Changes in the Covariability of Summer and Autumn Tropical Cyclone Frequency Over the Western North Pacific

ABSTRACT This study investigates interannual changes and long‐term trends in the covariability of summer (June–August) and autumn (September–November) tropical cyclone (TC) frequency over the western North Pacific (WNP) during 1961–2024. By using an empirical orthogonal function transformation, summer and autumn WNP TC frequency changes are converted to in‐phase and out‐of‐phase changes. In‐phase covariability is primarily controlled by the spring Pacific meridional mode, with more (fewer) summer and autumn TCs during its positive (negative) phase. Out‐of‐phase covariability is mainly linked to El Niño‐Southern Oscillation (ENSO) during the subsequent winter, with more (fewer) summer TCs but fewer (more) autumn TCs for a developing El Niño (La Niña). When considering different PMM‐ENSO relationships, in‐phase covariability occurs in PMM‐only and opposite‐signed PMM‐ENSO years, while out‐of‐phase covariability is found in ENSO‐only and same‐signed PMM‐ENSO years. These results can be explained by consistent (opposite) simultaneous correlations between TC frequency and the PMM (ENSO) index during summer and autumn. Additionally, given the increasing amplitude and influence of the PMM on ENSO in recent decades, same‐signed PMM‐ENSO years are occurring more frequently, leading to summer and autumn TC frequency exhibiting strengthened out‐of‐phase covariability but weakened in‐phase covariability, respectively.