Considering the Influence of Clouds on Preindustrial Southern-Ocean Carbon Uptake

The global carbon cycle mediates the effects of carbon on global climate and the surface ocean. On time scales between 10 and 1000 years, the ocean is the largest carbon reservoir on Earth. In particular, the Southern Ocean is an important sink of present-day carbon emissions, and models predict that this will continue to be the case in the future. For this reason, understanding the processes that control carbon uptake in the Southern Ocean is important for predicting carbon-driven changes to the environment and for understanding the global carbon cycle as a whole. Among modern numerical climate models, the representation of clouds and their radiative effects remains a source of uncertainty in the estimation of climate states - especially in the Southern Ocean. For example, cloud errors have been tied to sea surface temperature biases in this region. Here we show, using the Community Earth System Model version 1 with a diagnostic carbon cycle and biogeochemical tracers (CESM1-BGC), that observationally motivated cloud changes made to improve the spatial distribution of absorbed solar radiation increase the magnitude of the preindustrial Southern-Ocean carbon sink by 0.1 Pg/year. Furthermore, by decomposing changes in the air-sea carbon dioxide flux into thermally-driven and non-thermally driven components, we are able to attribute this increased carbon uptake to reduced radiative warming during summer.