2026 Articles

Contribution of La-Niña-Like Pacific Ocean Trend in Sea-Surface Temperatures to Western United States Wildfire Area, 1984–2022

Juang, Caroline S.; Williams, A. Park; Seager, Richard

In the western United States (US), the annual wildfire extent increased by 306% over 1984–2022, largely due to a 1,320% increase in forest-fire area. During this time, associated with the Pacific Decadal Oscillation (PDO), the tropical Pacific Ocean exhibited a "La-Niña-like" trend with an enhanced zonal sea-surface temperature gradient (gSST) between its warm western and cool eastern sides. In contrast, most atmosphere-ocean models simulate human-caused climate change should force an "El-Niño-like" gSST trend, a discrepancy from observations that is the focus of vigorous investigation. To understand the relevance of uncertainties in projections of gSST to western US wildfire activity, we statistically estimate the contribution of the observed increase in gSST of the past four decades. We compare the observed burned area outcome against a counterfactual scenario in which the observed La Niña-like gSST trend did not occur. We estimate that the strengthened gSST gradient accounted for 19% [11%, 33%] of the increase in western US wildfire area. In forest areas alone, we estimate the gSST trend accounted for 32% [21%, 45%] of the observed increase, due mostly to increased spring and summer vapor-pressure deficit (VPD). Thus, the strengthened gSST was not the dominant driver of the observed increase in wildfire area, but it appeared to play an important secondary role. Should the positive gSST trends not reverse, western US climate may remain more conducive to forest fire than would be expected based on current climate projections.

Keywords: western US, western United States, Pacific Decadal Oscillation, PDO, annual area burned, forest fires, zonal SST gradient, ENSO teleconnection, fire-climate relationship