Marine Chemistry 75 Ž2001. 17–32
www.elsevier.nlrlocatermarchem
Seasonal variation in total inorganic carbon and its controlling
processes in surface waters of the western North Pacific
subtropical gyre
Masao Ishii a,) , Hisayuki Y. Inoue a , Hidekazu Matsueda a , Shu Saito a ,
Katsuhiko Fushimi a , Kazuhiro Nemoto a , Toshihiko Yano b, Hideki Nagai b,
Takashi Midorikawa b,1
a
Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan
b
Climate and Marine Department, Japan Meteorological Agency, Tokyo 100-8122, Japan
Received 15 June 2000; received in revised form 28 February 2001; accepted 7 March 2001
Abstract
Seasonal variation in total inorganic carbon ŽTCO 2 . in surface waters of the western North Pacific Ž1378–1528E.
subtropical gyre was analyzed on the basis of measurements of TCO 2 and partial pressure of CO 2 Ž pCO 2 sw.. The
controlling processes including vertical mixing, horizontal advection, and net air–sea CO 2 transport, as well as biological
activity, were quantified. The seasonal increase in normalized TCO 2 ŽNTCO 2 . from autumn to winter, ranging from 19 to 37
mmol kgy1 in the northern part of the subtropical gyre between 248N and 308N, was predominantly accounted for by the
upward supply of TCO 2 due to enhanced vertical mixing. The contribution of horizontal advection, estimated from monthly
meridional NTCO 2 distributions and the monthly advection field of the Meteorological Research Institute ŽMRI.’s 3D-ocean
general circulation model, was insignificant. Analyses of the mixed-layer NTCO 2 budget revealed that biological activity
was playing an important role in the decrease in surface NTCO 2 from winter to summer. Annual net community production
reached 48 " 19 gC my2 between 248N and 308N, and 19 " 16 gC my2 between 158N and 238N. q 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Total inorganic carbon; North Pacific subtropical gyre; Seasonal variability; Net community production
1. Introduction
Carbon dioxide ŽCO 2 . in the atmosphere is closely
linked to the CO 2 in the ocean, and the ocean has
)
Corresponding author. Tel.: q81-298-53-8727; fax: q81-29853-8728.
E-mail address: [email protected] ŽM. Ishii..
1
Present address: Hakodate Marine Observatory, Hakodate
041-0806, Japan.
been widely regarded as an important sink for anthropogenic CO 2 Že.g. Takahashi et al., 1997; Rayner
et al., 1999.. A long-term increase in the CO 2 partial
pressure in surface seawater Ž pCO 2 sw. has been
observed in winter along the 1378E meridian in the
western North Pacific subtropical gyre; its growth
rate since 1984 between 158N and 308N is 1.8 " 0.6
matm yeary1 , which is similar to the growth rate of
the atmospheric CO 2 concentration ŽInoue et al.,
1995..
0304-4203r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 0 3 Ž 0 1 . 0 0 0 2 3 - 8
18
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
Large seasonal variability of pCO 2 sw has also
been documented for this region ŽInoue et al., 1987,
1995; Inoue and Sugimura, 1988; Murata and
Fushimi, 1996; Murata et al., 1998.. In winter,
pCO 2 sw is lower than CO 2 partial pressure in marine boundary air Ž pCO 2 air. over the western North
Pacific subtropical gyre; D pCO 2 Žs pCO 2 sw y
pCO 2 air. reaches approximately y60 matm between
258N and the Kuroshio Ž; 348N.. However, D pCO 2
ranges from 0 to 50 matm and exhibits large spatial
variability in summer. Therefore, with seasonal variability in mean wind speed taken into account, it has
been thought that the western North Pacific subtropical gyre acts as a strong CO 2 sink in winter Ž; y16
mmol my2 dayy1 . and a moderate source in summer
Ž; 4 mmol my2 dayy1 ..
The pCO 2 sw varies as a function of temperature,
salinity, total inorganic carbon ŽTCO 2 . and alkalinity. The large seasonal variation in sea-surface temperature ŽSST., from about 198C in winter to about
308C in summer, is an important factor in the large
seasonal variation in pCO 2 sw in this region. However, the amplitude of seasonal variation in pCO 2 sw
Žca. 100 matm. is no more than 70% of that calculated from the pCO 2 sw–temperature relationship
under conditions of constant salinity, TCO 2 and total
alkalinity ŽTA. Žthermodynamic temperature effect:
about 4.2% increase in pCO 2 sw per 18C.. Consequently, it has been deduced that the variation in
TCO 2 andror TA also has a significant effect on the
seasonal pCO 2 sw variation in this region ŽInoue et
al., 1995..
A significant contribution of variation in TCO 2
andror TA to the seasonal variation in pCO 2 sw in
the subtropics has also been suggested for the southwestern Indian Ocean ŽGoyet et al., 1991; Poisson et
al., 1993; Metzl et al., 1998. and for other regions in
the Pacific Ocean ŽStephens et al., 1995; Wong et
al., 1995; Landrum et al., 1996. from the apparent
pCO 2 sw–SST relationships. Large seasonal variations in TCO 2 in the upper water columns of the
subtropics have been observed at ocean time-series
stations in the northwestern Sargasso Sea off
Bermuda ŽKeeling, 1993; Michaels et al., 1994; Bates
et al., 1996. and in the central North Pacific off
Hawaii ŽWinn et al., 1994, 1998.. In contrast, seasonal TCO 2 variation in the subtropics was suggested to be insignificant in the tropical limbs of the
subtropical gyre in the central North and South
Pacific ŽWeiss et al., 1982..
It is important that we characterize the spatial and
temporal variation in oceanic CO 2 parameters and
their controlling processes in all the oceans in order
to evaluate the present status of net air–sea CO 2
transport and related phenomena in the carbon cycle,
as well as to enable prediction of future changes in
response to anthropogenic perturbations and climate
change. Seasonal and interannual dynamics of CO 2
parameters including TCO 2 have, however, been
documented only for limited regions of the global
oceans; the processes controlling pCO 2 sw variation
in the vast subtropics have yet to be understood.
We measured TCO 2 and pCO 2 sw, along with
hydrographic properties, in surface waters of the
western North Pacific subtropical gyre Ž1378–1528E.
between the equatorial region and the Kuroshio Ž;
348N. during five cruises conducted between July
1994 and October 1997. In this paper, we report our
results on the meridional distribution and seasonal
variations of these parameters of CO 2 chemistry in
surface waters. We then analyze the monthly distribution of surface NTCO 2 between 158N and 308N
along 1378E by coupling the observational results
with the empirical relationships between pCO 2 sw
and SST given by Inoue et al. Ž1995. and data sets of
monthly SST and sea-surface salinity. The roles of
vertical mixing, horizontal advection, air–sea CO 2
transport, and biological activity on the seasonal
variation in surface NTCO 2 are also quantified on
the basis of a simple transport model and a calculated budget of monthly NTCO 2 changes. Our study
provides an overview of seasonal variation in surface
NTCO 2 and the controlling processes that have a
significant effect on the seasonal variation in
pCO 2 sw in the western North Pacific subtropical
gyre.
2. Methods
We measured surface pCO 2 sw and TCO 2 along
transects in the western North Pacific once in summer, twice in autumn, and twice in winter ŽFig. 1..
Cruises RY9407 Žwhich was identified as the WOCE
section P9 one-time cruise ŽKaneko et al., 1998..,
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
Fig. 1. Map of sampling sites for total inorganic carbon in surface
seawater along the transects of RY9407 ŽJuly 9 to August 18,
1994. Ž`., KH-94-4 southbound leg ŽNovember 24–28, 1994.
Ž'., KH-94-4 northbound leg ŽFebruary 6–12, 1995. Žq.,
RY9701 ŽJanuary 24 to February 1, 1997. ŽI., and RY9709
ŽOctober 20–31, 1997. Žl.. Approximate positions of the major
oceanic currents are superposed, but they vary with time. STCC:
subtropical countercurrent; NEC: north equatorial current; NECC:
north equatorial countercurrent.
RY9701, and RY9709 were conducted aboard the
Japan Meteorological Agency’s RrV Ryofu-maru,
and cruise KH-94-4 was made aboard the University
of Tokyo’s RrV Hakuho-maru.
Underway pCO 2 sw measurements were made using a showerhead-type equilibrator and non-dispersive infrared gas analyzer described by Inoue and
Sugimura Ž1992. and Inoue Ž1999.. TCO 2 was measured by coulometric analysis ŽJohnson et al., 1985,
1987. using an automated CO 2 extraction unit ŽIshii
et al., 1998. and a coulometer Žmodel 5012, UIC,
USA.. For quality control, we used the Certified
Reference Material ŽCRM. for TCO 2 analysis
Žbatches 20, 25, 35. provided by Dr. A.G. Dickson
19
of Scripps Institution of Oceanography and a reference material we prepared from a batch of western
North Pacific oligotrophic water by a similar method
to that of Dickson Ž1991.. Precision Ž"1s . of analysis was estimated to be "1.1 mmol kgy1 in RY9407,
"2.5 mmol kgy1 in KH-94-4, "1.6 mmol kgy1 in
RY9701, and "1.0 mmol kgy1 in RY9709. Differences in our analytical results for the CRMs from
their certified values were y0.7 " 1.4 Ž1 s . mmol
kgy1 during these cruises.
During cruises RY9407, RY9701 and RY9709 of
RrV Ryofu-maru, surface seawater was pumped up
continuously from the bottom of the ship Ž; 5 m
depth. and analyzed hourly for pCO 2 sw. In RY9407
and RY9701, discrete samples for TCO 2 analysis
were taken from Niskin bottles on a CTDrrosette
sampler together with samples for the analysis of
salinity and other dissolved components such as
nutrients, and were analyzed on board immediately.
In RY9407, a vertical profile of TCO 2 was also
obtained at each hydrographic station. In RY9709,
samples for TCO 2 analysis were taken from the
seawater pumped up continuously from the bottom
of the ship. They were poisoned with mercuryŽII.
chloride and analyzed after the cruise in the laboratory on land. In cruise KH-94-4 of RrV Hakuhomaru, surface seawater pumped up continuously from
the bottom of the ship Ž; 5 m depth. was used for
the underway concurrent analyses of pCO 2 sw and
TCO 2 , which were made twice every 1.5 h, and for
SST and salinity measurements that were recorded at
1-min intervals.
We calculated TA in surface seawater from
pCO 2 sw, TCO 2 , temperature and salinity using the
solubility of CO 2 in seawater given by Weiss Ž1974.,
dissociation constants of carbonic acid given by Roy
et al. Ž1993. and equilibrium constants for other
acids and bases recommended in DOE Ž1994..
3. Results
3.1. Meridional distribution and seasonal Õariation
in NTCO2
Analytical results of TCO 2 and calculated values
of TA were normalized to a constant salinity Ž S s 35.
20
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
in order to remove the influence of variation in
precipitationrevaporation. Figs. 2 and 3 show the
summer–autumn and winter meridional distribution
of NTCO 2 , pCO 2 sw, NTA, temperature, and salinity in surface waters.
In summer ŽJuly–August 1994., surface NTCO 2
decreased gradually from 1945 mmol kgy1 at the
northern edge of the north equatorial countercurrent
around 58N down to 1932 mmol kgy1 at 178N, and
then began to increase northward up to 1960 mmol
kgy1 at 308N ŽFig. 2Ža... Surface NTCO 2 in the
stream of the Kuroshio at 32.88N Ž1945 mmol kgy1 .
was distinctly lower than that at 308N.
In winter ŽFebruary 1995., surface NTCO 2 again
decreased gradually from 1960 mmol kgy1 at the
northern edge of the NECC at around 58N down to
1945 mmol kgy1 at 158N ŽFig. 3Ža... Between 158N
and 208N, surface NTCO 2 was fairly constant, but
Fig. 2. Summer and autumn distributions in surface seawater of the western North Pacific of Ža. total inorganic carbon normalized at S s 35
ŽNTCO 2 .; Žb. partial pressure of CO 2 Ž pCO 2 sw.; Žc. total alkalinity normalized at S s 35 ŽNTA.; Žd. temperature; and Že. salinity.
Symbols denote sampling sites shown in Fig. 1.
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
21
Fig. 2 Ž continued ..
increased sharply northward from 208N to reach
1995 mmol kgy1 at around 308N immediately to
south of the Kuroshio. The meridional distribution of
surface NTCO 2 in early February 1995 was in agreement with that in late January 1997, within the
uncertainty of the analysis and in spite of their
differences in longitude, salinity, and year of sampling. The only exception was at 108N where a large
negative temperature anomaly was observed in January 1997 at depths shallower than 100 m due to a
cyclonic eddy caused by a stationary tropical cyclone
ŽJMA, 1997..
There was a significant seasonal variation in the
distribution of surface NTCO 2 . In the southern part
of the subtropical gyre between 78N and 208N where
a northward gradual decrease was observed, surface
NTCO 2 in winter was approximately 10 mmol kgy1
higher than that in summer ŽTable 1.. In the northern
part of the gyre where a northward increase was
observed, surface NTCO 2 in winter was also higher
than that in summer and their difference became
larger in higher latitudes Žapproximately 30 mmol
kgy1 at 308N.. In autumns ŽNovember 1994 and
October 1997., surface NTCO 2 was comparable to
values in summer or between those in summer and in
winter. The zone of the surface NTCO 2 minimum
observed to be around 158N in winter, moved a few
degrees north in summer.
3.2. Meridional distribution and seasonal Õariation
in pCO2 sw and NTA
In summer ŽJuly–August 1994., pCO 2 sw was
comparable to pCO 2 air in the southern part of the
subtropical gyre south of 208N, and was significantly
higher than pCO 2 air north of 208N ŽFig. 2Žb... The
D pCO 2 reached its maximum of 53 matm at around
278N. Between 208N and 278N, pCO 2 sw had a
tendency to increase northward with increasing surface NTCO 2 . However, pCO 2 sw decreased with
decreasing SST between 278N and 308N.
In winter Žin February 1995 and in January 1997.,
pCO 2 sw was comparable to pCO 2 air south of 128N
ŽFig. 3Žb... At the north of 128N, pCO 2 sw decreased
22
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
gradually toward the north with decreasing SST. The
D pCO 2 at around 308N immediately south of the
Kuroshio was largely negative Žy40 to y65 matm.,
indicating a remarkable CO 2-undersaturation of surface water, in spite of the highest surface NTCO 2 .
These results are consistent with previous reports
ŽInoue et al., 1987, 1995; Murata and Fushimi, 1996.
that have demonstrated the significant seasonal vari-
ability in pCO 2 sw in the western North Pacific
subtropical gyre.
In contrast with the distinct variation in surface
NTCO 2 and pCO 2 sw, spatial and temporal variation
in surface NTA was small and within the uncertainty
of the analysis Žsee Table 1, Figs. 2Žc. and 3Žc... The
uniformity of surface NTA in the subtropics has
been reported by Millero et al. Ž1998. on the basis of
Fig. 3. Winter distributions in surface seawater of the western North Pacific of: Ža. total inorganic carbon normalized at S s 35 ŽNTCO 2 .;
Žb. partial pressure of CO 2 Ž pCO 2 sw.; Žc. total alkalinity normalized at S s 35 ŽNTA.; Žd. temperature; and Že. salinity. Symbols denote
sampling sites shown in Fig. 1.
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
23
Fig. 3 Ž continued ..
their worldwide determinations by titration. The NTA
we calculated from TCO 2 and pCO 2 sw Ž2316 meq
kgy1 on average. was larger than that reported by
Millero et al. Ž1998. Ž2300 meq kgy1 .. The reason
for this discrepancy is not clear. Since the TArTCO 2
ratio in surface water of the western North Pacific
subtropical gyre is large Ž1.17–1.20., this discrepancy might be due to uncertainty in the dissociation
constants of carbonic acid used for the calculation
ŽLee et al., 1997..
Table 1
Averages of salinity-normalized Žat S s 35. total inorganic carbon ŽNTCO 2 . and total alkalinity ŽNTA. and partial pressure of CO 2 in
surface waters of the southern part and northern part of the western North Pacific subtropical gyre; mean " 1 s Žnumber of measurements.
Cruise
Month
Year
NTCO 2 Žmmol kgy1 .
NTA Žmeq kgy1 .
pCO 2 sw Žmatm.
7–208N
24–308N
7–208N
24–308N
7–208N
24–308N
1955 " 8 Ž3.
2322 " 4 Ž3.
2316 Ž1.
350.4 " 5.5 Ž154.
341.9 " 11.1 Ž74.
344.4 " 3.0 Ž51.
380.1 " 8.6 Ž69.
332.4 " 5.8 Ž36.
336.3 " 9.7 Ž26.
2312 " 6 Ž26.
2315 " 0 Ž2.
335.2 " 9.3 Ž78.
333.4 " 13.0 Ž85.
308.1 " 3.6 Ž31.
300.7 " 2.6 Ž41.
Summer and autumn
RY9407 July–Aug
KH-94-4 Nov
RY9709 Oct
1994
1994
1997
1937 " 4 Ž7.
1939 " 4 Ž53.
1940 " 2 Ž6.
1954 Ž1.
2318 " 3 Ž7.
2316 " 4 Ž26.
2319 " 2 Ž6.
Winter
KH-94-4
RY9701
1995
1997
1948 " 3 Ž60.
1949 " 8 Ž3.
1978 " 11 Ž26.
1973 " 13 Ž2.
2317 " 4 Ž60.
2313 " 1 Ž3.
Feb
Jan–Feb
24
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
4. Discussion
expressed as a function of surface NTCO 2 at time t
and the change in NTCO 2 :
4.1. Effects of seasonal Õariation in SST and surface
NTCO2 on pCO2 sw
NTCO 2 Ž t q 1 . s NTCO 2 Ž t . q DNTCO 2 Ž t .
As presented in Section 3.2, variation in surface
NTA was small in the western North Pacific subtropical gyre. Therefore, spatial and seasonal variation in
pCO 2 sw in this basin was basically attributable to
the combined effect of the variations in SST and
surface NTCO 2 , and somewhat to variation in salinity. As surface NTCO 2 was lower in summer than in
winter, the effect of seasonal NTCO 2 variation on
pCO 2 sw counteracted the thermodynamic effect of
seasonal SST variation. In the southern part of the
subtropical gyre between 78N and 208N, the difference in SST between summer and winter was 3.5 "
1.58C and that in surface NTCO 2 was 13 " 6 mmol
kgy1 . Computation of CO 2 chemistry for these
changes shows that the effect of seasonal variation in
SST was significant, but about 50% of the variation
was compensated for by that in surface NTCO 2 . In
the northern part of the gyre between 208N and
308N, the difference in surface NTCO 2 and SST
between summer and winter was 25 " 13 mmol kgy1
and 8.5 " 3.58C, respectively. In this region, compensation for pCO 2 sw variation due to the thermodynamic effect of SST variation by variation in
surface NTCO 2 was 30% on average. It is evident
from these estimations that the variation in surface
NTCO 2 , as well as the variation in SST, is an
important factor in the seasonal variation in pCO 2 sw,
hence for the net air–sea CO 2 flux, in the western
North Pacific subtropical gyre.
AD V
DNTCO 2 Ž t . s DNTCOVM
Ž t.
2 Ž t . q DNTCO 2
4.2. Processes controlling the seasonal Õariation in
surface NTCO2
Temporal variation in surface NTCO 2 at a given
site in the subtropics is potentially attributable to the
following four factors: NTCO 2 change due to physical dynamics of the upper layer of the ocean such as
. and horizontal advecvertical mixing Ž DNTCOVM
2
AD V .
Ž
tion DNTCO 2
, NTCO 2 change due to net CO 2
transport across the air–sea interface Ž DNTCO 2AS .,
and NTCO 2 change due to biological activity
Ž DNTCO 2BIO .. Surface NTCO 2 at time t q 1 can be
Ž 1.
where
q DNTCO 2AS Ž t . q DNTCO 2BIO Ž t .
Ž 2.
In the following subsections, we present the
monthly distribution of surface NTCO 2 along 1378E
as deduced from the results of the observations and
climatological data sets. We then discuss the importance of each component in Eq. Ž2. for the seasonal
variation in surface NTCO 2 .
4.2.1. Monthly meridional distribution of surface
NTCO2 along 1378E
Our observations of surface NTCO 2 in the western North Pacific subtropical gyre, made five times
in different months between 1994 and 1997 ŽFigs.
2Ža. and 3Ža.., are insufficient to discuss the processes controlling its seasonal variation. Therefore,
we attempted to deduce the monthly distribution of
surface NTCO 2 by combining the results of the
observations with climatological data sets.
First, we evaluated pCO 2 sw at each 18 latitude
along 1378E for each month from monthly SST data
ŽJMA, 1993. and the empirical relationships between
pCO 2 sw normalized at the year 1990 and SST ŽTable 3 in Inoue et al., 1995.. The calculation was
made for the zones between 148N and 318N where
the correlation coefficients for pCO 2 sw vs. SST
were larger than 0.9. The monthly meridional distributions of pCO 2 sw along 1378E obtained were then
translated into monthly meridional distributions of
surface NTCO 2 in the unit of mmol kgy1 on the
assumption, derived from the observations, that surface NTA remains constant Ž2316 meq kgy1 . over
time and space. Normals of the monthly SST ŽJMA,
1993., monthly sea surface salinity ŽWOD, 1998.,
and equilibrium constants for CO 2 solubility ŽWeiss,
1974. and for the carbonate system in seawater ŽRoy
et al., 1993. were also used for the calculations.
The computed monthly surface NTCO 2 distribution ŽFig. 4. was consistent with the observed surface
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
25
in the formation of the Subtropical Mode Water
ŽMasuzawa, 1969; Suga and Hanawa, 1990.. Therefore, surface NTCO 2 would be affected by the
entrainment of subsurface water in the course of
deepening of the mixed layer. We estimated the
contribution of vertical mixing to the variation in
. with a simple entrainsurface NTCO 2 Ž DNTCOVM
2
ment model ŽFig. 5.. DNTCOVM
in the unit of mmol
2
kgy1 monthy1 between two successive months t
and t q 1 is expressed as
DNTCOVM
2 Ž t.
s
Fig. 4. Seasonal variation in the computed surface NTCO 2 Žmmol
kgy1 . in the western North Pacific subtropical gyre along 1378E.
Hz z
MLŽ tq1 .
MLŽ t .
NTCO2 Ž July, z . r Ž July, z .
yNTCO 2 Ž t , ML . r ML Ž t . 4 d zrz ML Ž t q 1 .
NTCO 2 ŽFigs. 2Ža. and 3Ža.. and reproduced the
observational results that Ž1. the meridional gradient
was larger in winter, reaching ca. 50 mmol kgy1
between 178N and 308N in February; Ž2. there was
larger seasonal variation in higher latitudes, with an
amplitude reaching ca. 35 mmol kgy1 at 308N; and
Ž3. a shallow trough was present between 158N and
208N in February, which moved northward in summer. The computed values for the meridional gradient between 208N and 308N in summer Ž20 mmol
kgy1 . and for the amplitude of seasonal variation
between 158N and 188N Ž3 to 5 mmol kgy1 . were
somewhat smaller than the observational values.
These discrepancies may be explained by error in the
computed NTCO 2 Ž"5 mmol kgy1 . due to error in
the regression analysis of pCO 2 sw on SST and
potentially by the interannual variability in surface
NTCO 2 . In addition, the computed surface values of
NTCO 2 shown in Fig. 4 were 2–15 mmol kgy1
lower than those observed during 1994–1997 Žsee
Figs. 2Ža. and 3Ža... Since the empirical relationships
between pCO 2 sw and SST were determined on the
basis of the pCO 2 sw values normalized to the year
1990 by correcting for the increasing trend of
pCO 2 sw, the offset in the computed surface NTCO 2
is a reflection of the increasing trend of pCO 2 sw.
4.2.2. Vertical mixing
The northwestern part of the North Pacific subtropical gyre has a deep mixed layer in winter due
to the strong East-Asian monsoon and it results
rr ML Ž t q 1 .
Ž 3.
where z ML Ž t . and z MLŽ t q 1. are the depths of the
mixed layer at month t and at month t q 1, respectively; NTCO 2 ŽJuly, z . and r ŽJuly, z . are the
NTCO 2 in mmol kgy1 and the density of seawater in
Fig. 5. A schematic representation of the one-dimensional entrainment model. The depth of the mixed layer z ML varies according
to the climatological data Žsee text.. At a depth below the mixed
layer, the NTCO 2 in July was assumed to be maintained until the
depth became incorporated into the mixed layer in the following
months. When the mixed layer deepens, subsurface water between
the base of the mixed layer at month t Ž z ML Ž t .. and the base of
the deepened mixed layer at month t q1 Ž z ML Ž t q1.. Žshaded
area., which is richer in NTCO 2 , is incorporated into the mixed
layer.
26
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
kg my3 at a depth z in July, NTCO 2 Ž t, ML. and
r ML Ž t . are those in the mixed layer at month t; and
r ML Ž t q 1. is the density of seawater in the mixed
layer at month t q 1. During months when the depth
of the mixed layer is unchanged or becomes shallower, DNTCOVM
was assumed to be negligible.
2
Monthly variation in the mixed layer depth was
taken from Hanawa and Hoshino Ž1988., Suga and
Hanawa Ž1990., and Fukasawa Žprivate communication. Žthese comprise the same data set shown in
ŽFig. 9 in Inoue et al., 1995.. The vertical profile of
NTCO 2 in July at each 18 latitude was evaluated
from the vertical temperature profile along 1378E in
July from climatological data ŽJMA, unpublished
data. and the relationship between NTCO 2 and temperature, which was derived from the observations in
July 1994 during cruise RY9407 ŽWOCE P9. ŽFig.
6..
along 1378E
The monthly variation in DNTCOVM
2
is shown in Fig. 7Ža.. From February through Au-
Fig. 6. Vertical sections of Ža. temperature Ž8C. and Žb. NTCO 2 Žmmol kgy1 . in the top 300 m along WOCE P9 observed during July 19 to
August 18, 1994.
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
27
Fig. 7. Seasonal variation in DNTCO 2 Žmmol kgy1 monthy1 . in the western North Pacific subtropical gyre along 1378E due to Ža. vertical mixing, Žb. horizontal advection, Žc.
air–sea CO 2 transport, and Žd. biological activity.
28
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
gust, DNTCOVM
was mostly set at zero because the
2
depth of the mixed layer is unchanged or becomes
shallower. However, from September through January when the mixed layer deepens, there were
significant contributions of DNTCOVM
in the north2
ern part of the subtropics. Between 248N and 308N
where the thickness of the mixed layer is 30 m in
mid-summer but deeper than 140 m in mid-winter,
the total DNTCOVM
from September through Jan2
uary was estimated to average 31 " 6 mmol kgy1 .
The uncertainty was estimated by changing the depth
of the mixed layer by "5 m, i.e. the resolution of
the mixed layer depth in the climatological data.
4.2.3. Horizontal adÕection
Since the contours of monthly SST are almost
parallel to longitude and pCO 2 sw distributions show
little longitudinal variation ŽInoue et al., 1995; Murata and Fushimi, 1996. in the northern part of the
western North Pacific subtropical gyre, we assumed
that surface NTCO 2 was also longitudinally uniform
and considered only the meridional component of the
horizontal advection. Temporal variation in surface
NTCO 2 due to horizontal advection at month t
Ž DNTCO 2AD V Ž t .. is expressed by the product of horizontal advection velocity Ž Õ Ž t .. in the unit of m
monthy1 and the meridional gradient of surface
NTCO 2 ŽENTCO 2rE y . t in mmol kgy1 my1 :
DNTCO 2AD V Ž t . s yÕ Ž t . Ž ENTCO 2rE y . t
Ž 4.
The meridional component of the velocity for
horizontal advection Õ Ž t . was taken from the monthly
advection field of the Meteorological Research Institute’s 3D-ocean general circulation model ŽObata,
submitted for publication. and averaged over the
mixed layer depth. In this model, advection in the
surface layers is basically the Ekman flow constrained by the monthly wind stress given by Hellerman and Rosenstein Ž1983.. The meridional gradient
of surface NTCO 2 at each latitude was derived from
the monthly distributions of surface NTCO 2 Žsee Fig.
4. for the zones between 158N and 308N.
In the surface layer Ž0–2.8 m., the meridional
component of the Ekman flow often exceeds 0.1 m
sy1 and has significant effect on horizontal transport
of the surface water in the northern part of this
region. However, the thickness of the Ekman boundary layer is confined to the top ca. 20 m, and when
the lateral flow including geostrophic flow was averaged over the mixed layer, its meridional component
was usually smaller than 0.05 m sy1 . Consequently,
estimates of DNTCO 2AD V Ž t . exceeded "3 mmol
kgy1 monthy1 only at the northern limb of the gyre;
usually horizontal advection had a minor effect on
the seasonal variation in NTCO 2 ŽFig. 7Žb...
4.2.4. Air–sea CO2 exchange
The western North Pacific subtropical gyre has
been well studied with regard to pCO 2 sw and
pCO 2 air. The net CO 2 flux across the air–sea interface Ž FCO 2Ž t .. has been estimated on a monthly basis
using the gas transfer coefficient given by Tans et al.
Ž1990. and climatological wind data ŽInoue et al.,
1995.. We computed the change in NTCO 2 due to
air–sea CO 2 exchange Ž DNTCO 2AS Ž t . in the unit of
mmol kgy1 monthy1 . from FCO 2Ž t . Žin mmol my2
monthy1 ., mixed layer depth z ML Ž t ., and density in
the mixed layer r ML Ž t .:
DNTCO 2AS Ž t . s FCO 2Ž t . rz ML Ž t . rr ML Ž t .
Ž 5.
Uncertainty in these estimates would primarily
arise from the uncertainty in the gas exchange coefficient for the calculation of FCO 2Ž t ., which
was assumed arbitrarily to be 50%. Estimates of
DNTCO 2AS Ž t . ranged between y4 and q6 mmol
kgy1 monthy1 ŽFig. 7Žc... In summer, the northwestern part of the North Pacific subtropical gyre is
a moderate source of atmospheric CO 2 Žsee Fig.
2Žb.., hence DNTCO 2AS Ž t . is negative. In contrast, it
becomes a significant sink for CO 2 in winter and
spring Žsee Fig. 3Žb... Therefore, DNTCO 2AS Ž t . becomes positive and opposes to the decreasing tendency of surface NTCO 2 in these seasons.
4.2.5. Biological actiÕity
The contribution of biological activity to the
monthly changes in surface NTCO 2 Ž DNTCO 2BIO Ž t .
in the unit of mmol kgy1 monthy1 . was estimated
by balancing the DNTCO 2 Ž t . budget for each month
ŽFig. 7Žd..:
DNTCO 2BIO Ž t .
s DNTCO 2 Ž t . y Ž DNTCOVM
2 Ž t.
qDNTCO 2AD V Ž t . q DNTCO 2AS Ž t . .
Ž 6.
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
Subsequently, monthly net community production
ŽNCPŽ t . in the unit of molC my2 or gC my2 . in
the mixed layer was computed by integrating
DNTCO 2BIO Ž t . over the mixed layer:
NCP Ž t . s DNTCO 2BIO Ž t . z ML Ž t . r ML Ž t .
Ž 7.
Although the NCP thus evaluated is subject to
large uncertainty, the northwestern North Pacific
subtropical gyre is shown to be productive from
winter to spring ŽFig. 8.. The pattern of temporal and
spatial variation in NCP is fairly consistent with that
of chlorophyll derived from satellite imagery such as
CZCS and SeaWiFS, which shows higher levels of
chlorophyll Ž) 0.1 mg my3 . to the north of 258N
from November through April than in other months
ŽIshizaka, 1993.. The mean annual NCP, which
should be equivalent to the mean net annual production of organic matter, showed a significant meridional variation, i.e. 48 " 19 gC my2 yeary1 between
248N and 308N and 19 " 16 gC my2 yeary1 between 158N and 238N ŽTable 2.. More than 90% of
the production occurs during the 6 months from
November through April.
The mean annual NCP between 158N and 308N
was 32 " 17 gC my2 yeary1 , comparable to the net
annual production of organic matter in the upper 100
m of the ocean at station ALOHA near Hawaii
Ž19 " 11 to 32 " 20 gC my2 yeary1 ; Emerson et al.,
1997.. This study agrees to their conclusion that the
organic carbon flux in the subtropical ocean has been
underestimated, and may be responsible for a signifi-
Fig. 8. Seasonal variation in net community production ŽgC my2
monthy1 . in the mixed layer in the western North Pacific subtropical gyre along 1378E.
29
Table 2
Net community production in the mixed layer and net primary
production in the western North Pacific subtropical gyre along
1378E
Latitude Net community
production ŽgC my2 .
Net primary
productiona ŽgC my2 .
Nov–April May–Oct Annual Annual
24–308N 44"17
15–238N 19"14
15–308N 30"15
a
4"6
0"8
2"7
48"19 109
19"16 59
32"17
Longhurst et al., 1995.
cant part of the global-ocean biological organic carbon pump.
4.2.6. Importance of each process to the seasonal
Õariation in surface NTCO2
The contributions of vertical mixing, horizontal
advection, air–sea CO 2 transport and biological activity to the seasonal decrease in surface NTCO 2
from February to August and to its seasonal increase
from September to January are summarized in Table
3. Values listed in Table 3 are the means for the zone
between 248N and 308N, where the largest seasonal
variation in surface NTCO 2 was observed.
One development in this study is the description
of variability in the meridional distribution and its
use for estimating the effect of horizontal advection.
However, as long as the mixed-layer-averaged advection field from the 3D-ocean general circulation
model is used, its effect on seasonal NTCO 2 variation is minor. The decrease in surface NTCO 2 Žy25
" 5 mmol kgy1 . from winter to summer could be
partitioned basically into net CO 2 transport from the
atmosphere Ž13 " 7 mmol kgy1 . and biological production Žy35 " 15 mmol kgy1 .. Although net CO 2
transport from the atmosphere to the ocean is mainly
attributable to the effect of temperature on the solubility of CO 2 , this result suggests that biological
production is playing an important role in reducing
CO 2 emission to the atmosphere in summer.
The increase in surface NTCO 2 Ž25 " 5 mmol
y1 .
kg
from summer to winter is mainly explained by
enhanced vertical mixing Ž31 " 6 mmol kgy1 .. In
this estimate, the seasonal evolution of the vertical
NTCO 2 profile below the mixed layer has not been
well constrained. This is another source of error in
M. Ishii et al.r Marine Chemistry 75 (2001) 17–32
30
Table 3
Changes in NTCO 2 Žmmol kgy1 . from winter to summer and from summer to winter in surface seawater of the western North Pacific
subtropical gyre along 1378E
Period
DNTCO 2
DNTCOVM
2
DNTCO 2ADV
DNTCO 2GAS
DNTCO 2BIO
24–308N
Feb–Aug
Sep–Jan
Annual
y25 " 5
25 " 5
1a
1"1
31 " 6
32 " 7
y3 " 2
0"1
y3 " 3
13 " 7
5"3
18 " 9
y35 " 15
y11 " 15
y46 " 19
15–238N
Feb–Aug
Sep–Jan
Annual
y5 " 5
5"5
1a
1"1
8"4
9"5
1"1
0"1
y1 " 2
5"3
3"2
8"4
y11 " 10
y6 " 12
y18 " 11
a
Calculated from the long-term increase in pCO 2 sw ŽInoue et al., 1995..
estimating the contribution of vertical mixing, which
should be minimized through future observations.
between 158N and 238N. More than 90% of the
production occurred November through April.
5. Conclusions
Acknowledgements
We have presented the results of the observations
on the seasonal variation in oceanic CO 2 in the
northern part of the western North Pacific subtropical gyre and inferred the processes controlling the
seasonal variation in surface NTCO 2 there. The main
conclusions can be stated as follows.
Ž1. In this region, meridional and seasonal variability in pCO 2 sw in surface seawater is mainly
determined by the variability in temperature and
surface NTCO 2 . The amplitude of seasonal variation
in surface NTCO 2 was 19–37 mmol kgy1 in the
northern part of the subtropical gyre between 248N
and 308N. It compensated for about 30% of the
effect of seasonal variation in SST on seasonal variation in pCO 2 sw.
Ž2. Decreases in the surface NTCO 2 from winter
to summer are ascribed to biological production.
They are partly compensated for by net CO 2 transport from the atmosphere. Increase in the surface
NTCO 2 from summer to winter is explained by
entrainment of subsurface water in the course of
mixed layer deepening. Horizontal advection has
only a minor effect on the seasonal variation in
surface NTCO 2 .
Ž3. The mean annual net community production in
the mixed layer was 48 " 19 gC my2 yeary1 between 248N and 308N and 19 " 16 gC my2 yeary1
We thank Drs. K. Kawaguchi and M. Terazaki,
the chief scientists of the KH-94-4 cruise, and the
officers and crew of the RrV Hakuho-maru and
RrV Ryofu-maru for their help on board. We also
express our appreciation to Dr. I. Kaneko, Mr. M.
Fujimura and Mr. A. Obata for their helpful suggestions and for providing the output of the MRI 3D-ocean general circulation model. The thoughtful comments of anonymous reviewers were most helpful in
revising the manuscript. Data obtained on the
RY9407 cruise ŽWOCE section P9 one-time cruise.
are available from the WOCE Hydrographic Programme Office ŽInternet: http:rrwhpo.ucsd.edur
datar.. We will make data from other cruises available from the Japan Oceanographic Data Center
ŽJODC. and the Carbon Dioxide Information Analysis Center ŽCDIAC..
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