ZONALLY SYMMETRIC NORMAL MODES ASSOCIATED WITH THE AO/NAM

The Arctic Oscillation (AO) or the Northern Hemisphere Annular Mode (NAM), which is one of the important modes of climate variability, is usually represented by the leading EOF of Northern Hemisphere variability. However, this is not sufficient for an understanding of its dynamics, because EOF has only a statistical me aning and does not indicate whether it is really a physical mode or not. On the other hand, the theory of zonal mean flow eddy interaction merely gives the zon al mean structure of the zonal wind, and does not guarantee a zonal coherence to the variation of the zonal wind along a certain latitude. Therefore, it does no t imply a physical mode with a zonally symmetric structure can be organized thro ughout the hemisphere. So, fundamental issues of the mechanism of the AO are sti ll not well understood. In this paper, zonally symmetric or annular normal modes , which have the most direct linkage to linear dynamics around the basic state o f the atmosphere and interact with other modes due to nonlinear dynamics, are re garded to play a key role in our understanding of the AO/NAM. In order to invest igate the dynamical nature of the AO/NAM associated with these normal modes, thi s work is devoted to a numerical study on the zonally symmetric normal modes of the linear dynamics about the basic climate state of Northern Hemisphere winter. By use of a linerized pand spherical coordinate primitive equation, zonal ly s ymmetric normal modes with spatial structures resembling that of the AO/NAM are found, which demonstrates that the AO/NAM could be a reflection of the dynamical behavior of zonally symmetric normal modes at hemispheric scale.