by A series of atmospheric rivers bringing the threat of heavy downpours, flooding, mudslides and tornadoes to the Pacific Northwest and California in early 2024. While these storms are feared for the damage they can cause, they are also critical for 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 passage or filament of condensed water vapor carried in the atmosphere. It is like a river in the sky that can be 1,000 miles long. On average, the atmospheric rivers of the Amazon have about twice the regular flow.
When atmospheric rivers run up against mountains or run into local atmospheric dynamics and are forced up, the moisture they carry cools and condenses, so they can produce heavy rain or snowfall.
Atmospheric rivers are found throughout the world, often in the mid-latitudes. They form when large-scale weather patterns align to create narrow channels, or filaments, of intense moisture transport. These begin over warm water, usually the tropical oceans, and are directed towards the coast by low-level jet streams ahead of the cold fronts of extratropical cyclones.
On the US West Coast, the Pacific Ocean acts as a moisture reservoir for the storm, and the mountain ranges act as barriers, which is why behind the coast ranges and the Sierra Nevada sees so much rain and snow.
Why are back-to-back atmospheric rivers at high risk of flooding?
Consecutive atmospheric rivers, known as AR families, can lead to significant flooding.
The first heavy downpours soak the ground. As storms develop, their precipitation falls on soil that is unable to absorb more water. That contributes to more runoff. Rivers and streams fill up. Meanwhile, warm temperatures may lead to snowmelt, further increasing the risk of runoff and flooding.
California had nine consecutive atmospheric rivers over three weeks in December 2022 and January 2023. The storms helped bring most reservoirs back to historic averages in 2023 after several drought years, but they also created damaging floods and debris flows.
The familial cause of AR is an active area of research. Compared to individual atmospheric river events, AR families tend to be associated with lower atmospheric pressure heights across the North Pacific, higher pressure heights over the subtropics, longer and more zonal jet streams and air temperatures warmer tropical.
Large-scale weather patterns and climate phenomena such as the Madden-Julian Oscillation, or MJO, also play an important role in generating AR families. An active MJO transition occurred during events in early 2023, increasing the likelihood of atmospheric river activity over California.
A recent study by scientists at Stanford and the University of Florida found that storms within AR households cause three to four times more economic damage when the storms occur back-to-back than the storms themselves.
How important are atmospheric rivers to the West Coast water supply?
I am a research hydrologist, so I focus on the hydrological impacts of atmospheric rivers. Although they can lead to flood hazards, atmospheric rivers are also essential to the West’s water supply. Atmospheric rivers were responsible for ending more than a third of the region’s major droughts, including California’s severe drought of 2012-16.
Atmospheric rivers provide an average of 30% to 50% of the West Coast’s annual precipitation.
They also contribute to the snowpack, which provides a significant portion of California’s year-round water supply.
In an average year, the largest contributors to the snowpack in the Sierra Nevada are one to two large atmospheric rivers with snow. Together, the atmospheric rivers will contribute about 30% to 40% of the total snow accumulation in the normal season.
That’s why my colleagues in the Center for Weather and Western Water Extremes at the Scripps Institution of Oceanography, part of the University of California, San Diego, work on improving river atmospheric forecasts and predictions. Water managers need to be able to control reservoirs and figure out how much water they can save for the dry season and leave room in the reservoirs to manage flood risk from future storms.
What is the effect of global warming on atmospheric rivers?
As global temperatures rise in the future, we can expect more intense atmospheric rivers, leading to an increase in heavy and extreme precipitation events.
My research also shows that more atmospheric rivers are likely to occur concurrently during already wet conditions. Thus, the chance of major floods also increases. Another study, by scientists from the University of Washington, suggests a seasonal shift to more atmospheric rivers earlier in the rainy season.
There is likely to be more year-to-year variability in total annual precipitation, especially in California, according to a study conducted by my colleagues in the Extreme Western Weather and Water Center projects.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of 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 this, and has not disclosed any relevant connections beyond their academic appointment.
