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What is sudden stratospheric warming (SSW)?
23 February 2018

What is sudden stratospheric warming (SSW)?

A Sudden Stratospheric Warming (SSW) of the atmosphere refers to a rapid jump in temperatures in the stratosphere, the layer of the atmosphere extending from approximately 10 km to 50 km above the ground, which can then lead to the onset of cold weather in winter for the mid-latitudes. This rapid stratospheric warming (which can be up to about 50 degrees Celsius in a couple of days) is triggered by a disruption of the normal westerly flow by natural weather patterns or other disturbances in the lower atmosphere.

This disruption leads to a 'wobbling' of the jet stream. As these 'wobbles' or waves break, they can be strong enough to weaken or even reverse the westerly winds thus leading to easterlies. As this is happening, the air in the stratosphere starts to collapse and compress, leading to the rapid temperature increases.

The easterly winds in the stratosphere eventually sink to the troposphere, the lowest layer of the atmosphere. This can alter weather patterns in the northern hemisphere by weakening and displacing the tropospheric polar vortex – a large, semi-permanent cyclonic rotation of very cold air around the north pole. This in turn pushes the jet stream further south leading to the development of a blocking high pressure system at higher latitudes. Blocking highs can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time. If these blocking highs become established over Scandinavia or Greenland, such a synoptic pattern can lead to bitterly cold air from eastern Europe/Russia pushing in over Ireland. A SSW event in January 2013 led to a very cold month of March and some significant snowfall accumulations.

The figure below shows the average temperatures of the atmosphere over the Arctic at a function of height (pressure in hPa) between January 17th and February 16th 2018. As time progresses, the temperature anomaly in the stratosphere (around 200 hPa to 1 hPa) becomes strongly positive (red colour); this is known as a sudden stratospheric warming event. Figure credit: ECMWF.

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