However, while this average warming is the background for heatwaves, the extremely high temperatures are produced by the movements of the atmosphere we talked about earlier. We know increased carbon dioxide in the atmosphere is increasing Earth’s average surface temperature. Understanding the mechanics of what causes heatwaves is very important if we want to know how they might change as the planet gets hotter. What will climate change mean for heatwaves? These rain events are not just side effects, but they actively enhance and prolong heatwaves. When southeast Australia experiences heatwaves, northern Australia often experiences rain. Australian heatwaves are typically triggered by events in the Atlantic to the west of Africa.Īnother important feature of heatwaves is that they are often accompanied by high rainfall closer to the Equator. So for northwestern America, that’s the western Pacific. Importantly, just as for the recent event, the seeds for the Rossby waves that trigger heatwaves are located several thousands of kilometres to the west of their location. The breaking of the waves is intimately involved in making them stationary. Occasionally, the waves grow so large that they overturn on themselves and break.
#REEDER HEATING DRIVERS#
Rossby waves are the main drivers of weather outside the tropics, including the changeable weather in the southern half of Australia. We've learned a lot about heatwaves, but we're still just warming up These waves are very common, and they form when air is displaced north or south by mountains, other weather systems or large areas of rain. This leaves two questions: what makes a high-pressure system, and why does it stop moving?Īs we mentioned above, a high-pressure system is usually part of a specific type of wave in the atmosphere – a Rossby wave. In the northwestern US and western Canada, heatwaves are compounded by the warming produced by air sinking after it crosses the Rocky Mountains. 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. Heatwaves occur when the high-pressure systems stop moving and affect a particular region for a considerable time.
High-pressure systems are an intrinsic part of an atmospheric Rossby wave, and they travel along with the wave. The North American heatwave has seen fires spread across the landscape. Lytton in British Columbia hit 49.6℃ this week before suffering a devastating wildfire. This is where our innocuous rain event made waves again: 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. When the wave broke it created a region of high pressure that has remained stationary over the North American northwest for the past week. Then the wave was guided eastwards by the jet stream towards North America.Īlong the way the wave amplified, until it broke just like an ocean wave does when it approaches the shore.
There was nothing especially remarkable about this rain event, yet it made big waves twice.įirst, it disturbed the atmosphere in just the right way 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). Eight days ago, it rained over the western Pacific Ocean near Japan.
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