The Strong Index Change Period from January 1 to January 7, 1956

In continuation of a recent paper (Defant, Taba, 1957) a seven days period (January 1–7, 1956) has been investigated on a hemispheric scale. During this period strong world wide changes in the circulation type occurred. Such a circulation change from a meridional into a zonal type* has been studied for the first time by means of hemispheric tropopause maps and by using a special technique of dividing the total available material of atmospheric sound ings into three different groups as described in the previous paper. The following points summarize the results: 1. The hemispheric tropopause maps are most suitable for obtaining a clear picture of three dimensional structure of the atmosphere. The distribution of cold and warm air masses can easily be seen. Breaks in tropopause heights give an immediate impression of the location and the form of the two principal westwind maxima of the westerlies (polarfront jet and subtropical jet). The concentrations in the temperature field of the tropopause level are a further guide for the location of the main wind belts. This temperature field makes it possible to picture the meandering structure of the wind belts and shows the interaction of tropical and mid-latitude circulation. Clear distinction has to be made between subtropical and polar jet. 2. By the use of this method strong northward displacements of the tropical atmosphere have been shown. At the same time a strong meandering behaviour of the subtropical jet appears. These tropical impulses play a dominating role in producing pronounced changes in mid-latitude circulation and for the generation of large blocking anticyclones. 3. During the seven days period the character of soundings in the three selected groups (north of the polar jet, between the two jets and south of the subtropical jet) remained nearly unchanged. 4. Whenever the tropical atmosphere shifted northward stations inside deep troughs having a polar isothermal structure in the stratosphere showed a strong cooling in the higher stratosphere and at the end of the period even the polar stations showed the same feature. It is thought that this cooling is mainly due to advection from the tropical regions, but outgoing radiation and vertical motion may contribute. 5. A possible cyclic change and three dimensional interaction of the three different parts of the atmosphere is outlined at the end of the paper. DOI: 10.1111/j.2153-3490.1958.tb02008.x