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P. JAGANNATHAN and B. PARTHASARATHY

Abstract

Trends and periodicities in the annual rainfall of India were studied using data for 48 stations having record lengths of over 70 yr. Increasing or decreasing rainfall tendencies were found over large continuous areas in India. These trends, however, are not significant over all the stations in the areas but only at a few places distibuted at random. Wherever a trend is significant, it has persistance or a periodicity of more than 40 yr. Quasibiennial oscillation (QBO) is exhibited at several stations in the areas of increasing or decreasing trend. Similarly, the 11-yr cycle (solar cycle) is also exhibited in both areas. The QBO and the solar cycle are both present at only three stations, however. This casts a serious doubt on whether or not there is any physical relation between the QBO and the solar cycle, at least for rainfall.

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P. JAGANNATHAN and H. N. BHALME

Abstract

Despite the systematic nature of the monsoon rains over India, large year-to-year variations in the pattern of distribution of rainfall during the season occur. The yearly pattern of rainfall distribution during the monsoon season (May 31–October 2) for each of the years 1901–51 for a network of 105 stations over India is characterized by a set of six distribution parameters. A brief description of the spatial distribution of the different patterns is given to indicate the nature of the component patterns. Polynomial trend analyses of the time series of the distribution parameters indicate oscillatory features. Power spectrum analyses reveal certain significant periods corresponding to the sunspot cycle or some higher harmonics with regional preferences. The variation of distribution parameters in the different parts of the country with the different sunspot epochs is demonstrated. Studies of the distribution of surface pressure anomalies, frequency of storms and depressions, and the frequency of “breaks in monsoon” associated with the contrasting sunspot epochs suggest that the monsoon circulation features as well as the characteristics of the rainfall distribution have a periodicity nearing the sunspot cycle.

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