1961-2020年华北地区7-8月份极端降水频次趋势转折及可能原因分析OACSTPCD

Regional extreme precipitation(REP)is highly destructive and can cause geological disasters and economic losses.Using daily precipitation data,this paper investigates the REP frequency trend turning in July-August in North China from 1961 to 2020,and explores the possible causes of the varia-tion.The results show that there are three trend abrupt points in July-August REP days in North Chi-na,1986,1995 and 2003,with decreasing trends in the 1961-1985(P1)and 1995-2002(P3)periods,and increasing trends in the 1986-1994(P2)and 2003-2020(P4)periods.The anomalous divergence of moisture flux is present in North China,and anomalous high(Northeast high)is present in the upper troposphere from northeastern China to the Korean Peninsula at the REP event.The enhanced anoma-lous divergence of moisture flux in north China south of 38°N during the P2 and P4 periods compared with the P1 and P3 periods corresponds to the southward position of Northeast high,resulting in more REP days at the region.The anomalous moisture flux comes from the Indian Ocean and the northwest Pacific Ocean during P1 and P2,but only from the northwest Pacific Ocean in the latter two periods,which is the result of the weakening of the East Asian summer monsoon(EASM)and the strengthening of the western Pacific subtropical high(WPSH).The development of Northeast high during P1 and P3 is associated with the wave activity flux transmitted along the mid-latitude westerly jet stream(WJS).While the high-latitude activity center plays a role during P2 and P4 and the Northeast high is stronger in the seasonal background.The wave activity flux contributing to the development of Northeast high during P2 comes from both the WJS and the high-latitude activity center paths,while the wave activity flux during P4 propagates mainly along the high-latitude.The decrease of REP days in the P1 period is mainly the result of the significant weakening of the EASM,which is not conducive to water vapor transport to North China,while the P3 period is the result of the combined effect of the reduced EASM and the weakening of the WJS.The increase of REP days in the P2 period is due to the significant strengthening of the WJS,which is favorable to the development of Northeast high,while the P4 period,the strengthening of the WPSH ensures the water vapor transport to North China after the weakening of the EASM,and the reduction in the Arctic sea ice extent may establish a new path for wave activity flu-xes transmitted from high latitudes to northern China after the weakening of the WJS by influencing the high-latitude activity centers,both of which combine to increase the REP days.