The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.