Researchers from Stockholm University have studied the causes of these large variations in the Arctic sea ice extent. In a new study, published in Nature Climate Change, researchers show that warm and humid winds from southerly latitudes towards the Arctic in the spring are important for the amount of sea ice later in the summer.

In years when the summer sea ice extent is much below normal, the sky above the ice is often more cloudy in the spring than usual. The clouds warm up the surface, thereby increasing the ice melt. This results in the surface becoming darker due to the appearance of melt ponds and open water. In the summer, more solar radiation is then absorbed by the surface, which further increases the ice melt.
“Each year, the Arctic sea ice has reached its lowest extent in September, after the summer melt, but there are large variations in the ice cover from year to year. Our study indicates that, in years with a very small ice cover in September, the atmospheric conditions play an important role for the ice melt”, says Marie Kapsch from the Department of Meteorology, Stockholm University.

The transport of atmospheric water vapour to the Arctic plays a crucial role for the ice melt in the spring. A significant increase of water vapour leads to more clouds above the ice, and the warming effect of the clouds in turn leads to an increase in the ice melt. This is an important step in understanding the processes behind the large variations in the sea ice extent from year to year. 

 
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