Conterminous Unites States Temperature Extremes based on NLDAS-2 forcing data from 1979 to 2012

Over recent decades, high temperature extremes have become more frequent while cold extremes have been decreasing across the globe. Currently, there is active research to better understand climate extremes in relation to anthropogenic climate change. The purpose of this study is to characterize the temporal and spatial dynamics of temperature extremes across the United States using NLDAS-2 forcing data. The study presents the annual and seasonal exceedence rates of the 90th and 10th percentile-based temperature indices from 1979 to 2012. The indices are computed using the ClimDex procedure recommended by the Expert Team for Climate Change Detection Monitoring and Indices (ETCCDMI). The four temperatures indices examined are daytime 90th percentile (TX90) for the warm days, daytime 10th percentile (TX10) for the cool days, nighttime 90th percentile (TN90) for the warm nights, and nighttime 10th percentile (TN10) for the cool nights. Results for TX90 show a clear increasing trend in average exceedence rate from 26 days during the 1980s, to 29 days during the 1990s, and to 37 days between 2000 and 2012. The year 2012 had the highest TX90 exceedence rate of 61 days, while 1993 had the lowest exceedence rate of just 15 days. The exceedence rate of the TN90 also increased over time, but at a slower rate, from about 29 days on average before 1990, to more than 32 days between 2000 and 2012. During summer, the TX90 exceedence rate has been gradually increasing with an acceleration apparent since 1994. Fall has a steady increasing TX90 exceedence rate starting from 1997, with the highest rate observed in 1999 (13 days). Compared to summer and fall, spring has a tempered increasing TX90 exceedence rate during the study period. During winter, there is no discernible trend in the TX90 and TN90 exceedence rate. The study further examines the annual variation of spatial distribution and percentage area across the United States, where the exceedence rates were more than 100 days. Before 2000, the percentage area with more than 100 days of TX90 exceedence rate was typically less than 5%, however since 2000 this percentage area has sharply increased to 34% in 2011 and 72% in 2012. Except for 2008, the percentage area with TX90 exceedence rate above 100 day has been more 15% since 2006. For warm nights, the percentage area with more than 100 days of TN90 exceedence rate was typically less than 5%, apart years 1998 (7.7%), 2006 (8.4%), 2011 (7.2%), and 2012 (21%). The results of this study confirm that 2012 was an extraordinary year of widespread temperature extremes across the country. In general the occurrence of high temperature extremes has increased since 1979, and accelerated since the beginning of the twenty first century.