The Relationship between Natural Variability and Freezing Rain Frequency over Eastern North America

Freezing rain is one of the most disruptive winter precipitation types to society, ecology, and transportation. It occurs regularly in the cool season across North America, especially the eastern half of the continent. However, compared with other precipitation types, freezing rain is generally less studied and therefore the driving physical processes that govern trends and variability in freezing rain over time are not well understood.

Here, we investigate the temporal and spatial variability of freezing rain frequency using ERA5 reanalysis data from 1941-2020. We first evaluate the ability of ERA5 to represent observed variability using a long-temporal observational dataset, finding that the reanalysis does accurately reproduce the major features of inter-annual variability within all subdomains analyzed, once modifications to exclude suspected freezing drizzle are employed. We then explore relationships between time series of freezing rain frequencies, and modes of natural variability from seven subdomains over the eastern half of the US and Canada. We focus on modes that have relationships to cold-air outbreaks, atmospheric blocking, and hydro climate variability, including ENSO, the North Pacific Index (NPI), east Pacific-north Pacific oscillation (EP-NP), Pacific-North-American pattern (PNA), arctic oscillation (AO), and Atlantic multi-decadal oscillation (AMO). We explore compound modes of variability, where shorter-duration modes are evaluated concurrently with an underlying slower-varying mode.

Our findings indicate that there is reasonable consistency in the modes of variability that significantly impact freezing rain frequencies across the main axis region extending from the southern plains northeastward to eastern Canada. In this region, freezing rain is significantly more common when the North Pacific index is positive (weaker Aleutian low/anomalous ridge) and the PNA is negative - linked to blocked flow over the North Pacific and cold air intrusion into continental North America. More patterns however were significantly linked to lower freezing rain frequencies, including stronger Aleutian low (negative NPI), positive PNA, and neutral or negative EP-NP. There was some evidence that a positive AMO was generally associated with higher freezing rain frequency, but these findings were not statistically significant. In other regions, such as the northern Plains and central Canada, significant relationships to modes of variability were generally weaker, and linked more closely to negative NPI (stronger Aleutian low), and/or the positive phase of the AO, particularly during negative AMO for the latter. We further investigate these results through composite analysis of large-scale circulation during top and bottom freezing rain months and years.

In terms over overall trends since 1940, very few regions show significant trends. Of those that were identified, the most spatial consistent pattern of increasing trends were for the northern Plains (increase), with areas of decrease more disbursed and generally in or near the ice-axis in the midwest, south, and northeast. The fact that notable trends in freezing rain frequency have not yet occurred in light of global climate change indicates that the role of natural variability is the dominant driver of trends to this point, however the increase in the north and west of our domain is further evidence of a northward shift in freezing rain over time.