Sudden stratospheric warming
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A sudden stratospheric warming 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 Celsius.
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. However, a southern hemisphere warming has occurred on occasion as well.
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 exits 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 dedicated website at Columbia University.
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