Climatological features of rapidly intensifying tropical cyclones in the North Indian Ocean

The study investigates the climatological variation of rapidly intensifying tropical cyclones (RITCs) developed in the North Indian Ocean (NIO) using observational and reanalysis data from 1982 to 2020. The analysis reveals that out of 153 tropical cyclones (TC) developed in the NIO, 59 TCs underwent rapid intensification (RI). The monthly variation of RITCs exhibited bimodal distribution with a primary peak in November and a secondary peak in May. The percentage of RITCs during April-June is higher as compared to October-December. The study identifies the core zones of RI initiation in the Arabian Sea (AS) and the Bay of Bengal (BoB), respectively. The occurrences of RITCs over the AS have been observed to increase since 2000. Over 65% of RITCs occurred within 12 h of TC formation, and the duration of RI varies from 24 to 48 h. TCs with lifetime maximum intensity (LMI) less than 55 kt have not undergone RI. The trend in the 95th percentile of 24-h intensity changes is positive but insignificant (p-value >0.05) in the AS, and for the BoB, it is negative and significant (p-value <0.05). We infer that there has been an insignificant increase in the 95th percentile of 24-h intensity change at the rate of 0.32 kt per year in the AS and a significant decrease in the 95th percentile of 24-h intensity change at the rate of -0.21 kt per year in the BoB. The analysis further reveals that during the RI phase the composite SST of RITCs exceeds 30 degrees C in the core zone, and the mid-tropospheric (700-500 hPa) relative humidity at the centre of RITCs is high, preventing the intrusion of dry air from the west and aiding in the sustenance and intensification of TCs. The study not only provides insight into the climatology of RITCs in the NIO but also delineates the interplay of large-scale atmospheric and oceanic parameters in fostering rapid intensification of TCs.