What is an atmospheric river?
FOR CALIFORNIA it felt like déjà vu. Last year a procession of storms, fueled by atmospheric rivers, killed at least 21 people and caused nearly $5bn worth of damage across the state. On February 4th this year a similar storm again wreaked havoc. Hurricane force winds were hitting the central coast near Big Sur. In Los Angeles celebrities were showered when they arrived on the red carpet of the Grammy awards. Mansions in the Hollywood Hills were covered in mud. At least three people have died. What are atmospheric rivers, and are they a threat outside of California?
Atmospheric rivers are aptly named. They are like conveyor belts in the sky that carry a lot of water vapor across the oceans. No more than 400-500km across, they can be thousands of kilometers long; their near-surface winds move at least 12.5 meters per second (28 miles per hour). If all the water vapor in such a river were in liquid form instead it would be a layer at least 2 cm deep. It all adds up to a flow that can be greater than that of even the most powerful regular rivers. An atmospheric river that hits the west coast can deliver water at a higher rate than the Amazon discharging itself into the ocean, and at 15 times the rate of the Mississippi.
Over the past 30 years these filaments have been recognized as a key part of the Earth’s water cycle, directing much of the water vapor that flows from the tropics to the poles. The tropical origin explains the term “Pineapple Express”: California’s stormy weather is seen as coming straight from Hawaii.
Not all atmospheric rivers are dangerous. Coastal California receives up to half of its average annual rainfall from them. Although their winds can be as strong as a hurricane, they can also be as gentle as a spring shower. The American west coast is not the only part of the middle latitudes that feels their influence; the coasts of western Europe and South America also see them land (the west coast of Africa sees them less, as those that form in the Atlantic usually go south of the Cape of Good Hope). The storms often make landfall elsewhere on the west coast of North America, as well as the coasts of western Europe, southern Chile and Australia. They become dangerous when they come in quickly, and especially when the coast they hit is mountainous. After days of rain, soils can no longer absorb water, leading to floods and mudslides.
What explains the strength of the atmospheric river flooding California? The main factor is the warmth of the ocean. “A few degrees of warming in nearshore and offshore water temperatures means there’s more moisture in that lower atmosphere,” explained Daniel Swain, a climate scientist at the University of California, Los Angeles. And warm seas abound, thanks to global warming.Sea surface temperatures have been the highest on record every day since March last year.
Atmospheric rivers can be hailed as thirst quenchers. Last year’s march of storms ended a three-year drought in California and filled the state’s reservoirs. In 2017 an atmospheric river wiped out a punishing drought in Galicia, in the northwestern corner of Spain. At high elevations, moisture from atmospheric rivers falls as snow. The Central Sierra Snow Lab, a research station run by the University of California, Berkeley, found that 2023 will be the second snowiest year since records began in 1946. After the first deluge, skiers and -snowboards to higher resorts than ever before. to enjoy the new powder. Maybe ski bums, at least, are happy to repeat. ■