Atmospheric rivers are reaching farther north with greater frequency than they were 4 decades ago, according to new research. These lofted highways of water vapor are dumping rain on recovering Arctic sea ice during the winter, when ice should be at its peak.
At any given time, multiple atmospheric rivers are moving more than a Mississippi River’s worth of water from the equator to higher latitudes. When researchers first described the phenomenon several decades ago, it was seen as a midlatitude event, associated with flooding in California and snowmelt in the Pacific Northwest. But recently, atmospheric rivers have been snaking their way to the poles as well. A new study definitively links these extreme weather events with broader trends in Arctic sea ice loss.
Pengfei Zhang, an atmospheric scientist at Pennsylvania State University, began studying Arctic atmospheric rivers 2 years ago when he noticed an interesting trend. Whenever an atmospheric river stretched far enough north to reach the Arctic, sea ice immediately retreated. The effect was most pronounced in the dark months of winter, when Arctic sea ice was supposed to be recovering from summer losses to reach its maximum extent around late February or March.
“This means that the sea ice cover isn’t able to extend to the maximum amount allowed by freezing temperatures,” Zhang said.
Looking at satellite observations and meteorological data sets back to 1979, Zhang and his colleagues found that the number of atmospheric rivers that reached the Arctic had increased. At the same time, peak winter sea ice extent decreased.
Atmospheric rivers contribute to sea ice melting in a few key ways. The water vapor they contain traps more heat than dry polar air does, and this heat radiates down to the ice below. Heat is also released when water vapor forms rain or snow droplets. Finally, when rain reaches the surface, the warm droplets melt ice on contact.
The impact is pronounced enough that the ice’s retreat can be seen on satellite imagery within a few days of an atmospheric river storm, as though a giant blow-dryer were aimed at the ice margin. When atmospheric rivers become more frequent, the number of setbacks per season increases too.
Using a combination of the historic satellite data and climate modeling, the team determined that about a third of recent sea ice decline in parts of the Arctic could be attributed to the dampening effect that increasingly common atmospheric rivers have on sea ice recovery. The impact was most pronounced in areas with the highest number of atmospheric rivers, including the Barents and Kara seas.
Because of the storms, when the Arctic warms in spring and summer, there is less sea ice to melt, and the dark ocean surface is exposed faster. These waters absorb more sunlight, which drives faster warming and even more melting.
The researchers used large-ensemble climate models, including LENS2 (Community Earth System Model 2 (CESM2) Large Ensemble Community Project), to determine how much the increase in atmospheric rivers could be attributed to climate change. They found that 68% of the increasing trend in atmospheric river frequency could be attributed to human-caused climate change, though natural climate variability like the Interdecadal Pacific Oscillation also played a role.
The work is “impressive,” said Jonathan Wille, a polar meteorologist at ETH Zürich. “The authors of this study did an excellent job of quantifying the negative impacts of atmospheric rivers on early season sea ice growth.”
Bracing for a Wetter, Warmer Arctic
A wetter and warmer Arctic will have far-reaching consequences. Increasingly frequent atmospheric rivers will make the Arctic a stormier place, with bigger waves that could further hinder ice formation. This more extreme environment could have a number of impacts on Arctic ecosystems. More sunlight hitting open ocean as a result of less sea ice, for instance, is already causing phytoplankton blooms to begin earlier and end later, with impacts cascading across the food web. On a global scale, sea ice retreat could slow the ocean conveyor belt known as the Atlantic Meridional Overturning Circulation, which could lead to droughts or sea level rise thousands of kilometers from the Arctic.
“Unfortunately, there are many negative feedback loops related to sea ice melt, and this study highlights one of these processes,” Wille said.
—Rachel Fritts (@rachel_fritts), Science Writer