The Indian Ocean significantly influences the global carbon cycle but it is one of the undersampled regions with reference to surface ocean pCO2. As a part of the Regional Carbon Cycle Assessment and Processes-2 (RECCAP2) project, several approaches, such as interpolated observational climatology, hindcast model, observation-based surface CO2 (empirical models), and atmospheric inversion models have been employed for estimating net sea-to-air CO2 fluxes between 1985 and 2018. The seasonal, spatial and long-term variability in sea-to-air fluxes of CO2 were compared with observational climatology. The mean value of CO2 in the Indian Ocean (north of 37.5°S) for the period of 1985–2018 using all models is estimated to be −0.19 ± 0.1 PgC yr−1 and it is consistent with the observational climatology (−0.07 ± 0.14 PgC yr−1). The Indian Ocean north of 18°S is found to be the mean annual source (0.04 ± 0.05 PgC yr−1) whereas a net sink (−0.23 ± 0.11 PgC yr−1) in the south of 18°S. All models captured observed spatial patterns but underestimated the net source of CO2 in the Oman/Somalia upwelling, the Equatorial Indian Ocean and the Bay of Bengal whereas CO2 sink is overestimated in the South Indian Ocean. Overall, all models captured the seasonality in pCO2 levels and CO2 fluxes but overestimated the amplitude of their variability. All models suggested the strengthening of the sink over the period between 1985 and 2018 by 0.02 PgC yr−1 decade−1. A significant increase in the collection of surface ocean pCO2 and atmospheric CO2 measurements improves the model simulations in the Indian Ocean.

Key Points

Northern Indian Ocean is a source whereas South Indian Ocean (SIO) is a sink for CO2

Mean uptake of CO2 using all models comes to −0.18 ± 0.1 PgC yr−1

Net CO2 flux is underestimated off Somalia, Bay of Bengal and Equatorial Indian Ocean whereas sink is over estimated in SIO

Plain Language Summary

The Indian Ocean is under-sampled with reference to pCO2 levels and CO2 fluxes. We evaluated the CO2 fluxes simulated by different models based on observational CO2 flux climatology. The CO2 fluxes estimated by all models are close to climatological value; however under and/or overestimation of fluxes are noticed in several regions. Due to weaker monsoon mixing, accurate river discharge data and atmospheric deposition of pollutants in the model, they failed to reproduce actual CO2 fluxes. Inclusion of such processes in the model improves their performance in future.