Structure and Dynamics of the Boreal Late‐Summer East Asian Monsoon Interannual Variability

Abstract

Pronounced interannual variability of the late‐summer (July–August) East Asian summer monsoon exerts strong influences on flood and drought events across East Asia, yet the underlying processes governing this variability remain poorly understood. Using reanalysis data, we identify the dominant mode of circulation variability that captures the dominant interannual variability of the late‐summer East Asian monsoon system. The mode features a troposphere‐deep anticyclonic/cyclonic circulation, tilting northwestward with height, within the westerly jet exit region. This mode is dynamically maintained by extracting kinetic and potential energy from the background flow. The jet exit region enables the mode to gain eddy kinetic energy from the background field. Meanwhile, its northwestward‐tilting vertical structure facilitates the extraction of available potential energy from the meridional temperature gradient over East Asia. Vorticity budget analysis reveals that this northwestward‐tilting vertical structure arises from the vertical shear of climatological northerlies on the eastern flank of the summer South Asian High. In turn, the mode drives vertical motion anomalies via temperature advection, regulating precipitation anomalies of the late‐summer East Asian monsoon.