Changes in frequency of extreme temperature and precipitation events in Alaska

Decadal mean temperatures show a warming over the past 60 years at individual stations in Alaska. This research examines corresponding changes in frequencies of extremes of temperature and precipitation based on daily data from 26 Alaskan surface weather observing stations from 1950 to 2008. Extreme events were defined as the upper one percentile of events. Data were analyzed seasonally for each of six climate regions. Results indicate an overall increase in frequency of extreme maximum temperature events, a decrease in frequency of extreme minimum temperature events, and an increase in frequency of extreme three-day precipitation totals. Regional and seasonal variation is apparent, with the strongest trends occurring in the winter and spring seasons. Statistical significance is established through a series of Monte Carlo tests that preserve the distributions of the values. The winter and spring trends at many of the stations are significant at the 99% level. In spring, all stations in all regions, except the Southeast, display increases in frequency of warm extremes and decreases in frequency of cold extremes with at least 90% significance. Most stations in the Southeast displayed the same patterns with a high degree of significance. Winter had the next greatest increases (decreases) in the frequency of warm (cold) extremes, also with a high degree of significance. Precipitation results are highly variable and generally not as significant as patterns in temperature extremes. Decreases in frequency of extreme three-day precipitation events were found in the Arctic region for winter, spring, and summer, and are significant at the 90% level and greater. Significant decreasing trends are also seen in the Southwest in spring. The greatest increases in frequency of precipitation extremes are observed in the Southeast in winter. About half of all stations in the Southeast show moderately (80% or greater) significant increases during spring, summer and fall. Significant increases are also seen in winter and spring in the West Central region. Four of six regions (West Central, South Central, Southwest, and Southeast) show fewer minimum temperature extremes in winter and more maximum temperature and heavy precipitation extremes in summer during the more recent sub-period (1980-2008) relative to the earlier sub-period (1950-1979)