Atmospheric oxygen constraints on Southern Ocean productivity and drivers of carbon uptake

Ocean net primary production fixes dissolved carbon into organic matter while producing O₂, driving the biological carbon pump that contributes to ocean CO₂ uptake. The Southern Ocean plays a critical role in carbon export, yet its productivity estimates remain highly uncertain due to limited observations. Here we constrain Southern Ocean (south of ~44° S) net primary production by linking Coupled Model Intercomparison Project Phase 6 (CMIP6)-modelled productivity to modelled air–sea O₂ fluxes and applying O₂ flux estimates derived from airborne O₂/N₂ observations. We find an annual net primary production of 6.5 ± 1.36 PgC yr⁻¹, substantially higher than most CMIP6 model and satellite-based estimates, but consistent with Argo oxygen-based estimates. We show that CMIP6 models with underestimated productivity exhibit weak summer CO₂ uptake, with some also showing excessive summer temperature-driven outgassing. Together, these models produce incorrect seasonal CO₂ flux cycles with summer outgassing, whereas observation-based estimates indicate summer uptake. These errors may stem from inadequate model representation of ocean vertical mixing, which affects nutrient supply, stratification and heat redistribution. Our productivity estimates provide quantitative benchmarks that, combined with constraints from airborne CO₂ observations and surface ocean pCO₂ and temperature observations, reduce uncertainty in estimates of model-projected end-of-century Southern Ocean CO₂ uptake by 53%.