d. Stratospheric Sudden Warmings
Stratospheric sudden warmings are the most spectacular large-scale dynamical events to take place in the middle atmosphere; they occur in winter, and involve rapid rises of temperature in the polar stratosphere. These temperature changes
result in a reversal of the latitudinal gradient of the zonal-mean temperature (at l0 mb and below, and poleward of 60 ~ latitude, according to the WMO definitionof sudden warmings). They are also associated with a deceleration of the zonalmean westerly zonal wind in this region. If the winds reverse to become easterly, the warming is called "major"; if not, it is called "minor".There is considerable interannual and interhemispheric variability in the occurrence and morphology of sudden warmings. Major warmings occur on average in about one in two northern hemisphere winters, but have never been observed in the southern hemisphere winter (ja wel! in 2002, maar dat wist hij toen natuurlijk nog niet
. (They may therefore complicate the interpretation of the five-year climatology discussed above, which included some years with northern hemisphere major warmings, and some without.) Strong minor warmings have been observed in both hemispheres; in the Southern Hemisphere they tend to be confined to the middle and upper stratosphere, whereas in the Northern Hemisphere they usually involve the whole stratosphere.
After some sudden warmings the polar temperature falls again and the zonal winds accelerate, with the zonal-mean state reverting to roughly its previous form. Towards the end of winter, however, some sudden warmings lead directly into the "final warming", i.e., the change-over to summer conditions of warmer polar temperatures and easterly zonal winds. An interhemispheric comparison of the final warmings in 1982 is given by YAMAZAKI (1987). He finds that the final warming in the Southern Hemisphere in October is stronger and more rapid than that in the
Northern Hemisphere at the end of March. The two events also differ in their detailed structure and evolution.
Stratospheric sudden warmings are the most spectacular large-scale dynamical events to take place in the middle atmosphere; they occur in winter, and involve rapid rises of temperature in the polar stratosphere. These temperature changes
result in a reversal of the latitudinal gradient of the zonal-mean temperature (at l0 mb and below, and poleward of 60 ~ latitude, according to the WMO definitionof sudden warmings). They are also associated with a deceleration of the zonalmean westerly zonal wind in this region. If the winds reverse to become easterly, the warming is called "major"; if not, it is called "minor".There is considerable interannual and interhemispheric variability in the occurrence and morphology of sudden warmings. Major warmings occur on average in about one in two northern hemisphere winters, but have never been observed in the southern hemisphere winter (ja wel! in 2002, maar dat wist hij toen natuurlijk nog niet
. (They may therefore complicate the interpretation of the five-year climatology discussed above, which included some years with northern hemisphere major warmings, and some without.) Strong minor warmings have been observed in both hemispheres; in the Southern Hemisphere they tend to be confined to the middle and upper stratosphere, whereas in the Northern Hemisphere they usually involve the whole stratosphere.After some sudden warmings the polar temperature falls again and the zonal winds accelerate, with the zonal-mean state reverting to roughly its previous form. Towards the end of winter, however, some sudden warmings lead directly into the "final warming", i.e., the change-over to summer conditions of warmer polar temperatures and easterly zonal winds. An interhemispheric comparison of the final warmings in 1982 is given by YAMAZAKI (1987). He finds that the final warming in the Southern Hemisphere in October is stronger and more rapid than that in the
Northern Hemisphere at the end of March. The two events also differ in their detailed structure and evolution.
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