PNNL

Abstract

The Southern Ocean has absorbed most of the excess heat associated with anthropogenic greenhouse gas emissions. Since Southern Ocean observations are sparse in certain regions and seasons, much of our knowledge of ocean heat uptake is based on climate model simulations. However, climate models still inadequately represent some properties of Southern Ocean clouds, and they have not identified the mechanisms by which clouds may affect SOHU. Here we use the ERA5 and JRA-55 reanalyses to assess the influence of clouds and other atmospheric processes on SOHU over the past 42 years. We find that years with the highest SOHU between 45-65°S are dominated by ocean heat uptake anomalies during winter and spring, but not during summer or fall. Winter and spring cloud cover are up to 7% higher when SOHU is up to 5.5 W/m2 higher than the seasonal mean, with the largest increases in the South Pacific Ocean. Clouds are also optically thicker. These changes in cloud properties increases downwelling longwave radiation, amplifying ocean heat uptake. Cloud changes also increase heat uptake by maintaining a stable lower atmosphere, which suppresses turbulent heat fluxes out of the surface. Overall, we find that SOHU is likely not mediated by enhanced surface shortwave absorption over the observational time period. A better understanding of how atmospheric processes impact ocean heat uptake may help improve our understanding of ocean heat uptake mechanisms in the current generation of climate models.