(Bloomberg) — Coastal residents the world over could live without hurricanes and typhoons, New Yorkers could get to work and live quite nicely without nor’easters, but it’s hard to imagine life continuing in California without atmospheric rivers.
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A series of these powerful Pacific storms have battered the Golden State since December, killing at least 19 people and causing about $34 billion in losses and damages — and more rain from another atmospheric river is forecast over the coming days. But as destructive as they are, the meteorological forces at play are essential to the water cycle that fills the state’s reservoirs and aquifers and soaks the farmlands.
“Atmospheric rivers are the primary means for delivering rain and snow to California,” said Jonathan Rutz, an atmospheric scientist at the University of California, San Diego.
Swirling up deep in the tropical Pacific in what is also known as the “Pineapple Express,” atmospheric rivers carry enough water to match the Mississippi River’s flow at its mouth. The plumes of moisture can travel thousands of miles through the sky before dumping precipitation on North America’s western coast.
In 2021, an atmospheric river triggered a massive deluge in the province of British Columbia, dropping a month’s worth of rain over several days. The systems aren’t just confined to the West; atmospheric rivers carry water into the US Gulf Coast, South America, Greenland and even Antarctica.
For California and the US West, the storms bring the water that makes arid states livable year-round. It’s akin to the monsoons in India and the US Southwest that deliver a significant share of their water supply or summer thunderstorms in the Great Plains and Midwest, which carry moisture that helps crops survive the dry times.
But they also have a dangerous side, washing away homes and roads, causing evacuations, knocking out power, and in the worst-case scenarios, leading to deaths. And climate change is exacerbating the bad outcomes by making weather systems more erratic and catastrophic, according to the UN’s climate panel. The recent storms in California have claimed more lives than the past two wildfire seasons combined, according to Governor Gavin Newsom.
“California is soaked and even an inch more of rain can bring catastrophic impacts like flooding and mudslides,” Newsom said in a statement. “These conditions are serious and they’re deadly.”
California gets most of its water between October and March, and about 40% to 60% of the state’s average annual rain and snow comes from atmospheric rivers, according to Mike Anderson, the state’s climatologist. To have a good year, a lot of that precipitation has to arrive as snow in the Sierra Nevada, a mountain range in the eastern part of the state, which acts as a frozen reservoir until late spring and summer melts.
“The western United States, and particularly California, is largely dependent upon winter precipitation and accumulated snowpack to provide its annual water supply,” said Rutz.
The term atmospheric river was first coined in a research paper in the 1990s. But it wasn’t until about 2010 when exploration of the weather system really took off in growing recognition of its importance, Anderson said.
To advance that research, the National Oceanic and Atmospheric Administration has used its elite hurricane-hunting aircraft to fly reconnaissance missions into the rivers. In addition, a scale, resembling the famous five-step Saffir-Simpson hurricane system, was created to rank the storms from 1 to 5 by UC San Diego’s Scripps Instituton of Oceanography and government agencies.
The scale will need to be adjusted to take into account the cumulative effects of multiple atmospheric rivers arriving over a short time, as is happening this year, Anderson said. California is experiencing a historic drought so even weak storms are now causing tremendous damage, with dry and wildfire-scarred land prone to mudslides and flash floods.
At the same time, the severity of California’s drought, which began in the winter of 2019-2020, has begun to ease with the most severe categories of dryness being nearly erased from the most-populous US state, according to the US Drought Monitor. However, about 95% of the state remains parched and its largest reservoirs need more water.
“As hazardous as atmospheric rivers can be at times, without them, the water resources available to the West would be hugely reduced,” Rutz said.
“In order to sustain current levels of agriculture and population over the West, water would need to be brought from elsewhere, likely at a very large logistical cost and with much political difficulty.”
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