A massive ocean current in the Atlantic could influence storms thousands of miles away, potentially making California’s future atmospheric rivers wetter and increasing flood risks, according to new research.
Scientists at the University of California, Riverside, found that a weakening Atlantic Meridional Overturning Circulation — known as the AMOC — could reshape global wind patterns in ways that increase atmospheric river activity along the West Coast.
Researchers have found signs that the AMOC has slowed in recent decades, and climate models suggest the decline could continue as greenhouse gas emissions warm the planet.
But scientists have been working to understand how a changing Atlantic current could affect weather far beyond the ocean basin.
What is the AMOC?
The AMOC is a massive system of ocean currents that helps move heat around the planet. It acts like a global conveyor belt, carrying warm tropical water north through the Atlantic Ocean.
As that water cools and becomes denser, it sinks and flows back south through deeper ocean layers. The current helps regulate temperatures and weather patterns across the Northern Hemisphere, including contributing to Europe’s relatively mild climate.
Scientists have observed signs that the AMOC is weakening as climate change warms the planet.
“It is well known that the AMOC is a big player in the world’s climate system, and that it is slowing down,” said Mohima Mimi, a doctoral student in climate dynamics at UC Riverside and lead author of the study said in a release earlier this month. “What we didn’t know is exactly how the AMOC might impact atmospheric moisture and storms outside the Atlantic region.”
California’s atmospheric rivers could become more intense
Atmospheric rivers are narrow corridors of water vapor that transport moisture through the atmosphere. When they reach California, they can bring critical water supplies — but the strongest events can trigger flooding, mudslides and major damage.
The study found that a weaker AMOC could strengthen upper-level winds that guide storms across the Northern Hemisphere. Those stronger winds could then help steer more atmospheric rivers toward the West Coast.
The research found that by the end of the century, under a high greenhouse gas emissions scenario, a weakened AMOC could increase atmospheric river-related precipitation in California, particularly during winter.
The model showed the AMOC slowdown could increase winter atmospheric river precipitation by up to about 0.18 meters per year in California — roughly 7 inches — compared with a scenario where the current did not weaken. This could come as snow or rain.
Researchers said the impact would be especially significant because atmospheric rivers already produce many of California’s most extreme precipitation events.
“In California, atmospheric rivers are a double-edged sword,” Mimi said in the release. “They supply much of the state’s water supply, but as they become stronger, they’re likely to also bring widespread destruction.”
The Atlantic can influence Pacific storms
The study does not suggest the Atlantic current directly sends storms toward California.
Instead, researchers found that changes in the AMOC could alter temperature patterns and atmospheric circulation, strengthening prevailing westerly winds across the Pacific Ocean.
Those wind changes could make conditions more favorable for atmospheric rivers to reach the West Coast.
The authors said the AMOC’s influence is one piece of a much larger climate system that also includes natural patterns such as El Niño, the Pacific Decadal Oscillation and other ocean-atmosphere interactions.
What is an atmospheric river? How does it differ from a standard storm system?
On the West Coast, atmospheric rivers approach from the Pacific Ocean, carrying tremendous amounts of water. They occur most frequently during specific weather patterns, including El Niño, which can increase their frequency or intensity; however, they can also occur in other years.
These long, narrow corridors of air have been responsible for as much as 65% of the West’s extreme rain and snow events, USA TODAY formerly reported.
According to the National Oceanic and Atmospheric Administration, an atmospheric river — described as a “river in the sky” — transports water vapor outside of the tropics. While they vary in size and strength, the average system carries an amount of water vapor equivalent to the average flow of the Mississippi River at its mouth.
Past atmospheric rivers have caused widespread impacts. For example, earlier this year, two systems drenched Southern California, triggering severe flooding, tornadoes, avalanches, and mudslides. Cities including San Francisco and Redding received up to a month’s worth of rain, prompting evacuations and rescues from southern Santa Barbara to Orange County.
Is an atmospheric river the same as a 'pineapple express'?
A “Pineapple Express” is a type of atmospheric river originating from near Hawaii, bringing warm, wet air and heavy rainfall to areas such as California and the Pacific Northwest.
According to the National Weather Service in Portland, the current atmospheric river is forming as a strong weather system over the Gulf of Alaska, pulling in subtropical moisture and funnels it toward the region.
Meteorologists track these systems using a technique called integrated water vapor transport, which measures the amount of moisture moving through the atmosphere. High readings on this metric are a key indicator that an atmospheric river is approaching.
U.S. weather watches and warnings map
Stay informed. Get weather alerts via text
Brandi D. Addison covers weather across the nation as the Weather Connect reporter for the USA TODAY Network and contributes to Texas Connect coverage across the state. She can be reached at baddison@usatodayco.com. Follow her on Facebook here.
This article originally appeared on Palm Springs Desert Sun: Stronger atmospheric rivers in California? One study says it's possible
