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An unprecedented heatwave struck much of Western North America in June 2021. The unusual warming, caused by a phenomenon called a ‘heat dome’, may have been led by high-altitude winds known as planetary waves, according to a new study.
Planetary waves occur naturally in the mid-latitudes, largely due to the Earth’s rotation. It transfers heat from the tropics towards the poles and cold air towards the tropics. Under specific conditions, these waves stall in place for a few days, leading to extreme weather conditions such as heat waves.
Further, local soil moisture conditions also acted as a precursor to the 2021 heat dome event, which affected Oregon, Washington, Idaho, northern California, western Nevada, and British Columbia, the study published in journal Proceedings of the National Academy of Sciences noted.
Attribution studies suggested that climate change caused the event to be at least 1 to 2 degrees Celsius warmer. “It is clear that such a temperature anomaly is very rare and raises the question of whether there are other processes involved that are not properly resolved by current generation model simulations that form the basis of these attribution exercises,” the researchers wrote in the paper.
The 2021 heat dome event was attributed to a high-pressure system characterised by descending air. This inhibited cloud formation and enhances solar radiation at the surface. The high-pressure system may have resulted in the buildup of unsaturated water vapour, which likely led to an intense heatwave.
But this alone cannot explain the severity of the event. To gain further insights into the physical mechanisms driving the uniquely extreme nature of the heatwave event, the researchers focused on Oregon and Washington in the Pacific Northwest of the continental United States.
The team followed the events that led to the heatwave, from its origins in the beginning of June 2021, when a burst of heat generated upwind of the region contributed to an initial temperature peak. Soon, temperature anomalies (temperature deviations from normal) increased steadily throughout the month, with record-breaking values being observed on June 29.
The analysis revealed that the planetary wave over the Pacific Northwest amplified before the heat dome event. At certain frequencies and wavelengths (short-wavelength), these waves can be significantly amplified through resonance, resulting in heat extremes. This amplification is likely causing quasi-stationary atmospheric planetary waves, resulting in heat extremes.
The past two decades in particular have seen several record-shattering summer heat extremes across the Northern Hemisphere mid-latitudes, including the European heat wave of 2003, the Russian heat wave in 2010 and the Texas heat wave and drought in 2011.
The paper highlighted that each of these events was associated with high-amplitude quasi-stationary atmospheric Rossby waves or planetary waves.
This phenomenon, however, is not well captured in current-generation climate models, but it is becoming more prevalent due to high warming in the Arctic, the study noted.
Further, the planetary wave amplification likely led to a deficit of soil moisture. This created the conditions for atmospheric warming observed during the heat dome.
Higher temperatures increase atmospheric demand for water, resulting in increased evaporation and leading to a steady loss of soil moisture, the researchers explained. Neglecting such mechanisms in climate model analyses could underestimate the future likelihood or severity of extreme continental heat waves.
“Our findings hold the potential for more skillful predictions of low-probability yet impactful weather extremes that can have devastating consequences,” the scientists wrote.