Precipitation extremes over the continental United States in a transient, high-resolution, ensemble climate model experiment

Understanding future changes in the frequency, intensity, and duration of extreme events in response to increased greenhouse gas forcing is important for formulating adaptation and mitigation strategies that minimize damages to natural and human systems. We quantify transient changes in daily-scale seasonal extreme precipitation events over the U.S. using a five-member ensemble of nested, high-resolution climate model simulations covering the 21st century in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A1B scenario. We find a strong drying trend in annual and seasonal precipitation over the Southwest in autumn, winter, and spring and over the central U.S. in summer. These changes are accompanied by statistically significant increases in dry-day frequency and dry-spell lengths. Our results also show substantial increases in the frequency of extreme wet events over the northwestern U. S. in autumn, winter, and spring and the eastern U.S. in spring and summer. In addition, the average precipitation intensity increases relative to the extreme precipitation intensity in all seasons and most regions, with the exception of the Southeast. Further, most regions receive a greater fraction of total seasonal precipitation from extreme events. These results imply fewer but heavier precipitation events in the future, leading to more frequent wet and dry extremes in most regions of the U.S. Our simulations suggest that many of these changes are likely to become statistically significant by the mid-21st century. Given current vulnerabilities, such changes in extreme precipitation could be expected to increase stress on water resources in many areas of the U.S., including during the near-term decades.