Research On Explosive Stratospheric Warming And Its Abnormal Downstream Transmission

Based on ERA-interim daily reanalysis data,the variation characteristics of intensity,time,warming centers and tracks of 33 major stratosphere sudden warming(SSW)events and 22 minor SSW events in the northern hemisphere during 1979-2016 are analyzed.Besides,the time,intensity and circulation of the 25 anomalous downward propagation events of SSW are also analyzed.And in order to find out the possible factors affecting the anomalous downward propagation of SSW,the relationship between the nomalous downward propagation and the intensity of SSW events and the characteristics of circulation evolution and planetary wave with two anomalous downward propagation events of polar vortex displacementtype and vortex splitting type and twonone downward propagation events are analyzed.The results show:(1)The process length of SSW and the duration of major SSW both show a downward trend from 1979 to 2016.And the process length of major SSW events is significantly longer than that of minorones.The variation of temperaturealso shows a linear trend of downward.In major SSW events,there is a great correlation between the variation of meridional gradient of zonal mean geopotential height from 60°N to polarzonal and the mean zonal wind at 60°N in 10 h Pa SSW eventsmainly occur in February(average 1.4 times per year),and the minor eventsare mainly occur in the late winter and early spring.Most SSW events will appear the phenomenon of warmingfrom 10 h Pa to100 h Pa,but the number and the influence to stratosphere of magor events are much greater than minor events.Most of the warming occurs at 10 h P first,and then the signal would downward propagate to 100 h Pa with 6 hours to 20 days causing the sudden warming.(2)The initial warming centers of SSW diffuse southward from high latitudes near polar region of the eastern hemisphere to the lower latitude of the western hemisphere.The maximum warming centers at 10 h Pa havean obvious regional characteristic,mainly distribute in the Eurasian continent between 30°E and 120°EThe cumulative warming centersare mainly distribute between 90°W and 90°E,especially in 30°W-30°E,and major events mainly concentrate in the eastern hemisphere,minor ones are in the western hemisphere.The tracks of majority SSW events are from north to south,and the meridional spansare mostly between 0°-9°.Also the end points of SSW do not fall within the range of80°N-90°N.(3)Anomalous downward propagationevents of SSW occur in winter with the average of 18 days.The duration of downward has a slight decreasing linear trend at 1h Pa,with an average of 28.6 days.At 10 h Pa and 100 h Pa,it shows a linear trend of increasing,and the correlation is extremely high,and the duration is the longest with 47 days on average at 100 h Pa.The maximum anomalous downward propagation from 1h Pa to 100 h Pa shows a rising linear trend with16.4 days on average.The abnormal intensitiesare increased with time at 1h Pa,10 h Pa and 100 h Pa,and their variation trends are also consistent.The correlation between 1h Pa and 10 h Pa is the best.The anomalous downword intensity from 1h Pa to 100 h Pa is increasing with time,and the years with strong anomalous downword intensity are almost consistent with the years with strong abnormal in all layers.(4)15 polar vortex splitting types and 10 polar vortex displacementtypes are analysed in anomalous downward propagationevents of SSW.The variationsof the polar region are all changed from cold polar vortex to high pressure after warming and then returning to the cold polar vortex at 10 h Pa.It indicates that the polar warming may be required during the anomalous downward propagation of the SSW,but before the downward,the vortex displacementtype has a large range of small high pressure,and the cold polar vortex range is wider and there is a high temperature center in vortex splitting.During the period of downward,vortex displacement occurs in vortex displacement type,and the cold vortex center splits into two in the polar vortex splittingtype.In the later downward,there is still weak high-temperature center in vortex splitting.(5)Major SSW eventshave an important influence on the occurrence of anomalous downward propagation.Anomalous downward propagationin minor SSW always has a longer warming process.Before the warming,if the abnormal potential height is not appearing for a long time,it mayoccur downward,andif the duration of the positive abnormalyat 1h Pa is extremely long,it may not be downward.(6)Circulation characteristic of influencing the anomalous downward propagation on SSW: may be required: a relatively stable circulation in the early warming,especially in the vertical direction,during the warming,the circulation form should have a large change,especially in high latitude,and the intensity and the scope of positive anomaly should be obviously expanded and strengthened,and there should be multiple positive and negative anomalies in the lower stratosphere and the middle latitude of the troposphere,which often occurs in the late stage of the downword.In the vertical direction,the quasi-positive pressure structure is required.If there is an SSW event in the early warming,the downword may not go deep into the troposphere.(7)Planetary wave of influencing the anomalous downward propagation on SSW: in the early warming the EP flux in the stratosphere is increased,and reaches the strongest at the beginning of warming.At this time the area of EP flux convergence extends downward and poleward to the maximum and the intensity is strongest.And the west wind deceleration in the stratospheric polar is enhanced.During the warming,the EP flux uploading continues to weaken,and the difference between the front and the back is obvious,so that the planetary wave energy can be released enough.And the range of EP flux convergence gradually shrinks to the low latitude,and the westerly deceleration weakens,but the westerly circulation has been greatly weakened,even with a long-term conversion to the eastly circulation,which has a tendency to spread to the lower layers.Later,with a smaller upload EP flux and enhanced stratospheric EP flux divergence,the easterly circulation is weakened,and the westerly circulation is enhanced.However,due to the long-term westerly deceleration,the westerly circulation cannot recover quickly enough to the intensity of the pre-warming period.Ifthe energy of planetary wave breaking is large enough,and the EP flux convergence region can reach the troposphere converging with the tropospheric banding divergence zone,making eastly reach the troposphere,the anomalous downward propagationin SSW may reach the troposphere.