Changes in 20 th century extreme temperature 1 and precipitation over the western United States 2 based on observations and regional climate 3 model simulations

22 23 Trends in extreme temperature and precipitation in two regional climate model 24 simulations forced by two global climate models are compared with observed trends over 25 the western United States. The observed temperature extremes show substantial and 26 statistically significant trends across the western United States during the late 20th 27 century, with consistent results among individual stations. The two regional climate 28 models simulate temporal trends that are consistent with the observed trends and reflect 29 the anthropogenic warming signal. In contrast, no such clear trends or correspondence 30 between the observations and simulations is found for extreme precipitation, likely due to 31 the dominance of the natural variability over systematic climate change during the period. 32 However, further analysis of the variability of precipitation extremes shows strong 33 correspondence between the observed precipitation indices and increasing Oceanic Niño 34 Index (ONI), with regionally coherent patterns found for the U.S. Northwest and 35 Southwest. Both regional climate simulations reproduce the observed relationship with 36 ONI, indicating that the models can represent the large-scale climatic links with extreme 37 precipitation. 38 The regional climate model simulations use the WRF (Weather Research and 39 Forecasting) and HadRM (Hadley Centre Regional Model) forced by the ECHAM5 and 40 HadCM global models for the 1970-2007 time period. Comparisons are made to station 41 observations from the Historical Climatology Network (HCN) locations over the western 42 United States. This study focused on temperature and precipitation extreme indices 43 recommended by the Expert Team on Climate Change Detection Monitoring and Indices 44 (ETCCDMI). 45