Sea-air CO2 fluxes in the Indian Ocean between 1990 and 2009

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 CO₂ 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 CO₂ fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea-air CO₂ 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 CO₂ sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO₂ 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 CO₂ fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO₂ 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 CO₂ 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 pCO₂ was faster than the observed atmospheric CO₂ growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s.