Chlorofluorocarbons in the Western North Pacific in 1993 ... - terrapub

Journal of Oceanography Vol. 52, pp. 475 to 490. 1996

Chlorofluorocarbons in the Western North Pacific in 1993 and Formation of North Pacific Intermediate Water TAKAYUKI TOKIEDA1, SHUICHI WATANABE1,2 and SHIZUO TSUNOGAI1,2 1Department

of Chemistry, Graduate School of Fisheries Science, Hokkaido University, Hakodate 041, Japan 2Laboratory of Marine and Atmospheric Geochemistry, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan (Received 28 August 1995; in revised form 12 January 1996; accepted 12 January 1996)

Chlorofluorocarbons (CFC-11 and CFC-12) in the intermediate water having σθ between 26.4 and 27.2 were determined at 75 stations in the western North Pacific north of 20°N and west of 175.5°E in 1993. The intermediate water of 26.4–26.6σθ was almost saturated with respect to the present atmospheric CFC-11 in the zone between 35 and 45°N around the subarctic front. Furthermore, the ratios of CFC11/CFC-12 of the water were also of those formed after 1975. These suggest that the upper intermediate water (26.4–26.6σθ) was recently formed by cooling and sinking of the surface water not by mixing with old waters. The water below the isopycnal surface of 26.8σθ contained less CFCs and the area containing higher CFCs around the subarctic front was greatly reduced. However, the CFC age of the lower intermediate water (26.8–27.2σθ) in the zone around the subarctic front was not old, suggesting that the water was formed by diapycnal mixing of the water ventilated with the atmosphere with old waters not containing appreciable CFCs, probably the Pacific Deep Water. The southward spreading rate decreased with depth and it was one sixth of its eastward spreading rate of the North Pacific Intermediate Water (NPIW). 1. Introduction The North Pacific Intermediate Water (NPIW) has been given attention as a significant sink of the anthropogenic carbon dioxide (Tsunogai et al., 1993). NPIW has a salinity minimum at the isopycnal surface of around 26.8σθ, which is often referred to as a representative of NPIW. Wüst (1929) described that NPIW is formed by sinking of the low salinity and low temperature surface water in the Okhotsk Sea. Sverdrup et al. (1942) wrote that NPIW is formed in winter at the convergence between the Kuroshio Extension and the Oyashio. Reid (1965) noted the lack of outcrop of NPIW larger than 26.8σθ in the North Pacific and stated that it must be formed in the high-latitudes by vertical mixing across the pycnocline. Reiser and Swift (1989) found the outcrop of water denser than 26.8σθ in the Okhotsk Sea in winter, and suggested that NPIW is formed in the Okhotsk Sea having the outcrop in winter and transported to the North Pacific through the Bussol’ strait. Kitani (1973), Ohtani (1989) and Talley and Nagata (1991) also gave the same conclusion. Hasunuma (1978) suggested that the most characteristic property of NPIW, the salinity minimum, is formed in the region between the Oyashio front and the Kuroshio front, and Talley (1993) also pointed out that it is formed in the same region.

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Many studies have addressed the character of NPIW based on the distributions of water temperature, salinity, dissolved oxygen and potential vorticity (Reid, 1965; Kitani, 1973; Talley, 1988). Recently, transient tracers such as tritium and radiocarbon have been used for the investigation. For the distribution of bomb-derived tritium, Fine et al. (1981) found a great percentage of tritium in the intermediate layer of the North Pacific and concluded that the lateral flow of the intermediate water has a major role in the penetration of atmospheric gas components. Based on the two data sets of GEOSECS in 1973 and 1985 Long-Line Cruise, Van Scoy et al. (1991) found that the bomb-derived tritium peaks observed at the 26.8σθ surface in 1976 in the northeast subpolar region moved in the northeast subtropical gyre in 1982. They estimated the tritium peak crossed the subarctic front in about 1979, and that the residence time of NPIW in the subarctic region is less than 14 years, because the tritium peak at the surface was found in 1965–1966 (Fine and Östlund, 1977). Anthropogenic chlorofluorocarbons (CFCs) are now used as tracers of water masses moving with a time scale of several decades (Krysell and Wallace, 1988; Mantisi et al., 1991; Rhein, 1991). Warner and Weiss (1985a) measured the concentrations of CFCs in seawater along the east-west sections of 24°N and 47°N in the Pacific Ocean. Wisegarver and Gammon (1987) revealed that NPIW above the 26.8σθ surface is transported from the western North Pacific to the Station P (50°N, 145°W) with the transit time of 5–10 years. This study aims to determine the detailed distribution of CFCs in the western North Pacific, the source region of NPIW, in early 1990’s, and to make clear the formation and flow pattern of NPIW. In this study, we define the intermediate water as those having σθ between 26.4 and 27.2 including a salinity minimum in the North Pacific. 2. Sampling and Analytical Methods Seawater samples for the CFCs measurement were collected at 75 stations during 3 cruises, along the 5 north-south sections of 145°E, 155°E, 165°E, 170°E and 175.5°E in the western North Pacific (Fig. 1). The cruises, KH93-2 and HO59-3, were conducted in May to June and July to August, respectively, in 1993. The CFCs concentrations obtained during HO55-2 in May to June in 1992 are also included in the following discussion. The concentrations of CFCs were determined on board the vessels with a gas-chromatography equipped with an electron capture detector (Shimadzu GC-8A). The purging and trapping system of CFCs was similar to that of Bullister and Weiss (1988). The CFCs concentrations were calibrated against the 1983 calibration scale of Scripps Institution of Oceanography (Bullister, 1984). The samplers used were Teflon-coated 10 liter Niskin bottles and 1.2 liter Niskin bottles for the Hakuho-Maru and Hokusei-Maru cruises, respectively. The samplers were installed in a CTD-RMS system. Some contaminations from the samplers were not avoidable, but fortunately they were not highly at random. The CFCs data have been corrected by subtracting either their mean concentrations for waters below 2000 m for the Hakuho-Maru data, or the concentration differences between the Niskin bottles and the clean seawater samplers devised by Tokieda et al. (1994) for the Hokusei-Maru data. The correction values were 0.102 ± 0.033 pmol/kg and 0.067 ± 0.012 pmol/kg for the Hakuho-Maru data on CFC-11 and CFC-12, respectively, and 0.239 ± 0.029 pmol/kg and 0.177 ± 0.013 pmol/kg for the Hokusei-Maru data on CFC-11 and CFC-12. The uncertainties given above are the standard deviations of measured values for different samples. The uncertainties are larger than the standard deviations of replicate determinations of one sample, which were 0.7% and 0.9% for CFC-11 and CFC-12, respectively, at the concentration level of 0.5 pmol/kg.

Chlorofluorocarbons in the Western North Pacific in 1993 and Formation of North Pacific Intermediate Water

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Fig. 1. Map showing the sampling stations for chloroflorocarbons in the western North Pacific.

3. Results and Discussion 3.1 Distributions of CFCs in the western North Pacific The north-south sections of σθ and CFCs (CFC-11 and CFC-12) concentrations along 145°E and 165°E are shown in Figs. 2 and 3, respectively. In the surface water, the higher concentrations of CFCs were found in the higher latitudes, reflecting their larger solubilities at lower temperatures. In the subsurface water below 300 m depth, the concentrations decreased with depth in the western section along 145°E north of 30°N. The isopleth of 0.1 pmol/kg of CFC-11 exists in the depth range between 800 and 1300 m and shows no significant difference in the plots between

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(a)

(b)

Fig. 2. Vertical sections of σθ (a) and concentrations of CFC-11 (b) and CFC-12 (c) (in pmol/kg) sections along 145°E.


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(c)

Fig. 2. (continued).

both sections. On both sections, the isopleth of 0.1 pmol/kg of CFCs was deepened around 30°N and shallowed around 35°N. The distribution pattern of CFCs qualitatively coincided with that observed along 175°E in 1992 by Watanabe et al. (1994). Figure 4 shows the distributions of CFC-11 at the isopycnal surfaces of 26.4, 26.6, 26.8, 27.0 and 27.2σθ in the western North Pacific. The higher CFC-11 concentrations were distributed in the northern or northwestern part of the study area. The percent saturations with respect to the present atmospheric CFC-11 concentration calculated from its potential temperature and salinity are also shown in this figure. The concentration of CFC in the surface water depends on the water temperature and its atmospheric concentration which has increased with time. The percent saturations, therefore, depend only on its timing of formation and the mixing ratio with ambient waters, because the dependency of its solubility on temperature is eliminated. Consequently, we can regard the water almost saturated with CFCs as that recently formed not mixing with old waters. At the isopycnal surface of 26.4σθ (Fig. 4(a)), the water was almost saturated (95–102% saturation) with CFC-11 in the region near the subarctic front around 40°N. Since the recent increase rate of CFC-11 in the atmosphere is about 3%/year (Cunnold et al., 1994), the water must be younger than a few years old. The region containing CFC-11 with more than 95% saturation at the isopycnal surface of 26.6σθ (Fig. 4(b)) was also located around 40°N, but the area was greatly reduced and the zone more than 80% saturation occupied nearly the same area as the 26.4σθ. The distribution of CFC-11’s percent saturation at the isopycnal surface of 26.8σθ (Fig. 4(c)) was much different from those at the upper isopycnal surfaces and had a maximum off the south

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(a)

(b)

Fig. 3. Vertical sections of σθ (a) and concentrations of CFC-11 (b) and CFC-12 (c) (in pmol/kg)sections along 165°E.


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Kuril Islands. The distribution was favorable for the suggestion of Ohtani (1989), in that NPIW was formed in the Okhotsk Sea. This, however, is only for the 26.8σθ surface. The maximum CFC-11 saturation was greatly decreased with depth for waters below 26.8σθ, namely those of 27.0 and 27.2σθ (Figs. 4(d) and (e)). This finding suggests that the areas forming the intermediate waters above 26.6σθ and below 26.8σθ are different. The upper intermediate water was formed in the relatively longitudinally wide area around 40–45°N, loading much amount of the atmospheric gas components. On the other hand, the lower intermediate water may be formed in smaller area in the western North Pacific including the Okhotsk Sea. Figures 5 and 6 show the CFC-11/CFC-12 ratios at the isopycnal surfaces of 26.4–27.2σθ along 145°E and 165°E, respectively, together with corresponding CFC ages estimated from the equation by Warner and Weiss (1985b) and the temporal variation in the atmospheric CFCs mixing ratio (Smethie et al., 1988). At the isopycnal surface of 26.4σθ, the water having the CFC ratio of the present surface waters covers the entire latitudinal range studied here. According to Smethie et al. (1988), this means the water of 26.4σθ was formed after 1975. The CFC ratios decreased with increasing density, namely the CFC ages were older for the deeper intermediate water especially at the 145°E section (Fig. 5). The recently formed waters after 1975, however, exist even in the lower intermediate water of 27.2σθ in the latitudes between 40 and 45°N along 165°E (Fig. 6), although the water had a relatively low concentration and low percent saturation of CFC-11 (Figs. 4(e) and 7). This finding suggests that the water has not yet been subjected to the isopycnal mixing with older waters formed before 1975 by the same mechanism. The CFC-11/CFC-12 ages are plotted for the isopycnal surfaces of 26.8, 27.0 and 27.2σθ in

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Fig. 4. CFC-11 concentration (solid line, in pmol/kg) and percent saturation for CFC-11 relative to its atmospheric concentration in 1993 (dashed line and shaded area, in percent in italics) at the isopycnal surfaces of σθ of 26.4 (a), 26.6 (b), 26.8 (c), 27.0 (d) and 27.2 (e) in the western North Pacific.


(c)

(d)


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Fig. 5. Ratios of CFC-11/CFC-12 at the isopycnal surfaces of σθ of 26.4, 26.6, 26.8, 27.0 and 27.2 along 145°E.


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Fig. 6. Ratios of CFC-11/CFC-12 at the isopycnal surfaces of σθ of 26.4, 26.6, 26.8, 27.0 and 27.2 along 165°E.

Fig. 7. Vertical distributions of CFC-11 at the region around the subarctic front in the western North Pacific. HO552 Sta. 92082 (41°45N, 155°00E) (—), HO55-2 Sta. 92083 (41°00N, 155°01E) (+), HO59-3 Sta. 93080 (41°00N, 145°00E) (䊊), HO59-3 Sta. 93081 (45°00N, 155°00E) (䊉), KH93-2 Sta. C-14 (38°59N, 165°01E) (䊐), and KH932 Sta. C-15 (41°01N, 165°01E) (䊏).

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(b)

Fig. 8. Apparent ages of water after descending from the surface derived from the CFC-11/CFC-12 ratios at the isopycnal surfaces of σθ of 26.8 (a), 27.0 (b) and 27.2 (c) in the western North Pacific (in years).


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the western North Pacific (Fig. 8). The isopycnal surfaces of 26.4 and 26.6σθ are not included in Fig. 8, because all the waters at these surfaces were formed virtually after 1975 in the region. Even at the surface of 26.8σθ (Fig. 8(a)), most of the water in this region near the subarctic front was formed after 1975. The area of water of the younger CFC age was largely decreased, but it certainly existed even at the isopycnal surfaces of 27.0 and 27.2σθ (Figs. 8(b) and (c)), which is the same as the percent saturation given above. These support our previously stated conclusion that NPIW was formed in a wide area for the upper intermediate waters, but in a limited area for the lower one. Furthermore, they indicate that NPIW has two different origins, namely, the upper NPIW above the surface of 26.6σθ was recently formed by cooling at the surface without mixing, and the lower NPIW below the surface of 26.8σθ was formed by diapycnal mixing of the recent surface water with sufficiently old waters containing no appreciable CFCs, which was probably the Pacific Deep Water. This conclusion is in harmony with the hypothesis proposed by Talley (1993). 3.2 Southward spreading of NPIW in the western North Pacific To estimate the southward spreading rate of NPIW on the isopycnal surfaces denser than 26.8σθ, the decrease rates of the CFC-11/CFC-12 ratios toward the south were estimated from the regression lines given in Figs. 5 and 6 for the two meridional sections dividing into two latitudinal belts. One is 20–27°N and the other is 27–37°N. Since the CFC-11/CFC-12 ratio increased together with that of the surface seawater increased with a virtually constant rate of 0.064 year–1 before 1975 (Smethie et al., 1988), the mean southward component of spreading of

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Table 1. Southward spreading rate of NPIW.

Longitude

Latitude range

σθ

Slope*

145°E

20°N–27°N

26.8 27.0 27.2

0.049 (/deg.) 0.078 (/deg.) 0.103 (/deg.)

1.3 (deg./yr.) 0.82 (deg./yr.) 0.62 (deg./yr.)

140 (km/yr) 91 (km/yr) 69 (km/yr)

145°E

27°N–37°N

26.8 27.0 27.2

NC (/deg.) 0.004 (/deg.) 0.016 (/deg.)

NC (deg./yr.) 16.8 (deg./yr.) 4.0 (deg./yr.)

NC (km/yr) 1870 (km/yr) 440 (km/yr)

145°E

20°N–37°N

26.8 27.0 27.2

NC (/deg.) 0.035 (/deg.) 0.045 (/deg.)

NC (deg./yr.) 1.8 (deg./yr.) 1.4 (deg./yr.)

NC (km/yr) 200 (km/yr) 160 (km/yr)

165°E

27°N–37°N

26.8 27.0 27.2

0.029 (/deg.) 0.052 (/deg.) 0.082 (/deg.)

2.2 (deg./yr.) 1.2 (deg./yr.) 0.78 (deg./yr.)

250 (km/yr) 140 (km/yr) 87 (km/yr)

Southward rate

*The decrease rate of the CFC-11/CFC-12 ratio toward the south. NC: Not calculate due to the existence of water formed after 1975.

NPIW can be computed from the gradient. Table 1 lists the results of calculation. The southward spreading rates decreased with depth for both sections. The southward rate at the western section along 145°E is faster than that along 165°E in the latitudinal belt between 27–37°N. The rate was extremely small in the latitudinal belt between 20–27°N along 145°E. This is considered to be due to the boundary of the intermediate water caused by the Subtropical Gyre. Based on Fig. 8(a), we can obtain about 20 years for the traveling time of NPIW at the surface of 26.8σθ along 145°E from the subarctic front to the region around 20°N in the western Pacific, giving about 0.3 cm/ sec of the southward spreading rate. Wisegarver and Gammon (1987) estimated a faster eastward spreading rate of NPIW to be 2 cm/sec (630 km/yr) at the isopycnal surface of 26.8σθ based on the difference in the apparent CFC ages between the western and eastern subarctic North Pacific. Gargett et al. (1986) also reported a faster eastward spreading rate using bomb-derived tritium. The rate of southward spreading we obtained is only one sixth of the eastward spreading rate. 4. Summary and Conclusion We have obtained the following results from the CFCs observed in the western North Pacific: 1) The upper NPIW above the 26.6σθ surface was almost saturated with respect to the recent atmospheric CFC-11 in the region near the subarctic front. 2) The lower NPIW below the 26.8σθ surface decreased its CFC concentration with depth but the CFC-11/CFC-12 ratio was that of the surface water in the small area between 155–165°E around 40°N. 3) Based on the above results, we have concluded that NPIW has two different origins. The upper intermediate water above the 26.6σθ was formed by cooling at the surface in the relatively


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wide area around the subarctic front and the lower intermediate water below the 26.8σθ was formed by diapycnal mixing without direct ventilation with the surface water in a small limited area. 4) The southward spreading rate of NPIW is estimated to be one sixth of its eastward rate or it takes about 20 years for NPIW spreading from the subarctic front to the region around 20°N in the western Pacific. Acknowledgements We wish to express our hearty thanks to Drs. I. Yasuda, S. Nishino, S. Noriki, N. Tanaka and to our colleagues for encouraging us throughout this work and for giving support with the idea. The captains and crew of R/V Hokusei-Maru and R/V Hakuho-Maru are acknowledged for their assistance at the sampling. References Bullister, J. L. (1984): Atmospheric chlorofluoromethanes as tracers of ocean circulation and mixing: measurement and calibration techniques and studies in the Greenland and Norwegian seas. Ph.D. Thesis, Univ. of California. Bullister, J. L. and R. F. Weiss (1988): Determination of CClF3 and CCl2F2 in seawater and air. Deep-Sea Res., 35, 839–853. Cunnold, D. M., P. J. Fraser, R. F. Weiss, R. G. Prinn, P. G. Simmonds, B. R. Miller, F. N. Alyea and A. J. Crawford (1994): A global trends and annual releases of CCl3F and CCl2F2 estimated from ALE/GAGE and other measurements from July 1978 to June 1991. J. Geophys. Res., 99, 1107–1126. Fine, R. A. and H. G. Östlund (1977): Source function for tritium transport models in the Pacific. Geophys. Res. Lett., 4, 461–464. Fine, R. A., J. L. Reid and H. G. Östlund (1981): Circulation of tritium in the Pacific Ocean. J. Phys. Oceanogr., 11, 3–14. Gargett, A. E., H. G. Östlund and C. S. Wong (1986): Tritium time series from ocean station P. J. Phys. Oceanogr., 16, 1720–1726. Hasunuma, K. (1978): Formation of the intermediate salinity minimum in the Northwestern Pacific Ocean. Bull. Ocean Res. Inst. Univ. Tokyo, 9, 46 pp. Kitani, K. (1973): An oceanographic study of the Okhotsk sea—Particularly in regard to cold waters. Bull. Far Seas Fish. Res. Lab., 9, 45–77. Krysell, M. and D. W. R. Wallace (1988): Arctic Ocean ventilation studied with a suite of anthropogenic halocarbon tracer. Science, 242, 746–749. Mantisi, F., C. Beaugerver, A. Poisson and N. Metzl (1991): Chlorofluoromethanes in the western Indian sector of the Southern Ocean and their relations with geochemical tracers. Mar. Chem., 35, 151–167. Ohtani, K. (1989): The role of the sea of Okhotsk on the formation of the Oyashio water. Umi to Sora, 2, 63–83. Reid, J. L., Jr. (1965): Intermediate waters of the Pacific Ocean. Johns Hopkins Oceanogr. Stud., 2, 85 pp. Reiser, S. C. and D. D. Swift (1989): Ventilation and North Pacific Intermediate Water formation in the Sea of Okhotsk. Paper presented at the inaugural meeting, Oceanogr. Soc., Monterey, Calif. Rhein, M. (1991): Ventilation rates of the Greenland and Norwegian Seas derived from distributions of the chlorofluoromethanes F-11 and F-12. Deep-Sea Res., 38, 485–503. Smethie, W. M., Jr., D. W. Chapman, J. H. Swift and P. Koltermann (1988): Chlorofluoromethanes in the Arctic Mediterranean seas: Evidence for formation of bottom water in the Eurasian Basin and deep-water exchange through Fram Strait. Deep-Sea Res., 35, 347–369. Sverdrup, H. U., M. W Johnson and R. H. Fleming (1942): The Oceans: Their physics, Chemistry and General Biology, Prentice-Hall, Englewood Cliffs, New York, 1087 pp. Talley, L. D. (1988): Potential vorticity distribution in the North Pacific. J. Phys. Oceanogr., 18, 89–106. Talley, L. D. (1993): Distribution and formation of North Pacific Intermediate Water. J. Phys. Oceanogr., 23, 517– 537. Talley, L. D. and Y. Nagata (1991): Oyashio and mixed water regions as a formation area of the North Pacific Intermediate Water. Umi to Sora, 4, 65–74 (in Japanese). Tokieda, T., S. Watanabe, K. Namiki and S. Tsunogai (1994): Development of a clean seawater sampler for the determination of Chlorofluorocarbons. Bunseki Kagaku, 43, 827–829 (in Japanese).

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Tsunogai, S., T. Ono and S. Watanabe (1993): Increase in total carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon. J. Oceanogr., 49, 305–315. Van Scoy, K. A., R. A. Fine and H. G. Östlund (1991): Two decades of mixing tritium into the North Pacific Ocean. Deep-Sea Res., 38, Suppl. 1a, 5191–5219. Warner, M. J. and R. F. Weiss (1985a): Vertical distributions of anthropogenic chlorofluoromethanes in the North Pacific along 24°N and 47°N. EOS, 66, 928. Warner, M. J. and R. F. Weiss (1985b): Solubility of chlorofluorocarbons 11 and 12 in water and seawater. DeepSea Res., 32, 1485–1497. Watanabe, Y. W., K. Harada and K. Ishikawa (1994): Chlofluorocarbons in the central North Pacific and southward spreading time of North Pacific Intermediate Water. J. Geophys. Res., 99, 25195–25213. Wisegarver, D. P. and R. H. Gammon (1987): The ventilation and circulation of the Sub-Arctic North Pacific: Timescale from fluorocarbon transient tracers. EOS, 68, 1700. Wüst, G. (1929): Schichtung und Tiefenzirkulation des Pazifischen Ozeans. Veröff. Inst. Meersk. Univ. Beerlin, neue Folge, A, 20, 64 pp.