Precipitation‐Induced Reduction in Surface Ocean pCO 2 : Observations From the Eastern Tropical Pacific Ocean

Abstract Determining air‐sea CO 2 fluxes using pCO 2 disequilibrium requires knowing the gas transfer velocity and air‐sea pCO 2 difference. Most pCO 2 measurements are made from ships, whose uncontaminated seawater intakes are located at >5‐m depth to prevent ingestion of air. However, there could be bias in determinations of CO 2 fluxes if there is disagreement between pCO 2 measurements at the surface and 5 m. Seawater measurements made at the near surface and 5 m in the eastern Equatorial Pacific Ocean show that precipitation can dilute surface seawater salinity and lower the pCO 2 and dissolved inorganic carbon and raise pH of the same water, and that these changes in ocean chemistry are largely missed by measurements at 5 m. This finding implies that estimates of ocean CO 2 uptake might be underestimated, since rain will lower surface ocean pCO 2 in both source and sink regions and, hence, increase uptake in sink regions and decrease outgassing in source regions.