Atmospheric rivers - those long, narrow bands of water vapor in the air that bring heavy rains and storms to the US West Coast and many other regions - are shifting to higher latitudes, changing weather patterns around the world.
This shift is worsening droughts in some regions, intensifying flooding in others, and endangering the water supplies on which many communities depend. When atmospheric rivers reach far north into the Arctic, they can also melt sea ice, affecting global climate.
In a new study published in Science Advances, University of California, Santa Barbara, climate scientist Qinghua Ding and I show that atmospheric rivers have shifted about 6 to 10 degrees toward the two poles over the past four decades.
Atmospheric rivers in motion
Atmospheric rivers are not just a thing of the American West Coast. They originate in many parts of the world and provide more than half of the average annual runoff in these regions, including the southeast and west coasts of the US, Southeast Asia, New Zealand, northern Spain, Portugal, the United Kingdom and south-central Chile.
California depends on atmospheric rivers for up to 50% of its annual rainfall. A series of atmospheric rivers in winter could bring enough rain and snow there to end a drought, as parts of the region saw in 2023.
Although atmospheric rivers have a similar origin-moisture supply from the tropics-the atmospheric instability of the jet stream causes them to bend poleward in different ways. No two atmospheric rivers are exactly the same.
What climate scientists, and we too, are particularly interested in is the collective behavior of atmospheric rivers. Atmospheric rivers are commonly seen in the extratropics, an area between latitudes 30 and 50 degrees in both hemispheres that includes most of the continental US, southern Australia and Chile.
Our research shows that atmospheric rivers have shifted polarward over the past forty years. In both hemispheres, activity has increased along 50 degrees north latitude and 50 degrees south latitude since 1979, while it has decreased along 30 degrees north latitude and 30 degrees south latitude. In North America, that means more atmospheric rivers that soak British Columbia and Alaska.
A global chain reaction
A major reason for this shift is changes in sea surface temperatures in the eastern tropical Pacific. Since 2000, waters in the eastern tropical Pacific Ocean have tended to cool, affecting global atmospheric circulation. This cooling, often associated with La Niña conditions, pushes atmospheric rivers toward the poles.
The poleward movement of atmospheric rivers can be explained as a chain of interconnected processes.
During La Niña conditions, when sea surface temperatures cool in the eastern tropical Pacific, the Walker circulation - giant air loops that influence precipitation as it rises and falls over different parts of the tropics - strengthens over the western Pacific. This stronger circulation causes the tropical rain belt to expand. The extensive tropical rainfall, combined with changes in atmospheric vortex patterns, results in high-pressure anomalies and wind patterns that send atmospheric rivers further poleward.
Conversely, during El Niño conditions, with warmer temperatures at the sea surface, the mechanism works in the opposite direction, causing atmospheric rivers to shift so they are not as far from the equator.
The shifts raise important questions about how climate models predict future changes in atmospheric rivers. Current models may underestimate natural variability, such as changes in the tropical Pacific Ocean, which can significantly affect atmospheric rivers. Understanding this connection can help forecasters make better predictions about future precipitation patterns and water availability.
Why does this poleward shift matter?
A shift in atmospheric rivers could have major consequences for the local climate.
In the subtropics, where atmospheric rivers are becoming less common, the result could be longer droughts and less water. Many areas, such as California and southern Brazil, rely on atmospheric rivers for rain to fill reservoirs and support agriculture. Without this moisture, these areas could experience more water shortages, putting pressure on communities, farms and ecosystems.
At higher latitudes, atmospheric rivers moving toward the Arctic could lead to more extreme rainfall, flooding and landslides in places like the US Pacific Northwest, Europe and even the Arctic.
In the Arctic, more atmospheric rivers could accelerate the melting of sea ice, increasing global warming and affecting the animals that rely on the ice. A previous study I was involved in found that the trend in summer atmospheric river activity may contribute 36% of the increasing trend in summer humidity across the Arctic since 1979.
What it means for the future
So far, the shifts we've seen still mainly reflect changes due to natural processes, but human-induced global warming is also playing a role. Global warming is expected to increase the overall frequency and intensity of atmospheric rivers because a warmer atmosphere can hold more moisture.
How that might change as the planet continues to warm is less clear. Predicting future changes remains uncertain, largely due to the difficulty in predicting the natural fluctuations between El Niño and La Niña, which play an important role in atmospheric river shifts.
As the world warms, atmospheric rivers - and the critical rainfall they bring - will continue to change course. We must understand and adapt to these changes so that communities can continue to thrive in a changing climate.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Zhe Li, University company for atmospheric research Read more: Zhe Li 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.