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Many general circulation models fail to capture the observed frequency of atmospheric blocking events in the Northern Hemisphere; however, few studies have examined models in the Southern Hemisphere and those studies that have, have often been based on only a few models. To provide a comprehensive view of how the current generation of coupled general circulation models performs in the Southern Hemisphere and how blocking frequency changes under enhanced greenhouse gas forcing, we examine the output of 23 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that models have differing biases during winter, when blocking occurrence is highest, though models underestimate blocking frequency south of Australia during summer. We show that models generally have a reduction in blocking frequency with future anthropogenic forcing, particularly in the Australia-New Zealand sector with the number of winter blocked days reduced by about one third by the end of the 21st century.

Plain Language Summary Atmospheric blocking is a process in which an atmospheric wave breaks, diverting the jet stream and any incoming storms to the north or south. A blocking event is characterized by high pressure on the poleward side and low pressure on the equatorward side and persists for upward of 4 days. A key feature of blocks is their persistence, which can lead to events, which have significant impacts on society including heat waves in summer and periods of extreme cold in winter. Climate models often do not simulate enough blocking events, though most research on this problem has focused on the Northern Hemisphere and less is known about the Southern Hemisphere. We survey 23 climate models and show that during winter some models simulate too many blocking events, while others simulate too few, whereas during summer, almost all models simulate too few events to the south of Australia. We also show that with higher concentrations of greenhouse gases we expect there to be less blocking, particularly to the south of Australia and over New Zealand during winter.