Abstract
Through the analysis of the geopotential and wind fluctuation during the summer monsoon season over and around India, the predominant periodicity of about 40 days was confirmed especially in the lower and upper troposphere as a major active-break cycle of the monsoon activity. As well as the cloudiness fluctuation of the same period (Yasunari, 1979; Yasunari, 1980), these elements except the geopotential height in the upper level showed a marked northward propagation from the equator toward the Himalayan region. The geopotential height at 200 mb showed, in contrast, a standing oscillation over India. Cross-spectral analyses revealed that the vertical structure of the atmosphere is gradually modified during the northward phase shift of the anomaly cloudiness with the 40-day period. This modification is most apparent in the temperature field. The modification of the vertical structure from the south to the north over India seems to be attributed to the gradual enhancement of cumulus convection from the south to the north. It is also suggested that this cycle is caused by the periodical cold air outbreak over the middle and upper troposphere of the equatorial zone which may be associated with the index cycle of the westerly wave movements in the Southern Hemisphere. Finally, a multicell structure of the anomaly Hadley circulation regime over India through the Indian Ocean was proposed, which gives reasonable interpretations on various aspects of the active-break cycle of monsoon activity with this mode.
