Increasing environmental favorability for peak-monsoon cyclone genesis in the northern Arabian Sea

Tropical cyclones are among the most destructive natural hazards. Although only a small fraction of global tropical cyclones occur over the North Indian Ocean, their impacts can be severe because of the densely populated and low-lying coastal regions surrounding the basin. Cyclogenesis during the peak monsoon months of July and August is exceptionally rare over the Arabian Sea. Cyclone ASNA, which formed in August 2024, was the only peak-monsoon Arabian Sea cyclone identified during 1982–2024. Here, we investigate the oceanic and atmospheric conditions associated with its development. During August 2024, the Genesis Potential Index (GPI) exceeded its local 90th percentile over much of the northern Arabian Sea, with potential intensity providing the dominant positive contribution. A fixed-variable decomposition showed that this enhanced potential intensity was primarily associated with anomalously warm sea surface temperatures (SSTs). Regional SST anomalies reached approximately 2.1 °C and remained above 1.5 °C for more than a month. The warm anomaly developed under favorable surface heat-flux forcing and was subsequently sustained partly by horizontal advection and unresolved ocean processes, despite increasing surface heat loss. The precursor disturbance also encountered enhanced mid-tropospheric moisture and reduced vertical wind shear while crossing northwestern India and approaching the Arabian Sea. Lagrangian analysis identified the Arabian Sea, southern Indian Ocean, and terrestrial evapotranspiration as important diagnosed moisture sources for the associated heavy rainfall on 26 August 2024. Beyond the 2024 event, July–August GPI increased significantly over the northern Arabian Sea during 1993–2024, at approximately 1.215 × 10 −16 m −2 s −1 decade −1 ( p < 0.001). Trend decomposition showed that this increase was dominated by potential intensity, which was itself primarily associated with SST warming. These results indicate that the large-scale thermodynamic environment over the northern Arabian Sea has become increasingly favorable for peak-monsoon cyclogenesis.