Southern Ocean freshening stalls deep ocean CO2 release in a changing climate

The Southern Ocean critically mitigates global surface warming by taking up a large portion of the carbon released to the climate system by human activities. While models suggest this carbon sink should weaken with human-induced climate change due to an increased upwelling of carbon-rich deep water, such a strong decline has not been observed over the past decades. Here, using circumpolar hydrographic observations, we reveal that the freshening of the Southern Ocean since the 1990s has prevented the weakening of its carbon sink by trapping the upwelled carbon-rich deep water in the subsurface. The enhanced density stratification has prevented these CO ₂ -enriched waters from reaching the surface, and, thus hindered a saturation of the carbon sink in this period. Meanwhile, the surface layer became thinner, allowing the CO ₂ -rich circumpolar deep water to get closer to the surface, replacing winter water between 100 and 200 m. In this layer, the CO ₂ fugacity increased by ~ 10 µatm since the 1990s. Consequently, our findings imply that the model-predicted weakening of the Southern Ocean carbon sink has been delayed by the surface freshening and might emerge when the stratification weakens.