| In this paper,based on the 90 th percentile threshold of CN05.1 daily maximum and daily minimum temperature during 1979-2019,the summer extreme high temperature(EHT)in North China was divided into three types: independent warm day type(LWD),independent warm night type(LWN)and day-night compound(DNC)type.The variation characteristics of the three types of EHT in North China are analyzed,and their circulation characteristics and differences are studied.Finally,the possible relationship between the interannual variation of the three types of EHT in North China and the North Atlantic sea surface temperature anomaly is discussed.The main conclusions are as follows:(1)The temperature anomalies of the three types of EHT in summer in North China are different and mainly occur in different months.The daytime temperature anomaly of three types of EHT is usually LWD > DNC > LWN,night temperature anomaly is usually DNC > LWN > LWD.Daytime temperatures of LWD and DNC EHT can last into the night,while LWN can’t.The occurrence of LWD(LWN)EHT was mainly concentrated in early June to early July(late July to early August),and the occurrence of DNC EHT was mainly concentrated in mid-July.(2)Different distribution of local circulation anomalies affecting three types of EHT in North China in summer led to different daytime and nighttime temperature anomalies of three types of EHT,resulting in the occurrence of these types of EHT.The positions of500 h Pa anticyclonic circulation anomalies were different in North China when three types of EHT occurred.The anticyclonic circulation anomalies center were located over North China when LWD and DNC EHT occurred,while North China was located behind anticyclonic circulation anomalies center when LWN EHT occurred.The subsidence anomaly caused by anticyclonic circulation anomalies led to the decrease of cloud cover over North China when LWD occurred,thus absorbing a large amount of solar radiation,resulting in the largest daytime temperature anomaly of LWD among the three types of EHT.The rising motion of DNC low level leads to more cloud cover over North China than LWD,which results in less solar radiation absorbed during the day than LWD,so the daytime temperature anomaly is lower than LWD.When LWN EHT occurred,North China was located at the back of the anticyclonic circulation anomaly center,and the upward movement from low level to high level resulted in the largest cloud cover among the three EHTs,and the solar radiation absorbed by the ground was limited,so the daytime temperature anomaly was the lowest in three types of EHT.(3)The difference of large-scale circulation anomalies during the occurrence of three types of EHT in North China is the reason for the different local distribution of three types of EHT anomalies.When LWD(LWN)occurs,the northeast(south)wind anomaly of 850 h Pa in the south of North China blocks(favors)the transport of water vapor from sea surface to North China.The south(north)Western Pacific Subtropical High and the weak(strong)Westerly Jet lead to little(large)water vapor transport from the Bay of Bengal and the Western Pacific to North China,and this is also the reason why LWD(LWN)mainly occurs in summer from early June to early July(late July to early August).When DNC occurs,the anomaly intensity of 850 h Pa southerly wind,the location of WPSH and the intensity of WJS are between LWD and LWN,resulting in the water vapor content over North China is between the two.Dry air is not conducive to the maintenance of daytime temperature into the night,humid air is conducive to the maintenance of daytime temperature into the night,resulting in nighttime temperature anomalies is DNC > LWN > LWD.(4)The key regions of the North Atlantic sea surface temperature anomaly which are closely related to the interannual variation of the number of summer three types of EHT days in North China are discovered,and the possible mechanism of the influence of the key region sea surface temperature anomaly on three types of EHT is revealed.The key sea area affecting the interannual variation of summer LWD EHT days in North China is the distribution of positive,negative and positive tripolar sea surface temperature anomalies in the North Atlantic from south to north.The anomalies can affect the wave train spreading southeast from Siberia to North China in the middle and upper troposphere,leading to the occurrence of EHT in this region caused by anticyclonic circulation anomalies over North China.The key sea area affecting LWN is the distribution of negative and positive dipole sea surface temperature anomalies from south to north in the west of the North Atlantic Ocean,which can affect the wave train spreading downstream from the Caspian Sea to North China along the westerly jet stream.The key sea area affecting DNC is the distribution of negative and positive dipole sea surface temperature anomalies from south to north in the eastern North Atlantic Ocean,whose anomalies can affect the wave train spreading southeast from northern Europe to North China. |
