A New Year-Round Weather Regime Classification for North America

Large-scale, recurrent and persistent regional atmospheric circulation patterns – known as weather regimes – are increasingly used in subseasonal-to-seasonal prediction and in analysis of climate variability and change. Yet, the majority of existing studies have focused on regimes in the North Atlantic-European sector, with comparably fewer studies investigating North American regimes. An additional limitation stems from the seasonal dependence of existing North American regime classifications, which primarily focus on winter. Here, we normalize the seasonal cycle in daily geopotential height variance over North America and use empirical orthogonal function analysis combined with k-means clustering to define a new set of four year-round North American weather regimes: the Pacific Trough, Pacific Ridge, Alaskan Ridge, and Greenland High regimes. Additionally, we define a fifth ‘No Regime’ state to represent conditions closer to climatology than any of the regimes. Multiple statistical tests support the choice of four regimes, and the clustering solution is robust to various methodological choices. All four regimes typically persist for around one week, with instances of extremely long-lived regimes persisting for up to six weeks, while the No Regime state is mostly associated with regime transitions and typically persists for only a few days. Regime-associated temperature and precipitation anomalies are reported, alongside the link between regimes and the modulation of tornado activity across the United States. We also use the regimes to objectively quantify historical trends in the large-scale circulation over North America during 1979—2023, finding a large increase in the frequency and persistence of the Greenland High regime during summer, particularly since 2007.