Structure in the Phase Space of a General Circulation Model Deduced from Empirical Orthogonal Functions

Abstract

Recent studies of low-frequency variability have shown that at least two planetary-scale statistical flow regimes exist in the Northern Hemisphere winter circulation both in observations and in a general circulation model. This result was obtained from an analysis of a large-scale circulation index based on planetary-wave amplitude. In this paper, a 1200-day integration of the NCAR Community Climate Model (CCM0) in perpetual January mode is used as a case study to show that similar results in terms of multiple flow regimes can also be obtained from an empirical orthogonal function (EOF) analysis. Two modes are found in the probability density distribution in the subspace formed from the leading two EOFs of the model. There is an apparent correspondence between these modes and the two modes deduced from the previous wave-amplitude analysis.