By Agnieszka Gautier
Much like a conveyer belt, an atmospheric river transports moisture from the tropical and subtropical oceans and dumps it as rain or snow in cooler regions. These long, narrow bands of concentrated water vapor look like rivers in the atmosphere when viewed from satellite. When they hit coastal mountains or other high terrain, the air rises, cools, and water vapor condenses, resulting in rain or snow.
Atmospheric rivers are not a new weather phenomenon, but their name is. The broad term, atmospheric rivers, was coined in a 1994 study because of their likeness to terrestrial rivers. Decades earlier, subsets of atmospheric rivers were known by names such as the Pineapple Express or Rum Runner Express, based on their origin: Hawaii and Caribbean, respectively.
Atmospheric rivers are especially impactful along western coasts like the US western coast, France, Spain and Portugal, South America, Southeast Asia, and New Zealand. Often, they bring much-needed relief to drought-stricken coasts, but they can also result in flooding and landslides. Some of them that originate in the Atlantic make it all the way into the Arctic, impacting the coasts of Greenland, Norway and even the Svalbard Archipelago, lying near 80 degrees North latitude.
Atmospheric rivers carry much more water, as vapor, than the Earth’s largest terrestrial rivers—like the Amazon and Mississippi, often leading to extreme precipitation, damage, and even the loss of lives. For instance, atmospheric rivers are responsible for more than 80 percent of flooding across the US west coast, costing over $1 billion in damage annually.
Why are we hearing so much about atmospheric rivers?
According to John Cassano, lead scientist at the National Snow and Ice Data Center (NSIDC), there is growing interest in atmospheric rivers, driven by recognition that they are changing. “In the absolute sense, we are probably seeing more atmospheric rivers,” Cassano said, “but in terms of how unusual they are, relative to warmer weather the rest of the year, remains to be better understood.”
Cassano and NSIDC postdoctoral researcher Chen Zhang have attempted to better understand changes in atmospheric rivers. “Tens of different algorithms exist that help detect them,” Cassano said, “but most have been used in the mid-latitudes, so they do not translate well to polar regions.” Using data from the Atmospheric River Tracking Method Intercomparison Project (ARTMIP), Zhang quantified the degree of uncertainty in how atmospheric rivers are defined—either through very restrictive or non-restrictive parameters. “If you have a very restrictive definition, the impact of atmospheric rivers is gigantic in that moment,” Cassano said. “If you use a less restrictive method, the signal is smaller.” Another way to think of this classification is a matter of intensity. The more restrictive the definition, the more intense the event but the less frequent those events will occur.
A warmer climate means more water vapor in the atmosphere, fueling more frequent or larger atmospheric rivers. But the long-term impact may not be so clear cut. “If you look over a single season and add up the cumulative impact of all the weaker but more frequent atmospheric rivers compared to the less frequent but strong ones, what do you see?” Cassano asked. More frequent but less intense atmosphere rivers tend to have a bigger long-term impact because so many form over a long period.
“So, the conclusion, depending on what you’re studying or interested in doing,” Cassano added, “is that you need to be careful in how you identify atmospheric rivers and their impacts.” Research on their impact in the Arctic, however, is a bit clearer because the climate has already changed so drastically, with temperature increasing at two to four times the global average.
Atmospheric rivers in the Arctic
Atmospheric rivers bring in air that is warmer and moister than the air typically found in the polar regions. “If you get an atmospheric river in winter, it is more likely to fall as rain because of its warm temperature,” Cassano said. As this wet layer freezes, it forms an ice crust that can impede travel, starve animals unable to access vegetation beneath the crust, damage infrastructure, and cripple communities dependent on livestock.
Atmospheric rivers also indirectly influence the absorption and release of energy over sea ice and land. “In particular, atmospheric rivers create warm, liquid clouds that impact the radiation budget a lot,” Cassano said. Warm, liquid clouds emit long-wave radiation, which influences warming at the surface of the Arctic in winter.
When scientists discuss white surfaces like sea ice and snow reflecting solar energy, they are talking about short-wave radiation. By contrast, the Earth’s surface—its ice, snow, and land—emit long-wave radiation. The amount of long-wave radiation is proportional to temperature; the warmer the surface, the more energy radiates out. The atmosphere also emits long-wave radiation both downwards and upwards, but not well. “If there are no clouds in the atmosphere, the long-wave radiation coming down tends to be pretty small compared to what’s coming up from the ground,” Cassano said. So, in general, long-wave radiation cools the surface because more is emitted from the ground than gets absorbed from the atmosphere.
But when clouds come in, the energy budget changes. “Clouds are really good at absorbing and emitting long-wave radiation,” Cassano said. A cloud absorbs the long-wave radiation coming from the surface and emits it back to the surface, and an atmospheric river boosts cloud production. “Clouds warm up the surface in general,” Cassano said, “but atmospheric rivers are supercharged at warming the surface.”
During the Arctic’s wintertime polar night, clouds are especially important in warming the surface. Because of the severity of cold temperatures during winter, this does not mean that clouds can cause melt, but they can impede the growth and thickness of sea ice. “The consequence in winter is that it is slowing down sea ice growth,” Cassano said. So, when the sun does come up, sea ice is thinner. “On the fringes of winter, atmospheric rivers can certainly initiate melt, because it can be warm enough to bring the temperature above the melting point,” he added. That is especially important in the spring because they can melt off the snow, exposing the darker sea ice surface, which absorbs more solar energy and melts further.
Atmospheric rivers and ice sheets
Overall, in the Southern Hemisphere, atmospheric rivers bring heavy snowfall that help maintain the Antarctic Ice Sheet’s surface mass balance, defined as the total input of snowfall minus evaporation and runoff from surface melt. “It’s just so cold there that only the fringes of the continent may worry about the melt from the warming cloud effect,” Cassano said. On the Antarctic Peninsula, where temperatures are rising at three to four times the global rate, atmospheric rivers have been linked to melt events over the ice shelves, the extensions of land ice that float on the ocean.
For the Greenland Ice Sheet, atmospheric rivers are often tied to melt events. According to a 2024 study with Zhang and Cassano, western Greenland especially had significant influence of long-wave radiation from atmospheric rivers, amplifying surface warming year-round by 54 percent. This holds significance for melt events, particularly during summer.
Studying atmospheric rivers
Atmospheric rivers continue to be a blooming area of research. The international Arctic Rain on Snow Study (AROSS), led by NSIDC, for instance, studies atmospheric rivers in relation to rain-on-snow events. “There are definitely more rain-on-snow events from atmospheric rivers in the Arctic,” Cassano said. AROSS has compiled an inventory of such events from 1979 to present in a data set housed at NSIDC.
Cassano added, “The community has jumped on how atmospheric rivers are important. Let’s study them and give them attention, but careful thought needs to go into how we are defining them and the consequences of those parameters.” Cassano hopes to eventually define atmospheric rivers in a way that makes sense under a changing climate, to really see if they are morphing as well. By having a moving threshold of what constitutes an atmospheric river, scientists can begin to truly understand if bigger and more frequent storms are occurring beyond the Arctic. “We’re trying to remove the simple dynamic that a moister atmosphere leads to more atmospheric rivers,” Cassano said. “It’s not such an easy answer.”
References
Serreze, M. C., A. P. Barrett, and J. Voveris. 2021. Inventory of Arctic Rain on Snow Events: Meteorological and Surface Conditions. (NSIDC-0767, Version 1). [Data Set]. Boulder, Colorado USA. National Snow and Ice Data Center, doi:10.7265/f5bv-1z09. Date Accessed 04-23-2025.
Serreze, M. C., J. Voveris, A. P. Barrett, S. Fox, P. D. Blanken, and A. Crawford. 2022. Characteristics of extreme daily precipitation events over the Canadian Arctic. International Journal of Climatology, doi: 10.1002/joc.7907.
Venton, Danielle. 2024. Atmospheric rivers aren't new. Why does it feel like we're hearing about them more? National Public Radio. https://www.npr.org/2024/11/22/nx-s1-5198888/atmospheric-rivers-califor…;
Zhang, C., J. J. Cassano, M. Seefeldt, H. Wang, W. Ma, and Tung. 2024. Quantifying the Impacts of Atmospheric Rivers on the Surface Energy Budget of the Arctic Based on Reanalysis. EGUsphere, doi:10.5194/egusphere-2024-320.