The Pacific Northwest region of North America (PNW) has suffered from devastating summer heatwaves in past years. For example, at the end of June 2021, temperatures in the region reached record highs, dramatically increasing heat-related fatalities, economic losses, and wildfires. A growing body of literature has linked summer heat extremes in the PNW to the El Nino-Southern Oscillation (ENSO) and other variations of tropospheric circulation, such as stationary waves and atmospheric blocking. However, the extent to which summertime tropical sub-seasonal drivers influence heat extremes and fire weather conditions in the PNW remains elusive, as does our understanding of the underlying processes.
By examining the historical data combined with mechanistic analysis and model experiments, researchers from Pacific Northwest National Laboratory (PNNL) discovered the first comprehensive evidence linking BSISO, a large-scale eastward-propagating tropical system of clouds, rainfall, and wind in summer, to heat extremes and wildfire risks in the PNW. Notably, during BSISO phases 6–7, when the convective system is in the central-to-eastern North Pacific sector, the occurrence of heat extremes is significantly heightened by approximately 50–120% (2.2 times the seasonal probability; see Figures. 1a and d). As extremely high temperatures create favorable conditions for wildfires, the probability of dangerous fire-weather conditions is also more than doubled (by up to 2.2 times or ~120% as compared to the climatology) across much of Washington, northern Oregon, northern Idaho, and southwestern Canada during phases 6–7 (see Figures 1b and e). These more frequent dangerous fire-weather conditions are consistent with increased extreme vapor pressure deficit (VPD) in the PNW region (see Figures 1c and f). The high VPD indicates a low relative humidity and high temperature, conditions that cause increased evapotranspiration and the rapid drying of vegetative fuels, which increase fire risks.
The physical process underlying this effect is associated with BSISO-related diabatic heating in the tropical central-to-eastern North Pacific, which excites the northeastward propagation of Rossby waves driving the anomalous anticyclonic circulation anomaly in the Pacific North America sector (Figure 2). This propagation is supported by background westerly flow (associated with the central-eastern North Pacific summer subtropical jet), which acts as a waveguide for sub-seasonal Rossby waves entering North America. The resulting anomalous anticyclone associated with the Rossby wave train, in turn, strengthens the background summer stationary wave ridge over the PNW, leading to a prominent high-pressure system over the region. This high-pressure system then promotes surface warming by increasing anomalous downwelling shortwave radiation and enhancing adiabatic heating. Together, these processes elevate surface air temperatures, increasing the likelihood of extreme heat events.
This novel mechanism, which elucidates how BSISO modulates extreme heat events and weather conditions conducive to wildfires in the PNW, is robust and has been confirmed by both linear baroclinic model (LBM) experiments and Rossby wave ray tracing analysis. Diagnosis using the LBM reveals that heating from the tropical central-to-eastern North Pacific, which is linked to BSISO, substantially enhances the northeastward propagation of Rossby waves toward the PNW (see Figures 3a–c). Rossby wave ray tracing analysis further validates this by demonstrating that BSISO-induced heating in the tropical Pacific is the primary source of the northward wave energy, which is consistent with observed energy flux patterns toward the PNW. Overall, these findings indicate that persistent anticyclonic anomalies associated with the wave train induced by BSISO over the tropical central-to-eastern North Pacific are key drivers of the surface warming that occurs during heatwave events in the PNW.
In summary, our findings reveal that BSISO significantly influences heat extremes and fire risks across the PNW. This highlights a potential pathway for improving S2S predictions of heatwaves and wildfire risks in the region, namely, by refining the representation of BSISO-related diabatic heating over the tropical central-to-eastern North Pacific in the numerical weather prediction and climate models. If BSISO forecasts are made available two to three weeks in advance, they could then provide a three- to five-week lead time for predicting heatwaves and fire risks in the PNW. This extended lead time would give decision-makers an early-warning system to better prepare for the heightened risks of heat waves and wildfires across the PNW in the future. Further research is needed to understand how non-linear processes, such as local land-surface and SST feedback, influence BSISO's impact on heatwaves in the PNW, and how these impacts may change in a warmer climate.
Citation:
Lubis, S.W., Chen, Z., Lu, J. et al. Enhanced Pacific Northwest heat extremes and wildfire risks induced by the boreal summer intraseasonal oscillation. npj Clim Atmos Sci 7, 232 (2024). https://doi.org/10.1038/s41612-024-00766-3
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