秋季黄河中下游降水主模态及2021年极端降水的气候背景

The leading mode of autumn rainfall over the mid-lower reaches of the Yellow river and the climate background of extreme autumn rainfall in 2021

为探讨2021年秋季黄河中下游极端降水的气候背景,基于中国160站降水资料、NCEP/NCAR大气环流和NOAA海表温度(SST)再分析资料,分析了黄河中下游秋季降水主模态的时空特征及2021年极端降水的可能成因。经验正交函数分解表明,秋季黄河中下游降水主模态在空间分布上西至甘肃省东南部,东至山东省西部,主体位于黄河中下游地区,2021年是黄河中下游降水主模态的典型表现。采用黄河中下游秋季降水指数(ARYR)表征主模态时间变化,其年际、年代际变化分别与厄尔尼诺-南方涛动(ENSO)和太平洋年代际振荡(PDO)存在相关,拉尼娜年和PDO负位相时降水偏多。秋季黄河中下游降水偏多主模态与北太平洋SST异常偏高的区域和强度相关密切,选取关键区定义中纬度北太平洋SST(MNPSST)指数,当MNPSST偏高时,海洋性大陆(MC)区域存在强的上升运动,黄河中下游地区出现显著的低(高)空辐合(散),指数偏低则降水偏少。其中2021年MNPSST指数为1951年以来最高,2021年MC对流区北侧的中国南海北部有较强上升运动,中纬度北太平洋偏东气流异常偏强,水汽通量异常分为偏南和偏东两支到达黄河流域中游和下游。中纬度北太平洋SST正异常是2021年秋季黄河中下游极端降水的重要影响因子之一。

To investigate the climate background for the extreme rainfall anomaly in 2021 over the mid-lower reaches of the Yellow river,this study analyzes the leading mode of autumn rainfall over the mid-lower reaches of the Yellow river during 1951—2021 and its relationship with the extreme rainfall anomaly in 2021.Rainfall data collected at 160 stations in China and NCEP/NCAR atmospheric circulation reanalysis as well as NOAA sea surface temperature(SST)reanalysis are used.The result of Empirical Orthogonal Function analysis reveals a consistent autumn rainfall pattern from the southeast of Gansu province to the west of Shandong province,which covers the mid-lower reaches of the Yellow river.This pattern is regarded as the leading mode of autumn rainfall over the mid-lower reaches of the Yellow river.The time coefficient in 2021 is the maximum since 1951,consistent with the extreme precipitation in 2021 in the region.The extreme event in 2021 is a typical example corresponding to the leading mode.This study uses the time series of Autumn Rainfall over the Yellow river(ARYR)to represent the variability of this leading mode.Analysis reveals that the interannual and interdecadal variations of this mode are affected by El Niño-Southern Oscillation(ENSO)and Pacific Decadal Oscillation(PDO)with more rainfall during La Niña phase and negative PDO phase.Further,the leading rainfall mode over the mid-lower reaches of the Yellow river is closely related to SST anomalies in the mid-latitude North Pacific with more rainfall under higher SST.The mid-latitude North Pacific SST(MNPSST)index is calculated over the key region in the North Pacific,where the correlation is the most significant.The MNPSST index is the highest in 2021 since 1951.When the MNPSST index is high,the regressed high-level(low-level)anticyclone(cyclonic)shear occurs over the mid-lower reaches of the Yellow river,and strong upward motions develop over the mid-lower reaches of the Yellow river and the Marine Continent(MC)region.In the autumn of 2021,there are strong upward motions in the northern South China Sea,and anomalous easterly winds over the northern Pacific are obviously stronger.As a result,water vapor flux anomalies could split to southern and eastern branches,reaching the mid-lower reaches of the Yellow river basin.The leading mode of autumn rainfall over the mid-lower reaches of the Yellow river is strongly related to the intensity of SST anomaly in the North Pacific.Positive SST anomalies in the mid-latitude North Pacific are one of the most important factors affecting extreme rainfall over the mid-lower reaches of the Yellow river in autumn 2021.