Sudden stratospheric warmings
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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 frequeny of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly).