The role of natural climate variability in freezing rain occurrence across Central and Eastern North America

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 1941–2020. 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 non-linear relationships between freezing rain and natural variability. Results indicate that within the eastern continental U. S, 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 timescales but can modulate the effect of some of the other modes. When evaluating these patterns using random forest, non-linearities become apparent, particularly with the Arctic Oscillation and El Niño 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 are the dominant driver of historical variations.