Differential Impacts of Marine Heatwaves and Coldwaves on Air-Sea CO2 Flux Across Global Oceans

Zhao Meng

The contrasting impacts of marine heatwaves (MHWs) and marine cold waves (MCWs) on the ocean carbon cycle remain insufficiently understood. Based on observational and reanalysis data from 1990 to 2019, this study investigates the global-scale responses of air-sea CO2 fluxes (FCO2) to MHWs and MCWs. Results reveal that MHWs and MCWs exert opposing influences on FCO2, with the magnitude of MCW-induced changes approximately three times greater than those associated with MHWs. These responses exhibit pronounced spatial heterogeneity. Notably, the North Pacific subpolar region, Arabian Sea, equatorial central Pacific, and Southern Ocean subpolar region display patterns that deviate from global tendencies. Among them, the equatorial central Pacific emerges as a key driver of interannual variability in both global FCO2 and surface ocean partial pressure of CO2 (pCO2sea). In the Arabian Sea, FCO2 variability is primarily modulated by wind speed, whereas in the other three regions, pCO2sea plays a dominant role. While temperature variations largely control pCO2sea in most areas, non-thermal processes dominate in the four anomalous regions. In particular, freshwater fluxes from precipitation and evaporation, together with equatorial upwelling, are identified as critical regulators of FCO2 in the equatorial central Pacific. These findings advance our understanding of how extreme thermal events shape the ocean carbon sink and provide a scientific basis for improved carbon cycle assessments under ongoing climate change.