华南秋季干旱的年代际转折及其与热带印度洋热含量的关系

摘要

Based on the monthly Standardized Precipitation Evapotranspiration Index (SPEI),ocean temperature data from ORA-S4,and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data during 1961-2014,this study investigates the decadal shift of autumn drought in South China and its possible relationship with Heat Content(HC) in the tropical Indian Ocean.Results show that the main spatial distribution of autumn SPEI change in South China is in the same phase,with an obvious interdecadal change from wet to dry around 1988.It is further noted that there exists a significant positive relationship between SPEI in South China and HC in tropical western Indian Ocean in autumn,indicating that autumn drought in South China will be corresponded to a lower HC in tropical western Indian Ocean.The lower HC in the tropical western Indian Ocean plays an important role in the decadal drought in South China in autumn.There is an obvious zonal dipole pattern of HC anomalies shown as “＜” type in the tropical Indian Ocean in autumn.The positive HC anomalies in the tropical eastern Indian Ocean,which are possibly affected by the negative HC anomalies in the tropical western Indian Ocean,are closely related to the less OLR,more precipitation and higher sea surface temperature(SST) in the tropical eastern Indian Ocean and western Pacific(TEIWP) region.These changes will give rise to an enhanced East Asian Hadley cell,and then a subsidence motion anomaly over South China,resulting in a decadal drought in this region.In addition,the positive SST anomalies over TEIWP will make the northwestern Pacific subtropical high get larger than usual,and extend westward.It also induces cyclonic circulation a-nomaly around the northwestern Pacific.In such cases,South China is controlled by northerly wind anomaly,reducing the amount of water vapor transported to South China.As the interdecadal change of SST in TEIWP is an important physical process for the HC anomalies in tropical western Indian Ocean,which influences the decadal shift of autumn drought in South China,a series of numerical experiments using NCAR CAM5.1 verify that the decadal warming of SST in the TEIWP region plays an important role in causing the decadal autumn drought in South China.%采用1961-2014年逐月全球标准化降水蒸散指数(Standardized Precipitation Evapotranspiration Index,SPEI)数据集、ORA-S4海温资料及NCEP/NCAR再分析资料,对华南地区秋季干旱的年代际转折及其与热带印度洋热含量的关系进行了研究.结果表明:华南秋季SPEI主要表现为全区一致变化型,且具有明显的年代际变化特征,在1988年发生了年代际转折,转折后(前)为偏旱(涝)期.进一步分析表明,华南秋季SPEI与同期热带西印度洋海洋热含量变化呈显著的正相关关系,即当秋季热带西印度洋热含量偏低时,华南地区SPEI偏小,易发生干旱.热带西印度洋热含量异常影响华南秋季干旱的可能机制为:秋季热带印度洋热含量变化表现为“＜”型的东西向偶极子分布,即当热带西印度洋热含量偏低时,热带东印度洋热含量将会偏高;而热带东印度洋热含量偏高将会使热带东印度洋—西太平洋海表温度偏高、外逸长波辐射偏小、降水增多,凝结潜热释放增强,产生偏强的东亚Hadley环流,使华南地区存在异常下沉运动,不利于产生降水;热带东印度洋—西太平洋海表温度偏高,还会使西北太平洋副热带高压位置偏西、面积偏大,西北太平洋存在气旋性环流异常,使华南地区受偏北气流异常控制,从而削弱了向华南地区的水汽输送.热带东印度洋—西太平洋海表温度年代际变化是热带西印度洋热含量异常影响华南秋旱年代际变化的重要环节,因此用NCAR CAM5.1全球大气环流模式进行了热带东印度洋—西太平洋海表温度年代际变化的敏感性试验,证实该区海表温度年代际升高对华南秋季年代际干旱具有重要作用.