On the interconnection of marine heatwaves and the extremely severe cyclonic storms Mocha and Biparjoy in the Northern Indian Ocean

This study examines the two successive Marine Heatwave (MHW) events and their influence on the formation of two Extremely Severe Cyclonic Storms (ESCSs) in the Northern Indian Ocean (NIO). Strong MHWs were observed in the central and northern Bay of Bengal (BoB) before the genesis of the ESCS Mocha (09–15th May 2023). It was observed that the ESCS Mocha intensified rapidly upon encountering strong MHWs (mean intensity of $$\approx$$ ≈ 1.25 $$^{\circ }$$ ∘ C) along its trajectory. During the intensification phase of Mocha in the BoB, an anti-cyclonic circulation formed over the Arabian Sea (AS). This anti-cyclonic circulation strengthened in the presence of Mocha and persisted for some time even after Mocha made landfall, following which it gradually dissipated. Our study suggests that this anti-cyclonic circulation caused atmospheric subsidence, as evident from reduced cloud cover and high outgoing longwave radiation. Due to the reduced cloud cover, increased net shortwave radiation at the ocean surface was noted, which resulted in positive net heat flux, leading to the warming of the AS. This sustained warming was further reinforced by the presence of low-salinity plumes, which inhibited vertical mixing, as evident from shallower mixed layer depths. The combined effect of these processes led to the formation and gradual intensification of surface and subsurface MHWs, causing the Tropical Cyclone Heat Potential (TCHP) to increase over the same region. These strong MHWs and increased TCHP created a conducive environment for the cyclogenesis of the ESCS Biparjoy (06–19th June 2023) in the AS, which came shortly after Mocha. These findings underscore the need to assess the broader socio-economic and ecological impacts of interconnected natural hazards, especially in vulnerable coastal regions.