More frequent atmospheric rivers hamper Arctic sea ice recovery
Sea ice off the village of Kotzebue in Alaska. (Courtesy of Lamont-Doherty Earth Observatory)
The Arctic is rapidly losing its sea ice, even during the winter months when temperatures are below freezing and the ice is expected to recover from the summer melt. NOW, a new study in the review Natural climate change found that powerful storm systems called atmospheric rivers are partly responsible; the study finds that they increasingly reach the Arctic in winter, slowing sea ice recovery and accounting for one-third of winter sea ice decline.
“The decline in Arctic sea ice is one of the clearest evidences of global warming in recent decades,” said lead author Pengfei Zhang of Penn State University. “Although temperatures in the Arctic are well below freezing, the decline in sea ice in winter is still very significant. And our research shows that atmospheric rivers are a factor in understanding why.
Atmospheric rivers carry large amounts of water vapor in narrow, ribbon-like storm systems that can span thousands of miles and produce extreme rainfall and flooding when they make landfall. These storms regularly affect mid-latitude coastal regions such as California, where atmospheric river events in January dropped huge amounts of rain, leading to catastrophic flooding.
Using satellite observations and climate model simulations, scientists have found that human-induced warming has increased the rate of riverine atmospheric storms in the Arctic. They also found that a major mode of natural climate variability – the so-called Pacific Interdecadal Oscillation – has also contributed to atmospheric changes in rivers in recent years. The effects are particularly evident during the winter ice-growing season in the Barents and Kara Seas off the northern coasts of Norway and Russia.
“We often think of Arctic sea ice decline as a gradual process,” said study co-author L. Ruby Leung of Pacific Northwest National Laboratory. “This study is important because it reveals that the decline in sea ice is due to episodic extreme weather events, (which) have occurred more frequently in recent decades, in part due to global warming.”
The warm moisture carried by these storms increases what scientists call long-wave downward radiation – the heat emitted to earth from the atmosphere. It also produces rain. Either of these phenomena can melt the thin, fragile layer of ice that normally regrows during the winter months.
Using satellite remote sensing imagery, scientists observed the retreat of sea ice almost immediately after atmospheric and river storms, and found that the retreat persisted for up to 10 days.
“When this type of moisture transport occurs in the Arctic, the effect is not just the amount of rain or snow that falls from it, but also the powerful melting effect on the ice,” said Mingfang Tingprofessor at the Columbia Climate School Lamont-Doherty Earth Observatory and co-author of the study.
The melting of Arctic sea ice has wide implications. Open water is darker than ice-covered water and therefore absorbs more solar energy. this process feeds on itself, amplifying the warming of the polar region. Ice-free waters can open up new, more direct shipping lanes and access to minerals and other resources, but can also trigger international geopolitical struggles. Additionally, the melting of fresh water into the salty ocean can impact ocean circulation patterns. The rapid loss of sea ice is eroding Arctic coastlines, disrupting global weather patterns and disrupting Arctic communities and ecosystems, Ting said.
“This study, along with other work that has noted the presence of atmospheric rivers in the tropics, underscores that atmospheric rivers represent a global phenomenon,” said study co-author Bin Guan of the California Institute of Technology. .
Gang Chen, a professor at the University of California, Los Angeles, also contributed to the research. The project received support from the US National Science Foundation, NASA and the US Department of Energy.
Adapted from a Penn State University press release.
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