Changing patterns of monsoonal rainfall over Northwest India: a century-long perspective on rainfall trends, extremes, and associated dynamics

The Indian Summer Monsoon plays a vital role in shaping India’s socioeconomic landscape. Northwestern India, traditionally characterised by relatively low monsoonal rainfall during June to September (JJAS) and its high variability, is particularly sensitive to changes in rainfall patterns. This study analyses long-term climatology, variability, and trends of JJAS rainfall over Northwest India (NWI: 21°N-28°N, 69°E-77°E) using the India Meteorological Department’s rainfall dataset spanning 122 years (1901–2022). Our trend analysis of JJAS mean rainfall over NWI shows a modest increase in mean rainfall during the study period. Critically, a pronounced intensification is seen in the recent period (1999–2022). Specifically, average monsoonal rainfall over NWI increased from 508.14 mm (1901–1998) to 554.36 mm (1999–2022), representing an increase of about 46.22 mm (~ 8.7% of the long-period average). This increase is largely driven by increasing contributions from high-intensity rainfall events, with Extremely Heavy Rainfall (EHR) and Very Heavy Rainfall (VHR) events accounting for 36 and 32%, respectively, of the overall increase in seasonal rainfall between the periods. The increasing contributions are due to the recent increase in the frequency of associated wet days, with EHR and VHR events accounting for roughly 51 and 46% of this increase, respectively. Enhanced moisture transport from the Arabian Sea toward NWI supports the recent intensification of monsoonal rainfall. Spectral and wavelet analysis reveal significant decadal to multidecadal variability of NWI rainfall. A pronounced phase shift in recent decades, revealed by 31-year running correlations between NWI JJAS rainfall and major Sea Surface Temperature indices, likely underlies the decadal variability and recent increase in rainfall over NWI.