Climate Variations of Autumn Tropical Cyclone Genesis in the Western North Pacific: Roles of Tropical and Midlatitude SST Anomalies

Abstract

Tropical cyclones (TCs) in the western North Pacific (WNP) pose significant threats to socio‐economic systems, yet their variability during the autumn season remains relatively understudied compared to summer. This study examines the interannual and decadal variations of WNP TC genesis in autumn using reanalysis data sets and atmospheric general circulation model (AGCM) simulations. It is shown that sea surface temperature anomalies (SSTAs) in the tropical Pacific and Indian Oceans, as well as in the midlatitude North Pacific, are the primary drivers. On interannual timescales, SSTAs in the tropical Pacific and Indian Oceans—resembling El Niño conditions—suppress convection and induce anomalous subsidence over the WNP via atmospheric wave responses, and thereby inhibit TC formation. On decadal timescales, warm SSTAs in the midlatitude North Pacific play a dominant role. These anomalies induce anomalous upward surface turbulent heat flux, leading to increased diabatic heating and weakened atmospheric baroclinicity and transient eddy activity. The resulting anomalous atmospheric circulation is characterized by a positive geopotential height anomaly over the midlatitude North Pacific. Anomalous southward energy propagation and meridional overturning circulation further modify the upper‐tropospheric flow in the subtropics, leading to strengthened subtropical westerlies and enhanced vertical wind shear over the WNP, which are unfavorable for TC genesis. AGCM experiments forced with historical SSTAs reproduce these observed patterns well, supporting the proposed mechanisms. These findings underscore the distinct roles of tropical and midlatitude SSTAs in modulating autumn WNP TC activity on interannual and decadal timescales, with important implications for the prediction of TC genesis.