Atmospheric rivers, long bands of water vapor carried west to east across the Pacific Ocean by fast high-altitude winds, have cost the Western United States billions of dollars in flood damage over the last four decades.

In a study published this week in the journal Science Advances, scientists at the Scripps Institution of Oceanography at the University of California San Diego and the U.S. Army Corps of Engineers analyzed 40 years of flood damage data collected by the National Flood Insurance Program.

F. Martin Ralp and his colleagues at Scripps created a new intensity scale last year to describe the strength of atmospheric rivers. For the new study, Ralp and his research partners used the scale to better understand the relationship between atmospheric rivers and flood damage.

Like the scale used to grade hurricanes, the atmospheric river scale uses numbers 1 through 5 to characterize the river's strength. The scale also uses corresponding categories: weak, moderate, strong, extreme and exceptional.

Two main factors determine an atmospheric river's grade: the amount of water vapor it's carrying and the amount of time it takes for a river to move across a specific area.

The latest analysis confirmed what scientists suspected, that weak and moderate atmospheric rivers are responsible for minimal flood damage. Category 3 to 5 atmospheric rivers, however, cause billions of dollars in flood damage.

Scientists determined atmospheric rivers were responsible for 99 percent of the $51 billion in flood damages to western states over the last 40 years. Just ten atmospheric rivers were responsible for nearly half of the total, $23 billion.

"A small number of extreme ARs cause most of the flood damages in the West, and even modest increases in intensity could significantly increase their impacts," lead study author Tom Corringham, a postdoctoral researcher at Scripps, said in a news release.

Climate scientists predict atmospheric rivers will grow more intense as global warming continues. Flood damage totals are likely to increase as a result.

Authors of the new study hope ongoing research into the relationships between climate and atmospheric rivers will help scientists better predict the paths of these giant storms. Research into the dynamics of atmospheric rivers can also help policy makers craft better flood mitigation policy and develop more strategic disaster response plans.

"This is a reminder that weather and climate matter," Corringham said. "Every step we take now to stabilize the global climate system stands to reduce future adverse impacts on our economy."

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Marseille (AFP) Dec 2, 2019
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