Graham Taylor
In mid-June of 2021, weather forecasters in the Pacific Northwest saw an extraordinary area of high pressure build in the upper atmosphere weeks out in weather models. Given the decaying accuracy of weather models more than a week or so in the future, this potentially record breaking feature was assumed to be part of “fantasy land” – simulations of the atmosphere so far in the future that they are unlikely to become true. However, as the days grew closer, models did not show a decrease in the strength of the pattern, and forecasters and the public prepared for record breaking heat in a region commonly associated with mild temperatures. The feature showing up in models is referred to as a “ridge” – an area of relatively high atmospheric pressure, often visually identified as a large poleward bend in winds in the mid-troposphere. Underneath ridge patterns, large-scale sinking of air compresses and can lead to excessive heat, and ridge patterns have a tendency to become stuck in place for days in configurations known as blocks. In the case of the June 2021 Pacific Northwest heatwave, a massive ridge led to all-time record shattering high temperatures, in many cases near 10 degrees warmer than the previous record, with deadly results.
While climate scientists have an increasingly strong understanding of average global warming estimates, the effects of warming on these large-scale circulation patterns remains an active area of research. Summer hot extremes are not the only phenomenon affected by ridges – ridges are a common feature of normal variability, and while they are typically associated with warm and dry weather, they can also affect atmospheric rivers and water resources, and potential changes to these features would be highly impactful in the Pacific Northwest and across US. Common questions that scientists seek to answer include: How will atmospheric circulation features like ridges be affected by global warming? Will temperatures under future ridges warm at a faster rate than average temperatures? Will the seasonality of ridges change? In a general sense, these atmospheric waves form as a result of the atmosphere being a disproportionately heated fluid on a rotating sphere – if the magnitude and distribution of that heating changes then it is hypothesized that the typical patterns of atmospheric circulation may change as well.
Recent research set out to study the seasonal character of ridges over the Pacific Northwest, and how ridges behave in the latest suite of global climate models by the end of the 21st century. This is accomplished by first using reanalysis data, which is a comprehensive dataset of historical atmospheric conditions, to define the historical average shape and seasonal frequency of ridges over the Pacific Northwest. With the historical climatology of ridges defined, climate model projections for the time period of 2071-2100 are used to analyze differences between the projections of a future climate under a high emissions scenario and the historical climate. Confidence in the accuracy of these models is increased by recently published research that found climate models are able to accurately simulate the large-scale circulation patterns over the Pacific Northwest.
In general, the character of ridges over the Pacific Northwest in the future of models resembles the past – a result that may seem surprising, but contributes important knowledge to regional climate science. However, there are some subtle differences found. Models show a decrease in the number of ridges per season in winter, spring, and fall, but an increase in ridges over the interior west in summer, which may have implications for summer heat in an already hot region. Additionally, warm extreme temperatures scaled similarly to average warming in every season except summer, which saw more warming in the warm extremes than average temperatures, especially over the central part of the continent. While more research is needed to full investigate the dynamics of changing atmospheric circulation, this research is an important step in exploring how global warming may affect the large-scale circulation patterns that can be responsible for some of the most impactful midlatitude weather events. Such increased knowledge of our changing climate on a regional scale will help planners prepare for future challenges.
Further reading:
Gillett, N. P., Cannon, A. J., Malinina, E., Schnorbus, M., Anslow, F., Sun, Q., Kirchmeier-Young, M., Zwiers, F., Seiler, C., Zhang, X., Flato, G., Wan, H., Li, G., & Castellan, A. (2022). Human influence on the 2021 British Columbia floods. Weather and Climate Extremes, 36, 100441. https://doi.org/10.1016/j.wace.2022.100441
Loikith, P. C., Singh, D., & Taylor, G. P. (2022). Projected Changes in Atmospheric Ridges over the Pacific–North American Region Using CMIP6 Models. Journal of Climate, 35(15), 5151–5171. https://doi.org/10.1175/JCLI-D-21-0794.1
Neal, E., Huang, C. S. Y., & Nakamura, N. (2022). The 2021 Pacific Northwest Heat Wave and Associated Blocking: Meteorology and the Role of an Upstream Cyclone as a Diabatic Source of Wave Activity. Geophysical Research Letters, 49(8), e2021GL097699. https://doi.org/10.1029/2021GL097699
Taylor, G. P., Loikith, P. C., Aragon, C. M., Lee, H., & Waliser, D. E. (2022). CMIP6 model fidelity at simulating large-scale atmospheric circulation patterns and associated temperature and precipitation over the Pacific Northwest. Climate Dynamics. https://doi.org/10.1007/s00382-022-06410-1
https://oceanservice.noaa.gov/facts/rossby-wave.html
https://www.weather.gov/jetstream/basic
https://www.noaa.gov/stories/what-are-atmospheric-rivers
https://en.wikipedia.org/wiki/2021_Western_North_America_heat_wave
Graham Taylor is a PhD student in the Geography department at Portland State University. His research focuses on atmospheric patterns over the Pacific Northwest of North America, and how they may be affected by global warming. For any questions please contact graham.taylor@pdx.edu
Early Career Climate Network 