Which processes in the course of climate change are causing the threatening loss of sea ice in the far north? A study now shows that humid warm air streams from the south – so-called atmospheric flows – reach the Arctic more and more frequently in winter. As a result, they significantly impede the seasonal recovery of the sea ice cover there, the researchers report.
A particularly worrying view in the context of climate change is directed towards the far north of the earth: in the past few decades, hardly any other area has warmed up as much as the Arctic, which means there is a risk of climatic domino effects. The change is noticeable, among other things, through the ever smaller ice surfaces in the Arctic Ocean and its marginal seas. Basically, the expansion is subject to natural seasonal fluctuations: every year, the Arctic sea ice spreads from autumn until it shrinks again due to the milder temperatures in spring. In recent decades, however, not only the summer minimum of the sea ice extent has drastically reduced. In the winter months, the areas no longer expand as much and sometimes there are even thawing processes.
On the trail of atmospheric fluxes
The researchers, led by Pengfei Zhang of Pennsylvania State University in University Park, are now investigating to what extent an "infamous" atmospheric phenomenon could be involved in the lack of winter sea ice recovery. So-called atmospheric flows are bands of moisture-saturated air that are about 500 kilometers wide and up to several thousand kilometers long. When these torrents, often accompanied by storms, make landfall, their colossal amounts of moisture can cause extreme rainfall and flooding.
Atmospheric fluxes mostly extend from the equatorial region to our mid-latitudes – but sometimes they also "flow" into the arctic regions. The scientists have now investigated these particularly far-reaching versions by evaluating satellite data and other information covering the time frame from 1979 to 2021. In order to work out possible processes, they also carried out climate model simulations on the basis of the information.
As they report, their results show: Since 1979, atmospheric fluxes have increasingly reached arctic areas during the winter ice growth season. The analyzes show that the Barents and Kara Seas off the north coasts of Norway and Russia are particularly affected. There, the air masses can then ensure reduced ice growth in early winter and even renewed thaw effects: the satellite remote sensing images showed a decline in sea ice almost immediately after such atmospheric flow events, which often lasted for a few days.
Weakened Sea Ice Recovery
Various processes are at work here, the researchers explain: The warm moisture that is transported by these currents increases the heat loss from the atmosphere to the earth. In addition, rain can melt the ice cover, which is still thin in some cases, and even if the precipitation falls as snow, the effect is unfavorable: it acts like a blanket, which means that any heat from the underlying seawater can lead to thawing or a lack of ice formation. The researchers' models showed that these processes and the increasing frequency of events are significantly limiting seasonal sea ice recovery in the Arctic.
“The retreat of Arctic sea ice is one of the most obvious signs of global warming in recent decades. Our research now shows that atmospheric fluxes play a role in this,” says Zhang. His colleague Ruby Leung of the Pacific Northwest National Laboratory in Richland says: “We often think that Arctic sea ice retreat is a gradual process of global warming. However, our study now makes it clear that the sea-ice retreat is also due to episodic extreme weather events — atmospheric fluxes that have become more frequent in recent decades.”
Using their climate models, the researchers were able to show that human-caused warming has increased the rate of atmospheric fluxes all the way to the Arctic. However, it also became apparent that natural climate variability – the so-called Pacific Decadal Oscillation – contributes at least in part to the changes in atmospheric flux. "This study, along with other work that has established the presence of atmospheric fluxes in the tropics, shows that atmospheric fluxes are a global phenomenon," says co-author Bin Guan of the University of California at Los Angeles. Because of the far-reaching importance of these currents, they should continue to be the focus of research, according to the scientist.
Source: Pennsylvania State University, professional article: Nature Climate Change, doi: 10.1038/s41558-023-01599-3