Spatial variation of total CO2 and total alkalinity in the northern Indian Ocean: A novel approach for the quantification of anthropogenic CO2 in seawater (original) (raw)
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Journal of Marine Research, 1999
As part of a cooperative effort of the Joint Global Ocean Flux Study (JGOFS) and of the World Ocean Circulation Experiment (WOCE) program, we have measured total CO 2 (TCO 2 ) and total alkalinity (TA) along three sections in the northern Indian Ocean. One section through the Gulf of Aden to the Arabian Sea is parallel to the coast of Yemen. One section is across the Arabian Sea along the nominal 9N latitude and the other section is across the Bay of Bengal along the nominal 10N latitude. The measurements were performed on board R/V Knorr in September-October 1995. The primary purpose of this work is to understand the penetration of anthropogenic CO 2 along these ocean sections. Here, we present a novel approach to the calculation of anthropogenic CO 2 in the ocean based upon the fundamentals of water-sources mixing. Consequently, we rst describe the observations and mixing of water-sources before we describe the quanti cation of anthropogenic CO 2 concentrationsin these waters. The data show large spatial variations in surface seawater of both total CO 2 (up to 50 µmol kg 2 1 ) and total alkalinity (up to 40 µmol kg 2 1 ). The variations are mainly associated with physical processes characterized by water masses of different temperature and salinity. For example, at depths we observed low TCO 2 concentration at longitude 54E 6 2E associated with the low-salinity water mass owing northward. The contrasts between the sections across the Arabian Sea and the Bay of Bengal emphasize the large property differences between the two ocean basins. Multiparametric analyses on the data clearly show the relative contributions of different water-sources in each of the ocean sections. The mixing coefficients calculated from the multiparametric analyses are further used to quantify anthropogenic CO 2 concentrations in each water-source. The results indicate that the surface water-sources contain 47.8, 42.1 and 50.4 µmol kg 2 1 in the Gulf of Aden, the Arabian Sea and the Bay of Bengal, respectively. In the surface waters there is slightly more anthropogenic CO 2 across the Bay of Bengal than across the Arabian Sea. In contrast, anthropogenic CO 2 has penetrated signi cantly deeper in the Gulf of Aden than in the Arabian Sea and in the Bay of Bengal.
Journal of Marine Research, 1999
As part of a cooperative effort of the Joint Global Ocean Flux Study (JGOFS) and of the World Ocean Circulation Experiment (WOCE) program, we have measured total CO 2 (TCO 2) and total alkalinity (TA) along three sections in the northern Indian Ocean. One section through the Gulf of Aden to the Arabian Sea is parallel to the coast of Yemen. One section is across the Arabian Sea along the nominal 9N latitude and the other section is across the Bay of Bengal along the nominal 10N latitude. The measurements were performed on board R/V Knorr in September-October 1995. The primary purpose of this work is to understand the penetration of anthropogenic CO 2 along these ocean sections. Here, we present a novel approach to the calculation of anthropogenic CO 2 in the ocean based upon the fundamentals of water-sources mixing. Consequently, we rst describe the observations and mixing of water-sources before we describe the quanti cation of anthropogenic CO 2 concentrationsin these waters. The data show large spatial variations in surface seawater of both total CO 2 (up to 50 µmol kg 2 1) and total alkalinity (up to 40 µmol kg 2 1). The variations are mainly associated with physical processes characterized by water masses of different temperature and salinity. For example, at depths we observed low TCO 2 concentration at longitude 54E 6 2E associated with the low-salinity water mass owing northward. The contrasts between the sections across the Arabian Sea and the Bay of Bengal emphasize the large property differences between the two ocean basins. Multiparametric analyses on the data clearly show the relative contributions of different water-sources in each of the ocean sections. The mixing coefficients calculated from the multiparametric analyses are further used to quantify anthropogenic CO 2 concentrations in each water-source. The results indicate that the surface water-sources contain 47.8, 42.1 and 50.4 µmol kg 2 1 in the Gulf of Aden, the Arabian Sea and the Bay of Bengal, respectively. In the surface waters there is slightly more anthropogenic CO 2 across the Bay of Bengal than across the Arabian Sea. In contrast, anthropogenic CO 2 has penetrated signi cantly deeper in the Gulf of Aden than in the Arabian Sea and in the Bay of Bengal.
Sea–air CO2 fluxes in the Indian Ocean between 1990 and 2009
Biogeosciences, 2013
The Indian Ocean (44 • S-30 • N) plays an important role in the global carbon cycle, yet it remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea-air CO 2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea-air CO 2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea-air CO 2 uptake of −0.37 ± 0.06 PgC yr −1 is consistent with the −0.24 ± 0.12 PgC yr −1 calculated from observations. The fluxes from the southern Indian Ocean (18-44 • S; −0.43 ± 0.07 PgC yr −1 ) are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), overestimation in the northeastern region (Bay of Bengal) and underestimation of the CO 2 sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO 2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but overestimate the magnitude. The predicted sea-air CO 2 fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO 2 flux, whereas the atmospheric inversions predicted an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than that found by atmospheric inversions. Prediction of such weak interannual variability in CO 2 fluxes by atmospheric inversions was mainly caused by a lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990-2009 of −0.01 PgC decade −1 . This is inconsistent with the observations in the southwestern Indian Ocean that shows the growth rate of oceanic pCO 2 was faster than the observed atmospheric CO 2 growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s.
Sea-air CO2 fluxes in the Indian Ocean between 1990 and 2009
2013
The Indian Ocean (44 • S-30 • N) plays an important role in the global carbon cycle, yet it remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea-air CO 2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea-air CO 2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea-air CO 2 uptake of −0.37 ± 0.06 PgC yr −1 is consistent with the −0.24 ± 0.12 PgC yr −1 calculated from observations. The fluxes from the southern Indian Ocean (18-44 • S; −0.43 ± 0.07 PgC yr −1 ) are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), overestimation in the northeastern region (Bay of Bengal) and underestimation of the CO 2 sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO 2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but overestimate the magnitude. The predicted sea-air CO 2 fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO 2 flux, whereas the atmospheric inversions predicted an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than that found by atmospheric inversions. Prediction of such weak interannual variability in CO 2 fluxes by atmospheric inversions was mainly caused by a lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990-2009 of −0.01 PgC decade −1 . This is inconsistent with the observations in the southwestern Indian Ocean that shows the growth rate of oceanic pCO 2 was faster than the observed atmospheric CO 2 growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s.
Comparison of two approaches to quantify anthropogenic CO2 in the ocean' Results
2001
This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along the World Ocean Circulation Experiment cruise I1 (Goyet et al., 1999; Sabine et al., 1999). These two studies employed two different approaches to separate the anthropogenic CO2 signal from the large natural background variability. Sabine et al. (1999) used the AC* approach first described by Gruber et al. (1996), whereas Goyet et al. (1999) used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account tbr variations due to physical processes including mixing and the CO2 solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 conce...
Global Biogeochemical Cycles, 2004
1] We apply to the Indian Ocean a novel technique to estimate the distribution, total mass, and net air-sea flux of anthropogenic carbon. Chlorofluorocarbon data are used to constrain distributions of transit times from the surface to the interior that are constructed to accommodate a range of mixing scenarios, from no mixing (pure bulk advection) to strong mixing. The transit time distributions are then used to propagate to the interior the surface water history of anthropogenic carbon estimated in a way that includes temporal variation in CO 2 air-sea disequilibrium. By allowing for mixing in transport and for variable air-sea disequilibrium, we remove two sources of positive bias common in other studies. We estimate that the anthropogenic carbon mass in the Indian Ocean was 14.3-20.5 Gt in 2000, and the net air-sea flux was 0.26-0.36 Gt/yr. The upper bound of this range, the no-mixing limit, generally coincides with previous studies, while the lower bound, the strong-mixing limit, is significantly below previous studies.
Carbon budget in the eastern and central Arabian Sea: An Indian JGOFS synthesis
Global Biogeochemical Cycles, 2003
1] The carbon budget for the eastern and central Arabian Sea was constructed using results from the Modular Ocean Model and biogeochemical data collected largely under the Indian Joint Global Ocean Flux Study programme. The study region (east of 64°E and between 11°and 21°N) was divided into two vertical boxes; a surface box of the top 100 m that largely undergoes exchanges with atmosphere and exhibits relatively strong seasonal variability and a subsurface box between 100 and 1000 m. Water transport rates in surface layers were maximal (up to 83 Â 10 12 m 3 ) in the southwest monsoon season. Sinking from surface driven by convection (25 Â 10 12 m 3 ) largely supports lateral outflows of water in subsurface layers in the northeast monsoon. Surface waters are renewed 10 times faster (t = 0.8 years) than intermediate waters (t = 8 year). A net supply of 25 Tg C yr À1 is estimated to the upper 1000-m water column of the study area by the physical pump. Photosynthetic activity (234 Tg C yr À1 ) does not seem to support total carbon demands (1203 Tg C yr À1 ) by bacteria and microzooplankton and mesozooplankton in the surface layers. Carbon demand rate requires organic carbon nearly double that of the total living biomass production rate (644 Tg yr À1 ) suggesting that most of the demand might be met from internal cycling involving zooplankton grazing/excretion activities. Sinking flux (69 Tg C yr À1 ) from surface accounts for about 30% of the total photosynthetic production rate indicating intense remineralization of organic matter in the surface layers of the Arabian Sea. Grazing and excretion of carbon by the microzooplankton and mesozooplankton appear to easily sustain perennial supersaturation of carbon dioxide in surface waters of the Arabian Sea and emission of 32 Tg C yr À1 to the atmosphere.
Global Biogeochemical Cycles, 2001
This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along the World Ocean Circulation Experiment cruise I1 [Goyet et al., 1999; Sabine et al., 1999]. These two studies employed two different approaches to separate the anthropogenic CO2 signal from the large natural background variability. Sabine et al. [1999] used the AC* approach first described by Gruber et al. [1996], whereas Goyet et al. [1999] used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account t•br variations due to physical processes including mixing and the CO2 solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 concentrations estimated using the AC* approach average 12 + 4/•mol kg-• higher in this depth range than concentrations estimated using the MIX approach. Below •0800 m, the MIX approach estimates slightly higher anthropogenic CO2 concentrations and a deeper vertical penetration. Despite this compensatory effect, water column inventories estimated in the 0-3000 m depth range by the AC* approach are generally •020% higher than those estimated by the MIX approach, with this difference being statistically significant beyond the 0.001 level. We examine possible causes fbr these differences and identify a number of critical additional measurements that will make it possible to discriminate better between the two approaches.
Global Biogeochemical Cycles, 2001
This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along the World Ocean Circulation Experiment cruise I1 [Goyet et al., 1999; Sabine et al., 1999]. These two studies employed two different approaches to separate the anthropogenic CO2 signal from the large natural background variability. Sabine et al. [1999] used the AC* approach first described by Gruber et al. [1996], whereas Goyet et al. [1999] used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account t•br variations due to physical processes including mixing and the CO2 solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 concentrations estimated using the AC* approach average 12 + 4/•mol kg-• higher in this depth range than concentrations estimated using the MIX approach. Below •0800 m, the MIX approach estimates slightly higher anthropogenic CO2 concentrations and a deeper vertical penetration. Despite this compensatory effect, water column inventories estimated in the 0-3000 m depth range by the AC* approach are generally •020% higher than those estimated by the MIX approach, with this difference being statistically significant beyond the 0.001 level. We examine possible causes fbr these differences and identify a number of critical additional measurements that will make it possible to discriminate better between the two approaches.
Journal of Oceanography, 2013
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