Warmer Arctic temps weaken westerlies that bring warm, moist air to the continent
By Summit Voice
SUMMIT COUNTY — German scientists say they’ve found more evidence showing links between declining Arctic sea ice and shifting weather patterns, with cold, snowy winters more likely in Europe following summers when Arctic sea ice is low.
The researchers from the Alfred Wegener Institute for Polar and Marine Research say shrinking summertime sea ice cover changes the air pressure zones in the Arctic atmosphere, slowing westerly winds that usually transport relatively warm and moist air toward Europe.
If there is a particularly large-scale melt of Arctic sea ice in summer, as observed in recent years, two important effects are intensified. Firstly, the retreat of the ice leaves a darker ocean to warm up more in summer from solar radiation.
Secondly, the diminished ice cover can no longer prevent the heat stored in the ocean from being released into the atmosphere. As a result of the decreased sea ice cover the air is warmed more than it used to be, particularly in autumn and winter, because during this period the ocean is warmer than the atmosphere.
“These higher temperatures can be proven by current measurements from the Arctic regions,” said Ralf Jaiser, lead author of the publication from the Research Unit Potsdam of the Alfred Wegener Institute.
The warming of the air near the ground leads to rising movements and the atmosphere becomes less stable.
“We have analysed the complex non-linear processes behind this destabilisation and have shown how these altered conditions in the Arctic influence the typical circulation and air pressure patterns,” Jaiser explained.
One of these patterns is the air pressure difference between the Arctic and mid-latitudes: The so-called Arctic oscillation involving the Azores high and Iceland low.
If the pressure difference is high, it generates a strong westerly wind, carrying warm and humid Atlantic air masses to Europe. Without the westerlies, cold Arctic air can penetrate down through to Europe, as was the case during the last two winters.
Model calculations show that the air pressure difference with decreased sea ice cover in the Arctic summer is weakened in the following winter, enabling Arctic cold to push down to mid-latitudes.
Early this winter, that scenario did not develop in Europe, as mild temperatures dominated. But the recent and persisent outbreak of Polar air over the continent signals a return to those conditions.
“Many other factors naturally play a role in the complex climate system of our Earth which overlap in part. Our results explain the mechanisms of how regional changes in the Arctic sea ice cover have a global impact and their effects over a period from late summer to winter,” Jaiser said.
“Other mechanisms are linked, for example, with the snow cover in Siberia or tropical influences. The interactions between these influential factors will be the subject matter of future research work and therefore represent a factor of uncertainty in forecasts,” he concluded.
“Our work contributes to reducing the existing uncertainties of the global climate model and developing more credible regional climate scenarios – an important foundation to enable people to adjust to the altered conditions,” explains Prof. Dr. Klaus Dethloff, Head of the Atmospheric Circulation Section at the Research Unit Potsdam of the Alfred Wegener Institute.