The role of long-term trends and internal variability in altering fire weather conditions in the western United States

Abstract Atmospheric vapor pressure deficit (VPD) is closely related to fire activity in the western United States (US). Here, we demonstrate that the western US experiences an escalating VPD trend driven by increased atmospheric capacity to hold water vapor due to global warming, intensifying aridity and fire risk, especially in summertime. Atop these drying trends, the leading VPD mode, with a pronounced decadal component, displays a center-of-action in the southwest US. In its dry phase, this mode is characterized by reduced soil moisture, high-pressure systems, and La Niña-like conditions, and connected to decadal sea surface temperature (SST) variability in the Pacific and Atlantic basins. Leveraging large climate simulations, a backward model-analog technique reveals persistent SST signals a few seasons preceding the dry VPD mode. While state-of-the-art models can reasonably capture the spatial patterns of VPD, challenges emerge in representing the decadal component of the leading VPD mode, which predominantly arises from misrepresentations of the actual rather than saturation vapor pressure.