Clouds Amplify Mechanisms of Southern Ocean Heat Uptake

The Southern Ocean has absorbed most of the excess heat associated with anthropogenic greenhouse gas emissions. Since the Southern Ocean is poorly observed, much of our knowledge of ocean heat uptake is based on climate model simulations. However, climate models do not agree on how the atmosphere impacts the mechanisms controlling Southern Ocean heat uptake. In some models the peak in Southern Ocean heat uptake coincides with a peak in absorbed shortwave radiation at the ocean surface, implying a decrease in cloud cover drives heat uptake. Other models indicate that Southern Ocean heat uptake is controlled by increasing energy transport into the region, possibly from more downwelling longwave radiation from increased cloud cover. Here we use the ERA5 and JRA-55 reanalysis datasets to assess the influence of cloud cover and other cloud properties on Southern Ocean heat uptake over the past 42 years. We find that years with the most Southern Ocean heat uptake between 45-65°S have anomalously high ocean heat uptake during winter and spring, but not during summer or fall. Winter and spring cloud cover increase by up to 7% during high Southern Ocean heat uptake, with the largest increases between the Amundsen and Ross Seas. Clouds also become optically thicker. Changes in cloud properties increase downwelling longwave radiation, amplifying Southern Ocean heat uptake. Clouds also increase heat uptake by maintaining a stable lower atmosphere, which suppresses turbulent heat fluxes out of the surface. Overall, we find that Southern Ocean heat uptake is likely not mediated by enhanced surface shortwave absorption over the observational time period. A better understanding of how atmospheric processes impact Southern Ocean heat uptake may help reconcile the mechanisms of heat uptake in the current generation of climate models.