First discovered in 1952, a Sudden Stratospheric Warming (SSW) refers to a swift jump in temperatures in the stratosphere that is sometimes linked to the onset of cold weather in winter in certain regions.
What is an SSW?
The term SSW refers to what we observe - rapid warming (up to about 50°C in just a couple of days) in the stratosphere, between 10 km and 50 km up.
The stratospheric polar vortex and the Polar Night Jet
When the North Pole tilts away from the sun during the winter, the air above the pole becomes extremely cold, reaching temperatures as low as -80°C by December. Circulating around this cold pool is the stratospheric polar vortex with an area of low pressure at its centre. The vortex appears each winter in the stratosphere above the Arctic and exists until sunlight returns to the polar regions in the following spring. Due to the huge temperature difference between the cold polar air and the warm air at mid-latitudes, a jet stream flows around the perimeter of the vortex, known as the Polar Night Jet. This jet helps to lock the cold air in place within the stratosphere polar vortex. The Polar Night Jet flows west to east, sometimes in tandem with our own more familiar tropospheric jet stream some 20 km below.
Sometimes the westerly flow of the tropospheric jet stream can be disrupted. These disruptions are caused by upward-travelling planetary waves, also known as Rossby Waves. These are giant meanders in high-altitude winds and occur in the atmosphere due to the Earth's rotation. These types of waves are generated by flow over mountains and continental land-sea temperature contrasts. They can also be generated by year-to-year changes in large scale weather patterns such as El Nino. With large land masses in the Northern Hemisphere compared to the Southern this leads to more Rossby waves and so SSW events are largely a Northern Hemisphere focused phenomenon. There is only one known exception to this; in September 2002, a major warming was observed for the first time in the stratosphere in the Southern Hemisphere. Only Rossby waves with the greatest spatial scales are able to spread upward into the stratosphere. When the Rossby waves are strong, their amplitudes grow with height into the stratosphere where the waves break, just like on a beach. The breaking waves exert a drag on the Polar Night Jet, which weakens and distorts it. If these waves are strong enough, they may decelerate the jet sufficiently so that the westerlies turn easterly. Such a change in air flow disrupts the stratospheric polar vortex either by displacing it from its normal location over the pole or splitting it into two daughter vortices. Winds within the weakened jet slow down and turn toward the centre of the vortex. As the air converges in the centre of the vortex, it must then descend. This descent causes the air to compress and its temperature rises dramatically, sometimes by more than 50°C in just a few days. This descent also increases the pressure above the North Pole. Over the following weeks, the action of further Rossby waves allow the easterly winds to burrow down through the stratosphere. When they reach the tropopause, they often impact Atlantic weather systems and the jet stream.
Impact on weather in Northern Europe
Weather systems normally arrive over northern Europe from the west — with a flow of relatively mild air coming in off the Atlantic. But a weaker jet stream means less flow of mild Atlantic air, weakening areas of low pressure and moving the jet stream further south. This leads to high pressure over the North Atlantic, 'blocking' that flow of mild Atlantic air and dragging in cold continental air from the east. Exactly how cold it might be depends on the details of where the air comes from.
SSWs don't always result in this outcome — but a cold snap follows more often than not, so the SSW greatly increases the risk of wintry weather.
Impact on weather in North America
In February 2019, an SSW event caused the stratospheric polar vortex to split in two causing the polar vortex to move south into North America bringing extreme low temperatures to the Great Lakes region.
Impact on aviation
Abnormal weather conditions clearly will have an impact on the infrastructure that supports aviation operations resulting in reduced system capacity, runway closures etc.
Pilots need to reassess the risks associated with operations into familiar aerodromes. Where temperatures are much lower than is normally experienced, altimeter temperature error (for example) may well be significant.
Can we predict these events in advance?
Currently meteorologists can reliably predict individual SSWs about a week in advance, and can detect them early on with satellite and other observations.