In late June and early July of 2021, Pacific Northwest region over North America experienced an unprecedented heatwave, which results in approximately 1000 deaths and numerous wildfires. During the heatwave, a strong anticyclonic circulation of stationary wave in the upper troposphere, named as heat-dome-like stationary wave, plays an important role in shaping the regional weather and climate condition that are responsible for this extreme event, via large-scale subsidence which would lead to cloud-free condition and persistent downward shortwave radiation. Similarly, the 1988 drought over the central United States and the catastrophic 2003 European heatwave were also associated with abnormal stationary waves in the upper troposphere. However, future changes in summer stationary waves over Pacific Northwest and the underlying driving factors remain unclear. In this study, we investigate the projected changes in the anticyclonic stationary wave circulation over Pacific Northwest using data from the Coupled Model Intercomparison Project Phase 6 and diagnose the circulation changes using a stationary wave model. This study is publish on the npj Climate and Atmospheric Science.

Our findings reveal a significant 95% increase in the summer stationary wave amplitude over Pacific Northwest under the high-emission scenario in 2080–2099 relative to 1995–2014 (Figure 1). To quantify the intensity of the anticyclonic stationary wave circulation over Pacific Northwest, we define an eddy meridional wind dipole index as the difference in eddy meridional wind at 200 hPa between the western and eastern flanks of the anticyclonic circulation (boxed areas in Figure 1a). Increasing trends in the dipole index are found in both the reanalysis dataset (1979–2018) and historical simulations of 26 CMIP6 models and continue to increase in the future (Figure 1b). The dipole index is projected to increase by 95% (–36% to 226%) during 2080–2099 under the SSP5-8.5 scenario (Figure 1b).

To demonstrate the dynamical control of stationary waves on the heat extremes, we regress the detrended anomalies of summer daily maximum temperature (Tmax) onto the detrended anomalies of the dipole index from 1979 to 2014 in the reanalysis dataset (Figures 1c). The results show a significant positive anomaly of Tmax over Northwestern North America, suggesting that a stronger anticyclonic stationary wave circulation over Northwestern North America leads to a higher Tmax. Similar regression pattern of Tmax against the detrended dipole index is also obtained for the future climate (from 2050 to 2099) under both high-emission (Figure 1d). This is further corroborated by the significant correlation between the dipole index and the Tmax averaged over Northwestern North America across models (Figure 1e and Supplementary Figure 1c). Hence the projected strengthening of the stationary wave circulation over Northwestern North America will strongly influence the local extreme heat events.

            Diagnosis using the stationary waves model reveals the dominant role of the tropical Pacific heating in driving the enhancement of the stationary waves over Northwestern North America at the end of the 21st century (Figure 2). The diabatic heating changes over the tropical Pacific which is related to the El Niño-like warming pattern will induce a Rossby wave source in the northeastern tropical Pacific. Besides, in the warmer climate, a northward expanded waveguide in North America can be seen, favoring a more northward propagation of Rossby waves into Pacific Northwest region under the changing climate. As a result of both the enhanced wave source and the waveguide expansion, more northward wave activity flux is found over the Pacific coast of the United States under the changing climate (vectors in Figure 5c). This enhanced wave activity flux feeds into Pacific Northwest region, helping the buildup of the anticyclonic circulation there. These diagnostic results from the CMIP6 model simulations are consistent with the wave behavior in the stationary wave model forced by the tropical Pacific heating.

At the intraseasonal scale, we further analyze the daily outputs from CMIP6 models and find a significant increase in the frequency of occurrence of the positive dipole index during the boreal summer in 2080–2099 under the SSP5-8.5 scenario, compared to 1995–2014 (Figure 3). Interestingly, the robust increase is not found in other seasons. These suggest that we would experience more intense and frequent heat-dome-like weather patterns during the boreal summer over Pacific Northwest in the future.

            In summary, our study demonstrates a robust enhancement of anticyclonic stationary wave circulation over Pacific Northwest across the CMIP6 models. Although the underlying cause of the robust changes in the stationary wave circulation remains to be understood, it is likely tilting the circulation distribution over Pacific Northwest to be more favorable for heatwaves, thereby amplifying the mean warming effect of the increasing greenhouse gases. Further studies are needed to explore the stationary wave response and the associated effects over other regions.

Citation: Chen, Z., Lu, J., Chang, CC. et al. Projected increase in summer heat-dome-like stationary waves over Northwestern North America. npj Clim Atmos Sci 6, 194 (2023). https://doi.org/10.1038/s41612-023-00511-2