Enhanced Pacific Northwest heat extremes and wildfire risks induced by the boreal summer intraseasonal oscillation

Summer heat extremes in the Pacific Northwest (PNW; 42–57°N, 112–130°W) pose major risks to health, agriculture, wildfire activity, water resources, and ecosystems, as highlighted by the record-breaking 2021 heatwave and past large wildfires. While prior work has tied PNW heat extremes to ENSO, stationary waves, and blocking, the role of remote subseasonal tropical variability, specifically the boreal summer intraseasonal oscillation (BSISO), in modulating PNW heat extremes and fire weather remains uncertain. The BSISO, the dominant boreal summer tropical subseasonal mode, exhibits complex northeastward and eastward propagation and affects both tropical and extratropical weather. Teleconnections from BSISO convection vary by phase, and while impacts on Asian heat and precipitation extremes are documented, evidence for North American downstream effects has been limited. Motivated by this gap and hints from the 2021 event, this study asks whether BSISO phases systematically alter the likelihood of PNW heatwaves and fire-conducive conditions, and by what dynamical pathways. The purpose is to provide comprehensive observational evidence over 1979–2020 and to diagnose mechanisms linking BSISO convection/heating to North American circulation and surface heat extremes, with implications for S2S prediction.

Previous studies identify ENSO as a dominant interannual driver of PNW precipitation and heatwave frequency, with stationary waves and blocking modulating the likelihood of heat extremes through internal atmospheric dynamics and ENSO-forced Rossby waves. The BSISO strongly influences Asian monsoon regions, driving precipitation and heatwave extremes, but its North American teleconnections are less explored. Recent work suggested BSISO phase 7 latent heating aided S2S prediction of the unprecedented 2021 PNW heatwave, underscoring the potential importance of BSISO representation in models. Teleconnection patterns from the BSISO depend on convective location (Indian Ocean vs western North Pacific), with wave trains along the Asian jet or into the North Pacific. However, the specific mechanisms and pathways by which BSISO phases affect PNW heat extremes and fire weather had not been clarified before this study.

Data: ERA5 daily fields (geopotential height, 3D winds, temperature, humidity, surface fluxes, radiation components, latent and sensible heat fluxes, Tmax, etc.) at 0.25° covering 1979–2020. NOAA interpolated daily OLR (2.5°) for BSISO index calculation. Fire danger metrics (FWI, FDI) from ECMWF Climate Data Store. VPD computed from near-surface air temperature and dewpoint using standard exponential formulation (c1=0.611 kPa, c2=17.5, c3=240.978°C). BSISO index and events: A bimodal BSISO index constructed via extended EOF of 25–90 day bandpass-filtered OLR with lags −10, −5, 0 days, following Kikuchi (2021). The first two PCs define amplitude A=(PC1^2+PC2^2)^(1/2); active days have A≥1. Eight phases are defined in PC1–PC2 space; to increase samples, consecutive phases with similar convection are paired: 8–1, 2–3, 4–5, 6–7. Event criteria: amplitude ≥1 for ≥5 days; ≥10 days between events; start and end within JJA. Identified events (days): 8–1 (450), 2–3 (377), 4–5 (520), 6–7 (457). Composites and significance: Construct composites of anomalies (e.g., Tmax) relative to 1979–2020 daily climatology, focusing on pentads 0 to +1 (days 0–9 after BSISO phase onset) to account for Rossby wave propagation lag. Statistical significance via moving-block bootstrap (1000 resamples) at 95%. Heatwave definition: Heatwaves are ≥3 consecutive days with Tmax above the 90th percentile (TX90p) computed in a 30-day moving window during JJA over 1979–2020. Heatwaves classified as dry (RH<33%) or moist (RH>66%). Probability ratio (PR): Quantify phase-conditioned changes using PR = (N_HW|B/N_B) / (N_HW|S/N_S), where N_HW|B counts heatwave days within days 0–9 following a phase, and N_HW|S counts heatwave days in all JJA. PR also computed for extremes of FWI/FDI and VPD using local 90th/95th percentiles. Partition PR into dry and moist contributions. Dynamics diagnostics: Examine geopotential height anomalies (500 and 200 hPa) and streamfunction at 200 hPa; compute wave activity flux (WAF) per Takaya & Nakamura (2001) to diagnose Rossby wave energy propagation and its convergence; evaluate stationary waveguide via zonal wind and zonal stationary wavenumber (regions with Ks>0). Compute Rossby Wave Source (RWS) following Sardeshmukh & Hoskins to locate forcing by divergent outflow. Lead–lag cross sections averaged over 180°–110°W track meridional WAF, U200, streamfunction, and WAF divergence. Ray tracing: Rossby wave ray paths released from two BSISO heating lobes (western–central NP: 110°–175°E, 0°–30°N; central–eastern NP: 180°–120°W, 0°–30°N) across wavenumbers 1, 3, and 5 using background 200 hPa flow averaged from day −15 to +15 relative to phases 6–7. Linear Baroclinic Model (LBM): T42, 20 sigma levels, standard damping; forced with simplified BSISO-like diabatic heating centered over (i) tropical central–eastern NP and (ii) tropical western NP, using JJA mean state. Steady responses averaged days 5–15 after forcing onset. Sensitivity experiments vary forcing amplitude over the central–eastern NP to quantify PNW ridge response. Budgets: Surface energy budget anomalies (downward/upward longwave and shortwave, sensible and latent heat fluxes) averaged over PNW during pentad −1 (pre-onset) and pentad +1 (heatwave days) following phases 6–7. Temperature budget at 975 hPa decomposed into local tendency, horizontal advection, adiabatic (vertical motion) heating, and diabatic heating.

- BSISO phase dependence: The largest positive Tmax anomalies in the PNW occur during phases 6–7, with regional daily Tmax anomalies up to ~1.5°C (days 0–9), while phases 2–3 feature negative anomalies. - Heatwave likelihood: During phases 6–7, the probability ratio (PR) of heatwave days increases by ~1.5–2.2 (50–120%) relative to JJA climatology across much of the PNW, especially Washington and northern Oregon. In phases 2–3, heatwaves are less likely (PR<1). About ~70% of the increased heatwave probability during phases 6–7 is due to extremely hot and dry events. - Fire weather and atmospheric dryness: During phases 6–7, extreme fire weather conditions (FWI >95th percentile) and vapor pressure deficit (VPD >95th or 90th) more than double relative to climatology across Washington, northern Oregon, northern Idaho, and southwest Canada; far western Montana is also affected. FDI shows a similar but slightly smaller increase. - Teleconnection mechanism: Enhanced BSISO convection and diabatic heating over the tropical central-to-eastern North Pacific in phases 6–7 amplify the Rossby wave source north of the convection and generate a downstream barotropic Rossby wave train. This produces a strong anticyclonic ridge over the PNW, embedded within an arch-shaped tripole (cyclone over North Pacific, anticyclone over PNW, cyclone over NE North America). WAF diagnostics show strong poleward wave energy propagation from 180°–110°W into North America along a meridional waveguide supported by the subtropical jet, with WAF convergence over the PNW ridge. - Model and ray-tracing support: Ray tracing indicates that waves initiated near the central–eastern NP propagate poleward/eastward toward the PNW along arc-like routes (wavenumbers 1, 3, 5). LBM forced by central–eastern NP heating reproduces a significant positive 500 hPa height anomaly over the PNW; western NP heating also yields an anticyclonic response but weaker and more zonal. Increasing the central–eastern NP heating amplitude strengthens the PNW ridge response. - Surface and lower-tropospheric processes: During heatwave days (pentad +1), reduced cloud cover under the ridge increases downwelling shortwave radiation at the surface, while latent heat flux decreases due to soil drying and weaker winds; sensible heat flux remains comparable or slightly larger than latent heat flux. The 975 hPa temperature budget shows increased adiabatic heating (subsidence) and local diabatic heating, both contributing to surface warming. - Temporal evolution: Anticyclonic anomalies over the PNW persist up to ~15 days after onset during phases 6–7, consistent with elevated heatwave risk in the 0–9 day window.

The study directly addresses whether subseasonal tropical forcing modulates PNW summer heat extremes and wildfire-conducive weather. It shows that BSISO phases 6–7 substantially elevate heatwave and fire danger probabilities via a clear teleconnection pathway: enhanced diabatic heating over the central–eastern North Pacific intensifies the Rossby wave source and drives poleward wave energy along a jet-supported waveguide into North America, reinforcing the PNW ridge. This ridge reduces clouds, increases surface shortwave radiation, and enhances subsidence-induced adiabatic warming, jointly raising surface temperatures and atmospheric dryness (high VPD), which increases fire danger metrics (FWI/FDI). The mechanism is supported by WAF diagnostics, ray tracing, and LBM experiments, and clarifies the observed phase dependence (enhancement in phases 6–7, suppression in 2–3). These insights imply that S2S prediction of PNW heat and wildfire risks can benefit from accurate representation and prediction of BSISO convective heating, particularly over the central–eastern North Pacific. The results also suggest interactions with other modes: a secondary contribution from western North Pacific heating via zonal wave energy along the North Pacific jet, and possible modulation by ENSO and climate change, which may amplify eastern Pacific diabatic heating and strengthen teleconnections affecting PNW heatwaves.

This work provides the first comprehensive observational evidence that BSISO phases 6–7 markedly increase PNW summer heat extremes and wildfire-conducive conditions (PR ~1.5–2.2), primarily through enhanced diabatic heating over the tropical central–eastern North Pacific that triggers a poleward Rossby wave train and a persistent ridge over the PNW. The ridge drives surface warming via increased solar radiation and adiabatic subsidence, elevating VPD and fire danger metrics. Ray tracing and linear baroclinic model experiments corroborate the central–eastern North Pacific as the dominant teleconnection source. The findings highlight a tangible subseasonal pathway to improve S2S forecasts of PNW heatwaves and wildfire risk by better simulating and predicting BSISO convection/heating. Future research should quantify the roles of nonlinear land–atmosphere feedbacks, interactions with circumglobal teleconnection patterns, transient eddy feedbacks, local SST feedbacks, and anthropogenic forcing. Further work should assess how BSISO diversity and amplitude, and ENSO modulation, affect teleconnection strength and predictive skill, and evaluate model fidelity in representing the canonical BSISO propagation into the central–eastern North Pacific.

- Dynamics framework relies primarily on linear wave diagnostics (WAF, ray tracing) and a linear baroclinic model; nonlinear processes (land-surface feedbacks, interactions with circumglobal teleconnections, transient eddy feedbacks, local SST feedbacks) may modify responses and introduce asymmetries between phases. - WAF uses QG approximations that may introduce errors in regions where approximations break down, though less critical north of ~15°N. - Event sampling focuses on 1979–2020 JJA and days 0–9 after phase onset; results may vary with different windows or definitions. - Fire risk metrics (FWI/FDI) are meteorological indices and do not incorporate fuel load/ignitions; they indicate danger conditions, not realized fires. - Affiliated uncertainties from reanalysis and OLR datasets, and model skill limits in simulating BSISO propagation, especially its diversity. - The study emphasizes central–eastern NP forcing; secondary contributions from western NP heating exist but were not explored in depth.