A. series of atmospheric rivers It brings heavy showers, flooding, mudslides and avalanche threats to the Pacific Northwest and California in early 2024. Although these storms are feared for the damage they can cause, they are also vital to the water supply of the region, especially California. explains Qian Cao, a hydrologist at the University of California, San Diego.
What are atmospheric rivers?
An atmospheric river is a narrow corridor or filament of concentrated water vapor carried in the atmosphere. It is like a river in the sky that can be 1000 miles long. On average, atmospheric rivers have flows approximately twice the normal flow of the Amazon River.
When atmospheric rivers flow toward mountains or are forced to rise by colliding with local atmospheric dynamics, the moisture they carry cools and condenses, so heavy precipitation or snowfall can occur.
Atmospheric rivers occur throughout the world, most commonly in mid-latitudes. They form when large-scale weather patterns align to form narrow channels or filaments that enable intense moisture transport. These originate over warm water, usually tropical oceans, and are driven ashore by low-level jet streams ahead of the cold fronts of extratropical cyclones.
Along the West Coast of the United States, the Pacific Ocean acts as a reservoir of moisture for the storm and mountain ranges act as barriers; This is why the western sides of the coastal ranges and the Sierra Nevada get so much rain and snow.
Why do back-to-back atmospheric rivers pose a high flood risk?
Cascading atmospheric rivers, known as AR families, can cause significant flooding.
The first heavy downpours saturate the soil. As successive storms arrive, their precipitation falls on soil that cannot absorb any more water. This contributes to more current. Rivers and streams are filling up. Meanwhile, melting snow due to hot weather will also increase the risk of surface runoff and floods.
California experienced a historic influx of nine consecutive atmospheric rivers over three weeks in December 2022 and January 2023. The storms helped most reservoirs return to historical averages in 2023 after many dry years, but also caused damaging floods and debris flows.
The causes of AR families are an active area of research. Compared to single atmospheric fluvial events, AR families tend to be associated with lower atmospheric pressure highs across the North Pacific, higher pressure highs in the subtropics, a stronger and zonally longer jet stream, and warmer tropical air temperatures.
Large-scale weather patterns and climate events such as the Madden-Julian Oscillation, or MJO, also play an important role in the formation of AR families. An active MJO replacement occurred during events in early 2023, making it less likely that atmospheric river activity in California will increase.
A recent study by scientists at Stanford and the University of Florida found that storms in AR families cause three to four times more economic damage when back-to-back than the storms could cause on their own.
How important are atmospheric rivers for the West Coast’s water supply?
I am a research hydrologist, so I focus on the hydrological impacts of atmospheric rivers. Although they can pose flood hazards, atmospheric rivers are also essential to the West’s water supply. Atmospheric rivers have been responsible for ending more than a third of major droughts in the region, including the severe California drought of 2012-16.
Atmospheric rivers provide an average of 30% to 50% of the West Coast’s annual precipitation.
They also contribute to California’s snowpack, which provides a significant portion of its year-round water supply.
In an average year, one to two extreme atmospheric rivers filled with snow will be the major contributors to snowpack in the Sierra Nevada. Together, atmospheric rivers will contribute approximately 30% to 40% of the total snow accumulation in an average season there.
That’s why my colleagues at the Western Weather and Water Extremes Center at the Scripps Institution of Oceanography, part of the University of California at San Diego, are working to improve atmospheric river forecasts and forecasts. Water managers need to be able to regulate reservoirs and calculate how much water they can save for the dry season, while also leaving room in reservoirs to manage the risk of flooding from future storms.
How does global warming affect atmospheric rivers?
As global temperatures rise in the future, we can expect atmospheric rivers to become more intense, leading to an increase in intense and extreme precipitation events.
My research also shows that more atmospheric rivers are likely to form simultaneously under already wet conditions. Therefore, the possibility of extreme flooding increases. Another study by scientists at the University of Washington suggests that there will be a seasonal shift to more atmospheric rivers early in the rainy season.
According to research by my colleagues at the Western Weather and Extreme Water Center projects, there will likely be greater year-to-year variability in total annual precipitation, especially in California.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and analysis to help you understand our complex world.
Written by: Qian Cao, University of California, San Diego.
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Qian Cao does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations other than academic appointments.