The Variations Of The Stratospheric Meridional Circulation And Its Connections With The Tropospheric Weather And Climate Changes

The stratospheric large-scale meridional circulation,Brewer-Dobson circulation(BD circulation),plays an important role in stratosphere and troposphere interaction and exchange processes.Its change is closely connected with many stratospheric and tropospheric processes.Its effects on tropospheric weather and climate changes cannot be ignored.Based on a variety of reanalyses data and model simulations,firstly,we studied the basic characteristics of stratospheric meridional circulation and tropospheric meridional circulation as well as its impact factors.Then,we analyzed the covariations between stratospheric meridional circulation and tropospheric meridional circulation and their connections.Finally,we provided a possible mechanism of stratospheric BD circulation modulating tropospheric surface pressure.The main conclusions of this paper are as follows:(1)Firstly,using two reanalysis datasets and two CMIP6 models simulations,the trends of the width and intensity of the stratospheric BD circulation in historical period and different future scenarios are compared.The results show that,there is a consistent enhancement in the upward branch of the BD circulation in the historical period(1979–2014).The upwelling region becomes narrower in the lower stratosphere(below 50 hPa)and wider in the middle stratosphere(from 30 hPa to 10 hPa).Increasing in greenhouse gas emissions is the main cause of the changes in the BD circulation intensity and width in the historical simulations.In the future period in RCP8.5 scenarios(2015–2100),the width of BD circulation tends to be narrowing in lower stratosphere in both models outputs,while its trend in mid-stratosphere is opposite in two models,which is caused by their different planetary wave activities.In CanESM5 model simulations,the convergence of the planetary wave will propagate to lower latitudes in the stratospheric each layers under the influence of background wind field,as a result the upward branch becomes narrower.In CESM2-WACCM model simulations,the results in lower stratosphere is similar with that in CanESM5 model,while in mid-stratosphere,it is the convergence of the northern hemisphere planetary waves at higher latitudes,the weakening of the stratospheric planetary waves in the southern hemisphere and the strengthening of the gravitational waves in the middle latitudes,that cause the widening of the BD circulation ascending branches in two hemispheres.Analysis also shows that the mass flux of the upward branch at the tropopause is still increasing as the upward lift of the BD circulation.(2)Then,using six reanalysis datasets and six CMIP5 model simulations,the connection between the strength(HCS)and width(HCW)of the Hadley circulation was analyzed.The results show that,for the long-term trends,HCS and HCW are only negatively correlated in CMIP5 model simulations,this correlation do not exist in the reanalysis datasets.This inconsistency is caused by differences in the generation rate of the tropical zonal mean potential energy G(Pm).There is no significant trend in G(Pm)in the CMIP5 model simulations,while a significant increasing trend in the reanalysis datasets,which masks the negative correlation between the changes of HCS and HCW.When it comes to interannual variations,there is a negative correlation between HCS and HCW interannual variations in both the reanalyses datasets and CMIP5 model simulations,their correlation is constrained by tropospheric tropical zonal mean kinetic energy.(3)Next,using ERA-Interim and JRA-55 reanalysis data,the covariation between stratospheric BD circulation and tropospheric meridional circulation at different latitudes is then analyzed.the results show that the enhancement of the BD circulation in lower stratosphere(70 hPa)is accompanied by the enhancement of the ascending branch of the Hadley circulation in deep tropics,the weakness of the midlatitude Hadley circulation,and the equatorward shift of the Ferrel circulation.The positive correlation between BD circulation and the ascending branch of the Hadley circulation mainly occur in DJF and SON,these are the two seasons with the strongest BD circulation,the strengthened BD circulation may induce an enhanced Hadley circulation by global mass continuity.The connection between BD circulation and the subsidence of the Hadley circulation in DJF in southern hemisphere is mainly modulated by ENSO.Take the warm ENSO anomalies as an example,the upward and equatorward propagation of synoptic wave(higher than wave-3)in upper troposphere and lower stratosphere is enhanced at 30?S,the anomalous enhanced upward and equatorial synoptic wave fluxes at the upper tropospheric stratosphere cause an anomalous EP flux convergence in lower stratosphere and divergence in upper troposphere,as a result the enhancement of the lower BD circulation and the weakening of poleward branch of the Hadley circulation at 30?S.Moreover,when the BD circulation is enhanced,the Ferrel circulation shift equatorward in the northern and southern hemispheres in DJF and SON,respectively.Their covariations are modulated by the annular mode signal,and in addition the stratospheric BD circulation can also affect the Ferrel circulation at a lag of 2 months.(4)Finally,based on ERA-Interim and JRA-55 daily reanalysis datasets,a potential mechanism of BD circulation influencing surface pressure by modulating the distribution of air mass is analyzed.The results show that the variations in the surface pressure,particularly at middle and high latitudes,have a close correlation with the variations in the stratospheric BD circulation intensity.When the upwelling at 450 K intensifies,the surface pressure increases in the high latitudes(poleward of 60?)and decreases in the adjacent mid-latitudes in both hemispheres.And these correlations are significant during boreal winter in the Northern Hemisphere and during austral autumn in the Southern Hemisphere.It is found that the high latitude surface pressure changes follow the anomalous BD circulation by about 30 days.The increase in polar surface pressure associated with the stronger BD circulation is due to an increase in stratospheric air mass,while the decrease in mid-latitude surface pressure is related to a decrease in tropospheric air mass.