2009 - decadal increase of oceanic carbon dioxide in southern indian ocean surface waters...

8/11/2019 2009 - Decadal Increase of Oceanic Carbon Dioxide in Southern Indian Ocean Surface Waters (19912007)

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Decadal increase of oceanic carbon dioxide in Southern Indian Ocean surfacewaters (19912007)

Nicolas Metzl

LOCEAN/IPSL, CNRS, Universite Pierre et Marie Curie, Case 100, 4 Place Jussieu, 75252 Paris Cedex 05, France

a r t i c l e i n f o

Available online 13 December 2008

Keywords:

Carbon dioxide

Southern Ocean

Indian Ocean

Decadal variability

Airsea CO2fluxes

Southern Annular Mode

a b s t r a c t

The decadal variability of the fugacity of carbon dioxide (fCO2) at the sea surface is analyzed for the first

time in the south-western Indian Ocean and corresponding Antarctic sector. This study is based on

seasonal cruises (MINERVE and OISO) conducted onboard the R.S.S. Marion-Dufresneduring the period

19912007. Based on shipboard observations the average annual rate of the atmospheric CO2 was

1.72 ppm/yr, almost equal to the annual growth rate derived from high-quality measurements recorded

at monitoring stations in the Southern Hemisphere. An evaluation based on oceanic observations in the

Southern Indian Ocean (4201S), indicates that oceanicfCO2increased at a rate of 2.11 (70.07)matm/yrfor the period 19912007, i.e. about 0.4 matm/yr faster than in the atmosphere. In order to investigatethe processes that explain the oceanicfCO2variations (and the potential reduction of the ocean carbon

sink), the decadal variability is analyzed in detail in four regions (20351S, 35401S, 40421S and

50551S) for austral summer (DecemberMarch) and winter (JuneAugust). During austral summer, the

fCO2 increase is similar in the four regions (between +2.2 and +2.4 matm/yr). For austral winter thegrowth rate is lower north of 401S (+1.5 to +1.7matm/yr) than at higher latitudes (+2.2 matm/yr). Becausethese regions experienced different warming or cooling, the evolution of temperature normalized fCO2(fCO2

norm) has also been investigated. In the southern subtropical region (35401S), warming occurred in

winter, leading to a small change of fCO2norm (+0.6matm/yr). In this region, anthropogenic CO2 uptake

must be compensated by a reduction of dissolved inorganic carbon (DIC) in surface waters. At latitudes4401S, the observed cooling during winter leads to a rapid increase offCO2

norm (+3.6 to +4.7matm/yr),suggesting that the gradual import of DIC in surface water occurs in addition to anthropogenic CO2. The

contrasting variations observed north and south of 401S are likely related to the high index state of the

Southern Annular Mode (SAM) during the 1990s. The increase of the westerlies at latitudes 4401S could

have enhanced the vertical import of CO2-enriched deep waters in high-latitude surface layers, whereas

the decrease of the wind speed north of 401S would have reduced vertical mixing. Although this analysis

is limited to a relatively short period, 19912007, this is the first time that a link between the SAM and

the decadal reduction of the Southern Ocean carbon sink is suggested from in-situocean carbon dioxide

observations. This offers an encouraging result in the perspective of model validation and under-

standing of the future evolution of the ocean carbon sink and its coupling with climate change.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Anthropogenic emissions of carbon dioxide (CO2) into the

atmosphere from fossil fuel and land use change increased

dramatically from about 5.5PgC/yr (1 Pg 1015g) in 1970 to

8.4 PgC/yr in 2000 (Raupach et al., 2007) up to 9.9 PgC/yr in 2006

(Canadell et al., 2007). This is the consequence of increases in

population size and energy development. About half of these

emissions remain in the atmosphere, leading to significant recent

global warming (IPCC, 2007); the other half is stored in the ocean

and on land, but the partitioning between the ocean andterrestrial carbon sinks are uncertain (Sabine et al., 2003;

Stephens et al., 2007). For the last four decades, many scientists

have developed methods to estimate the global oceanic carbon

uptake. Whatever the method used (ocean observations, ocean

models, atmospheric inversion), the ocean carbon uptake is

estimated to be around 2 PgC/yr (range of 1.7 to 2.8 PgC/yr;

see review inLe Quereand Metzl, 2003). The most recent estimate

based on an international global ocean pCO2 data synthesis

indicates that total ocean CO2 uptake is 1.8 (70.7) PgC/yr for

year 2000 (Takahashi et al., 2009). This value represents about

20% of the anthropogenic emissions of year 2000. For the

anthropocene period (18001994) global ocean carbon inven-

tories derived from in-situ observations suggest that the ocean

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Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/dsr2

Deep-Sea Research II

0967-0645/$- see front matter& 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.dsr2.2008.12.007

Tel.: +33144273394; fax: +33144274993.

E-mail address: [email protected]

Deep-Sea Research II 56 (2009) 607619
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absorbed 48% of the emissions over the last 200 years (Sabine

et al., 2004). These authors concluded that the ocean has

constituted the only true net sink for anthropogenic CO2 over

the past 200 years and there are indications that in recent years

the ocean carbon uptake capacity has been reduced (Sabine et al.,

2004;Canadell et al., 2007). How the ocean carbon sink evolved in

the recent period (several decades) and will evolve in the future

(decades to century) are important questions regarding bothclimate change as well as acidification of the oceans and its

impacts on marine ecosystems (Feely et al., 2004).

In this context, observing the long-term change of oceanic

carbon dioxide in surface waters is crucial, not only to better

determine CO2 airsea fluxes at a global scale (Takahashi et al.,

2009), but also to understand how these fluxes will change in the

future under different environmental conditions, including higher

anthropogenic CO2emissions and climate change. The continuous

rise of sea-surface water concentrations of dissolved inorganic

carbon (DIC) and the partial pressure or fugacity of CO2 (pCO2 orfCO2) has been relatively well documented in the North Atlantic

and Pacific Oceans (e.g.,Bates et al., 1996;Bates, 2001;Feely et al.,

2002, 2006; Lefevre et al., 2004; Schuster and Watson, 2007;

Takahashi et al., 2006, 2009). The increase of DIC in sea-surfacewater is generally related to ocean uptake of anthropogenic CO2,

but decadal trends (both positive and negative) also have been

attributed to natural variation or climate change variability,

including evaporation anomalies (Dore et al., 2003), temperature

variation, and water mass transformation (Keeling et al., 2004;

Feely et al., 2006; Corbiere et al., 2007; Takahashi et al., 2009).

Changes in primary productivity also may be responsible for

long-term variations of pCO2, as suggested for the Bering Sea

where pCO2 has decreased for three decades (Takahashi et al.,

2006). However, the impact of the marine biological activity on

CO2 airsea fluxes decadal changes has never been clearly

established.

In the Southern Hemisphere, the long-term evolution of

oceanicpCO2is not well detected, mostly because historical data

are sparse in the remote oceans and the signal-to-noise ratio is

low (Lenton et al., 2006). In addition, during austral summer

when most data are available, the long-term variation of

biogeochemical properties, such as CO2, is often masked by large

spatio-temporal variability (Jabaud-Jan et al., 2004; Inoue and

Ishii, 2005;Breviere et al., 2006). Therefore, the detection of the

decadalpCO2 changes in polar waters requires an analysis over a

very long period (Inoue and Ishii, 2005) and should include winter

observations when the biological activity is low.

In the last 40 years, both greenhouse gas accumulation in the

atmosphere and ozone depletion induced significant thermal

contrast in the Southern Hemisphere (Thompson and Solomon,

2002) and changed the meridional atmospheric pressure gradi-

ents, leading to more positive state of the so-called Southern

Annular Mode (SAM) (Marshall, 2003). The variability of the SAMcan affect wind speeds, heat fluxes, ocean circulation and biology

at mid- and high latitudes (e.g., Lovenduski and Gruber, 2005;Sen

Gupta and England, 2006). Ocean carbon models (Lenton and

Matear, 20 07;Le Quereet al., 2007;Lovenduski et al., 2007;Verdy

et al., 2007) and inversions of atmospheric CO2 observations (Le

Quereet al., 2007) indicate that climate variability in the Southern

Hemisphere may dramatically impact the ocean carbon cycle and

CO2 airsea fluxes in temperate and high latitudes. The link

between surface ocean CO2 and climate variability (SAM and/or

ENSO) also has been recently investigated at regional scale based

on oceanic pCO2 observations conducted south of Tasmania in

19912003 (Borges et al., 2008). Although warming would result

in ocean CO2outgassing anomalies,Borges et al (2008)found that

positive (negative) CO2 airsea fluxes inter-annual anomalies areusually associated with negative (positive) SST anomalies. An

increase of the Southern Ocean carbon sink during warm events

has been occasionally observed and associated with higher

productivity during summer (Jabaud-Jan et al., 2004; Breviere

et al., 2006). How the Southern Ocean carbon sink evolves at

decadal scale has never been directly analyzed from in-situ

observations.

This paper describes for the first time the decadalfCO2changes

in the south-western Indian Ocean (20601S/3090

1E) based on

observations obtained during 19912007. Data were obtained

using consistent instrumentation and processing techniques since

1991. The paper starts with describing the methods and the

atmospheric CO2 trends recorded on board followed by a basin-

wide view of the oceanic fCO2 trends. The analysis is focussed on

four latitudinal bands where oceanic fCO2 variations are likely

driven by different processes in relation to climate changes in the

Southern Hemisphere. Finally, the results in the South-Western

Indian Ocean are compared with decadal fCO2 changes analyzed

in other ocean regions and discussed specifically when comparing

the contrasting patterns observed in the South Indian and South

Pacific oceans.

2. Data collection and atmospheric CO2 trends

Observations of sea-surface and atmospheric fCO2 were

obtained in the Southern Indian Ocean during 19911995

(MINERVE cruises) and 19982007 (OISO cruises) using the same

instrumentation and data processing (Poisson et al., 1993; Metzl

et al., 1995, 1999, 2006). During the cruises (Fig. 1), all conducted

onboard the R.S.S.Marion-Dufresne(IPEV/TAAF), sea-surface water

was continuously pumped and equilibrated with a thin film

type equilibrator, thermostated with surface seawater. After

passing through Peltier cold traps (35 1C), the CO2 in the dried

gas was measured with a non-dispersive infrared analyzer (NDIR,

Siemens Ultramat 5F). During all cruises three standards were

used for calibration with typical low, middle (near atmospheric),

and high CO2 concentrations (corresponding mole fractions

ranges of 250270ppm or mmol CO2 mol1, 350370ppm, and

470490ppm). The standards and atmospheric CO2 were mea-

sured every 67 h. The temperature in the equilibrium cell was

0.21.0 1C warmer than sea-surface temperature (SST) depending

on the location. Since 1991, the fCO2 measurements were all

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-66

-60

-50

-20

-30

-40

20

La Runion

Crozet

Kerguelen

Africa

Antarctica

Tracks of the MINERVE and OISO cruises

in the South-Western Indian Ocean (1991-2007)

Amsterdam

30 40 50 60 70 80 90

Fig. 1. Cruises (MINERVE and OISO) conducted in the south-western Indian Oceanover the period 19912007.

N. Metzl / Deep-Sea Research II 56 (2009) 607619608


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corrected to in-situ SST using polynomials given by Copin-

Montegut (1988, 1989). Based on an international at-sea inter-

comparison, the oceanicfCO2data used in this study are accurate

to about 72matm (Koertzinger et al., 2000).In order to investigate the decadal trend of oceanic fCO2, it is

important to evaluate the long-term accuracy of the measure-

ments made on board and associated data-processing used over

more than 15 years. The atmospheric CO2concentrations observedon board were regularly compared with those continuously

monitored at the station established since 1981 on the La

Nouvelle Amsterdam (Gaudry et al., 1983), a French island located

at 371480S771320E in the south-western Indian Ocean. Fig. 2

shows all the atmospheric CO2concentrations measured on board

south of 201S and those recorded at the La Nouvelle Amsterdam

since 1991 (RAMCES data communicated by M. Ramonet, LSCE/

IPSL). The cruises were conducted over a relatively large

latitudinal range (20601S) both near or far from the continents.

This explains why the atmospheric CO2 observations on board

show larger variability compared to the La Nouvelle Amsterdam

record (in this region the seasonality of atmospheric CO2 is very

low, if not the lowest in the world). The differences between the

ship-board measurements and those at the La Nouvelle Amster-dam have been interpreted as real signals, and high concentra-

tions are generally related to air mass trajectories coming from

land especially in the region 201301S influenced by the African

continent and Madagascar. For example, in January 2004 the high

atmospheric CO2 concentrations (4376 ppm, Fig. 2) were re-

corded around 201S401E in the Mozambique Channel. At higher

latitudes, atmospheric CO2 concentrations present much lower

variability (Metzl et al., 2006). Despite these differences, the mean

atmospheric CO2annual growth rate deduced from all individual

ship-board observations, +1.722 (70.004) ppm/yr over the period

19912007, is almost equal to that, +1.701 (70.003) ppm/yr

deduced from the continuous La Nouvelle Amsterdam data. It is

also comparable to the trends deduced from the 2D-Global-View

data set (1.711.73ppm/yr at latitude 20901S) (GLOBALVIEW-

CO2, 2007). The ship-board observations also showed that the

atmospheric CO2 growth rate increased significantly in recent

years from +1.64 (70.01) ppm/yr for 19912000 to +1.94

(70.01) ppm/yr for 20002007. These results derived from cruise

data are coherent with the changes evaluated at the global scale

based on high quality atmospheric observations (Canadell et al.,

2007). During the last OISO cruise in January 2007, the average

atmospheric CO2 in the Southern Indian Ocean was 380.3

(70.7) ppm, i.e. about 100 ppm higher than the preindustrial

value of 280 ppm. The comparisons of atmospheric CO2 trends

described above reflects the long-term quality of the data

obtained on board since 1991. As the same instrument (NDIR)

and its calibration were used for both atmospheric and oceanic

CO2 measurements, one could be confident to interpret the

oceanic fCO2 trends over the period 19912007. For comparingoceanic and atmosphericfCO2trends, a value of +1.7matm/yr willbe used as a reference in the atmosphere (trend estimated after

converting atmosphericxCO2data to fCO2at standard pressure).

3. OceanicfCO2 decadal variations

3.1. The oceanic fCO2 trend in the South Indian Ocean

In order to detect the oceanic fCO2trend, the continuous data

of all cruises conducted between 1991 and 2007 have been first

averaged on 1111latitudelongitude grids (this is the scale that

has been selected for constructing global Surface Ocean CO2Atlas

in the future, SOCAT project as discussed during the SOCOVVmeeting; IOCCP, 2007). In doing so, small-scale variability is

filtered out. In addition, during most cruises the sea-surface

continuous measurements were occasionally maintained at the

same location during several hours or days. Therefore the 1111grid product (hereinafter noted /fCO2S) helps to construct a

uniform weighted data set to analyze the long-term changes.

Because atmospheric pressure varies on seasonal and inter-annual

scales as well as between low and high latitudes, all /fCO2S

values have been normalized at standard pressure of 1013hPa

(hectoPascal 1 atmosphere), and trends of oceanic fCO2 can be

directly compared to the one estimated in the atmosphere

(+1.7matm/yr).To start the trend analysis, all /fCO2S values obtained since

1991 in the South Indian Ocean (20691S, 30901E), for all cruises

and seasons, are represented as a function of time inFig. 3(i.e. the

same way as for atmospheric data,Fig. 2). This overall view leads

to an average increase of ocean fCO2 of +2.11 (70.07)matm/yr(Fig. 3A), higher than in the atmosphere.

Since surface oceanicpCO2measurements started in the 1960s

(Takahashi, 1961; Keeling et al., 1965; Miyake and Sugimura,

1969), it is well known that the range of ocean pCO2is large, with

minimum/maximum values varying between 150 and 550matm inthe open ocean (Takahashi et al., 2002). In the South Indian Ocean,

the values range between 250 and 450 matm (Fig. 3A). The range(maximumminimum) of fCO2 in the South Indian Ocean varies

from 60matm (October 1996) to 140matm (January 2007). Despitethis large variability (spatial, seasonal and interannual), the

observations in the Southern Indian Ocean do show a positive

trend infCO2. Another estimate of the trend can be deduced froma direct comparison of/fCO2S values averaged for the first and

last cruises conducted in the South Indian Ocean. In January

February 1991 the mean of/fCO2S was 336 (718)matm against370 (730)matm in January 2007. This corresponds to an increaseof +34matm over 16 years (or +2.12matm/yr). The trendsestimated from all data and from the difference of two cruises

(austral summer 1991 versus 2007) are almost the same

(+2.1matm/yr). This suggests that since 1991, surface ocean fCO2increased faster than atmospheric CO2 (2.1matm/yr against1.7matm/yr) and consequently the driving force for ocean carbonsink would be reduced by 0.4matm/yr.

The observed CO2 increase at the sea surface should result

from anthropogenic uptake through air-to-sea gas exchange if the

oceanic processes such as thermodynamic, dynamic and biologi-cal activity are in steady state at decadal scale. However, based on

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350

355

360

365

370

375

380

385

1990

Year

xCO2(ppm) Ship Obs. Trend: 1.722 (+0.004) ppm/year

Ams. Station Trend: 1.701 (+0.003) ppm/year

1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. 2. Atmospheric CO2 concentrations (ppm) observed since 1991 on board in

the South Indian Ocean, south of 201S (open circles, bold line) and continuously

monitored at Nouvelle Amsterdam Island, 381S/771E, (black dots, thin line). Lineartrends and standard deviations calculated for both data sets are indicated.

N. Metzl / Deep-Sea Research II 56 (2009) 607619 609


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the same series of cruises, it is observed that sea-surface

temperature (SST) decreased at an average rate of 0.11

(70.03) 1C/yr in the South Indian Ocean. The fCO2 normalized at

a constant temperature (11.76 1C is the mean of all observations)

increased at a rate of 3.47 (70.43)matm/yr (Fig. 3B). The morethan 1 1C/decade cooling makes the thermodynamic effect onfCO2significant and without the cooling the underlying oceanic fCO2increase would be even greater. This result is obtained when

normalizing fCO2 at 20 or 5 1C. Dynamic and/or biological

processes must play a role in order to increase fCO2 in surface

waters and balance the cooling effect.

It is likely that the Southern Indian Ocean, from the subtropics

to the Antarctic zone, experienced regional forcing and circulation

variability at different latitudes and that different processes might

explain the decadalfCO2variations at a regional scale, as has beenshown in the North and Equatorial Pacific Oceans (Inoue et al.,

1995; Feely et al., 2006; Takahashi et al., 2006). In order to

understand the processes that control the fCO2 trends and the

evolution of the ocean carbon sink one has to analyze these data

at a regional scale.

3.2. Oceanic fCO2 regional distributions and changes

The south-western Indian Ocean is characterized by several

dynamic (e.g., frontal systems, gyres) and biogeochemical features

that create large spatial and temporalfCO2variability which have

been described in previous studies (Poisson et al., 1993; Metzl

et al., 1995, 1998, 1999, 2006; Jabaud-Jan et al., 2004). To helpin selecting the regions to investigate the decadal fCO2variations,

a short description of the seasonal distribution is presented here

for two different years. The continuous fCO2 measurements are

presented in Fig. 4 as a function of SST from four cruises

conducted in 1991 and 2000 during the same months (January

February and August).

For the austral summer, the fCO2/SST relationship in the

subtropical and sub-Antarctic zone (1527 1C) presents the same

structure in 1991 and 2000, but is shifted towards higherfCO2in

2000 (Fig. 4A). For example, in warm waters (SST420 1C) the

average fCO2was 370 (713)matm in 2000 against 347 (78)matmin 1991. The 23matm increase in fCO2 over 9 years is larger thanthe atmospheric increase (15matm over the same period). Duringaustral winter (Fig. 4B), the fCO2/SST relations decrease with SST

in both warm and cold waters, and are almost the same in 1991

and 2000 but shifted towards higherfCO2in 2000. In warm waters(SST4141C) the average fCO2was 318 (76)matm in August 2000against 302 (78)matm in August 1991. The increase of oceanicfCO2 based on winter data is 16matm, almost equal to theatmospheric increase.

In the cold waters, south of the sub-Antarctic region around

401S, the fCO2/SST relationship is also well expressed in winter

(Fig. 4B). The data show that fCO2 was higher in 2000 than in

1991. In the temperature range 26 1C, the average fCO2 was 370

(76)matm in August 2000 and 347 (711)matm in August 1991.The average SST was almost the same (3.4 1C in 1991 and 3.5 1C in

2000). Considering that biological activity is low and not changing

dramatically from year to year during austral winter, the fCO2increase of 23matm in the cold waters is likely a signature of

anthropogenic CO2but changes of dynamical processes could alsoexplain this rapid increase of +2.6matm/yr.

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trend = + 2.11 (0.07) atm/yr

250

270

290

310

330

350

370

390

410

430

450

1990

Year

fCO2

(atm)

trend = + 3.47 (0.43) atm/yr

100

200

300

400

500

600

700

1990

Year

fCO2

norm(

atm)

1992 1994 1996 1998 2000 2002 2004 2006 2008

1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. 3. Evolution of (A) surface water fCO2and (B) temperature normalized fCO2(at SST 11.761C, mean average of all data) based on all measurements conducted in the

South Indian Ocean (south of 201S) during the period 19912007. Each point corresponds to monthly average of the continuous measurement in a 1 111 la/long grid. All

seasons, covering different months are reported in these figures. Without selecting seasons and regions, the data lead to annual increasing rates of +2.11 and +3.47 matm/yrfor fCO2and fCO2

norm, respectively.



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As opposed to the winter observations, there is no clearfCO2/SST relationship during summer in cold waters at high

latitude (Fig. 4A). This is because the fCO2variability is large andmostly controlled by biological activity, which imprints large fCO2meso-scale gradients in polar waters. In this context, detecting

decadal fCO2 variations could be particularly difficult when

comparing only two cruises in summer. Inoue and Ishii (2005)

reported the same difficulty in the eastern sector of the Southern

Indian Ocean. One has to be very careful when selecting summer

data in order to detect long-term changes.

It is clear that direct cruise to cruise comparisons (1991 versus

2000) cannot lead to firm conclusions regarding the decadal

trends of oceanic fCO2. Indeed, it has been shown that for austral

summer interannual variability of fCO2 is large in the Southern

Indian Ocean (Jabaud-Jan et al., 2004; Inoue and Ishii, 2005;

Breviere et al., 2006; Borges et al., 2008). However, the results

presented inFig. 4suggest that both summer and winter data canbe used to evaluate the long-term trend of oceanic fCO2 over a

large region in the Indian Ocean, from the subtropics to the

Southern Ocean. This is now analyzed in more detail using all

observations in this region in summer and winter since 1991.

3.3. Regional analysis of oceanic fCO2 decadal trends

Based on the spatio-temporal fCO2 distribution, the dynamic

and biogeochemical characteristics of the Southern Indian Ocean

and recent knowledge of large-scale atmospheric and oceanic

changes, the decadalfCO2trends will be analyzed for four regions

(20351S, 35401S, 40421S and 50551S;Fig. 4). The variations of

SST could have a significant effect on fCO2. For this reason, thenormalized fCO2at constant temperature will be also investigated

(fCO2norm). In addition, because the seasonality is large (Fig. 4) and

cruises were not conducted for summer and winter every year, the

fCO2trends will be estimated independently for each season usingthe 1111monthly mean data described in Section 3.1 (/fCO2S,

/fCO2norm

S and /SSTS). Results are presented inFigs. 58for each

region andTable 1gives the values of trends and errors estimated

for SST, fCO2 and fCO2norm.

3.3.1. The subtropical zone, 20351S

The data in the subtropical region (20351S/40801E) have

been selected as oligotrophic warm waters of the western Indian

subtropical gyre, with SST420 1C in summer and SST4141C in

winter. A significant fCO2 increase over time is expected, as

observed in the subtropical zones of the North Atlantic and Pacific

Oceans (e.g.,Takahashi et al., 1983, 2006;Inoue et al., 1995;Bates,

2001). There is some evidence for this in the South Indian gyrewhen comparing the data from years 1991 and 2000 (Section 3.2,

Fig. 4A, B). Over the full period 19912007, the seasonality offCO2is well marked each year with higher fCO2 in austral summer

(Fig. 5B). This is clearly associated with the seasonality of SST

(Fig. 5A). For each season, the oceanic fCO2increase over 16 years

is also observed. The average growth rates are +1.8matm/yr forwinter and +2.2matm/yr for summer (Table 1), i.e. near or abovethe rate of atmospheric CO2 increase. Although the fCO2 appears

relatively stable for some periods (e.g., summer 19982002), the

trends estimated over 16 years suggest that the driving force for

the ocean sink (DfCO2 fCO2sea

fCO2atm) is decreasing. These

observations also suggest a slight decrease of temperature, almost

the same in summer and winter, of 0.03 and 0.04 1C/yr,

respectively. Therefore for both seasons the rate of increase forfCO2

norm is slightly higher than for fCO2 but not statistically

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250

300

350

400

-2

SST (C)

fCO2(atm)

Jan 1991

Jan 2000

20-35S35-40S40-42S50-55S

250

300

350

400

-2

SST (C)

fCO2(atm)

Aug 1991

Aug 2000

20-35S35-40S40-42S50-55S

2 6 10 14 18 22 26 30

2 6 10 14 18 22 26 30

Fig. 4. Continuous oceanfCO2measurements versus SST observed in the south-western Indian Ocean in (A) January and (B) August for years 1991 and 2000. The arrows

indicate SST range in the four zones selected to evaluate and discuss thefCO2 trends at regional scale.



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different (Table 1). Although the SST decrease is shown for both

seasons, and could be related to an advective process from the

south (import of high DIC and low alkalinity), the variations are

small and quantification of such small changes is not in hand at

present. In the subtropical Indian gyre, the data suggest that the

increase of fCO2 is mainly explained by progressive invasion of

anthropogenic CO2.

3.3.2. The transitional zone, 35401S

The region between 351S and 401S in the south-western Indian

Ocean represents a transitional zone bordered by the Aghulas

Frontal system and its associated return flow and the sub-

Antarctic front (e.g., Lutjeharms and Valentine, 1984;Belkin and

Gordon,1996). This region separates the oligotrophic warm watersof the subtropics and the more productive waters of the sub-

Antarctic zone and is characterized by large meridional fCO2gradients in all seasons (Fig. 4). In summer, for 15 1CoSSTo20 1C,fCO2 decreases sharply southward (gradients of 100matm arecommon, Fig. 4A). During winter, the SST decreases southward

from 16 to 13 1C andfCO2increases to the south. Gradients offCO2+50matm are regularly observed over the zone in winter, Fig. 4B).The SST presents a clear seasonality (Fig. 6A) but the fCO2seasonality is low (Fig. 6B). As opposed to the subtropical region,

this indicates that SST is not the main factor controlling the

seasonalfCO2 cycle south of 351S. The averaged data also clearly

reveal that fCO2 variability is much larger during summer than

winter. This is observed for all cruises and is probably related to

high primary productivity and shallow mixed-layers in summer,whereas deeper mixing and low biological production in winter

result in homogeneous biogeochemical properties in surface

water (DIC and fCO2). The comparison of continuous measure-

ments between 1991 and 2000 (Fig. 4) suggested a significant

fCO2 increase over 9 years. The full data set also shows a gradualfCO2 increase from 1991 to 2007 of +2.3 matm/yr for summer and+1.5matm/yr for winter (Fig. 6B, Table 1). These are about thesame rates as for the subtropical region. However, because the

data also recorded stable SST in summer (+0.01 1C/yr), but a

warming during winter (+0.07 1C/yr), the increasing rates of

temperature-normalized fCO2 are significantly different for sum-

mer (+2.1matm/yr) and winter (+0.6matm/yr). This suggests thatsince 1991 the oceanic processes that control surfacefCO2(as well

as DIC and perhaps alkalinity) were the same in summer, but

something changed during winter.

To balance the temperature effect that increased winter fCO2one possible scenario is an increase in primary productivity

during the winter months. Indeed, the observed warming in

winter could be related to a decrease of vertical mixing (related to

weaker winds), less import of DIC into the surface water and

increased productivity in illuminated surface waters during

winter (when nutrients are not limiting). Interestingly, based on

ocean-color satellite observations (CZCS 19791983 versus Sea-

WIFS 19982002), significant variations of the Chlorophyll-a

concentrations have been evaluated in the south-western Indian

region at these latitudes during winter but not in other seasons

(Antoine et al., 2005). It is also worth noting that the comparison

of the Chlorophyll-aconcentration in surface water and analysis of

the fCO2 trends are not for the same periods (19791983 vs.

19982002, and 19912007). Unfortunately, satellite color dataare not available for the period 19911997 and the period

ARTICLE IN PRESS

20-35S, 40-80E

280

330

380

430

480

1990

Year

fCO2(atm)

Dec-Feb

Jul-Sep

20-35S, 40-80E

10

15

20

25

30

1990

Year

SST(C)

Dec-Feb

Jul-Sep

20-35S, 40-80E

250

300

350

400

450

500

1990

Year

fCO2

norm(

atm)

Dec-Feb

Jul-Sep

1992 1994 1996 1998 2000 2002 2004 2006 2008

1992 1994 1996 1998 2000 2002 2004 2006 2008

1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. 5. Evolution of (top) sea-surface temperature (SST), (middle) sea-surface fCO2 and (bottom) temperature normalized fCO2 in the subtropical region of the south-

western Indian Ocean (20351S/40801E). The data obtained during 19912007 are selected only for summer cruises (DecemberFebruary, open circles) and winter cruises

(JulySeptember, filled circles). Linear trends are indicated for summer (dashed line) and winter (black line) and the trends are given in Table 1.



7/13

19982007 is short to analyze the impact of the primary

productivity changes on decadal fCO2 variations.

3.3.3. The sub-Antarctic zone, 40421S

The sub-Antarctic zone, the region bordered by the subtropical

front and the sub-Antarctic front, is characterized by large spatio-

temporal variability of fCO2, but on average the seasonality is

relatively well identified (Fig. 7). During summer fCO2is low due to

biological activity. In winter fCO2is higher due to deep mixing with

subsurface waters enriched in CO2 (Metzl et al., 1999). In order to

investigate the long-term trends, the data have been selected in a

relatively narrow band, 40421S, wherefCO2reaches a minimum insummer and is higher and rather constant in winter (Fig. 4). In this

region the fCO2 trends estimated for both seasons are almost the

same, +2.2matm/yr, which is faster than in the atmosphere. Asopposed to the regions north of 401S, the observations show a cooling

of surface waters for both seasons, 0.05 1C/yr in summer and

0.15 1C/yr during winter. This leads to a dramatic seasonal contrast

when comparing the decadal trends for fCO2norm (+2.9matm/yr in

summer and +4.7matm/yr in winter). The effect of cooling on fCO2must be offset by an increase of DIC, that could be associated with

deeper mixed-layers and higher winds speeds observed in this region

in recent years (in positive SAM period,Marshall, 2003).

3.3.4. The Polar zone, 50551S

The Polar Front occurs at about 501S in the south-westernIndian Ocean. Summer cruises reached as far south as 561S

(sometimes up to 691S), with winter cruises only reaching 561S on

a few occasions. Therefore, the decadal variations offCO2 will be

estimated over 50551S latitudinal band for summer and over

50521S for winter. Note that if one uses the summer data in the

band 50521S only, results are not dramatically different from

those for 50551S. In addition, the observations obtained in winter

showed that oceanic surface properties (fCO2, DIC, TA, nutrients)

were fairly constant over large distance in the region 50561S

(Metzl et al., 2006). Therefore the trends estimated in the band

50521S in winter are almost certainly representative of the

decadal changes in the POOZ (Permanent Open Ocean Zone). Data

over the Kerguelen Plateau (around 50521S), where strong

blooms create large fCO2 drawdown (Blain et al., 2007; Jouandetet al., 2008), have been filtered out to analyze the long-term

trends. Measurements in coastal waters around Kerguelen Island

are also excluded.

In the Southern Ocean, south of the Polar Front, it is now well

established thatfCO2is higher in winter than in summer (Metzl et

al., 2006;Takahashi et al., 2009). As in the SAZ, this is because the

effect of biological activity in summer, and vertical mixing in

winter dominate the effect of temperature changes on fCO2. The

seasonal amplitude is relatively small (about 20matm, Fig. 8B),much smaller than in the SAZ, and at these high latitudes the

ocean is not far from equilibrium with respect to atmospheric CO2.

The data in this analysis show that the average rate of fCO2increase of +2.4matm/yr in summer and +2.1matm/yr in winter,

are higher than in the atmosphere (+1.7 matm/yr). The rates ofincrease are similar for the SAZ (40421S, where the oceanfCO2is

ARTICLE IN PRESS

35-40S, 30-90E

250

300

350

400

450

fCO2(atm)

Dec-Mar

Jun-Oct

35-40S, 30-90E

10

15

20

25

SST(C)

Dec-Mar

Jun-Oct

35-40S, 30-90E

250

300

350

400

450

1990

Year

fCO2

norm(

atm)

Dec-Mar

Jun-Oct

1992 1994 1996 1998 2000 2002 2004 2006 2008

1990

Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

1990

Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. 6. Evolution of (top) sea-surface temperature (SST), (middle) sea-surfacefCO2and (bottom) temperature normalizedfCO2in the transitional zone of the south-western

Indian Ocean (35401S/30901E). The data obtained during 19912007 are selected only for summer cruises (DecemberMarch, open circles) and winter cruises

(JuneOctober, filled circles). Linear trends are indicated for summer (dashed line) and winter (black line) and the trends are given in Table 1.



8/13

well below equilibrium in summer, Figs. 4 and 7). As in the SAZ,

the SST observations in the POOZ present different trends for

summer and winter (Fig. 8A). A warming of about +0.02 1C/yr is

deduced from the summer data (19912007), whereas winter

data suggest a cooling of0.11C/yr for the period 19912000. This

rapid cooling may be caused by high SST observed in JuneJuly

1993 (Fig. 8A). However, when data for JuneJuly 1993 are filtered

out, the cooling in winter is estimated as 0.06 1C/yr. This leads to

a rate of increase for fCO2norm in winter between +3.6 and

+3.4matm/yr (depending the data selected, with or without1993 observations). Like in the SAZ, the thermal effect should be

balanced by an import of CO2 in surface waters, most likely

through a change in vertical mixing generated by a recent increaseof winds at high latitudes in the Southern Ocean.

4. Discussion

4.1. Evolution of the ocean CO2 sink in the South Indian Ocean

The regional analysis of thefCO2trends estimated in the south-

western Indian Ocean is summarized inFig. 9. For all regions and

seasons south of 201S the oceanic fCO2 increased during

19912007, and the oceanic growth rate is everywhere close to

or larger than in the atmosphere. The four regions analyzed using

summer and winter data lead to an average oceanicfCO2 increase

of 2.10 (70.3)matm/yr. This value is similar to the one derivedwhen using all observations (2.11matm/yr discussed in Section 3.1,

Fig. 3), suggesting this is a robust result. The increase corresponds

to a decadal change of +21 (73)matm/decade. The data obtainedsince 1991 suggest that DfCO2decreased by about 4 matm/decadein the south-western Indian Ocean. For a CO2 sink estimated as

0.4 to 0.7PgC/yr in the South Indian Ocean south of 201S

(Metzl et al. 1995; Takahashi et al., 2002, 2009), the decadal

change ofDfCO2would correspond to a reduction of the ocean CO2sink between 0.08 and 0.14 PgC/yr/decade. This estimate for the

Southern Indian Ocean gives only a measure of the potential

impact of the DfCO2change in terms of the integrated carbon flux

at the basin-wide scale and is similar to a 0.08 PgC/yr/decade

estimate for the Southern Ocean (4451S) for the period

19812004 based on atmospheric data (Le Quere et al., 2007).For an ocean modelling approach,Lenton and Matear (2007)and

Lovenduski et al. (2007) both concluded a reduction of the

Southern Ocean carbon sink would be around 0.1PgC/decade per

unit of change of the SAM. The numbers given here are not

directly comparable (South Indian versus Southern Ocean), but

offer some sense of how a change in observed DfCO2 would

translate in the global carbon budget.

4.2. A link between observed fCO2 decadal trends and the SAM?

In the south-western Indian Ocean, significant warming has

been observed in winter at 35401S, whereas south of 401S the

data indicate winter-time cooling since 1991 (Table 1). These

characteristics promote a strong regional contrast in the annualgrowth rates forfCO2andfCO2

norm in winter (comparison ofFig. 9A

ARTICLE IN PRESS

40-42S/ 30-80E

250

300

350

400

fCO2(atm)

Dec-Mar

Jun-Sep

40-42S/ 30-80E

5

10

15

20

25

1990

Year

SST(C)

Dec-Mar

Jun-Sep

40-42S/ 30-80E

200

250

300

350

400

450

500

fCO2

norm(

atm) Dec-Mar

Jun-Sep

1992 1994 1996 1998 2000 2002 2004 2006 2008

1990

Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

1990

Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. 7. Evolution of (top) sea-surface temperature (SST), (middle) sea-surfacefCO2and (bottom) temperature normalizedfCO2in the sub-Antarctic zone of the south-western

Indian Ocean (40421S/30801E). The data obtained during 19912007 are selected only for summer cruises (DecemberMarch, open circles) and winter cruises

(JuneSeptember, filled circles). Linear trends are indicated for summer (dashed line) and winter (black line) and the trends are given in Table 1.



9/13

and B). At 35401S the annual increase of fCO2norm is low,

+0.6matm/yr, whereas south of 401S the growth rate of fCO2norm

reaches 3.64.7matm/yr in winter. In order to balance thetemperature effect, there must be a decrease of CO2 in surface

water north of 401S and an increase of CO2 south of 401S.

The positive trend of the SAM in recent years (especially since

the 1990s, Marshall, 2003) may explain the observed decadal

trends in fCO2 and fCO2norm over 19912007, as it has been

recognized for physical properties and circulation changes.

A recent analysis based on altimetry data (19932005) showed

that in the south-western Indian Ocean (20801E the regioninvestigated here), the sea-level rise decreases south of 401S and

increases north of 401S(Morrow et al., 2008). This is interpreted

by stronger upwelling and cooling south of 401S and warming in

the northern region, as observed in our data set during austral

winter. Morrow et al. (2008) also have investigated the link

between the sea-level anomaly (SLA) and climate index (SAM and

ENSO), and they estimate that SLA was positively correlated with

the SAM during 19932005 in the south-western Indian Ocean

(30901E). In a positive SAM state, stronger westerly winds south

of 401S lead to surface ocean cooling and increase the physical

mixing (e.g., Sen Gupta and England, 2006) and would increase

the import of CO2 in surface waters. The opposite is observednorth of 401S where weaker westerly winds increase SST (increase

ARTICLE IN PRESS

50-55S summer, 50-52S winter, 40-70E

0

2

4

6

8

SST(C)

Dec-Mar

Jun-Aug


300

320

340

360

380

400

420

fCO2(atm)

Dec-Mar

Jun-Aug


300

320

340

360

380

400

420

1990

Year

fCO2

norm(

atm)

Dec-Mar

Jun-Aug

1992 1994 1996 1998 2000 2002 2004 2006 2008

1990

Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

1990

Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. 8. Evolution of (top) sea-surface temperature (SST), (middle) sea-surface fCO2and (bottom) temperature normalizedfCO2in the Permanent Open Ocean Zone (POOZ)

of the south-western Indian Ocean (50551S/40701E). The data obtained during 19912007 are selected only for summer cruises (DecemberMarch, open circles) and

winter cruises (JuneAugust, filled circles). Linear trends are indicated for summer (dashed line) and winter (black line) and the trends are given in Table 1.For summer datahave been selected in the latitudinal band 50551S and for winter in the band 50521S. See text for explanation of data selection.

Table 1

Annual trends and standard deviations of sea surface temperature (SST), fCO2and temperature normalized fCO2in selected regions of the south-western Indian Ocean, for

summer and winter..

Zone/period SST (1C/yr) fCO2 (matm/yr) fCO2norm (matm/yr) SST norm (1C)

20351S DecemberFebruary 0.03 (0.03) +2.18 (0.18) +2.54 (0.46) 23.90

20351S JulySeptember 0.04 (0.04) +1.75 (0.10) +2.28 (0.58) 18.75

35401S DecemberMarch +0.01 (0.03) +2.35 (0.28) +2.14 (0.30) 18.50

35401S JuneOctober +0.07(0.02) +1.52 (0.15) +0.62 (0.45) 14.30

40421S DecemberMarch 0.05 (0.04) +2.24 (0.32) +2.92 (0.76) 14.95

40421S JuneSeptember 0.15 (0.06) +2.18 (0.22) +4.72 (1.03) 11.09

50551S DecemberMarch +0.02 (0.01) +2.39 (0.16) +2.04 (0.26) 3.72

50521S JuneAugust 0.10 (0.03) +2.10 (0.26) +3.62 (0.43) 2.85

Based on observations conducted during 19912007 in all regions expect for 50521S in JuneAugust (period 19912000). Last column indicates SST used to normalizefCO2for each region and season.



10/13

SLR) and reduce the mixed-layer depth. Consequently during

positive SAM state, less DIC would be imported in surface layers

north of 401S. At large scale, this scenario has been proposed by

several authors, based on ocean carbon models, to interpret the

inter-annual to decadal CO2 airsea flux variability (Lenton and

Matear, 20 07;Le Quereet al., 2007;Lovenduski et al., 2007;Verdy

et al., 2007). The data presented in this study support the

modelled predictions although the analysis only focuses on the

south-western Indian Ocean.

4.3. Comparison with other ocean sectors

Compared to other oceanic sectors, the average growth rate

estimated in the south-western Indian Ocean for the period

19912007 is generally higher than those reported in other oceans

(Table 2, here quoting several studies based on multi-year

observations). The decadal variability of fCO2, or any other sea-

surface property, also depends on the period investigated. For

example, in the Equatorial Pacific, Feely et al. (2006)note that the

growth rate of ocean fCO2 was faster in recent decades (Table 2).

This clearly indicates that one could observe significant differences

in the decadal oceanic fCO2 growth rate depending on the

investigated period and region. It is also important to note that

few studies investigated the oceans in the Southern Hemisphere.Inoue and Ishii (2005) analyzed the long-term observations

of pCO2 in the Southern Ocean from a series of four cruises

conducted south of Tasmania (at 1401601E) in austral summer of

years 1969, 1984, 1995 and 2002. They deducedpCO2growth rates

that vary between +10 (75) and +15 (74)matm/decade in the SAZand Polar Front zone. The interannual variability could be large in

this sector, especially during austral summer (Breviere et al.,

2006) and could create difficulties to detect the trends. A recent

data synthesis obtained in the south-western Pacific (20551S,

1401801E) indicates that oceanic pCO2increased at a rate of +14.4

(73.0)matm/decade during 19842006 (Takahashi et al., 2009).

ARTICLE IN PRESS

0.0

0.5

1.0

1.5

2.0

2.5

3.0

AnnualRatefCO2s(atm/yr) 20-35S 35-40S 40-42S 50-55S

20-35S 35-40S 40-42S 50-55S

0.0

1.0

2.0

3.0

4.0

5.0

6.0

AnnualRatefCO2n

orm(

atm/yr)

Fig. 9. Annual mean trends of (A) sea-surface fCO2 and (B) temperature normal-

ized fCO2 in 4 regions of the south-western Indian Ocean. The open bars indicate

the growth rates estimated for summer and black bars for winter. Standard errors

associated to each trends are also indicated. In each figure, the dashed line

indicates the atmospheric CO2annual growth rate. Note that the larger scale in (B).

Table 2

Decadal trends of sea surface fCO2 observed in various regions of the oceans..

Region Period fCO2growth rate (matm/decade) Reference

Northern and tropical regions

North Atlantic

Subpolar Gyre 19932003 +18 to +28 Corbiere et al. (2007)

Subtropical 19832003 +17 (73) Bates (2007)

North Atlantic 4151N 19722006 +18 (74) Takahashi et al. (2009)

North Pacific 19702004 +12 (74.8) Takahashi et al. (2006)

West. North Pacific 19841993 +12 (79) Inoue et al. (1995)

Equatorial Pacific 19902003 +20 (72) Ishii et al. (2004)

Central Equ. Pacific 19751990 11 (710) Feely et al. (2006)

Central Equ. Pacific 19902005 +17 (77) Feely et al. (2006)

Southern Hemisphere

South-western Pacific

(451501S, SAZ) 19692004 +10 (75) Inoue and Ishii (2005)

(501551S, PFZ) 19692004 +15 (74) Inoue and Ishii (2005)

South-western Pacific (201551S) 19842004 +14.4 (73.0) Takahashi et al. (2009)

South-western Indian (201S421S) 19912007 +20.4 (73.3) This study, average summer/winter

Southern Ocean South of the Polar Front

Pacific 1401701E

Summer 19692004 +18 (72) Inoue and Ishii (2005)

Circumpolar

SST o6.5 1C winter 19862006 +21 (74) Takahashi et al. (2009)

Indian 30901E

Summer (50551S) 19912007 +23.9 (71.6) This study

Winter (50521S) 19912000 +21.0 (72.6) This study

This table presents some examples of the results based on regular monitoring and/or data synthesis.



11/13

The oceanic fCO2 increase of +20.4 (73.3)matm/decade in thesouth-western Indian Ocean (this study, 30901E/20421S) is faster

than values reported for the south-western Pacific. Interestingly, it

has been observed that since 1993, sea-level rise (SLR) decreased in

the south-western Indian Antarctic sector (stronger upwelling and

cooling), whereas the SLR increased in the south-western Pacific

(Morrow et al., 2008). The difference of decadal trend offCO2(as for

the SLA) could be explained by the contrasting response of theSouth Indian and South Pacific Oceans to the SAM and ENSO

variability (Morrow et al., 2008).

At higher latitudes, in the cold waters of the Antarctic Ocean to

the south of the Polar front, the trend in the south-western Indian

sector south was higher (+24 (72)matm/decade) in summer thanthe pCO2 trend of +18 (72)matm/decade reported by Inoue andIshii (2005). As for the SAZ, this could be explained by the regional

response of the ocean to the atmospheric variability, or because

the period investigated are different.

Takahashi et al. (2009) investigated the decadal change of

oceanic pCO2 in the circumpolar waters at high latitudes, using

only winter observations. For SST below 6.5 1C they evaluated that

the oceanic pCO2 increased at a rate of +21 (74)matm/decade

during the period 19862006. This is the same value as that forwinter data south of 501S in the south-western Indian Ocean since

1991, +21.0 (72.6)matm/decade. Takahashi et al. (2009) includedalmost all the winter data analyzed in this paper as well as several

other data sets, but they used another approach to detect the

trends (by evaluating trends for different SST ranges using all data

in circumpolar waters). This comparison is thus encouraging and

demonstrates that the oceanic pCO2 (or fCO2) increased faster

than in the atmosphere at high latitudes in several regions of the

Southern Ocean for the past two decades.

5. Conclusions

The decadal trends of oceanic fCO2have been investigated for

the first time in the south-western Indian Ocean based on

observations conducted during 19912007. The shipboard data

show an annual atmospheric CO2increase of 1.72 ppm/yr over the

period 19912007 with a more rapid increase in recent years,

+1.64 (70.01) ppm/yr for 19912000 to +1.94 (70.01) ppm/yr for

20002007. This is coherent with the analysis of atmospheric CO2at a global scale (Canadell et al., 2007).

In the ocean, the observations show a significant fCO2 increase

in all sectors of the south-western Indian Ocean, in warm and cold

waters. The mean rate is +2.11 (0.07)matm/yr, suggesting thatocean fCO2increased at about the same rate or faster than in the

atmosphere. For a constant gas transfer velocity, this would imply

an ocean carbon sink reduction of about 0.08 and 0.14PgC/yr/

decade in the South Indian Ocean.

At a regional scale, the fCO2 increase is detected for bothsummer and winter. It varies between 1.5 and 2.4 matm/yr, i.e.almost always equal or higher than in the atmosphere. The

regional analysis also shows that the annual increase of fCO2 is

rather similar in different seasons and regions (Fig. 9A), but that

the rates of normalized temperature fCO2norm present contrasting

values, north and south of 401S and especially during austral

winter (Fig. 9B). To balance the thermal effect, more CO2(DIC) has

to be imported in surface waters south of 401S, whereas CO2concentrations have to be reduced at 35401S (reduction of

vertical mixing and/or increased productivity in winter). This

meridional contrast could be explained by the change in climate

over the Southern Hemisphere associated to the upward trend of

the SAM index. This would be coherent with a recent scenario

based on modelling and suggestions that variability in theSouthern Hemisphere climate forcing (e.g., the SAM) impacts on

ocean circulation, by increasing the supply of DIC from the deep

ocean at high latitudes and reducing the oceanic carbon sink for

atmospheric CO2(Lenton and Matear, 2007;Le Quereet al, 2007;

Lovenduski et al., 2007, Verdy et al., 2007). This is the first time

that such scenario is suggested from in-situ data.

This study also shows that the growth rate of oceanic fCO2 is

higher in the subtropical and sub-Antarctic zones of the south-

western Indian Ocean (+20matm/decade) than in the south-western Pacific (+10 to +15matm/decade, Inoue and Ishii, 2005;Takahashi et al., 2009). The difference may be explained by

regional oceanic response to the SAM and ENSO since the 1990s,

as evaluated from altimetry data (Morrow et al., 2008), and

suggests that the decadal changes of the ocean carbon sink

for atmospheric CO2 may not be homogeneous around the

circumpolar region.

This work offers new findings, but clearly calls for a more

comprehensive analysis of the oceanic CO2 in the Indian and

Southern Oceans that should be conducted in the frame of the

international collaborative synthesis (IOCCP, 2007). As the data

are now available to evaluate the decadal changes of the ocean

carbon cycle, a strong interaction with the modelling community

is expected, not only to validate ocean models, but to helpunderstanding why the oceanic CO2 sink is changing and

presumably decreasing. For this issue, an important analysis

should be performed not only withfCO2data (as in this paper) but

by adding regular observations of DIC and alkalinity in surface

waters and at depth. The same is true for other oceanic regions

(e.g., the North Atlantic) where it has been shown that the oceanic

CO2sink has decreased over the past decade (Corbiere et al., 2007;

Schuster and Watson, 2007; Schuster et al., 2009). As the

reduction or saturation of the oceanic carbon sink has been

identified by several observational studies in the Northern and

Southern Oceans, this has major implications and provides a

strong motivation to pursue long-term oceanic CO2 observations

to better understand the global carbon budget, its evolution and

its coupling with climate change.

Acknowledgements

The long-term OISO observational program is supported by

three French Institutes INSU (Institut National des Sciences de

lUnivers), IPSL (Institut Pierre-Simon Laplace) and IPEV (Institut

Paul-Emile Victor). Warm thanks to the captains and crews of the

R.S.S.Marion-Dufresne, and many colleagues at the laboratories in

Paris, LPCM (Laboratoire de Physique et Chimie Marines,

19911996), LBCM (Laboratoire de Biogeochimie et Chimie

Marines, 19972004) and LOCEAN (Laboratoire dOceanographie

et du Climat: Experimentation et Approches Numeriques). This

work is part of the national program LEFE/Cyber/FlamenCO2, acomponent of SOLAS-France. Support from the European Inte-

grated Project CARBOOCEAN Contract 511176 (GOCE) is also

acknowledged. Un Grand merci to Andrew Lenton (LOCEAN/

IPSL) for his questions and comments in a previous version of this

manuscript, to Dorothee Bakker (UEA, Norwich) for a very helpful

review and Bronte Tilbrook (CSIRO, Hobart), co-editor, for

important comments on the revised manuscript.

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