Response Of Stratospheric Polar Circulation To ENSO In CMIP5/6 Climate Model

The Arctic stratospheric response to El Ni(?)o-Southern Oscillation(ENSO)is assessed using large samples from the historical simulations provided by the Coupled Model Intercomparison Project Phases 5 and 6(CMIP5 and CMIP6,respectively).CMIP6 models can well reproduce the ENSO signals in the Arctic stratosphere and have an ameliorated performance as compared to CMIP5 models.Specifically,El Ni(?)o events are associated with an intensified Pacific–North American pattern(PNA)and increased planetary wave activities in the troposphere and stratosphere,and thereby a weak and warm stratospheric polar vortex,which is the opposite for La Ni(?)a.In CMIP6 models,the ENSO-related stratospheric signal matures in late winter and spring(February–April)and increases with ENSO magnitude,regardless of the ENSO phase.However,such an increase is nonlinear.The stratospheric response to strong El Ni(?)o(La Ni(?)a)is weaker(stronger)than what should be achieved if the response changes linearly with the amplitude of El Ni(?)o(La Ni(?)a).An asymmetric time evolution of stratospheric signals exists between strong El Ni(?)o and La Ni(?)a events.The stratospheric response caused by strong El Ni(?)o is weaker from late winter to early spring but inversely stronger in middle and late spring compared to that caused by strong La Ni(?)a.By contrast,the Arctic stratospheric signal in moderate El Ni(?)o events is larger than that in moderate La Ni(?)a.Overall,compared to ENSO-neutral winters,the stratospheric sudden warming events would occur more(less)frequently under El Ni(?)o(La Ni(?)a)conditions,as simulated by CMIP6 high-top models.The CMIP6 models show that the Arctic stratospheric polar vortex is warming and weakening during the eastern Pacific El Ni(?)o,and have a strong downward propagation trend.During the central Pacific El Ni(?)o,the weakening of the polar vortex is not found in the previous winter,it is not until spring that there is a faint warming of the polar vortex and shows a weak downward propagation trend.This is due to the stronger and earlier development of PNA in the eastern Pacific El Ni(?)o event,resulting in the enhancement of the activity of planetary wavenumber 1.CMIP5 models can not well simulate the impact of two types of El Ni(?)o events on the northern hemisphere polar vortex.In the reanalysis data,when the central Pacific El Ni(?)o occurs,the spring polar vortex in the southern hemisphere is significantly weakened,and the anomaly is transmitted to the troposphere.During the eastern Pacific El Ni(?)o,the enhancement of the polar vortex is not significant.CMIP5 /6 models do not simulate the difference in the impact of two types of El Ni(?)o events on the polar vortex in the southern hemisphere in the reanalysis data.The impact of ENSO on surface climate is divided into two categories: tropospheric pathway and stratospheric pathway.In CMIP6,the stratospheric pathway of El Ni(?)o and La Ni(?)a is not related to the ENSO phase,but to the NAO(North Atlantic oscillation)of the negative phase.The response on the surface is that there are significant cold anomalies in northern Eurasia,and significant warm anomalies in Greenland,eastern Canada,most of China,the Middle East,and North Africa.The tropospheric pathway of El Ni(?)o is connected with the positive-phase PNA.There is a strong and significant warm anomaly in the northwest of North America and a strong cold anomaly in the south of the United States.La Nina’s troposphere pathway is the opposite.The stratospheric pathway of ENSO in CMIP5 models is not obvious compared with CMIP6 models.By isolating the tropospheric pathway,the stratospheric pathway of ENSO can be better observed.The influence of the stratospheric pathway on the surface in the CMIP6 models is very similar to the surface air temperature anomaly after the SSW occurs.In the absence of a stratospheric pathway,the tropospheric circulation field and surface response are almost exactly the opposite of those of the stratospheric pathway.When the stratospheric pathway in the CMIP5 models is present,only the signal in northern Eurasia is significant.