The Big Freeze

Thursday 09th Jan 2014 by theWeather Club

Most of Canada and a large swathe of the USA have been struck by an extreme arctic blast, blamed on a weather pattern known as the polar vortex. Weather records have tumbled in many places, with sub-zero temperatures even being recorded in parts of southern USA. The temperature plummeted as low as -38.3°C in Babbitt Minnesota and when you add in the wind chill factor, caused by gusts of up to 30mph, the temperature ‘felt’ more like -52°C. But it wasn’t just the temperatures making the headlines; parts of the Midwest were hit by two massive snow storms which dumped more than 60cm of snow.

The extreme conditions were caused by a polar vortex which has been pushed much further south than normal. The polar vortex tends to strengthen in the winter and weaken in the summer and can span a distance of more than 1,200 miles, with air circulating in an anti-clockwise direction around the North Pole. A similar vortex exists around the South Pole over Antarctica.

When the polar vortex is strong, it acts like a spinning bowl balanced on the top of the North Pole with the cold air tightly contained in an oval-shape with the jetstream forming the outermost boundary of the cold Arctic air. The jetstream is a discontinuous, narrow current of strong winds in the upper atmosphere which would normally flow from west to east holding the polar vortex in place over the Arctic. In early January, a sudden stratospheric warming event weakened and broke down the polar vortex, allowing fragments of cold air to push south into mid-latitudes. High pressure building up in the Arctic slowed down the jetstream, which caused it to buckle into deep folds and flow farther south than usual, introducing cold Arctic air into the central and eastern USA.

In recent years, climate scientists have noticed that changes in the position of the jetstream has led to outbreaks of colder weather in the mid-latitudes and milder temperatures in the Arctic, a so-called ‘warm Arctic-cold continents’ pattern. Whether this is related to a recent reduction in the amount of Arctic sea ice is not entirely clear, especially when considering individual events. But less sea ice and snow cover in the Arctic and relatively warmer Arctic air temperatures at the end of autumn has been linked with more variability in the orientation and strength of the jetstream.

On 24th December 2013 people reported hearing a loud ‘boom’ in and around Toronto. This sudden cracking noise was caused by a rare phenomenon known as a frost quake, or cryoseism, and occurs when the air temperature drops very rapidly. It is caused by water seeping down into the rock which freezes and expands in sub-zero temperatures. This expansion puts stress on a surrounding rock which builds until it is relieved explosively in a frost quake.