The Indian Summer Monsoon (ISM) is the primary source of freshwater for the Indian subcontinent, and even subtle changes in its intensity and timing can have profound implications for agriculture, water resources and disaster management. While numerous studies have investigated long-term variations in seasonal rainfall, comparatively less attention has been given to changes in the daily timing of rainfall and their relationship with land surface conditions. Using two decades (2001–2020) of GSMaP_ISRO rainfall observations, together with satellite-derived vegetation, soil moisture and aerosol datasets, this study reveals that the Indian monsoon has undergone significant regional changes in both rainfall characteristics and diurnal behaviour. The climatological rainfall distribution during the ISM season (Figure 1a) highlights the familiar rainfall maxima over the Western Ghats, Northeast India, the Indo-Gangetic Plain (IGP) and central India. However, comparison between the two decades (2001–2010 and 2011–2020) reveal distinct regional differences (Figures 1b and 1c). The anomaly map (Figure 1d) clearly demonstrates that west-central India experienced a significant increase in rainfall, reaching nearly 2 mm day⁻¹, whereas parts of eastern India showed a modest decline. Positive rainfall anomalies are also evident over the southern peninsula and portions of the IGP. These spatial changes suggest that the recent decade witnessed a redistribution of monsoon rainfall rather than a uniform increase across the country.

Figure 1 Spatial distribution of average rainfall during ISM (a) 2001 to 2020, (b) 2001 to 2010, (c) 2011 to 2020, and (d) mean rainfall anomaly with 95% significance between 2001-2010 and 2011-2020, respectively. (Red color circles) Rainfall increases over the west central India, southern region and IGP, while it decreases over Eastern region

To investigate the possible causes of enhanced rainfall over west-central India, vegetation changes were examined using MODIS-derived NDVI (Figure 2). The average NDVI maps (Figures 2a and 2b) show a noticeable increase in vegetation density during 2011–2020 compared with 2001–2010. The positive NDVI anomaly over west-central India (Figure 2c) closely matches the region of enhanced rainfall identified in Figure 1d. Furthermore, the NDVI time series (Figure 2d) indicates a persistent increase in vegetation throughout the study period. Increased vegetation enhances evapotranspiration, thereby supplying additional moisture to the lower atmosphere and strengthening local moisture recycling. This close correspondence between rainfall and vegetation suggests that land–atmosphere feedbacks may have contributed to sustaining the observed increase in monsoon rainfall over central India.

Figure 2 Mean NDVI during (a) 2001–2010, (b) 2011–2020 (c) anomaly (2011–2020 minus 2001–2010) and (d) NDVI time series over west-central India showing increasing vegetation

The role of land surface processes is further supported by soil moisture observations from the ESA Climate Change Initiative (Figure 3). The average soil moisture distribution (Figures 3a and 3b) indicates consistently wetter conditions over central India during the recent decade. The anomaly map (Figure 3c) reveals a marked increase in soil moisture across west-central India, while eastern India exhibits relatively drier conditions. The similarity between rainfall, vegetation and soil moisture anomalies indicates a coupled land–atmosphere response. Enhanced rainfall increases soil moisture, which subsequently promotes evapotranspiration and supports vegetation growth, thereby reinforcing atmospheric moisture availability. Such positive feedbacks would have likely contributed to maintaining favourable conditions for rainfall over west-central India during 2011–2020.

Figure 3 Mean soil moisture during (a) 2001–2010, (b) 2011–2020 and (c) anomaly (2011–2020 minus 2001–2010). Enhanced soil moisture is evident over west-central India during 2011–2020

While changes in rainfall amount are important, an equally significant finding of this study is the shift in the timing of maximum rainfall. Hourly GSMaP observations were analysed to determine the local time at which rainfall reaches its daily maximum. The climatological distributions (Figures 4a and 4b) show that rainfall generally peaks during the late afternoon over most land regions, consistent with solar heating and convective development, whereas coastal regions and the Western Ghats experience nighttime or early morning maxima due to land–sea breeze interactions and orographic lifting. Comparison between the two decades reveals a noticeable shift in rainfall timing (Figure 4c). The Indo-Gangetic Plain experiences an earlier peak in rainfall during the recent decade, whereas central India exhibits a delayed rainfall peak. These regional differences indicate that the diurnal cycle of the monsoon has evolved over the past two decades and that changes in rainfall timing are spatially heterogeneous.

Figure 4 Mean peak rainfall time during (a) 2001–2010, (b) 2011–2020, and (c) difference (2011–2020 minus 2001–2010), showing earlier peaks over the IGP and later peaks over central India

To understand the possible mechanisms responsible for these timing shifts, aerosol optical depth (AOD) was analysed (Figure 5). The average AOD distributions demonstrate substantially higher aerosol loading over the Indo-Gangetic Plain compared with central and southern India (Figures 5a and 5b). Aerosols modify the atmospheric radiation balance by scattering and absorbing solar radiation, thereby influencing atmospheric stability, cloud development, and convective processes. The coincidence of elevated aerosol concentrations over the IGP with the observed earlier rainfall peak (Figure 4c) suggests that aerosol-induced changes in atmospheric stability may have accelerated convective development. Conversely, relatively lower aerosol loading over west-central India, together with enhanced vegetation and soil moisture, is consistent with the delayed rainfall peak observed in that region. Although this study does not distinguish the effects of individual aerosol species, the observed spatial correspondence indicates that aerosol loading is likely one of the important factors modulating the changing diurnal characteristics of monsoon rainfall.

Figure 5 Spatial distribution of average AOD during (a) 2001 to 2010, and (b) 2011 to 2020, respectively

Taken together, the results reveal that the Indian Summer Monsoon has experienced a coherent regional transformation during the last two decades. The increase in rainfall over west-central India is accompanied by enhanced vegetation cover (Figure 2) and increased soil moisture (Figure 3), indicating stronger land–atmosphere interactions. Simultaneously, the timing of maximum rainfall has shifted across different regions (Figure 4), potentially influenced by changes in aerosol loading (Figure 5). These findings demonstrate that the recent evolution of the monsoon is not limited to changes in seasonal rainfall totals but also involves modifications in surface conditions, atmospheric composition, and the diurnal evolution of convection. Overall, the study highlights that the Indian monsoon is evolving as a coupled land–atmosphere system, where rainfall, vegetation, soil moisture, and aerosols interact to influence both the intensity and timing of precipitation. Understanding these interconnected processes is essential for improving weather prediction, climate projections, flood forecasting, agricultural planning, and water resource management under a changing climate.

For more details, please refer Geophysical Research Letters (2025), DOI: 10.1029/2024GL112697. Visit here.