Sudden stratospheric warming

A sudden stratospheric warming (SSW) is an event where the polar vortex of westerly winds in the Northern winter hemisphere abruptly (i.e. in a few days time) slows down or even reverses direction, accompanied by a rise of stratospheric temperature by several tens of degrees Kelvin. This is considered to be the most dramatic meteorological event in the stratosphere.

History of Sudden Stratospheric Warmings

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 (~40km). It was his persistence which led him to witness the first ever observed stratospheric warming on the 27th of January 1952. After his discovery, Scherhag created a team of meteorologists specifically to study the stratosphere at his university - 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 then 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.

Classification and Description of Sudden Stratospheric Warmings

Typically stratospheric meteorologists classify SSWs into three categories:
#Major Warming
#Minor Warming
#Final Warming

Sometimes a fourth type of warming known as a Canadian Warming is included because of its unique structure and evolution, which distinguishes it from the other warmings.

Major Warmings

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.

Minor Warmings

Minor warmings are similar to major warmings however they are less dramatic, the westerly winds are slowed, however do not reverse. Therefore a breakdown of the vortex is never observed.

Final Warmings

The radiative cycle in the stratosphere means that during winter the mean flow is westerly and during summer it is easterly. A final warming occurs on this transition, so that the polar vortex winds change direction for the warming, however do not change back until the following winter. This is because the stratosphere has entered the summer easterly phase. It is final because another warming cannot occur over the summer, so it is the final warming of the current winter.

Sudden Stratospheric Warming Dynamics

In a usual northern-hemisphere winter , several minor warming events occur, with a major event occurring roughly every two years. One reason for major stratospheric warmings to occur in the Northern hemisphere is because orography and land-sea temperature contrasts are responsible for the generation of long (wavenumber 1 or 2) Rossby waves in the troposphere. These waves travel upward to the stratosphere and are dissipated there, producing the warming by decelerating the mean flow. This is the reason that major warmings are only observed in the northern-hemisphere, with one exception. In 2002 a southern-hemisphere major warming was observed. This event to date is not fully understood.

There exists a link between sudden stratospheric warmings and the quasi-biennial oscillation: If the QBO is in its easterly phase, the atmospheric waveguide is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex, intesifying their interaction with the mean flow. Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly).

Plots of many physical quantities as well as animations of major, midwinter Stratospheric Sudden Warmings since 1950's can be found on a [http://www.appmath.columbia.edu/ssws dedicated website at Columbia University] .