What is an atmospheric river?

Atmospheric “rivers” are streams of concentrated moisture in the atmosphere, said Katerina Gonzales, a climate scientist at Stanford University’s Climate and Earth System Dynamics Group.

They’re called “rivers” partly because that’s what they look like on satellite images—long, relatively narrow bands of water—but also because they course through the atmosphere like a river moves across the landscape, Gonzales said.

These powerful rivers in the sky can carry massive volumes of water from the tropics toward the poles. When they bring moisture from regions near Hawaii to the West Coast, they’re often called the “pineapple express.”

“Atmospheric rivers often originate in the atmosphere over ocean basins and are carried along by different storm systems,” said Gonzales, who studies the causes and impacts of extreme precipitation in a warming climate. “When they cross over the ocean to land, they can induce extreme rates of rainfall, due to both the concentrated moisture and the strong winds in the atmospheric river.”

Atmospheric rivers have caused most of the flooding in the western United States, scientists say. These raging rivers in the sky will become more intense as the planet warms.

As global temperatures increase, atmospheric rivers will become wetter, longer and wider, a trend that scientists say is already apparent. Even modest changes in intensity could lead to “significant increases in damages” from flooding, scientists reported in a 2019 study in Science Advances. “The increase in exposure to risk over the coming decades, as population in the western coastal states continues to grow, is likely to drive damages even higher,” the authors warned.

For more information, the National Oceanic & Atmospheric Administration curates an Atmospheric River Portal that provides detailed information as well as current conditions and forecasts related to these weather features. 

Liza Gross

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