The Index Cycle and Its Role in the General Circulation

On the basis of accumulated aerological data on a hemisphere-wide scale, a reexamination is made of the problem of hemispheric zonal-index variations first raised in the late 1930's. These data lead to a theory explaining the important problem of how and why during each winter the zonal westerlies gradually fall to low strength and subsequently recover—the period of this “index cycle” consuming some four to six weeks. The theory postulates a mechanism for containment of air composing the polar and sub-polar cap by means of strong mid-troposphere westerlies created by large-scale confluence. In this manner cold air is produced and stored in northern latitudes, the atmospheric circulation operating as a vast condenser. The discharge of the condenser in the form of cold air outbreaks, long recognized as necessary for the atmospheric heat balance, is effected by a certain cellular-type blocking, which reaches maximum effectiveness in producing an extended index cycle only when the supply of abnormally cold air is abundant, usually in late February. The principal index cycles of the six years 1944 through 1949 serve as supporting evidence for the theory.