UI researchers Reveal Climate Forces Behind Devastating Atmospheric-River Storms

By Lynn Anderson Davy
University of Iowa Strategic Communication

Recent deadly mudslides that tore through communities in California were triggered in part by a brutal “atmospheric river” storm, a meteorological phenomenon characterized by heavy rainfall. Since the 1990s, scientists have been studying the physical forces behind these vast rivers of water vapor, and now researchers at the University of Iowa may have some answers.

In a study published this week in Proceedings of the National Academy of Sciences, two researchers with IIHR—Hydroscience and Engineering and the Iowa Flood Center detail the important role of atmospheric and climate conditions such as jet streams in predicting storm development and landfall, and identify three distinct clusters of atmospheric rivers, steps that could improve prediction and emergency preparation.

In an average year, West Coast states like California, Oregon, and Washington can witness 10–15 atmospheric-river storms, which account for 30–50 percent of annual regional rainfall. And while these storms—sometimes called “pineapple expresses” because of their tropical origin—usually bring much-needed precipitation for crops and water reservoirs, they can also produce torrential rainfall, massive snowpack, and devastating flooding.

“Atmospheric rivers have major socio-economic repercussions, sometimes worth billions of dollars, and yet we still don’t completely understand why one year might see more of these storms than another year,” says Gabriele Villarini, interim director of IIHR—Hydroscience and Engineering and co-author of the study. “Our goal was to identify the climatic conditions that drive these storms in order to help communities better prepare for them.”

However, as Villarini and study co-author Wei Zhang, an assistant research scientist at IIHR—Hydroscience and Engineering, advanced in their study of the storms, they discovered that atmospheric rivers have distinct characteristics in terms of storm path, point of landfall, and precipitation amount. They also realized the storms could be categorized into three main clusters based on these characteristics.

“What we found is that all atmospheric rivers are not created equally,” Villarini says. “In fact, there are three distinct spatial clusters of atmospheric rivers that affect California, Oregon, Washington, and the Southwest region of the U.S., and we were able to identify these main three clusters for the first time.”

Atmospheric rivers are characterized by long plumes of water vapor—250 to 375 miles wide on average—that travel across the sky like rivers do over land. Some scientists compare them to giant hoses with the power to discharge an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. Atmospheric rivers move as weather patterns develop and change, and they are present somewhere on Earth at any given moment.

To better understand atmospheric rivers, Villarini and Zhang analyzed NASA data to identify and track the storms that made landfall over North America in the September-to-March periods from 1980 to 2015. What they found was that two climate conditions, the Pacific-Japan Teleconnections/Patterns and the East Asian Subtropical Jet in particular, play important roles in producing atmospheric-river storms along the West Coast.

Basically, these climate conditions produce moisture and water temperature fluctuations, as well as strong winds and waves, that steer atmospheric rivers toward California, Oregon, and Washington, where a majority of these types of storms typically hit. Researchers found that other climate conditions, including the El Niño Southern Oscillation and Atlantic Meridional Mode, are more likely to drive atmospheric-river storms toward the Southwest region of the U.S., where they bring heavy rainfall and flash flooding to areas of Arizona, New Mexico, and Texas.

“But overall, it’s the Pacific-Japan Teleconnections/Patterns that has the biggest impact on the total frequency of atmospheric rivers making landfall over the western United States,” says Zhang. “And having this information could really help to predict and prepare for these storms and their impacts.”

A further review of NASA data revealed the paths of hundreds of atmospheric-river storms, which when taken together, form distinct patterns. From these patterns, researchers were able to identify three main clusters of atmospheric storms. One cluster originates north of Hawaii and affects California, Oregon, and Washington; a second cluster originates west of Hawaii and mostly affects Northern California; and a third cluster originates in the Pacific Ocean, close to the West Coast, and affects Southern California and Arizona.

Zhang cautions that despite this new information about atmospheric rivers and the clusters they form, there is still more research to be done to fully understand them and how they affect West Coast and Southwest communities.

“The information that we have now will enable us to make better predictions about long-term, seasonal atmospheric-river activity,” Zhang says. “But improved short-term prediction of these storms, in particular in relation to the mitigation of flood risk and the adequacy of water supply, is our ultimate goal.”

Gabriele Villarini (left) and Wei Zhang