LAUSR

Deadliest tropical cyclones (TCs) with category 4 or 5 intensity, which are referred to super TCs (STCs), often take place in Bay of Bengal (BoB) during pre-monsoon transition period. These STCs are often synchronized with the first-branch northward-propagating intraseasonal oscillation (FNISO) over BoB, but not all TCs synchronizing with FNISO were able to develop into STCs. The study analyzes the distinctive environmental conditions for regular and super TC groups. From the intraseasonal background, the most important difference appears in the intensity and relative location of FNISO. A stronger and closer ISO would lead to a colder upper-level air temperature and thus a more unstable stratification which promotes a greater TC intensity. Similar to intraseasonal timescale, upper-level temperature on the interannual timescale is also colder in the STC cases. The combined intraseasonal and interannual environmental temperature difference between lower and upper troposphere are responsible for significant difference between the STC and TC cases.