Finer aspects of spatio-temporal variations in Indian summer monsoon rainfall trend reversals over the last 120 years

Abstract

Prominent multidecadal rainfall trends and trend reversal points in the Indian summer monsoon rainfall during 1901–2020 across the four homogeneous regions of India have been examined beginning at the district level. Employing a robust rainfall pattern identification methodology, three significant rainfall trend reversal events have been identified during 1930s, 1960s, and 1980s. During the 1930s, central and northeast India witnessed a shift from increasing to decreasing rainfall trends, while the south peninsula experienced the reverse, resulting in a pronounced north-south asymmetry in rainfall pattern over India. In the 1960s, south peninsula and northwest India exhibited a reversal in rainfall trend from increasing to decreasing, with an opposite trend in northeast India, resulting in an east-west asymmetry in the rainfall pattern. Unlike the 1930s and 1960s, the rainfall trend reversal during the 1980s occurred over all four rainfall homogeneous regions. The three regions (south peninsula, central, and northwest) in India experienced rainfall trend reversal from decreasing to increasing trends, while the northeast experienced the opposite trend reversal, establishing an east-west asymmetry in the rainfall pattern. In terms of geographical extent, the rainfall trend reversal in the 1980s is the most prominent event during the last 120 years of Indian rainfall history as the maximum geographical area (~ 50%) experienced the rainfall trend reversal during this period. In terms of magnitude of rainfall amount variation, the rainfall trend reversal during the 1930s is the most prominent as more than 30% of the area had significantly higher (or lower) rainfall than the long-term average. Temporal changes are observed in the identified spatial asymmetry of rainfall pattern indicating that rainfall homogeneous regions in India must have changed over time.