Some Features of a Long Homogeneous Series of Indian Summer Monsoon Rainfall

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  • 1 Indian Institute of Tropical Meteorology, Poona-411 005, India
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Abstract

A long rainfall series for the contiguous Indian region for the summer monsoon season (June-September), when more than 75% of the annual rainfall occurs over large parts of the country, has been constructed by considering the rainfall data of a very large number of raingages since 1841 to present. The series from 1866–1970 has been found to be homogeneous. The statistical properties of this homogeneous time series have been investigated. The average monsoon rainfall of India is 88.75 cm with a standard deviation of 7.64 cm. Fisher's statistics g1, g2 and the chi-square statistic indicate that the that series is normally distributed. The years 1877, 1899, 1918, 1920, 1951 and 1965 were very bad monsoon years when the rainfall was below the 5th percentile of the distribution. The increase of 4.6% in the 30-year average from 1901–30 to 1931–60 is significant at the 5% level. The mean for the period 1931–60 is also significantly higher than the overall mean for the period 1866–1970 at the 5% level. During the period 1870–1920, the decade average was generally very steady. From 1921 onward, the decade mean increased and attained the maximum value of 93.17 cm during the decade 1941–50, and declined thereafter, the highest decline of 4.41 cm being from 1951–60 to 1961–70. The difference between the mean for the decade 1941–50 and the mean for the whole period of the series is close to the 5% significance point. Power spectrum analysis indicates the presence of a quasi-biennial oscillation in the time series. There does not appear to be a significant relationship between Indian monsoon rain and solar activity.

Abstract

A long rainfall series for the contiguous Indian region for the summer monsoon season (June-September), when more than 75% of the annual rainfall occurs over large parts of the country, has been constructed by considering the rainfall data of a very large number of raingages since 1841 to present. The series from 1866–1970 has been found to be homogeneous. The statistical properties of this homogeneous time series have been investigated. The average monsoon rainfall of India is 88.75 cm with a standard deviation of 7.64 cm. Fisher's statistics g1, g2 and the chi-square statistic indicate that the that series is normally distributed. The years 1877, 1899, 1918, 1920, 1951 and 1965 were very bad monsoon years when the rainfall was below the 5th percentile of the distribution. The increase of 4.6% in the 30-year average from 1901–30 to 1931–60 is significant at the 5% level. The mean for the period 1931–60 is also significantly higher than the overall mean for the period 1866–1970 at the 5% level. During the period 1870–1920, the decade average was generally very steady. From 1921 onward, the decade mean increased and attained the maximum value of 93.17 cm during the decade 1941–50, and declined thereafter, the highest decline of 4.41 cm being from 1951–60 to 1961–70. The difference between the mean for the decade 1941–50 and the mean for the whole period of the series is close to the 5% significance point. Power spectrum analysis indicates the presence of a quasi-biennial oscillation in the time series. There does not appear to be a significant relationship between Indian monsoon rain and solar activity.

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