Final warming of the Southern Hemisphere polar vortex in high‐ and low‐top CMIP5 models

The final warming date of the polar vortex is a key component of Southern Hemisphere stratospheric and tropospheric variability in spring and summer. We examine the effect of external forcings on Southern Hemisphere final warming date and the sensitivity of any projected changes to model representation of the stratosphere. Final warming date is calculated using a temperature‐based diagnostic for ensembles of high‐ and low‐top models from the fifth Coupled Model Intercomparison Project (CMIP5), under the historical, Representative Concentration Pathway (RCP4.5) and RCP8.5 forcing scenarios. The final warming date in the models is generally too late in comparison with those from reanalyses: around 2 weeks too late in the low‐top ensemble, and around 1 week too late in the high‐top ensemble. Ensemble Empirical Mode Decomposition (EEMD) is used to analyze past and future change in final warming date. Both the low‐ and high‐top ensemble show characteristic behavior expected in response to changes in greenhouse gas and stratospheric ozone concentrations. In both ensembles, under both scenarios, an increase in final warming date is seen between 1850 and 2100, with the latest dates occurring in the early twenty‐first century, associated with the minimum in stratospheric ozone concentrations in this period. However, this response is more pronounced in the high‐top ensemble. The high‐top models show a delay in final warming date in the late 21st century in RCP8.5 that is not produced by the low‐top models, which are shown to be less responsive to greenhouse gas forcing. This suggests that it may be necessary to use stratosphere resolving models to accurately predict Southern Hemisphere surface climate change.