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The decrease in δ13C of dissolved inorganic carbon (δ13CDIC) owing to uptake of anthropogenic CO2 (the oceanic 13C Suess effect) in the Southeastern Indian Ocean over the last decade was calculated using an extended multiparameter linear regression technique. Samples collected on the CROCCA‐2S (Coring to Reconstruct Ocean Circulation and Carbon Dioxide Across 2 Seas) cruise in November–December 2018 were compared to samples from the CLIVAR (Climate and Ocean: Variability, Predictability, and Change) and OISO (Océan Indien Service d'Observation) programs from 2007 to 2009. Surface ocean δ13CDIC decreased by an average of −0.53 ± 0.04‰ across this period, at an average rate of −0.053 ± 0.004‰ per year. This rate of δ13CDIC change is an increase from −0.021 ± 0.024‰ per year between 1994 and 2008. We find that the interior water mass most impacted by the oceanic 13C Suess effect between 2008 and 2018 was Subantarctic Mode Water (SAMW), within which δ13CDIC decreased by −0.044 ± 0.002‰ per year. Using previously published relationships between the oceanic 13C Suess effect and anthropogenic carbon, we estimate the annual storage of anthropogenic carbon within SAMW in the southeastern Indian Ocean has increased from ∼2.0 ± 0.2 μmol/kg per year between 1994 and 2008 to 5.5 ± 0.6 μmol/kg per year between 2008 and 2018. Plain Language Summary: We measured the ratio of carbon isotopes within seawater to identify the amount of atmospheric carbon sourced from the burning of biomass and fossil fuels which has been absorbed into the southeastern Indian Ocean over the last decade. We find that the rate at which this human‐sourced carbon is being absorbed by the ocean has increased up to 3‐fold over this time period, when compared with the period between 1994 and 2008. Key Points: The oceanic Suess effect disproportionately affects Subantarctic Mode Water (SAMW), within which δ13C has decreased by c. −0.44‰ from 2008 to 2018The observed δ13C change of −0.044‰/year represents roughly a 3‐fold increase in the rate of δ13C change compared to 1994–2008Between 2008 and 2018 SAMW in the SE Indian Ocean absorbed 5.5 ± 0.6 μmol/kg−1 of anthropogenic carbon per year [ABSTRACT FROM AUTHOR]

The spatiotemporal variability of upper ocean inorganic carbon parameters and air-sea CO2 exchange in the Indian Ocean was examined using inorganic carbon data collected as part of the World Ocean Circulation Experiment (WOCE) cruises in 1995. Multiple linear regression methods were used to interpolate and extrapolate the temporally and geographically limited inorganic carbon data set to the entire Indian Ocean basin using other climatological hydrographic and biogeochemical data. The spatiotemporal distributions of total carbon dioxide (TCO2), alkalinity, and seawater pCO2 were evaluated for the Indian Ocean and regions of interest including the Arabian Sea, Bay of Bengal, and 10°N-35°S zones. The Indian Ocean was a net source of CO2 to the atmosphere, and a net sea-to-air CO2 flux of +237 ± 132 Tg C yr-1 (+0.24 Pg C yr-1) was estimated. Regionally, the Arabian Sea, Bay of Bengal, and 10°N-10°S zones were perennial sources of CO2 to the atmosphere. In the 10°S-35°S zone, the CO2 sink or source status of the surface ocean shifts seasonally, although the region is a net oceanic sink of atmospheric CO2. [ABSTRACT FROM AUTHOR]