Dynamic Differences Of North American Cold Wave Outbreaks Under Different PNA Phase Transition

The anomaly of Pacific-North American type(PNA)atmospheric teleconnection pattern is one of the important factors affecting the outbreak of cold waves in North America,but there are significant differences in the development of different cold waves in the same PNA phase.In this paper,multi-scale subspace transform(MWT)and local multi-scale energy analysis(MSEVA)methods are used,combined with kinetic energy conversion(CK)and effective potential energy conversion(CP)formula between mean flow and disturbance flow,and based on ERA-5 reanalysis data,the evolution and differences of two record cold waves in North America in January 2019 and February 2021 under different PNA phase transition background are compared and analyzed.The results show that the PNA phase transition process of the two cold waves is opposite.The 2019 event represents a typical North American cold wave process,with the extreme cold weather from January 25 to 31 occurring in the Great Lakes region of northeastern North America under the previous PNA positive phase,while the 2021 event is an atypical process,with the extremely cold weather from 14 to 18 February occurring in Texas in the southwestern North America under the late PNA positive phase.By comparing and analyzing the difference between multi-scale reconstruction field and multi-scale energy distribution,the following conclusions are drawn:(1)The PNA phase of the 2019 event changed from positive to negative,and in the early stage of the event,the extremely cold weather developed in the Great Lakes of North America,and the wave series showed an east-west direction;in the later stage of the event,the disturbance cold vortex moved eastward,the new cold wave developed in the northwest of North America,and the wave series showed a northwest-southeast trend,and the corresponding relationship between the cold wave outbreak area and the PNA phase was basically consistent with the traditional research.The PNA phase of the 2021 event changed from negative to positive.In the early stage of the event,the cold wave developed in northwestern North America,moved to central North America and maintained,and the wave series showed a southwest-northeast trend;in the later stage of the event,the jet stream developed southward,a new cold and warm center in the Pacific Ocean developed,the wave train showed an east-west direction,and the extreme cold weather appeared in Texas in the southwest of North America.The corresponding relationship between the cold wave outbreak region and the PNA phase is quite different in the later stage and the traditional research.(2)In the early stage of the 2019 event,the cold vortex of the disturbance field in eastern North America obtained the effective potential energy from the background field to enhance the baroclinic property,and the kinetic energy obtained from the background field moved eastward;in the later stage of the event,the new disturbance field cold vortex developed in northwestern North America,and the effective potential energy and kinetic energy were obtained from the background field on both sides of the trough,and the intensity was weaker than that in the previous stage.In the early stage of the 2021 event,the cold vortex in the northwest of North America developed in front of the guided ridge in the northeast of the Pacific Ocean,and the cold vortex obtained the effective potential energy from the background field to enhance the baroclinic;the jets on both sides of the guided ridge upstream of the cold vortex obtained kinetic energy from the disturbance field,promoting the southward development of the jet stream in the Pacific.The late jet is maintained to the south of 40 °N,and the background field transports kinetic energy and effective potential energy to the disturbance field.The cold and warm center of the disturbance developing southward in the Pacific Ocean strengthens the wave train eastward along the jet stream,and the disturbance low pressure stabilizes in the southwest of North America,and the cold wave breaks out in Texas.Thus it can be seen that the potential height anomaly of the same positive PNA phase comes from the North American continent in the early stage of the 2019 event,when the center of the North American cold anomaly is located in the eastern part of North America,which is basically consistent with the traditional research,while in the late 2021 event,the dominant potential height anomaly of this phase comes from the Pacific Ocean,when the North American cold anomaly center is located in the southwest of North America,which is completely different from the traditional research.