In the memory of a climatologist, we had never seen that. In Portland, Oregon and Seattle, Washington, two large cities in the northwestern United States known for their cold and humid climates, the temperature reached its highest level ever since records began, in 1940. It was 46.1 ° C at the Portland airport on Monday afternoon.
But it is western Canada that still holds the palm. For the first time in its history, the country recorded temperatures above 45 ° C on its territory, two days in a row. In Lytton, a village northeast of Vancouver, the mercury climbed to 47.9 degrees on Monday. So much so that in places, schools and vaccination centers are closing, and residents are taking refuge in “refreshment centers”. Much of North America suffocates in a heat wave of rare intensity, popularized as the “heat dome”. Explanations.
What is a “heat dome”?
There are two types of atmospheric circulation during a heat wave: a movement of air mass from south to north, or the formation of a “dome”, which in scientific language is called a blocking atmospheric circulation. High pressures that trap hot air in an area. “This means that the descending air will compress the air in the lower layers, and this compression heats the air,” develops Philippe Drobinski, director of the dynamic meteorology laboratory. When you inflate a tire, you inject high pressure, and if you touch it right after, you will notice that it is hot. “
“This mass of air stagnates, and the heat reservoir thickens, increases in altitude, hence the expression dome,” explains Christophe Cassou, research director at the Climate, Environment laboratory in Toulouse. The records are broken day by day because the temperature drops less and less at night. The atmosphere is becoming more and more dry, the absence of clouds draws an amplifying pattern, it is a vicious circle. “
North America owes this warmth to the jet stream, the one that allows you to arrive in advance on a New York-Paris flight blowing from west to east. It appears on the American continent at the border between the United States and Canada, and arrives in Europe at the level of Scandinavia.
“The greater the temperature difference between the polar and subtropical regions, the stronger the jet stream. However, this gap weakens in summer. Instead of traveling in a straight line, the current will wave slowly ”, specifies Philippe Drobinski. The hot air thus stagnates in a specific place, hence the idea of a “dome” of heat. “These oscillations between North and South, like the Greek letter omega (Ω), currently bring fresh air to us, and warm air over there”.
Other climatologists prefer to speak of atmospheric blockage. “The anticyclone has created an area, which is measured in thousands of km, in which the sky is clear and absorbs a lot of solar radiation,” explains physicist Fabio D’Andrea.
What role does global warming play?
It amplifies a situation already known. Ripples existed long before climate change, but climate change tends to weaken the jet stream: by slowing down, ripples amplify and lengthen the duration of heat domes. “The blocking is quite classic. On the other hand, temperature records are associated with climate change, ”confirms Christophe Cassou. Even if the impact of global warming on the frequency of the phenomenon remains a subject of study, scientists already know that it contributes to heating more in a blockage situation.
According to a study published on May 31 in the journal Nature Climate Change, more than a third of deaths worldwide from heatwaves are directly attributable to global warming. The researchers draw up a toll of 100,000 deaths per year linked to heat and directly attributable to climate change. This figure could be underestimated due to a lack of data for certain regions of the world particularly affected by heat waves, such as Central Africa or South Asia.
How is this dangerous?
The episode will last several days. “A prolonged, dangerous and historic heat wave will persist throughout this week,” Environment Canada warned, issuing alerts for British Columbia, Alberta and parts of Saskatchewan, Northern Territories. West and Yukon, border with Alaska.
“We are the second coldest country in the world and the snowiest”, so little used to this “desert heat, very dry”, confided to AFP David Phillips, chief climatologist of Environment Canada. In humans, the body is put to the test. “Living several days at 45 degrees is not usual for these regions, the body is not able to regulate its temperature over time,” warns Philippe Drobinski.
The extreme heat wave also causes “thermal stress” for the ecosystem and the biosphere. With these Saharan temperatures, conifers and animals suffer greatly. In the video below, a family of bears look to cool off in a swimming pool.
“Twenty years later, French forests still bear the scars of the 2003 heatwave, which was also a dome of heat,” recalls Christophe Cassou. We will have to wait to see the consequences of this in the American West. “
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