Seasonal Variation of the Dominant Low-Frequency Variability Observed in the Barotropic Component of the Atmosphere: A Connection to the Arctic Oscillation

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.