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A one-two punch of precipitation resulted in damage to Oroville Dam’s main and emergency spillways pushing the second largest dam in California into a crisis in February 2017. Scripps Institution for Oceanography at UC San Diego and their colleagues have released a new report. study that they have identified the fingerprint of climate change in the events that triggered the incident. Issues with the dam’s spillways led to the evacuation of 188,000 people who lived in the floodplain of the Feather River some 70 miles north of Sacramento, Calif.
Dale Kolke / California Department of Water Resources
Though officials narrowly averted a sudden release of water from behind the emergency spillway holding back the top-most portion of California’s second largest reservoir, the incident demonstrates how difficult it is to balance water supply needs served by full dams with public safety that requires retaining sufficient empty space to capture floodwater. The consensus in climate science is that precipitation will become less frequent but more intense, increasing California’s hydroclimate instability.
“We expect atmospheric rivers (ARs) to become more potent in a warming climate. We had previously assessed ARs and their evolving contribution to hydroclimate change over western North America. It was done in global climate model projections,” saidStudy co-author Alexander Gershunov, a climate scientist at Scripps Oceanography, “but this is the first study that quantified the influence of global warming on a specific, real, recent, and impactful atmospheric river event.”
Researchers noted that in summer 2021, Oroville Lake’s record low level prevented hydropower generation for the first time since the dam opened in 1968.
“Climate projections for California and the U.S. West Coast suggest fewer, yet more intense, precipitation events in the future,” said study lead author Allison Michaelis, an assistant professor of meteorology at Northern Illinois University. “Such changes to the hydroclimate naturally come with heightened risks of both drought and flooding and potential for infrastructure damage.”
This research taps into growing knowledge about the weather phenomenon known as atmospheric rivers, which are potent flows of water vapor that provide California with up to half its annual water supply and account for more than 90 percent of the state’s flood damages. Scripps Oceanography scientists at the Center for Western Weather and Water Extremes (CW3E) have led much of the work onAtmospheric rivers, including the creation of the AR Scale. The scale categorizes atmospheric river storms in a spectrum between beneficial replenishing precipitation events deluges that can cause widespread flooding.
Previous research led by Scripps Oceanography has suggested that as the climate continues to warm, the state and much of the rest of the West Coast will grow increasingly dependent on atmospheric rivers for their water supply making accurate prediction of these storms increasingly important. CW3E scientists are trying to improve their ability predict when and whereabouts the atmospheric rivers will reach landfall and how much rain snow and runoff they will produce.
The new project was funded by the U.S. Bureau of Reclamation and the California Department of Water Resources (DWR). StudyThis article appears in the American Geophysical Union journal. Earth’s Future.
“The potential impacts of atmospheric rivers on water storage facilities were made clear by the 2017 Oroville Dam spillways incident,” said DWR Director Karla Nemeth. “DWR has been working with partners like CW3E and others to enhance tools to better forecast, prepare for, and manage climate change-induced extreme conditions that impact California’s water supply operations.”
An atmospheric river storm made landfall on February 6, 2017 in Northern California. An initial pulse of cold precipitation dumped snow onto an already large snowpack. The snow was then covered by a warm pulse, which poured rain onto the snow. Although the second pulse produced the same amount of precipitation as the first, it had the effect that it melted the snow on the ground and contributed to the inflow that Oroville Dam had difficulty controlling.
Michaelis, a former postdoctoral researcher scholar at CW3E was one of the researchers who modeled the February 2017 event. The simulation showed that human-induced warming caused an increase of precipitation by 11 percent during the first pulse, and a 15 percent increase in the second pulse. Researchers found that the warming predicted to occur would have a significant impact on precipitation in the second-pulse. This would have been almost 60%.
However, the first pulse didn’t experience as much increase in a warmer climate. Two components of one atmospheric river event behaved differently in simulations. This suggests that even though warmer air can hold more water, ARs can be stronger. This complicates weather prediction.
“We were surprised to see such different responses from the two pulses of this storm,” Michaelis said. “The different relationships between warming temperatures and precipitation increases opens up interesting research questions that we’re excited to explore with a larger sample of cases.”
Other than Michaelis and Gershunov the study authors are Tamara Shulgina, F. Martin Ralph of Scripps Oceanography and Meredith Fish, Scripps alumna Meredith Fish and Alexander Weyant. Alexander Weyant was an undergraduate math major at UC San Diego and contributed most of the data analysis. Since then, Weyant has joined Scripps Oceanography to be a first-year PhD candidate.
“I believe most students studying statistics in the math department would find it very exciting to join the research effort on weather events in the background of a nonstationary climate,” said Weyant. “They need only be made aware of this puzzling problem, which straddles the divide between structure and randomness, as well as that of short and long timescales.”
