In winter, the polar region is usually covered by a strong low pressure system, surrounded by the jet stream protecting us from the freezing cold. However, this low pressure system occasionally weakens, causing the cold air to reach further south. What's going on?
What is a polar vortex? Why don’t we hear about it every winter?
A polar vortex is a low-pressure system surrounded by strong high-level winds (the jet stream) that circles the Arctic from west to east, all around the Earth. When the polar vortex is strong, it prevents us from getting cold air from the Arctic. However, some winters, the polar vortex weakens and splits or gets displaced, and glacial Artic air can reach into the regions farther south, drastically decreasing temperatures in these locations.
Is there a polar vortex in Antarctica too?
Yes! But we hear less about it because there are less people affected by the potential disruption of the Antarctic polar vortex. Also, it is actually more pronounced and persistent than the one in the Arctic so it does not break as often.
Why does the polar vortex weaken and split sometimes?
It is not yet fully understood how the polar vortex splits are triggered but it is usually related to a rapid warming higher up in the atmosphere (the stratosphere). The consequence of this warming is a decrease in the temperature contrast between the poles and the regions further south, which weakens the polar vortex and makes the jet stream weaker and therefore wavier.
What does it mean for the winter weather in Norway?
A weaker jet stream is thought to favor the presence of persistent high-pressure systems called atmospheric blocking. During winter time, when Europe is under the influence of such an event, we experience dry but also cold weather (Brunner et al 2018). This is what happened in Norway, in March 2018. During these periods, air quality decreases, especially in urban areas. As we use more wood for residential heating, the emission of harmful pollutants is enhanced. This emphasizes the strong relationship between emissions and climatic conditions.
Will there be more polar vortex splits in a future warmer world?
This is still an open area of research. Up until now, there is no clear trend whether there are more or less of these events. We know that global warming increases the amount of Arctic sea ice melting during the summer months which in turn, warms the Arctic Ocean. This heat tends to reduce the contrast in temperature between the Arctic regions and the regions farther south, therefore leading to a weakened the polar vortex. What is clear however is that with global warming, winters will overall be milder. But if such polar vortex splits occur more often, we might get such punctual periods of very cold weather more frequently.