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Burned forest area has increased significantly in Western North America in recent decades, driven in part by climate conditions becoming more conducive to fire. This dissertation advances our understanding of the large-scale climate dynamics that drive drought and forest fire in Western North America and similar regions around the world. We also investigate the causes and implications of climate model biases in simulating natural climate variability, particularly for modes and phases of variability with large impacts on drought and fire in Western North America.
In the first chapter, we identify regional and global climate patterns in preceding seasons that typically influence the year-to-year variability of burned summer forest area in California. High vapor pressure deficit (VPD), high temperatures, low precipitation, high subsidence, high geopotential height, low soil moisture, and low snowpack and snowmelt anomalies all correlate significantly with July California burned area as far back as the January before the fire season. These climate anomalies occur as part of a hemispheric scale pattern with weak connections to the tropical Pacific Ocean. We explain how climate and weather anomalies, particularly extreme heat, drove the record-breaking burned-area year of 2018.
VPD, a function of temperature and humidity, is a skillful seasonal predictor of burned area in the southwestern United States (the ”Southwest”). Unexpectedly, despite the increased capacity of a warmer atmosphere to hold water vapor, near-surface specific humidity decreased from 1970 to 2019 in much of the Southwest, particularly in spring, summer, and fall. In Chapter 2, we identify declining near-surface humidity from 1970 to 2019 in the Southwest with reanalysis and in situ station data. Focusing on the interior Southwest in the months preceding the summer forest fire season, we find that an early spring decline in precipitation in the interior region induced a decline in soil moisture and evapotranspiration, drying the lower troposphere in summer. This prior season precipitation decline is in turn related to a trend toward a Northern Hemisphere stationary wave pattern that places a high pressure ridge over the interior Southwest. Using fixed humidity and temperature scenarios and the observed exponential relationship between VPD and burned forest area, we estimate that with no increase in temperature at all, the humidity decline alone would still lead to nearly one-quarter of the observed VPD-induced increase in burned area over 1984–2019.
Sea-surface temperature variability in the Pacific plays a powerful role in evolving global hydroclimate on decadal timescales, particularly in Western North America. Chapter 3 evaluates the ability of the current generation of climate models (CMIP6) to simulate realistic decadal sea-surface temperature variability in the Pacific and its teleconnections to circulation, precipitation, and aridity around the world. Using CMIP6 model large ensembles, we evaluate model fidelity in reproducing spatial and temporal characteristics of the Pacific Decadal Oscillation (PDO) and the Interdecadal Pacific Oscillation (IPO), and their hydroclimate teleconnections compared to observations. We find that models’ underestimation of decadal-scale Pacific SST variability is associated with their inability to produce large-amplitude decadal swings in precipitation in Southwestern North America that drive decadal variability in VPD. Finally, in Chapter 4, we investigate and contrast the possible climate drivers of observed increases in fire in the southwestern U.S. versus Eastern Australia, two of the most fire-prone regions of the world with significant, opposite-signed hydroclimate teleconnections to modes of Pacific decadal variability. We show that in both regions, VPD has increased with a large contribution from anthropogenic climate change. In the southwestern U.S., the recent trend in the tropical Pacific west--east sea surface temperature (SST) gradient contributed positively to the VPD-induced increase in burned forest area since 1984. However, in Eastern Australia, the tropical Pacific SST gradient trend has likely offset the anthropogenic climate change-induced increase in burned forest area.