Abstract Tropical cyclones (TCs) often lead to severe damage and loss of life, with their outer‐core size being a crucial factor in determining their destructive potential. In this study, we detected an abrupt decrease in the lifetime‐maximum size (LMS) of TCs in the western North Pacific since the late 20th century, based on data from various sources. Composite analyses and numerical simulations suggest that the decrease in TC LMS is mainly driven by changes of the large‐scale atmospheric humidity in the vertical direction. Increased mid‐level humidity could enhance the inner‐core deep convection while decreased near‐surface humidity could stabilize the atmosphere and suppress outer‐core convection, thus reducing TC LMS. The changes in vertical large‐scale atmospheric humidity are induced by an intensified Walker circulation associated with a La Niña‐like sea surface warming pattern developed since the late 20th century.

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