Climate extremes, including heatwaves, pose significant societal, economic, and human health risks. Since the 1950s, these events have intensified and become more frequent globally, with hot extremes showing the most pronounced changes. Record-breaking heatwaves have been extensively studied, including those in Australia (2012-2013), Central and Eastern China, Japan and Korea (2013), Western and Central Europe (2017), and Northeast Asia (2018). The June 2021 WNA heatwave was exceptional, reaching daily maximum temperatures exceeding 40°C and causing over 1400 deaths and numerous wildfires. Heat events are closely linked to atmospheric circulation anomalies, often manifested as anomalous high-pressure systems (heat domes) that trap hot air, enhance insolation, and transport warm, moist air from lower latitudes. The flow analog method quantifies the contribution of atmospheric circulation to extreme events. For instance, persistent anticyclones significantly contributed to the 2018 Northeast Asia heatwave. While a heat dome was crucial to the 2021 WNA heatwave, a quantitative assessment of its contribution and the influence of background warming remained unknown. This study uses the flow analog method to quantify the heat dome's contribution and investigate the influence of background warming on the 2021 WNA heatwave.

Several studies have investigated the role of atmospheric circulation in extreme temperature events. Teng et al. (2013) examined the influence of a subseasonal planetary wave pattern on US heat waves. Adams et al. (2021) analyzed the relationship between atmospheric circulation patterns and extreme temperature events in North America. Grotjahn et al. (2016) reviewed statistical methods, dynamics, modeling, and trends related to North American extreme temperature events and large-scale meteorological patterns. Sousa et al. (2018) studied European temperature responses to blocking and ridge regional patterns. Previous research also highlighted the role of human-induced climate change in increasing the likelihood of extreme heat events. Rapid attribution studies concluded that human-caused climate change significantly increased the probability of the 2021 WNA heatwave. While the observed and projected increases in heat events correlate with mean background warming, a quantitative estimation of the atmospheric circulation's contribution to the 2021 WNA heatwave was lacking. This study builds on this prior work by providing a quantitative assessment of the heat dome's role and analyzing the impact of background warming.

The study utilized observational data (GHCN-D, ERA5) and simulations from the Community Earth System Model version 1 (CESM1) Large Ensemble. The ERA5 data included daily maximum temperature (tmax), geopotential height at various pressure levels (z500, z300, z700, z850), and soil moisture from 1959-2021. CESM1 provided historical, RCP8.5, and preindustrial control simulations. The TXx7 index, representing the annual summer (June-August) maxima of the 7-day running mean of daily maximum temperature anomaly, was used to characterize the heatwave. The flow analog method was employed to quantify the contribution of atmospheric circulation to extreme temperatures. For each day of the 2021 heatwave (June 27-July 3), 20 analog days with the most similar eddy geopotential height anomalies (eddy z500) were selected from 1959-2020. The distribution of tmax anomalies conditional on the circulation was reconstructed. A similar approach was used for soil moisture, investigating its feedback on temperature extremes. Sensitivity tests examined the influence of domain size, circulation proxy, and event duration. Bias correction was applied to CESM1 simulations to improve comparability with observations. The probability ratio (PR) and fraction of attributable risk (FAR) were calculated to compare the probabilities of hot extremes in historical and preindustrial control simulations. Finally, projections of the frequency and population exposure to 2021-like heat extremes under different warming levels (1.5 °C, 2 °C, 3 °C) were made using the RCP8.5 scenario and SSP3 and SSP5 population projections.

The heat dome over the WNA explained over 50% of the observed temperature anomalies during the 2021 heatwave. The intensities of hot extremes associated with similar heat dome circulations increased faster than background global warming in both historical changes and future projections. This relationship was partly attributable to soil moisture-atmosphere feedback, with drier soil conditions exacerbating the heatwave through reduced evapotranspiration and decreased cloud cover. The probability of 2021-like heat extremes is projected to increase significantly due to background warming, enhanced soil moisture-atmosphere feedback, and a moderately increased probability of heat dome-like circulation. Population exposure to these extremes will also increase substantially. Limiting warming to 1.5 °C instead of 2 °C (3 °C) would avoid 65% (92%) of the increase in the frequency of 2021-like heat extremes and 53% (89%) of the increase in population exposure under the RCP8.5-SSP5 scenario.

The findings highlight the disproportionate impact of heat domes on extreme heat events under a warming climate. Even without significant changes in heat dome frequency or intensity, background warming and enhanced soil moisture-atmosphere feedback contribute to intensified extreme temperatures. This non-linear response implies that future heat extremes might be significantly more severe than simple extrapolations from current trends would suggest. The results underscore the importance of considering both background warming and atmospheric circulation patterns in climate change impact assessments. The study's projections demonstrate the substantial benefits of limiting global warming, highlighting the urgency of climate mitigation efforts to reduce the severity and frequency of future heatwaves.

This study quantitatively demonstrates the crucial role of heat domes in exacerbating extreme heat events in the WNA, particularly under global warming. The non-linear amplification of heat extremes due to soil moisture-atmosphere feedback emphasizes the need for comprehensive climate change mitigation strategies. Future research should investigate other feedback mechanisms and refine projections by incorporating uncertainties in future greenhouse gas emissions and population growth. Further research on the dynamics of heat dome formation and evolution is also warranted.

The study acknowledges limitations in the flow analog method, including potential sensitivity to domain size, circulation proxy, and event duration. Sampling uncertainty in analog selection may also influence results. The study did not explicitly consider other feedback processes, such as snow/ice albedo feedback, and the potential for autocorrelation in analog selection. Model dependence remains a potential concern, and the higher climate sensitivity of the CESM1 model used may lead to stronger warming projections compared to other models.