The Downward Extension Of Stratospheric Northern Annular Mode Anomalies And Its Influences On The Climate In China

The downward extension of stratospheric northern annular mode(NAM)plays an important role in the tropospheric weather forecast.In this study,the NCEP/NCAR,JRA-55 reanalysis datasets and observational datasets are used to diagnose the influence of tropospheric environment,QBO and ENSO on the downward extension of stratospheric NAM signals.Influences of stratospheric NAM on meteorological elements in winter time(November-March)over China are also diagnosed.1.Using the NCEP/NCAR reanalysis dataset,this study classifies stratospheric NAM anomalies during negative/positive phase into two categories—anomalies extending into the troposphere(referred as negative/positive TEs)and those not extending into the troposphere(referred as negative/positive NTEs),and the corresponding tropospheric environments during the TEs and NTEs are identified.Compared with that for the negative NTEs,the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs.Furthermore,the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation,which is manifested by fewer occurrences of negative refractive index squared in the mid-high latitude troposphere and stronger wave intensity in the mid-high latitude troposphere.However,the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave-propagation in modulating the upward wave fluxes into the stratosphere.Stronger and more persistent upward wave fluxes in the negative TEs,particularly wave-1 fluxes,are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors.These negative/positive geopotential height anomalies over the North Pacific/Euro-Atlantic are related to the positive/negative diabatic heating anomalies and the decreased/increased blocking activities in the mid-high latitudes.The subtropical diabatic heating could also impact on the strength of the mid-high latitude geopotential height anomalies through modulating horizontal wave fluxes.For positive NAM events,the results are roughly similar to those for negative NAM events,but with opposite signal.2.The impact of QBO on the downward extension of stratospheric NAM in positive and negative phases is examined.The results show that average number of days per month in which stratospheric negative NAM signals extend downward into the troposphere is more than twice as high during the easterly phase of QBO(EQBO)as those during the westerly phase of QBO(WQBO).Increasing likelihood of downward extension of negative NAM during the EQBO is related to three factors: enhanced upward wave-1 fluxes entering the stratosphere,anomalous poleward wave fluxes below 100 hPa and EQBO-induced residual circulation.It is also revealed that EQBO-induced subtropical tropospheric easterly anomalies lead to positive height anomalies over the subtropical North Atlantic,which are amplified by transient eddies and maintained by convection.These subtropical positive height anomalies further stimulate an anomalous wave train,forcing subpolar positive height anomalies over the Euro-Atlantic sector and enhancing wave 1.Over the Pacific,EQBO could also induce positive height anomalies in the tropical troposphere,and these anomalies are related to convection and vertical motion.These positive height anomalies can induce anomalous poleward-propagating waves,forcing negative height anomalies over the North Pacific at mid-high latitudes and enhancing wave 1.Fewer stratospheric positive NAM signals extend downward into the troposphere during the EQBO than during the WQBO,with mechanisms similar to those of negative NAM.3.The influence of ENSO events on the downward extension of stratospheric NAM signals is further analysed.The results show that Eastern-Pacific(E-P)El Ni?o events are more conducive to the downward extension of negative NAM anomalies,while E-P La Ni?a events are beneficial to the downward extension of positive NAM anomalies.During the E-P El Ni?o,the more occurrences and downward extension of negative NAM are related to anomalous upward and poleward propagation of planetary waves at mid-high latitudes.Anomalous upward planetary waves are mainly contributed by wave 1,and anomalous poleward planetary waves are caused by wave 1 and wave 3+.The tropical circulation and diabatic heating anomalies induced by E-P El Ni?o lead to an increasing geopotential height over tropical Pacific and enhanced subtropical westerly winds,which promotes anomalous poleward propagation of planetary waves.On the one hand,these poleward wave fluxes can weaken the mid-high latitude westerly winds,which is conducive to the development of negative NAM anomalies.On the other hand,they can also deepen the Aleutian low and lead to the strengthening of wave 1,which favours the formation and downward extension of negative NAM anomalies in the stratosphere.Planetary wave activities during the E-P La Ni?a events are roughly opposite to those during E-P El Ni?o.Influence of central-Pacific(C-P)ENSO events on the downward extension of NAM is not as obvious as those of E-P ENSO events,which is mainly caused by the difference in planetary wave activity between E-P ENSO and C-P ENSO events.4.Finally,this study analyses the influence of downward extension of stratospheric NAM signals on the near-surface meteorological elements in China during winter time.The results show that near-surface wind speeds increase(decrease)in the most areas of China when stratospheric negative(positive)NAM signals extend downward to the troposphere.These wind speed anomalies are related to the change of pressure gradient induced by NAM anomalies.During the period of negative NAM extending downward to the troposphere,near-surface air temperature decreases in northern regions of China as well as the middle and lower reaches of Yangtze River,and increases in southern China,the Southwest China,and the Qinghai province.During the period of positive NAM extending downward to the troposphere,near-surface temperature increases in Northeast China,Xinjiang,and Inner Mongolia,and other regions of China are dominated by anomalous cooling.These temperature anomalies are mainly attributed to the advection of meridional and zonal winds.The temperature changes caused by meridional advection term are greater than those of zonal advection term,and vertical motion contributes little to temperature changes.When negative NAM extends downward to the troposphere,precipitation in the middle and lower reaches of Yangtze River over China increases significantly but decreases significantly in Yunnan province.When positive NAM extends downward to the troposphere,precipitation increases significantly in southern China but decreases in the middle and lower reaches of Yangtze River.