Mechanisms of the interannual variability in occurrence numbers of the Southwest China vortex in summer

The Southwest China vortex (SWCV) is one of the dominant weather systems affecting the patterns of summer heavy rainfall in China. Yet, its interannual variability and physical mechanisms remain uncertain. Based on 6-hourly geopotential height data from China’s first-generation global atmospheric reanalysis (CRA-40), a SWCV dataset from 1979 to 2024 is established using the whole-process objective identification method. The results reveal 1,184 SWCV events over the past 46 summers, averaging 25 events annually. The SWCV occurrence numbers (SWCV_NUM) exhibit pronounced interannual variability, with nearly twofold differences between active and inactive years. SWCV-related 12-h maximum precipitation reached torrential rain intensity in 42% and heavy rain in 36%. Enhanced SWCV activity coincides with an extended rain belt from northern Southwest China to the Huaihe River basin, alongside suppressed convection over southern China. The cyclonic circulation over Mongolia (MONC) and the subtropical western North Pacific anticyclone (WNPAC) are closely related to interannual variability of SWCV_NUM. The MONC, primarily modulated by the circumglobal teleconnection pattern, exhibits a quasi-barotropic structure that enhances SWCV_NUM. In subtropical regions, the Indian Ocean Basin Mode-like warming generates anomalous low-level easterlies, inducing anticyclonic shear that strengthens the WNPAC. This circulation pattern enhances tropical moisture transport into Southwest China, promoting warm and humid convergence conducive to SWCV formation. The objectively historical dataset captures SWCV activity and enables extended analysis of its climatological features and interactions with other weather systems.