Chlorofluorocarbons (CFCs) in the North Pacific Central Mode Water: Possibility of under-saturation of CFCs in the wintertime mixed layer

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1 Geochemical Journal, Vol. 38, pp. 643 to 65, 24 Chlorofluorocarbons (CFCs) in the North Pacific Central Mode Water: Possibility of under-saturation of CFCs in the wintertime mixed layer TAKAYUKI TOKIEDA, 1 * MASAO ISHII, 1 TAMAKI YASUDA 1 and KAZUTAKA ENYO 2 1 Meteorological Research Institute, Nagamine 1-1, Tsukuba, Ibaraki 35-52, Japan 2 Climate and Marine Department, Japan Meteorological Agency, Otemachi 1-3-4, Chiyoda-ku, Tokyo , Japan (Received November 4, 23; Accepted October 7, 24) Analysis of chlorofluorocarbons (CFCs) data in seawater in the Kuroshio-Oyashio Transition Area obtained during the WOCE Campaign P-13 along 165 E in August to October 1992 and P-14 along 179 E in July to September 1993 revealed that CFC-12 and CFC-11 in the North Pacific Central Mode Water (NPCMW) were significantly under-saturated with respect to the atmospheric CFC-12 (75.5 ± 12.3% for P-13 and 85.9 ± 6.3% for P-14) and CFC-11 (72.9 ± 1.6% for P-13 and 82.6 ± 5.3% for P-14). Since the mode water is derived from the vertical convection in the surface layer in winter and is considered not greatly influenced by mixing with ambient water during the subsequent advection, the observed undersaturation of CFC-12 and CFC-11 in the mode water suggests that CFCs in the mixed layer in winter has already been under-saturated with respect to the atmospheric CFC-12 and CFC-11. A result from a simple one-dimensional model for the mixed layer also supports the entrainment of the water with lower degree of CFC-12 and CFC-11 saturation and the under-saturation in the wintertime mixed layer. Keywords: chlorofluorocarbon, North Pacific Central Model Water, under-saturation in winter, over-saturation in summer, wintertime mixed layer INTRODUCTION The Kuroshio-Oyashio Transition Area east of Japan has a complicated current and frontal structure (Fig. 1). The mode water is defined as a layer of vertically homogeneous water that is observed in the upper layer of the ocean below the seasonal thermocline. In the Kuroshio- Oyashio Transition Area, there are two well-known mode waters identified, the North Pacific Subtropical Mode Water (NPSTMW) (Masuzawa, 1969, 1972; McCartney, 1982; Talley, 1988; Suga and Hanawa, 199, 1995a, 1995b) and the North Pacific Central Mode Water (NPCMW) (Nakamura, 1996; Suga et al., 1997). The mode water is characterized as the potential vorticity (PV) minimum (Hanawa and Talley, 21). Among them, NPCMW is colder and denser. This is distributed in the central North Pacific, approximately 16 E to 15 W between the Kuroshio Extension Front (KEF), defined as 12 C isotherm at 3 m depth (Kawai, 1972) and the Kuroshio Bifurcation Front (KBF), defined as 6 8 C isotherm at 3 m depth (Mizuno and White, 1983), with potential density (s q ) between 26. and 26.5 and with PV less than m 1 sec 1 (Nakamura, 1996). In *Corresponding author ( ttokieda@mri-jma.go.jp) Copyright 24 by The Geochemical Society of Japan. this region the winter mixed layer is thicker than anywhere else in the North Pacific (Reid, 1982). The NPCMW is derived from the mixed layer in winter. Therefore, it is expected that a large amount of atmospheric gaseous components are shipped to the water with lower temperature of the deepened mixed layer during the formation and then transported to the ocean interior. However, little is known for the role of mode water on the transport of gases and others. Warner et al. (1996) and Mecking and Warner (21) focused on the relationship between the CFCs and oxygen maxima in the subsurface water and the NPSTMW, NPCMW and the North Pacific Eastern Subtropical Mode Water (NPESTMW, Hautala and Roemmich, 1998; Ladd and Thompson, 2). They found that the CFC maxima and subsurface oxygen maxima coincided with the PV minima associated with the NPSTMW in the western North Pacific. In a new light, because mode water is vertically homogeneous and is not greatly influenced by mixing with ambient water during the subsequent advection, a nature in the water of the mixed layer in winter of conservative compounds, such as CFCs, are well-conserved in mode water, when it is very difficult to observe by a vessel. This paper is the first attempt to get the information on the characteristic of the shipped CFC-12 and CFC-11 from the atmosphere to the water of the mixed layer in winter using CFC-12 and CFC-11 in the NPCMW. 643

2 P-13 P-14 Fig. 1. Map showing the schematic illustration of the near surface water masses, fronts and surface current systems in the Kuroshio-Oyashio Transition Area (Yasuda, 23). NPCMW: North Pacific Central Mode Water, NPSTMW: North Pacific Subtropical Mode Water, SAF: Subarctic Front, SAB: Subarctic Boundary, KBF: Kuroshio Bifurcation Front, KEF: Kuroshio Extension Front, EKC: East Kamchatka Current, OY: Oyashio Current, KB: Kuroshio Bifurcation and KE: Kuroshio Extension. DATA AND ANALYTICAL METHOD We utilized the CFCs data obtained from the World Ocean Circulation Experiment (WOCE) Hydrographic Program one-time survey along P-13 (165 E) in August to October 1992 and along P-14 (179 E) in July to September 1993 ( 1tim_pac.htm) for the analysis of CFC-12 and CFC-11 in the NPCMW (Fig. 1). Their precisions (1 standard deviation) for dissolved CFC-12 and CFC-11 measurements were about 1%. In this paper, we utilized the degree of CFCs saturation with respect to the atmospheric CFCs (% CFCs saturation), as one of the characteristics of the shipped CFCs from the atmosphere to the water of the mixed layer in winter. It was calculated as % CFCs saturation = [(CFCs) water ]/[(CFCs) sat. ] 1, where [(CFCs) water ] is a CFCs concentration in seawater and [(CFCs) sat. ] is a saturated CFCs concentration calculated using atmospheric mixing ratio and solubility of CFCs (Warner and Weiss, 1985). Data from the Conductivity-Temperature-Depth (CTD) sensors were used to calculate the PV. The PV were calculated using the CTD data with 2 db interval as PV = ( f q /r) (dr/dz), Fig. 2. The sections of temperature ( C) (a), potential density [s q ] (b) and potential vorticity ( 1 1 m 1 sec 1 ) [PV] (c) along 165 E during WOCE P-13 in In (a), the positions of the fronts are shown. KEF: Kuroshio Extension Front, KBF: Kuroshio Bifurcation Front and SAF: Subarctic Front. The Ocean Data View (Schlitzer, 22) is used to draw the sections. where f q is the Coriolis parameter at q N, r is the in-situ density of seawater and z is its depth. RESULTS AND DISCUSSIONS The North Pacific Central Mode Water during the WOCE campaign in early 199s The south-north sections of seawater temperature, s q and PV between 3 N and 5 N along 165 E (P-13) and 179 E (P-14) are shown in Figs. 2 and 3. The Kuroshio Extension Front located about 34 N and 36 N, and the Kuroshio Bifurcation Front located about 39 N and 44 N, in the sections of P-13 and P-14, respectively, following the definition by Kawai (1972) and Mizuno and White 644 T. Tokieda et al.

3 Fig. 4. The sections of CFC-12 (a) and CFC-11 (b) concentration in seawater along 165 E during WOCE P-13 in The Ocean Data View (Schlitzer, 22) is used to draw the sections. Fig. 3. As in Fig. 2 but along 179 E during WOCE P-14 in (1983). There were lower PV waters with relatively vertically homogeneous temperature around 3 m depth centered at 37 N in the section of WOCE P-13 and around 2 m depth between 35 N and 45 N in the section of WOCE P-14 between the Kuroshio Extension Front and the Kuroshio Bifurcation Front. The value of PV less than m 1 sec 1 have been defined as the NPCMW by Nakamura (1996) and Suga et al. (1997). In this study, however, the PV value less than m 1 sec 1 was used to define the NPCMW, to remove the influence by the mixing with ambient seawater after the formation of the mode water. The degree of CFCs saturation in the NPCMW during the WOCE campaign The sections of CFC-12 and CFC-11 concentration and its degree of saturation in seawater between 3 N and 5 N along 165 E (P-13) and 179 E (P-14) are shown in Figs. 4 to 7. The CFCs concentration and its degree of saturation are higher in the surface layer and decreased with depth. Like water temperature and potential density, the CFCs concentration and its degree of saturation showed smaller vertical gradient in the water with lower PV. The data of marine atmospheric CFCs in winter, when the most intensive CFCs exchange between the mode water and the atmosphere is considered to occur, are not available. Therefore, the degree of saturation of CFCs was calculated using the atmospheric CFCs estimated by Walker et al. (2). Considering ~3 %/yr of the atmospheric CFCs growth rate at the time, the uncertainty in the estimate of the atmospheric mixing ratio we used is estimated to be 5% at maximum. In addition, the uncertainties in the estimate of CFCs solubility (Warner and Weiss, 1985) and of the measurement of CFCs in seawater are 2~3% and 1%, respectively. Adding the variability of atmospheric pressure that influences the solubilities of gases, total uncertainty in the calculation of the degree of saturation of CFCs are estimated to be 8%. In the near-surface water, CFCs was slightly oversaturated (11 11%). Saturation anomalies for various dissolved gases in the near-surface water, which are deviations from solubility equilibrium with the atmosphere, have been reported by many investigators (Bieri et al., 1968; Craig and Weiss, 1971; Kester, 1975). This impli- CFCs under-saturation of CFCs in the wintertime mixed layer 645

4 Fig. 5. The sections of the degree of CFC-12 (a) and CFC-11 (b) saturation in seawater along 165 E during WOCE P-13 in The Ocean Data View (Schlitzer, 22) is used to draw the sections. cation will be discussed elsewhere. Below the surface, the degree of CFCs saturation is less than 1%. The mean degree of saturation of the NPCMW were calculated to be 75.5 ± 12.3% and 85.9 ± 6.3% for CFC-12 and 72.9 ± 1.6% and 82.6 ± 5.3% for CFC-11 during WOCE P-13 in 1992 and P-14 in 1993, respectively. Since the NPCMW is considered to be not significantly influenced by the mixing with ambient water, there are two reasons which can explain the significant under-saturation of CFCs observed. The one reason is that the lower PV water is old water and another is that the water has already been under-saturated when the NPCMW was formed. According to Warner et al. (1996), we estimated the apparent CFCs age from the degree of saturation of CFCs and the time history of atmospheric CFCs mixing ratio on the assumption that the water was saturated with CFCs (1%) when the water was formed. Because the mixing ratio of CFC-12 around the year 1984 was 76% of that in 1992 and around the year 1987 was 86% of that in 1993 and the mixing ratio of CFC-11 around the year 1984 was 73% of that in 1992 and around the year 1986 was 83% of that in 1993 (Walker et al., 2), the CFCs ages for the NPCMW were apparently estimated to be about 8 or 6 years for CFC-12 and 8 or 7 years for CFC-11. However, it is not acceptable that 6~8 years have elapsed since the water had subducted from the mixed layer, because Fig. 6. As in Fig. 4 but along 179 E during WOCE P-14 in ) the water existed at the depth of the wintertime mixed layer, 2) the sites of observation was in or near the formation area of the NPCMW, as has indicated by Nakamura (1996) and 3) it would be difficult for the water mass to keep the homogeneous state for 6~8 years. In other words, the CFCs age found in the NPCMW would not indicate the real age of water. The observed significant under-saturation in the NPCMW during WOCE P-13 and P-14 thus strongly suggests that the NPCMW has already been significantly under-saturated for CFCs since it has been formed. The simulation of the degree of saturation of CFC-12 in the mixed layer In order to examine if CFC-12 could be undersaturated in the wintertime mixed layer, we reconstruct the degree of saturation of CFC-12 in the water of the mixed layer. The temporal change in the CFC-12 standing stock in the mixed layer (DICFC ML ) can be expressed as DICFC ML = DVM + DGAS + DADV, where DVM is the net change in CFC standing stock by vertical mixing, DGAS is the net change by air-sea gas exchange and DADV is the net change by horizontal 646 T. Tokieda et al.

5 (a) MLD (m) SST ( C) 5 5 < > < 1993 > (b) 15 5 Fig. 7. As in Fig. 5 but along 179 E during WOCE P-14 in Wind Velocity (m/sec) 1 5 < > < 1993 > Gas Exchange Rate ( cm/ hr) advection. Though a lot of data for CFCs in seawater has been obtained by the effort of the WOCE campaign and others (Watanabe et al., 1994; Tokieda et al., 1996) in 199s, the spatially high-density data set for CFCs in seawater is still not available and CFCs is a transient tracer. And unfortunately, we do not have sufficient information on the horizontal advection and diffusion in this area at the present. Therefore, in this study, we tried to reconstruct the variation of the degree of saturation of CFC-12 in the mixed layer by one-dimensional simulation. A temporal change in the CFC-12 standing stock in the mixed layer by the vertical mixing was estimated using a simple entrainment model (Ishii et al., 21). This model has an assumption that the water under the mixed layer is entrained into the mixed layer when the mixed layer deepens, and the water at the bottom of the mixed layer leave from the mixed layer when the depth of mixed layer shallowens. Namely, when the mixed layer depth deepens (MLD(t + dt) MLD(t) > ), DVM = {MLD(t + dt) MLD(t)} CFC UML, and, when the mixed layer depth shallowens (MLD(t + dt) MLD(t) < ), DVM = {MLD(t + dt) MLD(t)} CFC ML (t), Fig. 8. The parameters at 179 E, 4 N in 1992 and 1993, the mixed layer depth in meter [MLD] (solid) and sea surface temperature in degree C [SST] (dashed) in (a) and wind velocity in m/sec (solid) and calculated gas exchange rate in cm/hr (dashed) in (b), used in the simulation of the degree of CFC-12 saturation in the water of the mixed layer. The sources are described in text. where MLD(t) is the thickness of the mixed layer at a time t, CFC ML (t) is CFC-12 concentration in the mixed layer at t and CFC UML is CFC-12 concentration under the mixed layer. A change in the CFC-12 standing stock in the mixed layer by air-sea gas exchange is expressed as CFC-12 flux using the gas exchange rate and the difference of mole fraction of CFC-12 between air and sea surface. That is, DGAS = k(t) (CFC air (t) CFC ML (t)/sol CFC (t)) where k(t) is a gas exchange coefficient for CFC-12 calculated with water temperature and wind velocity (Wanninkhof, 1992), CFC air (t) is atmospheric CFC-12 mixing ratio (Walker et al., 2) and Sol CFC (t) is a solubility of CFC-12 in the surface water (Warner and Weiss, 1985), at the time t. In this simulation, we utilized the monthly sea surface temperature, the mixed layer depth and the wind velocity from two sources over 24 months in 1992 and CFCs under-saturation of CFCs in the wintertime mixed layer 647

6 Simulated Degree of CFC-12 Saturation (%) % 11% 1% 9% 8% < > < > Fig. 9. The simulated variation of degree of CFC-12 saturation in the water of the mixed layer. The legends in the figure show the initial degree of CFC-12 saturation on 1992/1/ Data of the monthly mean sea surface temperature and the wind velocity were taken from the National Centers for Environmental Prediction and National Centers for Atmospheric Research (NCEP/NCAR) reanalysis (Kanlay et al., 1996). The monthly mean mixed layer depth was estimated as the depth where the water temperature was sea surface temperature minus 1. C and using the Scripps Institution of Oceanography (SIO) upper ocean temperature reanalysis (White, 1995) (Fig. 8). The simulation for the degree of saturation of CFC-12 in the mixed layer was conducted for a location at 4 N, 18 E and a period from January 1 in 1992 to December 31 in We started the simulation for the degrees of CFC-12 saturation in the mixed layer with various initial conditions on January 1 in 1992, assuming that the water below the mixed layer had a CFC-12 concentration equal to that of the surface water saturated in the previous midwinter (Fig. 9). The simulated degrees of CFC-12 saturation in the mixed layer from all the initial conditions coincided with each other after June, suggesting our model can obtain the result enough stabilized in run for half a year. The result showed a seasonal variation with undersaturation in winter and super-saturation in summer. This result is consistent with observational results that CFC- 12 is considered to be under-saturated in the mixed layer in winter. However, the degree of CFC-12 saturation in the lower PV water at 179 E in 1993 during WOCE P-14 was about 9%. Such a large under-saturation is not reproduced in this simulation. The change in degree of saturation by gas exchange is insensitive. It is possibly due to the assumption that the water below the mixed layer had a CFC-12 concentration equal to that of the surface water saturated in the previous mid-winter. Actually, there is not the water with 1% of saturation for CFC-12 below the mixed layer. Simulated Degree of CFC-12 Saturation (%) % 9% 8% 7% 5% < > < > Fig. 1. The simulated variation of degree of CFC-12 saturation in the water of the mixed layer. The legends in the figure show the ratios of concentration under the mixed layer to the concentration in the surface water saturated with atmospheric CFC-12 in last mid-winter. In the next, we performed the simulation of the degree of CFC-12 saturation in the mixed layer with various concentrations below the mixed layer to reproduce a large under-saturation in the wintertime mixed layer (Fig. 1). When the water below the mixed layer was saturated with CFC-12 under the condition of the previous midwinter, the degree of saturation is indicated by 97%. To reconstruct 85% of CFC-12 saturation in the wintertime mixed layer, the water beneath the mixed layer has to be less than 5% of saturation of CFC-12. However, the degree of CFC-12 saturation observed in the wintertime mixed layer (~35 m) was larger than 6% (Fig. 5 or 7). It is necessary that the water with lower CFC-12 saturation is derived by horizontal advection/mixing, as well as the vertical mixing in winter. When a water mass with saturated with CFCs was mixed with other water mass with different temperature and with saturated with CFCs, a water mass formed has over-saturated CFCs (Wada and Hattori, 1991), because of the non-linearity of the CFCs solubility against the water temperature (Warner and Weiss, 1985). Therefore, when we consider the possibility of the horizontal mixing for the lower CFCs saturation in the NPCMW, it needs the existence of a water mass with considerably lower CFCs saturation around the Kuroshio-Oyashio Transition Area. The monthly changes in CFC-12 concentration in the mixed layer due to vertical mixing and gas exchange VM + GAS and in the monthly change in saturated CFC- 12 concentration due to the change in water temperature SATURATE are shown in Fig. 11. When VM + GAS exceeds SATURATE, the degree of CFC-12 saturation increases. The period when VM + GAS is less than SATURATE is usually the period when sea surface tem- 648 T. Tokieda et al.

7 Concentration Changes in the Mixed Layer (pmol/kg/month) VM+GAS SATURATE < > < > Fig. 11. The variations of simulated monthly CFC-12 concentration change by vertical mixing plus gas exchange [VM + GAS] and monthly concentration change by change in temperature in the surface water. perature decreases, i.e., saturated CFC-12 concentration increases. In this period, the increase of CFC-12 due to the influx from the atmosphere is smaller than the increase of CFC-12 saturation concentration by cooling of surface water. The method of partial pressure of CFC (p CFC ) for the estimation of CFC age since leaving the source region (Doney and Bullister, 1992) has a critical assumption that CFC was saturated in the mixed layer in winter. If CFC is under-saturated in the surface or the mixed layer water, the CFC age is over-estimated and the formation rate is under-estimated for the NPCMW. SUMMARY AND CONCLUSION We have obtained the following results: 1) CFCs concentration as well as water temperature and potential density showed small vertical gradient in the NPCMW with lower PV. CFCs in the NPCMW was significantly (73~85%) under-saturated with respect to the atmospheric CFCs. It is considered that CFCs has been under-saturated in the wintertime mixed layer. If such CFCs under-saturation in winter generally happened in the regimes of ventilation, CFC age in the interior of the ocean as estimated from the CFCs concentration should have been over-estimated for 6 to 8 years. 2) A simple one-dimensional model in the mixed layer showed the under-saturation of CFC-12 in winter and over-saturation in summer. This variation is caused by the slower air-sea gas exchange that can not respond to the faster change in the CFCs saturation concentration due to the seasonal change in the mixed layer temperature. Acknowledgments The authors would like to express their sincere thanks to members of the Geochemical Research Department, MRI, for their encouragement throughout this work. We also thank Prof. H. Y. Inoue, Hokkaido University, for his helpful and useful discussions. The constructive comments of the anonymous reviewers are also gratefully acknowledged. This work is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through Grantin-Aid. REFERENCES Bieri, R. H., Koide, M. and Goldberg, E. D. (1968) Noble gas contents of marine waters. Earth Planet. Sci. Lett. 4, Craig, H. and Weiss, R. F. (1971) Dissolved gas saturation anomalies and excess helium in the ocean. Earth Planet. Sci. Lett. 1, Doney, S. C. and Bullister, J. L. (1992) A chlorofluorocarbon section in the eastern North Atlantic. Deep-Sea Res. 39, 1,857 1,883. Hanawa, K. and Talley, L. D. (21) Mode Water. Ocean Circulation and Climate: Observing and Modelling the Global Ocean (Siedler, G., Church, J. and Gould, J., eds.), , Academic Press, London. Hautala, S. L. and Roemmich, D. H. (1998) Subtropical mode water in the Northeast pacific basin. J. Geophys. Res. 13, 13,55 13,66. Ishii, M., Inoue, H. Y., Matsueda, H., Saito, S., Fushimi, K., Nemoto, K., Yano, T., Nagai, H. and Midorikawa, T. (21) Seasonal variation in total inorganic carbon and its controlling processes in surface waters of the western North Pacific subtropical gyre. Mar. Chem. 75, Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R. and Joseph, D. (1996) The NCEP/NCAR 4-year reanalysis project. Bull. Am. Meteorol. Soc. 77, Kawai, H. (1972) Hydrography of the Kuroshio Extension. Kuroshio, Its Physical Aspects (Stommel, H. and Yoshida, K., eds.), , Univ. of Tokyo Press, Tokyo. Kester, D. R. (1975) Dissolved gases other than CO 2. Chemical Oceanography, Vol. 1 (Riley, J. P. and Skirrow, G., eds.), , Academic Press, New York. Ladd, C. and Thompson, L. (2) Formation mechanisms for North Pacific central and eastern subtropical mode waters. J. Phys. Oceanogr. 3, Masuzawa, J. (1969) Subtropical Mode Water. Deep-Sea Res. 16, Masuzawa, J. (1972) Water characteristics of the North Pacific central region. Kuroshio, Its Physical Aspects (Stommel, H. and Yoshida, K., eds.), , Univ. of Tokyo Press, Tokyo. McCartney, M. S. (1982) The subtropical recirculation of Mode Waters. J. Mar. Res. 4 (Suppl.), Mecking, S. and Warner, M. J. (21) On the subsurface CFC maxima in the subtropical North Pacific thermocline and their relation to mode waters and oxygen maxima. J. Geophys. Res. 16, 22,179 22,198. CFCs under-saturation of CFCs in the wintertime mixed layer 649

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