North American heatwave 2021 - Rossby waves

The story: Average daily highs for June in places like Lytton are around 16.4°C (61.5°F) but on June 28th the village reported a high of 47.9°C (118.2

North American heatwave 2021 - Rossby waves

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• The story: Average daily highs for June in places like Lytton are around 16.4°C (61.5°F) but on June 28th the village reported a high of 47.9°C (118.2°F), beating its record from the previous day of 46.6°C which was 1.6°C hotter than any temperature recorded anywhere in Canada, ever. It was also the highest temperature ever recorded at a latitude above 50° North. Canada’s west coast and much of America’s Pacific north-west were baking in a heatwave.
• Understanding it technically: The heatwave was caused by a phenomenon called “heat dome”, in which an area of high pressure in the atmosphere stops the air beneath it escaping. Heatwaves occur when there is high air pressure at ground level. The high pressure is a result of air sinking through the atmosphere. As the air descends, the pressure increases, compressing the air and heating it up, just like in a bike pump. Sinking air has a big warming effect: the temperature increases by 1 degree for every 100 meters the air is pushed downwards.

1. High-pressure systems are an intrinsic part of an atmospheric Rossby wave, and they travel along with the wave. Heatwaves occur when the high-pressure systems stop moving and affect a particular region for a considerable time.
2. When this happens, the warming of the air by sinking alone can be further intensified by the ground heating the air – which is especially powerful if the ground was already dry. In the northwestern US and western Canada, heatwaves are compounded by the warming produced by air sinking after it crosses the Rocky Mountains.

• Rains in Japan: Aroung 26th June 2021, it rained over the western Pacific Ocean near Japan. It kicked off the North American heatwave by two events

1. It disturbed the atmosphere to set off an undulation in the jet stream – a river of very strong winds in the upper atmosphere – that atmospheric scientists call a Rossby wave (or a planetary wave). Then the wave was guided eastwards by the jet stream towards North America.
2. The wave amplified, until it broke just like an ocean wave does when it approaches the shore. When the wave broke it created a region of high pressure that remained stationary over the North American northwest for many days.
3. This is where the rain event's importance was seen - the locked region of high pressure air set off one of the most extraordinary heatwaves we have ever seen, smashing temperature records in the Pacific Northwest of the United States and in Western Canada as far north as the Arctic.

• Rossby waves: A high-pressure system is usually part of a specific type of wave in the atmosphere – a Rossby wave. These are very common, and form when air is displaced north or south by mountains, other weather systems or large areas of rain. Rossby waves are the main drivers of weather outside the tropics, including the changeable weather in the southern half of Australia. Occasionally, the waves grow so large that they overturn on themselves and break. The breaking of the waves is intimately involved in making them stationary.

1. The seeds for the Rossby waves that trigger heatwaves are located several thousands of kilometres to the west of their location (as they were for the June-July heatwave). So for northwestern America, that’s the western Pacific. Australian heatwaves are typically triggered by events in the Atlantic to the west of Africa.
2. Another important feature of heatwaves is that they are often accompanied by high rainfall closer to the Equator. When southeast Australia experiences heatwaves, northern Australia often experiences rain. These rain events are not just side effects, but they actively enhance and prolong heatwaves.
3. Higher anthropogenic carbon dioxide in the atmosphere is increasing Earth’s average surface temperature, but while this average warming is the background for heatwaves, the extremely high temperatures are produced by the movements of the atmosphere in the instant cases.
4. A major cause for heat domes in North America was a sharp difference in temperature between the east and west of the Pacific over the preceding winter.  The water movements creating such gradients may be changing because of global warming, but the link isn’t clear. But it is clear that warmer climate overall (average global temperatures having risen by at least 1°C from pre-industrial levels) make the temperature spikes experienced in heatwaves that much more unbearable. [as per America’s National Oceanic and Atmospheric Administration NOAA]

• Climate and weather studies: Big questions remain - How will events that seed Rossby waves change? How will the jet streams change? Will more waves get big enough to break? Will high-pressure systems stay in one place for longer? Will the associated rainfall become more intense, and how might that affect the heatwaves themselves? Some of the key processes involved are too detailed to be explicitly included in current large-scale climate models, which agree that global warming will change the position and strength of the jet streams. But the models disagree about what will happen to Rossby waves. New computer models of the world’s climate are needed, that explicitly include some of the fine detail (about a km in size) of weather. 

 

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