Abstract
In this study, EOF analyses are conducted for the barotropic component of the atmosphere in the Northern Hemisphere for each season to investigate the dominant low-frequency variability in the atmosphere. It shows that the structure of the Arctic Oscillation (AO) appears in winter and spring as a dominant mode. In summer and fall, however, the dominant modes are not the annular pattern, but show more localized structures. The seasonal variation of the dynamical SVD-1 mode is analyzed using the climate basic states based on the singular eigenmode theory by Tanaka and Matsueda (2005) to compare with the observed EOF-1 for each season. It is found that the AO-like structure appears robustly in DJF when hyper diffusion is used in the model. The AO-like pattern obtained by observed EOF-1 appears in MAM when the viscosity is changed to the bi-harmonic diffusion. But the structures obtained as the SVD-1 in JJA and SON are different from that of the observed EOF-1. From this result, we may conclude that the AO in winter is explained by the singular eigenmode theory. However, the dominant low-frequency variability in other seasons are unlikely to be the singular eigenmode of the dynamical system explained by the SVD analysis.
