Geochemical Journal, Vol. 38, pp. 643 to 650, 2004
Chlorofluorocarbons (CFCs) in the North Pacific Central Mode Water:
Possibility of under-saturation of CFCs in the wintertime mixed layer
TAKAYUKI T OKIEDA,1* MASAO ISHII ,1 T AMAKI YASUDA1 and K AZUTAKA ENYO2
1
2
Meteorological Research Institute, Nagamine 1-1, Tsukuba, Ibaraki 305-0052, Japan
Climate and Marine Department, Japan Meteorological Agency, Otemachi 1-3-4, Chiyoda-ku, Tokyo 100-8122, Japan
(Received November 4, 2003; Accepted October 7, 2004)
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 ± 10.6% 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
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
(2001) 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, 2000). 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.
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 KuroshioOyashio 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, 1990, 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, 2001). Among them,
NPCMW is colder and denser. This is distributed in the
central North Pacific, approximately 160∞E to 150∞W
between the Kuroshio Extension Front (KEF), defined as
12∞C isotherm at 300 m depth (Kawai, 1972) and the
Kuroshio Bifurcation Front (KBF), defined as 6–8∞C isotherm at 300 m depth (Mizuno and White, 1983), with
potential density (sq) between 26.0 and 26.5 and with
PV less than 2.0 ¥ 10–10 m–1sec–1 (Nakamura, 1996). In
*Corresponding author (e-mail: [email protected])
Copyright © 2004 by The Geochemical Society of Japan.
643
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, 2003).
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 (http://whpo.ucsd.edu/data/tables/onetime/
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.] ¥ 100,
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 = ( fq/r) ¥ (dr/dz),
644 T. Tokieda et al.
Fig. 2. The sections of temperature ( ∞C) (a), potential density
[sq] (b) and potential vorticity (¥ 10–10 m–1sec–1) [PV] (c) along
165∞E during WOCE P-13 in 1992. 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, 2002) is used to draw the sections.
where fq 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 1990s
The south-north sections of seawater temperature, sq
and PV between 30∞N and 50∞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
Fig. 4. The sections of CFC-12 (a) and CFC-11 (b) concentration in seawater along 165∞E during WOCE P-13 in 1992. The
Ocean Data View (Schlitzer, 2002) is used to draw the sections.
Fig. 3. As in Fig. 2 but along 179∞ E during WOCE P-14 in
1993.
(1983). There were lower PV waters with relatively vertically homogeneous temperature around 300 m depth
centered at 37∞N in the section of WOCE P-13 and around
200 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
2.0 ¥ 10–10 m –1sec–1 have been defined as the NPCMW
by Nakamura (1996) and Suga et al. (1997). In this study,
however, the PV value less than 1.5 ¥ 10–10 m–1sec–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 30∞N and
50∞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. (2000). 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 (101–110%). 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
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
1992. The Ocean Data View (Schlitzer, 2002) is used to draw
the sections.
cation will be discussed elsewhere. Below the surface,
the degree of CFCs saturation is less than 100%. 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 ±
10.6% 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
(100%) 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., 2000), 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
646 T. Tokieda et al.
Fig. 6. As in Fig. 4 but along 179∞ E during WOCE P-14 in
1993.
1) 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 (DICFCML) can be expressed as
DICFCML = 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
(a)
300
25
250
20
15
150
10
SST (°C)
MLD (m)
200
100
5
50
0
<
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><
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>
0
Year
Fig. 7. As in Fig. 5 but along 179∞ E during WOCE P-14 in
1993.
15
50
40
10
30
20
5
10
0
<
19 92
><
1993
>
Gas Exchange Rate (cm/ hr)
Wind Velocity (m/sec)
(b)
0
Year
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
1990s, 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., 2001). 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) > 0),
DVM = {MLD(t + dt) – MLD(t)} ¥ CFCUML,
and, when the mixed layer depth shallowens (MLD(t +
dt) – MLD(t) < 0),
DVM = {MLD(t + dt) – MLD(t)} ¥ CFCML(t),
Fig. 8. The parameters at 179 ∞E, 40∞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, CFCML(t) is CFC-12 concentration in the mixed
layer at t and CFCUML 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) ¥ (CFCair(t) – CFCML(t)/SolCFC(t))
where k(t) is a gas exchange coefficient for CFC-12 calculated with water temperature and wind velocity
(Wanninkhof, 1992), CFCair(t) is atmospheric CFC-12
mixing ratio (Walker et al., 2000) and SolCFC(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
120
110
100
120%
110%
100%
90%
80%
90
80
70
<
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><
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>
Year
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 1992/1/1.
1993. 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 1.0∞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 40∞N,
180∞E and a period from January 1 in 1992 to December
31 in 1993.
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 CFC12 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 90%. 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 100% of saturation for CFC-12 below the mixed layer.
648 T. Tokieda et al.
Simulated Degree of CFC-12 Saturation (%)
Simulated Degree of CFC-12 Saturation (%)
100%
90%
80%
70%
50%
130
110
105
100
95
90
85
<
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><
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>
Year
Fig. 10. 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.
10). 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 50% of saturation of CFC-12. However, the degree of CFC-12 saturation observed in the wintertime
mixed layer (~350 m) was larger than 60% (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 CFC12 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-
Concentration Changes
in t he Mixed Layer (pmol/ kg/m onth)
partment, 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.
0.2
0.1
0
-0.1
-0.2
-0.3
VM+GAS
SATURATE
<
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><
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REFERENCES
>
Year
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 (pCFC) 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 De-
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