<p>The persistent winter–spring dryness or wetness variations in Southwest China (SWC) pose a significant challenge for climate prediction, with underlying mechanisms not fully understood. This study reveals that the in-phase variations of dryness/wetness between winter and spring over SWC can be displayed by the first leading mode of season-reliant empirical orthogonal function (S-EOF1) analysis on the standardized precipitation evapotranspiration index. A negative (positive) phase in the first principal component (PC1) corresponds to persistent winter–spring dry (wet) conditions over SWC on an interannual timescale. Results suggest that the local anticyclone anomalies in the mid-upper troposphere over the Tibetan Plateau (TP) and SWC lead to reduced precipitation through descending motion, along with enhanced temperature and potential evapotranspiration via adiabatic warming and diabatic heating processes, thereby contributing to prolonged dry conditions during winter and spring. Reduced winter snow cover on the TP (TPSC) plays a critical role by inducing local tropospheric warming, which strengthens the meridional temperature gradient and transient wave activity around 35°–50°N, leading to mid-upper tropospheric anticyclone anomalies over the TP–SWC during winter. These anomalies persist into spring due to the continued heating effects of reduced TPSC, exacerbating precipitation deficit and elevated temperature, thereby sustaining dryness over SWC across both seasons. Numerical experiments using the liner baroclinic model confirm the above physical processes. During years with excessive TPSC, the processes are reversed. Additionally, winter Arctic Oscillation and North Atlantic sea surface temperature anomalies also exert significant influences on persistent winter–spring dryness/wetness variations.</p>