By Danielle VentonNovember 23, 2024 at 02.00 (NPR)

An aerial view of the Los Angeles River swollen by runoff from a prolonged atmospheric river storm in February 2024 in Los Angeles, California. Photo: Mario Tama/Getty Images/Getty Images North America

California is in the middle of a strong atmospheric river that has caused flooding, evacuations, road closures, and the mention of it is worldwide news and social media. And this comes on the heels of twoprevious winters where the Golden State saw similarly damaging storms. If you feel like you’ve started hearing the term a lot more in recent years, you’re not alone. You’re not even wrong.

In recent years, “atmospheric flooding” has become much more common in scientific journals and in media coverage. According to experts who study climate and weather, a couple of reasons can explain why. Technical weather terms in general are now used more in the news. Atmospheric rivers are a thriving area of ​​research, more of which can filter into media coverage. And these storms are also expected to intensify and become more damaging as the climate warms—which means more attention to them.

What exactly is an atmospheric river?

Before we get into why we hear about them more, let’s go over the basics of what an atmospheric river is.

These storms have always existed. They occur around the worldoften on the west coasts of the middle latitudes, where an ocean meets a land mass. They are long filaments of concentrated water vapor in the lower atmosphere that occur along with strong winds – and they are the primary means of moving water horizontally. In California, a normal winter can see five of these types of storms and as many as 20 can occur during wet winters. A typical one might be 300 miles wide, a mile deep and 1,000 miles long. When plotted on a map or viewed from a satellite in space, they looked just like rivers.

For a long time they were colloquially and scientifically referred to as things like Pineapple Express or Rum Runner Express. However, they turned out to be only a subset of atmospheric rivers, those originating near Hawaii or in the Caribbean en route to Europe. Not all ARs are very hot or start in these places.

“So the term ‘atmospheric river’ is the broader envelope,” said Daniel Swain, a climate scientist at the University of California.

The term was coined in a 1994 newspaper by two researchers at the Massachusetts Institute of Technology.

“And it turns out they’re very comparable to terrestrial rivers in terms of how much water moves in them,” Swain said. “In fact, sometimes they are significantly greater than a fraction of the flow of the largest terrestrial rivers on Earth,” including the Mississippi or the Amazon River.

The way we talk about the weather has changed

Swain believes that one reason people are suddenly hearing more about atmospheric rivers is that those who communicate about weather to the public have made a transition to using terms that the scientific community uses.

“I think a lot of it probably has to do with the media landscape and the popularization of certain technical weather terms,” ​​he said, pointing to “bomb cyclone” and “bombogenesis” as other examples. These are formal, quantitatively defined meteorological terms, “and everyone assumes it’s just some invention of the social media hype era.”

In fact, he says, these appear to date back to the 1940s during World War II when meteorologists advised Allied forces in the North Atlantic Theater.

Atmospheric flooding, he says, is similar.

“Instead of just making something up out of the air,” says Swain, “there’s been an interest in what are actually meaningful, technically correct scientific terms to describe various weather phenomena, which I’m not so sure is a bad thing. “

Scientists have done a lot to better understand atmospheric rivers

In recent years, AR has been a flowering area of research, some of as is filtrationto mediareporting.

Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, has been a pioneer in the field and is often quoted in the press.

Scientists like Ralph have helped discover how important atmospheric rivers are, both to California but also to storms around the country and the world. As early as 2004, the subject had fallen out of favor, says Ralph. But with new data collected by aircraft and satellites, he showed scientists how to see the storms in a new way, allowing scientists to observe them from the inside out.

“I sort of resurrected the subject after an early setback,” Ralph said.

This now-vibrant area of ​​research has made some new discoveries, Ralph says, including how to better predict their effects, how they affect both snowfall and snowmelt in the polar regions, and links between AR intensity and climate change.

“Because a warmer atmosphere contains more water vapor, and water vapor is the fuel in atmospheric rivers, AR can transport more water vapor,” Ralph says. “And there are studies now showing that we can expect to see somewhat more extreme AR and more common, in some cases, just because of that.”

The weather news in California has turned from being about drought to being about storms

What might add to the impression that atmospheric rivers are a new thing is that for much of the past decade California was in severe drought and didn’t get them. Then in early 2023, several AR storms followed one after the other, resulting in flood around California and 22 dead.

“In both cases, it’s a story of atmospheric rivers, in one case a deficit of atmospheric rivers, not enough of them, and the other case an abundance — too many atmospheric rivers at once,” Swain said. “California’s water lives and dies by this.”

Atmospheric rivers are wrong in more than 80 percent floods in the west. On average, these storms cause $1 billion in damage each year.

A look at Google Trendsreveals an early interest in atmospheric rivers in early 2011, almost nothing below drought years 2012-2016then more blisters in 2017, 2019 and 2021 coinciding with storms and flooding on the West Coast. And finally, big spikes in interest in 2023 and 2024. So far, fall has only brought one AR to California, but it’s a record a.

A major development for the future of atmospheric river research, Ralph says, is the ability to improve our forecast up to two weeks before a storm.

Legislation introduced Wednesday by Sens. Alex Padilla (D-Calif.) and Lisa Murkowski (R-Alaska) seeks to secure funding to increase airborne reconnaissance — using aircraft to fly through the storms — to learn more about atmospheric rivers.

“The more we sample these storms, the more accurate the forecasts become,” Ralph said.

Felt around the country

Lest you think these storms are purely a West Coast phenomenon, scientists are increasingly appreciating AR’s role in fueling and controlling nor’easters, strong storms that affect the East Coast.

“It’s entirely possible that AR recon in the Gulf of Mexico and off the East Coast will actually be able to improve forecasting of the track and intensity of nor’easters,” Ralph said, “which folks in the East know very well, is a very important detail for to determine whether the big cities are affected.”

The NPR audience first heard about atmospheric rivers year 2013when Jon Hamilton offered “Tips for Surviving a Mega Disaster.”