Precipitation-Induced Reduction in Surface Ocean pCO(2): Observations From the Eastern Tropical Pacific Ocean

Determining air-sea CO2 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 CO2 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 CO2 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.