Enhanced Interannual Variability and Extreme Freezing Rain Events in Late Winter over Southern China since the Early 21st Century

This study explores the evolving spatiotemporal characteristics of winter freezing rain (FZR) over Southern China (SC) and the underlying dynamic mechanisms, based on hourly ERA5 data (1979–2025). Particular focus is placed on the regime shift in interannual variability and extreme events observed since the early 21st century. Regionally averaged, winter FZR totals 2.7 d and 8.7 mm annually, with late winter (January–February) constituting the primary peak (85%). Analysis reveals a pronounced interdecadal shift in late-winter FZR. In Period 1 (P1: 1979–2002), FZR frequency showed a significant weakening trend. In Period 2 (P2: 2003–25), this trend stabilized or transitioned into an increase, accompanied by a near-doubling of the interannual variance and a clustering of extreme events; notably, the five most severe FZR extreme events on record all fell within P2. The atmospheric modulators for these P2 changes differ between January and February. January variability is dominated by heightened fluctuations in high-latitude systems, including the Siberian High and Ural blocking. Conversely, the February variability is driven by an interdecadal strengthening and westward extension of the western North Pacific subtropical high (WNPSH), which alters moisture transport into SC.