The Influence of Direct Radiative Forcing versus Indirect Sea Surface Temperature Warming on Southern Hemisphere Subtropical Anticyclones

Abstract:

Southern hemisphere subtropical anticyclones are projected to change in a warm climate during both austral summer and winter. A recent study of CMIP 5 & 6 projections (Fahad et al. 2020) shows a combination of local diabatic heating changes and static-stability-induced changes in baroclinic eddy growth as the dominant drivers. Yet the underlying mechanisms forcing these changes still remain uninvestigated. This study aims to enhance our mechanistic understanding of what drives these Southern Hemisphere changes during both seasons. Using an AGCM, we decompose the response to CO₂-induced warming into two components. The first component is the fast, direct atmospheric response to CO₂ radiative forcing, and the second component is the slower atmospheric response due to indirect sea surface temperature warming. We further investigate the influence of tropical diabatic heating with AGCM added heating experiments. As a complement to our numerical AGCM experiments, we also analyze the Atmospheric and Cloud Feedback Model Intercomparison Project experiments that similarly isolate the influence of direct CO₂ and indirect sea surface temperature warming. We find indirect sea surface temperature warming to be the dominant mechanism forcing the tropical diabatic heating changes, static stability changes, and the baroclinic eddy growth changes associated with the anticyclones. Direct radiative forcings of CO₂ acts as a secondary mechanism. Notably, the associated tropical diabatic heating changes only play a significant role in driving changes in static stability and the anticyclones during austral winter, implying that extra-tropical surface temperature change controls summer static stability change in the Southern Hemisphere. An opposing tug of war response due to the opposing influence of direct CO₂ radiative forcings and indirect sea surface temperature warming is seen for the South Atlantic anticyclone during austral winter.