夏季北半球环状模态及其周期分析

对NCEWNCAR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 annularmode，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2a和准6a的年际周期，同时还具有准11a和准21a的年代际周期，其年际周期信号占主要部分。

Summer Northern Hemisphere Annular Mode and Its Periodic Analysis

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.