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 CO 2 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 CO 2 accumulation, we examined anomalies of the surface seawater CO 2 fugacity (fCO 2 ) 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 (nTCO 2 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, nTCO 2 and nTA showed negative anomalies of −4.0 and ‒0.5 μmol kg −1 , respectively. The combined effects of the nTCO 2 and nTA anomalies on fCO 2 made the observed fCO 2 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 TCO 2 is opposite to that for TA. We estimated that the pIOD modulated the air–sea CO 2 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 CO 2 dynamics could be explained by the anomalous salinity conditions associated with upwelled water and the freshwater balance.