Hydroclimatic Regime Shift Weakens Snowmelt Regulation of Summer Heat Extremes in Northern China

Changes in precipitation phase and seasonal water supply can modify land‐surface moisture memory, but their implications for summer heat extremes remain insufficiently understood in midlatitude dryland transition zones. Northern China has experienced substantial hydroclimatic changes in recent decades, yet it remains unclear whether these changes have altered the efficiency with which spring snowmelt regulates subsequent summer heat extremes. Using daily observations from 348 meteorological stations during 1980–2019, we examined long‐term changes in precipitation phase and seasonal hydroclimatic conditions and assessed how effective cumulative spring snowmelt ( M AM ) is linked to summer heat extremes. We identified a hydroclimatic transition around 2000, with northern China shifting from a ‘warming‐without‐wetting’ to a ‘warming‐wetting’ regime, mainly associated with increased summer rainfall. This transition was spatially heterogeneous: northwestern China experienced persistent wetting with increases in both snowfall and rainfall, northeastern China became wetter primarily because of increased rainfall, whereas North China retained a warming‐drying tendency. Against this background, response analysis showed that greater M AM was generally accompanied by fewer and weaker summer heat events. The regulatory association of M AM was consistently stronger for T max than for N hot . Pathway analysis further suggested that antecedent upper‐root‐zone soil moisture and surface energy partitioning mediated the snowmelt–heat linkage, with additional modulation by summer rainfall. After 2000, mean effective cumulative spring snowmelt decreased, soil moisture and latent heat flux decreased, while sensible heat flux and summer heat metrics increased. These changes indicate a weakened M AM ‐related regulation of T max and a reduced efficiency of snowmelt‐derived moisture buffering under the post‐2000 hydroclimatic background. These results suggest that recent hydroclimatic change in northern China has altered not only seasonal water availability but also the land‐surface regulation of summer heat extremes, with implications for understanding how climate warming reshapes regional water–heat interactions and amplifies heat risk.