Eastern US Getting More Extreme Winter Weather From Atmospheric Rivers

Chaotic winter weather is becoming the norm as natural phenomena known as atmospheric rivers get more and more common in the U.S., research has found.

Over the past 40 years, these long, narrow atmospheric systems have increased in frequency over the eastern U.S., mainly due to changes in the jet stream, the study in the journal Science Advances reveals.

Atmospheric rivers carry large volumes of water vapor, which can be released as huge amounts of rain or snow as the system passes over land. According to the National Oceanic and Atmospheric Administration (NOAA), the average one carries the same amount of water as the average flow at the mouth of the Mississippi River, but the most powerful can contain 15 times that.

"Using multiple observational datasets, we find that there is a significant increase in AR [atmospheric river] frequency over the EUS [eastern U.S.] during the past four decades. This trend is mainly driven by changes in large-scale circulations, particularly the poleward shift and strengthening of the mid-latitude jet stream over North America," the researchers wrote in the paper.

From Snowfall to Flash Floods

Atmospheric rivers usually form in the tropics, moving north towards the U.S. mainland, where they can result in severe flash flooding and landslides, which can lead to damage to infrastructure and danger to life. The Pineapple Express is a common nickname for a type of strong atmospheric river that forms near Hawaii, bringing moisture over to the U.S. West Coast.

"While they act as the drought buster along the US West Coast: 33 to 74 percent of the persistent droughts from 1950 to 2010 were ended by the arrival of landfalling AR storms, they are also the primary drivers of flood damages there: ARs generated most of the observed high daily runoff in the western US (WUS) during water years 1949–2015," the authors wrote.

In the early months of 2023, several successive Pineapple Express brought huge amounts of snowfall to the west, resulting in record-breaking snowpacks in the Rocky Mountains and the Sierra Nevadas. When spring came, this then led to widespread chaos as the snow melted into water, leading to flash floods.

In the eastern states, atmospheric rivers form over the Gulf of Mexico, pulling moisture northwards. They are more common there than in the west, but are usually less powerful and lead to less rainfall.

"Approximately 30 to 35 percent of annual precipitation over a broad area of EUS can be attributed to AR activity, and ARs are responsible for over 60 percent extreme precipitation events and more than 70 percent flood events across the EUS," wrote the authors.

Impact of Jet Stream Shift

Now, however, atmospheric rivers over the eastern states are becoming more frequent, due to changes in the Pacific/North America (PNA) climate pattern that have caused the jet stream to move further north.

"The PNA pattern has been found to play a critical role in this transition. As the PNA pattern shifts toward a more negative phase, it favors the northward transport of warm and moist air from the Gulf of Mexico and the Atlantic, associated with more [atmospheric rivers] affecting the [eastern U.S.]," the authors wrote in the paper. "Our study revealed that the PNA pattern has a stronger influence on AR variability over the EUS than other climate modes."

With increased numbers of atmospheric rivers being flung across the eastern U.S, this could lead to increased frequencies of winter storms, and resultant damage.

"There are numerous hazards as we can see from recent events, snow, ice, strong winds, coastal erosion and high water levels when impacting the coastline, thunderstorms, heavy rainfall, tornadoes and hail," Esther Mullens, an assistant professor of geography at the University of Florida, told Newsweek.

A Tool for Flood Management

The authors state in the paper that this finding is important in predicting how precipitation and flooding will change in the coming years.

"With the promising prediction skill of the PNA index at lead time of one to two months during winter, it may provide some potential for subseasonal to seasonal forecasting of AR activity over the EUS, which could empower decision-makers to proactively prepare for potential AR-associated impacts, developing effective strategies to manage water resources and mitigate the effects of extreme weather events."

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