Changes in Twentieth-Century Extreme Temperature and Precipitation over the Western United States Based on Observations and Regional Climate Model Simulations

Trends in extreme temperature and precipitation in two regional climate model simulations forced by two global climate models are compared with observed trends over the western United States. The observed temperature extremes show substantial and statistically significant trends across the western United States during the late twentieth century, with consistent results among individual stations. The two regional climate models simulate temporal trends that are consistent with the observed trends and reflect the anthropogenic warming signal. In contrast, no such clear trends or correspondence between the observations and simulations is found for extreme precipitation, likely resulting from the dominance of the natural variability over systematic climate change during the period. However, further analysis of the variability of precipitation extremes shows strong correspondence between the observed precipitation indices and increasing oceanic Nino index (ONI), with regionally coherent patterns found for the U.S. Northwest and Southwest. Both regional climate simulations reproduce the observed relationship with ONI, indicating that the models can represent the large-scale climatic links with extreme precipitation. The regional climate model simulations use the Weather Research and Forecasting (WRF) Model and Hadley Centre Regional Model (HadRM) forced by the ECHAM5 and the Hadley Centre Climate Model (HadCM) global models for the 1970-2007 time period. Comparisons are made to station observations from the Historical Climatology Network (HCN) locations over the western United States. This study focused on temperature and precipitation extreme indices recommended by the Expert Team on Climate Change Detection Monitoring and Indices (ETCCDMI).