Decadal increases in anthropogenic CO2 along 20°S in the South Indian Ocean

Abstract

We used high‐quality data for dissolved inorganic carbon and related water properties in the Indian Ocean along 20°S (World Ocean Circulation Experiment Hydrographic Program line I3) and 24°S (I4) obtained 8 years apart (1995–2003/2004) to estimate decadal‐scale increases of anthropogenic CO2 in the interior of the South Indian Ocean. Significant increases were detected to about 1800 m depth in the longitude range 35–45°E. In the upper thermocline subtropical subsurface water and Indian Central Water, anthropogenic CO2 increased an average of 7.9 ± 1.1 and 7.7 ± 0.5 μmol kg−1, respectively, whereas in the lower thermocline Antarctic Intermediate Water, the increase was 3.8 ± 0.7 μmol kg−1. A significant increase was also detected in Circumpolar Deep Water (2.5 ± 1.0 μmol kg−1). The estimated uptake rate of anthropogenic CO2 along the I3/I4 line over this time interval was 1.0 ± 0.1 mol m−2 a−1. Seasonal variations, which are influential in this ocean because of the Indian monsoon, did not affect detection of the anthropogenic CO2 signals. Comparisons with previous studies showed that increases of anthropogenic CO2 became larger in the most recent decade and that the CO2 uptake rate was similar to that in the South Pacific (1.0 ± 0.4 mol m−2 a−1) but higher than those in the South Atlantic (0.6 ± 0.1 mol m−2 a−1) and North Pacific (0.5 ± 0.1 mol m−2 a−1) Oceans. Deep penetration of anthropogenic CO2 is possibly associated with the higher uptake rate.