In January 2016 the Arctic experienced unprecedented warming, with air temperatures as high as 8°C, some 20 degrees higher than average for the time of year. Extreme warming events like this have become more common in the Arctic, contributing to the strong warming trend observed in the region. But why is the Arctic warming faster than the rest of the world? A new study indicates that warmer sea surface temperatures in the western North Atlantic may be an important driver of Arctic warming.
Currently scientists are unable to agree on the major driver of Arctic warming. A number of contributors have been suggested, including decreased surface reflectivity due to less snow and ice, increased surface turbulent heat fluxes due to thinner ice, more downward longwave radiation due to additional water vapour and clouds over the Arctic region, and poleward transport of energy via atmospheric waves.
Recent observations have shown a correlation between sea surface temperature in the western North Atlantic and surface air temperature over the Barents-Kara Seas – two seas inside the Arctic Circle, lying to the north of Scandinavia and Western Siberia. But the exact climate mechanism by which these two regions are linked has remained elusive. In order to investigate, Baek-Min Kim from the Korea Polar Research Institute and his colleagues used sea surface temperature and climate data going back to 1979, modelling the atmospheric dynamics during early winter (October to December), when Arctic warming is most pronounced.
The researchers showed that warmer sea surface temperatures in the western North Atlantic intensified the jet stream – the high-level river of wind that carries weather systems in an easterly direction around the northern hemisphere – and shifted it slightly further north, creating ideal conditions for Arctic warming. In addition, the intensified storm activity increased the chances of triggering stationary waves – disturbances that last for an entire season – that travelled into the Atlantic Arctic Ocean.
"This induces a significant warm advection, increasing surface and lower-level temperature over the Eurasian sector of the Arctic Ocean," Kim and his colleagues wrote in their paper in Environmental Research Letters (ERL).
The warming observed in the Arctic is already having knock-on effects elsewhere. For example, Kim and his colleagues previously showed that the reduction of sea ice in the Barents-Kara Seas due to Arctic warming brings cold winters to east Asia.
"The reduction in sea ice increases heat input into the atmosphere, and this generates atmospheric waves that propagate into the stratosphere," explained Kim. "These waves break and weaken the stratospheric polar vortex and allow cool air to intrude to mid-latitudes because of the meandering polar vortex."
Understanding what is driving Arctic warming, and identifying the precursors to extreme warming events, should help meteorologists develop better seasonal forecasts for the northern hemisphere, and give advance warning of extreme weather. However, nothing remains static in climate science, and as Arctic ice continues to melt it may alter feedback mechanisms, perhaps reducing the significance of sea surface temperatures in the western North Atlantic as a driver of Arctic warming. But for now the western North Atlantic is the place to watch if you want to know what kind of season lies ahead for the Arctic.
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