Using observational station data, the frequency of cold surges across the contiguous United States and southern Canada is calculated and analyzed for much of the twentieth century. Subsequently, the large scale and surface environment favorable for cold surges is investigated. Twelve carefully selected representative stations are chosen throughout this region. Climatologies and composites are performed for surges at each station in order to identify the key upper-level and surface features responsible for an outbreak of cold air. Using daily maximum temperature data for 50-104 years, moderate and strong cold surges are identified for the twelve representative stations using the criteria of some critical temperature fall, individualized to each station, between day 1 and day 2. This allows for the identification of potential trends in the annual and decadal frequency of surges.
Moderate cold surge frequency has decreased at all but three stations, Houston (HOU), San Francisco (SFO), and Boulder (BOU). The North Central and Midwest stations have exhibited the most distinct decrease in frequency. The most statistically significant decrease has occurred at Huron, South Dakota (HON). Seven stations, including all three southern Canadian cities, have actually shown an increase in frequency of strong cold surges. So while the frequency of cold surges has decreased, there has been a greater number of strong surges.
Potential contributors to the high variability in cold surge frequency and intensity are investigated here, particularly yearly changes in surface temperatures, daily temperature variability, sea surface temperatures (SSTs), snowcover in the source regions, and the upper-level jet circulation. Warming has occurred at seven stations, while seven of the twelve stations have also shown a decrease in daily temperature variability. On the long time-scale of a century, snowfall has increased across most of Canada, although a decrease has occurred in Canada and all of the Northern Hemisphere in the past two decades. Finally, the North American jet stream circulation during the cold surge season of November through March has shifted from a weakly negative towards a more positive Pacific North American index (PNA) with a more zonal nature. All of these climate variations contribute to the variability in cold surge frequency, intensity, and regionality in some manner