Tropical Weather Exacerbates Heat and Wildfire Threats in the Pacific Northwest
A better understanding of how the tropical weather variability influences heatwaves and fire hazards in the Pacific Northwest offers a potential pathway to improving forecasts for these impacts weeks to months in advance.
Image by John Smith | Forest Service, USDA
The Pacific Northwest region of North America (PNW) has experienced devastating summer heatwaves in past years. Many studies have associated summer heat extremes in the PNW with the El Nino-Southern Oscillation and other variations of circulation in the lower atmosphere (troposphere). However, the effect of summertime tropical sub-seasonal drivers, which influence heat extremes and fire weather conditions in the PNW, remains elusive, as does the understanding of the underlying processes. Through a detailed examination of historical data supported by mechanistic analysis and model experiments, researchers unveil that boreal summer intraseasonal oscillation (BSISO)—a large-scale, tropical eastward-propagating system of clouds, rainfall, and wind during summertime—intensifies heat extremes and wildfire risks in the PNW. This particularly occurs when the BSISO’s convection is located over the tropical eastern North Pacific Ocean.
The occurrence of heat extremes and fire risks in the PNW is heightened by 50–120% (2.2 times the seasonal probability) during the BSISO phases, where convection over the tropical western North Pacific moves northeastward. The physical process behind this effect is primarily linked to BSISO-induced heating over the tropical eastern North Pacific, which acts as a source for atmospheric Rossby waves that traverse through Earth's atmosphere and influence weather patterns downstream in the PNW. The results suggest that improving the representation of BSISO-induced heating over the tropical eastern North Pacific in weather and climate models could help enhance forecasts of heatwaves and wildfire threats in the PNW from weeks to months in advance.
Heat waves can cause large socioeconomic and environmental impacts in the PNW, but understanding the factors triggering these events remains incomplete. This study investigates whether and how a sub-seasonal remote tropical driver influences the occurrence of heat waves in the PNW. Researchers found that the occurrence of heat extremes and associated fire-conducive weather conditions in the PNW is significantly heightened during the BSISO phases 6-7 by ~50–120% relative to the seasonal probability. Sensitivity experiments using a linear baroclinic model isolated the diabatic heating from the tropical eastern North Pacific as the dominant driver. Rossby wave trajectory analysis further confirms this by demonstrating that BSISO-induced heating in the tropical eastern North Pacific is the primary source of the northward wave energy. This result provides a robust and distinct dynamical pathway of the BSISO teleconnection impact on the PNW. The results of the current study underscore a potential route for improving sub-seasonal to seasonal predictions (from weeks to months in advance) of the PNW heat and wildfire extremes by improving the representation of BSISO-related diabatic heating over the tropical eastern North Pacific in the numerical weather prediction and climate models.