Impact of additional freshwater around Antarctica on the Southern Ocean carbon cycle : an inter-model comparison

The ongoing increase of global mean temperature, caused by anthropogenic CO₂ emissions, will most likely lead to enhanced melting and calving of Antarctic ice shelves in the coming decades. As a consequence, the freshwater input into the Southern Ocean is expected to increase as well. The resulting change in ocean salinity could have significant consequences for ocean circulation, water column stratification, and water mass formation in the Southern Ocean, which are all expected to affect the capacity of the surface ocean to remove CO₂ from the atmosphere, and the sequestration of carbon in the deep ocean. However, the magnitude and spatio-temporal patterns of these changes and their links to freshwater forcing are not yet well understood. To reduce these uncertainties, increase our understanding, and better quantify the feedbacks on the climate system, the international SOFIA initiative (Swart et al., 2023) defines freshwater input protocols for consistent use in various Earth System Models. Here we study the impact of additional freshwater around Antarctica on circulation and carbon fluxes in a steady preindustrial climate state using four Earth System Models. Most of the models show a decrease in the uptake of CO₂ by the surface of the Southern Ocean, caused by a strengthened outgassing of natural CO₂ between 50°S and 60°S. The stronger outgassing can be attributed to an increase in sub-surface dissolved inorganic carbon concentration south of the Antarctic Circumpolar Current that is associated with a redistribution of water masses in the Southern Ocean. Furthermore the reduction of the production and downward flow of Antarctic Bottom Water is leading to a decrease of its volume, and the expansion of carbon-rich Circumpolar Deep Water, which increases the carbon content at depth and thus weakens the overall CO₂ uptake. However, the models disagree in terms of the intensity of the weakened Southern Ocean CO₂ uptake. This difference seems to be mainly linked to the model resolution and the representation of the ocean mean state, e.g. the strength of the stratification, which is a determining factor for the redistribution of the additional freshwater to depth. To pursue this work, experiments with additional freshwater forcing in various climate states are conducted to analyse the ocean carbon cycle’s response and quantify potential climate feedbacks.