 1. Seasonal probability maps of thermal fronts in the East China Sea between 1985 and 1996 (Hickox et al., 2000; courtesy of I.
Belkin; the color bar is frequency scale in percents).
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1638
used. The fugacity of CO2(fCO2) was measured by means of a manual system composed of a LICORTM (model 6262) non-dispersive infrared analyzer (NDIR) and an equilibrator (Wanninkhof
and Thoning, 1993). The precision of the measure-ments was72 matm. The fugacity of CO2was also computed from TA and DIC data in line with the procedures of Lewis and Wallace (1998). The
ARTICLE IN PRESS
Fig. 2. Distribution of climatic surface salinity (S), temperature (T), nitrate (NO3), phosphate (PO4) and silicate (SiO2) in the Taiwan Strait and its vicinity in August and February (modified fromChen, 2003).
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1639
difference between the measured and calculated fCO2was less than 10 matm.
3. Results
Fig. 4shows the continuous temperature, salinity and st records for surface seawater obtained from the OR II-806 cruise. The discrete sample values are also marked, and these closely agree with the continuous records. According to Tang and Su (2000), a salinity front is empirically defined as having DS/DXX0.05/n mile. It is clearly shown that between Stns. C and E, salinity drops considerably by almost 0.9 (in the Pratical Salinity Scale) with the sharpest decline (DS/DXffi0.1/n mile) being found between Stns. C and D. For the most part, temperature remains steady near the salinity front, but worth bearing in mind, the temperature sensor on the Autosalinograph malfunctioned after Stn. E.
As for st, after a slight rise, it falls dramatically between Stns. C and E, this largely a product of the somewhat parallel decline in salinity (Fig. 4).
Fig. 5 shows the cross-section for y, S, st and apparent oxygen utilization (AOU) in the northern section of the Taiwan Strait in summer (Chen and Hsing, 2005). It is immediately clear that the surface layer is uniformly warm, with temperatures above 28 1C across the Strait and with no discernable thermal front. On a broad scale, this mirrors the findings from the Autosalinograph discussed above and that of Hickox et al. (2000) based on satellite data. With respect to salinity in the surface layer, the maximum, with a value higher than 33.7, is also found at Stn. C, but this decreases rapidly to below 33.1 at Stn. D, and then steadily declines to 32.7 towards the Fujian coast. This, no doubt, stems from land runoff from continental China. The maximum levels of the surface layer st and AOU
ARTICLE IN PRESS
Fig. 2. (Continued)
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1640
ARTICLE IN PRESS
T (CTD)
Longitude (°E )
120.0 120.4 120.8 121.2 121.6
T (°C )
Fig. 4. Continuous surface temperature, salinity and stfrom the ORII-806 cruise.
BA
118 119 120 121 122 123 118 119 120 121 122 123
Longitude (°E) Longitude (°E)
Fig. 3. Study area and the location of the sampling stations.
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1641
are also at Stn. C, highly indicative of an upwelling of, or a mixing with the subsurface waters.
Interesting to note is that Stn. C seems to be vertically well mixed and that there is neither a thermocline nor a halocline. To the west of Stn. C, a themocline and ahalocline come together at a depth of 30–40 m, and the fresher surface layer becomes thicker towards the coast. To the east of Stn. C, the themocline and halocline are slightly deeper. With Stn. C taken as the focal point, the subsurface T, S, st and AOU contours on both sides all become more and more shallow the farther away they are from that station. More to the point, at Stn. C, there are horizontal T, S and st maxima, but an AOU minimum for subsurface waters.
Similarly shaped contour lines are evident for NO3
, NO2
, PO34 , SiO2and chlorophyll a (Figs. 6
and 7;Chen and Hsing, 2005). These parameters all show a distinctive horizontal minimum for subsur-face waters at Stn. C. On the other hand, like many other fronts found elsewhere (e.g. Olson, 2002 and references therein), the surface layer at Stns. B and C has enhanced primary production, as evidenced in the higher chlorophyll concentrations (Fig. 7a). The Degree of Nutrient Consumption (DNC, Chen et al., 2004) based on chlorophyll a, (DNCC¼0.7 chlorophyll a/(NO3+NO2+0.7 chlorophyll-a)), nitrate and nitrite (DNCN¼[dissolved organic nitrogen (DON)+particulate organic carbon (PON)]/(NO3+NO2+DON+PON)) as well as phosphate (DNCP¼[dissolved organic phosphorus (DOP)+particulate phosphorus (PP)]/(PO4+ DOP+PP)), as obtained by Chen and Hsing (2005), are shown in Fig. 7. The minimum DNCC
ARTICLE IN PRESS
120.0° 120.4° 120.8° 121.2° 121.6°St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1642
and DNCN values, indicative of newly upwelled waters (Chen et al., 2004; Chen and Hsing, 2005), are found near the surface at Stn. C., whereas the maximum DNCC, DNCN and DNCP values are found in the deep waters at Stn. C., which is a clear sign that the waters are older near the bottom of Stn. C when compared with the waters on either side.
Figs. 8 (a)–(d) show the cross-section of pH, normalized TA (NTA ¼ TA 35/S), normalized DIC (NDIC ¼ DIC 35/S) and fCO2, respectively.
Each of the pH and NTCO2contour follows those of temperature, with pH exhibiting a subsurface maximum but with NDIC exhibiting a subsurface minimum at Stn. C. Both the NTA and fCO2 cross-sections closely resemble that of S. While the NTA cross-section does not show a clear minimum, the
fCO2 cross-section shows a clear subsurface mini-mum at Stn. C.
It is quite clear that a front exists between Stns. C and D, with the relatively more oxygenated, nutrient-rich water having mixed downward below 30 m and the relatively older, oxygen-and-nutrient-poor water having mixed upward above 30 m at Stn. C.
The y/S plot (Fig. 9a) clearly shows that the y/S properties at Stn. C should be categorized in the same group as those at Stns. A and B, while those at Stns. D through I are grouped in a second group.
The NO2
data (Fig. 6b) also suggest that the mid-depth waters at Stns. A and B slip down to the bottom at Stn. C. As for S, (Fig. 9a) there is a large, distinct gap between the two groups. To our surprise, each group is represented by a straight
ARTICLE IN PRESS
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1643
y/S line, which makes us suspect there is a simple two end-member mixing process for each. Even so, one question still arises: Exactly what are the end-members, and where do they originate?
4. Discussion 4.1. Water masses
Each and every paper in the extant literature, starting with the landmark 1935 charts from the Japanese Hydrographic Office of the Imperial Navy (Fig. 2inNitani, 1972) chronologically on up to the reports of Ninno and Emery (1961), Chu (1961, 1971), Wyrtki (1961), Nitani (1972), Fan (1979, 1982), Fan and Yu (1981), Chuang (1985, 1986), Guan (1984),Wang and Weng, 1987;Chao (1991),
Wong et al. (1991),Zhang et al. (1991),Fu and Hu (1995), Wang and Yuan (1997),Fang et al. (1998), Wang et al. (2000), Jan et al. (2002), Chung et al.
(2001), Ichikawa and Beardsley (2002), and Liang et al. (2003), and still others, has, to the authors’
credit, pointed out that the currents in the Taiwan Strait flow northward in summer. For this very reason, it seemed logical for us to first look for waters in the southern part of the Taiwan Strait with comparable properties.
Fig. 9b displays the y/S plots for a cross-section between Kaohsiung and Kinmen, the locations of which are shown in Fig. 3, (the ORIII-721 cruise;
August, 2001). Needless to say, except for the westernmost stations, particularly Stns. M and N, the y/S plots are all curved, a characteristic typical of the West Philippine Sea (WPS) and the South
ARTICLE IN PRESS
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Fig. 7. Cross-section of (a) chl-a; (b) DNCC; (c) DNCN; and (d) DNCPfrom the ORII-806 cruise (modified fromChen and Hsing, 2005).
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1644
China Sea (SCS). Fig. 10ashows the y/S plots for the OR-I 418B cruise in the WPS at about 211450N from 1201 to 1301E in May, 1994. At about 1221E, there is a front which separates the outflowing SCS tropical and intermediate waters from the north-ward-flowing WPS waters (Chen and Huang, 1996;
Chen, 2005). The maximum salinity to the east of the front is saltier than its counterpart to the west;
conversely, the minimum salinity to the east of the front is fresher than that to the west (Fig. 10a). This is attributed to the fact the waters west of the front experience the effects of outflowing waters from the SCS where intensive upwelling and vertical mixing tend to homogenize the water column; hence, this accounts for the fresher maximum salinity and saltier minimum salinity layer to the west of the front.
The y/S plots from the OR-I 508 cruise in the SCS in Nov., 1997 are shown inFig. 10b. The waters at
the stations farther to the north (north of 181N) are very much influenced by the WPS waters (higher salinity in the maximum salinity layer), unlike the waters from the SCS proper south of 181N which have lower salinity in the upper water column. In a word, almost all of the y/S plots obtained from the stations in the southern cross-section of the Taiwan Strait (Fig. 9b) are comparable to those of the SCS. The curved-shape strongly suggests that the whole upper water column moves into the Penghu Channel (located as shown inFig. 3) without much vertical mixing. Two coastal stations M and N on the OR III 721 cruise (shown in Fig. 9b) have a considerably more distinctive y/S structure in that the bottom waters are significantly colder and fresher than those at the other stations. These y and S signatures cannot be attributed to coastal upwelling, but they can be explained as remnant winter waters. This is the subject of a future study.
ARTICLE IN PRESS
120.0° 120.4° 120.8° 121.2° 121.6°St. H G F E D C B A
Longitude (°E)
120.0°St. H G120.4°F 120.8°E D 121.2°C 121.6°B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Longitude (°E)
120.0° 120.4° 120.8° 121.2° 121.6°
St. H G F E D C B A
Fig. 8. Cross-section of (a) pH; (b) NTA; (c) DIC; and (d) fCO2 from the ORII-806 cruise.
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1645
In light of the above comparisons of the y/S characteristics, it should come as no surprise that waters from the SCS flow into the southern Taiwan Strait and that they continue to flow northwardly in August, 2001. Above and beyond all this, a problem arises, however. Since the sill depth in the middle of the Taiwan Strait is less than 60 m, most of the northward-flowing water below that depth is blocked. True, the waters in the upper water column continue to flow northwardly, but in all likelihood, such factors as strong tidal movements, bottom friction and heavy winds tend to mix these waters.
Once they finally reach the northern cross-section,
the once-curved y/S lines turn into straight lines at Stns. D–I (Fig. 9a). These straight lines are consistent with the group of y/S curves on the left-hand side in the southern cross-section (Fig. 9b), which suggests the mixing sustained by winds and tides.
It must be pointed out that the straight y/S lines at Stns. A–C (Fig. 9a) are, nevertheless, entirely inconsistent with any of the other y/S lines in the southern part of the Taiwan Strait and equally out of line with typical y/S curves in the WPS and the SCS. It follows then that another mixing process must have played a role in this, and the most
ARTICLE IN PRESS
31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0
15
Fig. 9. Potential temperature (y) vs. salinity from (a) the OR-II 806 cruise (modified fromChen and Hsing, 2005) and (b)the OR-III 721 cruise. Station locations are given inFig. 3.
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1646
probable candidate for this is the mixing of the subsurface Kuroshio waters with the waters of the ECS shelf itself.
4.2. Upwelled subsurface Kuroshio waters
It is generally an accepted fact that the subsurface waters of the Kuroshio are driven by large-scale circulation and, as a result, upwell onto the ECS shelf. This process is particularly strong in the region northeast of Taiwan given that the topo-graphy of the bottom forces the northward-flowing Kuroshio to turn northeastwardly. Offshore of the
shelf break is a retrograde shelf-break front (Olson, 2002) with isopycnals sloped in a direction opposite of that of the topography (Wong et al., 1991;Chen et al., 1995;Chen, 1996and references therein). This retrograde feature shoreward of the shelf break front is clearly visualized in the contour lines at Stns. A–C (Figs. 5–8). The upwelled waters subse-quently mix with the shelf waters of the ECS.
Also evident is that owing to the strong tidal mixing and other physical factors, these waters mix well, and the once curved y/S plots eventually become straight lines. Finally, as evidenced by the very short y/S line (Fig. 9a), stratification is
ARTICLE IN PRESS
Fig. 10. Potential temperature (y) vs. salinity from (a) the OR-I 418B and b) the OR-I 508 cruises.
C.-T. Arthur Chen, S.-L. Wang / Continental Shelf Research 26 (2006) 1636–1653 1647
weakened in the vicinity of Stn. C, hence vertical mixing is enhanced.Fig. 11 presents an example of such y/S curves for a cross-section northeast of Taiwan (Chen et al., 1995). The mixing lines are linear in the upper water column and correspond strikingly to the y/S lines at Stns. A–C. Simply put, the salinity front near Stn. C separates the Chinese coastal and Taiwan Strait waters from those originating in the Kuroshio. What this highlights is that the oft-cited northward-flowing Taiwan Strait waters in summer may not in fact cover the whole northern part of the Strait at all. The reason is simple: the easternmost segment may have actually originated in the northeast!
Earlier, we reported that the curved-shape of the y/S plots is a strong indicator that the whole upper water column moves into the Penghu Channel. Yet, another key point in this regard concerns the conversion of kinetic energy carried by the bottom currents in the Penghu Channel to potential energy.
Arguably, the kinetic energy (0.5 mv2, where m is mass and v is velocity) carried by the bottom currents in the Penghu Channel may be converted to potential energy (mg0Dz, where g0is reduced gravity and Dz is the difference between the depth of the ridge and that of the incoming water), meaning that waters deeper than 60 m may be able to ascend the ridge, and then redescend to deeper depths north of the ridge (Wang and Chern, 1992). Taking
v to be 0.32 m s1 (Chuang, 1986), northward-flowing waters between 60 and 65 m in depth can indeed edge up and pass over the ridge. Likewise, waters at 80 m can equally achieve the same provided that v is increased to 0.63 m s1 during strong tides. For these reasons, it is most reasonable that the 80-m deep bottom waters between Stns.
DI might very well originate in the south. It is, nevertheless, very unlikely that the bottom waters at Stns. A and B could have come from the south as most tidal currents are not strong enough for waters deeper than 120 m to surmount the 60-m deep ridge.
As Wang et al. (1988) and Jan et al. (1994) reported, after the northward-flowing surface and bottom waters from the Penghu Channel veer to the west in front of the Chang-Yuen Ridge to the north of the Penghu Channel (Fig. 3), the originally vertically stratified waters are transformed, becom-ing uniform in the eastern portion but maintainbecom-ing their stratified nature in the western portion. But their findings only hold true for areas south of 251N where the water depth is less than 80 m and for areas that the Kuroshio subsurface waters do not reach.
North of 251N, with the entry of the Kuroshio subsurface waters, the waters on the eastern side become stratified again (Fig. 5). The vertically mixed waters (Stn. C) divide the two stratified waters on both sides. This should have been obvious as no 18 1C waters (e.g. that found at Stn. B in the
ARTICLE IN PRESS
Table 1
Average value of various parameters for the 65-m thick water column at Stns. A–E
Average A B C D E
T(1C) 27.7 28.8 27.6 26.1 26.2
Salinity 33.688 33.663 33.735 33.512 33.452
st 21.430 21.065 21.570 21.921 22.926
DO(mmol/L) 189 186 187 188 170
AOU(mmol/kg) 14 15 17 22 38
NO3+NO2(mmol/L) 2.6 1.2 1.1 2.5 2.8 Chl-a(mg/L) 0.18 0.248 0.26 0.223 0.16 PO34(mmol/L) 0.39 0.18 0.14 0.244 0.35
SiO2(mmol/L) 6 4 4 7 8
DON(mmol/L) 5.45 3.17 3.44 2.85 4.25
DOP(mmol/L) 0.1 0.13 0.18 0.24 0.11
DNCC 0.24 0.23 0.18 0.26 0.22
DNCN 0.72 0.76 0.78 0.71 0.71
DNCP 0.24 0.48 0.62 0.59 0.36
NTA(mmol/kg) 2306 2303 2301 2313 2315
NTCO2(mmol/kg) 2003 1988 1989 2030 2028
pH(tot) 8.07 8.089 8.092 8.041 8.043
fCO2(mmol/kg) 402 387 377 414 410
33.0 33.5 34.0 34.5 35.0
T,°C
Fig. 11. Potential temperature (y) vs. salinity from the OR-I 179
Fig. 11. Potential temperature (y) vs. salinity from the OR-I 179