夏季北大西洋海温异常与长江流域极端高温日数年际变化关系的年代际减弱

Interdecadal weakening of the relationship between summer North Atlantic sea surface temperature anomalies and extreme high temperature days in the Yangtze River basin

摘要:

基于1961—2020年Hadley海温资料、CN05.1逐日最高气温资料和NCEP/NCAR再分析资料,利用奇异值分解(singular value decomposition,SVD)和回归分析等统计方法,研究了夏季北大西洋海温异常与长江流域极端高温日数年际变化关系的年代际变化。研究表明,当夏季北大西洋中纬度海温正(负)异常、副热带海温负(正)异常,即北大西洋偶极型(North Atlantic dipole,NAD)海温异常正(负)位相时,长江流域极端高温日数增加(减少)。二者关系在20世纪80年代末期之后发生年代际减弱。进一步的研究结果表明,在1961—1988年,夏季NAD海温异常可以激发从北大西洋出发沿欧亚中高纬向东传播的大气遥相关波列,在东亚上空产生反气旋/气旋环流异常。东亚反气旋/气旋环流异常引起长江流域上空气流垂直运动变化,并通过垂直运动引起的热力作用等,影响长江流域极端高温日数的变化。而在1989—2020年,夏季NAD海温异常激发的欧亚中高纬大气遥相关波列强度减弱,东亚上空的反气旋/气旋环流异常的强度也相应减弱,从而使得NAD海温异常与长江流域极端高温日数之间的联系减弱。NAD海温异常激发的欧亚中高纬遥相关波列强度在20世纪80年代末期之后的减弱可能与NAD海温异常振幅的减弱有关。

Abstract:

The Yangtze River basin(YRB) is a key region for the occurrence of extreme high temperatures (EHT),which has significant impacts on both human society and ecosystems.The number of EHT days (EHTD) in the YRB exhibits notable interannual variability.Using sea surface temperature (SST) data from the Hadley Center,daily maximum temperature datasets from CN05.1,and reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR),this study investigates the relationship between summer North Atlantic SST anomalies (SSTA) and EHTD in the YRB through singular value decomposition (SVD) analysis.Results reveal that the first SVD mode demonstrates a strong relationship between North Atlantic Dipole (NAD) pattern (characterized by positive (negative) SSTAs in the midlatitude and negative (positive) SSTAS in the subtropical North Atlantic) and EHTD in the YRB,accounting for 63.43％ of the total squared covariance.The correlation coefficient between the time series of the first SVD mode for NAD SSTA and EHTD is 0.51,statistically significant at the 99％ confidence level.An interdecadal shift in the relationship between NAD SSTA and EHTD in the YRB occurred in the late 1980s.From 1961—1988,NAD SSTA showed a strong connection with EHTD in the YRB.However,after the late 1980s,this relationship weakened.Further analysis suggests that the interdecadal changes in the NAD SSTA-EHTD relationship are primarily driven by differences in atmospheric circulation anomalies over Eurasia,triggered by NAD SSTA.Before the late 1980s,positive (negative) NAD SSTA induced an atmospheric teleconnection pattern extending from the North Atlantic to East Asia in the middle and upper troposphere,featuring two positive (negative) geopotential height anomaly centers over the Ural Mountains and East Asia,and two negative (positive) centers over Greenland and Lake Baikal.This teleconnection pattern,linked to Rossby wave energy propagation,led to an anticyclone/cyclone circulation anomaly over East Asia,inducing anomalous descending/ascending motion over the YRB,which in turn provided favorable/unfavorable thermal conditions for EHT occurrences.The East Asian anticyclone/cyclone circulation anomaly had a significant influence on EHTD in the YRB.However,after the late 1980s,the aforementioned teleconnection pattern excited by NAD SSTA weakened,reducing the East Asian anticyclone/cyclone circulation anomaly and weakening the NAD SSTA-EHTD relationship.The differences in atmospheric circulation anomalies between the two periods are closely related to the reduced amplitude of NAD SSTA.The reasons behind these weakening of NAD SSTA amplitude remain to be explored.These findings offer valuable insights for understanding and predicting EHTD in the YRB.