The United States has been affected by extreme cold weather due to what is referred to as a polar vortex. Professor Robert Kerr from our School of Engineering, Department of Mathematics and Centre for Scientific Computing explains what a polar vortex is and why it causes this kind of effect.

Professor Kerr said: "What is being called the “polar vortex” is technically something that always exists to one degree or another, especially during winter. It exists because the mid-latitude jet stream flows from west to east, and the sub-polar low north of 70deg goes the other way. As a result, storms that originated in the freezer called the Canadian prairie (we called them “Alberta Clippers”) head southeast to the Great Lakes, then almost due east. If in Nov-Dec this often results in ice storms, one of which brought down first the electric grid throughout Quebec province with influence over all of New England. In Jan-Feb its just plain cold, perfect for hockey.

"After leaving N. America these storms get recharged by the warm Gulf Stream. This underlay the movie 'Perfect Storm'. Then the vortex, usually centred over Greenland, sends the storms northeast. Usually skirting western Iceland, dumping snow in Norway. Those storms that hits the west of the British Isles are the source of the greatest monetary loss from weather. And then from the point of view of the civilised world, they disappear into the Arctic.

"They don’t disappear. Instead the sub-Arctic low usually catches them and they go back west, then south to Alberta and the cycle starts again. This takes about a week and the UK being on the edge of this, has heavy rains with a recurrence of about a week.

"For some reason, over the past decade, the modestly strong low pressure over Greenland has been getting stuck further west, approximately between Hudson Bay and Quebec City, becomes significantly stronger and the return route isn’t the long way around through Norway, but essentially straight north over the Labrador Sea and Greenland, recycling the same cold air over the Alberta freezer and generating these extremely low temperatures.

"When this last happened over several years, about 2014 +/-2, while the extreme polar vortex was in control, the UK, especially the northwest got repeated heavy flooding. That doesn’t seem to be happening this time as it looks like there is a short wavelength Rossby wave being shed which is feeding cold, but not hyper-cold, air from Greenland our way.

"It is the fad to blame the extreme weather events on global warming. There is less sea ice, but I haven’t seen a dynamical mechanism connecting sea ice to the extreme polar vortex. To me, it is just as likely that the extreme heat in the S. Hemisphere (Australia) is pushing up against the N. Hemisphere jet stream resulting in extreme meandering."

31 January 2019

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