In the 1990s, scientists began to use the term "atmospheric river" to refer to strips of water vapor in the air that can be as narrow as a few hundred miles wide, but thousands of miles long. They carry massive amounts of water that can cause huge deluges that lead to flooding and other disruptions and damage. Usually, they move water vapor from the subtropics to the relatively cooler midlatitude and polar regions. But they also move heat and energy around the globe. Now, researchers have shown that atmospheric rivers do not only cause torrential downpours, the heat they transport can also lead to extreme heat waves and warmer winters. The findings have been reported in Nature.
"We're seeing temperature anomalies associated with atmospheric rivers that are five to ten degrees Celsius (nine to eighteen degrees Fahrenheit) higher than the climatological mean. The numbers are astounding," study co-author Juan Lora, an assistant professor at Yale University.
Until now, atmospheric rivers ave been defined by the amount of moisture they transport, noted first study author Serena Scholz, a graduate candidate in the Lora lab. While scientists knew they contained heat, the moisture they contain has been the focus of interest, until now.
In this work, the researchers assessed weather data from the past forty years, which was compiled by algorithms that track atmospheric rivers, and information from NASA's MERRA-2. They focused on how these atmospheric rivers affected temperatures on hourly scales, and heat waves on a sclae of more than three humid days.
"There was no doubt—atmospheric rivers are really impactful for both timescales," Scholz said.
The study also determined that these impacts are more significant in the winter compared to summer.
This year, California and the Pacific Northwest have been seriously impacted by a number of atmospheric river events. These are usually known as the Pineapple Express, because they take water from the tropics and move it to the west coast, leading to heavy rain and snow.
These events also alter the energy balance on the Earth's surface, according to this new study. Cloudy conditions caused by these atmospheric rivers can trap thermal radiation near the surface of the planet, which can enhance the greenhouse effect. While this can balance out the lack of sunlight, it does not seem to lead to temperature spikes.
The research concluded that the warm temperatures that come with atmospheric rivers are simply due to the movement of warm air from one place to another.
"As we tried to understand why this is happening, we were expecting to find a transient greenhouse-type effect going on," Scholz said. "But it's just heat moving from one area to another, via the river."
Sources: Yale University, Nature
