By Ed Brennen
If it seems like you’ve been hearing a lot about “atmospheric rivers” in the news lately, you’re not mistaken.
The weather phenomenon — which is a large, narrow corridor of water vapor traveling through the atmosphere — was responsible for dumping nearly 18 feet of snow on California’s Sierra Nevada mountain range in December. A few weeks earlier, an atmospheric river triggered record rainfall and massive flooding in Washington state and British Columbia.
“If you are in space looking down at Earth, they look like long rivers of clouds,” says Asst. Prof. of Environmental, Earth and Atmospheric Sciences Christopher Skinner, who researches atmospheric rivers.
Skinner and a colleague at Yale University, Juan Lora, are collaborating on research to understand the importance of atmospheric rivers in Earth’s climate history — work that was funded by a three-year, $225,000 National Science Foundation grant in 2019. Using a supercomputer run by the National Center for Atmospheric Research in Cheyenne, Wyoming, they have been conducting a series of lengthy simulations to understand how atmospheric rivers respond to changes in our planet’s temperature, carbon dioxide levels and other variables.
“In today’s climate, atmospheric rivers are hugely important for moving water from low latitudes to high latitudes. We don’t know whether this has always been the case,” says Skinner, who recently took time to discuss atmospheric rivers — a term that was coined by MIT researchers in the mid-1990s and has gained steam in the mainstream media of late.
Q: For those who aren’t steeped in science, what are atmospheric rivers?
A: Atmospheric rivers are storms that carry a tremendous amount of water. They’re typically about 300 miles wide and more than 1,500 miles long. The flow of water in an atmospheric river is equal to about two and a half times the flow of water that discharges from the Amazon River. On average, there are about five to 10 atmospheric rivers moving through the atmosphere at any one time, and each one lasts for a few days before weakening and falling apart.
Q: How are atmospheric rivers different from hurricanes, which we experience on the East Coast?
A: Atmospheric rivers and hurricanes both form over the oceans, where there is a lot of evaporation, and both can cause extreme precipitation and high winds. However, the similarities end there. Hurricanes are powerful cyclones that form in the tropics. They weaken very quickly once they leave the tropics because they need warm ocean waters to survive. Atmospheric rivers, on the other hand, form outside of the tropics and play a really big role in the weather of the midlatitudes. In terms of impacts, hurricanes generally have much stronger winds and damaging storm surge. Most of the damage from atmospheric rivers occurs in the form of flooding and the associated landslides.
Q: Are atmospheric rivers harmful or helpful?
A: It’s a good question, and the answer is “both.” Occasionally, we can get “stuck” in certain weather patterns. If you’re stuck in a dry pattern, you won’t see very many atmospheric rivers. That can be a problem in a place like California that relies on atmospheric rivers for about 50% of its snowpack, drinking water, agricultural water, etc. However, if you’re stuck in a pattern with persistent atmospheric rivers, it can be a scenario where you’ve got too much of a good thing. One of these patterns plagued the Pacific Northwest this fall, dropping over 2 feet of rain in less than a month on Washington and British Columbia. These types of active atmospheric river patterns can be especially dangerous in regions that experience summer wildfires because of the increased risk for landslides on top of the burned, treeless areas.
Q: Are atmospheric rivers becoming more frequent? If so, is that because of climate change?
A: In a warmer world, the atmosphere above the ocean contains more water vapor. Because the Earth has warmed nearly 2°F over the past 150 years, atmospheric rivers now transport greater amounts of water vapor than they used to. The consequence of this is that when an atmospheric river makes landfall, it can release a greater amount of precipitation, potentially causing greater damage. We expect the world is going to continue to warm this century, and atmospheric rivers will result in more extreme precipitation. Unfortunately, warmer temperatures also mean that the atmospheric river precipitation will increasingly fall as rain instead of snow, even in mountainous regions, leading to greater flood risk and new water management challenges.
Q: What have you learned so far in your research?
A: One of the interesting things we’ve found is that atmospheric rivers have indeed been an important component of Earth’s climate even during periods of Earth’s history that were very different from today. For example, about 20,000 years ago, Earth was in the midst of an ice age. A massive ice sheet covered nearly all of Canada and parts of the northern United States, including Lowell — which was underneath nearly a mile of ice. We’ve found that the margins of the ice sheet acted like mountain ranges, forcing atmospheric rivers upwards in the atmosphere. When atmospheric rivers move higher in the atmosphere, the water vapor they hold condenses and falls to Earth. Though we are still working out all the details, we think atmospheric rivers were really important contributors to the growth of the ice sheet edges in certain parts of North America. We are preparing a manuscript detailing this work now.
Q: Why is it important to understand atmospheric rivers?
A: Knowledge of atmospheric rivers is critical for water resource management, as well as flooding and landslide preparation. The Scripps Institution of Oceanography recently developed a 1-5 scale for atmospheric rivers, similar to the Saffir-Simpson Hurricane Wind Scale, which is a very effective hazard warning system. It assigns a number to impending landfalling atmospheric rivers based on their intensity and potential for beneficial and hazardous impacts that makes preparation for the event easier. More broadly, we need to understand these systems as completely as possible to increase our ability to forecast them on a range of timescales.