Long-term changes in rainfall epochs and intensity patterns of Indian summer monsoon in changing climate

The rainfall spell patterns of the Indian Summer Monsoon (ISM) and their respective intensities have been analyzed in light of changing climate by using climatology of gridded rainfall data during the period 1901-2022. The active and break days over the monsoon core zone are identified for peak monsoon months for each year based on the standard criteria. In general, maximum of four active spells are observed within the monsoon season (99% of the time) with occasional occurrences of additional spells. A significant change is observed from 1901 to 2022 in the contribution of each active spell to the respective cumulative ISM rainfall; but the sum of all the active spells rainfall contributions remained the same. The spell-1 rainfall contribution to the total ISM rainfall is decreasing (-0.4 +/- 0.15 mm/decade); while it is increasing (0.22 +/- 0.11 mm/decade) in spell-2. Also, it is found that the number of break days between the first and second active spells is declined by 11 days over 122 years along with the enhancement in the number of second active spell days. It is proposed that these may be partly due to the observed decline in the northward propagation speed of the monsoon and partly due to significant shift from positive to negative cloud radiative forcing over the Indian ocean region during the first-half (June-July) of the monsoon to the second-half (August-September) of the monsoon. In addition, the probability density distribution of active spell-4 is observed to be distinctly different from the rest of the active spells. In association, the monsoon trough axis is observed to be shifting southward along with the weakening of low level winds (at 850mb level) from beginning to the end of the active monsoon season. Concurrently, in-situ observations of drop size distributions of low, moderate and extreme rainfall events suggests that the controlling microphysical processes shift from size controlled to number controlled drop size distributions with extreme rainfall events being primarily number controlled. Thus, the significance of the present study may provide the better understanding of the role of duration active spells to the monsoon in changing climate.

Automated summary

The study analyzes the rainfall patterns and intensities of the Indian Summer Monsoon, and finds significant changes in the contribution of each active spell to the total rainfall. The first and second active spells show decreasing and increasing rainfall contributions, respectively. The study suggests that the observed decline in monsoon propagation speed and the shift in cloud radiative forcing may partly explain these changes in the monsoon. The findings provide a better understanding of the role of active spells in the changing climate.