Strong Southern Ocean carbon uptake evident in airborne observations.

The Southern Ocean plays an important role in determining atmospheric carbon dioxide (CO ₂ ), yet estimates of air-sea CO ₂ flux for the region diverge widely. In this study, we constrained Southern Ocean air-sea CO ₂ exchange by relating fluxes to horizontal and vertical CO ₂ gradients in atmospheric transport models and applying atmospheric observations of these gradients to estimate fluxes. Aircraft-based measurements of the vertical atmospheric CO ₂ gradient provide robust flux constraints. We found an annual mean flux of –0.53 ± 0.23 petagrams of carbon per year (net uptake) south of 45°S during the period 2009–2018. This is consistent with the mean of atmospheric inversion estimates and surface-ocean partial pressure of CO ₂ ( Pco₂ )–based products, but our data indicate stronger annual mean uptake than suggested by recent interpretations of profiling float observations.