Decadal increase of oceanic carbon dioxide in Southern Indian Ocean surface waters (1991–2007)

Abstract

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-Dufresne during the period 1991–2007. 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 (>20°S), indicates that oceanic fCO₂ increased at a rate of 2.11 (±0.07) μatm/yr for the period 1991–2007, i.e. about 0.4 μatm/yr faster than in the atmosphere. In order to investigate the processes that explain the oceanic fCO₂ variations (and the potential reduction of the ocean carbon sink), the decadal variability is analyzed in detail in four regions (20–35°S, 35–40°S, 40–42°S and 50–55°S) for austral summer (December–March) and winter (June–August). During austral summer, the fCO₂ increase is similar in the four regions (between +2.2 and +2.4 μatm/yr). For austral winter the growth rate is lower north of 40°S (+1.5 to +1.7 μatm/yr) than at higher latitudes (+2.2 μatm/yr). Because these regions experienced different warming or cooling, the evolution of temperature normalized fCO₂ (fCO₂^norm) has also been investigated. In the southern subtropical region (35–40°S), warming occurred in winter, leading to a small change of fCO₂^norm (+0.6 μatm/yr). In this region, anthropogenic CO₂ uptake must be compensated by a reduction of dissolved inorganic carbon (DIC) in surface waters. At latitudes >40°S, the observed cooling during winter leads to a rapid increase of fCO₂^norm (+3.6 to +4.7 μatm/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 40°S are likely related to the high index state of the Southern Annular Mode (SAM) during the 1990s. The increase of the westerlies at latitudes >40°S 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 40°S would have reduced vertical mixing. Although this analysis is limited to a relatively short period, 1991–2007, 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-situ ocean carbon dioxide observations. This offers an encouraging result in the perspective of model validation and understanding of the future evolution of the ocean carbon sink and its coupling with climate change.

Introduction

Anthropogenic emissions of carbon dioxide (CO₂) into the atmosphere from fossil fuel and land use change increased dramatically from about 5.5 PgC/yr (1 Pg=10¹⁵ g) 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 and terrestrial 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 in Le Quéré and Metzl, 2003). The most recent estimate based on an international global ocean pCO₂ data synthesis indicates that total ocean CO₂ uptake is −1.8 (±0.7) 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 (1800–1994) global ocean carbon inventories derived from in-situ observations suggest that the ocean 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 both climate 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 air–sea 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 CO₂ emissions 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 or fCO₂) has been relatively well documented in the North Atlantic and Pacific Oceans (e.g., Bates et al., 1996; Bates, 2001; Feely et al., 2002, Feely et al., 2006; Lefèvre et al., 2004; Schuster and Watson, 2007; Takahashi et al., 2006, Takahashi et al., 2009). The increase of DIC in sea-surface water 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; Corbière 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 air–sea fluxes decadal changes has never been clearly established.

In the Southern Hemisphere, the long-term evolution of oceanic pCO₂ is 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; Brévière et al., 2006). Therefore, the detection of the decadal pCO₂ 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 gradients, leading to more positive state of the so-called Southern Annular Mode (SAM) (Marshall, 2003). The variability of the SAM can 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, 2007; Le Quéré et al., 2007; Lovenduski et al., 2007; Verdy et al., 2007) and inversions of atmospheric CO₂ observations (Le Quéré et al., 2007) indicate that climate variability in the Southern Hemisphere may dramatically impact the ocean carbon cycle and CO₂ air–sea 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 1991–2003 (Borges et al., 2008). Although warming would result in ocean CO₂ outgassing anomalies, Borges et al (2008) found that positive (negative) CO₂ air–sea fluxes inter-annual anomalies are usually 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; Brévière 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 decadal fCO₂ changes in the south-western Indian Ocean (20–60°S/30–90°E) based on observations obtained during 1991–2007. 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.