Summer Q&A: a new week, a new question!

Why was it so cold in the United States last winter?

Do you remember the extremely cold temperatures that hit the United States last January? If not, you might remember the provocative words of their president, wondering what was going on with Global Warming! The polar vortex was identified as the big culprit all over the news, but how come this Arctic feature can have such an impact at our latitudes?

Figure 1: The science behind the polar vortex (

Let’s begin from the start. The Arctic polar vortex is a naturally occurring low-pressure weather system, that forms every winter in the stratosphere, about 50 km above the Arctic. It is partly caused by the temperature gradient between the pole and the equator and results in the counter-clockwise circulation of very strong winds, trapping the very cold Arctic air. The jet stream, another atmospheric feature, occurring at mid-latitudes, consists in a narrow band of fast flowing winds in the high troposphere, separating polar air and mid-latitude warmer air.

From time to time, the Arctic polar vortex weakens and splits apart during what is called a Sudden Stratospheric Warming (SSW). As a consequence, the jet stream moves further south and gets wavier, which allows freezing air outbursts further south than normal and leads to extremely cold events in mid-latitudes. This is what happened last winter in Canada and the United States.

As you might have read in last week Q&A, the average surface temperature in the Arctic is warming twice as fast as anywhere else because of the Arctic amplification. This will result in a decrease of the temperature gradient between the Equator and the Arctic, and therefore, weaken the polar vortex, which might then be more subject to SSWs. However, this is still subject to debate. Some studies based on climate models report that there is no evidence of statistically significant future changes to the behaviour of the vortex because there are other atmospheric mechanisms at stake that could have a counterbalancing role.

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Story by Caroline Jacques.