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In order to study the physical nature of the Arctic Oscillation (AO) or the Northern Hemisphere annular mode (NAM), the linear stochastic model, constructed empirically from the output of a GCM run, is used to investigate the modal aspects of this climate variability mode. Theoretical analysis on the dominant modal response to the stochastic forcing is performed in terms of the maximum variance contribution. As the modal aspect of AO/NAM, two dominant normal modes are found to resemble the spatial structure of the AO/NAM represented by the leading EOF. These stochastically forced normal modes are regarded to be physical modes and thus can explain many fundamental features of the AO/NAM such as the barotropic annular dipole, tropospher–stratosphere coupling, and its dominance over the wintertime Northern Hemisphere. It then follows that the origin or physical nature of the AO/NAM can be attributed to the behaviors of the dominant annular normal modes. A key distinction of this study from previous eddy driving theories is that, to drive the variability, eddy forcing needs not to have a spatial structure completely coinciding with the pattern of the NAM, since the latter is mainly decided by the linear normal modes. [ABSTRACT FROM AUTHOR]

This paper describes a numerical study of the role of zonally symmetric normal modes in the dynamics of the Arctic Oscillation (AO) or Northern Hemisphere Annular Mode (NAM) with seasonally varying climatological fields. A general theory of the mechanism for normal mode selection involving a fully developed non-linear interaction between different modes is proposed, and this explains not only the prominence of the AO/NAM in the Northern Hemisphere, but also the existence of the Antarctic Oscillation (AAO) in the Southern Hemisphere. Following this normal mode selection mechanism, zonally symmetric normal mode with spatial structures resembling those of the AO and AAO is extracted by use of ECMWF data and the linear operator of a two-dimensional model with a seasonally varying basic state. The interannual variability of this mode is also investigated in the light of this theory. [ABSTRACT FROM AUTHOR]