Abstract/Summary

Northwestern India (NWI) received anomalously heavy rainfall from 21 to 30 August, 2024, despite the monsoon being in a break phase. This study reveals how midlatitude dynamics triggered this unusual event. Using reanalysis, we perform a moisture budget analysis and local wave activity (LWA) diagnostics and find that the enhanced mid-tropospheric baroclinic instability >1.1 day^-1 (1.34σ) peaked during 13–17 August in the North Atlantic, 7–10 days before the NWI rainfall event. This instability fueled the North Atlantic westerly jet and reinforced a “Ω” shaped blocking high over the Ural Mountains (“Ural block”) that developed over 21–22 August. This block initiated a wave train that propagated eastward with increasing amplitude over subsequent days. The downstream distorted Rossby waves broke, triggering a pronounced Caspian Sea trough that deepened by 25–26 August, strengthening a subtropical jet streak north of the Tibetan Plateau, with maximum geostrophic wind speeds exceeding 120 . The NWI was in the jet streak's right entrance region, where the upper-level divergence and compensatory upward motion intensified a low-pressure system that had been steered northwestward into the region. Despite monsoon break conditions, the elevated mid-tropospheric humidity allowed sustained deep convection, which combined with the quasigeostrophic ascent arising from the jet streak entrance region and led to extreme rainfall over NWI. Our analysis provides a case study showing the importance of the growing influence of mid-latitude circulation changes traceable to alterations in the North Atlantic for Indian monsoon variability. The physical linkages demonstrated here may be crucial for improving monsoon behavior in changing climate.