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.