Increasing frequency of very severe cyclonic storms over the northern Indian Ocean driven by anthropogenic greenhouse gas forcing

This study identifies a significant increasing trend in the frequency of cyclonic storms (CS; maximum sustained winds exceeding 34 kts) over the northern Indian Ocean (NIO), primarily contributed by a pronounced rise in the number of very severe cyclonic storms (VSCS; maximum sustained wind speed exceeding 64 kts) since 1979. The observed increase in the VSCS frequency is closely associated with the enhanced lower-middle tropospheric relative humidity due to the vertical moisture advection processes. The strengthened upward motion is primarily caused by anomalous positive vorticity advection by mean westerly flows. Through analysis of Detection and Attribution Model Intercomparison Project (DAMIP) experiments, it is demonstrated that the mid-level cyclonic circulation trend is predominantly forced by greenhouse gas emissions, while anthropogenic aerosols exert a dampening effect. These findings underscore an escalating risk of stronger cyclones for the densely populated coastal nations around the NIO under continued climate change. The increasing frequency of VSCS over the NIO poses escalating threats, including more destructive storm surges, extreme winds, and heavy rainfall that exacerbate coastal flooding and infrastructure damage. These intensified storms also endanger critical ecosystems, like mangroves and fisheries, impacting vulnerable coastal communities. Addressing these risks requires improved early warning systems, climate-resilient infrastructure, and accelerated emission reductions to mitigate long-term intensification of VSCSs.

Key points

• The frequency of very severe cyclonic storms over the northern Indian Ocean has an increasing trend.
• The increase in very severe cyclonic storms is primarily driven by the lower-middle tropospheric
• The observed moistening trend in the lower-middle troposphere is largely attributable to the anthropogenic greenhouse gas forcing.