Decadal frequency and trends of extreme excess/deficit rainfall during the monsoon season over different meteorological sub-divisions of India

The monsoon season (June-September) is the major source of rainwater in all the meteorological subdivisions of India, except the Tamil Nadu & Pondicherry sub-division. In this study, the observed trends in monsoon rainfall, monthly as well as seasonal, were investigated and compared, and the variability and frequency of extreme excess and deficit rainfall during 1871-2008 were also analysed. The magnitude of a trend was estimated using Sen's slope method. The trend results, using the Mann-Kendall and Spearman's Rho trend tests, were similar for all the 30 meteorological sub-divisions of India, as well as for the Indian region, for monsoon months and for the monsoon season. In the majority of sub-divisions, a decreasing trend of monsoon rainfall during the study period was found. The decadal trends in frequency and the spatial variability of occurrence of extreme excess/deficit rainfall years were also investigated. The results show that the decadal occurrence of number of extreme excess rainfall years over the eastern coastal sub-divisions has an increasing trend, while 17 sub-divisions show decreasing trend. However, the occurrence of total extreme deficit rainfall years shows an increasing trend over the eastern and peninsular sub-divisions, except for Kerala and Gangetic West Bengal.