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Quantifying the Contribution to Western United States Wildfire Area Burned due to Trends in the El Niño Southern Oscillation and the Pacific Decadal Oscillation

Presentation Date
Wednesday, December 13, 2023 at 2:10pm - Wednesday, December 13, 2023 at 6:30pm
MC - Poster Hall A-C - South



Warming and drying climate trends across the western United States (US) from natural and anthropogenic drivers have increased annual area burned ~300% since 1984. A dominant mode of natural climate variability in the western US is the El Niño-Southern Oscillation (ENSO) and its related decadal variability, the Pacific Decadal Oscillation (PDO). From the 1980s-2000s to the 2000s-2020s, the tropical Pacific shifted from a predominantly warm-phase state with a weak sea-surface temperature gradient (SSTg) across the tropical Pacific to a predominantly cool-phase state with an enhanced SSTg, likely enhancing fire-related hydroclimate beyond the effects from anthropogenic regional warming alone. Importantly, coupled atmosphere-ocean climate models generally simulate a weakened tropical Pacific SSTg in response to anthropogenic greenhouse-gas emissions, which are in contrast to the observed trends thus far. Contrasts between modeled and observed trends, potentially arising from systematic model biases in equatorial Pacific ocean-atmosphere dynamics or in other regions (e.g. the Southern Ocean) that influence the tropical Pacific, indicate a wider range of uncertainty in future western US hydroclimatic trends than is implied by climate model projections alone. In this research, we quantify the contribution and characteristics of the tropical Pacific SSTg’s influence on annual wildfire area by geography and vegetation type. Using this model, and comparing results of the observed strengthening SSTg trend to a scenario removing the trend, we found that the strengthening SSTg was partly responsible for increases in area burned over four decades in forested regions, but counteracted the increase in observed area burned in the interior southwest. The result has implications for projecting future changes in area burned given the ongoing debate as to causes of the recent tropical Pacific SSTg trend, how it will evolve in the future, and whether climate models can simulate this.

Funding Program Area(s)