Stratospheric Midwinter Warmings

Abstract

The cold cyclonic vortex occasionally breaks down in midwinter, with the result that the Arctic becomes warmer than middle latitudes and the westerlies are replaced by easterlies. This phenomenon is referred to as a “Major Midwinter Warming”. It was discovered by Scherhag (1952), and a review of its characteristics is given by Labitzke (1981). We illustrate the breakdown of the vortex by maps of the temperature and height fields on a day during the cold stage preceding the breakdown and on a day at the peak of the warming (Fig. 1a-d). The maps in Fig. 1 are representative of conditions in the stratosphere; in the mesosphere the sequence is reversed: while the warming progresses in the stratosphere the mesosphere cools and the stratopause descends, as illustrated schematically in Fig. 2 and by rocketsondes in Fig. 3. These figures show only the general features of a major midwinter warming since no two warmings are alike.