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...pach et al., 2007) and up to 8.4 PgC yr−1 in 2006 (Boden et al., 2012). Of the total anthropogenic emissions, about half remain in the atmosphere, leading to warming of the globe in the recent years (=-=IPCC, 2007-=-), and the remaining half is stored in the ocean and on land. The Indian Ocean is unique compared to the other two major ocean basins as it is completely closed in the north by the Indian sub-continen...

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...ndian Ocean also experiences strong variability driven by the Indian Ocean Dipole (IOD), and is additionally influenced by the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) (=-=Saji et al., 1999-=-; Murthurgude et al., 2002; Thompson and Solomon, 2002). Several physical/hydrological/circulation factors (e.g. variability in atmospheric forcings, open boundaries such as ITF; Fieux et al., 1996; C...

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...on of the Indian Ocean in the Southern Hemisphere (18–44◦ S) had excellent agreement between ocean inversions and atmospheric inversions, more so with TRANSCOM T3L3 interannually resolved inversions (=-=Baker et al., 2006-=-) than with TRANSCOM T3L1, which only solved for mean fluxes (Gurney et al., 2003). Though the actual reasons are unknown for why interannual inversions agreed better with the bottom-up estimates than...

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...e inversions; light grey: one or two inversions). We note that the temporal period over which the atmospheric data were collected is not the same for all the stations. SSTs (sea surface temperatures; =-=Murtugudde and Busalacchi, 1999-=-; Xie et al., 2002). The southern tropical and subtropical Indian Ocean are also under the influence of water discharged from the Pacific via the Indonesian Through Flow (Valsala and Ikeda, 2007). The...

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...inity transport and the biogeochemical cycling of carbon and nitrogen. This zone is one of the most productive regions in the world, accounting for 15–20 % of global ocean primary productivity (e.g., =-=Chavez and Barber, 1987-=-; Behrenfield and Falkowski, 1997). Many previous studies on the ocean CO2 system in the Indian Ocean were concentrated in the northwestern Indian Ocean (Arabian Sea; e.g., George et al., 1994; Kumar ...

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...M) (Saji et al., 1999; Murthurgude et al., 2002; Thompson and Solomon, 2002). Several physical/hydrological/circulation factors (e.g. variability in atmospheric forcings, open boundaries such as ITF; =-=Fieux et al., 1996-=-; Coatanoan et al., 1999) influence sea–air CO2 fluxes and the carbon budgets (Sarma, 2006). The estimated sea–air CO2 fluxes at regional and basin scales suggest strong seasonal and interannual varia...

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...nd land use changes. These activities led to increased accumulation of CO2 in the atmosphere from ∼ 4.0 PgC yr−1 in 1970 to 6.8 PgC yr−1 in 2000 (Raupach et al., 2007) and up to 8.4 PgC yr−1 in 2006 (=-=Boden et al., 2012-=-). Of the total anthropogenic emissions, about half remain in the atmosphere, leading to warming of the globe in the recent years (IPCC, 2007), and the remaining half is stored in the ocean and on lan...

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... Sea; e.g., George et al., 1994; Kumar et al., 1996; Goyet et al., 1998; Sarma et al., 1996, 1998, 2003) and southwestern Indian Ocean, south of 35◦ S (e.g., Metzl et al., 1991; Poisson et al., 1993; =-=Metzl et al., 1995-=-, 1998; Metzl, 2009). The northeastern region (Bay of Bengal) receives a significant amount of freshwater and is strongly stratified compared to the northwestern Indian Ocean, leading to contrasting b...

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...ospheric and ocean inversions. 2.2.1 Observations Overall the Indian Ocean remains one of the least sampled basins in the world ocean with respect to CO2 measurements both in terms of space and time (=-=Pfeil et al., 2013-=-). Within the Indian Ocean there are only two zones where data are available: the Arabian Sea, where seasonal data are available, and southwestern Indian Ocean (near Kerguluen Islands), where longer-t...

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...esponsible for interannual variation over a group of years. Studies in the southwestern Indian Ocean based on long-term CO2 observations describing the spatiotemporal pCO2 variations for that region (=-=Goyet et al., 1991-=-; Metzl et al., 1991, 1995, 1998; Poisson et al., 1993, 1994; Metzl, 2009). In the period 1991–2007 Metzl (2009) calculated an oceanic pCO2 growth rate of 2.11± 0.11 µatm yr−1, which is ∼ 0.4 µatm yr−...

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...hott et al., 2009). These subducted water masses then travel to the northern Indian Ocean through a shallow meridional overturning circulation known as the cross-equatorial cell (Miyama et al., 2003; =-=Schott et al., 2002-=-). The Indian Ocean remains poorly sampled with respect to CO2 spatially, and more importantly temporally. At present only about 30 % of the region is sufficiently sampled to observationally resolve t...

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...10a; Sarma et al., 2012). The pCO2 in this region shows large seasonal variations associated with the monsoonal circulation, with maxima during summer and winter and minima in the transition periods (=-=Sarma et al., 1998-=-, 2000, 2012; Goyet et al., 1998; Sarma et al., 2003.) In addition to the geographical features, the atmospheric forcing of the Indian Ocean is also unique. The predominant westerly winds can be seen ...

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...by coastal upwelling in the northwestern Indian Ocean, i.e., off Somalia and off the Oman coast, where pCO2 levels as high as > 600 µatm have been reported during peak southwest (SW) monsoon periods (=-=Kortzinger and Duinker, 1997-=-; Goyet et al., 1998). All models underestimated these fluxes with the exception of NEMO-PLANKTON5, which overestimated the flux over the entire NIO. The differences in the response of the OBGMs in th...

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..., 1987; Behrenfield and Falkowski, 1997). Many previous studies on the ocean CO2 system in the Indian Ocean were concentrated in the northwestern Indian Ocean (Arabian Sea; e.g., George et al., 1994; =-=Kumar et al., 1996-=-; Goyet et al., 1998; Sarma et al., 1996, 1998, 2003) and southwestern Indian Ocean, south of 35◦ S (e.g., Metzl et al., 1991; Poisson et al., 1993; Metzl et al., 1995, 1998; Metzl, 2009). The northea...

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...9; Murthurgude et al., 2002; Thompson and Solomon, 2002). Several physical/hydrological/circulation factors (e.g. variability in atmospheric forcings, open boundaries such as ITF; Fieux et al., 1996; =-=Coatanoan et al., 1999-=-) influence sea–air CO2 fluxes and the carbon budgets (Sarma, 2006). The estimated sea–air CO2 fluxes at regional and basin scales suggest strong seasonal and interannual variations (Bates et al., 200...

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...the entire Indian Ocean using interpolation of surface partial pressure of CO2 (pCO2), dissolved inorganic carbon (DIC) and total alkalinity (TA) data collected during the Indian Ocean cruises (e.g., =-=Louanchi et al., 1996-=-; Sabine et al., 2000; Bates et al., 2006). These studies suggest that the region north of 20◦ S acts as a strong source of CO2 to the atmosphere (0.367 PgC yr−1) with a sink of atmospheric CO2 betwee...

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...ubsurface features than at the surface, marking the boundary between the lowoxygen, high-nutrient waters of the northern Indian Ocean and the high oxygen, low nutrient values of the subtropical gyre (=-=Wyrtki, 1973-=-). This front marks the northernmost extent of the low surface pCO2 values. This boundary occurred mostly north of 10◦ S by many models. In addition to this, the subtropical convergence zone is the mo...

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...fferences in pCO2 are likely related to challenges of simulating the mixing and biological production in the Arabian Sea in coarse-resolution OBGMs coupled to relatively simple biogeochemical models (=-=Friedrichs et al., 2007-=-). However, these higher pCO2 values do not translate to larger CO2 fluxes and in fact the OBGMs significantly underestimated sea–air CO2 fluxes in the Arabian Sea. This is because in the OBGMs the la...

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... in the northwestern Indian Ocean (Arabian Sea; e.g., George et al., 1994; Kumar et al., 1996; Goyet et al., 1998; Sarma et al., 1996, 1998, 2003) and southwestern Indian Ocean, south of 35◦ S (e.g., =-=Metzl et al., 1991-=-; Poisson et al., 1993; Metzl et al., 1995, 1998; Metzl, 2009). The northeastern region (Bay of Bengal) receives a significant amount of freshwater and is strongly stratified compared to the northwest...

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...uptake (30◦ N–44◦ S) from the biogeochemical ocean models and observations (dashed line) (PgC yr−1). Negative values reflect sea–air fluxes into the ocean. non-monsoon periods (Louanchi et al., 1996; =-=Sarma et al., 2000-=-). Encouragingly, OBGMs in the Arabian Sea do capture the observed net sea–air CO2 flux, although the strength of the source varies among the models. In contrast several of the ocean prognostic models...