Human Activities May Have Boosted the West's 'Precipitation Roller Coaster'

The Western U.S. is a hotspot for studying climate change impacts on the hydrological or water cycle. Despite lower-than-average total precipitation in 2021, the contrasting dryness and wetness in the Western U.S. has been widely described as a “precipitation roller coaster.”

A new report from a team of climate scientists suggests the 2021 extreme “precipitation roller coaster” event that affected California and Utah is largely attributable to the combined impacts of greater precipitation intensity and dry spell length, with precipitation intensity playing a dominant role. However, the 2021 precipitation event in other Western states exhibited divergent contributions from precipitation intensity and dry spell length.

The research, which has been published in the journal Geophysical Research Letters, may set the stage for a more complete understanding of the origin of contrasting dry/wet conditions in the Western U.S. The study’s lead author, Wei Zhang, an assistant professor of climate science at Utah State University, said the analysis is a new angle for understanding a real-life problem experienced not only in 2021 but as part of the West’s climate regime and more intense wet and dry periods.

“It is quite challenging to quantify the ‘precipitation roller coaster,’” Zhang said. “We accomplished the task through an index, which includes metrics to quantify both wetness and dryness. … We quantified it using a product of the average precipitation intensity during wet days and the length of dry spells.”

The team identified the contributions of precipitation intensity and dry spell length to the extreme 2021 event.

“We saw increasing precipitation intensity and dry spell length based on comparison to the historical data,” said co-author Robert Gillies, director of the Utah Climate Center at USU. “This points to an increasing intensity of the hydrological cycle.”

The team noted that in the midst of a “megadrought,” the West’s hydrologic cycle is complicated by heat waves, wildfires, flash flooding events, and air quality issues and assessed the role of human activity in forcing extreme hydroclimatic intensity events.

Zhang and Gillies’ paper in the American Geophysical Union journal has been referenced this week in stories about California beginning the new year with extreme precipitation and winds that have damaged structures and caused flooding, mudslides, sinkholes and widespread power outages. A Business Insider story, “California's sudden jump from withering drought to violent floods is a warning for the whole world,” includes a link to the paper and tells the larger story about projected increases in extreme and dangerous weather events.

The USU researchers found that extreme hydroclimate events are 7.3 times more likely to occur when greenhouse gas emissions, land use and land cover changes, and fine particulates in the atmosphere factor into the models.

Gillies said the scientists’ next step is to figure out the dynamic and thermodynamic forcing of changes in the wetness and dryness so we can have a better picture of how our future will unfold.