Last Stratospheric Spring Warming In The CMIP5/6 Climate Model And Its Links To The Arctic Oscillation And May Precipitation

This paper evaluates the performance in simulating the stratospheric final warming(SFW)events that lead to the final collapse of the stratospheric polar vortex in spring in both hemispheres based on the historical experimental data provided by the Coupled Model Intercomparison Project Phases 5 and 6(CMIP5/6).Overall,the CMIP5/6 models can reproduce the main characteristics of the occurrence of SFWs.However,the SFW onset date in the models is 7 days and 9 days later than the observation in the Southern and Northern Hemispheres(SH,NH),respectively.In addition,the intensity of the SFWs in the models is50–70% of that in the observation.Compared with CMIP5,the CMIP6 models' ability in the simulation of NH SFWs improves greatly.This is not manifested as significantly earlier SFW onset in CMIP6 than that in CMIP5,but more intense stratospheric planetary wave activities before the SFW and a larger interannual variability of the SFW onset date in CMIP6 that is much closer to the observation.Different from in the NH,the CMIP6 models' ability is roughly the same to simulate the SH SFWs as CMIP5,whereas it is even slightly weaker than that of the CMIP5 model in the simulation of SFW onset date and the stratospheric planetary wave activities before the onset.The performance of CMIP6 high-top models is better than the low-top models.Specifically,in the NH,the simulation in the high-top models is considerably improved in terms of the intensity of circumpolar zonal wind around the SFW onset and the stratospheric planetary wave activities before the SFW onset.By contrast,in the SH,the improvement in the high-top models is mainly manifested in the fact that the simulated SFW onset date is fairly earlier than that in the low-top models by 11 days,which is closer to the observation.As for the connection between SFWOD and the Arctic Oscillation(AO),the model can simulate the spatial pattern of AO roughly.And it can simulate the significant positive correlation between SFWOD and AO index above 150 h Pa in April and May.However,there was no significant correlation between SFWOD and AO index in the middle and lower troposphere.Further analysis of the frequency and the intensity of AO index in the early and late SFW in observation shows that there may be an asymmetry between the SFWOD and the AO in May.However,there is a positive correlation between them in the extreme years of SFW.In general,the tropospheric AO anomalies corresponding to SFW anomaly years in the models are significantly stronger than observation.There is a significant positive correlation between SFWOD and precipitation in Southwest China in May.Overall,the CMIP6 models can not simulate such a relationship.Only Can ESM5 and CESM2-WACCM can reproduce the significant sliding positive correlation between the SFWOD and the precipitation in Southwest China in May.Among them,CESM2-WACCM performs better and can reflect the relationship between SFWOD and precipitation in The Arabian Sea region in May.In terms of circulation,CESM2-WACCM can well simulate the positive(negative)geopotential height anomalies over the Iranian Plateau at 200 h Pa and 500 h Pa in the late(early)SFW,the westerly(easterly)circulation anomalies over the Southern Arabian Sea and the Indian subcontinent at 850 h Pa and the southwest(northeast)wind from Indochina Peninsula to Southwest China,resulting in increased(decreased)precipitation in Southwest China.