Multiple weather regimes in the summertime North Atlantic circulation

Summertime weather regimes are examined over the North Atlantic sector based on a 44-year series of daily 700 hPa height anomalies for the period 1951-1994 in order to deepen our understandings of weather regime dynamics in the real atmosphere. Weather regimes are identified by examining the probability density function (PDF) in three-dimensional phase space, which is spanned by the three leading empirical orthogonal functions (EOFs) of the anomaly fields. Five weather regimes are obtained. Each weather regime corresponds to a distinct region with statistically significant large PDF values compared with multivariate Gaussianity. These include two extreme phases of the North Atlantic Oscillation (NAO), and flow patterns with intensified jet streams over the North Atlantic. The characteristics of weather regimes in summer are described in comparison with those in winter. The similarity in the circulation pattern of weather regimes in both seasons suggests the robustness of summertime weather regimes against the seasonal cycle. We discuss the persistence and transition of summertime weather regimes in connection with the bifurcation property of weather regimes elucidated in simple atmospheric models. The interannual variation of the occurrence frequency of weather regimes is also discussed.