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.