2021年5月浙江旱涝急转特征及强降雨过程分析

Analysis of Characteristics of Precipitation and Mechanism of an Abrupt Drought-Flood Alternation in Zhejiang during May 2021

为探究2021年5月浙江旱涝急转事件发生的原因,利用奇异值分解(Singular Value Decomposition, SVD)分析方法、NCEP/NCAR再分析资料、NOAA海表面温度资料等,分析总结了浙江旱涝异常特征,以及旱涝形势下大气环流、水汽输送和海温背景场等的变化特征。结果表明:1) 2021年5月第5候(第29候)浙江发生了一次较严重的旱涝急转事件,旱涝急转前浙江全省处于严重气象干旱状态,旱涝急转后全省平均降水异常偏多。2) 旱涝急转前后大气环流和水汽条件发生了明显的变化。南亚高压北抬增强,500 hPa西太平洋副高持续加强,高低空急流的垂直耦合使上升运动加强。同时,浙江上空850 hPa西南风持续增强,来自孟加拉湾的水汽输送与副高西侧的水汽辐合增强,为降水提供了有利的水汽条件。3) SVD分析表明,前期秋季赤道中东太平洋海温异常升高,则浙江5月降水很可能会随之增加。To investigate the causes of the abrupt drought-flood alternation in Zhejiang that occurred in May 2021, we employed Singular Value Decomposition (SVD) analysis, along with NCEP/NCAR reanalysis data and NOAA sea surface temperature data. This analysis summarized the abnormal characteristics of abrupt drought-flood alternation in Zhejiang, as well as the changes in atmospheric circulation, moisture transport, and background sea temperature fields under drought and flood conditions. Results showed that: 1) The abrupt drought-flood alternation occurred in Zhejiang in the 5th pentad of May 2021. The precipitation was below normal, and severe drought in the city was found before the alternation, while abnormally high precipitation occurred afterward. 2) There were significant changes in atmospheric circulation and moisture conditions before and after the abrupt drought-flood alternation. The South Asian high-pressure system intensified and moved northward, while the 500 hPa western Pacific subtropical high continued to strengthen. The vertical coupling of the low-level and upper-level jet streams enhanced upward motion. Meanwhile, the southwesterly winds at 850 hPa over Zhejiang strengthened, leading to increased moisture transport from the Bay of Bengal and enhanced moisture convergence on the western side of the subtropical high, which provided favorable moisture conditions for precipitation. 3) SVD analysis indicates that warmer sea surface temperature anomalies in the eastern equatorial Pacific during the preceding autumn are likely to lead to increased precipitation in Zhejiang in May.