Decomposing tropical Pacific air-sea CO2 flux anomalies during marine heatwaves and cold spells: thermal versus non-thermal drivers

Marine heatwaves (MHWs) and marine cold spells (MCSs) are extreme thermal events that can significantly influence air-sea CO2 fluxes (FCO2), yet their impacts in the tropical Pacific remain poorly constrained. Using multi-source observations and machine-learning models (Random Forest, XGBoost, LightGBM), we assessed FCO2 anomalies during MHWs and MCSs from 1990 to 2019. Results show that MHWs enhance oceanic CO2 uptake by ~0.53 mol C m-2 yr-1, whereas MCSs reduce uptake by ~0.28 mol C m-2 yr-1. Taylor decomposition indicates that non-thermal factors, especially dissolved inorganic carbon (DIC), dominate pCO2sea anomalies. LightGBM provides superior predictive skill and reveals that FCO2 variability is shaped more strongly by spatial structure than by temporal fluctuations and that wind variability plays a particularly strong role among the physical processes driving the non-temperature-related FCO2 anomalies. These findings improve our mechanistic understanding of how thermal extremes regulate ocean-atmosphere carbon exchange and inform projections of ocean carbon sinks under climate change.