Abstract

Temperature extremes show substantial and statistically significant trends across the US West during the late 20th century, with consistent results among individual stations. 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 obserations and simulations is found for extreme precipitation, likely due to the dominance of the natural variability over the period. However, an analysis of the variability of precipitation extremes shows strong trends in observed precipitation indices with increasing Oceanic Niño Index (ONI), with regionally coherent patterns found for the northwest and southwest.

Both regional climate simulations reproduce the observed trends with ONI, indicating that the models can represent the climatic links with extreme precipitation. The regional climate model simulations use the WRF (Weather Research and Forecasting) and HadRM (Hadley Centre Regional Model) forced by the ECHAM5 and HadCM 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 for Climate Change Detection Monitoring and Indices (ETCCDMI).