During the boreal cool season regional climate in the United States is strongly impacted by extreme temperature regimes (ETRs), heavy rain events and snow/ice storms. In particular, extreme temperature regimes such as cold air outbreaks (CAOs) significantly impact energy consumption and human safety (via exposure). Using NCEP-NCAR and MERRA reanalyses, we study the statistical characteristics of CAOs and warm waves (WWs) over three distinct geographical regions: the Midwest (MW), Northeast Megalopolis (NE), and Deep South (SE). We examine the regional long-term variability in the frequency and amplitude of ETRs and quantify their regional modulation by prominent natural modes of low frequency variability.

ETR behavior is assessed using 3 different ETR metrics for each cool season: 1) Number of extreme (cold or warm) Days, 2) a cumulative “Impact Factor” and 3) Peak normalized anomaly Value. Linear correlation and regression analysis is used to isolate associations with the seasonal mean state of the North Atlantic Oscillation (NAO or AO), Pacific-North American, Pacific Decadal Oscillation and the El Nino-Southern Oscillation. This is extended into a multiple linear regression analysis to determine how much of the long-term ETR variability is attributable to each of the various low frequency modes.

We find a significant downward trend in SE WW events between 1949 and 2011 in terms of both Number of Days and Impact Factor metrics. Otherwise, no significant trends are found for either CAOs or WWs in any of the regions considered. Thus our analysis produces the interesting null case that there continues to be no significant reduction in either the amplitude or frequency of cold air outbreaks over the US. In particular, the past two winters (2009/10 and 2010/11) over the SE region each rank in the top 5 in terms of annual CAO impact. Finally, we find several statistically significant associations between ETR metrics and the seasonal-mean behavior of natural modes of low frequency variability. Such low frequency modulation appears to be most common for ETRs occurring over the SE region.