Study On The Interannual Variation And Causes Of The Coupling Relationship Between Large-scale Low Temperature Events And Stratospheric Air Mass Circulatio

Cold air outbreak(CAO)is the most frequent and wide-impacted disastrous weather in winter.It is necessary to improve its sub-seasonal forecast skills in winter operational forecasts.The sub-monthly variation of the stratospheric polar vortex in the Northern Hemisphere winter,related to the stronger stratospheric warm air transport into the polar stratosphere(defined as a PULSE event),always couple with the equatorward cold air mass transport at lower levels through the deep planetary waves as the link,thus affecting the near-surface CAOs.The downward influence of the stratospheric signal makes it a new sub-seasonal predictability source for sub-seasonal forecasts of cold air outbreak events.However,the relationship between PULSE and CAO events in Northern Hemisphere winter is not stable from year to year and the causes are not fully understood,which greatly limits the effective application of the stratospheric signal in sub-seasonal forecasting.Using ERA5 Reanalysis data covering winters(November-March)in the period 1979-2015,and according to the one-to-one correspondence between the CAO and PULSE,the CAO events in the midlatitudes of Eurasia and North America are classified into events that are coupled and decoupled with the PULSE.The coupled CAOs are further categorized into events that are respectively lead-coupled and lag-coupled with PULSEs.Then,the characteristics of CAO events with different types are examined,and the temporal variations of the ratio of different types of CAO events to the total number of events in winter(i.e.,coupling rate)are analyzed.Finally,the influencing factors and underlying mechanisms are investigated.The main results are as follows:(1)Compared with decoupled CAO events,the intensity and affected area of extremely cold temperatures tend to be larger during CAO events that are coupled with PULSEs,especially lag-coupled events in North America and lead-coupled events in Eurasia.The winter coupling rate has significant interannual and interdecadal changes.(2)The interannual and interdecadal changes of coupling rate are found to be closely related to the El Nino-Southern Oscillation(ENSO).The North Pacific-North America wave train and Aleutian low induced by El Nino intensifies the upward planetary waves propagating to the troposphere,which is favorable for the higher lag-coupling rate of CAO in North America and lead-coupling rate of CAO in Eurasia.The ENSO signals related to the interdecadal changes of CAO coupling rate in winter can be traced back to the previous winter,while an ENSO phase transition from the previous winter to the current winter is closely related to the interannual changes of CAO coupling rate.(3)The interannual changes of PULSE-CAO coupling in winter is not closely related with Pacific Decadal Oscillation(PDO)/North Pacific Gyre Oscillation(NPGO),which is the first/second leading mode of the monthly or seasonal mean North Pacific sea surface temperature anomaly in the entire period from 1979 to 2015.However,it is found that their relationship reversed from significantly in-phase/out-of-phase in the period 1979-2000 to outof-phase/significantly in-phase in the period 2000-2015 for PDO/NPGO and the rates of lagcoupled CAO in North America,suggesting the rising role of the NPGO on the interannual variability of the PULSE-CAO coupling relationship in recent decades.Such increasingly important role of NPGO on PULSE-CAO coupling from 2000-2015 is related to the impact of the interannual component of NPGO on the strengthening of the upward propagation of planetary waves originated in the Eurasian and the downward reflection of planetary waves toward the America and North Pacific.