2021年陕西省超强秋汛环流特征及其驱动因子分析

Analysis of the Circulation Characteristics and Driving Factors of the Super Strong Autumn Flood in Shaanxi Province in 2021

利用观测和再分析资料,诊断分析了2021年陕西省9~10月超强秋汛的极端性及其年际异常的关键环流和驱动因子。结果表明:(1)2021年陕西省秋汛期累计降水量为1961年以来第一位,超强秋汛主要与西北太平洋反气旋(WNPAC)的异常偏强和西伸、环渤海高压及巴尔喀什湖槽偏强有关,三者导致冷暖空气在陕西省辐合,形成持续时间较长的秋汛。(2)2021年9~10月赤道中东太平洋冷海温加强,第二次La Niña事件发展,导致中太平洋对流层下沉气流加强,进而加强WNPAC,增强环渤海高压及巴尔喀什湖槽,在东亚中高纬度形成“东高西低”型环流异常。(3)夏秋季热带大西洋海温异常偏暖,通过增强热带北大西洋—热带中太平洋纬向反转流激发WNPAC。La Niña发展和热带北大西洋异常暖海温二者协同作用驱动了2021年的陕西省超强秋汛。

In September–October 2021,Shaanxi Province experienced record-breaking rainfalls,resulting in severe floods and disasters.This study analyzed the extreme characteristics of the super strong autumn flood that occurred in 2021 over Shaanxi Province and demonstrated the interannual variations in its associated circulation and the driving factors behind the flood using observational and reanalysis data.The results showed that:(1)The cumulative rainfall over Shaanxi Province during the 2021 flood had been the heaviest since 1961.Moreover,this flood was closely related to the extremely strong anomalous anticyclone developed over the western North Pacific(WNPAC),the development of high pressure in Bohai Bay,and the formation of a significantly low trough over Lake Balkhash.As a result of these phenomena,cold/warm air converged over Shaanxi Province,resulting in the formation of a strong autumn flood that sustained for a long time.(2)The strengthening of the cold east equatorial Pacific sea surface temperature(SST)during the periods of September and October 2021 and the development of a second La Niña event led to an intensification of the downdraft in the troposphere and Central Pacific.This resulted in the strengthening of the WNPAC,BSH(Bohai Sea High),and BLT(Balkhash Lake trough)and the formation of a type of“west–low and east–high”circulation anomaly in the middle–high latitude East Asia regions.(3)The WNPAC was stimulated by the zonal reversing circulation of the tropical North Atlantic–Central Pacific Ocean,which was enhanced by the abnormally warm tropical Atlantic SST in summer and autumn.Thus,this work reveals that the synergy between the development of La Niña and abnormally warm SSTA(SST anomaly)in the tropical North Atlantic caused the super strong autumn flood in Shaanxi Province in 2021.