A long-time series (1895–1984) of mean areally averaged winter temperatures in the contiguous United States depicts an unprecedented spell of abnormal winters beginning with the winter of 1975–76. Three winters during the eight-year period, 1975–76 through 1982–83, are defined as much warmer than normal (abnormal), and the three consecutive winters, 1976–77 through 1978–79, much colder than normal (abnormal). Abnormal is defined here by the least abnormal of these six winters based on their normalized departures from the mean. When combined, these two abnormal categories have an expected frequency close to 21%. Assuming that the past 89 winters (1895–1984) are a large enough sample to estimate the true interannual temperature variability between winters, we find, using Monte Carlo simulations, that the return period of a series of six winters out of eight being either much above or much below normal is more than 1000 years. This event exceeds the calculated return period of the three consecutive much colder than normal winters (1976–77 through 1978–79) all falling into a much below normal category, i.e., one that is expected to contain approximately 10% of the data. The more moderate winters of 1981–82 and 1983–84 can also be considered abnormal by relaxing the limits necessary for an abnormal classification, but this gives a return period of 467 years for the spell of eight abnormal winters in the nine consecutive winters 1975–76 through 1983–84.
1 National Climatic Data Center, Asheville, NC 28801.
2 Climate Analysis Center, Washington, DC 20233.