Final warming of the Southern Hemisphere polar

6 The final warming date of the polar vortex is a key component of 7 Southern Hemisphere stratospheric and tropospheric variability in spring 8 and summer. We examine the effect of external forcings on Southern 9 Hemisphere final warming date, and the sensitivity of any projected changes 10 to model representation of the stratosphere. Final warming date is cal11 culated using a temperature-based diagnostic for ensembles of highand 12 low-top CMIP5 models, under the CMIP5 historical, RCP4.5, and RCP8.5 13 forcing scenarios. The final warming date in the models is generally too 14 late in comparison with those from reanalyses: around two weeks too late 15 in the low-top ensemble, and around one week too late in the high-top 16 ensemble. Ensemble Empirical Mode Decomposition (EEMD) is used to 17 analyse past and future change in final warming date. Both the lowand 18 high-top ensemble show characteristic behaviour expected in response to 19 changes in greenhouse gas and stratospheric ozone concentrations. In 20 both ensembles, under both scenarios, an increase in final warming date 21 is seen between 1850 and 2100, with the latest dates occurring in the 22 early twenty-first century, associated with the minimum in stratospheric 23 ozone concentrations in this period. However, this response is more pro24 nounced in the high-top ensemble. The high-top models show a delay in 25 final warming date in RCP8.5 that is not produced by the low-top mod26 els, which are shown to be less responsive to greenhouse gas forcing. This 27 suggests that it may be necessary to use stratosphere resolving models to 28 accurately predict Southern Hemisphere surface climate change. 29