Spatiotemporal Variability in South Asian Summer Monsoon Rainfall During 2005–2025: A CHIRPS and ERA5-Land Assessment

The South Asian summer monsoon is a critical climate system that influences water availability, agricultural productivity, livelihoods, and disaster risks for millions of people across the region. Increasing climatic variability, the rising frequency of extreme weather events, and the occurrence of extreme rainfall events highlight the need for a comprehensive assessment of recent monsoon rainfall patterns that are critical to the people and agricultural economy. This study assesses the spatiotemporal variability and trends of South Asian monsoon rainfalls during 2005–2025. CHIRPS v2.0 satellite–station precipitation data and ERA5-Land atmospheric reanalysis data were integrated with GIS-based spatial analysis and statistical methods. This integration was used to examine rainfall distribution, anomalies, temperature, and temporal variability across six major monsoon-affected countries: Bangladesh, Bhutan, India, Pakistan, Nepal, and Sri Lanka. The results reveal pronounced spatial contrasts and temporal fluctuations in monsoon rainfall across South Asia. Positive rainfall anomalies were observed over major river basins during several wet years, coinciding with documented flooding in some areas, while other regions experienced rainfall deficits associated with dry conditions. Autocorrelation diagnostics for the original and Sen-detrended regional rainfall series did not detect statistically significant serial correlation at intervals of 1–3 years. Trend analysis using the standard Mann–Kendall test and Sen’s slope estimator indicated a positive tendency in mean monsoon rainfall (2.94 mm year−1); however, this trend was not statistically significant at the 0.05 significance level (p = 0.156). The analysis also highlighted substantial interannual variability in recurrent wet and dry monsoon conditions across the study region. A formal analysis of the ERA5-Land temperature series also showed a positive but non-significant tendency in regional June–September 2 m air temperature (Sen’s slope = 0.0114 °C year−1; Mann–Kendall p = 0.176). The rainfall–temperature association was not statistically significant. These findings contribute to an improved understanding of recent monsoon rainfall and temperature variability and provide information to support regional water-resource management, improve coping capacities and adaptation planning. The study demonstrates the value of integrating satellite precipitation, atmospheric reanalysis, GIS, and statistical methods for early-warning systems, disaster-risk reduction, and regional adaptation strategies across the region.