Environmental controls on the seasonal carbon dioxide fluxes in the northeastern Indian Ocean (original) (raw)
Related papers
A sink for atmospheric carbon dioxide in the northeast Indian Ocean
Journal of Geophysical Research, 1996
Intensive observations in the northeast Indian Ocean (Bay of Bengal) during the presouthwest and northeast monsoon seasons of 1991 reveal that freshwater discharge from rivers of the Indian subcontinent exerts the dominant control over total carbon dioxide (TCO2) and pCO 2 distributions in surface waters. Low pCO 2 levels occur within the low-salinity zones, with a large area in the northxvestem bay acting as a sink for atmosl:heric CO 2. Only a part of the observed pCO 2 variation can be accounted for by the effect of salinity, and biological production suppoaed by external nutrient inputs in conjunction with strong thermohaline stratification may be more important in lowering surface water pCO 2 by > 100 patm relative to that in the atmosphere. The pCO 2 distribution is seasonally variable and appears to be controlled by the spreading of fresher waters by the prevailing surface circulation.
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.
Journal of Oceanography, 2013
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Variability of Atmospheric CO2 Over India and Surrounding Oceans and Control by Surface Fluxes
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2012
In the present study, seasonal and inter-annual variability of atmospheric CO 2 concentration over India and surrounding oceans during 2002-2010 derived from Atmospheric InfrarRed Sounder observation and their relation with the natural flux exchanges over terrestrial Indian and surrounding oceans were analyzed. The natural fluxes over the terrestrial Indian in the form of net primary productivity (NPP) were simulated based on a terrestrial biosphere model governed by time varying climate parameters (solar radiation, air temperature, precipitation etc) and satellite greenness index together with the land use land cover and soil attribute maps. The flux exchanges over the oceans around India (Tropical Indian Ocean: TIO) were calculated based on a empirical model of CO 2 gas dissolution in the oceanic water governed by time varying upper ocean parameters such as gradient of partial pressure of CO 2 between ocean and atmosphere, winds, sea surface temperature and salinity. Comparison between the variability of atmospheric CO 2 anomaly with the anomaly of surface fluxes over India and surrounding oceans suggests that biosphere uptake over India and oceanic uptake over the south Indian Ocean could play positive role on the control of seasonal variability of atmospheric carbon dioxide growth rate. On inter-annual scale, flux exchanges over the tropical north Indian Ocean could play positive role on the control of atmospheric carbon dioxide growth rate.
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.
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.
Role of biology in the air–sea carbon flux in the Bay of Bengal and Arabian Sea
A physical-biological-chemical model (PBCM) is used for investigating the seasonal cycle of air–sea carbon flux and for assessing the effect of the biological processes on seasonal time scale in the Arabian Sea (AS) and Bay of Bengal (BoB), where the surface waters are subjected to contrasting physical conditions. The formulation of PBCM is given in Swathi et al (2000), and evaluation of several ammonium-inhibited nitrate uptake models is given in Sharada et al (2005). The PBCM is here first evaluated against JGOFS data on surface pCO 2 in AS, Bay of Bengal Process Studies (BoBPS) data on column integrated primary productivity in BoB, and WOCE I1 data on dissolved inorganic carbon (DIC) and alkalinity (ALK) in the upper 500 meters at 9 • N in AS and at 10 • N in BoB in September–October. There is good qualitative agreement with local quantitative discrepancies. The net effect of biological processes on air–sea carbon flux on seasonal time scale is determined with an auxiliary computational experiment, called the abiotic run, in which the biological processes are turned off. The difference between the biotic run and abiotic run is interpreted as the net effect of biological processes on the seasonal variability of chemical variables. The net biological effect on air–sea carbon flux is found to be highest in southwest monsoon season in the northwest AS, where strong upwelling drives intense new production. The biological effect is larger in AS than in BoB, as seasonal upwelling and mixing are strong in AS, especially in the northeast, while coastal upwelling and mixing are weak in BoB.
Atmospheric Environment, 2011
This study reveals that landesea breezes, atmospheric stability and influence of net ecosystem metabolism for the conversion of organic carbon to atmospheric CO 2 are the major driving forces behind the variation of atmospheric CO 2 at the landeocean boundary, northeast coast of India. The seasonal variation of partial pressure of CO 2 (pCO 2 ) and its efflux from the coastal water were several fold higher in the pre-monsoon (1807.9 AE 757.03 m atm; 579.03 AE 172.9 mM m À2 h À1 ) than in the monsoon (1070.5 AE 328.5 m atm; 258.96 AE 185.65 mM m À2 h À1 ) and the post-monsoon (615.7 AE 121.6 m atm; 53.27 AE 19.24 mM m À2 h À1 ). The mean photic zone productivity to column respiration ratio was 0.12 AE 0.08, revealing predominance of heterotrophic processes. Community respiration was at minimum during monsoon (38.82 AE 8.63 mM C m À2 d À1 ) but was at maximum (173.8 AE 111.8 mM C m À2 d À1 ) during pre-monsoon and intermittent (125.07 AE 11.97 mM C m À2 d À1 ) during post-monsoon. Diurnal variations of atmospheric CO 2 concentration were determined by local air circulations and atmospheric stability. Seasonal variations of atmospheric CO 2 bear a significant signature of biological processes occurring in the coastal water by means of airesea exchange, markedly affected by the net ecosystem metabolism. Important predictors of coastal atmospheric CO 2 in decreasing order of explained variability are wind direction, stability, CO 2 efflux and wind velocity.
A window for carbon uptake in the southern subtropical Indian Ocean
Geophysical Research Letters, 2012
Atmospheric CO 2 sinks in the southern Indian Ocean was examined for their seasonal, interannual and interdecadal variability. Two distinct zones of CO 2 uptake are identified; located in the 15 S-35 S band is the northern box where CO 2 uptake is dominated by the solubility pump, and in the latitudinal range of 35 S-50 S is the southern box where both the solubility and biological pump are equal players. The anthropogenic CO 2 inventories appear to be a result of deepening subduction of CO 2 and subsequent invasion into the northern domain. The seasonal and interannual variability of CO 2 sinks to the north are rather surface trapped, while the deep CO 2 variability is found to be coherent with the atmospheric forcing, consistent with decadal wind stress curl anomalies. This is a step towards separating the secular trends of deep ocean CO 2 from its natural variability in the analysis region.