Substantially underestimated winter CO₂ sources of the Southern Ocean

The size and control mechanism of the Southern Ocean's carbon fluxes remain highly uncertain due to sparse winter observations. Here, we integrate satellite light detection and ranging (LIDAR) measurements with machine learning to assess the Southern Ocean air-sea CO₂ fluxes between 2007 and 2020. We reveal that CO₂ outgassing south of 50°S was underestimated by up to 40% in previous studies. While the midlatitude Southern Ocean (30° to 50°S) strengthens as a carbon sink, the high-latitude region (50° to 90°S) shows Southern Annular Mode (SAM)-modulated alternation between uptake and outgassing. The air-sea CO₂ partial pressure difference (ΔpCO₂) increasingly dominates flux variability over wind-driven transfer velocity. We propose a framework involving three latitudinal loops with differing pCO₂ controls: (i) Antarctic (salinity/sea ice), (ii) polar front (atmospheric CO₂/chlorophyll), and (iii) subpolar (sea surface temperature/CO₂). The findings underscore the winter processes' critical role and necessitate year-round observations to understand Southern Ocean's global carbon cycle impact.