Stationary waves, large-scale atmospheric circulation anomalies lasting a month or longer, significantly influence regional climates and contribute to extreme weather events. The unprecedented 2021 Northwestern North American heatwave exemplifies this, linked to an anomalous anticyclonic circulation. While extensive research exists on future projections of wintertime stationary waves, summertime responses, especially at mid-latitudes, remain less understood. Previous studies project weakening tropical summertime stationary waves due to increased ocean moisture and reduced land-sea contrast, while mid-latitude responses are more complex. Over Northwestern North America, a projected increase in heat-dome-like anticyclonic circulation probability under high-emission scenarios is noted, but the underlying mechanisms remain unclear. This study aims to address this gap by identifying the dominant physical processes driving projected changes in Northwestern North America's stationary wave circulation, focusing on the robust strengthening of anticyclonic circulation in the upper troposphere by the end of the 21st century under intermediate and high-emission scenarios. By using a stationary wave model and Rossby wave ray tracing, the researchers aim to attribute these enhancements to specific forcing mechanisms, improving our understanding of future heatwave changes.

Existing literature highlights the significant role of stationary waves in shaping regional climates and causing extreme weather events. Studies focusing on wintertime stationary waves generally project weakening circulations at mid-high Northern Hemisphere latitudes under high-emission scenarios. In contrast, summertime stationary wave projections are less explored. Research indicates a projected weakening of tropical summertime stationary waves, linked to increased ocean moisture and reduced land-sea contrast in moist static energy. However, mid-latitude responses are more varied, with some studies suggesting increased probability of heat-dome-like anticyclonic circulations over Northwestern North America under high-emission scenarios. These projections are often based on limited datasets and lack detailed mechanistic explanations. The impact of stationary waves on heat extremes is well documented, but the projected changes in stationary wave circulation over Northwestern North America and their driving mechanisms remain a significant research gap, motivating this study.

This study employs a multi-faceted approach. First, it utilizes data from the Coupled Model Intercomparison Project Phase 6 (CMIP6), specifically focusing on 26 models' historical simulations (1979-2014) and future projections under high-emission (SSP5-8.5) and intermediate-emission (SSP2-4.5) scenarios. Monthly and daily data from the European Centre for Medium-Range Weather Forecasts' fifth-generation atmospheric reanalysis (ERA5) are also used for comparison and validation. An eddy meridional wind dipole index, defined as the difference in eddy meridional wind at 200 hPa between the western and eastern flanks of the anticyclonic circulation, is used to quantify the intensity of the anticyclonic stationary wave circulation. To understand the underlying dynamics, an idealized stationary wave model is employed. This model solves the three-dimensional nonlinear primitive equations for deviations from basic states, forced by zonally asymmetric forcings (diabatic heating, transient vorticity fluxes, and transient divergence fluxes). The model uses 3D basic states derived from CMIP6 model outputs for both historical (1995-2014) and future (2080-2099) periods. Sensitivity experiments are conducted by isolating the effects of diabatic heating, transient vorticity forcing, and transient divergence forcing. Rossby wave ray tracing is used to analyze wave propagation paths, and a stationary zonal wavenumber is calculated to characterize the Rossby waveguide. Regression analysis is used to assess the relationship between the dipole index and summer daily maximum temperature (Tmax).

The study's key findings include a projected 95% increase in the summer stationary wave amplitude over Northwestern North America by 2080–2099 under the high-emission SSP5-8.5 scenario, relative to 1995–2014. This significant increase is robust across multiple CMIP6 models. The analysis reveals a strong positive correlation between the intensified anticyclonic circulation and higher summer daily maximum temperatures (Tmax) in both present-day and future climate projections. Sensitivity experiments using the stationary wave model clearly demonstrate that diabatic heating changes over the tropical Pacific are the dominant driver of the projected stationary wave response. Specifically, increased diabatic heating in the tropical Pacific leads to an enhanced Rossby wave source in the northeastern tropical Pacific. This is further amplified by a poleward expansion of the waveguide over North America, facilitating northward propagation of Rossby waves into Northwestern North America. The increased wave activity flux contributes to the build-up of the anticyclonic circulation, consistent with both CMIP6 model simulations and the idealized model's response to tropical Pacific heating. Projection uncertainty in the stationary wave circulation is linked to uncertainty in projected changes in tropical diabatic heating, potentially influenced by air-sea interactions and El Niño-like warming patterns. Further analysis of daily CMIP6 data reveals a significant increase in the frequency of occurrence of positive dipole index events during boreal summer under the SSP5-8.5 scenario.

The findings strongly suggest that the projected intensification of heat-dome-like stationary wave circulation over Northwestern North America will significantly exacerbate heatwave risk in the region. The identification of tropical Pacific diabatic heating as the primary driver provides crucial insights into the underlying mechanisms, highlighting the long-range teleconnections that influence regional climate extremes. The consistency between CMIP6 model simulations and the idealized stationary wave model strengthens confidence in the projection. The observed expansion of the waveguide amplifies the effect of the enhanced Rossby wave source, further underscoring the importance of considering both wave generation and propagation pathways. The link between projection uncertainty and uncertainty in tropical diabatic heating emphasizes the need for improved understanding of tropical air-sea interactions and their role in shaping future climate projections. The increased frequency of intense heat-dome-like events underscores the urgency of addressing climate change to mitigate the risks associated with extreme heat.

This study robustly demonstrates a projected substantial increase in heat-dome-like stationary wave circulation over Northwestern North America by the end of the 21st century, particularly under high-emission scenarios. The dominant role of tropical Pacific diabatic heating in driving this change, coupled with the expanded waveguide, provides a comprehensive mechanistic explanation. These findings highlight the crucial need for improved climate models and further research into tropical air-sea interactions to reduce uncertainties in future climate projections and inform effective adaptation strategies to mitigate the growing risk of extreme heat events.

The study's findings are based on CMIP6 climate models, which have inherent limitations and uncertainties. The idealized stationary wave model simplifies the complex interactions within the atmosphere, potentially leading to discrepancies between model results and real-world observations. The focus on diabatic heating as the dominant driver doesn't fully account for potential contributions from other factors, such as land-atmosphere interactions. Further research is needed to better quantify the relative contributions of various factors to the projected changes in stationary wave circulation and heatwaves.