Understanding the intermodal spread in CMIP5 Projections of Southern Hemisphere Subtropical Anticyclones and Hadley Cell Edge

Several studies have investigated how the atmospheric circulation associated with the Southern Hemisphere (SH) Subtropical Anticyclones (SA) will change in climate model projections. The winter season has however remained largely uninvestigated, as well as the extent to which the mechanisms proposed in the literature explain the inter-model spread in projections. This study aims to address these gaps focusing on projected changes in the intensity and area of the SH SAs under global warming during both the local summer and winter season, and the spread in responses seen across models. Using the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive, our analysis shows that the strength of all the SH SAs intensifies in the center and poleward flank during both seasons. Model differences in diabatic cooling over the subtropical ocean explain the inter-model variability in projected SLP change during austral summer over the South Pacific SA. Whereas model difference in tropospheric static stability, and hence the conditions promoting baroclinic eddy growth, appear to largely explain the South Atlantic SA, South Indian SA, and zonal mean inter-model spread of SLP change on the poleward flank. During austral winter, a combination of diabatic cooling changes over the subtropical oceans and changes in the conditions promoting baroclinic instability explain the inter-model spread of SLP change over most of the SH subtropical to mid-latitude region. These findings point to a robust relationship between projected static stability changes and the strengthening of Southern Hemisphere Subtropical Anticyclones along their poleward flank.