The Role of Natural Climate Variability in Freezing Rain Occurrence across Central and Eastern North America

Abstract Freezing rain is highly disruptive to society, ecology, and transportation; however, the driving physical processes governing variability and trends in freezing rain have not been extensively studied. This work investigated the temporal and spatial variability of freezing rain occurrence across eastern North America, using ERA5 reanalysis data from 1941 to 2020. After validation of freezing rain frequencies over time using 132 station sites, relationships between freezing rain and several modes of natural variability that influence North American temperature and hydroclimate are investigated across subdomains. Additionally, random forest regression analysis was used to examine nonlinear relationships between freezing rain and natural variability. Results indicate that within the eastern continental United States, freezing rain is more common with a weaker Aleutian low/anomalous ridge, or a negative Pacific–North American pattern—linked to Alaskan/North Pacific ridging—and cold air intrusion into continental North America. The long-duration modes (e.g., decadal oscillations) generally exert weaker influence at the studied time scales but can modulate the effect of some of the other modes. When evaluating these patterns using random forest, nonlinearities become apparent, particularly with the Arctic Oscillation and El Niño–Southern Oscillation, where highly positive and negative values are linked to more freezing rain in some domains. The analysis also revealed a generally limited signal for significant trends in freezing rain over time, though northernmost domains show increases. The fact that notable trends in freezing rain frequency (particularly decreasing trends in the south) have not yet occurred considering global climate change indicates that the role of natural climate variability is the dominant driver of historical variations. Significance Statement The purpose of this study was to examine long-term trends in freezing rain across central and eastern North America. The study first evaluated frequencies over time, noting that there are limited overall trends (increasing and decreasing). One of the main reasons for this is due to the important role that natural climate variability plays in modulating the occurrence of freezing rain events. Natural climate variability that promotes higher atmospheric pressure over the Pacific sub-Arctic regions appears to be strongly indicative of favorable conditions for freezing rain, particularly in the United States. This work extends our knowledge of the important factors that drive seasonal to multiyear freezing rain variability.