基于时空综合分型的El Niňo事件对中国东部降水的影响差异

Different impacts of two types of El Niňo events on precipitation in eastern China based on spatiotemporal diversity

基于1961—2022年中国高分辨率降水格点资料、合成分析和经验正交函数分解方法,揭示了时空综合分型得到的生命史较长-异常中心位置偏东的低频-东部(LF-EP)型和生命史较短-异常中心位置偏西的准两年-中部(QB-CP)型El Niňo事件对中国东部降水的不同影响。结果表明,LF-EP型事件对降水的影响显著且稳定:自发展年秋季到衰减年夏季长达近4个季节,长江以南区域降水持续地显著偏多,且异常雨带中心自衰减年春季起逐步北抬。QB-CP型事件发生时降水异常特征变化更加复杂多变:发展年夏秋季其空间分布与LF-EP型事件中的特征大致相反,长江以南区域降水整体偏少,冬季开始长江以南逐渐有正异常降水出现,且春季以后异常雨带表现出逐渐南退特征,至衰减年夏季发展为华北-长江中下游-华南地区降水异常“正-负-正”分布。进一步通过比较不同类型事件中大尺度水汽输送的差异探讨了其影响不同的可能机制,发现由海温异常纬向位置差异导致的西太平洋区域大气环流直接响应以及衍生模态响应差异是造成中国东部降水异常空间分布特征差异的重要原因。同时,两类时空事件持续性和转相时间的差异也会使得对降水影响的时间尺度出现差别。

El Niňo significantly influences precipitation in eastern China,and it has been demonstrated that the diversity of El Niňo characteristics leads to inconsistent impacts.Traditional identification of El Niňo diversity focuses only on difference in spatial distributions of anomalous features,overlooking the temporal discrepancies among types.This leaves it unclear whether these time-scale differences change El Niňo's influence.Therefore,utilizing high-resolution grid data of precipitation in China from 1961 to 2022,this study revisited the El Niňo influence on precipitation in eastern China based on a newly proposed classification method that considers both spatial and temporal characteristics.We identified historical El Niňo events into two primary spatiotemporal types:the low-frequency eastern-Pacific(LF-EP)type,characterized by a long lifecycle and located in the tropical eastern Pacific,and the quasi-biennial central-Pacific(QB-CP)type,with a shorter lifecycle and located relatively west.In the evolution of LF-EP-type events,the LF mode is dominant,while the QB mode is relatively weak.In QB-CP-type events,the LF mode is in the phase transition stage,while the QB mode is dominant.Composite analysis results indicate these two different spatiotemporal types of El Niňo have distinct impacts on the evolution of precipitation anomalies in eastern China.LF-EP-type events have a stable influence on precipitation in eastern China,with a nearly four-season-lasting anomaly starting from autumn of the development year to summer of the decay year,resulting in above-normal rainfall south of the Yangtze River.Moreover,the center of the anomalous rain belt migrates northward starting in the spring of development year.In contrast,QB-CP-type events exhibit more frequent changes in precipitation anomaly characteristics.During summer and autumn of the development year,rainfall is generally less south of the Yangtze River,contrary to LF-EP-type events.Positive precipitation anomalies begin to emerge south of the Yangtze River in winter and show a gradual southward retreat,culminating in a“positive-negative-positive”distribution across North China,the Yangtze River basin,and South China by the decay year's summer.This study also compared large-scale moisture transport differences between the two spatiotemporal types of El Niňo to investigate the potential mechanisms behind their differing impacts.The results show that the difference in the zonal positioning of SST anomalies is a key factor leading to distinct atmospheric circulation responses.Due to the shift in the latitudinal position of SST and convective anomalies,the positions of anticyclonic circulation anomalies in the Northwest Pacific Ocean vary among different types of El Niňo in summer and autumn,resulting in different moisture transports towards eastern China.In winter,the different types of El Niňo induce a similar anomalous Walker circulation,whose sinking branch in the maritime continent region leads to relatively consistent anticyclonic water vapor transport towards eastern China.However,the related circulation anomaly dominated by the LF ENSO mode persists significantly longer due to its longer lifecycle.That is to say,the differences in persistence and phase transition timing between the two types of spatiotemporal events result in changes in the timescale of their impact on precipitation in eastern China.Additionally,during the spring and summer of the decay year,LF-EP El Niňo can also indirectly develop anticyclonic water vapor transport in the Northwest Pacific to maintain its influence on precipitation in eastern China through the Indo-western Pacific Ocean“capacitor”effect and through the nonlinear interaction with the tropical Pacific annual cycle.