Cluster analysis of multiple planetary flow regimes

A modified cluster analysis method has been developed to identify spatial patterns of planetary flow regimes and to study transitions between them. This method has been applied first to a simple deterministic model and second to northern hemisphere (NH) 500‐mbar data. The dynamical model is governed by the fully nonlinear, equivalent‐barotropic vorticity equation on the sphere. Clusters of points in the model's phase space are associated either with a few persistent events or with many transient events. Two stationary clusters have patterns similar to unstable stationary model solutions, zonal or blocked. Transient clusters of wave trains serve as way stations between the stationary ones. For the NH data, cluster analysis was carried out in the subspace of the first seven empirical orthogonal functions (EOFs). Stationary clusters are found in the low‐frequency band of more than 10 days, and transient clusters are found in the band‐pass frequency window between 2.5 and 6 days. In the low‐frequency band, three pairs of clusters determine EOFs 1, 2, and 3, respectively. They exhibit well‐known regional features, such as blocking, the Pacific/North American (PNA) pattern, and wave trains. Both model and low‐pass data show strong bimodality. Clusters in the band‐pass window show wavetrain patterns in the two jet exit regions.