Polar Vortex & SSW
The Polar Vortex is a phenomenon seen in the stratosphere, in the winter hemisphere only. It occurs at a height of around five to thirty miles and forms because air above the winter pole cools markedly in response to the lack of solar radiation experienced there at that time of year. This cooling makes the air more dense, leading to a high pressure area near the surface but a low pressure area aloft, as illustrated by Image 1. Remember that the air pressure at a particular point is related to the weight of the air column above that point.
Whenever there is a pressure difference between two places, movement of air (wind) results, initially from high to low pressure. Just as with systems at the surface in the northern hemisphere, the air circulates anticlockwise around a low pressure area and clockwise round a high pressure area. This situation is what is seen in the stratosphere above the Arctic in winter; a pool of cold air (Polar Vortex) is bounded by a stream of fast moving westerly winds (Polar Night Jet). Such a state of affairs can remain stable and persistent throughout the winter months when the temperature difference between poles and tropics is greatest. However, on some occasions, and for reasons that are still being actively researched, it can be perturbed and change quite rapidly over a period of just a few days. The effects of that may later be felt far down in the troposphere, where our weather is generated.
Sudden Stratospheric Warming
One such situation where a disturbance to the Polar Vortex results in tangible effects on the weather over mid-latitudes, including the UK, is known as a Sudden Stratospheric Warming (SSW). When this happens, the temperature in the stratosphere warms by as much as 50⁰C in a matter of a few days. The immediate consequence is that the Polar Vortex becomes less pronounced and its westerly winds (5 to 30 miles above the surface) reduce and may even reverse direction. In addition, the Vortex can move away from being directly over the pole and sometimes it splits. The latter is often the precursor to particularly cold weather in the UK.
The mechanisms by which the effects of the warming aloft are transferred down to the troposphere and our local weather are still not well understood. However, they can result in changes to the pattern and location of the mid-latitude jet stream which drives weather between 40⁰N and 60⁰N and is often referred to in television weather forecasts. Cold outbreaks over Europe, Asia and North America are possible outcomes, but it should be noted that there is a great deal of variability in the detail of SSW events which makes accurate prediction of their consequences difficult.
Nevertheless, despite the fact that we cannot see what is going on tens of miles above the poles with the naked eye, we do occasionally get to feel the consequences, especially when those include the arrival of a ‘Beast from the East’!
Source: Tracking the Stratosphere‐to‐Surface Impact of Sudden Stratospheric Warmings; Richard J. Hall et al; JGR Atmospheres. https://doi.org/10.1029/2020JD033881