Heavy rain also brings heat, scientists say

The environmental threat posed by atmospheric rivers (long ribbons of water vapor suspended in the sky) does not only occur in the form of concentrated torrential rains and severe flooding typical of these natural phenomena. They also cause extremely hot and humid heat waves, according to a new study from Yale University.

Researchers Selena Scholz and Juan Lora say atmospheric rivers – horizontal plumes that transport water vapor from warm subtropics to cooler regions across the world’s mid-latitudes and polar regions – also transport heat. . As a result, atmospheric rivers may have a greater impact on global energy transfer than previously realized.

“We’re seeing temperature anomalies associated with atmospheric rivers that are 5 to 10 degrees Celsius (9 to 18 degrees Fahrenheit) higher than the climate average. The numbers are staggering,” Yale University’s Global said Laura, assistant professor of planetary science. in Arts and Sciences and is a co-author of the new study.

The results of this study are published in the journal Nature.

Scientists began using the term “atmospheric river” in the 1990s. Now there are between three and five of them meandering through each hemisphere at any given time.

It can be thousands of miles long but only a few hundred miles wide. The amount of water vapor they carry is about 7 to 15 times the same amount of water that the Mississippi River discharges each day. Frequent heavy rains can cause significant damage and disruption, such as the Oroville Dam crisis in California in 2017 and the UK floods in 2019-2020.

“Historically, they have been defined by how much water they carry,” says Scholz, a graduate student in Lora’s lab and lead author of the study. “People knew that warmth was inherent, but with so much rain, the focus has been on moisture.”

New research suggests it’s also worth looking at atmospheric river temperatures.

Scholz and Lora analyzed 40 years of global weather data from NASA’s MERRA-2 reanalysis and seven publicly available algorithms that track atmospheric rivers around the world. Specifically, we examined the temperature increases associated with atmospheric rivers on two timescales: hourly temperature spikes and moist heat heat waves lasting more than three days.

“There was no doubt about it: atmospheric rivers have a large impact on both time scales,” Scholz said.

The researchers noted that this phenomenon has a more dramatic impact in winter than in summer. This trend actually helped inspire the project in the first place. Laura noticed that Connecticut’s winters have been particularly mild and wet in recent years, so he and Scholz decided to investigate heat transport in atmospheric rivers.

“And the numbers were so interesting that it turned into a worldwide study,” Laura said.

Although other studies have addressed the role of temperature in atmospheric rivers at high latitudes, this is the first study to focus on mid-latitude regions, where there are several “hot spots” for atmospheric rivers. be. These hotspots include the east and west coasts of North America, Western Europe, Australia, and the southern regions of South America.

Perhaps the best-known circulating atmospheric river is the “Pineapple Express” system, which brings warm moisture from the tropics and brings rain and heavy snow to the west coasts of Canada and the United States.

A new study shows that when atmospheric rivers occur, they alter the energy balance at the Earth’s surface in several ways, researchers say. For example, cloudy conditions prevent sunlight from entering, but those clouds trap more heat radiation near the surface, creating a temporarily enhanced greenhouse effect. This heating offsets the loss of sunlight but does not cause the temperature to rise.

Rather, the main cause of atmospheric river temperatures rising is simply the movement of warm air near the water surface from one region to another.

“When we tried to understand why this was happening, we expected to find that there was a temporary greenhouse effect going on,” Scholz said. “But that’s just heat moving from one area to another through the river.”