Elucidation of Indian Summer Monsoon: Impact of “ASHOBAA” and “KOMEN”

Tropical Cyclones Ashobaa and Komen over the North Indian Ocean in 2015 represent a rare and remarkable event that formed during the monsoon season, making their occurrences uncommon. Ashobaa developed over the Arabian Sea during the transition phase, just prior to monsoon onset, while Komen formed over the Bay of Bengal during the active monsoon phase. Notably, Komen was the first system during the monsoon month of July to intensify into a cyclonic storm during the active monsoon period in the past 25 years.

The current study investigates the ocean-atmospheric conditions that facilitated the genesis and intensification of these systems into cyclonic storms. The associated dynamic and thermodynamic characteristics during their formation and evolution are analyzed to elucidate their interaction with the monsoon circulation. Our analysis reveals that warm sea surface temperature anomalies and weak surface winds over the northern Arabian Sea provided conducive conditions for the genesis of cyclonic storm Ashobaa, despite forming during the monsoon onset phase. The intensification of Ashobaa was aided by low vertical wind shear, high tropical cyclone heat potential, and enhanced moisture availability. These favorable conditions enabled the monsoon vortex to intensify unusually into cyclone Ashobaa. On the other hand, oceanic influence was comparatively weaker during the active monsoon phase. The evolution of cyclonic storm Komen was primarily driven by high low-level relative vorticity, enhanced moisture convergence, and a gradual increase in surface wind energy over the Bay of Bengal. Although both cyclones developed under low to moderate wind shear, their genesis and intensification processes were different. The 2015 monsoon variability over the North Indian Ocean was modulated by Komen through enhanced atmospheric forcing and monsoon–land interactions, whereas Ashobaa was largely driven by the oceanic parameters. The vital role of the ocean surface and the subsurface in the genesis and the intensification highlights the importance of incorporating accurate ocean initial conditions (surface and sub-surface) in the operational cyclone forecasting framework.