Intra-Seasonal Variabilities Associated With Tropical Cyclones in Eastern and Southeastern Asian Regions and Their Recent Change

This study focuses on the diverse intra-seasonal variabilities (ISVs) exhibited by tropical cyclone (TC) occurrence frequency affecting southeastern and eastern Asian regions, classified as SEATOFs and EATC occurrence frequency (EATOFs). Both groups exhibit pronounced 8–20-day variability, particularly in EATOFs, whereas SEATOFs demonstrate an additional 20–60-day peak. The corresponding 8–20-day atmospheric ISVs are focused on, which closely mirror the tropical quasi-biweekly oscillation (QBWO) but differ in spatial structure. SEATOFs are tied to a zonally elongated, westward-propagating QBWO (W-QBWO), while EATOFs are linked to a west-northward-propagating QBWO (NW-QBWO) with a slight southwest-northeast tilt and intensified perturbations along the East Asian coast. Notably, EATOFs and their 8–20-day ISV amplitude have concurrently increased since the late 1990s. This shift can be attributed to an interdecadal shift of QBWO dominance from W-QBWO to NW-QBWO. These results highlight the QBWO's substantial influence on TC activities across both Asian regions, particularly along the East Asian coast. © 2025. The Author(s).