. Modeling experiments with supercooled cloud liquid increase the Southern Ocean carbon sink

Abstract
Uptake of carbon dioxide by the ocean causes ocean acidification and reduces global warming. The sea-air flux of carbon dioxide in the Southern Ocean depends on sea-surface temperature, sea ice concentration, biological processes, surface winds, and ocean circulation. The limited observations in this region and the complexity of modeling the influence of these factors make constraining the carbon flux difficult - neither the sign nor the magnitude of this flux is established. We use a fully-coupled climate model to demonstrate that the magnitude of the carbon flux in the Southern Ocean depends on the amount of supercooled liquid in low-level convective clouds over this region. We find that the Southern Ocean carbon sink of 0.19 PgC/year that is present in a preindustrial control simulation increases to 0.28 PgC/year in a simulation with increased, more realistic amounts supercooled cloud liquid. The globally integrated flux is balanced by increased outgassing in the tropics. Increased supercooled cloud liquid increases the albedo of Southern Ocean clouds. This increased cloud brightness reduces the solar radiation absorbed at the ocean surface and alters surface ocean properties - such as sea-surface temperature and biological activity - that are important for sea-air gas exchange. This result shows that representation of clouds in climate models can alter the base rate of carbon flux in the Southern Ocean in those models, which will affect the predicted timing of destructive amounts of ocean acidification and atmospheric warming.