Weather Regimes in the Pacific from a GCM. Part II: Dynamics and Stability

Abstract

The Pacific weather regimes found by Haines and Hannachi from a GCM perpetual January 10-year run, identified as ±Pacific?North American (PNA), are examined for stability both within a model-derived EOF phase space and the full phase space for the 500-mb flow level. The authors also examine the behavior of the 500-mb streamfunction tendency based on the barotropic vorticity equation model projected onto the EOF phase space. Normal mode and nonnormal mode analysis of these regimes are performed. It is shown in particular that the +PNA state is less stable than the ?PNA, which can explain previous results concerning the greater robustness in finding the ?PNA state as a fixed point in the attractor. Of particular interest is the local character of the +PNA regime, which indicates fast growth rates within the EOF phase space of the order 3?4 days.