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The interannual variability of the stratosphere in the northern winter is discussed, mainly on the basis of zonal harmonic wave analyses of the daily height and temperature fields in the middle stratosphere. Comparing 12 winters, common and distinctive characteristics of the midwinter disturbances have been evaluated and three types of midwinter warmings have been identified. It is shown that the main difference between disturbed and undisturbed winter months in the middle stratosphere lies in the different development of zonal harmonic wave 2, and that the development and vertical extent of this wave depend on the vertical structure of the cold trough over Canada.
K. Labitzke (Wed,) studied this question.