Trends in weather type frequencies across North America

While 20th century changes in many individual meteorological variables are well documented, the trends in multivariate synoptic-scale air masses-or weather types-largely remain unexplored. Utilizing a recently developed gridded weather typing classification system, this research investigates the changes in the frequency of weather types (WTs) across North America, 1979-2017. Averaged across the study domain as a whole, Humid Warm WTs are occurring 22 more days per year, while Dry Warm WTs have increased by 10 days/year. These increases are offset mostly by decreased frequency of Dry Cool (-17 days/year) and Cool WTs (-21 days/year). The largest absolute changes are in the Canadian Archipelago, where the Warm WT is occurring 42 more days/year and the Cool WT is occurring 48 fewer times per year. In western Canada all humid types are occurring more frequently, including a Humid Cool type that is occurring 16 more days/year. The Desert Southwest US and northern Mexico show significant increases in Dry Warm WTs (+33 to +40 days/year). Cold front and warm front passages show increases in most of the US and decreases in most of Canada. Describing these secular changes to the frequency of intuitive weather types may be an effective means of communicating these climate trends to policymakers and the general public, especially considering their large magnitude.