Variation And Mechanism Of Summer Extreme High Temperatures In Northeast Asia Based On Circulation Classification

The intensified extreme high temperature events(HTEs)in the past few decades have brought great damage to Northeast Asia.This paper statistics and characterizes the features of HTEs in Northeast Asia from 1979 to 2021,and objectively classifies extreme high temperature events into ACA type with anticyclone-cyclone-anticyclone circulation anomalies and CA type with cyclone-anticyclone circulation anomalies.The study used observational statistics,dynamic diagnosis,and numerical simulations to investigate the key atmospheric signals of these events,their interannual variations,and mechanisms.The study also revealed the decadal variations and possible connections between different types of HTEs and the Atlantic Multidecadal Oscillation(AMO)and Pacific Decadal Oscillation(PDO).The main conclusions of this paper are as follows:(1)The summer HTEs in Northeast Asia were objectively classified based on largescale circulation anomalies.This study used the standard deviation threshold to analyze 425 HTEs in Northeast Asia during summers from 1979 to 2021.Results showed that these events were mainly accompanied by quasi-latitudinal wave anomalies from the eastern Ural Mountains to Japan,but the circulation anomaly was weak and insignificant in the upstream region.Based on the circulation anomalies at 300 h Pa over Eurasia(0°—180°,30°—80°N)during HTEs,the study proposed an objective classification method.This study defined HTEs accompanied by a wave train anomaly of the East European anticyclone,the Siberian anticyclone,and the Northeast Asian anticyclone as ACA type events(accounting for 42.4% of total events),and those accompanied by anticyclone anomalies over the Urals and cyclone anomalies over Siberia as CA type events(accounting for 34.6% of total events).The remaining 23% of HTEs did not exhibit significant large-scale circulation anomalies.The study found that the upward trend of ACA type events from 1979 to 2021 was more than twice that of CA type events.Although the strength of ACA type and CA type events was similar,ACA type events had weaker persistence,especially for events lasting five days or more,where the probability of CA type events was16.7% higher.This was related to the higher probability(about 18.8%)of Northeast Asian atmospheric blocking anomalies accompanying CA type events.(2)The early crucial atmospheric signals of two types of HTEs are investigated through observational statistics and numerical simulations.The composite analysis of circulation patterns indicates that the local circulation anomalies over Northeast Asia are similar prior to the two types of HTEs,but there are significant differences in upstream signals.The circulation anomalies are mainly distributed in the mid-high latitudes from low to high altitudes.The key precursor signal for ACA type HTEs is the warm atmospheric anomaly over southern Greenland,while CA type events are associated with cold anomalies over the Ural Mountains region.Using the linear baroclinic model,this study further reproduced the responses of the circulation anomalies to precursor signals for both types of events and found that the precursor signals can explain 42.4% and 36.7% of the circulation anomalies for ACA type and CA type events,respectively.(3)The interannual variations of two types of HTEs and the related key circulation factors are investigated.This study identified the interannual variation of two types of HTEs and their key atmospheric circulation factors.The two types of events exhibit significant interannual variability and are consistent with changes in the tropospheric thickness.The atmospheric circulation patterns associated with the interannual variability of ACA type events are related to the two wave trains over the Eurasian continent,accompanied by weakened subtropical jet stream and strengthened polar jet stream.The interannual variability of CA type events is characterized by a meridional dipole structure over the Eurasian continent and is significantly correlated with the interannual variability of the frequency of blocking events in the Siberian region.Using the CAM5.3 model,this study further verified the differences in the frequency of occurrence of the two types of events during years with abnormal jet stream and blocking patterns.The results showed that abnormal jet stream(blocking)can increase the annual frequency of ACA(CA)events by 31.8%(36.8%).(4)The interdecadal variations of ACA and CA HTEs and their relationship with AMO and PDO are revealed.Both types of HTEs show a "more-less-more" interdecadal variability pattern,but there are significant differences in the timing of transitions.Correlation and synthesis analyses reveal that the interdecadal variability of ACA(CA)events is consistent with AMO(PDO),which is further validated by information flow analysis.Positive AMO provides a favorable circulation background for the occurrence of ACA events by promoting anomalous anticyclones over the Eastern Europe and Northeast Asia regions and maintaining the activity of the two wave trains over the Eurasian continent,while negative PDO enhances the frequency of blocking events over the Siberian region,thereby promoting CA events.CAM5.3 simulations confirm the proposed mechanisms and show that the combined impact of positive AMO and negative PDO on ACA event frequency(77.1%)is greater than the impact of positive AMO alone(54.2%),while the interdecadal increase of CA events is mainly driven by negative PDO(87.4%)with little sensitivity to AMO.