Printer-friendly Version Interactive Discussion Observed Increase in Springtime Surface Partial Pressure of Co 2 in the East Equatorial Indian Ocean during 1962–2012 Bgd Printer-friendly Version Interactive Discussion

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Rapidly rising atmospheric CO 2 and global warming may have been impacting the ocean, and, in contrast, the response of surface CO 2 partial pressure (pCO 2) in the equatorial Indian Ocean is poorly understood. In this study, we attempted to evaluate the variation of springtime sea surface pCO 2 in the east equatorial Indian 5


Introduction
Atmospheric carbon dioxide (CO 2 ) levels have risen from ∼ 280 ppm (parts per million in volume) in the pre-industrial revolution ear to a 2012 value of ∼ 393 ppm (Tans and Keeling, 2013) due to quickly increasing human activities, which might have caused global climate change (Denman et al., 2007).Meanwhile, the ocean as a CO 2 sink to the atmosphere plays a key role in alleviating the increase of CO 2 in the atmosphere (Sabine et al., 2004;Takahashi et al., 2009), absorbing roughly half of anthropogenic CO 2 from fossil fuel burning and cement production (Sabine et al., 2004).Ocean CO 2 uptake, however, is not benign.As we know that the uptake of anthropogenic CO 2 by the ocean will raise CO 2 concentrations in seawater, hence lower pH and CaCO 3 saturation state, and change speciation of carbonate chemistry (i.e., ocean acidification, OA) via a series of chemical reactions when CO 2 was dissolved into seawater (Millero, 2007).Furthermore, OA will induce negative feedbacks on oceanic CO 2 uptake and even significant impacts on marine organisms (especially for calcifying organisms) and ecosystems (Doney et al., 2009).
Sea surface partial pressure of CO 2 (pCO 2 ) is primarily determined by temperature, salinity, dissolved inorganic carbon (DIC) and total alkalinity (TA) (Millero, 2007), and to a large extent could directly reflect the change in seawater CO 2 concentration (e.g., Xue et al., 2012) and even the status of OA.Furthermore, sea surface pCO 2 data is relatively easy to obtain (e.g., via underway measurements on the volunteer observing ships) and a large number of data have been accumulated since 1960s (Sabine et al., 2013).Thus, sea surface pCO 2 is often used for characterizing the capability of the ocean to absorb atmospheric CO 2 , and for monitoring the change in seawater CO 2 concentration and the response of sea surface pCO 2 to rapidly increasing atmospheric CO 2 .During recent years, more attention has been therefore paid to studies on the oceanic pCO 2 change.For example, long-term increases in seawater CO 2 levels have been observed in a variety of oceanic zones such as subtropical North Pacific and North Atlantic (Bates, 2007;Lenton et al., 2012), the equatorial Pacific (Takahashi et al., Introduction Conclusions References Tables Figures
In the Indian Ocean, a lot of CO 2 related studies such as carbonate parameter distributions and related controlling processes as well as its role in the global carbon cycle were conducted primarily propelled by several national or international programs such as JGOFS (Joint Global Ocean Flux Study) and WOCE (World Ocean Circulation Experiment) (e.g., Naqvi, 2002).For instance, Sabine et al. (2002) found that the DIC and TA distributions in the Indian Ocean were dominated by the large-scale circulation patterns and the strength of the biological pump.Bates et al. (2006) examined the spatial and temporal variability of upper ocean carbonate parameters and air-sea CO 2 exchange in the Indian Ocean, and pointed out that the Arabian Sea, Bay of Bengal, and the equatorial belt (10 • N-10 • S) were perennial sources of CO 2 to the atmosphere.Sardessai et al. (2010) found the northeastern Indian Ocean showed significant intraseasonal variability in CO 2 fluxes, acting as a strong sink of CO 2 during southwest monsoon, a weal sink during fall intermonsoon and a weak source during winter monsoon.
In contrast, studies on the variation of the Indian Ocean surface water pCO 2 are relatively sporadic due to lack of enough pCO 2 measurements (Takahashi et al., 2009;Lenton et al., 2012;Sabine et al., 2013;Sarma et al., 2013).For example, Metzl (2009) for the first time investigated the decadal variability of the CO 2 fugacity (f CO 2 , a parameter very close to pCO 2 in meaning and magnitude) at the sea surface in the southwestern Indian Ocean (a CO 2 sink) and corresponding Antarctic sector, and observed that oceanic f CO 2 increased at a rate of 2.11 ± 0.07 µatm yr −1 during 1991-2007.In addition, related studies based on models were also conducted in the Indian Ocean.For instance, it was reported that the Arabian Sea and Southern Peninsular India interannual CO 2 emission anomalies were related to the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation (ENSO) (Valsala and Maksyutov, 2013).A more recent study using the ocean biogeochemical models suggested a small enhancement in the CO 2 sink over the period 1990-2009 in the southern Indian Ocean (Sarma et al., Introduction Conclusions References Tables Figures

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Full 2013), which is not consistent with the observations in the southwestern Indian Ocean by Metzl (2009).Therefore, more studies based on in-situ observations are needed to confirm the model results.
Significant increases in sea surface pCO 2 in the equatorial Pacific have been indicated (Feely et al., 1999;Takahashi et al., 2003;Ishii et al., 2009).The equatorial Indian Ocean is different from the Pacific in a number of ways.For instance, the Indian Ocean lacks steady equatorial easterlies and hence there is no climatic equatorial upwelling in the eastern part (Schott et al., 2009).As a result, the east equatorial Indian Ocean (EIO) does not have a significant source of CO 2 as in the eastern tropical Pacific (Takahashi et al., 2009).However, under the forcing of rising atmospheric CO 2 and global warming, the response of surface pCO 2 in the EIO, which serves as a perennial CO 2 source to the atmosphere (Bates et al., 2006;Takahashi et al., 2009), is poorly understood.
In this paper, we attempt to check the variability of springtime surface pCO 2 in the EIO (Fig. 1), which is relatively better occupied, using data collected during May 2012, together with the historical data since 1962 integrated by Takahashi et al. (2013) (i.e., LDEO_Database_V2012), and to explore the possible factors responsible for it by analyzing the effects of temperature, salinity, mixed layer depth (MLD), biological production and ocean circulation.At last, the potential influences of oceanic pCO 2 increase were simply discussed.

Study site
The study site was located in the EIO, extending from 5 • N to 5 • S along 90 • E and 95 • E (Fig. 1).The wind field in the equatorial Indian Ocean is unique relative to the Pacific and Atlantic, with weak and westerly annual mean, and strong semiannual annual cycle.Consequently, the equatorial currents are quite different in features from Introduction

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Full those in the other oceans (Schott and McCreary, 2001).The surface currents in the equatorial Indian Ocean reverse directions seasonally, with a weak westward flow during winter and summer, and a strong eastward flow during the intermonsoon periods in spring (April/May) and fall (October/November) (Schott and McCreary, 2001).The strong eastward currents are now widely known as Wyrtki jets (Wyrtki, 1973;Schott and McCreary, 2001).Another important current in the equatorial Indian Ocean is the transient Equatorial Undercurrent (Knauss and Taft, 1964).It exists only during part of the year due to lack of sustained easterlies, typically February-June with an easterly component.

Data sources
Sea surface pCO 2 data in the MOMSEI cruise (Monsoon Onset Monitoring and its Social and Ecosystem Impact) were collected during 1-9 May 2012.Sea surface pCO 2 was continuously measured every 15 min with a HydroC ™ CONTROS sensor.More details on pCO 2 measurements were described by Saderne et al. (2013).In brief, the CO 2 mole fraction in a headspace behind a membrane equilibrator is measured using a two-wavelength non-dispersive infra-red detector (NDIR).The equilibrator consists of a flat silicone composite membrane, the NDIR unit and additional sensors for pressure, temperature and relative humidity measurements.These additional sensors are used for correcting the NDIR signal and calculating the partial pressure.In addition, regular zeroings are automatically performed to correct the drifting of the instrument with time by scrubbing CO 2 from the internal gas stream.An overall accuracy is estimated better than ±1 % of the upper range value (Saderne et al., 2013).At the meanwhile, during the MOMSEI cruise sea surface temperature (SST) and salinity (SSS) were collected every 15 min using a SBE 21 Seacat Thermosalinograph.have been examined individually as they were integrated into this database.The uncertainty of the pCO 2 data is estimated about ±2.5 µatm on the average, given differences in equilibrator designs, calibration methods and some interpolated parameters (Takahashi et al., 2013).Atmospheric CO 2 data in this study are from monthly average values measured as the mole fraction in dry air on Mauna Loa, Hawaii (Table 2), where the longest direct measurement of atmospheric CO 2 is recorded.The atmospheric pCO 2 were corrected to 100 % humidity at the mean SST (29.60 •C) and SSS (34.16) during the investigation period, following Jiang et al. (2008).In addition, several ancillary parameters such as SST, MLD and chlorophyll a (Chl a) (Table 2) were used.

Change in surface pCO 2 since 1962
Figures 2 and 3 show all the pCO 2 data collected since 1962 in the EIO.Spatially, pCO 2 distributions are relatively homogeneous in each cruise, with standard deviation less than 9 µatm (Fig. 2 and Table 1).Latitudinally, sea surface pCO 2 showed some common features (Fig. 3).For instance, high pCO 2 values with large variability on a small regional scale were observed between 0 • N and 3 • N in 1963, 2007 and 2012, and surface pCO 2 decreased towards the south from around 2 in 1963, 1995, 1999 and 2012, probably associated with temperature (Fig. 3).In addition, high pCO 2 values in 1995 and 1999 were found in the southwestern part of the study area (Figs. 2 and 3), possibly due to high salinity (Fig. 3).Temporally, gradual increase in sea surface pCO 2 with time was the most evident feature (Figs.2-4, Table 1), although an exception occurred in July 1987.Sea surface pCO 2 in July 1987 was similar to, or larger than that in February 1995, probably due to high salinity in the eastern part of the study area in July 1987 (Figs. 2     and 3).Generally, the mean value of sea surface pCO 2 in the investigation area in-Introduction

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Full  Table 1).If data from all cruises during 1962-2012 were considered, sea surface pCO 2 would have an increase rate of 1.70 µatm yr −1 (Fig. 4).
Considering that not all the cruises were conducted in the same season and areas, we checked the pCO 2 variation using data collected at Location A, B, and C (these data were almost collected during the same season (April/May) and at the same location, Fig. 1), which removes the possible bias due to spatial and temporal variability.
Results showed that from 1963 to 2012 during spring (April/May), sea surface pCO 2 had a mean increase rate of ∼ 1.75 µatm yr −1 (Fig. 4).Also we compared the surface pCO 2 between April 1963 and May 2012 along the equator from 89.5 • E to 94.5 • E, and found during the past 50 yr since the International Indian Ocean Expedition (IIOE, from 1960(IIOE, from -1965)), surface pCO 2 increased from 307 ± 4 µatm in April 1963 to 392 ± 6 µatm in May 2012 (Fig. 4), with a mean rate of 1.70 µatm yr −1 .The increase rate of surface pCO 2 , ∼ 1.7 µatm yr −1 , obtained using all data measured during 1962-2012, and data just from fixed points (Location A, B and C) as well as via difference between 1963 and 2012 are almost entirely the same, indicating that this mean increase rate is reasonable and reliable.This pCO 2 increase rate in the EIO is similar to that in the subtropical North Atlantic (Bates, 2007), higher than that in the western tropical North Atlantic (Park and Wanninkhof, 2012), and lower than that in the equatorial Atlantic and Pacific (Oudot et al., 1995;Takahashi et al., 2006) (Table 3).
However, assessing the trend of surface pCO 2 in our study area is challenging due to lack of continuous data.Surface pCO 2 might also be modulated by some climate events such as the ENSO and the IOD.For example, Takahashi et al. (2003) investigated the effects of the Pacific Decadal Oscillation phase shift on decadal pCO 2 variation, and found during the decade before the shift, surface pCO 2 in the central and western equatorial Pacific reduced at a mean rate of about -20 µatm per decade, whereas after the shift, it rised at ∼ 15 µatm per decade.Also model studies showed that the Arabian Sea and Southern Peninsular India interannual CO 2 emission anomalies were related to the IOD and ENSO (Valsala and Maksyutov, 2013).In our study, Introduction

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Full influence of climate events are not taken into account due to lack of continuous data since 1962.Especially, there were no data available during 1963-1987, and we don't know what happened to sea surface pCO 2 during this period.These may cause uncertainties for the determination of pCO 2 trend.However, these climate signals will fade away when timescales lengthen (Fay and McKinley, 2013).Thus, our study at least provides the mean rate of sea surface pCO 2 increase from 1962 to 2012, which is important for understanding the variation of ocean CO 2 system (carbon speciation) and its potential effects on marine organisms.It also implies that big changes in carbon chemistry (e.g., OA) and even other marine biogeochemical processes may have taken place in the Indian Ocean since the IIOE.Thus, it is very essential to conduct IIOE-2 (planned in 2015-2020) from the viewpoint of global change, besides the fundamental developments in technology.

Possible factors responsible for sea surface pCO 2 increase
It was obvious that the observed increase of springtime sea surface pCO 2 with time was not due to the local uptake of CO 2 via air-sea exchange, given that the EIO was almost always a CO 2 source to the atmosphere (Fig. 4, Bates et al., 2006;Takahashi et al., 2009).Although atmospheric CO 2 was rapidly rising since 1962, which has the potential to increase seawater pCO 2 via air-sea equilibrium and reduce the CO 2 release from the ocean to the atmosphere, pCO 2 at the sea surface in the EIO was still larger than that in the atmosphere during 1962-2012 (Fig. 4), indicating no local uptake of CO 2 from the atmosphere in the EIO (Bates et al., 2006;Takahashi et al., 2009).Since sea surface pCO 2 was mainly influenced by physical and biological processes such as temperature, horizontal and vertical mixing, advection, ocean circulation, and biological activity (e.g., Sabine and Key, 1998;Xue et al., 2012), we will check the effects of SST, SSS, MLD, ocean circulation and Chl a (as a proxy of biological production) on surface pCO 2 in the following sections.Introduction

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Full Generally, seawater pCO 2 increases with temperature (e.g., Takahashi et al., 1993).However, we found that sea surface pCO 2 increase in the EO did not correspond to SST rise.For instance, surface pCO 2 in 1987-2012 was much higher than that in July 1962and April 1963, whereas SST in July 1962and April 1963 was almost at the same level as that in 1987-2012 (Fig. 5a).This suggests temperature may not be responsible for surface pCO 2 increase during the study period.Further, we checked the trend of SST from 1962 to 2012 using ERSST data (Extended Reconstructed Sea Surface Temperature, Table 2) and found SST significantly increased with an increase rate of 0.013 • C yr −1 (Fig. 6a).That is to say, during this period surface ocean warming occurred.It will cause surface pCO 2 to increase at a rate of ∼ 0.1 µatm yr −1 alone based on the pCO 2 -temperature relationship (i.e., (αpCO 2 /αT )/pCO 2 = 0.0423 (Takahashi et al., 1993).This is coherent with the increase rate of pCO 2 normalized to a constant temperature of 29.60 • C (npCO 2 ), the mean temperature of all data.We found surface npCO 2 still increased at a mean rate of 1.62 µatm yr −1 , i.e., the nontemperature effect (Fig. 4b).Thus, surface pCO 2 increase during the period 1962-2012 was partially trigged by surface ocean warming, and was essentially due to change in seawater CO 2 concentration.In general, surface pCO 2 in the open ocean rises with increased salinity as shown in Figs. 3 and 5. From Fig. 3, we found SSS variation may not be responsible for surface pCO 2 increase.For example, SSS between 2 • S and 2 • N in May 1999 was almost the same (34.2-34.6)as that in May 2012, but npCO 2 in May 2012 was clearly higher than that in May 1999 (Figs. 3 and 5).Although no significant trend in SSS from Fig. 6b  MLD deepening will bring CO 2 -rich waters to surface layer, leading to high pCO 2 level (e.g., Dumousseaud et al., 2010).We find MLD thickness showed an increase trend during 1969-1985, a decrease trend during 1985-1997 and then an increase trend during 1997-2009 (Fig. 6c).Overall, there is no significant trend for MLD during 1969-2009.However, surface pCO 2 showed a clear increase trend during 1962-2012, especially during 1987-2012.Thus, although MLD variation may lead to increase in surface pCO 2 , for example, during 1997-2009 when MLD deepened, it was not the primary factor causing pCO 2 increase over the period 1962-2012.
Biological activity could also affect seawater pCO 2 (e.g., Zhang et al., 2010).We found from 1998 through 2012 there was no significant trend for Chl a (Fig. 6d).Model studies also indicated that there were no significant changes in net primary production, particle export production and export efficiency from 1960 to 2006 in this region (Laufkötter et al., 2013).Thus, it revealed that biological activity was not the main factor leading to pCO 2 rise.
In all, surface ocean warming (SST increase) partially contributed to springtime sea surface pCO 2 increase on interannual timescales, while the effects of salinity, MLD, and biological activity were not significant.

Influence of ocean circulation
The increase of the EIO pCO 2 with time may be primarily due to the transport of carbon accumulated in the CO 2 sink region (to the atmosphere) towards the CO 2 source region on a basin scale via ocean circulation.We know that the region between 15 • S and 50 • S in the Indian Ocean, a major subduction zone of the Indian Ocean (Schott et al., 2009), is a significant sink of atmospheric CO 2 (Fig. 1a, Takahashi et al., 2009;Valsala et al., 2012) and, therefore, anthropogenic CO 2 will be accumulated in this region.For example, it was reported that the inventories of anthropogenic CO 2 were largest in this zone in the Indian Ocean (Sabine et al., 2004), and the oceanic increase in carbon storage roughly kept pace with atmospheric CO 2 increase (Sabine et al., 1999).Introduction

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Full On the other hand, ocean circulations (e.g., a shallow meridional overturning circulation) in the Indian Ocean allow the carbon accumulated in the CO 2 sink region to be transported towards the equatorial Indian Ocean (Fig. 1, Schott et al., 2002Schott et al., , 2009;;Valsala et al., 2012).For instance, subducted water masses first move westward in the South Equatorial Current (SEC), which splits into the Northeast and the Southeast Madagascar Currents, then part of the subducted water masses merge into the East African Coast Current (EACC), and finally a fraction of the northern SEC/EACC inflow returns eastward just south of the equator during the winter monsoon with the South Equatorial Countercurrent (SECC), which exists year round, except that it is subsurface during the summer monsoon (Fig. 1, Schott and McCreary, 2001;Schott et al., 2009 and references therein).At this time, at the eastern end of the SECC, the South Java Current (SJC, Fig. 1) flows southeastward (Schott and McCreary, 2001).In addition, the strong eastward Wyrtki jets (Wyrtki, 1973;Schott and McCreary, 2001) in the surface layer and the eastward Equatorial Undercurrent (Knauss and Taft, 1964) in the subsurface layer may also play an important role in transporting waters eastward (Xuan et al., 2012).Thus, the southern subtropical Indian Ocean acted as a window for carbon uptake (Valsala et al., 2012) and part of the absorbed CO 2 could be ultimately transported to the northern Indian Ocean including the equatorial belt.Similar mechanism was also proposed in the equatorial Pacific by Feely et al. (1999), who pointed out that the entrained subtropical water was injected into the upwelled water at the equator.
Nevertheless, the pathways of ocean circulations are very complicated, and there must be other ways to increase pCO 2 in the equatorial Indian Ocean.For instance, the Red Sea-Persian Gulf Intermediate Water formed in the northwestern Indian Ocean carry anthropogenic CO 2 signals and spread equatorward (Sabine et al., 2004;Alvarez et al., 2009 and references therein), which also contributes to the increase of equatorial waters pCO 2 .Thus, further studies on carbon transport should be conducted in the Indian Ocean in the future.Even so, we speculate that the CO 2 sink region to the atmosphere may be a window for CO 2 uptake (e.g., Valsala et al., 2012), and the mixing between waters in the CO 2 sink region and the CO 2 source region via large Introduction

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Full scales ocean circulation may be driving the pCO 2 trend in the global ocean to tend towards that in the atmosphere (Fay and McKinley, 2013).Thus, signal of increasing atmospheric CO 2 was transmitted to the global ocean this way.

Implications and impacts of surface pCO 2 increase
Increase in surface pCO 2 will impact oceanic CO 2 uptake.In the EIO, the mean pCO 2 increase rate during spring in the ocean (∼ 1.7 µatm yr −1 ) is larger than that in the atmosphere measured at Mauna Loa, Hawaii (1.41 µatm yr −1 , Fig. 4a).It indicates that the driving force of CO 2 air-sea exchange (∆pCO 2 = pCO 2 sea − pCO 2 air ) became larger than before.Thus, more and more CO 2 in this CO 2 source region would be released from the ocean to the atmosphere, which may cause the decrease of global oceanic carbon sink, if the gas transfer velocity keeps constant.Also a decrease in oceanic carbon sink has been observed in other oceans such as in the North Atlantic (Lefvre et al., 2004;Schuster et al., 2009) due to the faster increase of oceanic pCO 2 than atmospheric pCO 2 .
In addition, oceanic pCO 2 increase may alter the speciation of seawater carbon chemistry, probably exerting negative effects on marine organisms and ecosystems (Doney et al., 2009).Oceanic pCO 2 rise means the increase in seawater CO 2 and HCO − 3 concentrations, and decrease in CO 2− 3 and pH levels (i.e., OA).Given that in surface waters of most oceans the CaCO 3 saturation state is larger than 1 and CaCO 3 dissolution yet does not begin (Feely et al., 2004), TA at the surface may not change due to CaCO 3 dissolution (note that CO 2 air-sea exchange does not change TA).Thus, we can assume that TA, salinity and temperature keep constant during the period 1962-2013, and estimate pH (on NBS scales) and aragonite saturation state (Ω arg ) from surface pCO 2 , TA, temperature and salinity employing CO2sys (Lewis and Wallace, 1998) and adopting the CO 2 system coefficients of Mehrbach et al. (1973) as refitted by Dickson and Millero (1987).In fact, temperature rise and salinity decrease have happened as described in Sect.3.2.1.They both will lower pH, since temperature Introduction

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Full (mean surface pH in the 2012 MOMSEI cruise was 8.23 ± 0.02), and correspondingly aragonite saturation state (Ω arg ) would drop from 4.4 to 3.9.Reduction in pH (0.07 pH units) during the past 50 yr  in the EIO is surprising, considering that the average surface ocean pH has just declined by about 0.1 units since the 1700s due to the absorption of anthropogenic CO 2 (Raven et al., 2005).Noticeably, sea surface pCO 2 increase has been observed in various oceanic zones, some of which serve as CO 2 sinks and some as CO 2 sources to the atmosphere (Table 3).It seems that ocean pCO 2 is rising on a global scale due to the forcing of rising atmospheric CO 2 and global warming (Fay and McKinley, 2013).It also demonstrated that OA may have taken place in the global ocean, not just limited to the CO 2 sink region.

Summary and conclusions
Results showed that sea surface pCO 2 during spring in the EIO significantly increased from1962 to 2012, with a mean rate of ∼ 1.7 µatm yr −1 .Assessing the trend of surface pCO 2 in the study area is full of challenge due to lack of continuous data, and more observations with good temporal and spatial resolutions are needed in the future.However, this study not only provides direct evidence for OA in the Indian Ocean, but also implies that changes in other marine biogeochemical processes may have taken place in the Indian Ocean since the IIOE conducted during 1960-1965.
Given that the EIO during the study period was almost always a CO 2 source to the atmosphere, it was obvious that the observed increase of springtime sea surface pCO 2 with time in this region was not due to the local uptake of CO 2 via air-sea exchange, although quickly increasing atmospheric CO 2 had the potential to increase seawater

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Full pCO 2 .We found surface ocean warming partially contributed to sea surface pCO 2 increase, whereas the effects of salinity, mixed layer depth, and biological activity were not significant.The pCO 2 increase in the equatorial waters was probably due to the transport of carbon accumulated in the CO 2 sink region (to the atmosphere) towards the CO 2 source region on a basin scale via ocean circulation.Additionally, our study showed that more and more release of CO 2 from the ocean to the atmosphere and big pH reduction (0.07 pH units) in the past 50 yr (from 1963 to 2012) may have occurred in the EIO.In conjunction with the studies conducted in various oceanic zones about interannual variations in sea surface pCO 2 , it seems that sea surface pCO 2 increase on interannual timescales occurs in both CO 2 source and sink regions, and possibly that ocean pCO 2 is rising on a global scale due to the forcing of rising atmospheric CO 2 level and global warming (Fay and McKinley, 2013).It also demonstrated that OA may have taken place in the global ocean, not just limited to the CO 2 sink region.

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Full  Center.Details on ERSST.v3 are described in Smith et al. (2008).Monthly mean data in the study area ( 5• N to 5 • S along 90 • E and 95 • E) during April/May from 1962-2012 are selected.b This mixed layer depth is in fact a temperature-mixed layer depth, or isothermal layer depth.Mixed layer depths are computed as the depth with a 0.2 • C absolute temperature difference from 10 m temperature (Keerthi et al., 2013) Full      2 for more details.
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The historical data before 2012 were integrated by Takahashi et al. (2013) (i.e.Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | creased from ∼ 308 µatm in April 1963, through ∼ 373 µatm in May 1999, to ∼ 387 µatm in May 2012 (Fig. 4a and Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | could be found, more recently Subrahmanyam et al. (2011) reported a significant freshening trend in SSS during 2002-2007 in the EIO, which indicates a decrease of surface pCO 2 .Thus, we inferred that SSS change may not be the major factor triggering pCO 2 increase.Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |rise will raise the concentration of hydrogen ion, and salinity decline will decrease TA.If we use the mean surface TA (2243 µmol kg −1 ) estimated byLee et al. (2006), SSS (34.08),SST (29.60 • C) and mean surface npCO 2 in April 1963 and May 2012, surface pH at 25.0 • C in the EIO would decrease from 8.33 in April1963 to 8.26 in May 2012 Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Denman, K. L., Brasseur, G., Chidthaisong, A., Ciais, P., Cox, P. M., Dickinson, R. E., Hauglustaine, D., Heinze, C., Holland, E., Jacob, D., Lohmann, U., Ramachandran, S., Da Silva Dias, P. L., Wofsy, S. C., and Zhang, X.: Couplings between changes in the climate system and biogeochemistry, in: Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Discussion Paper | Discussion Paper | Discussion Paper |
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Table 2 .
Sources of data used in this study.ERSST.v3bisthe most recent version, from US National Oceanic and Atmospheric Administration, National Climatic Data a . Monthly mixed layer depth data during April from 1969 to 2009 in the study area are used.Atmospheric CO 2 data were measured as the mole fraction in dry air on Mauna Loa, Hawaii, the longest record of direct measurements of CO 2 in the atmosphere.Monthly mean values during 1962-2012 were used.Introduction c Chlorophyll a data during April/May from 1998 to 2012 are used.d See specific cruise information in Table 1.e