ENSO‐Driven Pacific Ocean Carbon Sink Variability During Boreal Autumn Over 1985–2021

The global ocean absorbs approximately one‐quarter of anthropogenic carbon dioxide (CO 2 ) emissions, but its uptake varies substantially on interannual timescales. The El Niño‐Southern Oscillation (ENSO) strongly modulates equatorial Pacific CO 2 exchange, yet the controls on the basin‐wide variability in carbon flux remain incompletely understood. This study integrates a 37‐year observation‐based air‐sea CO 2 flux product with carbonate‐system decomposition and CESM1 sensitivity experiments to quantify and diagnose the Pacific response during boreal autumn. Basin‐integrated regression analysis yields an ENSO sensitivity of −115 ± 28 Tg C yr −1 per standard deviation of the Niño 3.4 index. This response arises mainly from ENSO‐related wind‐stress anomalies that modify Ekman pumping and surface ocean p CO 2 through changes in salinity‐normalized dissolved inorganic carbon (nDIC). During El Niño, weakened equatorial upwelling limits the supply of carbon‐rich subsurface waters, reducing the nDIC contribution to surface ocean p CO 2 and thereby decreasing CO 2 outgassing despite surface warming. This study provides critical insights for projecting Pacific carbon uptake under future climate variability.