Modulation of Surface Seawater CO2 System at 80°E: Impacts of the Positive IOD in 2019

Abstract

To elucidate impacts of an Indian Ocean Dipole (IOD) event on surface seawater CO₂ dynamics, we analyzed data collected along the World Ocean Circulation Experiment Hydrographic Program I08 N line (latitudes 20°S–6°N at 80°E) in December 2019, when a strong positive IOD (pIOD) event occurred. After removing the effects of anthropogenic CO₂ accumulation, we examined anomalies of the surface seawater CO₂ fugacity (fCO₂) from the climatology in relation to other marine properties. At latitudes 11°S–6°S, where horizontal advection of upwelled water off Sumatra was observed, dissolved inorganic carbon and total alkalinity, both normalized to a salinity of 35 (nTCO₂ and nTA) showed positive anomalies of +11.4 and +8.1 μmol kg⁻¹, respectively. At latitudes 5°S–5°N, where distinct low-salinity water was observed because of the pIOD, nTCO₂ and nTA showed negative anomalies of −4.0 and ‒0.5 μmol kg⁻¹, respectively. The combined effects of the nTCO₂ and nTA anomalies on fCO₂ made the observed fCO₂ anomalies small, +3.2 and −6.6 μatm for 6°S–11°S and 5°S–5°N, respectively, because the direction of the Revelle factor for TCO₂ is opposite to that for TA. We estimated that the pIOD modulated the air–sea CO₂ flux by +0.45 and −0.55 mmol m⁻² d⁻¹ on average within11°S–6°S and 5°S–5°N, respectively. The impacts of the pIOD on the CO₂ dynamics could be explained by the anomalous salinity conditions associated with upwelled water and the freshwater balance.