与北极涛动或北半球环状模相关的纬向对称的正规模态

作为一个全球气候变率的重要模态,北极涛动或北半球环状模态(AO/NAM)一般由北半球海平面气压变率的EOF1来表示。但是通常认为EOF1仅有统计学意义而并不能够表明它是否是一种实际的物理模态。另一方面,现有的波-流相互作用理论也仅能给出纬向风的纬向平均状况,而不能够保证沿着某一纬圈的变化,所以它并不意味着一个具有半球尺度的纬向对称的相干结构能够组织起来。因此AO/NAM的形成机制仍然是一个具有争议的问题。文章提出纬向对称的或环状的正规模态与基本气候态附近的线性(进而非线性)动力学之间有着最直接的联系,因而可能在AO/NAM动力学的理解中扮演重要角色。为了深入探讨AO/NAM的动力学本质,使用p-坐标球面原始方程计算了冬季北半球及全球气候基本态附近的线性动力系统纬向对称的正规模态,发现在半球尺度上的纬向对称正规模态具有AO/NAM的经向偶极子及垂直方向近似正压的空间结构特征。而这些纬向对称正规模的时间变化尺度取决于与其他非纬向对称正规模间的相互作用。从而说明了AO/NAM可能是半球尺度上纬向对称正规模态的动力学行为的反映。

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 meaning 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 zonal 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 not imply a physical mode with a zonally symmetric structure can be organized throughout the hemisphere.So,fundamental issues of the mechanism of the AO are still 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 of the atmosphere and interact with other modes due to nonlinear dynamics,are regarded to play a key role in our understanding of the AO/NAM.In order to investigate the dynamical nature of the AO/NAM associated with these normal modes,this 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 p-and spherical coordinate primitive equation,zonally symmetric 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.