华西秋雨趋势变化的年代际转折及其成因分析

Analysis on the Interdecadal Transition and Its Causes of the Autumn Precipitation Trend in West China

华西地区(25°N^35°N,100°E^110°E)是中国秋季降水主要地区之一。本文根据华西地区72站月平均降水资料、NCEP/NCAR再分析资料和哈德莱中心海温及海冰资料,利用相关和回归等分析方法研究了1961~2014年华西地区秋雨的年代际变率及其与大气环流和海温的关系。华西秋季降水年代际变率分解为呈现显著下降趋势的P1时段(1964~1998年)和呈现上升趋势的P2时段(1998~2014年)发现,对应P1时段降水下降趋势的华西区域大气位势高度异常场具有西正东负结构,大尺度环流场显示为从大西洋东传经北极巴伦支—喀拉海区至东亚的准纬向波列,该波列体现了上游负位相NAO(North Atlantic Oscillation)的调制作用。对于P2时段的降水上升趋势,其位势高度场配置与P1时段相反,而大尺度波列结构在欧亚大陆的部分呈西北—东南走向,且整体偏西,体现了上游正位相NAO的调制作用。这种环流结构导致华西区域西北侧形成负异常中心,有利于西南暖湿气流进入研究区域。影响华西秋雨趋势转折的海温关键区位于热带中东太平洋和热带印度洋。在P1时段,华西秋雨降水趋势与同期热带中东太平洋和印度洋海温呈显著正相关关系。而在P2时段,华西秋雨与前冬热带中东太平洋和印度洋海温存在显著负相关,前冬西北太平洋海温正异常也同时影响了华西秋雨的上升趋势。

The West China(25°N-35°N,100°E-110°E)is one of the main autumn precipitation regions in China.In this study,the monthly precipitation data of 72 stations in West China,NCEP/NCAR reanalysis data,and sea surface temperature and sea ice data of Hadley Centre were analyzed.Accordingly,the interdecadal variability of autumn rain in West China from 1961 to 2014 and its relationship with atmospheric circulation and sea surface temperature were found through correlation and regression methods.The decadal variability of autumn precipitation in West China was decomposed into two periods P1(1964-1998)with a significant downward trend and P2(1998-2014)with an upward trend.Results show that the anomalous field of the atmospheric geopotential height,corresponding to the downtrend of the precipitation in P1,has a structure of the positive to negative in west to east inside the study region.The large-scale circulation field shows the quasi-zonal wave train originating from the eastern Atlantic Ocean to East Asia via the Barents-Kara Seas and Kara Sea areas,which reflects the modulation effect of the upstream negative-phase NAO.In P2,the geopotential height field configuration related to the downward precipitation trend is opposite to that in P1.The largescale wave train structure shows a westward-shifting northwest-southeast pathway over Eurasia,which reflects the modulation effect of positive-phase NAO at the upstream.This circulation structure leads to the formation of a negative anomaly center over the northwest of West China,which is favorable for the southwest warm and humid airflow to enter the study area.The key SST zones that influence the autumn rain in West China are located in central and eastern tropical Pacific and Indian Ocean.In P1,the precipitation trend of autumn rain transition in West China was positively correlated with the SST over tropical Middle East Pacific and Indian Ocean during the same period.In P2,there exists a significant negative correlation between the autumn rain and SST in the tropical Middle East Pacific and Indian Ocean during the last winter.In addition,the positive SST anomaly in the northwest Pacific Ocean during the previous winter also affects the increasing trend of the autumn rain inWest China.