Factors That Influenced the 2021 Pacific Northwest “Heat Dome”

Heatwaves have become more common in recent decades, and intense heat waves are more frequent in the US West. In the summer of 2021, the Northwest experienced a historic heatwave, breaking high-temperature records by several degrees and triggering concerns over climate extremes and their impacts on human health and society. 

The anomalous Pacific Northwest “Heat Dome” event in June 2021 registered temperatures surpassing 116°F (47 °C) in Portland, Oregon, and 107°F (42 °C) in Seattle, Washington. Hot temperatures linger from late June to early July.

Science News reports that the average temperatures in Oregon, Washington, Idaho, northern California, western Nevada, and British Columbia during this anomalous heatwave were 3.6°C warmer than the average summer temperature worldwide from 1951 through 1980 and the hottest in a millennium.

As residents grappled with sweltering temperatures, scientists explored the causes and reasons behind this weather anomaly. 

Researchers from the University of Pennsylvania and the Potsdam Institute for Climate Impact Research (PIK) delved into how resonant planetary wave amplification was the precursor to the 2021 Pacific Northwest Heat Dome event, a record-breaking incident.

The authors of the paper “Role of atmospheric resonance and land-atmosphere feedbacks as a precursor to the June 2021 Pacific Northwest Heat Dome event,” published in the Proceedings of the National Academy of Sciences (PNAS), looked at how certain atmospheric conditions and interactions between the land and the atmosphere contributed to record-breaking temperatures during the heat incident.

“Our study shows that anomalous summer jet stream behaviour—which we know is favoured by human-caused warming yet isn’t well captured by current generation climate models—contributed to the unprecedented 2021 Pacific Northwest’ Heat Dome” event,’ says Mann, Presidential Distinguished Professor and co-author of the paper (Magubane, 2024).

The planetary waves, also known as the Rossby waves, are large-scale oscillations of the midlatitude atmospheric flow. Because of the planet’s rotation, they usually form a snaking shape over the Earth. These waves lead to persistent surface weather systems, often resulting in extreme events such as the change of weather over the Pacific Northwest.

Researchers find the planet wave amplified due to resonance, a process where certain atmospheric conditions align to reinforce the wave’s strength and persistence. This wave amplification resulted in the soil moisture deficit, contributing to record-shattering heat during the 2021 heat dome event. According to the authors, these extreme water conditions do not occur in isolation but because of the complex interaction between the Earth’s atmosphere and its terrestrial landscape.

“Natural processes include atmospheric dynamics like air pressure and temperature variations, ocean currents, and natural climate cycles, while human-induced climate change involves alterations to the Earth’s atmosphere due to emissions of greenhouse gases, deforestation, and urbanisation. It is the synergy between these natural and human-driven processes that shape the frequency, intensity, and characteristics of these weather phenomena” (Magubane, 2024)

“The team’s work provides insights into the mechanics of this heatwave and pinpoints the significant roles of amplified atmospheric circulation patterns and soil moisture, an interplay that created the perfect conditions for a heat dome to form (Magubane, 2024).”

Researchers note that the phenomenon of resonant planetary wave amplification interacting with the land surface feedback as a catalyst for creating extreme heat events is not well represented in state-of-the-art climate models, which could potentially cause underestimates in the future likelihood or severity of extreme continental heat waves, highlighting the importance of including these preconditioning feedback mechanisms in climate model analyses.

The authors concluded that their findings hold the potential for more skilful predictions of low probability yet impactful weather extremes that can have devastating consequences.

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