The Critical Role of Snowmelt Onset‐Driven Vapor Pressure Deficit Variations in Wildfire Dynamics of Northern Latitudes

Abstract Interannual variability in snowmelt onset triggers dynamic responses in processes like surface energy exchange and the hydrological cycle through interactions between vegetation, soil and atmosphere, significantly affecting subsequent fire weather. However, it still remains unclear how shifts in snowmelt onset regulate fire weather and thereby wildfire dynamics during the following seasons. We analyze snowmelt onset dates and wildfire data for northern latitudes (>40°N) and find that interannual variability in snowmelt onset critically influences wildfire dynamics during post‐snowmelt periods, with earlier snowmelt tending to increase wildfire incidence. This influencing role is primarily associated with the snowmelt onset‐induced variations in vapor pressure deficit, which is geographically pronounced in 47.5% of the northern latitudes, contributing 57.1% of the total influencing role, approximately doubling the contribution from plant water deficit or fuel moisture content and fuel availability. Mechanistically, compared to late snowmelt onset, early vegetation spring green‐up caused by an earlier occurrence in snowmelt leads to early moisture consumption. This, in turn, amplifies plant water deficits, which limit evaporative cooling and increase sensible heat fluxes, exacerbating atmospheric dryness and creating favorable conditions for wildfires during the post‐snowmelt period. The reveal of these mechanisms has important implications for assessing wildfire risk, enhancing wildfire simulations and forecasting in regions vulnerable to ongoing and future climate change, and projecting carbon‐climate feedback.