Sudden Stratospheric Warming (SSW)
Every so often in winter time, as a result of a sudden warming in the stratosphere, the Polar Vortex can weaken to the extent that its normal circulation breaks down entirely. The usually strong westerly winds can be disrupted and can even go into reverse. This disruption of winds can work its way down through the atmosphere to impact the jet stream. While there is not a direct cause and effect, SSW events can increase the chances of severely cold outbreaks of weather across mid-latitude regions of Asia, Europe and North America.