a series of atmospheric rivers brings the threat of heavy rains, flooding, mudslides and avalanches to the Pacific Northwest and California this week. While these storms are feared for the damage they can cause, they are also essential to the region’s water supply, especially in California, as Qian Cao, a hydrologist at the University of California, San Diego, explains.
What are atmospheric rivers?
An atmospheric river is a narrow corridor or filament of concentrated water vapor transported in the atmosphere. It is like a river in the sky that can be 1,000 miles long. On average, atmospheric rivers have about twice as much flow as the Amazon.
When atmospheric rivers run into mountains or encounter local atmospheric dynamics and are forced to rise, the moisture they carry cools and condenses, so they can produce intense rainfall or snowfall.
Atmospheric rivers occur all over the world, mostly in mid-latitudes. They form when large-scale weather patterns align and create narrow channels or filaments of intense moisture transport. These begin over warm waters, typically tropical oceans, and are guided offshore by low-level jet streams ahead of cold fronts of extratropical cyclones.
Along the US West Coast, the Pacific Ocean acts as the reservoir of moisture for the storm, and the mountain ranges act as barriers. That’s why the western sides of the coastal areas and the Sierra Nevada see so much rain and snow.
Why do successive atmospheric rivers pose a high flood risk?
Successive atmospheric rivers, known as AR families, can cause significant flooding.
The first heavy rain showers saturate the ground. As successive storms arrive, precipitation falls on ground that can no longer absorb more water. That contributes to more drainage. Rivers and streams are filling up. In the meantime, snow may melt due to warm temperatures, further increasing the risk of runoff and flooding.
California experienced a historic series of nine consecutive atmospheric rivers in a three-week span in December 2022 and January 2023. The storms helped return most reservoirs to historical averages by 2023 after several years of drought, but also caused damaging flooding and debris flows.
The cause of AR families is an active area of research. Compared to individual atmospheric river events, AR families are generally associated with lower atmospheric pressure heights in the North Pacific, higher pressure heights over the subtropics, a stronger and more zonally extended jet stream, and warmer tropical air temperatures.
Large-scale weather patterns and climate phenomena such as the Madden-Julian Oscillation, or MJO, also play an important role in generating AR families. During the early 2023 events, an active MJO shift occurred, shifting the odds toward increased atmospheric river activity over California.
A recent study by scientists at Stanford and the University of Florida found that storms within AR families cause three to four times more economic damage if the storms arrive one after the other than they would have caused on their own.
How important are atmospheric rivers to the West Coast’s water supply?
I’m a research hydrologist, so I focus on the hydrological effects of atmospheric rivers. Although they can lead to flooding hazards, atmospheric rivers are also essential to the Western water supply. Atmospheric rivers are responsible for ending more than a third of the region’s major droughts, including California’s severe drought from 2012-2016.
Atmospheric rivers provide an average of 30% to 50% of the annual precipitation on the West Coast.
They also contribute to the snowpack, which provides a significant portion of California’s water supply year-round.
In an average year, one to two extremely atmospheric snowpack rivers will be the dominant contributors to the Sierra Nevada snowpack. Together, atmospheric rivers will contribute about 30% to 40% of the total snow accumulation there in an average season.
That’s why my colleagues at the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, part of the University of California, San Diego, are working to improve atmospheric river forecasts and predictions. Water managers need to be able to regulate reservoirs and figure out how much water they can save for the dry season while still leaving room in the reservoirs to manage flood risk from future storms.
How does global warming affect atmospheric rivers?
As global temperatures rise in the future, we can expect more intense atmospheric rivers, which will lead to an increase in heavy and extreme precipitation events.
My research also shows that more atmospheric rivers are likely to form at the same time in already wet conditions. The risk of extreme flooding therefore also increases. Another study, conducted by scientists at the University of Washington, suggests that a seasonal shift to more atmospheric rivers will occur earlier in the rainy season.
There will also likely be more year-to-year variation in total annual precipitation, especially in California, as shown by a study by my colleagues at the Center for Western Weather and Water Extremes projects.
This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.
It was written by: Qian Cao, University of California, San Diego.
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Qian Cao does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.