A series of atmospheric rivers is bringing the threat of heavy downpours, flooding, mudslides and avalanches to the Pacific Northwest and California this week. While these storms are dreaded for the damage they can cause, they are also essential to the region’s water supply, particularly 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.
- On average, atmospheric rivers have about twice the regular flow of the Amazon River.
- When atmospheric rivers run up against mountains or run into local atmospheric dynamics and are forced to ascend, the moisture they carry cools and condenses, so they can produce intense rainfall or snowfall.
Why are back-to-back atmospheric rivers a high flood risk?
- Consecutive atmospheric rivers, known as AR families, can cause significant flooding.
- In the meantime, there may be snowmelt due to warm temperatures, further adding to the runoff and flood risk.
- California experienced a historic run of nine consecutive atmospheric rivers in the span of three weeks in December 2022 and January 2023.
- Compared with single atmospheric river events, AR families tend to be associated with lower atmospheric pressure heights across the North Pacific, higher pressure heights over the subtropics, a stronger and more zonally elongated jet stream and warmer tropical air temperatures.
- An active MJO shift occurred during the early 2023 events, tilting 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 when the storms arrive back to back than they would have caused by themselves.
How important are atmospheric rivers to the West Coast’s water supply?
- Although they can lead to flood hazards, atmospheric rivers are also essential to the Western water supply.
- Atmospheric rivers provide an average of 30% to 50% of the West Coast’s annual precipitation.
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, work on improving 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 is global warming affecting 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.
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 beyond their academic appointment.
