Substantially underestimated winter CO 2 sources of the Southern Ocean
- Siqi Zhang
- Peng Chen
- Kelsey Bisson
- Cédric Jamet
- Paolo Di Girolamo
- Davide Dionisi
- Yongxiang Hu
- Zhenhua Zhang
- Kun Shi
- Delu Pan
2025-11-05
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 CO 2 fluxes between 2007 and 2020. We reveal that CO 2 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 CO 2 partial pressure difference (Δ p CO 2 ) increasingly dominates flux variability over wind-driven transfer velocity. We propose a framework involving three latitudinal loops with differing p CO 2 controls: (i) Antarctic (salinity/sea ice), (ii) polar front (atmospheric CO 2 /chlorophyll), and (iii) subpolar (sea surface temperature/CO 2 ). The findings underscore the winter processes’ critical role and necessitate year-round observations to understand Southern Ocean’s global carbon cycle impact.