Sudden stratospheric warming

A sudden stratospheric warming (SSW) is an event where the polar vortex of westerly winds in the winter hemisphere slows down or even reverses direction over the course of a few days. The change is accompanied by a rise of stratospheric temperature by several tens of kelvins.

The first continuous measurements of the stratosphere were taken by Richard Scherhag in 1951. He used radiosondes to take reliable temperature readings in the upper stratosphere (~40 km). It was his persistence which led him to witness the first ever observed stratospheric warming on 27 January 1952. After his discovery, Scherhag assembled a team of meteorologists specifically to study the stratosphere at the Free University of Berlin. This group continued to map the northern-hemisphere stratospheric temperature and geopotential height for many years using radiosondes and rocketsondes. In 1979 when the satellite era began, meteorological measurements became far more frequent. Although satellites were primarily used for the troposphere they also recorded data for the stratosphere. Today both satellites and stratospheric radiosondes are used to take measurements of the stratosphere.

Typically stratospheric meteorologists classify the vortex breakdown into three categories: major, minor, and final.

Sometimes a fourth category, the Canadian warming, is included because of its unique and distinguishing structure and evolution.

These occur when the westerly winds at 60N and 10hPa (geopotential height) reverse, i.e. become easterly. A complete disruption of the polar vortex is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole.

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