Classification and description
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.
[edit] Major
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.
According to the World Meteorological Organization's Commission for Atmospheric Sciences (Mclnturff, 1978): a stratospheric warming can be said to be major if 10 mb or below the latitudinal mean temperature increases poleward from 60 degree latitude and an associated circulation reversal is observed (that is, the prevailing mean westerly winds poleward of 60 latitude are succeeded by mean easterlies in the same area).
[edit] Minor
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.
Mclnturff states: a stratospheric warming is called minor if a significant temperature increase is observed (that is, at least 25 degrees in a period of week or less) at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly to easterly is less extensive.
[edit] Final
The radiative cycle in the stratosphere means that during winter the mean flow is westerly and during summer it is easterly (westward). 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.
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link to en.wikipedia.org]
