Is Blocking a Circulation Regime?

The relationship between Pacific blocking and large-scale circulation regimes is investigated. The large-scale circulation regimes are obtained by cluster analysis using the k-means method and tested against significance and reproducibility. Pacific blocking is described using two different methods. In a direct approach, blocking is described by a recently developed blocking index, which is defined in terms of potential temperature anomaly on a surface of constant potential vorticity. In an indirect approach, the occurrence of extreme events is used as a proxy for blockings. Between the two methods there is a causal relationship; the direct one is an indication of the occurrence of the blocking, while the indirect one is a measure of some of the effects caused by the blocking. The results indicate that large-scale circulation regimes are related to but not necessarily tightly coupled to blocking and weather extremes in the Pacific–North America region.