The Role of Anthropogenic Forcing in Western United States Hydroclimate Extremes

Study key findings & significance

• HYINT exhibited extremely high values in parts of the western U.S. in 2021, mainly caused by average precipitation intensity

• Hydroclimatic intensity shows a significant rising trend in most of the southwestern U.S. mainly tied to a rising dry spell length trend

• The extreme hydroclimatic intensity event is more likely to occur under anthropogenic forcing than natural forcing alone

Plain Language Summary

The western U.S. is a hotspot for studying climate change impacts on the hydrological cycle. Despite lower-than-average total precipitation in 2021, the contrasting dryness and wetness in the western U.S. has been widely reported as a “precipitation roller coaster”. In this paper we quantified the “precipitation roller coaster” using an index (HYINT) - a product of average precipitation intensity during wet days and dry spell length. The study found that the 2021 extreme hydroclimatic intensity event was largely attributable to the combined impacts of precipitation intensity and dry spell length in California and Utah, with precipitation intensity playing a more important role. In contrast, the 2021 precipitation event in other western states exhibited divergent contributions from precipitation intensity and dry spell length. The southwestern U.S. has been identified as a hotspot for increasing hydroclimatic intensity, which is tied more to the increasing dry spell length than the precipitation intensity. The trends in dry spell length and precipitation intensity may have played a key role in driving the 2021 extreme hydroclimatic intensity event. Using climate model experiments with and without anthropogenic forcing, an extreme hydroclimatic intensity event in the western U.S. is more likely to occur with anthropogenic forcing.