Observed Changes in Indian Summer Monsoon Rainfall at Different Intensity Bins during the Past 118 Years over Five Homogeneous Regions

In the present paper, the variabilities and long-term trends of summer monsoon rainfall for different intensity bins (dry, low, moderate, high, very high, and extreme) are studied for five homogeneous regions, namely Northeast India (NEI), Northcentral India (NCI), Northwest India (NWI), Westcentral India (WCI), and Peninsular India (PI) for 118 years (1901-2018). The study was carried out based on gridded rainfall data from the India Meteorological Department (IMD). The rainfall characteristics such as number of rainy days, percentage contribution, and periodicity of rainfall intensity classes are analysed and found to be different in different homogeneous regions. The long-term trend (1901-2018) of total rainfall showed a significant increasing trend (19.9 mm decade(-1)) in NEI and significant decreasing trends in NCI (9.6 mm decade(-1)) and PI (4.9 mm decade(-1)). Analysis on rainfall intensity indicates a significant increasing trend for high, very high, and extreme classes in NEI, a significant increasing trend for dry, and a decreasing trend for moderate and very high classes over NCI and PI. From correlation analysis among the homogeneous regions, it is found that the rain events in different intensity classes show different relationships, which indicate the regional heterogeneity in rainfall characteristics. It is also important to note that an increase in rainfall contribution from very high and extreme classes was found over NEI, NWI, and WCI in the multidecadal period of 1991-2018, while NCI showed a decrease during this period; however, in NCI, a drastic increase for these intensity bins is distinct during the 1961-1990 multidecadal period. In addition to the trends and variabilities, we also explored spatial heterogeneity of different rainfall intensity categories, and found remarkable differences from one homogeneous region to another.