The Role of Long-Term Trend and Internal Variability in Altering Fire Weather Conditions in the Contiguous United States

Fire hazards can have catastrophic effects on human society and ecosystems, posing pronounced threat to public health and natural resources. Previous studies have shown that atmospheric vapor pressure deficit (VPD: difference between the saturated and actual vapor pressure) is closely related to fire activity. In this study, we first examine the observed long-term VPD trend in the contiguous United States (US), considering an evolving background climate. Supporting previous studies, our results suggest that anthropogenic climate change notably elevates the risk of fire activity in the western US especially for warm seasons. The elevated VPD trends in the western US are mainly associated with changes in temperature. While debates persist regarding the dominant internal dynamical drivers that exacerbate fire events, our findings, based on observations and a 30-member single-model large ensemble, indicate that La Niña-like sea surface temperature anomalies in the Pacific basin and a positive Atlantic multidecadal variability are favorable for the development of fire weather conditions in the western US. Such fire-favored conditions are also associated with a vertically enhanced high-pressure system above the western US. Our research hypothesis posits that oceanic variability could establish favorable pre-conditions that might modulate the propagation of atmospheric circulations and teleconnection patterns. This interaction could potentially result in more occurrences of high-pressure systems over the western US, thereby exacerbating the already challenging fire-weather conditions in that region.