Through a quantitative analysis of tropical rainbelts, determination of the critical behavior of the rainbelt as to its position and/or intensity is found. The behavior is related to the rainfall variability in the semi-arid regions of tropical Africa, the Sahel and northern Kalahari regions. Two parameters describing the rainbelt are used; its center of gravity and total rainfall. These parameters indicate the center and intensity of the connective activity of the ITCZ, respectively.
The reduction in total rainfall of the rainbelt during the northern summer solstice season is the critical factor determining the continuous reduction of the Sahelian rainfall during the period from the early 1960's to the mid-1980's. On the other hand, the northward advance and increase in the total
rainfall of the rainbelt are equally important for the wet conditions of the Sahel during the 1950's. For northern Kalahari, the reduction in total rainfall of the rainbelt during the southern summer solstice season is also responsible for the below-normal rainfall during the period between the late 1950's and the early 1970's. The southward advance and increase in the total rainfall of the rainbelt resulted in the wet conditions during the 1950's.
These results demonstrate that the poleward expansion of the tropical annual rainfall zone during the wet period of the 1950's (Shinoda, 1989) was caused by the concurrent poleward advance and increase in total rainfall of the rainbelt during the northern and southern summer solstice seasons. The equatorward contraction of the rainfall zone after this period (Shinoda, 1989) resulted from the reduction in the total rainfall of the rainbelt for each season.