Temperature Regimes, Climate Extremes, and the Waning of Winter in Minnesota

Changes in the frequency and magnitude of extreme weather events are perhaps the most visible sign of climate change. Yet long term variation in average temperature extremes proves to be a persistent climate change signal in the observational record. Relative to all other climate variables, average minimum temperatures have shown the greatest increase, with spatial coherence throughout the United States. On the seasonal scale, winters throughout the US are becoming less cold at a faster rate than summers are warming. These changes are evident across the Upper Midwest and are epitomized in Minnesota, a state known for its extreme winter climate characteristics. Against the backdrop of winter warming are notable events of extreme cold, including the “high-profile” polar vortex outbreaks of 2014 and 2019. The present study seeks to address a gap in existing literature around winter-specific seasonal changes by examining century-scale climate observations to explore the nature of winter climate changes. With a focus on Minnesota, we present an analysis of daily, monthly, and seasonal data from individual stations and NOAA state climate divisions. We characterize seasonality of winter changes, their trend and variability characteristics, possible regime-like behavior as well as the possibility of extreme event frequency. Drawing on the strongest climate change signals detected in Minnesota, we leverage data from the broader region to characterize the spatial extent of these changes in the context of larger scale mid-latitude climate patterns. With the year 2020 and the new 30-year normals period rapidly approaching, this is an excellent opportunity to reconsider what constitutes an extreme event. Since Minnesota and Upper Midwest climate stakeholders have an economic and environmental dependence on a historic winter profile, this research aims to shed light on what is the new winter “normal” in the climate change era.