The Synergistic Impact Of Intraseasonal Changes In Mid- And High-latitude Circulation Systems On Winter Low Temperature Events In Northeast My Countr

Based on minimum air temperature data derived from China Meteorological Data Network and the NCEP/DOE reanalysis data,the intra-seasonal scale circulation characteristics and evolution of extreme cold events in the Northeast China during the winter of 1979-2018 are explored.The synergistic influence of stratospheric and tropospheric key circulation systems such as stratospheric polar vortex and Siberian High on cold events is discussed.Besides,two cold events under the condition of strong and weak polar vortex in the early stage are selected to compare the differences of their circulation systems.The main conclusions are as follows:(1)The winter daily minimum air temperature in the Northeast China presents a significant low-frequency period of 10-30 d.During the cooling phases,a pair of cyclone-anticyclone in the lower troposphere moves southeastward accompanying the intensifying Siberian High and leads to the abnormal northerly;the developing wave trains in middle troposphere result in enhancing and maintaining cold air.There are two low-frequency wave trains in the Eurasian at 200 hPa level.The northern one moves southeastward through the Ural Mountains to the coast of East Asia,with the upstream wave activity flux dispersing to the Northeast China,inducing the development of the northeast cold vortex.The southern one transmits the weak wave activity flux,the low temperature in Northeast China is mainly affected by the low frequency variability of middle-high latitude system.The upper troposphere dominates by weak East Asian polar front jet and strong East Asian subtropical jet,guiding cold air outbreak and accumulate in the NE-N region.(2)In the process of low temperature,the deep cold air transports from north to south,the cold air column tilted southward at 70-850 hPa.In the early stage,the strong cold air in the upper level is located near Novaya Zemlya,and the intensity of the cold air in the lower layer is relatively weak.With the invasion and replenishment of the high potential vorticity,the high potential vorticity column continues to pass down along the incline isentropic surface.The low vortex system in the troposphere moves eastward and the deepening makes the cold air invade southward with the north wind behind the trough.On the other hand,the air flow with the high potential vorticity moves down along the isentropic surface,causing the southward and strengthening of the Siberian High,which leads to the occurrence and development of cold events.(3)The key circulation systems affecting the Northeast China cold events include the stratospheric polar vortex,the East Asian subtropical jet in the upper troposphere,the Ural blocking high in the middle troposphere,the northeast cold vortex,the Siberian High in the sea level pressure field.Except for the stratospheric polar vortex,the other systems are weak in the early stage.With the weakening and splitting of the polar vortex,the strengthening of the high potential vorticity in the Kara Sea leads to the development of the Ural blocking high,which combined with the high over east side of the Sea of Japan,conducive to the strengthening of the northeast cold vortex.The barrier effect of the subtropical jet makes the cold air accumulate over the Northeast and North China,resulting in the decrease of temperature.The polar vortex leads 5 phases earlier than low temperature,Ural blocking high advanced low temperature of 3-4 phases,the rest of the systems are basically in phase with low temperature.The synergistic effect of circulation systems will lead to extended range of cold anomalies in Northeast and North China.(4)Two typical cold events are screened out according to the background of strong and weak polar vortices.The event corresponding to the strong polar vortices(the 18th cold event,denotes as Case18)lasting longer than that of the weak polar vortices(the 27th cold event,denoted as Case27),and the cooling range during phase 7 is larger.With the upper potential vortex incursion into the lower level,the air column rotates strongly,resulting in the strengthening of circulation systems in the troposphere.Moreover,the system moves slowly,and the enhanced phase of is earlier than Case27.Therefore,it takes a long time for cold air to accumulate and erupt,the temperature will be lower.