2021年盛夏中国东部极端降水月际演变成因及可预测性

Causes and predictability of the inter-month evolution of extreme precipitation over eastern China in midsummer 2021

2021年7—8月中国东部雨带演变特征与气候平均季风北推进程存在显著差异。其中,7月降水正异常中心位于江淮-华北地区,8月则南移至华中地区。2021年中国东部降水异常偏多且存在月际差异主要与7(8)月西北太平洋副热带高压(西太副高)偏北偏东(偏南偏西)、东亚副热带西风急流偏北(偏南)以及南亚高压持续东伸相关联。进一步研究表明,热带对流的活跃位置和北大西洋的增暖加强是影响其降水中心南移的主要原因。2021年7月热带大气低频振荡(MJO)在海洋性大陆地区活跃对应其热带海洋性大陆对流异常偏强,激发北传的类太平洋-日本(PJ)型遥相关波列,使得西太副高偏北偏东,有利于西北太平洋水汽在江淮-华北地区辐合,导致其降水偏多。8月,新发展MJO在热带印度洋上空对流异常持续偏强,加强局地经向环流,使得中国35°N以南至西北太平洋地区出现异常下沉运动,有利于西太副高南移西伸。此外,2021年8月北大西洋海温(SST)异常偏暖激发对流层高层向东南传播的Rossby波,有利于南亚高压加强和东亚副热带西风急流加强南移。因此,8月降水中心南移至华中地区。CFSv2预测系统(6月起报)结果能预测7月江淮-华北大部分地区降水偏多,但预测的8月华中南部地区降水偏少与实况相反。这可能是由于模式能够较好再现7月海洋性大陆热带对流活动影响江淮-华北地区降水的过程,但不能预测2021年8月热带印度洋对流活动和北大西洋海温异常偏暖对华中地区降水的影响。

In July-August 2021,the evolution of the rain belt in eastern China deviated significantly from the typical northward progression of the climatological monsoon.The center of positive precipitation anomalies was located in Jianghuai-North China region in July and shifted southward to the Central China region in August.These regions experienced the highest and second-highest average precipitation anomalies on record since 1979,respectively.The inter-month differences in precipitation anomalies were primarily associated with the northeast(southwest)displacement of the western Pacific subtropical high(WPSH),the northward(southward)shift of the East Asian subtropical westerly jet,and the continuous eastward extension of the South Asia high(SAH)in July(August)2021.The active tropical convection and enhanced warming of the North Atlantic Ocean were key factors influencing the southward shift of the precipitation center.In July 2021,the Madden-Julian Oscillation(MJO)was active over the Maritime Continent,intensifying tropical convection and triggering the northward propagation of a Pacific-Japan-like wave train.This caused the WPSH to shift northeastward,promoting water vapor convergence in the Jianghuai-North China region and resulting in increased precipitation.In August,the MJO reactivation over the tropical Indian Ocean strengthened local meridional circulation,leading to anomalous downward motion from southern China to the northwest Pacific Ocean.This favored the southward and westward extensions of the WPSH.Additionally,anomalously warm North Atlantic Sea surface temperature(SST)in August 2021 simulated the southeastward propagation of Rossby waves in the upper troposphere,intensifying the SAH and causing the East Asian subtropical westerly jet to strengthen and shift southward.Consequently,warm and moist air from the tropical western Pacific converged in Central China,shifting the precipitation center southward in August.The Climate Forecast System,version 2(CFSv2,reported in June),accurately predicted positive precipitation anomalies for July 2021 in most parts of the Jianghuai-North China.However,it incorrectly predicted negative precipitation anomalies for August in southern China.The model successfully reproduced the northward movement of the WPSH and the influence of tropical convective activity on the Maritime Continent during July precipitation in the Jianghuai-North China.However,it failed to predict the effects of tropical Indian Ocean convection and anomalously warm North Atlantic SSTs in August 2021.Consequently,the model could not replicate the southward and strengthened deviation of the East Asian subtropical westerly jet or the intensified and eastward deviation of the SAH,leading to inaccurate precipitation predictions for Central China in August.The extreme precipitation anomalies in eastern China during midsummer 2021 were significantly influenced by the frequency and intensity of typhoons.Additionally,the predictive efficiency of dynamic models for the inter-monthly evolution of midsummer precipitation was limited.Developing an effective prediction model that integrates dynamic and statistical approaches is necessary to improve monthly-to-seasonal climate predictions in the future.