Chicago has been affected by temperatures as low as -30°C in January this year. When the wind chill is added to this, it was said to feel like -50°C. The reason behind these unseasonally low temperatures is the polar vortex. The polar vortex refers to upper level winds which normally keep very cold winds in the region around the North Pole. (It is a winter equivalent to the mid-latitude jet stream which sometimes breaks down forming a blocking anticyclone that brings heatwaves to temperate areas.) The cause of the polar vortex ironically appears to have been higher surface temperatures in the Arctic region displacing colder area to the south. Rising temperatures in the Arctic lead to a decline in Arctic sea ice. As the ice melts, there is a change in albedo. The darker ocean surface absorbs more insolation than the reflective ice cover, and some of this heat is released into the atmosphere in winter, leading to the formation of some wind systems that can displace the vortex.
The vortex split into two parts – one over North America, affecting cities such as Chicago, and one over Europe and northern Asia. The one affecting Europe has not produced such cold conditions. The polar vortex in North America has led to at least twelve deaths; schools and businesses have closed; trains, buses and flights have been cancelled; postal services have been disrupted; people have been warned not to stay outside for more than 15 minutes and if they do go outside not to talk or breathe deeply!
Scientists studying the polar vortex have noticed an increase in the number of cases where it has affected large urban areas. They suggest that as the planet warms due to global climate change, the jet stream that meanders over North America and Eurasia is slowing down and its meandering is becoming more pronounced. This may result in cold air being dragged southwards or warm air being dragged northwards. When the meandering breaks down completely, it results in a blocking high (blocking anticyclone) whereby unusual weather patterns may persist for some time.
The polar vortex may become a more frequent event if it is linked to global climate change.
For the formation of a polar vortex visit https://scijinks.gov/polar-vortex/
For images and a diagram explaining the formation of the vortex visit https://www.vox.com/science-and-health/2019/1/29/18201196/polar-vortex-2019-des-moines-midwest-record-cold-windchill