Relationship Between Planetary Wave Activity And Monthly Decadal Variations In The Arctic Stratospheric Winter At The End Of The 20th Centur

In order to investigate the influence of planetary waves activity on stratospheric temperature and circulation during the late 20 th century,this paper used ERA5 reanalysis data and WACCM model data to study changes of stratospheric temperature and circulation,as well as their relationship with planetary waves activity.The study studied these factors through the lenses of climatology,decadal changes,and dynamic forcing.This paper first studied the decadal changes in winter temperature and zonal wind in the extratropical stratosphere of the northern hemisphere during winter months(DJF)in the late 20 th century,and evaluated the performance of WACCM model in these contexts.This paper then focused on the dynamic effect of planetary waves with different wave numbers.And then,employed various methods to separate the transient planetary waves from the quasi-stationary planetary waves,in order to study their climatology,decadal changes and relationship with the stratosphere.Finally,this paper separated the transient planetary waves into high frequency and low frequency waves,and identified the key cycle range of the transient planetary waves.The main conclusions of this paper are summarized as follows:(1)In the late 20 th century,there was a clear evolution in the decadal changes of stratospheric temperature and circulation in the northern Hemisphere.The polar vortex was relatively warm and weak in December,but relatively cold and strong in February.WACCM has reproduced the monthly mean climatology and the opposite decadal changes of temperature and circulation in December and February.Through an analysis of the effects of planetary waves with different wave numbers,it was found that changes in stratospheric temperature and circulation in December and February are related to the strengthening or weakening of the stratospheric planetary waves.The wave with wave number 1 or 2 play a decisive role in the strengthening or weakening of the planetary waves,respectively.(2)The climatology and decadal changes of planetary waves activity show that the climate states and decadal changes of planetary waves activity in the troposphere and stratosphere at the end of the 20 th century were mostly dominated by quasi-stationary waves,with transient waves only weakening the influence of quasi-stationary waves.However,when total planetary waves activity decreased in January and February,the role of transient waves could not be ignored.By analyzing the eddy heat flux of planetary waves,it was found that the increase of planetary waves uploading in December was mainly caused by quasi-stationary waves,but the decrease of tropospheric planetary waves uploading in February near the tropopause was mainly caused by the trend of transient waves.This indicates that the changes in stratospheric temperature and circulation in February are largely related to the dynamic forcing of transient waves.(3)Further studies of transient waves reveal that low-frequency transient waves with longer cycles have a greater impact on the climatology and decadal changes of transient planetary waves.On the other hand,high-frequency transient waves have a smaller influence on the activity of transient waves and can be ignored in the stratosphere.Although the dynamic effect of high-frequency transient waves also exhibits significant month-to-month differences,their influence is much smaller than that of the low-frequency transient waves and is almost negligible.