The Trend Of Stratospheric Quasi-Biennial Oscillation And Its Impact On The Tropical Madden-Julian Oscillation

Quasi-Biennial Oscillation(QBO)of the tropical stratospheric zonal winds is the most stable quasi-periodic signal within the stratospheric atmosphere,while Madden-Julian oscillation(MJO)is the dominant sub-seasonal mode within the tropical tropospheric atmosphere.In the past few decades,a large number of studies have been investigated on QBO and MJO respectively,however,the interaction and influence mechanism between them have not been deeply understood,especially whether the characteristics of the interaction between QBO and MJO will also change in the background of climate change is still unclear.Based on the sounding observation data,reanalysis data and CMIP5 model output data,this work analyzes the long-term variation trend and the interdecadal oscillation period of QBO amplitude firstly,and on this basis,the influence of the variation of QBO amplitude on the interdecadal characteristics of the tropical MJO is analyzed.The paper further discusses the variation of the tropical lower stratosphere temperature,the static stability of the Upper Troposphere Lower Stratosphere(UTLS)region and the tropical tropopause height on the response characteristics of QBO,which play an important role in the correlation between QBO and MJO,and then we analyzed the long-term trends of these three factors under the QBO easterly and westerly phases of the wind under the warming scenario,and obtains the following main conclusions:(1)During the period 1956-2020,the QBO amplitude of tropical stratospheric latitudinal winds still shows a steady weakening trend,especially in the lower stratosphere.During the northern hemisphere winter,the strongest weakening trend of the 50 h Pa QBO amplitude is found in the sounding data,and the weakening trend is significantly stronger after 1998 than before.The interdecadal oscillation period is about 14-20 years,which will gradually become longer as the QBO amplitude weakens.(2)In the Northern Hemisphere winter,there is a stable negative correlation between the QBO index and the MJO amplitude during 1979-2018,but after 1998,the strength of the negative correlation between the two gradually decreases.Prior to1998,the QBO easterly phase(EQBO)acted as an enhancement to the MJO,while the QBO westerly phase(WQBO)acted as a suppression.After 1998,the QBO amplitude weakened significantly,thus the enhancement of EQBO weakened and the suppression of WQBO weakened.That is,the MJO amplitude weakens,the number of active MJO days decreases,the MJO signal propagation eastward narrows,and the period of MJO events shortens in the following 20 years under the EQBO phase compared to the previous one,while the interdecadal variation of the MJO features under the WQBO phase is the opposite.Before 1998,the MJO convective signal and the associated precipitation were stronger and longer in duration under the EQBO phase compared to the WQBO phase;the dynamic and thermal factors closely related to the development and eastward propagation of the MJO,i.e.,the convergence anomaly of the horizontal wind field in the lower troposphere and the dispersion anomaly in the upper troposphere,the vertical velocity anomaly and the specific humidity anomaly in the middle troposphere,were all ahead of the appearance of the MJO deep convective signal in spatial location in terms of intensity center,which was favorable to the eastward propagation and development of the MJO convection,while this ahead signal was not obvious during the WQBO.After 1998,compared with the previous period,the signals of both MJO dynamical and thermal factor strength centers ahead of MJO deep convection are significantly weaker under both the QBO easterly and westerly phases;under the EQBO phase,the MJO convection signals and dynamical and thermal factors become weaker,while the changes are enhanced during WQBO.(3)During the winter of 1958-2018,the three main factors of QBO affecting the MJO,i.e.,tropical low stratospheric temperature,UTLS regional hydrostatic stability,and tropical tropospheric top height,become weaker in response to both EQBO and WQBO after 1998 compared to before.That is,in general,compared to the WQBO phase,the tropical low stratosphere is colder,the UTLS region is more unstable,and the tropopause is higher during the EQBO,but after 1998,the cold anomaly in the tropical low stratosphere during the EQBO weakens,the stability of the UTLS region increases,and the tropopause height decreases,such changes are not favorable to the development of tropical deep convective activity,but these three factors show opposite changes favoring the development of deep convection under the WQBO phase.The four climate models in CMIP5 that can simulate QBO autonomously all underestimate the amplitudes of the above three relevant factors and their relationships with QBO.Compared with the other three models,the MPI-ESM-MR model is better,and the simulation results of this model under different GHG emission scenarios also indicate that the modulation of the above three correlates by QBO easterly and westerly winds is weaker during the period of stronger QBO amplitude reduction.In a climate of persistent and increasing global warming,the long-term trends of stratospheric cooling and tropospheric warming,enhanced instability in the UTLS region,and raising of the tropopause persist and intensify during EQBO and WQBO,and the enhancement is stronger under westerly winds than during easterly winds.