Late 1990s’ cool season climate shift in eastern North America

Abstract

Changes in the frequency of cold and warm synoptic weather types within eastern North America indicate the occurrence of a cool season climate shift in the late 1990s. This aligns with the timing of shifts toward earlier spring ice out on freshwater lakes within the region and less frequent cold air mass passage over the North American Great Lakes. While a preliminary shift occurred in the early 1980s, a pronounced shift toward more frequent warm weather types during November through April occurred after 1997, when prominent warming of the lower atmosphere over the region began. The late 1990s’ climate shift appears linked to middle atmospheric circulation change associated with phase changes for two influential low-frequency climate teleconnections originating within the Atlantic and Pacific Oceans, the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. The climate shift, approximately 20 years after warming within the global climate system commenced, further illustrates the importance of climate system variability within anthropogenically driven climate change.