Increasing Frequency of Extremely Severe Cyclonic Storms in the North Indian Ocean by Anthropogenic Warming and Southwest Monsoon Weakening
North Indian Ocean (NIO) has shown an increase in the frequency of extremely severe and higher-category cyclonic storms (ESCS), with maximum frequency during May. Analyses of data sets and coupled model experiments reveal that anomalous increase in Potential intensity (PI) and ocean heat-content with higher increase during May and weakening summer monsoon circulation has led to increasing ESCS with peak frequency during May. PI is a function of environmental conditions that influence thermodynamic atmosphere-ocean disequilibrium and thermodynamic efficiency. Increase in air-sea disequilibrium by the accelerated warming of NIO dominates the PI trend, contributes more than 70% of PI trend, remaining by the thermodynamic efficiency from increased tropical-tropopause layer cooling. Additionally, weakening of summer monsoon circulation weakens vertical wind-shear, weakens southward ocean heat-transport and enhances heat accumulation, leading to increase in ESCS. Projected changes in PI and wind-shear may further increase ESCS, making densely populated NIO region vulnerable to climate change.