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On 7 February, water vapor streamed from Hawaii on an atmospheric river called the Pineapple Express.

NOAA Earth System Research Laboratory Physical Sciences Division

California rains put spotlight on atmospheric rivers

California has no mighty rivers like the Mississippi, but rivers of a kind are flooding the state. Since the new year, more than a meter of precipitation has fallen in some places, unleashing floods, triggering landslides, and, last week, bringing the emergency spillway of the Oroville Dam, the tallest in the nation, to the brink of failure. The seemingly endless string of storms has also been a boon, because after five crippling years, drought has been vanquished from all but the southern corners of the state. It’s all because of atmospheric rivers: long, narrow ribbons of water vapor rushing across the sky.

Just a few hundred kilometers wide, atmospheric rivers stretch thousands of kilometers from the tropical oceans toward the poles, carrying up to 20 times as much water as the Mississippi River. That moisture gets tugged along by the windy paddle wheels of spinning storms ahead of its path. When the atmospheric rivers make landfall and the vapor condenses, they can release a staggering amount of rain and snow.

Scientists are now working to unravel their physics so that they can provide better forecasts, now and in a future, hotter world. For dry midlatitude regions, any changes could have a profound impact. “They make or break precipitation in places like California,” says Marty Ralph, an atmospheric scientist at the University of California, San Diego, and director of the Center for Western Weather and Water Extremes.

The rivers strike coastlines around the world, from California to Europe to Antarctica. A study in this week’s issue of Nature Geoscience provides a rare look at their global impact. By teasing out the fingerprints of atmospheric river events in global weather data collected between 1997 and 2014, the researchers found these storms produced up to half of all extreme weather events in midlatitude regions, bringing high winds as well as intense precipitation. In Europe, 14 of the 19 events associated with the greatest insurance losses due to wind damage were linked to atmospheric rivers. “It’s wind transporting moisture, so the stronger the wind, the stronger the atmospheric river,” says Duane Waliser, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead author of the study.

In California, where atmospheric rivers deliver roughly half the state’s annual precipitation in a handful of intense bursts, they present a special challenge to dam managers. California law requires managers to maintain reservoir storage space during the winter rainy season in case of an impending storm, regardless of the weather outlook—which can force them to release water needlessly. At Lake Mendocino, scientists and managers are experimenting with using forecast-based decisions instead, in the hopes of reducing unnecessary discharges. Forecasts of atmospheric river events are now pretty reliable out to about 5 days, says Michael Dettinger, a hydrologist at the U.S. Geological Survey in Carson City, who is leading the project with Ralph.

Ralph and others are still working to improve the lead time and accuracy of these forecasts, using models and research aircraft that fly through atmospheric rivers and measure water content, wind speed, and other variables. Ralph says managers need to know exactly where an atmospheric river will strike, down to the individual watershed. Also important are long-range forecasts—akin to seasonal predictions of El Niño or La Niña events—that would give clues about whether the year will bring many atmospheric rivers, or few. “That’s the holy grail,” Dettinger says.

Researchers are exploring possible connections to large-scale climate features, like the so-called Madden-Julian oscillation (MJO). The MJO is a disturbance in tropical climate that marches eastward around the planet every few months. When the MJO stirs up more rainfall in the western Pacific, it may help spawn atmospheric rivers that slam into the western United States—a relationship that could help forecast wet periods.

Another potential culprit are the large-scale planetary waves called Rossby waves, wiggles in high-altitude winds that are linked to the position of high- and low-pressure systems around the globe. Ashley Payne, an atmospheric scientist at the University of Michigan in Ann Arbor, has found that when Rossby waves “break” against North America like ocean waves on a beach, they create strong, low-pressure centers that encourage intense atmospheric rivers.

Atmospheric rivers are likely to grow more intense in coming decades as climate change warms the atmosphere, enabling it to hold more water, says Christine Shields, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado. It’s still unclear how their frequency and landfall location will change around the world, although several studies show that southern California could see an increase.

And that would mean this year’s barrage is a glimpse of the future. With more storms set to clobber the coastline this week, managers are worrying about coping with the sudden shift from drought to surplus. “You can have too much of a good thing,” Ralph says.