Global carbon dioxide flux into the surface ocean intensified by rain-induced dilution

Rain impacts the flux of carbon dioxide (CO2) across the air–sea interface by three mechanisms: wet deposition, enhancement of surface gas exchange through turbulence and changes in the temperature and salinity of the ocean’s top microlayer. Understanding of these effects has progressed to enable improved estimations for the impact of rain on global ocean carbon uptake, but substantial epistemic uncertainty in estimates of rain-induced salinity dilution limits further constraint on the ocean carbon sink. Here we present a global analysis of rain effects on CO2 exchange employing the most realistic available model of rain-induced dilution for high-resolution satellite datasets, yielding a total rain-induced increase in ocean uptake of 13.6% relative to a wind-only effect. Dilution is found to be responsible for about two-thirds of the total additional uptake due to freshening-induced repartitioning of the carbonate system. This doubles the previously reported upper bound on rain-enhanced ocean carbon uptake, highlighting the importance of saltwater dilution by rain and the corresponding need to improve quantitative understanding of the surface ocean during and after rain. We estimate that observational coverage of carbon in the surface ocean implicitly captures 30% of the dilution effect and find that current global carbon budget estimates are probably underestimating the ocean carbon sink by 10 ± 5% due to their exclusion of rain effects.