The Research On The Dynamical Mechanism Of Extreme Cold Events Based On The Three-Pattern Decomposition Of Global Atmospheric Circulation

In contrast to previous studies that mainly focused on the influence of horizontal circulation on extreme cold wave events(extreme cold events),this thesis reveals the dynamic synergistic effects of local horizontal circulation,meridional circulation and local zonal circulation on extreme cold events based on the newly developed method in recent years named three-pattern decomposition of global atmospheric circulation(3PDGAC).As to overcome the expression error of the traditional polar vortex indices under global warming,this thesis first defines the polar vortex indices using the streamfunction in 3P-DGAC and analyzes the advantages of the new indices in indicating the surface air temperature(SAT)changes on weather and climate time scales.On this basis,the dynamical influence mechanism of three-pattern circulation for the extreme cold events in East Asian and North America is investigated,highlighting the influence of local meridional and zonal circulation and their dynamic synergistic effects with the horizontal circulation.Finally,the differences in the three-dimensional structure of the related atmospheric circulation for persistent extreme cold events are explored,and the dynamic synergistic effects of the three-pattern circulation in typical persistent extreme cold event are revealed.The main conclusions are obtained as follows:(1)A new definition of polar vortex indices based on the streamfunction in 3PDGAC is proposed,and the dynamic advantages of the newly defined indices in indicating the occurrence and development of weather and climate events are demonstrated.Under the influence of the Arctic amplification,the air temperature from the lower to middle troposphere over the high latitudes in boreal winter rises significantly,causing a significant increase of geopotential height in the middle troposphere,which leads to errors in the definition of the tropospheric polar vortex indices using a fixed threshold of geopotential height in traditional studies in practical applications.However,the streamfunction of horizontal circulation in 3P-DGAC can always maintain stable monotonicity in the troposphere,which is less susceptible to global warming.The streamfunction of horizontal circulation in 3P-DGAC matches better with the relative vorticity of actual atmosphere than the mass streamfunction of the traditional two-dimensional decomposition method,which is a suitable tool for describing the evolution of large-scale circulation.Therefore,the polar vortex indices defined by the streamfunction in 3P-DGAC not only conforms to the dynamic characteristics of the polar vortex,but also clearly separate the relatively cold and warm centers,and is applicable in both the stratosphere and troposphere.In addition,the new polar vortex indices are closely related to the Arctic Oscillation(AO)and North Atlantic Oscillation(NAO)teleconnection than the traditional definition of geopotential height,and are more indicative for SAT at the weather scale.Further analysis of the effects of climate change in polar vortex area reveals that there are differences in the roles of stratospheric and tropospheric polar vortex areas on SAT in different regions,showing that the cooling trend of SAT in February in the mid-high latitude East Asia is caused by a significant increase in the stratospheric polar vortex area,while the cooling trend of SAT in the mid-low latitude North America is caused by a significant increase in the tropospheric polar vortex area.(2)The dynamic influence mechanism of local horizontal circulation and local vertical circulation on the intensity and persistence change of extreme cold events in East Asian is revealed.During the extreme cold events in East Asia,the enhanced Ural Mountains blocking high pressure(Ural blocking)causes a southward deformation of the East Asian polar vortex edge,which together with the enhanced Siberian high induces cold air outbreak from high latitudes.The analysis of the climatology characteristics of the local meridional circulation in winter shows that there is a broad Ferrel cell and Hadley cell over East Asia and its upstream region,but the Polar cell is not obvious.Only the Hadley cell is evident in the downstream region,and the meridional circulations are irregular in the mid-high latitudes.The Ferrel cell over East Asian upstream has no meaningful impact on the SAT in East Asia,but the weakening of the Ferrel cell over East Asia leads to the southward movement of cold Arctic air,which directly affects the intensity of the extreme cold events.The Hadley cells in East Asia and its downstream region are synergistically changing,and the weakening of the latter is conducive to the intensifying the sinking motion over the mid-latitudes of downstream region,which in turn strengthens the Siberian high and indirectly affects the duration and intensity of the extreme cold events in East Asia.The climatology of the local zonal circulation over mid-latitude East Asia exhibits a clockwise rotation with rising in the west and sinking in the east.When it strengthens over the east side of East Asia,inducing the occurrence of anomalous counterclockwise rotation(rising in the east and sinking in the west)of the zonal circulation over the west side of East Asia.The common rising branch of both causes adiabatic cooling of the East Asian air mass,while their respective sinking branchs intensify the high-pressure systems over the East Asian upstream and downstream from the peak of the extreme cold to its later stage,prolonging the duration of the extreme cold events.Additionally,the vertical velocity of the zonal circulation dominates the total vertical velocity over the region with the strong sinking motion in the upstream and downstream,and its regulation on the highpressure systems is more important than that of the meridional circulation,which has not been involved in previous studies.The above conclusions suggest that the dynamic synergistic effects of the three-dimensional atmospheric circulation need to be considered simultaneously when analyzing the influence mechanism of the extreme cold events.(3)The pathways of local horizontal circulation and local vertical circulation affecting the occurrence and development of extreme cold events in North America are clarified,and the key role of mid-latitude vertical circulation in the development of extreme cold events in North America is pointed out.The strengthening of the northeast Pacific blocking forces the tropospheric polar vortex over North America to shift southward,resulting in the cold air outbreak.Compared with the Siberian high,the location of the North Atlantic high pressure is southerly,which prevents the anomalous North American cyclone from moving eastward and prolongs the duration of extreme cold events.The Ferrel cell over North American covers the mid-high latitudes in boreal winter,but the standard three-cell modes exist over the upstream and downstream of North America,representing the differences in dynamic influence mechanism of extreme cold events between East Asia and North America.The weakening of the Polar cell over the upstream of North America favors the strengthening of the northeast Pacific blocking,which plays a key role in the occurrence and development of the extreme cold events.Subsequently,the weakened Ferrel cell over North America followed by low-level anomalous northerly wind guides the cold Arctic air to the midlow latitudes.The strengthening of the Polar cell over the downstream of North America leads to a combined transport of cold air from polar high altitudes and lowlevels to eastern North America.Furthermore,there are local zonal circulations with rotating clockwise(rising in the west and sinking in the east)over the west and east sides of North America respectively,in which the significant weakening of the western zonal circulation is conducive to strengthening the northeast Pacific blocking and delaying the collapse of the blocking.while the strengthening of the eastern zonal circulation not only enhances the high pressure over downstream of North America through the anomalous sinking motion and prolong the duration of the extreme cold events,but also causes the low-level anomalous easterly winds to guide the cold air at high altitudes that sinking with the Polar cell over the downstream towards North America.This part of the thesis improves the understanding of how local meridional and zonal circulations act on the extreme cold events.(4)The dynamic mechanism of the local zonal circulation affecting the persistence of the extreme cold events and the local meridional circulation affecting the intensity of the extreme cold events at mid-high latitudes are explained.The study about the persistent extreme cold events in East Asia and North America finds the strong blocking highs over the upstream in both regions lead to a relatively early onset,and the strong highs over the downstream prolongs the transit time of the weather system,both of which play a key role in the maintenance of extreme cold events in the middle and late stages.The anomalous signals of the local zonal circulation in both regions are always strong at the peak of extreme cold and its late stages,and has a significant regulatory role on the high-pressure systems over the upstream and downstream.Therefore,although the horizontal circulation has a direct influence on the duration of the extreme cold events,the modulating role of the indirect influence of the zonal circulation is most critical.Similarly,the analysis finds that the local meridional circulation usually affects only the intensity of cold air outbreaks and has little effect on the duration.A detailed analysis of the typical persistent extreme cold event further demonstrates that the influence of local vertical circulation on the extreme cold events is essential.The anomalous local zonal circulation triggered by the horizontal circulation can in turn enhance the block high and further prolong the duration of the extreme cold events.Moreover,the intensity of the cold events could be larger if the local Polar cell strengthens and widens in the same configuration as the mid-latitude meridional circulation.The dynamic synergistic effects between the three-pattern circulations are directly responsible for the occurrence and development of the persistent extreme cold events.