The spatiotemporal variations of winter severity over North America

Winter severity affects many aspects of life, including traffic, public health, and the behaviour of animals and plants. The newly developed accumulated winter season severity index (AWSSI) is used in this study to examine the changes of winter severity across North America (NA) during recent decades. The results indicated that the winter severity experienced a notable interdecadal transition in 1965, characterized by increasing AWSSI before 1965, and decreasing AWSSI after 1965. This study also investigates the relationship between the winter severity and the atmospheric circulations over NA. The variations of winter severity are mainly controlled by temperature, while the large-scale forcings (i.e., ENSO, PDO, and NAO) also play an important role. In particular, PDO is mainly associated with the opposite variation of winter severity between the Southeastern United States and Northwestern NA, while the NAO leads to the opposite variation of winter severity between the Eastern United States and Eastern Canada. Under the influence of ENSO, the variations of winter severity over Southern and Interior Alaska, the Pacific Northwest, and the Northern Great Plains are opposed to that over the Southern United States and Northern Canada.