Stagnation of atmospheric circulation leads to historically prolonged extreme rainfall event over northwestern India in August 2024

Mahendra, N., Huber, M., Wang, L., Hunt, K. M. R. ORCID: https://orcid.org/0000-0003-1480-3755, Chilukoti, N. and Chowdary, J. S. (2025) Stagnation of atmospheric circulation leads to historically prolonged extreme rainfall event over northwestern India in August 2024. Journal of Geophysical Research: Atmospheres, 130 (21). e2025JD044227. ISSN 2169-8996 doi: 10.1029/2025JD044227

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.

Item Type	Article
URI	https://centaur.reading.ac.uk/id/eprint/127003
Identification Number/DOI	10.1029/2025JD044227
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > NCAS Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
Publisher	American Geophysical Union
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