Extreme hot and dry summer conditions in the Pacific Northwest (PNW) region of North America pose significant risks to human health, agriculture, water resources, and the environment, particularly through increased wildfire activity. The 2021 heatwave and the 2014 Carlton Complex fire serve as stark examples of the devastating consequences. The Intergovernmental Panel on Climate Change (IPCC) projects an exacerbation of these heat extremes in a warming climate. While previous research has linked PNW summer heat extremes to climate modes like El Niño-Southern Oscillation (ENSO), stationary waves, and atmospheric blocking, the role of the subseasonal remote tropical driver, particularly the boreal summer intraseasonal oscillation (BSISO), remains unclear. Understanding this influence is crucial for improving subseasonal-to-seasonal (S2S) predictions of heatwaves and wildfire risks. This study aims to fill this knowledge gap by providing comprehensive observational evidence linking BSISO to summer heat extremes and wildfire risks in the PNW, and by proposing a plausible mechanism explaining the observed teleconnection.

Existing literature extensively links PNW summer heat extremes to ENSO and variations in tropospheric circulation, such as stationary waves and atmospheric blocking. ENSO, as the dominant mode of interannual variability, is known to influence precipitation and heatwave frequency in northwestern North America. Similarly, interannual variability in stationary waves and blocking events heighten the probability of heatwaves. While these factors have been thoroughly studied, the influence of remote tropical subseasonal drivers, especially the BSISO, on PNW heat extremes and fire weather conditions has remained elusive. Recent work hints at the latent heat release during BSISO phase 7 contributing to the 2021 PNW heatwave prediction, emphasizing the importance of accurate BSISO representation in S2S models. However, empirical evidence and a mechanistic understanding of BSISO's teleconnection effects on PNW heatwaves and fire weather are still lacking.

This study utilizes daily data from the European Centre for Medium-Range Weather Forecasts' fifth-generation atmospheric reanalysis (ECWMF-ERA5) dataset (1979-2020) and NOAA/NESDIS daily interpolated outgoing longwave radiation (OLR) data (1979-2020). The BSISO subseasonal variability was analyzed using a daily bimodal index based on Kikuchi's methodology, which captures the northward and eastward propagation of BSISO convection. Four pairs of consecutive BSISO phases (8-1, 2-3, 4-5, and 6-7) with similar convective patterns were examined. Composite analyses were conducted for outgoing longwave radiation (OLR) and diabatic heating anomalies, daily maximum temperature (Tmax) anomalies, probability ratio (PR) of heatwave occurrences, fire weather index (FWI), fire danger index (FDI), and vapor pressure deficit (VPD). Statistical significance was assessed using a moving-block bootstrap test. To understand the dynamical mechanisms, composites of geopotential height (GPH) anomalies, stream function (SF), wave activity flux (WAF), and Rossby wave source (RWS) were analyzed. A linear baroclinic model (LBM) with specified heating profiles was used to examine the generation of Rossby wave trains. Finally, surface and 975 hPa temperature budget analyses were performed to elucidate the thermodynamic contributions to surface warming during heatwave events. The heatwave was defined as at least three consecutive days exceeding the 90th percentile of daily maximum temperatures (TX90p) during the warm season (JJA). Dry and moist heatwaves were further classified based on relative humidity thresholds.

The study found that during BSISO phases 6-7, the likelihood of heatwaves and dangerous fire weather conditions in the PNW significantly increased, with PR values reaching 1.5-2.2 times the climatological probability. This heightened risk was particularly pronounced in Washington, Oregon, northern Idaho, and western Montana. The Tmax anomalies reached up to 1.5 °C during these phases. Conversely, phases 2-3 showed a decrease in Tmax and heatwave frequency. The increased heatwave frequency during phases 6-7 was predominantly associated with extremely hot and dry conditions. Analysis of large-scale circulation patterns revealed an arch-shaped tripole GPH anomaly pattern during phases 6-7, with an anticyclone over the PNW, consistent with a barotropic Rossby wave train emanating from the tropical central-to-eastern North Pacific. This ridge strengthens the summertime stationary waves, promoting stable conditions and heat accumulation. Conversely, phase 2-3 showed an opposite pattern with a low-pressure anomaly over the PNW. WAF analysis confirmed northward wave energy propagation from the tropical central-to-eastern North Pacific toward the PNW during phases 6-7, supporting the development of the anticyclone. The LBM experiments confirmed the crucial role of diabatic heating in the tropical central-to-eastern North Pacific in generating the Rossby wave train. Temperature budget analysis showed surface warming during heatwaves was primarily due to increased surface radiative heating and enhanced adiabatic warming from anomalous subsidence.

This study's findings significantly advance our understanding of PNW heatwaves and wildfire risks by highlighting the critical role of BSISO. The identified teleconnection mechanism, involving enhanced diabatic heating in the tropical central-to-eastern North Pacific triggering a Rossby wave train, offers a new perspective on subseasonal predictability. The results are consistent with previous findings highlighting the importance of eastern North Pacific heating for northward energy transport. While the study emphasizes the primary role of eastern North Pacific BSISO heating, other factors like land-surface feedback, non-linear interactions with circumglobal teleconnection patterns, extratropical transient eddy feedback, and local SST feedback could also play a role and warrant further investigation. The interplay between internal variability (ENSO) and external forcing (anthropogenic climate change) on the BSISO's impacts also requires further study. Preliminary analyses suggest that ENSO enhances the BSISO's influence on PNW heatwaves during phases 6-7.

This research provides the first comprehensive observational evidence linking BSISO to summer heat extremes and wildfire risks in the PNW. The study reveals a key teleconnection mechanism driven by enhanced diabatic heating in the tropical central-to-eastern North Pacific. This mechanism offers a promising avenue for improving S2S prediction of PNW heat extremes and wildfire risks by focusing on more accurate BSISO simulation and prediction. Future research should investigate the impact of anthropogenic climate change on this mechanism and further explore the role of other contributing factors. Improving the representation of the BSISO in numerical models is crucial for enhancing the skill of subseasonal-to-seasonal prediction of heatwaves and wildfires in the PNW.

The study primarily focuses on linear wave dynamics. Non-linear processes, such as land-surface feedback, non-linear interactions with recurrent summer circumglobal teleconnection patterns, extratropical transient eddy feedback, and local SST feedback, are not fully explored and could influence the results. The analysis is also limited to a specific period (1979-2020) and might not fully capture long-term trends or interdecadal variability. The simplified representation of BSISO forcing in the LBM experiments might not fully capture the complexity of real-world conditions.