On The Response Of Earth’s Neutral Atmosphere To Winter Stratospheric Disturbances And The Corresponding Coupling Mechanisms

The winter stratospheric disturbance is a large-scale atmospheric phenomenon,which is induced by the energy released in the stratosphere by the breaking of the upward propagating stationary planetary wave(SPW)excited in the troposphere.The research in this paper mainly involves three kinds of winter stratospheric disturbance events,namely the sudden stratospheric warming(SSW),the Canadian stratospheric warming(CSW)and the upper stratosphere/lower mesosphere(USLM)disturbance.During the winter stratospheric disturbance,the zonal mean temperature and wind in the polar stratosphere will undergo dramatical changes in a short interval,and the polar vortex will become unstable due to the sudden enhancement of the planetary wave activity.Since the Earth’s atmosphere is a highly coupled system,changes in the background atmosphere during winter stratospheric disturbances can influence the troposphere,mesosphere,and thermosphere/ionosphere system,as well as the equatorial region and summer hemisphere,through various coupling processes.Therefore,winter stratospheric disturbance events,as a natural window to study the coupling mechanism of the Earth’s atmosphere,have gradually attracted extensive attention from researchers.The research on this topic is of great significance for improving the accuracy of surface weather and space weather forecasts and deepening the understanding of the variability of the middle and upper atmosphere.This doctoral dissertation will focus on the excitation mechanism and interhemispheric coupling effect of secondary planetary waves in the middle and upper atmosphere during the SSW event,the atmospheric dynamics-photochemical coupling effect during the CSW event,and the impact of winter upper stratospheric disturbance on the lower stratospheric polar vortex and other aspects,comprehensively studying the response of Earth’s neutral atmosphere to winter stratospheric disturbance events,and revealing multiple atmospheric coupling mechanisms in these processes.During SSW events,large-amplitude planetary waves often appear in the mesosphere and lower thermosphere(MLT)region,which are called secondary planetary waves.This type of planetary wave is dominated by westward propagating modes with zonal wavenumbers 1(W1)and 2(W2),among which the W1 mode is the most common.At present,the secondary planetary wave has been captured by a large number of ground-based and space-based observations,but its excitation mechanism is still unclear.In order to study the excitation mechanism of the secondary planetary waves,we need to clarify the climatology of the W1 planetary waves widely existing in the Earth’s atmosphere first,and the climatology of the W1 quasi-6-day wave is still very vague.Based on the multiyear global geopotential height observations of the microwave limb sounder(MLS)onboard the Aura satellite,we conducted spectral analysis on the data in the Northern and Southern Hemispheres,respectively.The averaged spectra show that the W1 quasi-6-day wave is amplified four times in the MLT region before and after the two equinoxes,and the amplitude in the summer hemisphere is much larger than that in the winter hemisphere.The SD-WACCM-X simulation results indicate that the seasonal variation of the relative position of the W1 quasi-6-day wave critical layer and the atmospheric baroclinic/barotropic instability in the summer hemisphere mesosphere middle and high latitudes is mainly responsible for the above-mentioned climatology: the intersection of the two only occurs before and after the two equinoxes.This background atmospheric condition allows the W1 quasi-6-day wave excited in the winter stratosphere to move across the equator and reach the summer hemisphere mesosphere to be overreflected by the critical layer and amplified in the unstable region,and enter the low thermosphere with larger amplitudes.This part of the study lays the important foundation for the following discussion of the secondary planetary waves during the SSW.Based on the MERRA-2 reanalysis and the Aura/MLS geopotential height measurements,we extracted the W1 planetary waves during all the SSW events in the Northern Hemisphere from 2005 to 2020,and found that among the W1 planetary waves with periods of 2-12 days,the planetary wave with period of 8 days occurs most frequently.This result is quite different from the distribution of the climatological period of the W1 planetary wave in the Earth’s atmosphere,and the periods of the W1quasi-6-day wave and quasi-10-day wave seldom shift to around 8 days.Further analysis indicates that the W1 quasi-8-day wave is excited in the Northern Hemisphere polar stratosphere,and the wave perturbations are mainly confined to the middle and high latitudes of the Northern Hemisphere stratosphere and mesosphere.The global structure of the quasi-8-day wave is related to the relative position of the critical layer and instability,and it may differ from event to event.Furthermore,we find for the first time that the reversal and recovery of the background wind during the SSW is equivalent to a kind of oscillation,and the zonal wavenumber of this type of zonally symmetric oscillation is 0(S0).Before the W1 quasi-8-day wave is excited,an S0 wave with the same period of about 8 days can be observed in the background atmosphere.The nonlinear interaction between the S0 quasi-8-day wave and the large-amplitude SPW with zonal wavenumber 1(SPW1)during the SSW occurs in the polar stratosphere,which generates the child wave of the W1 quasi-8-day wave.This excitation mechanism well links the characteristics of the SSW itself with the type of the secondary planetary wave.During the rare Antarctic SSW in 2019,the W1 7.4-day wave,6.0-day wave and9.6-day wave were simultaneously captured in the MLT region of both hemispheres,and the W1 7.4-day wave peaked 10 days earlier than the other two W1 planetary waves,suggesting that this SSW caused an interhemispheric coupling of the Earth’s atmosphere.After careful comparison of the spatial structure and occurrence time,it is confirmed that the W1 7.4-day wave is actually a climatological event of the W1 quasi-6-day wave before the September equinox,and the other two W1 waves are secondary planetary waves during the SSW,their wave sources in the Southern Hemisphere stratosphere are all amplified by the atmospheric instability during the SSW.Through calculation,we find that the frequencies and zonal wavenumbers of the three W1 planetary waves satisfy the wave-wave nonlinear interaction theory,and thus deduce that the other parent wave is an S0 32.2-day wave.Moreover,this S0 32.2-day wave was also successfully discovered in the polar stratosphere of the Southern Hemisphere during the SSW,indicating that the W1 6.0-day wave and 9.6-day wave during the SSW are child waves generated by the nonlinear interaction between the W1 7.4-day wave and the S0 32.2-day wave.The strong responses of the Northern Hemisphere MLT to these waves are mainly due to the inherent characteristics of the W1 7.4-day wave as a climatological W1 quasi-6-day wave event.The above two excitation mechanisms of secondary planetary waves during SSW both highlight the role of zonally symmetric oscillations.For the large-amplitude W1 10-16-day secondary waves that have a good one-toone correspondence with the sudden stratospheric final warming(SSFW)in the Northern Hemisphere,we find that the phase of the equatorial quasi-biennial oscillation(QBO)in the middle stratosphere plays a crucial role in the trans-equatorial propagation of the secondary waves.During the 2015 and 2022 SSFW in the Northern Hemisphere,the middle stratospheric QBO is in the eastward phase,and the W1 10-16-day wave can smoothly move across the equator and trigger a strong response in the Southern Hemisphere Mesosphere.During the SSFWs in 2005,2014 and 2016,due to the strong westward phase of the middle stratospheric QBO,the secondary waves are blocked by the critical layers and the negative waveguides when they propagate to the equatorial region,thus the trans-equatorial propagation does not occur,and the secondary wave cannot be captured in the Southern Hemisphere.The different response highlights the importance of large-scale oscillations in the equatorial region in the interhemispheric coupling of the Earth’s atmosphere.In terms of dynamic-chemical coupling during winter stratospheric disturbances,we start with a rare CSW event in the Northern Hemisphere in 2016/2017,and investigate the modulation of the secondary planetary wave on the mesospheric water vapor.The results show that during the 2016/2017 CSW event,the polar mean temperature in the lower and middle stratosphere only increase by a few kelvins,but the background wind reverses to westward,and the degree of the wind reversal is similar to that caused by the two subsequent major SSW events in the same winter,which is quite rare among CSW events in recent decades.More importantly,this CSW event also triggers a large-amplitude W1 secondary wave with period of about 12 days,and the secondary wave in the dynamical parameters successfully modulates the water vapor mixing ratio in the mesosphere.Reconstructions of wave perturbations in water vapor data reveal that meridional advection plays a major role in the imprint of waves from dynamical parameters into atmospheric composition,while vertical transport plays a relatively minor role.For the downward coupling effects of the middle and upper atmosphere,we explore the impact of winter stratospheric perturbations that occur at higher altitudes on the lower stratospheric polar vortex.Prior to the rare Arctic ozone depletion and anomalously strong polar vortex in the lower stratosphere in March 2011 and 2020,a specific disturbance sequence occurs in the USLM region during the early and middle stage of the two boreal winters,namely a USLM disturbance in early January and a minor SSW event in early February.The combination of these high-altitude disturbances will decelerate the eastward wind in the upper stratosphere,so that the peak height of the polar night jet around the polar vortex will continuously descend during the early and middle winter,and the uppermost boundary of the SPW1 positive waveguides also descends to the lower and middle stratosphere,forming a reflection configuration for the subsequent upward SPW1.The persistently weaker planetary wave forcing during the late winter makes the polar region anomalously cold,and the polar vortex in the lower stratosphere remains extremely stable.In addition,SDWACCM-X simulations show that the eastward gravity wave drag in the MLT region does not move downward after the two winter stratospheric disturbances to trigger an elevated stratopause event,which is also an important factor for the prolonged high strength of the polar vortex.This study reveals a mechanism that the dynamical processes in the upper stratosphere and even mesosphere are coupled downward to influence the dynamics and chemistry of the lower atmosphere.