Shifted Diurnal Cycle of Indian Summer Monsoon Rainfall
About the work
Rainfall over land typically peaks in the late afternoon or evening, but the exact hour depends on convection, circulation and local conditions. Using hourly self-recording rain-gauge data from 81 India Meteorological Department stations, the study compares the time of peak monsoon (June–September) rainfall in a past epoch (1969–1991) with a recent one (1992–2014).
- Widespread shifts: 45 stations show a delayed peak, 23 an advanced peak and 13 no shift; 63 stations have statistically significant changes (95% level).
- Regional pattern: central, north and south peninsular India (about 55% of stations) shift later, typically by 1–3 hours; western and northeast India shift earlier. The pattern agrees with IMERG satellite data.
- Cause: the shift mainly reflects changes in the frequency of rainy hours, with changes in heavy rainfall events contributing most at the majority of stations: more heavy events later in the afternoon over central India, and changes in heavy-rain intensity over western India.
- Possible drivers: rising aerosols and lower CAPE over central India (alongside more break days) delay convection, while higher CAPE and lower CIN over western India favour an earlier onset.
The findings point to a large-scale shift in the monsoon diurnal cycle with climate change, and offer a useful test for climate and forecast models.
Read the paper (doi:10.1029/2025GL116401)Chutia, T., Chakraborty, A., Bhat, G. S., & Rajeevan, M. (2025). Shifted diurnal cycle of Indian summer monsoon rainfall. Geophysical Research Letters, 52, e2025GL116401. Open access under CC BY-NC-ND; the sketch above is an original illustration and does not reproduce the paper's figures.