Changing status of tropical cyclones over the north Indian Ocean

Climatologically, the frequency of tropical cyclones (TCs) in the Bay of Bengal (BoB) is higher relative to that over the Arabian Sea (ARB). However, recent years exhibit a greater number of TCs forming in the ARB than in the BoB. During the study period (1982–2019), a significant increasing trend in the intensity, frequency, and duration of cyclonic storms (CS) and very severe CS (VSCS) is observed over the ARB. There is a 52% increase in the frequency of CS during the recent epoch (2001–2019) in the ARB, while there is a decrease of 8% in the BoB. Over the ARB, increment in CS duration is 80% and VSCS is almost threefold in the recent epoch as compared to the past epoch (1982–2000). Also, lifetime maximum intensity and accumulated cyclone energy have increased over the ARB implying an increase in the strength of TCs. The increase in TC duration over the ARB is prominent during May, June, and October and a decrease over the BoB is noted during November. The increase in the duration of TCs in the ARB is associated with an increase in mid-level relative humidity and column averaged (950-150 hPa) moist static energy, which is significantly correlated to an increase in sea surface temperatures and tropical cyclone heat potential in the basin. In the recent epoch, TC genesis is observed at lower latitudes (< 8° N), which is another factor contributing to longer durations of TCs. This increases the probability of TC intensification with the support from other favourable environmental parameters. Significant changes in TC tracks are also noted in May, June, and October due to changes in steering currents.