夏季北半球环状模态及其周期分析

Summer Northern Hemisphere Annular Mode and Its Periodic Analysis

对NCEP/NCAR再分析资料中的北半球夏季(6—8月)海平面气压(sea level pressure,SLP)场进行EOF分解,其主要模态之一为夏季北半球环状模态(Northern Hemisphere annular mode,NAM),体现出大气的纬向对称结构,SLP异常场在北极和中高纬地区呈现反相变化分布。通过经验模态分解和希尔伯特变换可知,夏季的环状模态具有准2 a和准6 a的年际周期,同时还具有准11 a和准21 a的年代际周期,其年际周期信号占主要部分。

By use of the empirical orthogonal function（EOF） method,we can get the Northern Hemisphere annular mode （ NAM ） of summer sea level pressure （ SLP ） fields during 1948--2005 from NCEP/NCAR reanalysis data. As one of main modes of SLP fields, summer NAM, exhibiting an annular structure in the Northern Hemisphere, involves a seesaw between the Arctic basin and parts of the surrounding zonal ring, which displays the zonal symmetry of high degree. Using the empirical mode decomposition（EMD） and Hilbert-Huang transform （HHT） methods, we conclude that the summer NAM index has the interannual periods of quasi-2 yr and 6 yr, and the interdecadal periods of quasi-11 yr and 21 yr,in which the interannual signal is the primary part.