Dynamical life-cycle and teleconnections of North American and Euro-Atlantic weather regimes

Weather regimes are recurrent and quasi-stationary large-scale atmospheric circulation patterns, typically linking to surface weather. They have been defined over multiple regions and used in a range of practical applications, including forecasting, the study of atmospheric circulation trends and climate model evaluation. Despite their widespread use, the extent to which regimes reflect physical modes of the atmosphere is seldom investigated. Similarly, the statistical and physical connections between weather regimes defined over different geographical regions have been largely left unstudied. Here, we consider two widely-used sets of weather regimes: North American and Euro-Atlantic regimes. By leveraging dynamical systems theory, we find that when the atmospheric flow is assigned to a regime, it displays persistent characteristics and a lifecycle-like temporal evolution. We further find that these characteristics are enhanced when the atmospheric flow displays a comparatively strong projection onto the cluster-mean of the regime to which it is assigned (while the reverse is true for a weaker projection). We argue that this is evidence supporting the physical relevance of both North American and Euro-Atlantic weather regimes. We next consider the connection between the two sets of regimes. Specific pairs of regimes show a close visual and statistical correspondence. Moreover, the joint analysis of the two sets of regimes can provide medium-range statistical predictability for anomalies in their occurrence frequencies. Conditioning on North American weather regimes also results in anomalies in both the large-scale circulation during specific Euro-Atlantic regimes, and the associated European surface weather. We conclude that there is a benefit in conducting joint analyses of North American and Euro-Atlantic weather regimes, as opposed to considering the two in isolation.