Impact of atmospheric processes on mechanisms of Southern Ocean heat uptake

Abstract:

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 atmospheric processes impact 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. Using reanalyses, satellite observations and climate models, we assess the influence of cloud cover and other cloud properties on Southern Ocean heat uptake over the past 40 years. We find that Southern Ocean clouds increase ocean heat uptake by suppressing turbulent heat fluxes out of the surface while increasing downwelling longwave radiation. At least half of the increase in surface longwave absorption that accompanies greater Southern Ocean heat uptake is due to increased cloud cover and liquid water path between 45-60S. Opaque clouds have the strongest impact on ocean heat uptake, especially in fall and winter. 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 mechanisms of heat uptake in the current generation of climate models.