Projections of synoptic anticyclones for the twenty-first century

Synoptic anticyclones are a common feature of subtropical and midlatitude climate and are associated with descending air and clear conditions, while associated anticyclonic circulation anomalies can contribute to temperature extremes. When anticyclones are tracked in both the ERA5 reanalysis and 10 global climate models from the 5th Coupled Model Intercomparison Project (CMIP5) using a common grid, the CMIP5 models consistently underestimate the observed frequency of anticyclones in the southern hemisphere, while overestimating anticyclone frequencies in the northern hemisphere. Under a high emissions scenario, the overall frequency of anticyclones is projected to decline over the twenty-first century. Declines are largest in the southern hemisphere subtropics, where projected changes in anticyclone frequency can be linked to the projected poleward shift in the Southern Annular Mode. Stronger and more robust declines are projected for the subset of quasi-stationary anticyclones that move less than 4 degrees over 24 h. Using the Australian region as a case study, regionally downscaled models show very similar projected changes to the driving CMIP5 models, adding little additional value for understanding projected changes in anticyclones.