Impact And Mechanism Of The Northern Hemisphere Stratospheric Polar Vortex On Extreme Low Temperature Events In East Asi

Extreme cold event(ECE)is the main disastrous phenomenon in mid-latitude Asia in winter.It is important to study the characteristics of ECE and formation mechanisms to improve the understanding and predictability of ECE.Therefore,by combining the data of National Centers for Environmental Prediction(NCEP)and Modern-Era Retrospective Analysis for Research and Applications version 2(MERRA-2),using meteorological statistical methods and synoptic dynamic analysis,this study systematically analyzed the variation characteristics of ECE in mid-latitude Asia and the influence of stratospheric polar vortex on the ECE.The main conclusions of this study include:(1)During 1959/1960-2019/2020,the mean surface temperature in the winter significantly increased(0.25℃/decade)and the intensity of ECE significantly weakened(0.15℃/decade)in East Asia.The frequency of ECE has significantly decreased in East Asia.And ECEs occurred more frequently from 1960 s to 1970 s and around 2010/2011.We consider the entire process of the movement of large-scale cold air in the China as an extensive ECE.The frequency and cumulative days of extensive ECE lasting for 3 days or longer decreased significantly,but the frequency and cumulative days of extensive ECE lasting for 1-2 days did not change significantly.Extensive and persistent ECE(lasting for 8 days or longger)mainly occurred in January.(2)Based on the duration,we divide extensive ECEs into those with longer durations(lasting for 8 days or longer)and those with shorter durations(lasting for 3-7 days).We further studied the dynamic mechanism related to these two kinds of extensive ECEs with different duration.Compared with extensive ECEs with shorter duration,the stratospheric polar vortex extended to the Atlantic-Europe region during the extensive and persistent ECEs,which strengthened the gradient of temperature between the mid-latitude and high-latitude regions and further led to the enhancement of the zonal wind in corresponding region.The propagation of enhanced zonal wind in the Atlantic-Europe region from the lower stratosphere to the troposphere resulted in the reduction of the 500 h Pa geopotential height over the Atlantic-Euro region,which is beneficial to the strengthening and maintenance of the Ural ridge and the East Asian trough.The northerly wind in front of the Ural ridge resulted in the enhancement and southward movement of the surface anticyclone circulation,and led to the outbreak of extensive and persistent ECEs in China.However,during the extensive ECEs with shorter duration,the intensity of the Ural ridge and the East Asia trough were relatively weaker than that in extensive and persistent ECEs.(3)We analyze an ECE with the longest duration in Siberia from 1980/1981 to 2019/2020 and the influence of minor SSW(sudden stratospheric warming;SSW)event on it.This ECE occurred from 30 December 2000 to 10 January 2001,which was in the recovery phase of a minor SSW event.The stratospheric polar vortex gradually strengthened along with the increasing of the zonal winds in the stratosphere over the Ural region.The enhancement of the zonal wind resulted in the reflection of planetary waves in the stratosphere in Atlantic-Europe and Siberia region,which led to the strengthening of the troughs in these two regions.The strengthening the meridional flow contributed to the development and maintenance of the Ural ridge.The cold advection in front of the Ural ridge led to the outbreak of ECE.When the stratospheric polar vortex did not continue to strengthen,the cold air weakened and moved towards northeast China.We also found that the contribution of minor SSW events to the frequency of ECE was relatively less than that in major SSW events,but the ECE intensity caused by major and minor SSW events is basically similar in the past 40 years.Moreover,more than 80% of ECEs related to minor SSWcould lasted longer than 5 days.(4)According to the intensity,we divide extensive ECEs into extensive and strong ECEs(1% threshold)and extensive and weak ECEs(10% threshold).We analyzed the impact of strong stratospheric polar vortex(s SPV)and weak stratospheric polar vortex(w SPV)events on the frequency of extensive and strong ECEs and extensive and weak ECEs.The frequency of extensive and strong ECEs(extensive and weak ECEs)caused by the s SPV and w SPV events were really similar.The proportion of the frequency of extensive and strong ECE related to w SPV and s SPV events in the total frequency of extensive and strong ECEs was 38.5%,which was greater than 31.5% of that in extensive and weak ECEs.That is to say,compared with extensive and weak ECEs,w SPV and s SPV events are more important to the frequency of extensive and strong ECEs.We also found that during day2-day7 of the extensive ECEs caused by the w SPV event,the weakening of the stratospheric polar vortex was favor for the enhancement of the intensity of extensive ECEs.(5)During 29 December 2020 – 1 January 2021 and 6–8 January 2021,two ECEs broke out in eastern China.During 29 December 2020–1 January 2021(the first ECE),the tropospheric polar vortex migrated to mid–latitude East Asia and East Asian trough strenghthened,which resulted in more planetary wave propagated into the stratosphere.Then,the stratospheric polar vortex weakened and extended to mid-latitude East Asia from 2 January.And,a SSW event occurred on 5 January.The easterly zonal wind accompanied by the deformation of stratospheric polar vortex over East Asia tended to strengthen the East Asian trough via reflecting the planetary wave from the stratosphere to troposphere.Simultaneously,the strengthening and southward shifting of the Ural ridge resulted in the southward migrating of extremely strong East Asian trough,which led to the outbreak of second ECE.Therefore,the ECE in early January was jointly caused by the combined effects of deformation of stratospheric polar vortex during SSW event and the southward shifting of Ural ridge.