July 28, 2021

What is the dome of heat that suffocates the northwest of the Americas?

Several hundred sudden deaths, hospitalizations on the rise and the increase in forest fires: western Canada and the United States are suffocating under the effect of unbearable temperatures. This unprecedented heat wave is due to a known climatic phenomenon, called the heat dome, which has grown exceptionally large this year. Events set to multiply with climate change.

  • What is a heat dome?

It is linked to a large and very powerful blocking high pressure system which persists in one place, causing the air masses to stagnate. This high pressure zone also promotes subsidence, that is to say the descent of air which, under the effect of compression, heats the atmosphere. The phenomenon is amplified by drought, important in the northwest of the American continent, and strong sunshine. As the trapped heat continues to heat up, the system acts like a lid on a pot, earning it the name of a heat dome.

This zone prevents the circulation of air masses from west to east, and deflects the jet stream, these strong winds located at altitude and which revolve around the Earth. As they go around the dome, the cooler air masses form an omega circulation, shaped like the Greek letter.

Research conducted by la National Oceanic and Atmospheric Administration (NOAA) show that heat domes are more likely to form during years with a La Niña phenomenon such as in 2021, when the waters are cool in the eastern Pacific and warm in the western Pacific. This temperature difference creates winds that blow warm air from the West Pacific towards the North American continent.

  • Is this heat wave unprecedented?

This blocking situation is “Classic for the region and the heat waves there are often associated with this system”, but the temperatures reached this year are “Unpublished”, comments Aurélien Ribes, Météo France researcher at the National Meteorological Research Center, specializing in extreme events and heatwaves. “These extraordinary temperatures are completely out of the statistics”, replies the climatologist Robert Vautard, director of the Pierre-Simon-Laplace Institute.

Many records fell this week: over 45 ° C at 50e parallel, 49.6 ° C in Canada, at Lytton (British Columbia), while the previous record was 45 ° C, in 1937, 44 ° C at the Portland airport in the United States. Records were broken for days in a row and temperatures hardly dropped at night, exceeding daytime averages for the end of June. This heat dome is also exceptional in that it occurs very early in the year.

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