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Rivers in the sky prevent the seasonal recovery of Arctic sea ice

The Arctic is currently warming faster than the global average and this is contributing to the seasonal melting of sea ice, ice sheets and glaciers. However, winter temperatures in the region are still well below freezing and this should give the ice an opportunity to recover from the summer melt. However, a new study led by scientists from Pennsylvania State University has identified that powerful storms, brought by atmospheric rivers, are increasingly reaching the Arctic in winter and are slowing down the recovery of the sea ice. 

“Arctic sea ice decline is among the most obvious evidence of global warming from the past several decades,” said Pengfei Zhang, assistant research professor of meteorology and atmospheric science at Penn State and lead author of the study. “Despite temperatures in the Arctic being well below freezing, sea ice decline in winter is still very significant. And our research shows atmospheric rivers are one factor in understanding why.”

Atmospheric rivers are typically long, narrow bands in the atmosphere – like rivers in the sky – that carry large amounts of condensed water vapor away from the tropics towards lower latitudes. Likened to great conveyer belts of water in the sky, they play a central role in the global water cycle. The water-laden plumes move with the weather and, when they make landfall, often release this water vapor in the form of rain. 

Although atmospheric rivers come in many sizes, those that contain the largest amounts of water vapor and bring the strongest winds can give rise to extreme rainfall and floods. The very largest of these carry a greater flux of water than the Amazon River. There are typically 3–5 atmospheric rivers present within a hemisphere at any one time, and they have increased slightly in intensity over the past century. 

Using satellite observations and climate model simulations, the Penn State scientists and their colleagues found that these storms are increasingly reaching the Arctic – particularly the Barents and Kara seas off the northern coasts of Norway and Russia – during the winter ice-growing season. The rain that they bring is slowing sea ice recovery and accounts for a third of all winter sea ice decline. These findings are published in the journal Nature Climate Change.

“We often think that Arctic sea ice decline is a gradual process due to gradual forcings like global warming,” said study co-author L. Ruby Leung, Battelle Fellow at Pacific Northwest National Laboratory. “This study is important in that it finds sea ice decline is due to episodic extreme weather events – atmospheric rivers – which have occurred more frequently in recent decades partly due to global warming.”

According to the researchers, the storms brought by atmospheric rivers affect ice melt in two important ways. Firstly, warm moisture carried from the tropics falls on the Arctic ice at a time when the thin, fragile ice cover is regrowing during winter. Secondly, the storms increase long-wave radiation, or heat, emitted back to Earth from the atmosphere. Both these factors slow down the recovery of sea ice during the cold months. 

Using satellite remote sensing images, the scientists observed that sea ice retreated almost immediately following atmospheric river storms and that the retreat persisted for up to 10 days. Because of this melting and because the storms are becoming more common, atmospheric rivers are slowing down seasonal sea-ice recovery in the Arctic, the scientists said.

“When this kind of moisture transport happens in the Arctic, the effect is not only the amount of rain or snow that falls from it, but also the powerful melting effect on the ice,” said study co-author Mingfang Ting, a professor at Lamont-Doherty Earth Observatory, Columbia University. “This is important since we are losing Arctic sea ice fast in the past few decades that brought many unwanted consequences such as Arctic warming, erosion of Arctic coastlines, disturbance to global weather patterns and disruption to the Arctic communities and ecosystems.”

The loss of Arctic sea ice has broad implications, the scientists said. Open waters may enable new, more direct shipping routes but also trigger geopolitical concerns between countries. Additionally, freshwater melting into the salty ocean may impact oceanic circulations patterns that stabilize global temperatures. From a biological perspective, sea ice cover is important for the successful reproduction of many animal species, and reductions in cover will negatively impact breeding success. 

“Those factors make this study especially important from a science perspective, but also from social and security perspectives,” said study co-author Professor Laifang Li of Penn State. “Sea ice melting has a big impact for the climate system and for society, and our study finds the Arctic is an open system and that climate change is way more complicated than temperature change alone can explain.”

Using large-ensemble climate models, the scientists determined that human-induced warming has increased the rate of atmospheric river storms in the Arctic. But they also found that one major mode of natural climate variabilities – the so-called Interdecadal Pacific Oscillation – also contributes to atmospheric river changes.

“This study, together with other work that noted the presence of atmospheric rivers in the tropics, highlights that atmospheric rivers represent a global phenomenon,” said Bin Guan, Earth systems scientist at the University of California, Los Angeles and Jet Propulsion Laboratory, California Institute of Technology. 

“Since they were discovered relatively recently – in the 1990s, and even more recently in terms of recognizing their societal impacts – atmospheric rivers provide an opportunity for potentially coordinated research and applications globally, that is, with today’s computational and technological capabilities.”

By Alison Bosman, Staff Writer

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