How Rivers in the Sky Travel Across the Ocean
Winter in California is a time of promise and peril. We’re desperate for rain, only not too much please. Our fate swings from drought to floods, depending largely on whether or not we get rainstorms called atmospheric rivers. These ribbons of extraordinarily wet air shoot across the Pacific Ocean, dropping the moisture they carry upon landfall.
The Bay Area’s latest “wet” season began with the bang of a record-breaking atmospheric river in late October but then fizzled out. These storms have been so scarce in the last few months that the state is facing a third year of deepening drought.
Atmospheric rivers typically begin over oceans in the tropics, where it’s so warm that water evaporates readily, filling the air with moisture. Then all it takes to start an atmospheric river is a storm called an extratropical cyclone, which spins over the ocean and sweeps up the wet air.
“Atmospheric rivers are seeded by convection storms that move water vapor from the surface to a couple of kilometers high,” says Alan Rhoades, a hydroclimate scientist at Lawrence Berkeley National Laboratory.
Atmospheric rivers that form over the Pacific Ocean are then launched and propelled toward the West Coast by strong winds. “You need a driver like a low-level jet stream,” says Zhenhai Zhang, an atmospheric scientist at Scripps Institution of Oceanography. “The jet carries the moisture and moves fast in a narrow corridor.”
This airborne stream of water vapor then traverses a series of low and high pressure regions that are strung across the ocean. These regions set the path of the atmospheric river, which can be so long that it extends halfway across the Pacific. “A low grabs the water vapor and moves it northward, and a high then steers it toward the West Coast,” Rhoades says.
He likens this process to a conveyor belt, where the low and high pressure systems are like rollers that direct the atmospheric river. Another way to look at this process is that the low and high pressure areas are like the pegs of an enormous pinball machine, guiding and redirecting the flow of the atmospheric river and so shaping its trajectory toward land.
Other Recent Posts
UC Berkeley’s Brilliant Breakthrough in Carbon Capture
Researchers have developed COF-999, a new material that absorbs CO₂ directly from the air without rapidly degrading — a game-changer for carbon capture.
Coho Salmon Remain Afloat Four Years After CZU Fire
At the southern end of their range, coho salmon in Scott Creek are adapting to wildfire and warming.
How Two East Bay Teachers Are Fighting the Climate Crisis
Climate literacy and sustainability resolutions are changing how East Bay schools tackle teaching about climate science and solutions.
California Makes Biggest Downpayment Ever for One Region’s Climate-Ready Projects
A NOAA grant will fund flood mitigation, wildfire risk reduction, and habitat restoration — and green job creation — across Santa Cruz-Monterey.
Collecting and Unifying Regional Metrics on Wetland Health
By standardizing and coordinating data collection, the Wetlands Regional Monitoring Program will supercharge new analyses of restoration projects.
Two Workshops Daylight Alameda-Oakland Shore Solutions
A collaborative planning committee is shopping strategies to safeguard East Bay shores from sea level rise, groundwater, and stormwater flooding.
Vote Cinches Robust Regional Response to Sea Level Rise
BCDC adopted a Regional Shoreline Adaptation Plan in December 2024, setting the stage for local governments to address growing flood threat.
If You Like What You’re Reading, Pay it Forward!
In the bomb cyclone of bad and fake news, KneeDeep is a refuge.
Training Future Farmers To Grow More Than Food
At a Bay Area collective for BIPOC and queer farmers, Brooke Porter and Alexa Levy are fighting to build an inclusive food system from the soil up.
Don’t Tidy, Leave Winter Homes for Insects
Your messy garden might be saving beneficial insects. Before you reach for the rake, learn about how dead leaves and stems help pollinators overwinter.
Instruments on this Sonoma County ridge measure precipitation, temperature, humidity, winds, barometric pressure, and more. Scientists use the data to connect the hydrology of the Russian River watershed to atmospheric river forecasts. Photo: Scripps, UCSD.
That trajectory is hard to predict, however. “Atmospheric rivers are often called whips or hoses because their path can shift in different directions,” Rhoades says. This is partly because these storms depend on wind, which changes from hour to hour. Atmospheric river speeds are difficult to pin down but may vary from 20 to 100 kilometers per hour.
And while the path can look fairly smooth at a global scale, up close it’s a different story. “At a small scale, it’s challenging to predict the exact path of an atmospheric river,” Zhang says, adding that an atmospheric river’s size, shape and intensity are also in flux.
Taken together, all this variability makes it hard to know exactly how strong an atmospheric river will be and exactly where it will make landfall. Another complication is that atmospheric rivers can be just a few hundred kilometers across. This is very close to the uncertainty in predictions of where they will hit, which can be off by a couple hundred kilometers. “That’s the difference between landfall in Los Angeles or in San Diego,” Zhang notes.
