Spatial-temporal variations of polar vortex anomaly over Northern Hemisphere in winter

Based on the NCEP/NCAR reanalysis monthly height data from 500 to 10 hPa(total 12 layers) in winter from 1948/1949 to 2009/2010,three polar vortex circulation indices of Northern Hemisphere are calaulated,which are the area index(S),the strength index(P),and the center position index(λc,φc) including longitude and latitude.The vertically seasonal,monthly and abnormal changes of Northern Hemisphere polar vortex are analyzed.Results show that the changes of polar vortex area and strength are different not only in the different heights,but also in the appearance time of the intraseasonal maximum.Correlation between the polar vortex area and strength is not significant,therefore they can be seemed as two independent indices.There are also differences of the area and strength interdecadal trends between in the upper layer and in the lower layer,and their abnormal changes are complicated.The changes of the center positions in both the upper and lower layers are identical.The changes are mainly located in the north of 80°N of the eastern hemisphere.Around the North Pole,the polar vortex longitude change is much more severe than the latitude change.The interdecadal change of latitude is significant,and the polar vortex is obviously southward in recent 10 years.On the whole,the position of polar vortex center from 500 hPa to 10 hPa varys in counterclockwise direction,and the radius increases with height.